Cultural Resources SeriesReport No. CELMN/PD-RN

U.S. Army Corpsof EngineersNew Orleans District

PHASE II CULTURAL RESOURCE INVESTIGATIONOF SUBMERGED ANOMALIES, BRETON SOUNDDISPOSAL AREA, PLAQUEMINES PARISH, LOUISIANA

September 1993

FINAL REPORT

R. Christopher Goodwin & Associates, Inc.5824 Plauche StreetNew Orleans, LA 70123

PREPARED FOR:

U.S. Army Corps of EngineersNew Orleans District --.---........

P.O. Box 60267New Orleans, LA 70160-0267

Unclassified. Distribution is unlimited.

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I Form ApprovedREPORT DOCUMENTATION PAGE OMB No. 0704-0188

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4. PERFORMING ORGANIZATION REPORT NUMBER(S) 5. MONITORING ORGANIZATION REPORT NUMBER(S) COELMN/PD-93/11

6a. NAME OF PERFORMING ORGANIZATION 6b. OFFICE 7a. NAME OF MONITORING ORGANIZATIONR. Christopher Goodwin & Associates, Inc. SYMBOL U.S. Army Corps of Engineers

(if applicable) New Orieans District

6c. ADDRESS (City, State, and ZIP Code) 7b. ADDRESS (City, State, and ZIP Code)5824 Plauche St.,New Orleans, LA 70123 P.O. Box 60267, New Orieans, LA 70160-0267

8a. NAME OF FUNDING/SPONSORING 8b. OFFICE 9. PROCUREMENT INSTRUMENT IDENTIFICATION NUMBERORGANIZATION SYMBOLU.S. Army Corps of Engineers (if applicable) DACW29-92-D-0011, Delivery Order 0003New Orleans District CELMN-PD-RN

8c. ADDRESS (City, State and ZIP Code) 10. SOURCE OF FUNDING NUMBERS

P.O. Box 60267 PROGRAM PROJECT NO. TASK NO. WORK UNITNew Orleans, LA 70160-0267 ELEMENT NO. Civil Works ACCESSION NO.

N/A Funding

11. TITLE (Include Security Classification)PHASE II CULTURAL RESOURCE INVESTIGATION OF SUBMERGED ANOMALIES, BRETON SOUND DISPOSAL AREA, PLAQUEMINESPARISH, LOUISIANA

12. PERSONAL AUTHOR(S)Jack Irion, Ph.D., and Paul Heinrich, M.A., A.B.D.

13a. TYPE OF REPORT 13b. TIME COVERED T 14. DATE OF REPORT (Year, I 15. PAGE COUNTFinal FROM TO Month, Day)1993, September 80

16. SUPPLEMENTARY NOTATION

17. COSATI CODES 18. SUBJECT TERMS (Continue on reverse if necessary and identify by block number)

FIELD GROUP SUB-GROUP anomaly chemical remnant magnetism remote sensing

biological remnant magnetism Louisiana side-scan sonar05 06 Breton Island magnetometer underwater archeology

Breton Sound Plaquemines Parish

19. ABSTRACT (Continue on reverse if necessary and identify by block number)

Five clusters of magnetic anomalies were recommended for groundtruthing following Phase I remote sensing survey of the proposed Breton SoundDisposal Area, Plaquemines Parish, Louisiana. The present project concerned Phase II investigation of these clusters. Research methods involvedmagnetic survey of the five clusters at an interval of 15 m (50 1t), and examination of four of the clusters by divers. Search and excavation procedureswere employed to locate the source of magnetic perturbation. This investigation was conducted for the U.S. Army Corps of Engineers, New OrleansDistrict, pursuant to Delivery Order 0003 of Contract DACW 29-92-D-001 1.

Diving investigations and subsequent geophysical tests determined that the magnetic anomalies were caused by discreet areas of biological orchemical remnant magnetism associated with methane or natural gas seeps common throughout coastal Louisiana. The areas of anomalous magnetismare confined to a small zone around the gas seep and produce a magnetic signature that closely resembles that produced by a shipwreck.Recommendations were made to the New Orieans District to add sub-bottom profilers to the suite of equipment deployed on cultural resource surveys.Sub-bottom records will show gas vents as a blank area or "blow out," which, when correlated with a magnetic anomaly, will suggest that the source isgeologic rather than anthropogenic.

20. DISTRIBUTION/AVAILABILITY OF ABSTRACT 21. ABSTRACT SECURITY CLASSIFICATIONX UNCLASSIFIED/UNLIMITED _ SAME AS RPT. Unclassified

DTIC USERS

22a. NAME OF RESPONSIBLE INDIVIDUAL 22b. TELEPHONE (Include Area 22c. OFFICE SYMBOLJoan Exnicios Code) '504-862-1760 CELMN-PD-RN

DD Form 1473, JUN 86 Previous editions are obsolete. SECURITY CLASSIFICATION OF THIS PAGE

PHASE II CULTURAL RESOURCE INVESTIGATION OFSUBMERGED ANOMALIES, BRETON SOUND DISPOSAL AREA,

PLAQUEMINES PARISH, LOUISIANA

FINAL REPORT

# R. Christop erG ydnPh.D.

IPrjincil1 nv r

By

Jack B. Irion and Paul V. Heinrich

R. Christopher Goodwin & Associates, Inc.5824 Plauche Street

New Orleans, LA 70123

September 1993

For

U.S. Army Corps of EngineersNew Orleans District

P.O. Box 60267New Orleans, LA 70160-0267

Contract No. DACW29-90-D-0011Delivery Order No. 0003

ii

TABLE OF CONTENTS

REPORT DOCUMENTATION PAGE ......................................... i

T IT LE PA G E .......................................................... ii

LIST O F FIG URES ..................................................... v

LIST O F TA B LES ...................................................... vii

ACKNOW LEDGMENTS ................................................ viii

DESCRIPTION OF THE PROJECT AREA .................................... 1Introd uction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1O rganization of the Report ................................................ 1N atural S etting ... .. . ... . ... ... .. .. ... .. .. ... .. .. .. .. .. . .. . .. . .. . . .. . .. 1

Hydrology of the Project Area ........................................... 1Pedology of the Project Area ............................................ 4Geology of the Project Area ............................................. 6

Unnamed Marine Complex ........................................... 6Chandeleur Island Com plex .......................................... 6St. Bernard Delta Com plex .......................................... 11Prairie C om plex ...................... ........................... 12O lder Tertiary Strata ............................................... 13

Historical Context of the Project Area ....................................... 13Previous Investigations ................................................. 17

M ETHO DO LO GY ..................................................... 28Critique of M ethodology ................................................. 29

II1. RESULTS OF INVESTIGATIONS ......................................... 32Results of Magnetic Contouring Survey ...................................... 32Results of Diving Investigations ............................................ 33Sources of Anomalous Magnetism ......................................... 40

Detrital Remnant Magnetization ......................................... 41Chemical Remnant Magnetization ....................................... 41Bacterial - Biological Remnant Magnetization ............................... 42

Investigations into the Source of Anomalous Magnetism in Breton Sound ............. 43Results of Thin Sectioning ............................................. 43

C luster A . .. . .. .. .. ... .. .. ... .. .... . ... .. .. .. .. .. .. . .. .. . . . .. . .. 43C luster G . .. .. .. .. .. .. .. ... .. ... .. ... .. ... .. .. .. . .. .. . .. . .. . .. . . 44Discussion of Thin Sections ......................................... 44

Stable Isotope Testing ................................................ 45C arbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5O xygen .................................. ..................... 46

The Formation of Carbonate-Cemented Sediments in the Northern Gulf of Mexico ....... 47Central Texas Inner Shelf ............................................. 47Louisiana Continental Slope ........................................... 48

iii

Louisiana Barrier Islands .............................................. 50A labam a - Florida Shelf ............................................... 51H ypothesis . .. .. .. .. ... .. .. ... .. .. . ... ... .. .. .. ... .. .. .. . .. . .. . .. . . 52

Summary of the Origin of Magnetic Anomalies in the Project Area .................. 52

IV. THE ARCHEOLOGICAL SIGNIFICANCE OF CHEMICALAND BIOLOGICAL REMNANT MAGNETISM ................................ 54The Origin of Magnetic Anomalies in the Project Area ........................... 54Archeological Significance ............................................... 55

REFERENCES CITED ................................................. 57

SCOPE OF SERVICES ........................................... Appendix A

iv

LIST OF FIGURES

Figure 1. A location map showing the Breton Island project area ....................... 2

Figure 2. The Loop Current (from Ichiye et al. 1973) ................................ 3

Figure 3. A map showing the areal distribution of sedimentary environmentswithin the Breton Island region (from Curtis 1960:476) ....................... 5

Figure 4. A geologic cross-section of the Chandeleur Barrier Island Chain(modified from Penland et al. 1985) ..................................... 7

Figure 5. A diagrammatic cross-section of Holocene deposits underlying shelfeast of the Chandeleur Islands (constructed from data and figuresby Penland et al. 1985) ............................................. 8

Figure 6. A diagram illustrating shoreline changes in the Chandeleur BarrierIsland Chain between 1870 and 1978 (from Penland et al. 1985) ............... 9

Figure 7. A map showing the distribution of facies associated with ChandeleurIsland Chain (modified from Suter et al. 1988) ............................ 10

Figure 8. A map showing the faults, and oil and gas fields located within theBreton Island project area (modified from Bechtel [1974] and Stanfield[19 8 1]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 9. Magnetic contour map of the Breton Sound Disposal Area ................... 18

Figure 10. Magnetic anomalies comprising Cluster A ............................... 22

Figure 11. Magnetic anomalies comprising Cluster C ............................... 23

Figure 12. Magnetic anomalies comprising Cluster D ................................ 24

Figure 13. Magnetic anomalies comprising Cluster F ............................... 25

Figure 14. Magnetic anomalies comprising Cluster G ............................... 26

Figure 15. Acoustic anomaly at Cluster G ....................................... 27

Figure 16. Magnetic contour map of Cluster A .................................... 34

Figure 17. Magnetic contour map of Cluster C .................................... 35

Figure 18. Magnetic contour map of Cluster D .................................... 36

Figure 19. Magnetic contour map of Cluster F .................................... 37

V

Figure 20. Magnetic contour map of Cluster G .................................... 38

Figure 21. Magnetic contour map showing the wreckage of a shrimp trawler inthe Gulf of Mexico (from Garrison et al. 1989:11-212, Figure 11-98) .............. 39

vi

LIST OF TABLES

Table 1. Magnetic anomaly clusters identified in Breton Sound ....................... 19

Table 2. Coordinates of anomalies derived from Phase I survey compared tocoordinates of actual anomaly source .................................. 32

vii

ACKNOWLEDGMENTS

The authors gratefully acknowledge the assistance of those individuals who contributed to theproject. Ms. Joan Exnicios and Dr. Edwin Lyon, both of the U.S. Army Corps of Engineers, New OrleansDistrict, ably served as Contracting Officer's Technical Representative and as Contracting Officer'sRepresentative, respectively. Without their assistance this project would not have been possible.

Several scholars contributed their expertise and advice in the preparation of the report and inguiding our research in the right direction. Dr. Paul Aharon, on the Faculty of the Department of Geologyand Geophysics at Louisiana State University, and his graduate assistant, Mr. Fu Baoshun, provided keyresearch on the phenomenon of Biological Remnant Magnetism. Mr. Robert Floyd, a marine archeologistwith Consella, Cook & Associates, Inc., in Lafayette, provided substantial knowledge from his years ofexperience in surveying oil fields throughout Louisiana and Texas. Mr. Thomas A. Oliver, ChiefHydrographer for Gulf Ocean Services, Inc., kindly provided information relating to the presence ofanomalous magnetism in the Chandeleur Islands.

We also acknowledge the contributions of the captains of the two vessels employed during theproject. Both Joe Kuljis, Captain of the Omeco III, and Ken Tischler, at the helm of the TGIF, are superblyskilled watermen whose enthusiastic assistance contributed greatly to the project.

The dive team for this project consisted of Dr. Jack Irion; David Beard, M.A.; Richard Swete, M.A.;Thomas Fenn, B.A.; David Robinson, B.A.; and, John Neville, M.A. Dr. Irion, and Paul V. Heinrich, M.A.,A.B.D., co-authored the report; R. Christopher Goodwin, Ph.D., provided editorial guidance. DavidCourington, B.A., and Shirley Rambeau, A.A., illustrated the report; Martha R. Williams, M.A., M.Ed., andDavid S. Robinson, B.A., edited it; and, it was produced by Kimberly S. Busby, M.A.

viii

CHAPTER I

DESCRIPTION OF THE PROJECT AREA

Introduction

This report documents the results of Phase II testing of five clusters of submerged magneticanomalies located within the proposed 1,600 ac (64.7 ha) Breton Sound Disposal Area in the Gulf of Mexicoeast of Breton Island, Plaquemines Parish, Louisiana (Figure 1) (see Irion et al. 1993 for detailed descriptionof the U.S. Army Corps of Engineers proposed project). These targets were selected for investigationbased on recommendations made following Phase I survey of the project area in 1992 (Irion et al. 1993).The proceeding investigation had concluded that these five clusters of magnetic anomalies, designated A,C, D, F, and G, afforded the greatest potential for containing historically significant cultural remains, i.e.,shipwrecks in the project area. Based on this analysis, the U.S. Army Corps of Engineers, New OrleansDistrict, funded Phase II testing to search for and identify the source of anomalous magnetism in the fivetarget areas.

Organization of the Report

This report is organized according to the format used in previous U.S. Army Corps of Engineers,New Orleans District, Phase II reports. The remainder of Chapter I describes the nature of the project, andpresents the natural and historic context of the project area. The methods and theories applied to data-gathering during the Breton Sound Phase II project are examined on Chapter II. The results of the divinginvestigations are presented in Chapter III. Specific recommendations that will alter substantially thetheoretical framework for detecting and analyzing magnetic anomalies off the coast of Louisiana arecontained in Chapter IV. The remote sensing data acquired during Phase I are compared to those obtainedduring Phase II to test the efficacy of close interval magnetic survey during the testing phase. In addition,the array of remote sensing devices currently employed in underwater survey in coastal Louisiana also isexamined in Chapter IV. The Scope of Services is presented in Appendix A.

Natural Setting

The proposed Breton Sound Disposal Area is located in the Gulf of Mexico. It lies in PlaqueminesParish, approximately 1.6 km (1 mi) east of Breton Island, the westernmost of the Chandeleur Island chain.The area measures 1.6 km (1 mi) wide and 4 km (2.5 mi) long. The project area is bounded byLouisiana South State Plane coordinates 2,690,588.912E/307,815.476N; 2,695,296.359E/307,905.019N;2,695,547.466E/294,749.345N; and, 2,690,838.315E/294,659.775N. Water depths in the project area rangefrom 2.4 to 7.6 m (8 to 25 it).

Hydroloaqy of the Proiect Area

Currents in the Gulf generally are complex and characterized by an "offshore," or open Gulf, andan "inshore," or shelf energy, regime. The open Gulf is influenced by the Loop Current (Figure 2). Theshelf circulation shows strong influence from secondary flows of the Loop Current (Garrison et al. 1989:11-51). Currents along the Louisiana coast flow in a predominantly eastward direction. Longshore currentsin the project area generally were light to moderate and ranged from approximately 0.5 Id to 1 kt. The

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strongest currents were observed in the northern part of the project area and coincided with a flood tide.The majority of the project area lay in the lee of Breton island and was unaffected by the flood tide fromthe Mississippi River.

Winds in the project area are dominated by easterly trades that flow from the southwest in thesummer and from the northeast in winter. Waves associated with winds in the Gulf generally are 1 to 1.5m (3 to 5 ft) in height. Cold fronts, termed "northers," often produce waves 3 to 4 m (5 to 16 ft) in height,and midwinter fronts can produce waves as high as 7 m (23 if). Tropical storms and hurricanes canproduce extreme conditions throughout the project area. Hurricane-generated waves over 30 m (100 1f)high have been calculated in the Gulf of Mexico just off the Mississippi Delta (Abel et al. 1988).

Pedoloaqy of the Proiect Area

Two principal sedimentary environments exist within the project area: the "open inlet lagoon" andthe "reworked Mississippi delta." The majority of the project area lies within the open inlet lagoonalsedimentary environment. The southernmost portion of the project area lies in the reworked MississippiDelta sedimentary environment (Figure 3).

The open lagoonal inlet sedimentary environment occupies two open tidal inlets that have cut intothe reworked surface of the St. Bernard Delta. One inlet lies between Breton Island and the modernMississippi Delta to the southwest, while the other lies between Breton Island and Gosier Island to thenortheast. The northern tidal inlet, which starts at the strait between Breton Island and Gosier Island, turnsabruptly southward, crosses in front of Breton Island, and merges with the southern tidal inlet. This inletis characterized by strong tidal currents and by a firm bottom of either sand, silty clay, or an erosional shelllag. Between the islands, the depth of the tidal inlet, 7 to 11 m (23 to 36 1f) below mean sea level, issignificantly greater than that of the adjacent shelf and sound. As it spreads seaward, the tidal inletdecreases to depths ranging from 3 to 4 m (10 to 13 if) (Shepard 1956:Figure 5).

Surficial sediments of the open lagoonal inlet sedimentary environment found within the northernchannel consist primarily of clayey silt (Shepard 1954). Its medium-grain size varies between 0.004 to0.0625 mm (8 to 4 phi) in diameter. The percentage of sand present varies from 1 to 20 percent within thecenter of the inlet, to about 80 percent at the edge of the inlet. Similarly, the amount of clay varies from30 to 50 percent within the center of the inlet, to less than 10 percent at its edge. The sediment consistsprimarily of detrital clastic grains with I to 10 percent shredded wood and a highly variable percentage ofwhole and fragmentary shells. Other grains often found in marine environments, such as foraminifera,carbonate grains, glauconite, and fragments of echinoids either are absent or occur in trace amounts(Shepard 1956).

The remainder of the project area lies outside of the open tidal inlet channels and consists of thereworked Mississippi Delta sedimentary environment. This former surface of the St. Bernard Delta Complexhas been eroded deeply and reworked by shelf currents and waves. The surficial sediments consist of sand(Shepard 1954). The medium-grain size of these sediments ranges from 0.0625 to slightly over 0.125 mm(4 to less than 3 phi) in diameter. Typically, these sediments consist of greater than 80 percent sand, andthey lack clay altogether. These sands contain an average of only 0.3 percent shredded wood and 1.0percent fragmentary shell material; thus, they consist almost entirely of detrital clastic grains (Shepard1956).

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Geology of the Project Area

The sediments that underlie the shallow shelf bottom of the project area consist of a complexassemblage of Pleistocene and Holocene deltaic, nearshore marine, and coastal sedimentary deposits.Unconformities and other discontinuities divide the Holocene deposits into three major sedimentarysequences that are informally designated as the St. Bernard, Chandeleur Island, and unnamed marinecomplexes. The oldest Holocene sedimentary sequence lies on a significant package of older, LatePleistocene fluvial and deltaic sediments, i.e., the Prairie Complex.

Because of the complex heterogeneous nature of the sediments that underlie the project area, theyhave been defined into sedimentary sequences on the basis of regionally mappable unconformities ratherthan by differences in lithology. Each of these sedimentary sequences is called an alloformation, which isa mappable body of sedimentary rock or unconsolidated sediments that is defined on the basis of boundingdiscontinuities. A bounding discontinuity can be either an erosional unconformity or a construction surface(North American Commission on Stratigraphic Nomenclature 1983).

Within the Louisiana Shelf and adjacent Mississippi River Delta, these allostratigraphic units neitherhave been defined adequately nor named formally. Because they represent informal stratigraphic units,the units are termed complexes. A complex is an alloformation that has not been defined formally; itconsists of a single depositional sequence that is composed of sediments deposited within differentenvironments that are located between distinct, regionally mappable bounding discontinuities. After acomplex is named and described as a formal allostratigraphic unit, the use of the term "complex" shouldbe abandoned (Whitney J. Autin, personal communications 1992; Autin et al. 1990, 1991).

Unnamed Marine Complex. The area mapped as the reworked Mississippi Delta sedimentaryenvironment is underlain by the Unnamed Marine Complex. This complex consists of sands and silty sandsthat have been eroded and continue to be eroded. Storm and tidal currents transport and deposit thesesediments seaward of the barrier islands on the continental shelf. Tidal, wave, and geostrophic currentscontinually rework these sediments to create a blanket of scattered subaqueous bars composed of relativelyclean sand that covers the continental shelf. The base of this unit is formed by a ravinement surface. Itis a regionally mappable erosional unconformity formed by shoreface erosion (Nummedal and Swift 1987;Penland et al. 1985).

Chandeleur Island Complex. The Chandeleur Island Complex consists of an unconformallybounded package of lagoonal, barrier island, and tidal channel deposits (Figures 4, 5, and 6). The basalcontact of this complex consists of a low-relief erosional unconformity that separates its basal lagoonaldeposits from the deltaic deposits of the underlying St. Bernard Delta Complex. This unconformity hasbeen and continues to be formed by the transgression of the inner shoreline of Breton and ChandeleurSound over the delta plain of the St. Bernard Delta.

Seaward of Breton Island, the upper contact of the Chandeleur Island Complex is a marine erosionsurface termed a ravinement surface. The continuing westward migration of the shoreface of theChandeleur Barrier Island system is eroding the deposits of both the Chandeleur Island Complex and,eventually, the uppermost sediments of the St. Bernard Delta Complex, to form this ravinement surface.This unconformity forms the surface of this complex seaward of the barrier islands (Figure 5). The speedof this landward migration is indicated by the rapid rate at which the associated barrier islands of theChandeleur Barrier Island System have moved landward (Figure 6) (Penland et al. 1985, 1987; Suter et al.1988).

Typically, the basal portion of the Chandeleur Island Complex consists of 1 to 2 m (3.3 to 6.6 ft)of lagoonal sediments unconformably overlying the deltaic sediments of the St. Bernard Delta Complex(Figures 5 and 7). These basal lagoonal sediments consist of bioturbated silty clays and contain both shell

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10

fragments and sand lenses. The silty clays that accumulated within the lagoon were formed behind theChandeleur Islands as a result of the continued subsidence of the former deltaic plain. Because of the slowrates of sedimentation and high biological activity, these deposits have become highly bioturbated. Theobserved sand lenses found throughout the area are remnants of lag deposits formed during infrequentstorms (Heerden et al. 1985:193-194).

Beneath and adjacent to the landward edge of the barrier islands, a meter or more of thinlyinterbedded silty clays and silty sands overlie the lagoonal silty clays. Within this unit, the coarser layersgenerally increase in thickness upward, with a corresponding decrease in the thickness of the finer layers.These sediments commonly are either parallel or cross-laminated and have been bioturbated to a minordegree. They represent the distal edge of washover deposits that form the leading edge of the landward-moving barrier island system (Heerden et al. 1985:194).

The barrier islands within the Chandeleur Barrier Island Chain consist primarily of silty sands (Figure5). These sands are highly bioturbated and display very few primary sedimentary structures; they oftencontain organic-rich root horizons and thin clay layers. These silty sands represent washover deposits thatform the core of the barrier island, and they often are overlain by an additional 1 to 1.5 m (3.3 to 4.9 ft) ofclean parallel-laminated and bioturbated sands. The clean sands represent both dune and washoversediments. Periodically, hurricanes erode and wash these sands from the seaward portion and shorefaceof the barrier island and transport them over the barrier island into the lagoon or onto the island. The resultof this process is the landward migration of Breton Island and other islands of the Chandeleur Barrier Islandchain. The landward migration of the island and its associated shoreface eventually results in the erosionof the entire barrier island-lagoonal depositional sequence. As a result, any historic or prehistoricarcheological deposits associated with the subaerial sediments of these barrier islands almost certainly havebeen dispersed (Heerden et al. 1985:194; Penland et al. 1985).

The seaward edge of the barrier islands of the Chandeleur Barrier Island system consist of 10 to15 m (33 to 49 ft) of sandy, tidal inlet, channel fill deposits (Figure 7). These sediments consist of upwardfining, horizontally bedded to bioturbated sands that contain numerous shells, and they represent thecoarse-grained fill of abandoned tidal channels. Because they are often 10 to 15 m (33 to 49 ft) thick, thebasal parts of these deposits lie below the level of the ravinement surface. Hence, they commonly willconstitute the only portion of the Chandeleur Island Complex to survive shoreface erosion (Penland et al.1985, 1987).

St. Bernard Delta Complex. Underlying the deposits of the Unnamed Marine and Chandeleur Islandcomplexes are the deltaic sediments of the St. Bernard Delta Complex. The St. Bernard Delta Complexis an allostratigraphic unit that is bounded by a lower marine erosional surface, called a "ravinementsurface," cut across the Prairie Terrace, and by erosional surfaces cut to varying depths across its deltaplain. Between these bounding unconformities, this complex consists of a basal layer of transgressivesediments, a middle unit of fine-grained progradational deltaic sediments, and an upper unit ofaggradational, deltaic natural levee and marsh sediments. Internally, a minor unconformity formed by abrief period of nondeposition, called a "diastem," separates the deposits of the individual delta lobes withinthis complex. The sediments of either the Chandeleur Island Complex or the Unnamed Marine Complexunconformably overlie the St. Bernard Delta Complex (Figure 4).

Within the project area, the deltaic sediments lying between the erosional boundaries of the St.Bernard Delta Complex are approximately 45 to 50 m (148 to 164 ft) thick (Figure 4). This depositionalsequence consists of a basal layer of transgressive deposits less than 5 m (16 ft) thick, which in turn isoverlain by 35 to 45 m (115 to 148 ft) of progradational deposits. About 2 to 5 m (6.6 to 16 ft) ofaggradational swamp and marsh deposits overlie the progradational deposits and form the surface of theSt. Bernard Delta Complex (Frazier et al. 1978).

11

The landward movement of the shoreline over previously subaerial coastal plain formed the basalerosional unconformity of the St. Bernard Delta Complex. As the shoreline migrated landward, the beachshoreface typically cut deeply into the underlying Pleistocene sediments of the Prairie Complex. As aresult, the upper meter to several meters of this coastal plain was eroded almost uniformly and was reducedto a transgressive sand lag. During the period of time that lapsed between the submergence of an areabeneath the Gulf of Mexico and the influx of deltaic sediments, sediments eroded from this coastal plainwere reworked into a thick transgressive sheet sand. Clayey silts and silty clays accumulated upon thebasal sand lag as the water depth increased (Frazier et al. 1978).

As the St. Bernard Delta prograded into the Gulf of Mexico, a thick sequence of progradationaldeposits accumulated. Initially, clay was deposited from suspension to form a thick blanket ofunfossiliferous, parallel-laminated, and fine-grained sediments called the "prodelta facies." As this deltaprograded seaward, the accumulating prodelta facies became siltier and developed parallel and lenticularlaminae of silt. As progradation continued, laminated silts and clays with thin sand layers, called the "deltafront facies," accumulated as part of the St. Bernard Complex. Locally, distributaries deposited interbeddedsilts and silty sands that display a wide variety of sedimentary structures associated with currents andwaves at their mouth. These sediments are called "distributary mouth bar facies" (Coleman 1982; Frazieret al. 1978).

Once this delta had built up to sea level, natural levee and marsh sediments accumulated upon thesubaqueous progradational deposits to create a subaerial delta plain of the delta complex. The depositionof sediment by floodwaters formed low ridges, called "natural levees," that bordered the distributary channel.Through breaks in the natural levees, floodwaters built crevasse splays upon the adjacent delta plain andsubdeltas; these flood episodes filled in the adjacent interdistributary bays (Coleman 1982).

The natural levee and crevasse splay deposits consist of silts, sandy silts, silty sands, and very finesands that are characteristically small-scale, cross-laminated, and rippled with intensively bioturbated zones.These sediments generally have oxidized and contain abundant digenetic materials such as ironsesquioxide and carbonate nodules and cements. Organic marsh deposits accumulated within theperiodically flooded land away from the main distributaries (Coleman 1982).

Eventually, long-term delta lobe progradation led to the overextension of the distributary network,and to a decrease in hydraulic efficiency. The decrease in hydraulic efficiency eventually caused anupstream diversion of the trunk channel. As a result, the channel switched to a shorter, more efficientcourse with a steeper gradient, thus generating a new delta complex, and abandoning the St. Bernard Deltasystem (Fisk 1960).

Wave action started to destroy the delta lobe when the sediments needed to maintain theabandoned delta complex were diverted to building a new delta. In addition, tectonic and compactionalsubsidence and eustatic sea level rise caused the delta plain of the St. Bernard Delta to sink beneath theGulf of Mexico. Initially, a landward-migrating beach formed in front of the shoreface along the edge of theeroding St. Bernard Delta. Longshore currents moved sand away from the center of the eroding headlandformed by this delta lobe to create spits and barrier islands that flanked either side of it. Eventually, thesubsidence submerged the delta plain of the St. Bernard Delta and created Chandeleur Sound and BretonSound. These sounds detached the beaches from the St. Bernard Delta Plain to create the ChandeleurBarrier Island System (Penland et al. 1985, 1987; Suter et al. 1988).

Prairie Complex. As defined by Autin et al. (1991), the Prairie Complex consists of two, possiblythree, depositional sequences and possible alloformations that underlie the Holocene deltaic deposits ofthe St Bernard Delta Complex. Each of these depositional sequences consists of an indistinguishable andheterogeneous assemblage of deltaic, shallow marine, and strandplain deposits that vary fromSangamonian to Middle Wisconsinan in age. The Prairie Complex is the uppermost portion of some 900

12

m (2,950 ft) of sedimentary strata consisting of Late Quaternary fluvial, shelf phase, and nearshore stratathat comprise the eastern Louisiana Shelf.

Within the project area, the upper contact of the Prairie Complex consists of a formerly exposedportion of what was once the Louisiana coastal plain. However, it now lies buried beneath youngerHolocene deltaic and nearshore deposits at a depth of 50 to 60 m (164 to 197 ft) below sea level (Figure4). In the subsurface, the top of the Prairie Complex is marked by the occurrence of a typically truncatedweathering zone that developed within its uppermost sediments when this coastal plain was subaeriallyexposed during the Wisconsinan glaciations. This weathering zone is distinguished from the overlyingHolocene deposits by a mottled orange, tan, or greenish gray color, an abrupt increase in stiffness andshear strength, and the presence of pedogenic calcareous nodules (Autin et al. 1991; Fisk and McClelland1959; Frazier et al. 1978).

Available radiocarbon dates indicate that the former coastal plain that formed the surface of thePrairie Complex was flooded sometime after 10,000 to 9,000 radiocarbon years B.P. As a result, it couldhave been occupied during the initial stages of human occupation of this area. However, shoreface erosionduring the Holocene submergence of the survey area by the Gulf of Mexico apparently has eroded thesurface of this coastal plain deeply. This erosion probably has destroyed the majority of archeologicaldeposits that would have been present on its surface (Fisk and McClelland 1959; Frazier et al. 1978;Nummedal and Swift 1987).

Older Tertiary Strata. The original continental shelf on which Quaternary and Pliocene sedimentsaccumulated consists of numerous stacked Miocene period shelf-margin deltas of the ancestral MississippiRiver. These shelf margin deltas consist of thick progradation wedges of deltaic strata broken by east-westtrending growth faults. These faults formed contemporaneously with the deposition of the deltaic deposits.The abundant hydrocarbon traps associated with roll-over and other geologic structures created by thesefaults create the numerous oil and gas fields within the Breton Sound area (Curtis 1970; Winker 1982)(Figure 8).

Historical Context of the Project Area

Beginning with Lemoyne d'lberville's first voyage to French Louisiana in 1699, many ships steerednear Breton Island on their way to the mouth of the Mississippi River. Located at the southern end of thebow-shaped Chandeleur Island chain, this small sandy island occasionally has been at the center ofimportant maritime activity. French vessels coming from such Gulf Coast ports as Mobile and Biloxi sailedclose to it as they plied toward the mouth of the Mississippi River en route to New Orleans. The Frenchalso anchored warships at Breton Sound during the late eighteenth century to protect French shipping. TheBritish similarly stationed their armada off the island in December 1814 as they prepared to assault the city.of New Orleans. Rum runners anchored near the island, during the early 1920s since it lay near the threemile limit and thus beyond the jurisdiction of American Coast Guard cutters. Therefore, this small southerntip of the Chandeleur chain played a minor but insignificant role in the maritime history of the northwesternsection of the Gulf of Mexico.

The French explorer, d'lberville, was one of the first Europeans to visit Breton Island. Leaving theport of Brest in 1698, he eventually reached Mobile Bay and then relocated his three-ship fleet to ShipIsland. D'Iberville then explored the coastline along the Chandeleur Islands, including Breton Island.Leaving his fleet, the Frenchman set out in small boats across "a headland of black rocks," entered themouth of the Mississippi River, and proceeded up the river (Crouse 1954:171; McWilliams 1981:5).

In May 1699, d'lberville initiated the second of his three voyages to French Louisiana. As henavigated small boats up the mouth of the Mississippi River, Spanish ships encountered the two French

13

.........

.......

........ ,,.,

.BRETON

ISLAND ,, .. -".. .

............. ........... .' ... ".. .. ..... ..... ... .. ..

.....

_..

C - ................

-TAy

C;)'

SGAS FIELD

OIL FIELD 0 15

MILES............. FAULT LINE

Figure 8. A map showing the faults, and oil and gas fields located within the Breton Island project area

(modified from Bechtel [1974] and Stanfleld [1981]).

14

frigates anchored at Ship Island. The Spanish vessels opted not to challenge their French counterparts andsailed away. However, one of the Spanish ships was wrecked on Chandeleur Island and lost all its cargo(McWilliams 1981:9)

Antonine Simon le Page du Pratz penned one of the earliest descriptions of Chandeleur Island.During a voyage to Louisiana in 1718, Du Pratz investigated the island which he dubbed "CandlemasIsland." According to Du Pratz, the island was so flat that it was barely visible even when his ship was onlyone league away. He also noted that the water near the island was four fathoms deep.

Another of the few historical descriptions of Breton Island was authored by Colonel Samuel HenryLockett, a professor of engineering at Louisiana State Seminary at Pineville, Louisiana. In 1869, Lockettinitiated a topographical survey of the state. The Chandeleur Islands, he recorded, were sandy and marshywith an occasional group of pine and oak trees. He wrote that Breton Island was crooked and low, witha general northeast and southwest orientation, and was about 11 miles long. According to Lockett, therewas a "good channel" between Breton and Grand Gosier islands, which lay 5 miles west (Lockett1969:127).

Although Breton Island had a good channel in Lockett's estimation, many Gulf islands did not offersafe locations for anchorage. The frequent and violent hurricanes that arrived near the end of summeraltered the shapes and shorelines of these barrier islands. One violent storm partially destroyed Ship Islandin 1701, and another obstructed the channel of Massacre Island in 1717. A French missionary summedup the problem in a 1711 letter to his uncle, lamenting that "our coast changes shape at every moment;what was a muddy ridge becomes an island, and what was an island becomes a muddy ridge" (Giraud1974:65-66). Consequently, a good, usable, and safe channel could quickly become a shallow anddangerous one.

The vast deposits of sediments at the mouth of the Mississippi River formed a virtual mud blockadethat prevented all but the smallest vessels from ascending the river. Late winter and spring floodwatersinundated the waterway with even more sediment, thereby reducing the depth of water further andgrounding vessels for months at a time. Some French captains ignored orders-even when accompaniedby threats of violence-to proceed up the treacherous river channel. For example, Captain Le Gac of theDromedaire secured a signed certificate from another captain stating that it would be easier for an elephantto go through the eye of a needle than for the Dromedaire to move up the river. The first sea-going shipto attempt to enter the river was an English vessel, which the French turned away in the famous English-Turn incident in 1700. It was not until 1718, when the Neptune entered the Mississippi River, that anothership dared to sail up the river (Lowrey 1964:233-234).

The establishment of New Orleans in 1718 soon led to an increase in water traffic between the newcity and the French ports of Biloxi and Mobile. This development meant that more vessels would pass nearBreton Island as they sailed between Gulf ports and the mouth of the Mississippi River. These shipscarried such products as silk, tobacco, rice, indigo, sassafras, quinine, and lumber (Surrey 1916).

Though some vessels plied between Gulf ports and the mouth of the Mississippi River, many otherstook the shorter, safer route across the Rigolets, through Lake Pontchartrain, and down Bayou St. John toNew Orleans. This way was preferable over the Mississippi River route because the sand and mud barsat the entrance of the river made navigation slow and treacherous, and because the swift river currentsmade heading upstream difficult. It sometimes took as long as a month for a ship to travel from the mouthof the Mississippi River to New Orleans. Despite the hazards involved in navigating the Mississippi Deltaapproaches, many French and Spanish vessels landed at the mouth of the river. For example, a Spanishship arrived from Havana in 1725 and another appeared two years afterward (Giraud 1974:347). In contrastwhen Lake Pontchartrain overflowed it created a clear water route to the Gulf. This route easily wasaccessible to a substantial number of vessels, and it became the path of "ordinary communications"

15

between Gulf ports and New Orleans during the early eighteenth century (Giraud 1974:347; Pearson1989:88-89; Surrey 1916:33).

During the early 1720s, the French did a considerable amount of work at the mouth of the river tomake it a more usable route to New Orleans. Their efforts centered on the harbor at Balise, located at thepoint where vessels entered the river, where they constructed various buildings to house products from Gulfports, such as pitch and tar from Mobile. When ships arrived at Balise, pirogues helped to lead them intothe channels, and pilots took them over the bar (Giraud 1974:155-156, 347).

Both Spain and Britain stationed warships off the Chandeleur Islands, including Breton. In a 1794military report to the Spanish government on Louisiana and western Florida, Governor Carondelet wrotethat war vessels were anchored "with all safety, on Ship [Navios] Island, the Chandeleurs [Candalaria], andBreton Island" (Robertson 1911:1:320). Two decades later, during the War of 1812, the British anchoredtheir armada off the Chandeleur Islands as they prepared to invade New Orleans. Shortly after arriving,the fleet came under attack from six American gunboats. The British returned fire and reportedly sank oneof the vessels (Grummond 1962:330-332).

Many ships continued to ply the Mississippi River during most of the nineteenth century, althoughnavigation was difficult and dangerous. The New Orleans and Ship Island Canal Company, which lobbiedfor a canal through the Rigolets, complained of the delays and damages to vessels entering the river. Thecompany noted in March 1859 that there were 35 vessels waiting to egress; three were grounded on thebar; and, 17 were outside the entrance (The New Orleans and Ship Island Canal 1869:1-7).

Toward the close of the nineteenth century, the State of Louisiana and the Federal governmentfinally took steps to alleviate the navigational difficulties at the mouth of the river. The city of New Orleanswanted to construct a canal from Fort St. Philip into Breton Island Sound. Benjamin Buisson, the stateengineer for Louisiana, initially had formulated this plan in 1832 (Lowrey 1964:246). However, in 1875,Congress instead authorized the construction of the Eads jetties at South Pass, one of four main entrancesto the river. The project, completed in 1879, gave the Mississippi River a 35-foot deep channel (Roberts1946:274-275). Of course, the deepening of the river channel led to more traffic up the waterway andtherefore to more vessels passing by Breton Island.

The initiation of Prohibition in 1920 led to the emergence of a "Rum Row" off of Breton Island andthe Chandeleur Islands, as vessels loaded with alcohol assembled there just beyond the United States'three mile limit. Most of these rum ships, many of which were British, were under foreign registry, thougha sizeable number were owned locally. Coming from Cuba and British Honduras, the rum boatsrendezvoused at Breton Island and the Chandeleur Islands with contact or "mosquito" boats which took thealcohol through Lake Borgne, Lake Pontchartrain, or the passes at the mouth of the Mississippi River.Coast Guard cutters and customs boats initially were too slow to catch these vessels as they skirted for .the American coast (Jackson 1978:277-278).

Rum runners played cat and mouse games with Coast Guard and customs vessels until about1925. For example, in September 1925, the New Orleans Times-Picayune reported that patrol cutterspositioned off Chandeleur Island were picketing a British schooner loaded with 9,000 cases of whiskey.According to the paper, the ship was 18 miles east-southeast of the lower end of the Chandeleurs and nearthe three mile limit. Later in the month, a Coast Guard vessel fired upon and sank the power vessel, EmiliaG, in Breton Sound off Errol Island. Activity in this area soon wound down as rum runners relocated to apoint off Timbalier Light that provided access through Barataria Bay and Bayou Lafourche (Jackson1978:277-278; New Orleans Times-Picayune 1925).

Breton Island was often in the vortex of maritime activity in the Gulf despite being small andincompatible from a commercial standpoint. Many vessels steered near the tiny, sandy island since it lay

16

along the heavily traversed route from the mouth of the Mississippi River to such important Gulf Coast portsas Biloxi, Mobile, and Pensacola. Therefore, it was the location of the island, rather than its intrinsic value,that made it a significant site on the Louisiana coast.

Previous Investigations

A remote sensing survey of the Breton Sound Disposal Area was conducted by Goodwin &Associates, Inc., in 1992 (Irion et al. 1993). A total of 78 magnetic anomalies (Figure 9), ranging from low-amplitude perturbations of short duration to moderately strong and strong anomalies of significantly longduration, were recorded during this survey (Irion et al. 1993:Table 1).

Because it was not feasible economically to examine all 78 anomalies, archeologists proposed atesting strategy that utilized criteria for selecting test areas based upon the results of numerous similarinvestigations (Arnold 1980; Bevan 1986; Garrison, 1981, 1986; Irion 1986; Mistovich and Knight 1983;Saltus 1980; Watts 1980; Weymouth 1986).

It should be remembered that any analysis of magnetic anomalies is speculative at best - partscience, part intuition, and part experience. At the 1990 meeting of the Council on Underwater Archaeologyin Tucson, Arizona, researchers gathered to discuss the problem of shipwreck signature characterization.They overwhelmingly concluded that no means presently exist to discriminate confidently betweenshipwrecks, ferrous debris, and natural occurrences of anomalous magnetism. Faced with budgetrestrictions that preclude the physical examination of every anomaly, cultural resource managers thereforehave sought to develop a sampling strategy that accounts for the two most prevalent characteristics:duration and spatial frequency.

In summarizing the work of previous researchers, Garrison et al. (1989:11-223) compiled thefollowing list of magnetic and acoustic traits that are characteristic of historic shipwrecks:

1. multiple peak anomalies or spatial frequency;2. differential amplitude anomalies;3. areal distribution > 10,000 M 2;

4. long gradients and duration;5. axial or linear orientation of anomalies;6. scour areas associated with anomalies;7. a geometrically complex exposed structure associated with anomalies; and,8. relative locational permanence.

Based upon these criteria, an analysis of the Breton Sound anomalies revealed that 58 percent of the totaluniverse of 78 anomalies could be discounted as isolated occurrences.

The remaining 42 percent of the anomalies were grouped into seven clusters based on a modeldeveloped by Mistovich and Knight (1983:154). This model assumes a shipwreck site in a high-energyenvironment will yield a clustering of three or more anomalies within an area <50,000 M2 at a survey lanespacing of 50 m (164 ft). Using this criterion, it was found that weak anomalies with durations of less than15 seconds comprised two of the seven clusters (B and E). These also were eliminated from furtherconsideration. The remaining five clusters, A, C, D, F, and G, were adjudged to possess some potentialfor containing historic shipwreck remains (Irion et al. 1993:50) (Table 1). Additional testing of these fiveclusters was recommended to identify the source of anomalous magnetism in each area and to determineif the potential cultural resource possessed the qualities of significance as defined by the National Register

17

D5

B20 C5 002

134 930 1 0a0

A ý1 K 33 00

B2 0 F1080 F,086

0 K93

BA569

A 5 . . . .15 4 _ _

I E L41

K78 ýv 70 aDJ42 0 030 Q67 Q

A80 9ý?5

K67 cis J

58

C)E V52~~ @F72)

0 U 6 AA50

o4 0

A19FF38Q I

o GG0149 KK246J98

H13

H1

A18 8 • El7 Fh7 Sil

23. 0

L. 0OJ1219 I

QEI

Figure 9. Magnetic contour map of the Breton Sound Disposal Area.18

Table 1. Magnetic Anomaly Clusters Identified in Breton Sound.*

DURATIONANOMALY nT (IN SEC.) TYPE

CLUSTER A

G-12 61 30 Bi-polar

H-93 27 13 Negative

H-91 20 9 Negative

J-14 7 30 Multi-component

CLUSTER B_____________ ____

1-34 13 11 Positive

J-25 9 11 Negative

K-93 9 15 Multi-component

CLUSTER C1-62 34 20 Positive

J-42 14 10 Positive

K-78 13 7 Bi-polar

L-41 16 7 Multi-component

N-203 13 5 Bi-polar

CLUSTER D____ ____

A-80 18 9 Bi-polar

C-68 568 30 Bi-polar

D-59 20 5 Positive

CLUSTER E

J-56 16 6 Bi-polar

K-63 13 3 Bi-polar

K-64 17 3 Bi-polar

K-67 12 7 Multi-component

L-58 15 3 Bi-polar

19

Table 1, continued

DURATIONANOMALY nT (IN SEC.) TYPE

CLUSTER F

A-183 11 20 Multi-component

B-124 10 6 Positive

E-17 15 35 Multi-component

G-123 15 6 Bi-polar

J-117 13 1 Positive

CLUSTER G_________ _____________ ____

T-111 1,476 100 Multi-component

U-16 436 33 Bi-polar

S-11 20 11 Bi-polar

From Irion et al. (1993), Phase I report on the Remote Sensing Survey of Mississippi River-Gulf Outlet,Breton Sound Disposal Area, Plaquemines Parish, Louisiana.

20

of Historic Places cdteda of significance (36 CFR 60.4 [a-d]). Testing consisted of a close-interval magneticsurvey to identify the central area of magnetic disturbance; a physical examination of that area then wasconducted.

Cluster A (Figure 10) was considered to have the most potential for producing significant culturalremains. It comprised four anomalies on three tracks, two of which were of significantly long duration; oneexhibited a multicomponent signature, which usually is indicative of multiple objects. The contour plotexhibited by Cluster A was remarkably similar to that produced by small wooden-hulled vessels of theexploration or colonial periods of American history. These vessels often carded small amounts of iron and,after wrecking, their magnetic properties declined significantly as exposure to the corrosive effects of seawater metamorphosed magnetic iron to non-magnetic oxides. As a result, moderate to low amplitudeanomalies (10 to 100 gammas) have been recorded over very early historic shipwreck sites, such as the1554 wreck of the San Esteban in Texas (Clausen and Arnold 1975).

Five anomalies comprised Cluster C (Figure 11). Anomaly 1-62 exhibited a moderately longduration of 20 seconds. The historical prdcis developed for the Breton Sound project area suggested thatthe potential presence of a small undocumented fishing craft probably would produce a magnetic readingsimilar to that observed at Cluster C.

Cluster D (Figure 12) contained one of the strongest perturbations recorded during survey, AnomalyC-68. With a duration of 20 seconds and an amplitude of 568 gammas, Anomaly C-68 was presumed topossess a substantial ferrous mass. The location of this anomaly was near the reported site of the 1923loss of the yacht Fidget (Berman 1972:168).

Five magnetic anomalies comprised Cluster F (Figure 13). Two of the contributing anomalies, E-17and A-183, exhibited multi-component signatures of moderately long duration (35 and 20 seconds,respectively). It was hypothesized that the cluster potentially represented a small wooden-hulled vesselthat lacked massive ferrous components.

Three magnetic anomalies and an acoustic target comprised Cluster G (Figures 14 and 15).Anomaly T-1 11 of this cluster exhibited a complex multi-component signature of long duration (100 seconds)With a peak-to-peak amplitude of 1,476 gammas. The sonogram depicted an anomalous discontinuity ofthe bottom in the same location. Historical research indicated that the U.S. Coast Guard had issued Noticeto Mariners No. 96 on November 25, 1971, warning of "a large piece of unidentified wreckage ... trailinga length of wire rope" near this location. It was hypothesized that Cluster G represented the location of thiswreckage (Irion et al. 1993:53).

21

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.t:~ 625 1A-49709. 0 i--''- . Z' ' '-. S-t131 497139.5C h-"1162j 47055 -' ' N.-. 4a11.2319 ý,4971 0.

-,!,I11629- 49608.5 r- 0i ~'-137-914 11628 )4-sitn4i' , -'l r'~"4 I ' 12310- '497081 54113 4967.065T ' '1'4-11 tl23118.i 497110 5

~'l624 ýi49700;5'Ti.---t111625. 49702.0. Z i~ .~j I . It t*4*Fd12312:fr49?118 0

w~~ll636.a~49708,0't,- t 'L" Ir *Igt12314 .449711 5,-,

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-h1t16345A'497l&,5-77.r~~ i 64 12332t-.e9707; 5. n-77, '

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t11652~~~~s49693U32 1, 4139e497067 .5-I'i'rt

1 a1uhl123425A4971l0W

141-l659"'49706A 5;;4w N-'Hbd~ljI ' s~st l2349A--497 Itt 5"n#tmr

,-tI?84ao49? 125.nt*.tg4j .iimmt158490ke.a..h.a~llVG~is4976 5h,..~ ~"112355k*497lt5A7'....

i*41lB4'n497t5.5 " a136~'9l 5~~

i*I-144178%49708.......'-I =12584471L0.,.

41171~497t.5 'ffif~ II. .44."I,12335r-497096i~ 'T ~. '.s" mO-1t2406 49708. 4 ¾

4t~tfl5t~497065.Z7'4..c A ( 12339*4970 at atoss .WI;-11128-u&4971t.5 "' .irtlR4l? 49?00.0 ei1 11729a'49707.0 'T >f41123412#a 49789.5 077si1.11730 49710.5 3o*f - 4112341949709.541It65i 4C9jGr9444-QnPpZ'

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&& 2249Pý4M

'L I~a *1 ; 4 LZ...I U

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N :~112254 -- 49712i5a ' 'i.1l2255ý. 49712.5.J

( a '-, A1l22586-!49710,0\ I it 112259-.49710.5 N

t I ~123009-w49709. 0-A¼.112391-:49713Q S. 24 77i

t.-v12302-.497l1.5 i;*A1X9 .49708.5"t12384- 49713. e--

i:It123061; 49713.0

-, *i-.tNt~123137-r49713.5..-,i It@a112315 49712.5

I',!'.-112310 -49717.9z'~.i *" ~ a '.1t2316-i49798.5-

Vvh122G..49714L 5 '

t'~ '7\7., &II -1123214.&49799.9 -:--i i *..

I . *..'F-.~-I-*tI. '.,S..- c112315:i49712i5-. '.4' .. £\~'ni'his I~,'9~t. It~dnkc,112316-,449714.5-- flji. .'alp., a 4 ,4"I

4&.iu.' ~'~i-A12317.-i497175.0 ~ t~a.

!-A1l2318i;ý497659 i.-~t~ 2*-1%& .I.-

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"Itd12351,a&49?18 9.cti 443...-

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V I"" 'a~I""r~ aM 12482*449789iONZi- -4 - *e-'a

'2~~~a h12484Ma497lh1 4.a 1'.wh *l 123 5485 4 9 7 99lO;5 Z.n2 t .i '. '2i ,fi'W-n-' - - ' ' ' k &

mak1123496ai"497195',...i' .- ' 2-'v -'l.#.

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re ~ ~ ~ ~ a t~w.,. Udl24 180A) 49709 18.usý i.*;a2 2 '~~. 421 . ~~~~~ LaI'4tel124 11'iik 49709915.,y.a"n . \ ij.I 1s1M't'

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-4~12418,V,49709.5 -

N- 112419 ;14979.~ ____

I H93 J14

22

i13109 49741.0 ... .... .12940 49715.51131113 49739.5 : : *112941-49?13.-- .

-113112 49741 8\.-112942:,,49707,0 ,--"i ,113113 49739.5 :(: *'129434~49705.0~,7-i f, */,113114 49741.5 A~ ".18945,*49711.5 7113115 49738.5 ,112946,.49709.5 A.113116 49738.5 I: ..112947 -.. . i i113117 49740.0 8! 1129 49709.0 .113119 4974.0 8 .............. 112949 497125113128 49739. 8 **../ 1129SO,47l. 1>:

13121 49741.011295249711"113122 49748.5 .,.12953!e497O885.,..,. . . -..113123 49739.5 .112954,i49789.57--i---3124 49742.0 '12955.,-.49?09.5113126 49740.08 / It -2956',:49711.0113127 49740 5 2 .1129 5 Z ..4 9 70 ý113128 49741.0 8 , .,:5B ." ,7O 5-.'"113129 49740.5 8 . .. .' .: . .. 4971..0 i.-..113130 49742.0 5 ' .113601 t4971•. ,

-113131 '49740.5 :1130ea 149709.5....113133 49740.5 : :1130809 49709 -.113134 49738.5 -*413884 49718.0113135 49739.0 NA : .1138-14 49705.0 i i V

;113136 49740.5 : 13007-49711.A .-- i .113137 49741.0 8 :1388o, 4971. " -.4."113138 49739.5 :,113 IN9N-49709.0.7-7e .i I-r, . i.113148 49740.5 :- .1491 11. o.5

"113141 49739.0 : / '.113811-,49717.8 .,. .t : *.113142 49739.5 I- -V113812-#49714.5- i,.*,-..., •.113143 49739.8 1 ,13014-,49715.5 :"- i. , .. .113144. 49741.0 11G815M-49?l2.0 *~'.t.'*113145 "49738.5 1 t113016-.49712.5 i i,113146 49738.0 8 J113017--49?15.0 7"- .. i113148 49737.5 I .1 80 49 4..?, ,i4, 0 7,-i-,

'113151 49737.5 ( -'1382a-49715.e -7.113150 49737.80 It..20-.49?. 1.,113151 49737.5. ,,1 19 ,4 15 " "i* i , . ! \ • • ,: ,. ".113152 49738.5 •.I 3 2 • 9 1 . ,-•,- ",L, ,••...•.

113153 49737.0 . /1 .Ul02449715.0.., -. •..,.-- , ,113155 "49739. 8 ' i18025-t.49710." .113156- 49748.5 I\ tt1382&-*49?1045 .i , i . "113157 49742.5 - 118827-,049711;0,-,113158 49747.5 •'•"0 P-9-,4971365.':•,• /i.. ",i d,•i:....

'113159 49757A8 1 4M1883 49718.8 " i .-•.,-..."113201, 49764.5 "it tS831'•49217:8 . .,.. .-.. .,.It13262 -49771 ý 0 dl.113032 449?22.5." i,. I ,113283 49769.0 -/1'18033• 49724;5... ' .5 ,113204 49764.0 8%M1 034/*49718.0 M . , ',113295. 49760.0 8 t-303 4 . .4-, i13286 49757.0' -1'4 .,1133V• 49?12.8 • ... ,.. .,, ,

3208 49752.80l83491~.i7'11"1•29 49749.0 8uti03949710 ---

3218 49747.0 118904 *.49714.1 -e ,i.,,L ; .4- l¶I.... .• 132 1 4 74. tw,t 18,04l•e49711h8 ' • ' .i,' -.'...i.(,,,•-, ''i.• '321 49744.! -13212 -49744.5 w•t 1.3e43 049M1.0 ,6 •,7 , .. i •..,i-.,.;•....

'113213 49744A ' ,,1,3844.49218.8.-113215 49742.5 /".. . -1845-4971g:0,-.-..... ..."'113216 49741.5 5 / -,113846"49711•5 • 4j,4 ..:1132127 :49741. : ( I-11384?k-4-49714 0 7.7--. j +i. ' - .113218 :49742.0 e k e48,44971t.5 i . . -- , '..

113228 49741.0 I "'113051 -49711 0 . r....j'113222 49739.5 1 .. •13052,49716,. .... '-.,.

113223 49740.5 QtL.3853,,P49714wO.--'-.--.113224 -49742.0 -4.113054,.-49712.0 ;- . . -.-.-.. ,.113225 49740.5 t3055-49713. 5.-..113226 49741.5 r-II3056,497fE5-113227 49741.0 l18.5..4971t.o....113228 4974805 . .i•,113059,-49719.5'=

"113231 49748.5 .'

1113232 49742.8 -(411I@49711-3113101,.4971S,-2 g- .,i . . - ., -113232 49742.0 :\ .!131021849712.0..7103233 49739.0 .4 II 3 490.5=113234 '49742.5 11 3 1 8 5- 4971408.5•-. 0 -,. i . .".1 3235 49742.0 1 1 5• 9 1 • -•.i•'!; -i i ,. .113237 49739.5 -"'113106-49709.0 -- "i 4-7113238 49741.0 I 113107 49711.5

1!13108 49710.5 -

162 J42

Figure 11. Magnetic anomalies comprising Cluster C.

1216101 s49 .u __It: L3229734.0 -121683 -49733.5 Z7.i 13224 .936-.~64.4973745 4" .132244 -497325

,.. . ... a16e66*49734t5 L4.32245.' 973351. .. ~.,'tJ*..., ..121667.- 49736.5 .. 326~92

'..~.12I618~-b49734-5. I 1.H,'.'' "132246 '- 49733, 5 i-l..1a609.-4973540e.T_ 7' i*-.132251*'.4973M~

-I .. t .~ ....121609 w 497335i52-z-.i... - -125.493.'1l65~.974.Oý11322549ý49732, 7.

-121610-49734315. :~.,125 .93A I' -. 12161 434132250 -A9733. 6

~l2118.4975~8,.1 . *iT\-. ~.'--... ..132251 ..4973b i.j -.121614 -~49734~~-.t. i, 'i'* h.132252 -49732 t6

.*.116224978.6~ ~ .... 32230 .49733 53- 1216153-.49734.5 0.-~~; 1.326 49730*121616 -. 49734.-'~... 'z *-. 4,432254- 49733... .

I. t121617-49733.5'.,,t,, i 1, 1322855u49732 50( .12126 M4?38.5 ~.-"132257 .497325...0

'.-A4 ,.2 16113 -497325,~ 0T\ i. s-132256 -497336.

aa 1325 -414-1421621.449727L.5,77- i,.,.. ., ; -/ 4-.1323011,.49732.5

<C .21632.-49788&i. 0 i, i -132301 A~9733.5* ~ '~,i.,I~t.,..cu132302..49733.0'

.- ~..ral13231215- 49733.5 -im114.49736.5,,...-. .- \i -. i. -332.~92

~-121644.b~4934.B-~. I ~.132306 A~9732.5, -.- * . .,A.21628t-4973257. -i-- 1 .- .,.~1a2.~93

.12 162 9.--497 35 10 Z_ "- A .'-.132308,4-497329A

mt2l6364L49?39a5.i V.. o .132309..49733 1i 'I..'d.U '4 *. L-1--.t.,13231 8.,t497305

i-'121632 .4978a?.0 ...-.. Ž-i,~ .4'. U-' ... 323129.49731.5

441S-.93. 1--i V i.-..-..',. u-432313249729.0-fl *~'.41*.~s 1216353.49733.5Z=.... 4 ., .. 5~I, .,. ill,33-493 6

'-1~ 42654.:4939~Th...+.~ ',-..-4-132314 -w49736ilB' ~'- ."..,-~ ~ -M.-3135_419733ý6 N

~i ~.t..N i.1216387-, 497346 a 5 7,-.. i.-t.t. - IN 1;' o, t..--;4r 32316 '49735. 79.~.1a658-4936.5~. ). t 22.~A. . ..'132337.-4973l 0

.- ;,A5..473. It~ 132338 -49734 5,-.121640 -&49734A. U ;Vi. ,*i i.-.. *t134.4966s.* 132320-,"9734, 7-~~I *t'*-.442164a-,&49734...t--~,-, ! 6-= alsIG12342'.,49741&5-121643-49735 i-I.A 1 I3 3a.4780 _.:-

-121644.-i-49734;e5 .i,, I..-4 i: i 4; k-323432 '49729. 57NA ~ ~ ~ ~ .I-21645'49736.9.7-i'. ,:-i *-%' i 't..~-..132344.-.4973 65~~fl ~ ~ ~ ~ ~ .121e646.4973?6 0 i.. -4- I-N .. ~ .335.93

.4217?...97345 ~ *A~ A,432347 -. 49732A5~I~t4-8d~4~~-.4t*12167- .49739& 5'*' 6 ~ .. i' . 1232346.A9733.5

..12l?648*49732.0~ -I.1..32349..-49735 8-A .2l1660 +49737. 05-7. i .. ,..: -. 1323529-49736:5

..12~l4~4978.6. ... .-A323313 --49735&8

'4216.52-49739.0~. 132335. 49732.6blta "127653.49748.5 U~- Q .. 32356 49734.507

-121717-. 49736.5 .-- 132357 -49736. 57ct:'. .. 42?1..47335~> I--1323358-497326O;.12167.-1-49736.5 W.;, i ý i- 132359-.49735. eu-.121659-~-49736.s 5. i.. 13243 -4197361.6w.1216591&49732.5 1.-'ý n! ~ 132338a- 49732 8

v it [49735. i=.328.493-'-; 4217809? p T~ ..l . .t tt.123240 49736.8

-- V127249738.5 ~7 .. '132341-49732.0

14217e29 *4745;1323429.49733.5ii121736 449744.5 4**T..32343 .,49732.5-

K7888473L N203248473.

-- ta eg-4732 0 _-j i,13249 .:49723 e

1~*IeqII .uII IOc9 i S

121603 49733.5 13P-243 49734._

"".121604 4973,a5 74 4-132244,-49732.5, - .7,,,.. 1216O94•49?37=5. . .-.. i - ", .. . .132246-4,49732.5 - : , (..,.121697-.49730.5 i .. ,132245. '49733.52

,,13P249;,i497329.B -, 421610.b4973415 -Z• - ',,. - .i .IiS'i ,='b -. 13225 9 .,\ .-12169 '-49713, -, .h ..=.•.. .• i .... A t32250 -t;497313-.5..,•.1 161 -•.973 ,5 ,.-"•.i.÷ ,;.,- iV i.,• -= . .... -. ,13 251.%- 49731= & 7--"; .,•• .'~,i•.i.4 .•

.121614,49734 -"-... - i. • . . i. .- 132252-,,4973240 -- I ..*,121615.-49734.0 = !#132253 -49734.0 .... .7-.121616 49734.9. .. . . .e. .," &, t322544ý49734. --i 1 ....121617, ..4 9 51322557.f49732.0--

*1216t8 -. 49735, , i.. i . 1 , - i. *1. . .i...-13225 8.-49736.9 0•121619-u 497•22i5 '-i ,--, ; i . ... ;-,., ,.1322_59 -49734.B ---0 .. . •{• • .•.,, ;•121621-".49727,..-"S., ,-- .. .i 4".132300.-49732.5-121622-.49740.9 -*•.,- - .. ,132301 ',49733.5 i. ' .. ,.121623-- 49733.•47-7..;.. I .i i,132392-49733.0 -_ . ,.,i ,,.....

•-121624-,49736. , i--. .. ... i . .. ,i. ... ".132304 -49733.5 "-.i .. ,, •. .. •...•~.- 12 11625:w497a66 ;77--i ... 132305,-49733.5 - , **-,.

A121626 *49738.5,,-i.. t.. ' ,- , . .- -+,t32306 -49732.5,....,I.. ,*.121628 L 49732,5.-, &-.-, ,L.1323071M49733., -7 - .. • , ,132398. *,49733,9 =0 . -• - .. :' i j • •t*,.121629.,o497351 8 - ",132308 4,49729, 0 -7.i , i ." .i-'4. 1. , i •M'i•12 16 a , ;4 9 7 37.5 • iJE;,J': , ,i i,• 5.. , Z o,1132-3099.49732,5,-7'',, i .,."- ,i, .•*,, -. i

1 4 , " , .- '. ..- .... .--1132311-& 49730 -4 5 .1-12 1632 .-4978 ,.- e I•, A-1...32312-.49731.5 -- - i- . I -

,121635 49733 , .. -. i...32314-.49729.0 .... . . ". . .. ,...,i ... j, .. i'a,

,1 16 9,49 • ;--- . T, ( - ,132319,,49731 .--. -. -. h. . , .-...t1.'48,-49734,e.7--.i* f> 1.41323164,49735.0 77 : .... i , ,I.,

,4216438-49734 , - -.. . II .- i. A-- 132321,, 49?3; 45., • i . i,9 .. i -i- - ,-1216439&.49735,5 --..... r-.,,.. i ! . , -,. •.43231a9..4973145 ,7 !-- 4W.,..-412L64.4 49734A 69- . 7.i- - . . i /i...1 .,.132326*i497329-7,,• ,1;1645--49732.9 !5 i--.i .. .*•-.. = ,•132321,.,49735t5 ",-12164641.49735, 8 -'.i*** .SIC- . -. , .• .i.,132329.. 49736 - -... i'.• '• i.', .... ..: 2 f616•49739,5.a-•.i...-i. -,.u i . ' -" -', ,.132323, 49733.5 .4-12 1645. 49733.9',a . . .. . -. ... F - i.4'-013232W -49735.45 - ,

.121653 ,4973345 ;77-.-.i...'. ( ... 13233 -t 49732 - " i . .-.. ,'121•654.49739a5 77-i...4.. i,'-.- . .-. 132334 49735.B . i.i-. ,• .121654.- 497328 -. i...0i!...-• 1.'. •'.-. 132335 .49735.5 .7 i, .i ,-, ..-,121650 -497 367- .. ,i:,., I .. i ,.-.. .,132330. -,49739. - .. ,• , . ,

-.. 132335 .. 49731. - i !, .. N ii.. 4-4.1216571..4973 =.-, ' i. ' ..-- ,..- ..-13233 ,. 49735.9 -

-421652-~49739.5 (1 1/ .. - ' - , 132332 '.497335.0...~

*..12t958,-49736.9,,'..,-...a..-i... i I. r i..i, .. '.. . .,132334.,49731.o --. ii , . ....

-. 121653 - 4973.7;. ; .i .... i ,,19,43 ., ,.. -. , 132338 -.49734.0 - .. ,

~~ .a.t323349..49729.6 _I

-421655 ;,49737.,•.',...i N-' I ... 132335 -49734.0-.!2?1 .4973~5., . .... . ..132337 -49731.5 - V* i. .- 2

i ...49.36 . ;...,'.-,. , " . ,,*,12174..4973. . - . , .... .,132338 .49734.5 ".,.1217e5- 49735.5 , .--%32354, 4973A ..... ....

,121716 ' 49749.S -k. 132344.1,49732.0 .5,

• 121?17.,,497.34-5 .132342),49733.5 ,4•21t7G84973..5. , ,.. . 132345 -4973..5-121?29,•49736 *-"--•1,., i 1323548,.49732.0 "7,121?10-49734.Z,--i- ; .43235-.49731.5 I

-421tea3 4973 -.5 • l . ..Wo... i -i .,13234 8 49734.8 -I ...l . .I ...121723.'49732.9 - - i %-.-,',132349,.49732.Q 7 L_' "-4"7., 132350 ,.49734.g t, i..0, ..

.1217 ,4 .49735. 5 . l323 -49734.5 .'-421713-.49736. 1 e- - i --.132352 -49734.5 i..121M279.49733.5 -4 i,. 132354 49731.0,12P15 8- 49734: ..-- ~ ;; e i'i••.- .. 132355 49732.0 5-- i i -ii i.• .:! --... "

-12172 ',497380.5 .. .- 132456 49734.5 -- , .i.., i .

.121727• 49736.5 t.132435?49730.5 .i ,

-121739 4973445 : • .. 1324395 49734_5

,121723 49732. ::•.0 77 !, .- •-130- 49732.0 7•-. ,•

K78 N203

23

102808849746.5 49694.0102889 49747.08k954:499. 49747.5 .. :. V '885248-kI496gl.5 __1028 11-49748.0 *: eQ5249ww-496926 7.9 1,-102812 '49747.5 ::~9254649T

102813 .49748.8 W@~85245aA49694.r55.182815 '49747.0 / U.8524?.vi4995.5102816 --49747.5182818- 49747.5 )144: "085248,C-49696. 7182817 49747.5 : ~885258 a.4 9693. 5 -',;102819 49746.5 . W9885251 - 49693. 81802828, 49747.8.,6055~4918I2822 49747.8 i M-885253-496995:102823 49747.5 M-.~8852564 a4969e. 5 =.102824'. 49747.5 wu6b*8S25-49690.5-.-. i-102825'ý. 49745.5 /

*829.h99VB2825r 49748.8 .a-! .- B28*49 818B2827 i49748;0 N. m~e655890.i49693A7.182829 --49747.8 h.98S5381*.1 496915 5,-182830 49747.5 8883&96.182831 .4A9749.0

B59fM99.~ .102832 -49746.5 7~Q94d

96 V. Q,'182833,4945.6 / e'853906" 692~, 0111-1823449746.5 N~98~o9~j182836 ':49747.5)w~85e84~

2 ~102837.f 49747.8 / 85981i.49671 6.f;-102838 '49746. -"48531 lik49692.8- 0 i1028339--49747.5 : 8 51a9%,T182848--49745.5 -7 1 / R958e*496963.*r,*r-182842 '-49745.0 . l08515-4966?. 5,7Z',182843 '49743.5 mxmM K: I ~986&4

9679.5~,7-i.,77-102844,!,49747.5 ' M*8583f449?26.,, f .. ý.lB32 845-ýT49745. 0 - .

1'02846$ 49745. 09F8 A (3- il -L L852J492O~182847-- 49747.80 1:.-~'992~94.54~~-182849 --49746.8 : :G 5 a4.915,,-ATle2859l-.49745.e Ri I. 0.85958GL49634 ;5 Zm i--1 82851,:,.497435 8 - : : :- IR85S98&a.4944;? 9 , ~

-1028531,049746.0 - -G05326A*49462.5X...4: I: -~L' SIW53294~49509. e.7.i..-..

.102857,1,497468-; I: "0&953 30Ap4g595 0 * 4 e. Z.-:85ý 4947 : *":--v~95081IW49623.

W599332* 4964 7.B -"*102859.ýA9746.5-' . (..... ...... beIRSlG854-.49 6 5 4 .5...102980 ý-49745.8 0Me3~~96 ~'102981T'49746;8

1028344975. l4G8S83fiam4g66 9 . e*..5 Owe883G4sms . .

Kid,.. =*1e029134""49746. 5 ::10295,:!49746;.0 N'1 82 9 1 8- 4 9 7 5 64

6.. v a 283 4 i. g g A) i-. .~ 1 8 2 9 1 1 Q 4 9 7 6 2 . 5 7 ; .5 i . h c j .102908 -149759 g 1' e 4 8 ~ ~ 9 9 ~ .,.'1829 1 e .49756.9 0

&888a99.'~18029114 '49749.2 : .

*182916;.49747;.5-110

2917)y 49746.5 4969/:.A10291'-7 49746. 5 WS5-499.77i182919 -49745.5

A", au85354imw49698., I102920 49746.8 Wi-O8$585A.ý49695:5ýZ7

182921: 49745.5 1" 9s 6 5 I 4 6 .9eji. -~~ 98 53 5 6NL49696~s.1182924:-7497465'......

I82927Z49745.0 - - 685:.: a'Q8 49a~ 6 96A.,.102928-49745ý5 ' ' 943a9 9 .6.~182929-ý49747g . 9hw95g.~6987.6182932- '4974685 .*884 49695a-.~,

18e2933 49744.0 3:. 9 6 8"slý469.@=102934 49974 6.5 :: ~ I :I~959..998

.18N95 ~7 5 :I L-.--854 I O 49r,95 s-5 77

A29411 49(F96:9

A8006

Figure 12. Magnetic anomalies COMprisi ng Cluster D.

.A85239 49694.0 - :I /~2£~85240:469. 0, I 9201'

-&e -4849694.0 ' (, 62io"895249miA19692.M~ Ii i 920i'J".rB5244*i-49694;5 92(

ait" 95R45aw49694.8I e (e22-ý7985246-6ý49695.5 A3 II 9202,

r&1385248'a 49696. 0 -'09202

Zappo"&G.49693 5 -- 7 89202-".si9925 t a-4969-3.5 e 92020

e8 65252.- 49693 8 08.92o2'mi~e65253-~49695.5 87 9202:Mll6e5254 dL9690. 5 (.-e 9293,"ee95256-49690. 5 I i98"N65257&&4969aA j 6-)9203

wi885P56&49693 .4. 0 - /: i 89203

mw~'9S3eO0,i4 9 6 9 1. egeof 628"0i.85301.&49685.5 7,7 99283

"Q~.Q5904AW4 969 U. 0~ 77 M6929

" e9 B9 W8954969 29 ~ . l;. 1 ' 39204ý"ui.85908mk4969 1 5 ,7 8 92f84CLAM10&496? t59 7.j9iA20

6A9204

W10853 t5.49679.5 -77p69205

809284

A692C4

,sit6B5316A*49462,5.~ ef28

eul853l8A4495089.~' A09210

2e94532-Am49647.0.~ _7 i 2-"e~895983"49654. 5 l= I892bk9e5925i~4966a9.~ e i992

i~.6952~4959. 9'-i' 092ti1 1* 53 1299210,ý, --

-.69211

~9a~32M496?.9 -- o - 9211

4iý6859349&a49654.5_7- i_ j_21wiSS595325.49692. 5 -'T -

oa~98a5926&49669?;0 i 99w~e85395ab49699.5-1-- -i P092t

ad!eeS3549~i49698 5_ ""- ,92\ I

a~ieO9857-4449696 .'- kZ7'i .- IL9212.e.92 I

k"5345ei49694;0Z9-7-i_ a,92iiI.68534 a-~49692.5 6 e. a f; 2 e,ýAQ85348t-i49696.- '62i

;&we8534@449692.T8; ~kwSS48064*49698. 57 j=. i, i Y OM9216 e.68525l~a'4969 A .8.a i j692to-el[53509 -49696 --. ;7 i ie9214e~8534 I 0*L49695.-i" 5 e 921-1

AR941 t 49696:9-> e 92tAZ

C68 I 62

0a9i01 49691.0zi92iii19.4A9694.5a ,. (-A W M -eA' ~ 9 6 9 4 .5'~-- ýir Ne92020#.i49G92-.1392021-&4969I 5. .,

A992022.-49692.5e92023 +*49691.5

...... -- 92824 149692.0e92026,.49693.8

.99202?..4*,49693.ef92eR98,-49G93,08

.0i92833v64i96925 5'

e92035-,49692. 75Nv e92036 -- (.9692. 5

ýA92835-a49692. 7/ '92083,6,96912 8-

8i92e84wo4969C0-9

iA92044-149691.

*892048--aý9691 ~i 0 I -

-09209-41 9691 ý 0.-.

Me9284419693. 0 ;.I8920515bi49691. 5-:1-092052"~9689; 5

*-~ -ýý 9 9 18 -

ed9 2 149aig449691.8.i@992t58es'I9692-3

77- 40~~9295a&4496890 5-- Z;4i*4

"Q"092858M=90'9 L 5

1392156.&.49GIB9 5

. 8-92i.5?.,49697-5~8 r

ie92859JS'496935.,.5. . '.

Re92tR8i,,496e4;5-*~.-~'0.92126,ti496993 9;i

u6496~983A Wtj0921R8,*&496938.JI -0.

.u89219ii.a496619*.. S.t.i 'I

*e9a12314496795-8 7

e~9921294j4967? L5,4e992I24ia49682. e.

JIl892I25a;..4%84 a5 -Oil w.eqe2I26*496739&5-

* ~-' .092139'449849,Z7,'1'139P404189 6i81.5968

e9214131-4496815 .a '

-.e9214E!-k496181.5-7'

092143. 49688.0 __AcQ5.4'9 ~Qc?7Ej

D59

24

%3. :W 3 ̀ 5 3 490/i4.0 -185431 45 72b.90453 44 5 q j i ii,:, 41054132 49727.C )

104538 49732.5 98325549690.8-T .. -.- ,i.i,-: 19543349726. /520454 493.543.,' 0383255 496929 -- ' ." . i- I ', ,9•5435 4972.5

184541 49731._ - . -913325 14968505 10,,: ,:' .i [05435 49725.5 E

104542 49732.5 .. ,083256 49692.8 e. 205437 49724 5

204543 (92 . ,,.g 394.,96 h ...i: i i I"• i.''i.ri••• t 1854438 43726 . /184541 49731.0 .: 170412735.5 384960.5 " i - 15439 49725.5394545 49731.5 0 .. 993825 ,49680 .. i M i , i05448 49724 .5

304547 49730.8 / : ,..9 33889.49691 ,8 "3i"! i•\, •.';.iwi. .. ,'•,.)•, i=, i 285442 497278.e. •-.384548 49731.0 :983314 49691.5 4... 185443 49726.

3054-.78. 803386' 49689,5'•: .- I I(/ '•'*"i,'.i•• 205444 497265104558 49738.5 w-098330 ?.49681.8 . I ,. .• w,.i..? 285445 49725. 5 ;"304552 49729.5 . . . .... 033138 496 8"5 -"•3 i ." ,o .ij t.i'r.9330 95446 497,2

204552 49732.0 5 -683302 4969310 0. i.,i,• .i 305447 49728.5

30453 31~4961.5'~'~" 054 49 49726.7

294556 49730.0 . .- 9833103 49693.0 77 .i 085452 49725.0204557 49738.5 0 . : . .083304 49691.5 Z8 i..i,• P' ,• j, 1 i0'A 285454 49725.0

28458.4932.•N.883315.•496892.5.--'i. iV •/,-i•'i~ l'J'-I 285453649724.5

504559 49730.5 " b8033026-496998 .a-5 1 * i i, 1,.i •I,3i• 105454 49725.5

204603' 49732. ..8 :N ::•• ,.,8033I8.4960B,, 5. •:••Z :'" '"' 395456 43728.8 ' N3846 5 105457 49727.5104603 49733.5 07.9B320 49697,58 7 i, ,. ii ..a $.W.tI 0e5450 49738.5204654 49732.8 "...833230. 4968905 - .i .. ,.i.iu 105449 4972.6.

28460549731.5 .... ' 4 0558 49728.5 7 710460749730.5 "..183324.. 496911 * .-• i. 3,.,. .. 2;a•;A 2855834 3731.0 :

3045563 49729.5 ( h983310 49693.5 10...' I~25583 49726.05

30468949729.5 -01333 .. 49689.9 .2 10558449732.124629 "49723.9 - 604873327154969i,5".I.. i \, 4 , 205585 49726.538 4 N 093328-.49692.5 : . 205506 49727.5

204628 49738.5 * ' 329.49694t5 -- . • i ' i, re., ;v 105517 49726.9

104635 49730.0 -4.083331•,4978915.0 . I.. . ;oi. 1 i i aP• -• f 105588 429716.5

204654 49732.8 e .: , 083315 - 49692.0 7 I� i"I F3" 105538 49724.284636 49731.5 0883331.49694.5.'.'. i ., . 105 497b i 285532 49729.5104631 49732.5 (o 0-'8334 7.49694.5".i.I / , 185522 49720.5 :"184638 49731.05 .:2. 158334549698.5 -.. I'i ( I.W""114ii3 305523 4973728.0

38"1"473.1,.9833368 496 01 5-'. i p t ý l .,1iI.ajI 305452 497327.

.104628 49733.8 ... 9337.46928.. ..... .. I I.I 4I'6

.'i185536 497327.8" :184629 49731.0 "-83332 ' 4969580 i- i " 285537972831I4623 49736.545 49730.83338,-49685, I-.i,., I ,,l.,•V1 J.iAI3i 1

184604 49734.8 .... •.83q if 10559849726.5 /

104625 49735.5 . -. 7 083341-.49687.5 ."- ,...i "-.. .. ,385519 49728.5 N284606 49732.5 1 2 ,83343 .49691.5 1. ,, 5.,.1 .,-358497329.5

47 4: 5 03324..496"9 I i 105523 49738.118460 4972.5 ,983325%49689 8 i ./., i 20552i4•4977i.

..d98332?.a.4969L.5,- k.,, lI *II40j35525 49738.5

284630 •49729.88"".] 1455e5 79726.S104633 49732.5 I i:49833428•49692,5 . , X'',', ' 185526 49727.5

4 6 49728.8 .- 8933429 49694.- ,5 .85527 497 7. E2" ' 105528 49718.5

204633 49730.8 .a., 903353.1 4967895 7 1.;,% iI1i 0553, 4973-.8.8",4.284635 49732.5 . .' .0334 .4969 ,28 ,i.,1- ,f4irl 285532 49729.5

.20463749.27.5.. ,9-h83354 149694,5 Z-.i %" .i - 8553249729.5124638 49728.0 -1 3it - .. 5533 49729.124639 49728.5 -983334 .49694.5 7 1 #49304648 49727. le.5083358.i4968915..-":' • ,i.i.'.-ii•ii 385532 49728.5204641 49729.0 .. 983351,%49689,0 - i , i,, i .44*';i i A 10553 49739A :

38464 49729.0 1 .9,83348 .49698i .!-, i i! .- ,. i,.W-.4•..r 185531 49731.5

k'083345 -496998 .. ' .30. 3i5539 4 9 .5:

104640 49729.0 3. 1-2055 4973

104641 49727.0 " ./ j. I• 5. 185541 49728.5184642 49732.0 '". " .3441 4 496905 i i 1...

•..8 340 • 96898 .7-' 'i. i .-i .,i,: t€,• •215542 49727.5204640 49729.5 z'. 34.'4965 i N,,II., i 105541 49729.9104649 49728.5 "9.83u3424-,49690.5 I , A ,'[,- ,-,i'

184647 49728.8 =,.6483435 -49686,S ----... ! , .,.i 1855420.49738.0 i

194651 4972..5 5 - ' i .3-8.i-,,,8 0l. 185545 49729.58,

204652 497390. ' :'803459-49699.5 =i - .it i .,...*, 915546 497328.204653 49729.8 /*,08347. &49689.5 7_j ' ' ,', ., 25547 49733.8104653 49737.0 ,-,983431.496988-- , i . 205548 497380820465 49732.0 083...341 - 49692.8 i 1 8554924738.5

104656 49728.5 : 0..833493- 49689.8 5 05551 497265 A204657 149728.5 "84"599. 185552 49733.5184673 49728.5 4 1.983450..49699.5 1 1 8 , ,if 15528 497338.!

•, 8 41 ,-49699.0 7- - , d •, i;l'.

284706 49728.5 1 .,49335. -9341,498 i730A

10716 w908.352 : '. "-".•. 34 n4969B, 1 , e : i i- , . 1055 93 .104635 49725.5 ---. 4 v.i.r "; 5531 49729.5

.14703 49728.8 1*,, -83428'. 49688. 95 2204704 49727.8 0 : '98343-2 4968600 555.3 49728.5

104706 497P7 -935a,11986705 I 56

A183 B124 El,

Figure 13. Magnetic anomalies comprising Cluster F.

1053 473.

ilu85' 47., . 1-46116 49673.5 :4"I 185433 49726.5 / I .104647... . 49677A. ..

e. .,i 014..I.,.1, 185435 49725. e - I..0460..4967"5. 5 '77 , .,.. ... " ....5 105436 49725.5 • 104651 49676.5 4-l :.,t,.- ..

-*i i -- , . ,1 543 49724.5 1 465 . 496 5 .5 . . . . ,5 ;. ..., va.4-# 4 105438 49726.6 |"10465 ..496?6.8 -. ,..i., ,..• &wAi, b..i.S : - i ., . i, - . I -.L , . . 1 0 5 4 3 8 4 9 7 2 6 . 5 0.. 1 9 4 6 5 3 .. 4 9 6 76 3 .0 .. -i, . -. .

5 -- , , a 1 45443 104655 49659 3, *-. i . i,, o,O -. , 185442 49727.e 104657., 49 '. i. eS i [ .j/ i.•,i , ' •.1 d'J:' 185444 49728.5 ,N -. 184659., 49673.5 T I.---,,Mi.,,,r•..I : •, • • ••5 $ P1t, .e,.i. 105443 49725.5 "6be * 49675 4- -5 ;- " "".,,i,'i,,,:' 185446 497228. • ' ._ - ,.104659. 49677.5T'7*-i,.J,|...... •.iqivz,.u•

8 V. I......~~~185444 49728.8!872 ,97,5 ..S .''.I154-4768 /104659.1 ~496735. -ia - \ .. 185445 49725.5--. . . 0 10470 - 49677.5 ... .. .. ,5 1t 1054,16 49722.8 1'0 .164701. 49675.0 ,- ... 4i. . :. . b"ima

5 185445 49724.0 .1 4702 t, 49675 5 i d...., .,.. . , ii .. 'I'I.' " i"Ia.:,iu.; v 185449 4972.80 . . ...,104703 . 49675.0 . .'.....I,- 105456 49726.85 . N5 - !. • i o,-di- - ', , 105451 49725.40 85459 4 6-1047064 49609.E .-.4. .,I.. -i.-..k , 'i4a, J

5........ - .. , 105452 49727.57 .. 04714 ,.49676.5 ,-- .. 4... ., . Jiwi 8,-. ,,185453 49724." .I047 05.4967 5 ... i.j . ..e L AM iai~j.. ,

545.. 1047C9 .49675,0 si,,. . , .8 -- ."_ ' i ( - I,..,jkI.h4rA'sI 185456 4972.8 ',.,104717 ,,49674,5 77 Ji; ;,,ir IF I 'S • ; - . "-..' *" 4 ~.' A.• eT 18 5 4 5 7 4 9 7 2 7 . 5 J

.1 - V, i A5i57 4 . . . .W;] 194718 ,°49674;5 . i.. ,

104849370.1052 468.

i:-,•I "• . *' ! •.~ •,iz? _; 104713 , 49677 15 - A,. ,A.-i .. •.,wl'•, ,~,, I t , i',i. . 18558 49726.55 " . 104714 -496.?75 7."- I. i ,'_. ?At " ,,

.. U~.Aij185511 49729.5 .:..1:l .l• " .19I4712".49665.0 :TG•,*I.,i••:h..s •••h•,

. . .. .t JJ ,.t•i '185503 49728.5. ,I..- 105 7 717,.49674.5 - .-.. i,

i... . 54t A• 1055034 49732. 1 f 104716,Q49604 . I . jiw.a -' i:. i 150,.4i. -- 1647219 49671.5 --Z i.-&i Aw. ;,i;," A ;,4r

i•. ; 185519.49728., N .- , 104720 49675. i.,, ,..a' -& ' "858926 /..143 96.5 • i..i-e,' i•.•...'. . ,,4 ~ i

105585 49736.t5 105506 49727.5,. 1,5500 4 1726. C 184721.- 49674.5 ,. o i.." ' si M. ;

1877 ,iAi : 5523 49728.15 ... 14723 .49608.1 * . .,.l i,,,,i-" i .U.-

5 r. i 1852 4978. 134724 49670.4.-* ... ..

185526 49726.5.5 I 1.1-87447255 • / . 49673.0 t. 11 .-" , 185512 49728.5 . .- 10-1472- 49675.8 e' i- -. i ,

5'; 105513 49730.8 E. ' .174. .49673.8 .- , .i

ilt &4jw19i:36-49677A _-..";.0 . j. .1" i .,ij,'.-h i•'Th.- l85 351449737 .5 .... 194749 -.49673.5 _ "-Ti.. t•*, *.u .. Ai..I-=kii,,l.AJ•,-e -

85 .... 144752. 49674.9 .'..h oi ,; .. j.....

"1"85548 49738.'6 ..1,475, -49674.8 -. 4,,.j .115417 4972.8. IfS..146 .4665 i..

., ',,,l 185514 49727.5 " 4 _

j ,.- I-•- ', t ".'54,4 7I55 2 19728 ... 4"18554, 49732.6; i., .i.I8i4d. .49612459728.5

"F'>i ' i, i:i l• 185524 49733.8 I,-184901 .496765.0 T=•i .. ! ,,i.i~~,w~m ki7 i 105518 49738.8 (1. 104732 4967-. 5 h.5 .

085519 49726.5.' 85549 49738.5 .�,r/ .�104784. 4967.5 1.-..4, , .. ,.,•a.1- ,,e4-- .. 15.. 5.-2 49733 - ... 1047395 .49676.' 7 -. ,, . -. i.S .. : / . .L:.i Hj148 4.i.e *8105525 49729.5 -. 194848 , 49673.5. T ,.i. ;-,,i1•,",Jj = , '

•85553 49738.:5185554 49723.A .194749 49670.5, i.8.. w 'T , 105552 49729.5 .... 194814 .4961.9.-I. i.185539 497238.5 -I "04•,1. 49673.5 -. '"i"5"'"

1855368- 49731.5 -104743 496743 0 =-i-4 .B 45532 49729.5 G-2104745 -49677.8 "_ hi i3,; ll~b|o~jl) ' : •" i• ' ... • *-•,,- v. • .,1-1 4746 -49675.0 77 j.•f,.. i I ;ýt li~~•. ,

I ,:'-i i - i u- 1 1-0:i •,i t | 5533 49720.5 'S13 49731.0425 --1 4 7, 49672.5 -'77,.i-i,.,-.• .•im ,- J~,#4 lL•Li-- : .,!,,,i .. |.•|10 i 5535 -49737j.' I .. , 047 49 1, 49673.5 i,,-.! " . '! •*, ,i',d • • z

'-'-'; I •" •t . ... .•;li•185542 497,?. .5 =.,.104757 .4 6 .0 -4 i ý.•i , i ,, • ,-i:••~h • .J .• .. t, , , ,i,105537 49731.0 q8&W-- " • ! .',l "105538 49731.5 ' ..1• J • "•.::! , 0475 1,,496767.5 ,"•.•'I .( •. ", .k ,,l i L , lt

S. ... ,.~~~~~~~~~~~104752 ,,49674.0 T , ;;'-;;;;•;;~i••ilv .,•:. i. ." i" : N,k;.', .;, I95539 4973 .0 :, : : ",.I 4 I.49674.0 1';. i.'b,,•-, ,,• ,,u ,S.•. ..... ,:. 185540 49730.e • ,,f •p .. 1047400 . 49673.5 u i 1 a, ,•,, 4,,&,. .,,•, 105542 49727.5 " 6I41214757 49673.0 .,,-', i-,•( •l/,i -4 4~~k ,S i } ;.,-, .,:.., •n.•185544 49729.8 . . ' .777 .. i 164 !,, • ,• i 1855 49 3 .' e 1497 '8•496676 5 ."I-•. 4,•'104,0A.;,"•: i &, &A, 64ZS i . , ;,,."L.,--il105545 49732.5' •1 4759 ,496735,--i.0 .. J ~•,••,• l,~••,d,,,• i, i i, i i. • .•! 185546 4'9732.5 .069 9 7 • L" 'i ./ l,.,w•4 • [ ,'':t.:,;S i " i . . i ,1055 47 4 97 33 .A AN l-0 1248 1 .4967 2 .5 :. . H , L .i .- l i il .,• . ,.• . ;

497 . i iý ."i.18 58 49• ... .105519 49730.0 -. 10 12480 49673.5 7 . &-•' .;ti ,A|s •, • iA.05 9 49 3 . 043 4 4 67h w,•. ,:• •L • : : : i~~~05552 49733.

....:e.

- i : :'77- " 1046 6 .49674.5"7-T-i 'Y'4: ..iý;,ii ,

II7 055 497 "7 A966& -7 4%"

05554 4972. G12

1946l20 -97: 49705o 5*.104649 :49677.0 .... L

0..1464e. 49678.7 .*-..5*--114921-.49712,.-:--184659.9674.5~~;7.4 ~ '~.l4922-u497M.~5- i.g,.A*..1e4650:: 49673.5 1 ... 14923349707i,5 .- T7 .

1065 49675.0 7- i.. 4 i-M.~

19474927-497e7&5..' ;7iM0415479.349675.0 'I.ii4,4w 4 -14937.-497055.,I'.'e ~

I04655 1 93, -.114929- 497025 i.- .104167., 4967.8.0........~''" '--43~479.. 194650. .49674.~5 ji *.'I94 9755

.1483496775. i :V.,, :ýL -4149342 4979~5;7 -8 -* 1478 1.-49675.0 i 1hg4a s~~ I .149 3 '49704,-7_i- 9194769 '496745, 5 wJi ..I aJ.i&1..1149364 ~49707. 5 i.. i .*....10.I47I23 49675.9 0 .d.~ s4&i -1 14937A45 75 5 7 ,I -.

104705-A9677& '114938 1 497065 .5~ i-1476.49674.6 7... ht.14959449795,5 "- i A.,Ai~. U.

-1047e?.' 49674i.5.'' .1149541 49?05~ 4.,. i. ;Z i. 104 703 496715. 0 . A I u14942.,49707.8 7 i....~; I A,~ . .. ... ..

N4847239 .49675.0 -. ,. 4w-. I. &-,,d,.4 .WA&iaol I49593'149704. i -. j,,,

10I473 1.ý,49674,5 1.. 9. 4 7657)ý,19e4732 49675.5 -7 ._- i.. j.m~. H49.~..u. 15-9..497 Z79.iI,. ...i. ...

8947135 .49677.at..i.' .. ~.i&.m~.'188~99.1017114:3649677;5 -. I .. 1 4 l19I8.497e4.5...;.,,%.ie I97a 5 49674. 777. .,,1 441513.993s., #+pil ... i-..i,

..104716iI49674.0 -i. .11 ... b*~~ .1114950* 4970596s... 1047417- 49673. 6 7ý .149591 &497035 Q

...104743. 49675. 6 : e, 11~.~t M ) ~ .. 1 5 1 . 4 7 49 .A .. i"14 ..''184744~~~~~~~ t467. 495 a. ~.15 49794eg , -. jj...1474 .4 9677. . a.- 49~*...a.Jb,- ~ 1 53,9149707,o5.. ;

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Figure 14. Magnetic anomalies comprising Cluster G.

-- Cii

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26

Figure 15. Acoustic anomaly at Cluster G.

27

CHAPTER II

METHODOLOGY

The search and groundtruthing methodologies used during this project were designed to maximizepersonnel, equipment and time. Four steps were involved in the process of investigating the magneticanomalies: reacquisition, contouring, refinement, and identification. The methods utilized are, for the mostpart, common practices for Phase II underwater archeological projects. Arnold (1982:83); Dean et al.(1992:128-144); Gearhart et al. (1990:29); Irion and Bond (1984:33); Marx (1975:123-124); Mistovich andKnight (1983:208); and Wafts, (1986:109-110) all recommend variations of the methodologies employedduring this project. The one major departure from common practice was the utilization of a SchonstedtGAU-20 magnetic gradiometer in place of either a magnetometer or standard metal detector for refining thelocations of anomalies. The benefits of this method over the others will be discussed in a later section.

Survey and diving operations were conducted from two different vessels during the course of theproject. During the first phase of the project, a 19.8 m (65 ft) long wooden-hull, live-aboard, sport fishingboat, Omeco III, was utilized. It provided a stable offshore work platform and saved considerable traveltime because it had the capability of anchoring overnight near the project area. The vessel also wasequipped with two small on-board fiberglass skiffs that could be used as chase boats and for refining thelocations of anomalies. However, due to delays in the project caused by adverse weather conditions,Omeco III was forced to return to her home port of Biloxi to honor previous commitments.

During the second phase of the project, the TGIF, a 7.9 m (26 ft) fiberglass Aquasport dive charterboat based in Grand Isle, was utilized. While TGIF did not provide as much deck space as Omeco i1l,TGIF, its stern-mounted dive platform made entry and exit from the water by divers much more efficient.TGIF also was much more maneuverable, allowing for more rapid refinement of the locations of theanomalies. There were only two main logistical drawbacks in the use of this vessel. First, it requiredproject personnel to find lodging onshore, necessitating approximately two hours per day of travel to andfrom the project area. Second, the wheelhouse of the boat in which the electronic equipment was mountedwas not enclosed fully, risking the threat of having operations interrupted by rainy weather. Fortunately,this did not occur, although one additional day was lost to high winds that resulted in six- to seven-footseas.

The reacquisition of the anomaly clusters involved a 15 m (49.2 ft) interval survey with an EG&GGeometrics Model 866 proton precession magnetometer. Positioning and trackplotting were controlled viaa Magnavox MX200 Differential Global Positioning System (DGPS), supplied by Offshore Navigation, Inc.(ONI) of New Orleans. Differential GPS depends upon the mobile unit receiving pseudorange correctionsfrom a static shore-based receiver to achieve position accuracies of <5 m. ONI maintains its owndifferential stations around the Gulf of Mexico; for most of the project, ONI's station on Grand Isle was used.Unfortunately, a tornado destroyed this station midway through the project, requiring ONI to transmit asignal from its headquarters in New Orleans until the station was repaired. The positioning system wasinterfaced with an Apple Macintosh Classic II running Navigate software to provide navigation control anddata logging. Anomalies located during Phase I of this project first were plotted on the Navigate program.A survey grid with track lines spaced 15 m (49.2 ft) apart then was superimposed over the target, whichwas placed in the center of the grid. Guided by the navigation software, the magnetometer was towedalong each lane. Since water depths in the project area did not exceed 6 m (20 ft), the sensor was towedon the surface. The data was acquired on both magnetic media and a paper printer. The data wascontoured on an IBM-compatible laptop computer running the grid and topo programs of the GoldenGraphics Surfer package. The resulting contours were analyzed to determine the most significant magnetic

28

perturbation within each target cluster. A position for the center of each target was derived and a buoy wasplaced at that location.

Once a buoy was dropped near the point of greatest magnetic deviation, a Schonstedt GAU-20gradiometer was deployed to refine further the center of the anomaly.: During the course of the project,several methods were employed for this step. The first involved extending the gradiometer sensor from thebow of a small skiff and making a series of passes around the buoy. If the gradiometer indicated that theobject was away from the buoy, then a second buoy was dropped and the process was repeated until thebuoy marked the strongest signal.

The second method entailed towing the gradiometer sensor within a weighted PVC sled behind thesurvey vessel. The sled had a buoy attached to it so that its position could be monitored from the surface.As with the previous method, a series of passes was made near the target buoy. When the strongestreturn signal was achieved, the survey vessel slowly backed toward the gradiometer sensor and a secondbuoy was dropped near the sensor.

The third method involved divers deploying the gradiometer sensor at different points away fromthe target buoy and then slowly pulling it toward the buoy anchor. A series of radii around the buoy wassurveyed in this fashion. Upon encountering a strong signal, the diver was advised by surface personnelmonitoring the gradiometer control box to investigate the area near the sensor.

The final step in the process sought to identify the source of the anomaly. Once the position of thestrongest gradiometer reading was refined, divers were deployed to investigate the area. All diving wasconducted using standard open-circuit SCUBA equipment with the addition of full-face EXO-26 band masksfitted with wireless diver-to-diver/surface communication units. Due to the low water temperatures,(approximately 130 C [550 F]) dry diving suits were utilized by all personnel.

The first stage of the process of sourcing the anomaly involved attaching a search line to the buoyanchor and conducting a series of visual and tactile circle searches around the buoy to determine if thesource of the anomaly protruded above the sea floor. Each sweep extended the search radius by 1.5 m(5 tt) out to a distance of 15.2 m (50 if). In the event that no-above surface features were encountered,a series of radii were surveyed with the gradiometer until the point of greatest magnetic deviation waslocated. All targets were probed with a small diameter 2.84 m (9 ft 4 in) stainless steel antenna. All probeswere made to the length of the probe or to refusal. Refusal resulted either from a hard-packed stratum thatunderlay the more loosely consolidated sands, or from the source of the anomaly. More intensive probingwas used to determine the horizontal extent of anomalies and their depths below surface.

The last step in identifying the source of buried anomalies was excavation. Excavations wereconducted using a 10 cm (4 in) diameter hydro-induction dredge powered by 5 cm (2 in) diameter, 5 hpHonda water pump. Excavations were continued until the source of the anomaly was identified, or until theexcavation became too deep for divers to work safely.

Critique of Methodology

In the past few years, a number of Phase II testing projects similar to the Breton Sound Projecthave been conducted, making it now possible to analyze the effectiveness of groundtruthing methods. TheBreton Sound project employed what has become standardized groundtruthing methodologies for PhaseII underwater archeological projects. The sole exception was the use of the magnetic gradiometer in placeof a magnetometer for refining the point of highest magnetic deviation for magnetic anomalies. As will beseen by the following discussion of several similar projects, the results and success rates are similar.

29

One of the first major projects of this kind was conducted prior to the construction of theTennessee-Tombigbee Waterway in Alabama and Mississippi. Larry Murphy and Alan Saltus (1981)identified a total of 21 Study Areas, each one consisting of multiple magnetic anomalies. Many anomalieswere determined to require further work; these include five anomalies identified as vessels or associatedvessel remains; 15 anomalies were characterized as modern debris; four could not be identified becausethey were buried too deeply or could not be located; and, eight were not investigated because they felloutside of project impact areas.

J. Barto Arnold III (1982) in his survey of Matagorda Bay, Texas, identified 12 magnetic anomaliesas having characteristics requiring further investigation. Of these, three were the probable remains ofhistoric shipwrecks; two were probable modern shipwrecks; two were modern debris; and, five could notbe located or a source determined.

One of the more successful projects of this kind (100 percent identification of anomalies) wasconducted in Mobile Bay, Alabama (Irion and Bond 1984). Of the 11 anomalies investigated, nine wereidentified as modern debris; one was a series of pilings associated with the Confederate obstructions in theupper bay; and, one was a brick-filled shipwreck, also part of the Confederate obstructions.

However, environmental conditions can influence the efficacy of these methodologies in theidentification of anomalies. For example, Robert Gearhart et al. (1990) encountered very deep sand in theirattempts to identify positively the site of the 1554 Spanish vessel, Santa Maria de Yciar, near PortMansfield, Texas. These sediments prevented a positive identification of the seven anomaly clusters tested.However, two of the clusters, were believed to be associated with the wreck because of the characteristicsof their magnetic signatures.

In general, the combined use of advanced magnetic detection equipment and traditional line searchtechniques have proven successful in locating sources of anomalous magnetism in highly turbid

environments where visual search techniques are extremely limited. Magnetic detection equipmentbecomes even more important when one considers that the majority of shipwrecks in Louisiana watersprobably show no debris above the bottom. Garrison et al. (1989:11-167) observed that of 47 significantmagnetic anomalies in Texas waters, only 13 percent, or six cases, showed debris above the bottom thatwas detectable utilizing side-scan sonar. With no surface wreckage to aid divers in acquiring target,magnetic detection equipment becomes a requirement.

The magnetic gradiometer proved to be a useful tool for refining the locations of magneticanomalies. The gradiometer incorporates the benefits of both a magnetometer and a hand-held metaldetector. The gradiometer sensor can be towed behind a survey vessel or attached to a boom extendingfrom the bow in shallow water, much like a magnetometer. Unlike the magnetometer, however, thegradiometer does not detect the anomaly unless it passes almost directly over it. A magnetometer beginsto detect localized disturbances in the magnetic field from many meters away, resulting in a much largerfield area.

The gradiometer also can be manipulated by divers on the bottom. This involves pulling or towingthe gradiometer sensor in a controlled pattern around the target buoy. The divers, however, must be carefulnot to let the gradiometer get too close to them as any ferrous objects in their equipment will be detected.Although not as easily maneuvered as a hand-held metal detector, the gradiometer has the sensitivity todetect deeply buried ferrous metal or magnetic objects that a standard metal detector might miss.

During the present project, it was found that the gradiometer, long in use among oilfield divers tolocate buried pipelines, was a useful and effective archeological tool as well. Through the combined useof this instrument deployed first from a small boat to make the initial contact and, second, carried by a diver

30

on the bottom to refine the position, it was possible to acquire and define the target area in a very shortamount of time.

Finally, the Navigate program proved to be extremely accurate for relocation purposes. When abuoy was dropped on the dive site coordinates, its position was annotated on the computer screen with aspecial icon. As testimony to the accuracy of this program, when one buoy was inadvertently sunk andanother had to be dropped, divers later reported that the buoy weights were less than 10 ft away from oneanother.

31

CHAPTER III

RESULTS OF INVESTIGATIONS

Results of Magnetic Contouring Survey

Prior to initiating diving investigations, a 50,000 m2 area was re-surveyed at a 15 m (50 ft) intervalat each of five target areas (A, C, D, F, and G). The methods and equipment employed in this task havebeen described in detail in Chapter II. The close-interval survey fulfilled one of the primary tasks of theScope of Services and was intended to assist in site definition and target selection. However, this exerciseappeared to be largely unnecessary at this stage of the investigations. Positions derived from close-intervalsurvey only differed slightly from those reported for the clusters following Phase I survey (Table 2). Theefficacy of close-interval survey has been demonstrated clearly when applied to actual historic sites(Clausen and Arnold 1975; Garrison 1986; Gearhart 1988) however, it can be argued that, while providingvaluable information about the anomaly, such an exercise may be unnecessary at the Phase II level.

Table 2. Coordinates of Anomalies Derived from Phase I Survey Compared to Coordinates of Actual

Anomaly Source.

CLUSTER DESIGNATION PHASE I COORDINATES J PHASE II COORDINATES

A 29029'31.20" N 29*29'31.94" N89*09'34.80" W 89*09'35.58" W

C 29*29'03.60" N 29*29'03.68" N89*09'30.60" W 89009'32.89' W

D 29028'53.40" N 29°28'52.53" N89*09'42.60" W 89'09'42.00" W

G 29027'46.29" N 29*27'45.62" N89'09'17.04" W 89009'16.68" W

All positions in WGS-84 geographic coordinates.

Magnetic survey at intervals of <15 m (<50 ft) has been applied to buried shipwreck sites todistinguish significant features within those sites and to analyze the extent of associated wreck scatter. Theintent of close-interval survey as applied during the present study was twofold: first, to isolate the mostspatially significant anomaly within a cluster for diving investigation, and, secondly, to characterize thesignature of the target for the purpose of theory-building aimed at distinguishing "treasure" from trash. Byconducting close-interval surveys of target anomalies, it was hoped that pattern recognition could bedeveloped to assist resource managers in developing a strategy for deciding which anomalies must begroundtruthed and which ones can be ignored. The target clusters were selected for investigation basedupon initial similarities between their characteristics of amplitude, duration, and signature and thoseexhibited by historic shipwrecks. It was theorized that by expanding the available data base relating to themagnetic characteristics of groundtruthed marine debris, a preliminary hypothesis concerning the magneticpatterning of submerged historic sites within the waters of the New Orleans District could be formulated.The results of this exercise were surprising.

32

Based on the initial analysis of the contour plots of the five clusters (Figures 16, 17, 18, 19, and20), it was surmised that two could represent potential shipwreck sites; two clearly comprised sources; and,one could be geologic in nature. These suppositions, discussed below, illustrate the potential pitfalls ofanomaly characterization based upon magnetic data alone.

Cluster A (Figure 16) was thought to possess some potential as a shipwreck site. The contourplot showed a clustering of anomalies characteristic of the debris field of a vessel containing only minimalferrous components. The anomalies seemed to concentrate in an elongated pattern in the east-central zoneof the 50,000 m2 survey area. This type of clustering has been described in the literature as representinga typical shipwreck pattern (Garrison et al. 1989:11-223).

After an examination of the contour plot, Cluster C (Figure 17) did not appear to be promisingas a potential cultural resource. The magnetic field within the survey area appeared to contain numeroussmall isolated anomalies, but it lacked any apparent focus. The signature of this cluster appeared to begeologic in nature, much as one would expect to see in a disconformity of the bottom caused by dredging.As far as could be determined, however, no activity of this type has taken place in the project area.

Cluster D (Figure 18) was displayed as a single large point source anomaly. Three other briefpoint source anomalies with a magnetic perturbation >100 gammas also appeared within the block. ClusterD was not characteristic of the model developed for distinguishing shipwrecks from isolated occurrencesof anomalous magnetism. In the field, it was speculated that this cluster represented isolated targetslacking in the qualities of National Register significance.

Cluster F (Figure 19) consisted of several isolated monopolar anomalies. Monopolar anomaliesgenerally are indicative of the presence of an elongated body of sufficient length that one pole is near themagnetometer sensor and the opposite pole is removed effectively to infinity (Breiner 1973:20). This typeof signature has not been associated with shipwreck material. The Scope of Services called for diverinvestigation of Cluster F only if time allowed. Owing to adverse weather conditions in March and April, thiswas not possible. However, it is felt that Cluster F may be eliminated from further consideration based onthe patterning of its magnetic signature, and does not represent a National Register-eligible archeologicalsite.

Cluster G (Figure 20) appeared to resemble what has been described as the "classic" shipwrecksignature. The contour of Cluster G showed a broad-based anomaly that contained several significantpeaks. Lesser anomalies trailed away from the main body in an elongated shape. The contour closelyresembled that of the remains of a shrimp trawler identified by Garrison et al. (1989:11-212; Figure 11-98) insize, amplitude, shape, and duration (Figure 21). Based upon the magnetic signature and the presenceof an anomalous acoustic disconformity (Irion et al. 1993:Figure 9), Cluster G was interpreted as theremains of steel hull wreckage reported by the Coast Guard in 1971 (Irion et al. 1993:55).

Results of Diving Investigations

Following the close-interval resurvey of the five target anomaly clusters, two (A and G) wereinterpreted as potential shipwreck remains. However, the diving investigations of these targets revealeda very different source for these anomalies, which has important ramifications for all submerged culturalresource surveys in the waters of the New Orleans District.

Following the methods outlined in Chapter II, Clusters A, C, D, and G were examined usingvisual circle search techniques, gradiometer searches, probing, and excavation. Using the gradiometer,it was possible to refine accurately the area of magnetic moment and to concentrate probing and excavationin those areas.

33

00

CONTOUR INTERVAL 20 GAMMAS

Figure 16. Magnetic contour map of Cluster A.

34

0

00C)

0 0

D 0o

CONTOUR INTERVAL = 10 GAMMAS

Figure 17. Magnetic contour map of Cluster C.

35

©0

.CONTOUR INTERVAL 100 GAMMAS

Figure 18. Magnetic contour map of Cluster D.

36

00

CONTOUR INTERVAL 10 GAMMAS

Figure 19. Magnetic contour map of Cluster F.

37

a

0

CONTOUR INTERVAL 100 GAMMAS

Figure 20. Magnetic contour map of Cluster G.

38

Site 23, Line 305 GA 332-SP10Magnetic Contour Map

Norlhing

3186450 3186590 3186730 3186875 3187015 3187160 3187300291425

291230

S 291030

(nM

29083

290445

* I I290250negative anomaly

Contour Interval 2 Nanoleslas Positlve anomaly

Figure 21. Magnetic contour map showing the wreckage of a shrimp trawler in the Gulf of Mexico (fromGarrison et al. 1989:11-212, Figure 11-98).

39

Cluster G was the first cluster to be examined. After the position was refined by thegradiometer, divers probed around the target area. They encountered a solid resistant surface at a depthof 1.4 m (4.5 ft). This surface appeared to extend at a consistent level over an area approximately 3 x 9m (10 x 30 ft). Excavation of a trench to a depth of 1.8 m (6 it) revealed that this resistant surfacecomprised a densely packed natural oyster shell reef. Individual oysters comprising the reef were foundin growth positions, and no artifacts were encountered. The gradiometer continued to register strongreadings when the sensor was placed in the trench, although no anomalous ferrous objects wereencountered. No explanation was immediately apparent for the anomalous magnetism. The oyster shellcertainly was not magnetic, and the only other objects encountered in the trench were large, flat platelettesof calcium-cemented sandstone. The anomalies in Cluster G do not appear to have originated from culturalmaterials; therefore, Cluster G does not constitute a National Register-eligible site.

When similar results were encountered during ground-truthing at Clusters A, C, and D, wheresubmerged cultural resources were absent and only sandstone rocks were observed, it was decided toconduct an experiment to determine what effect these sand concretions had upon the gradiometer.Concretions that had been collected from Clusters A and G were brought in contact with the gradiometersensor. In each case, the instrument registered an increase in magnetic gradient. Confident now that thesandstone concretions were related to the pockets of anomalous magnetism, it was concluded that ClustersA, C, D, and G represented natural, and not cultural, features for which consideration for National Registereligibility was unnecessary. Therefore, fieldwork ended, and research was initiated to explain this strangephenomenon.

Sources of Anomalous Magnetism

A magnetic anomaly is any mappable departure from the normal magnetic field of the earth.These measurable departures from the normal magnetic field of the earth can be caused either by naturalsediments or cultural materials. A natural magnetic anomaly is caused by a body of sediment or bedrockwith magnetization that is different from the magnetization of the surrounding sediment or bedrock.Geologists refer to the body of sediment or bedrock as either a "magnetic contrast" or an "anomalousmagnetization;" the surrounding and differently magnetized sediment or bedrock is known as "backgroundmagnetization." Cultural magnetic anomalies are caused by the presence of ferromagnetic alloys lying uponor buried within the bottom sediments of shelf and sounds. In the Offshore Continental Shelf, thepredominant cultural materials are modern ferromagnetic debris, shipwrecks, and components of theinfrastructure, associated with the exploitation of natural resources such as pipelines and offshore platforms(Garrison et al. 1989; Machel and Burton 1991a).

The natural magnetization of sediments, either background or anomalous, is called the "naturalremnant magnetization" (NRM) and consists of four major components. The detrital remnant magnetizationcomponent (DRM) is the magnetization associated with grains of magnetic minerals derived from theerosion of pre-existing volcanic, igneous, metamorphic, and sedimentary rocks. Chemical remnantmagnetization (CRM) is the magnetization associated with authegenic minerals created by the precipitationof new mineral grains and the alteration of pre-existing grains. Bacterial-biological remnant magnetization(BRM) is the magnetization associated with magnetic minerals created by aerobic and anaerobic bacteria.Finally, viscous remnant magnetization component (VRM) is the magnetization carried by low-coercitygrains. Except for the VRM,-all of these components are capable of producing magnetic contrasts withinthe sediments that underlie the offshore areas of Louisiana (Machel and Burton 1991a, 1991b).

40

Detrital Remnant Magqnetization

One potential cause of magnetic contrasts within deltaic, barrier island, and marine sedimentsthat may produce magnetic anomalies is detrital remnant magnetization (DRM). Due to its high specificgravity, the mechanical sorting of sand-size material by currents often creates concentrations of heavy,(e.g., magnetic) minerals. Within beach environments, wave action creates concentrations of heavy mineralgrains of high specific gravity called "beach concentrates." Within fluvial systems, fluvial processes produceblack sands composed of heavy minerals called "placers," which consist of magnetite, ilimenite, hematite,chromite, garnet, zircon, spinel, and other heavy minerals (Greensmith 1986).

However, only minor amounts of magnetic heavy minerals occur within the sand transported bythe Mississippi River (Russel 1937). As a result, it is very unlikely that sediment transport processes cangenerate either placers or beach concentrations large enough to generate significant magnetic anomalies.Of the numerous studies of the sediments that compose the Mississippi River Delta complexes, theChandeleur Barrier Island Complex, and the Unnamed Marine Complex, not one has reported theoccurrence of either placers or beach concentrations large enough to generate magnetic anomalies likeClusters A, C, D, F, and G.

Another source of DRM is fine-grained (less than 0.1 pm in size) magnetic sediment carried insuspension by wind and water currents. This fine-grained magnetic sediment consists of variousproportions of minerals such as magnetite, hematite, maghematite, or gregite. Water-borne, fine-grainedmagnetic sediments accumulate preferentially within natural levee sediments and, to a lesser extent, withinupper point sediments because of their fine size. It will be absent from channel and lower and middle pointbar sediments. The magnetic contrasts and the associated magnetic anomalies that these sedimentscreate would be related closely to the fluvial and distributary channels in occurrence, shape, and size.Because of their fine size, these magnetic sediments can be transformed easily by digenetic conditionseither into other magnetic materials or into nonmagnetic minerals (Allen 1985; Machel and Burton 1992;Oldfield 1991, 1992).

Wind-borne, fine-grained magnetic sediments were distributed across the Louisiana CoastalPlain. As a result, they accumulated noticeably only on stable, relict geomorphic surfaces, such as thePrairie Terrace, which were exposed for long periods of time (Oldfield 1991). As these sedimentsaccumulated, they were admixed by pedogenic processes with the sola of the soils developing within thesediments that were forming the geomorphic surface. Consequently, wind-borne magnetic sediments wouldbecome part of the regional background magnetism related to paleosols such as those present at the topof and within the Prairie Complex. Subsequent pedogenic processes very likely transformed a significantamount of this sediment into other minerals that are either magnetic or nonmagnetic (Machel and Burton1992).

Chemical Remnant Maqnetization

Four processes can create chemical remnant magnetization (CRM) and can generate positivemagnetic contrasts within the fluvial-deltaic, coastal, and marine sediments that underlie the project area.These processes are pedogenesis; the formation of well-drained swamp environments; dehydration of ferrichydroxides; and, hydrocarbon seepage. Depending upon the process involved, CRM may create eitherregional background magnetization or magnetic anomalies of differing sizes and shapes.

Pedogenesis may create either regional background magnetization or magnetic contrastsassociated with natural levee sediments. The formation of soils processes may produce abundant nodules,cements, and fine-grained sediments within subaerially exposed surfaces, e.g., the Prairie Terrace andnatural levees (Machel and Burton 1992; Oldfield 1991, 1992; Schwertmann and Taylor 1977). The

41

production of iron oxides, such as hematite, magnetite, maghematite, and goethite, creates a CRMcomponent that further enhances the magnetization of regional geomorphic surfaces like the Prairie Terraceand specific landforms like the subaerial natural levees of the Mississippi River and its deltaic distributaries.As previously noted, soil formation processes may produce regional background magnetization, or localizedmagnetic contrasts and associated magnetic anomalies within a configuration related to the shape and sizeof an associated channel system.

Like pedogenesis in natural levee deposits, digenesis within the sediments of well-drainedswamp environments produces significant amounts of iron oxides. Iron oxides occur as small 0.5 to 6 mm(0.002 to 0.026 in), fairly well-cemented nodules. These nodules vary in shape from irregular masses towell-rounded clasts. Iron oxides also occur as rims around plant rootlets and as disseminated cement(Coleman 1966). Depending on the mineralogy of these iron oxides, well-drained swamp deposits may formlarge magnetic contrasts associated with buried river valleys.

Two processes may create CRM within the sands that comprise channel fills and point bardeposits. First, fluvial systems apparently transport ferric hydroxide colloids as dilute suspensions that areabsorbed on the surface of clay minerals. After deposition, ferric hydroxides spontaneously dewater toform limonite and, with complete dehydration, hematite. Secondly, the weathering of such iron-rich primarysilicates as pyroxenes, amphiboles, and iron chlorites, by either soil or groundwaters, may release iron thateventually will form various iron oxides. Depending on the specific minerals formed, the formation of ironoxides may cause fluvial and distributary sands to acquire a distinct magnetic contrast (Pettijohn et al.1987).

Hydrocarbon seeps, including biogenic methane, may create CRM produced by digeneticpyrrhotite and magnetite. Depending on the relative amounts of magnetite, pyrrhotite, or other magneticminerals produced and the amount of hematite destroyed, the plume of hydrocarbons may create positive,negative, or no magnetic contrast relative to the background magnetization. The type, amount, and stabilityof magnetic mineral or minerals created by hydrocarbon seeps depend primarily upon Eh, Ph, concentrationof hydrogen sulfide, and the concentration of bicarbonate (Machel and Burton 1991 a, 1991 b).

Bacterial - Biolo-gical Remnant Magqnetization

Several types of microbes, mainly bacterial, are known to form magnetic minerals that aredeposited at their death. So-called "iron bacteria" take up iron compounds leaving iron upon their cellsurfaces. This process results in the precipitation and accumulation of various types of iron oxides andhydroxides. Upon the death of the bacteria, these iron compounds are deposited in a variety of soils,lacustrine sediments, and marine deposits. A variety of anaerobic and aerobic bacteria also forms verysmall grains of magnetite. Upon their death, they contribute very fine-grained magnetite to soils within theshallow to deep marine and brackish to hypersaline lacustrine environments in which they lived. Certaintypes of these bacteria actually may produce magnetite during the oxidation of hydrocarbons in theanaerobic environment.

There are some bacteria that generate iron sulfides by generating reduced sulfur that later reactswith dissolved iron to form sulfides. Within the Gulf Coastal Plain, numerous studies, (e.g., Posey et al.[1987], Sassen [1980, 1987], Sassen et al. [1988, 1991] and others), have shown that the formation ofmetallic sulfides is associated with the microbial reduction of sulfate and the oxidation of thermogenic gasand crude oil. In addition, they note that the same geochemical environment that favors the formation ofmetallic sulfides within salt domes also supports the precipitation of 13C-depleted carbonate minerals.Sassen et al. (1991) imply that bacterial methane oxidation and sulfate reduction are responsible for theformation of the carbonates that comprise carbonate buildups and the metallic sulfides associated withthem.

42

Investigations into the Source of Anomalous Magnetism in Breton Sound

Nodules of carbonate-cemented sandstone instead of historic cultural materials were found attwo magnetic anomalies with signatures suggestive of historic shipwrecks. The magnetic anomalies layalong the boundary between the open lagoonal inlet and the reworked Mississippi Delta sedimentaryenvironments. Thus, the nodules of carbonate-cemented sandstone occurred within bottom sands that hadbeen reworked frequently by tidal, wave, and geostrophic currents. Also, they had formed within sedimentsthat had accumulated only within the last thousand years.

Samples of these nodules were collected for analysis from two of these clusters, A and G, todetermine if these nodules could be the source of the magnetic anomalies. Preliminary testing of themagnetic susceptibility of the nodules clearly demonstrated that their magnetic susceptibility was equivalentto typical carbonated-cemented sandstone, and was insufficient to produce the observed magneticanomalies (Chad McCabe, personal communication 1993).

Additional analysis was performed to determine if the composition of these nodules could berelated to the magnetic anomalies in some way. For petrographic analysis, a commercial companyproduced petrographic thin sections from three different nodules. During preparation, these were stainedpartially with Alizarin Red to identify the composition of the carbonate cement. In addition, samples fromthe same nodules were submitted for stable isotope analysis of the carbon and oxygen that form thecarbonate cement.

Results of Thin Sectioning

Cluster A. At Cluster A, carbonated-cemented sandstone nodules were buried beneath the loosesands that comprise the local sea floor. The nodules recovered from the area of the magnetic anomalywere flat cobble-size clasts that measured approximately 0.63 to 1.9 cm (0.25 to 0.75 in) in thickness and9.5 to 13 cm (3.75 to 5 in) in length. As defined by Folk (1980), they ranged from very bladed to veryelongated in shape. Bubbles of gas were observed bleeding from the sea floor bottom within the area fromwhich the nodules were collected.

Two samples of these nodules were collected from Cluster A. Both consist of dark gray (5Y 4/1)and very dark grayish (2.5Y 3/2), carbonate-cemented sandstone of variable hardness; one is very friableand the other well-indurated. In the friable sample, the sand faction is well-sorted and very fine-grained;in hand specimen, neither bioclasts nor sedimentary structures could be identified. In contrast, the well-indurated sandstone nodule is well-rounded and irregular in shape, and it is has been bored by pelecypodsand encrusted by bryozoa and serpulid worms. Many of the borings still contain the shells of the pelecypodthat made them. The friable sandstone nodules lack any encrustations or borings. Thin Section B12-MA1was prepared from a friable sandstone nodule, while Thin Section B12-MA2 was prepared from a well-rounded and indurated sandstone nodule.

Standard petrographic examination of both thin sections indicated that both sandstone nodulesfrom Cluster A consist of calcite-cemented subarkose. This subarkose is composed entirely of a grain-supported framework of very fine-grained, well to moderately well-sorted sand that lacks any silt or claymatrix. Although highly variable in roundness and sphericity, the sand generally is subangular and ofmoderate sphericity. Thin Section B12-MA2 exhibits distinct cross laminae; similar laminae are absent inThin Section B12-MA1.

The sandstone nodules from which Thin Sections B12-MAl and B12-MA2 were prepared aresubarkoses as defined by Folk (1980). Straight and wavy quartz are estimated to comprise about 82 to 88percent of the sand. Potassium feldspars and plagioclase are estimated to comprise about 8 to 10 percentof the sand, and mica averages about 1 percent of the sand. Trace amounts of heavy minerals such as

43

zircon were observed. Opaques, apparently pyrite and altered pyrite, comprise about 2 percent of the sandin Thin Section B12-MAl and about 4 percent of the sand in Thin Section B12-MA2. Rare, scatteredforaminifera tests occur within Thin Section B12-MA1. Within Thin Section B12-MA2, the foraminifera aremore abundant; unidentifiable fragments of mollusc shells also are present. In both thin sections, thesebioclasts had been altered considerably by digenesis.

Staining of the thin sections with Alizarin Red demonstrated that precipitated calcite cementsthe sandstone nodules. In Thin Section B12-MAI, the calcite cement occurs as a thin, often discontinuous,lining that leaves most of the original porosity unfilled. The sandstone nodule is cemented only by verysmall, occasional, scattered patches of solid cement. In Thin Section B12-MA2, the sandstone is cementedsolidly by precipitated calcite so that its pore space is filled almost completely. The calcite cement hascorroded many of the quartz grains and replaced both feldspar and mica grains; it is uncertain whether thecalcite cement has replaced pyrite or vice versa.

Cluster G. Carbonate-cemented sandstone nodules also were found at Magnetic Anomaly G,buried beneath the loose sands that comprise the sea floor. The single nodule examined from Cluster Gis a cobble-sized clast about 12.7 cm (5 in) long and 3.0 cm (1.2 in) thick. As defined by Folk (1980), itis very elongated in shape. It consists of dark gray (5Y 4/1), carbonate-cemented, indurated sandstone.Its sand faction is well-sorted and very fine-grained. In hand specimen, no bioclasts, sedimentarystructures, encrustations, or borings could be seen. Thin Section B12-MG1 was prepared from this nodule.

Standard petrographic examination of Thin Section B12-MG1 indicated that the sandstonenodules from Cluster G consist of calcite-cemented subarkose. This subarkose is composed entirely of agrain-supported framework of fine-grained, well-sorted sand that lacks any silt or clay matrix. Althoughhighly variable in roundness and sphericity, the sand generally is subangular and of moderate sphericity.

The sandstone nodule from which Thin Sections B12-MG1 was prepared is a subarkos asdefined by Folk (1980). Straight and wavy quartz are estimated to comprise about 88 percent of the sand.Potassium feldspars and plagioclase are estimated to comprise about 8 to 10 percent of the sand, and micaaverages about 1 percent. Trace amounts of heavy minerals such as zircon were observed. About 1percent of the sand consists of opaques, most likely pyrite and altered pyrite. Rare bioclasts consist ofhighly altered foraminifera tests and unidentifiable fragments of mollusca shell.

Alizarin Red staining of the thin section demonstrated that precipitated calcite cements thesandstone nodules. In Thin Section B12-MG1, the cement consists of a thin lining of microcrystalline calcitethat leaves the majority of the original porosity unfilled. A patch of solidly cemented sandstone, in whichsignificant filling of the intergranular porosity occurs, is present within the interior of the nodule. The calcitecement has corroded many of the quartz grains and replaced both feldspar and mica grains. Currently, itis uncertain as to whether the calcite cement has replaced pyrite or vice versa.

Discussion of Thin Sections. Four main observations support the in situ formation of thesecalcite-cemented sandstone nodules. First, most calcite-cemented sandstone nodules are so poorlycemented that it is unlikely that they would have survived shoreface erosion and reworking. Second, mostof these nodules lack the rounding and other physical evidence of having been eroded and transported.Third, most of these nodules lack any evidence of encrusting fauna that would have formed had they beenexposed at the surface for any length of time. Finally, the framework grains that compose these nodulesreflect the composition of the sediments from which they were recovered. The one nodule recovered fromCluster A that is well-rounded, well-indurated, and encrusted by marine organisms simply may result fromlocalized erosion and reworking of sediments by tidal, wave, or geostrophic currents.

The available evidence indicates that the occurrence of the calcite-cemented sandstones isrestricted to small patches on the seafloor. Sedimentologists, biologists, and paleontologists such as Curtis

44

(1960), Heerden et al. (1985), Parker (1956), Phleger (1955), Shepard (1956), Suter and Penland (1987)and Suter et al. (1988), who have studied the sedimentology and fauna of the eastern Mississippi Delta,have sampled the sea bottoms of Breton Sound, Chandeleur Sound, and the adjacent continental shelfextensively. Despite the large number of bottom and subsurface samples taken by these researchers, noneof them has reported the presence of carbonate-cemented nodules similar to those found at the BretonSound Disposal Area. Given the intensity of sampling within the sounds and shelf of the eastern MississippiDelta (e.g., Schroeder et al. 1988a), widespread distribution of carbonate-cemented sandstone clasts wouldhave been detected. Therefore, the lack of previous reports concerning the presence of calcite-cementedsandstone nodules implies that they are concentrated within very small patches on the seafloor bottom.

Stable Isotope Testinq

Understanding of the potential geological origins of the calcite-cemented sandstones, and of theprocesses by which they were formed, was enhanced by determining their specific isotopic components.This information was derived from a series of isotopic analysis of the stones, conducted by Paul Aharonof the Louisiana State University Geology Department. Results of these tests are presented below.

Carbon. Carbon (C) is one of the most abundant elements in the universe and the basis for theexistence of life on earth. Consequently, it is the most important element in the biosphere, and it alsooccurs as a significant component within the earth's crust and mantle, the hydrosphere, and atmosphere.In its reduced form, carbon comprises organic compounds that are the basis of life, as well as crude oil,natural gas, and methane (CH4). In its oxidized state, carbon occurs as carbon dioxide (CO2) andbicarbonate (HCO3) in aqueous solution, and as various types of carbonate minerals within the crust of theearth. Carbon has two stable and one radioactive isotopes. The stable isotopes, 12C and 13C, respectivelycomprise about 98.89 percent and 1.11 percent of the nonradioactive terrestrial carbon. In addition,radioactive 14C is naturally occurring due to its formation by the interaction between 14N and cosmic rays(Faure 1986).

The stable isotopes of carbon are fractionated by a variety of natural processes. Isotopefractionation is a consequence of the fact that certain thermodynamic properties of molecules depend upon,and are sensitive to, the masses of the atoms of which they are composed. Consequently, isotopicfractionation occurs during several different types of chemical reactions and physical processes.These include: (1) isotopic exchange reactions involving the redistribution of the isotopes of anelement; (2) undirectional reactions in which reaction rates depend upon isotopic compositions; and,(3) various physical processes such as evaporation and condensation, melting and condensation, anddissolution and precipitation. As a result, the isotopic abundance of stable carbon isotopes varies by about11 percent within the various types of organic compounds and minerals containing carbon. Because ofisotopic fractionation, the relative degree of either enrichment or depletion of 13C relative to 12C withinorganic compounds and carbonate minerals give important clues concerning the sources of carbon and theprocesses that created these materials (Faure 1986).

The isotopic composition of carbon-bearing organic compounds and carbonate minerals isexpressed in terms of delta notation. This notation is a measure of the degree of either enrichment ordepletion of 13C relative to 12C as compared to a known standard. The parameter is defined as:

[(13C/12C)samole-(13C/12 C)standard51C= ('3C/12C)standard)] x 103

45

Consequently, positive values of 813C represent enrichment of a sample in 13C, whereas negative valuesof 813C imply depletion of a sample in 13C, relative to some reference standard (Faure 1986).

Arbitrarily designated carbon-bearing compounds generally are used as reference standards forcarbonate isotopes. For carbonates, the typical reference standard consists of carbon dioxide (CO2) gas.Carbon dioxide (CO2) gas is obtained by reacting belemnites (Belemnitella americana) from the Pee DeeFormation of South Carolina with 100 percent phosphoric acid at 250C. This is the PDB (Pee DeeBelemnite) standard of the University of Chicago. Another standard, used by many to determine the relativeisotopic composition of materials is the National Bureau of Standard No. 20 Reference Standard (NBS-20).This standard consists of Solenhofen Limestone from Germany (Craig 1957; Faure 1986).

Oxygen. Oxygen (0) is the most abundant element in the crust of Earth. It is an important rock-forming element for silicates, carbonates, oxides, and other minerals. It also is the principle component ofwater and a significant component of some organic compounds. Oxygen has three stable isotopes whichare 160,170, and 180. Their approximate abundances are 160 = 99.36 per cent, 170 = 0.0375 per cent, and180 = 0.1195 per cent. Like the carbon isotopes, the oxygen isotopes are fractionated by specific chemical,physical, and biological processes. In Holocene carbonates, oxygen isotopes are indicative of thetemperature at which minerals precipitate and the source of the water, e.g., fresh, brackish, or marine, fromwhich they precipitate (Faure 1986).

The isotopic composition of oxide and carbonate minerals and water also is expressed in terms ofdelta notation. This notation is a measure of the degree of either enrichment or depletion of 180 relativeto 160 as compared to a known standard. The parameter is defined as:

f(180/ 160)sample-(' 8 0/'60)standard(180/160)standard)] x 103

Consequently, positive values of 8180 represent enrichment of a sample in 180, whereas negative valuesof 8180 imply depletion of a sample in 180, relative to some reference standard (Faure 1986).

The reference standard used for oxygen isotopes depends upon the material analyzed. Theisotopic composition of oxygen within water generally is reported in terms of differences of 180/160 ratiorelative to a standard called "SMOW," which stands for Standard Mean Ocean Water. This standard, NBS-1, consists of a large volume of distilled water distributed by the National Bureau of Standards. The oxygenisotope standard for carbonates is the PDB (Pee Dee Belemnite) standard. The PDB standard consistsof carbon dioxide (CO2) gas produced by the reaction of 100 percent phosphoric acid with Cretaceousbelemnites (Belemnitella americana) from the Pee Dee Formation of South Carolina at 250C. Forcarbonates, a sample is analyzed as carbon dioxide (CO2) gas using mass spectrometers equipped withdouble collectors. Carbonate samples usually are reacted with 100 percent phosphatic acid at 250C toproduce the carbon dioxide gas. The relation between 8180SMOW and 8'8OPDB is given by the equation:5180SMOW = (1.03086) (8180PDB) + 30.86 (Faure 1986).

Preliminary analysis of the calcite-cemented sandstone nodules from the Breton Sound areasuggests that they should be depleted significantly in 13C, but have 8180 values typical of the mean

seawater for the water temperature and depth of the survey area. Unfortunately, definitive analysis of thestable isotopes of carbon and oxygen are still in progress. However, when these analyses are completed,the association of the magnetic anomalies with methane seeps will be tested firmly and the source of themethane possibly will be determined.

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The Formation of Carbonate-Cemented Sediments in the Northern Gulf of Mexico

The calcite-cemented sandstones found in association with Clusters A, C, D, F, and G are atypicalof siliciclastic, particularly deltaic, depositional environments (Coleman 1982). Early lithification ofcarbonate sediments such as oolites, intraclasts, bioclasts, pellets, and micrite, by carbonate minerals ofthe calcite, aragonite, and dolomite groups, is typical of and well-studied for many carbonate depositionalenvironments. However, the early lithification of siliciclastic sediments by carbonate minerals is much rarerand less well-documented. The paucity of siliciclastics with early carbonate cements associated with themodern sea floor, such as the sandstones found in the project area, results from the facts that: (1) the poreand marine waters associated with clastic environments generally are undersaturated in respect tocarbonate minerals, and (2) the sedimentation rates of clastic environments overwhelm the production ofcarbonate cements by precipitation and the production of carbonate by organisms (Roberts et al. 1987;Weiss and Wilkinson 1988).

Holocene carbonate-cemented siliciclastic sediments have been reported from four regions withinthe northern and northwestern Gulf of Mexico. Sandy limestones and carbonate-cemented shellysandstones have been described by Weiss and Wilkinson (1988) from the Inner Shelf offshore of the TexasGulf Coast. Carbonate-cemented sediments and nonbiologically precipitated carbonate buildups arecommon to the western Louisiana Continental Slope (Roberts et al. 1987). Kocurko (1984) and others havestudied pebble and cobble-sized clasts of carbonate-cemented siliciclastic sediments associated with thebarrier islands of South Louisiana. Schroeder et al. (1988a) described carbonate-cemented sediments onthe Florida-Alabama Shelf that occur as hardbottoms consisting of carbonate buildups and extensive areascovered by rock rubble. By examining the four processes by which siliciclastic sediments form in theNorthern Gulf of Mexico, it will be demonstrated that sediments in the project area are unique. Theformation of these sediments can only be the result of nonbiologic carbonates.

Central Texas Inner Shelf

The carbonate-cemented sediments found within the Central Texas Inner Shelf and along theCentral Texas Coast consist of pebbles and cobbles of shelly sandstones to sandy limestones, called"sandy biosparites," and shelly limestones, called "shelly biomicrites." They occur as prominent componentsof beach sediments along the rapidly eroding deltaic headlands of the Texas coastline, and as gravelscattered across over 3,000 square km (1160 square mi) of the adjacent Inner Shelf. This gravel coversthe Inner Texas Shelf from the Rio Grande to Sabine Pass and from the modern shoreline to 15 to 20 km(49 to 66 mi) offshore (Weiss and Wilkinson 1988).

These shelly sandstones to sandy and shelly limestones occur as well-rounded, very bladedpebbles and cobbles. The gravel consists of very bladed to very elongated clasts that range in size froma few millimeters to a few centimeters in length. They typically have a maximum diameter of 20 to 40 cm(8 to 16 in) and a thickness of I to 10 cm (0.4 to 3.9 in). Almost all of this gravel has been bored by clams,sponges, and other marine invertebrates and encrusted by other marine invertebrates, such as barnacles,bryozoans, and oysters. Based on petrographic, trace element, and oxygen isotope data, these gravelshave been classified into two distinct lithologic groups, sandy biosparites and shelly biomicrites (Weiss andWilkinson 1988).

The sandy biosparites consist of variable proportions of quartz sand, silt, shells, and shell debriscemented by a precipitated low-magnesium calcite cement. The shells associated with the sandybiosparites consist of mixed marine estuarine to nonmarine molluscan fauna whose aragonitic shell materialeither has been dissolved or calcitized. Some of the micritic cement have very distinct caliche textures.Sandy biosparites have average stable isotope values of -3.11 per mil 5180 (PDB) and -4.21 per mil 813C(PDB). Trace elements, stable isotopes, caliche textures, mixed and nonmarine shell fauna, and

47

calcitization of aragonitic shell material all argue that the cementation occurred in response to ground watermovement during the subaerial exposure of the central Texas Inner Shelf during the Late Wisconsinanlowstand (Weiss and Wilkinson 1988).

The shelly biomicrites consist of well-preserved molluscan debris encased in a sandy micritic matrix.The shells associated with the shelly biomicrites consist of mixed marine estuarine to nonmarine molluscanfauna. -The micritic matrix is somewhat pelleted and contains intraclasts of micrite containing greater orlesser amounts of shell debris or fine sand. The sediment is cemented by high-magnesium calcite. Shellybiomicrites have average stable isotope values of 0.01 per mil 5180 (PDB) and -13.08 per mil 513C (PDB).Trace elements, stable isotopes, pelleted textures, mixed and nonmarine shell faunas, and intraclasts allargued that the shelly biomicrites consist of inner shelf shell lags formed by the erosion of Pleistocenefluvial-deltaic deposits, admixed with high-magnesium calcite mud, and cemented by 13C-depleted calcitewithin anoxic bottom muds at some relatively shallow depth below the shelf bottom (Weiss and Wilkinson1988). Because of the circumstances needed to create them, this type of shelly biomicrite can form onlywithin wave-dominated continental shelves, e.g., Texas Inner Shelf, characterized by low sedimentationrates.

Neither the sandy biosparites nor the shelly biomicrites are associated with iron sulfides, ironsulfates, and magnetic minerals. The detailed petrographic analysis of these rocks by Weiss and Wilkinson(1988) contains no reference to pyrite, hematite, pyrrhotite, or any other magnetic iron-bearing mineral.Because of the surficial nature of the processes that formed these rocks, it is unlikely that they would beassociated with any significant magnetic anomalies. Furthermore, such an association is unlikely becausethe highly rounded, bioeroded, and encrusted nature of these nodules indicates that they have been erodedand dispersed from their point of origin.

Louisiana Continental Slope

Recent research within the Louisiana Continental Slope has revealed expansive areas of carbonate-rich and frequently lithified bottom sediments, as well as enormous reeflike buildups composed ofnonbiologic carbonates. The carbonate-rich sediments often are lithified sufficiently to form induratedseafloors called hardbottoms. The carbonate buildups are mounds composed of solid, nonbiologicallyprecipitated high-magnesium calcite and dolomite. They range in relief from a few meters to over 20 m (afew feet to over 65 ft) above the adjacent bottom. Details concerning the petrography of these depositshave been published in a number of papers, e.g., Roberts et al. (1987, 1988, 1989). The shallower buildupsmay be capped by relict Pleistocene reefs. Carbonate buildups typically are associated with the crests andflanks of diapirs. They often form lineaments with mud vents and depressions that appear to be associatedwith faults (Roberts et al. 1989).

Unfortunately, specific details concerning the occurrence and petrography of the carbonate-richsediments and associated hardgrounds are lacking in the published literature. From the LouisianaContinental Shelf, Paul Aharon (personal communication, 1993) has recovered calcite-cemented sandstonesand sandstone nodules similar and identical to the ones recovered from the project area. Unfortunately,comparative data for these sandstones are lacking.

These carbonate-rich sediments and carbonate buildups are created by the seepage of either oneor a combination of different types of hydrocarbons, including biogenic methane, thermogenic methane,natural gas, and crude oil. Biogenic methane is formed by the decay of woody, algal, and other botanicalmatter buried within sediments. Thermogenic methane is formed by the thermal degradation of organicmatter at depths of hundreds of meters (Roberts et al. 1988, 1989).

48

The presence of hydrocarbons within sediments can cause the precipitation of carbonate cementsby means of a complex process. The oxidation of methane and degradation of crude oil within thesesediments increases the alkalinity of the pore fluids. The increase in alkalinity creates exceptionally highpCO 2 in the interstitial pore fluid of the gas-charged sediment which may subsequently result in carbonateprecipitation. This process has been discussed in detail by a number of studies, namely Iversen andJorgensen (1985); Suess and Whiticar (1989); Whiticar and Faber (1986); and, Jorgensen (1992).

Methane oxidation is believed to occur in either the anoxic environments generated by sulfate-reducing bacteria or in oxic environments as a result of the activities of aerobic bacteria. Carbon dioxideis generated by the reaction:

CH4 + 202 = CO2 + 2H20

The carbon dioxide, thus created, dissolves within the water to form bicarbonate ions (2HC0 3 ) that can beincorporated into the carbonate cements. Within anoxic environments, the action of reducing bacteria maycause the precipitation of carbonate cements by causing the reaction (Jorgensen 1992; Suess and Whiticar1989; Whiticar and Faber 1986):

CH4 + S0 4= = H2S + H20

and then:

C03= + H20 + CO 2 = 2HCO&3

Through these processes, the extremely 513C depleted carbon isotopes characteristic of biogenicmethane, thermogenic methane, or crude oil are inherited, in part, by the carbonate cements that theiroxidation generates. As a result, methane-derived carbonates are remarkably depleted in 13C. Carbonatesassociated with the oxidation of biogenic methane can produce carbonate cements with 813C valuestypically ranging from -45 to -55 per mil (PDB) and as much as -70 per mil PDB. The degradation of crudeoil will produce 813C values of around -25 to -35 per mil (PDB). The 813C values for thermogenic gas willbe intermediate between biogenic methane and crude oil. The actual 513C per mil values for carbonatesand carbonate cements can be larger because of the admixture of contributions of bicarbonate from eithermethane or crude oil with bicarbonate from less 13C-depleted inorganic sources within the interstitial porewater (Jorgensen 1992; Kocurko 1984; Roberts et al. 1989).

Preliminary analysis of the calcite-cemented sandstone nodules from the Breton Sound areasuggests that they should be depleted significantly in 13C, but should have 180 per mil values typical of themean seawater for the water temperature and depth of the survey area. Unfortunately, definitive analysesof the stable isotopes of carbon and oxygen are still in progress. When these analyses are completed, theassociation of the magnetic anomalies with methane seeps will be tested firmly, and the source of themethane possibly will be established.

In theory, (Machel and Burton 1991a), the same hydrocarbon seeps that form 13C depletedcarbonate cements and buildups have the potential to form magnetic anomalies. However, within the Gulfof Mexico almost nothing has been published concerning the association, or lack of association, betweenmagnetic minerals and magnetic anomalies and carbonate buildups and cements associated withhydrocarbons seeps. The paucity of published research may reflect either a lack of such studies or theproprietary nature of studies that are associated with the discovery of commercial hydrocarbon deposits.However, ongoing work by Paul Aharon (personal communication 1993) and Fu Baoshun (personalcommunication 1993) has demonstrated that carbonate-cemented sediments and carbonate buildupsassociated with hydrocarbon seeps often are associated with magnetic anomalies. In addition, they havefound that the carbonate-cemented sediments and carbonate buildups typically contain various iron sulfidesand sulfates. Some of the samples from these deposits showed moderate magnetic susceptibility that

49

disappeared upon exposure to air and dehydration. This very phenomenon was observed in the samplesrecovered from the project area. Unfortunately, the lack of published data concerning sediments underlyingsurface buildups and deposits leaves the exact source of the magnetic anomalies open to question (PaulAharon, personal communication 1993; Fu Baoshun, personal communication 1993).

Exposures of the Miocene Lucina Limestone within Italy provide revealing cross-sections ofcarbonate buildups formed by hydrocarbon seeps. Like the carbonate buildups in the Gulf of Mexico, theseisotopically depleted carbonates contain abundant iron sulfides. These exposures exhibit the presence ofconcentrated accumulations of iron sufides lying a few meters beneath the buildups (Aharon et al. 1993;Paul Aharon, personal communication 1993). Analogous accumulations of iron sulfides, e.g., pyrrhotite,underlying areas of carbonate precipitation caused by hydrocarbon seepage in the Gulf of Mexico, couldproduce the magnetic anomalies observed to be associated with them.

Louisiana Barrier Islands

The carbonate-cemented sediments found along the Louisiana Barrier Islands consist of crusts andsandstone nodules associated with barrier islands along the north and northwestern coast of the Gulf ofMexico. The crusts have been reported from the Chandeleur Island System, from Elmer's Island in SouthLouisiana, and from Mustang and Padre Islands along the Texas Coast (Kocurko 1984; Morgan andTreadwell 1954; Roberts and Whelan 1975). The nodules typically occur as a crust associated with algalmats that cover supratidal fiats lying between marshes and dunes on the leeside of barrier islands. Thesecrusts are a thin layer of sand and silt cemented by high-magnesium calcite. Individual layers, which canform in about two months, typically are about 5 mm (0.2 in) thick. If a number of layers are superimposedon one another, the crust can be as thick as 30 mm (1.2 in). They frequently exhibit various types ofsedimentary structures such as cross-lamination, root casts, and burrows, that are possessed by the sandsin which they form. The cements that form the surficial crust consist of high-magnesium calcite with about35 to 50 mole percent magnesium and minor amounts of aragonite. They have average stable isotopevalues of 0.80 per mil 8180 (PDB) and -1.00 per mil 813C (PDB). The high magnesium content and stableisotope values indicate that evaporation and algal growth control the formation of the crusts (Kocurko 1984).

After burial, a number of changes occur within the carbonate crusts. First, the aragonitedisappears, and the dolomite appears as an accessory mineral. The amount of magnesium present alsodrops to about 17 to 33 mole percent Mg. Finally, the crusts become depleted in 180 and 13C to about -0.06 per mil 5180 (PDB) and -4.00 per mil 6

13C (PDB). These changes result from the dissolution andreprecipitation of carbonate in an environment from which the influence of active algal mat growth, thepresence of decaying organic matter, and the influence of marine waters have been removed (Kocurko1984).

The nodules consist of well-indurated calcite-cemented sandstone. Morgan and Treadwell (1954)reported that the occurrence of nodules ranged in size from small pebbles to small boulders. However, allof the nodules observed by later investigators ranged in size from pebbles to cobbles. These nodules varygreatly in shape from contorted, irregular masses and donut shaped pieces, to rounded, moderatelyspherical pebbles or cobbles. Typically, this type of carbonate-cemented sandstone nodule is "slab shaped"flattened parallel to the bedding of the sand bed in which it formed. These nodules exhibit small-scalecross-lamination, burrows, and root casts, and contain inclusions of clay balls, intraclasts, numerous piecesof wood, or other organic materials inherited from the original sand bed (Kocurko 1984; Roberts and Whelan1975).

These nodules contain varying proportions of sand, silt, and shell debris cemented by calcitic micriteand microspar. The sediment cemented to form these nodules consists typically of either well-sorted finesand, very fine sand, or silt. Less commonly, the sediment consists of poorly sorted molluscan debris

50

admixed with fine sand. The sandstone nodules from the Chandeleur Islands are cemented by calcite thatoccasionally is associated with aragonite; these nodules are depleted significantly in 13C with values thatrange from -24.1 to -40.0 per mil 513C (NSB-20). The sandstone nodules from Elmer's Island are cementedby calcite, which is associated with minor amounts of dolomite; they have average stable isotope valuesof 0.60 per mil 8180 (PDB) and -37.0 per mil 813C (PDB). The carbon isotope values range from -25.9 to -48.1 per mil 613C (PDB). Unfortunately, little if any information is given about the presence or absence ofiron sulfides and sulfates in these rocks (Kocurko 1984; Roberts and Whelan 1975).

Stable isotopes of carbon and oxygen demonstrate uniform origin for the carbonate cements ofthese sandstone nodules. Their oxygen isotope values indicate that the carbonate cement precipitated ina uniform, marine pore fluid. The severely depleted carbon isotope values are characteristic of methane-derived carbonate cements. Because of the extreme depletion of 13C, biogenic methane is the methaneassociated with the formation of these cements. The apparently uniform dispersion of carbonate nodulesthroughout the sediments of the Chandeleur Island Complex and other barrier islands further indicates thatubiquitous biogenic methane, not localized plumes of thermogenic methane, initiated the formation of thesecarbonate cements (Kocurko 1984; Roberts and Whelan 1975).

Alabama - Florida Shelf

Extensive hardbottoms composed of carbonate-cemented sediments are common on the seafloorof the continental shelf off of Alabama and northwest Florida. Three types of carbonate-cementedsediments have been identified within these hardgrounds: (1) shelly sandstone; (2) massive to nodularsiderite-cemented sandstones and mudstones; and, (3) calcite-cemented calcirudite. The shelly sandstonesoccur as rubble fields in areas such as the Southeast Banks area and rock outcrops, and the SouthwestRock area. The siderite-cemented sandstones and mudstones occur within the rubble fields at the 17Fathom and Southwest Rock areas (Parker et al. 1993; Schroeder 1988a, 1988b).

The rubble fields that are composed of shelly sandstones and siderite-cemented sediments consistof scattered, rounded, and abraded pieces and irregular slabs. They vary in size from small pebbles toboulders, ranging from I to 2 m (3 to 6 ft) in diameter. The rocks also range in color from buff to yellowishgray, dark gray, and dark brown. Many of the sandstones are well-indurated, whereas shelly sandstonesand coquina are friable. These rocks are bored extensively by the bivalve Lithophaga and the spongeClinoa. Abundant encrusting epifauna, e.g., soft corals, barnacles, bryozoa, sponges, and serpulid worms,cover the larger slabs, although the smaller, abraded rocks lack encrusting organisms (Parker et al. 1993;Schroeder et al. 1988a, 1988b).

Two rock outcrops are associated with the rubble fields at the Southwest Rock area about 17.5 km(10.5 mi) south of Dauphin Island at water depths of 20 to 22 m (66 to 72 ft). One rock outcrop measures.about 7 to 9 m (23 to 30 ft) across and 1 to 1.5 m (3 to 5 it) high; the second outcrop is about 1.5 to 3.5m (5 to 11.5 It) across. The rocks consist of a well-indurated, medium gray, shelly sandstone (Schroederet al. 1988a, 1988b).

The shelly sandstones comprise submature, siliciclastic sandstones containing variable amountsof shell debris. They include a full spectrum of rock types ranging from sandy biosparites to eitherforaminiferal and ostracodal sandstones or quartzose sandstones. Mollusk shells, foraminifera tests, andechinoderm fragments commonly comprise the shell debris. Associated with the shelly sandstone areabraded and bored fossil shells, such as, oyster (Crassostrea virginia) and hardshell clam (Mercenariamercenaria), characteristic of estuarine environments. The stable isotope values of these carbonate-cemented sediments range from -25 to -45 per mil 813C (PDB) (Parker et al. 1993; Schroeder et al. 1988b).

51

Siderite-cemented sandstone and mudstone rock rubble consist of clastic sediments cemented bysiderite. These rocks are well-indurated and dark gray with a rusty-brown exterior rind. Their sedimentarystructures either are lacking or indicative of intensive bioturbation. Unlike the shelly sandstones, pyrite ispresent as the nuclei of some of the siderite grains. The stable isotope values of these siderite-cementedsediments are about -12 per mil 513C (PDB) (Parker et al. 1993; Schroeder et al. 1988b).

Recent studies by Shroeder et al. (1988b) and Howard (1990) indicate that the carbonate- andsiderite-cemented siliciclastics of the Alabama-Florida shelf are a transgressive lag derived from thedestruction of Late Pleistocene and Early Holocene deposits. The highly fragmented and rounded natureof these rocks suggests that they have been reworked by shoreface erosion during the currenttransgression. The presence of abundant relict shells of oysters (Crassostrea virginia) and hardshell clams(Mercenaria mercenaria) demonstrates the former presence of bay, sound, or lagoon deposits associatedwith these rocks. The severe depletion of 13C within the carbonate cements and other evidence indicatesthat they formed within shallow subsurface sediments heavily charged with biogenic methane (Howard1990; Parker et al. 1993).

Hypothesis

From the comparison of the calcite-cemented sandstones nodules found in the project area withthe known occurrences of carbonate-cemented siliciclastics, it is proposed that these nodules consist oflocal shelf sands cemented in place by calcite cements. Because they are cemented by methane-derivedcalcite and lack caliche-like cements, the nodules from both magnetic anomalies clearly differ in origin fromthe shelly sandstones to sandy limestones of the Inner Texas Coast. As previously noted, the poorlycemented, often friable nature of these nodules at both magnetic anomalies indicate that these noduleshave not been reworked from older deltaic, lagoonal, or fluvial sediments as have carbonate nodules foundwithin barrier islands of the South and Eastern Louisiana coast. The lack of primary, carbonate mud clearlydifferentiates these nodules from the shelly limestones that have formed in place within the Inner TexasCoast. Because of the available data and associated magnetic anomalies, it is proposed that these nodulesare shallow water examples of the carbonate-cemented and carbonate-enriched sediments that have beenstudied extensively within the Louisiana Continental Shelf.

Summary of the Origin of Magnetic Anomalies in the Project Area

Since it was not feasible to collect core samples of sediments from the magnetic contrastsresponsible for creating magnetic anomalies in the project area, an absolutely precise origin for theseanomalies cannot be determined. However, because of their size, shape, and apparent association with13C-depleted carbonate-cemented sediments, it is hypothesized that the magnetic anomalies are causedby Chemical Remnant Magnetization and Bacterial-Biological Remnant Magnetization created by localizedplumes of methane. Although unknown in composition, the gas that was observed to be bubbling activelyfrom the seafloor presents additional evidence for the association of these magnetic anomalies withmethane plumes generated within the sediments underlying the seafloor. If such methane seeps arepresent, then the most likely source for the associated magnetic anomalies would be the presence of eithermagnetite or pyrrhotite with minor amounts of other magnetic minerals produced within the nearsurfaceportion of a methane plume (Machel and Burton 1991a, 1991b).

At this time, it is unclear whether these plumes are formed by thermogenic methane escaping fromoil or gas accumulations within the underlying Miocene strata, or by biogenic methane derived from thedecomposition of organic matter within underlying deltaic sediments. The presence of nearby major gasfields (Figure 8) demonstrates that the Miocene strata underlying both magnetic anomalies containthermogenic methane and natural gas. However, the presence of a major deltaic lobe underlying both

52

Breton Island and the magnetic anomalies also suggests that biogenic methane leaking from these deltaicsediments could be another likely cause of the hydrocarbon seeps. In either case, hundreds, if notthousands, of magnetic anomalies similar to those investigated in the project area should exist within theLouisiana Continental Shelf; theoretically, magnetic anomalies of similar origin also may occur within LakePontchartrain and elsewhere within the boundaries of the New Orleans District.

Presently, no regional maps exist that depict the distribution and types of magnetic anomalies onthe scale of Magnetic Clusters A through G. Regional maps of magnetic anomalies within the Gulf ofMexico have been produced; however, these anomalies reflect only those magnetic contrasts related tolarge-scale differences between lithologic units and geologic structures within the thick wedge of Cenozoicand Mesozoic deposits and the underlying crust. The precise distribution and types of magnetic anomalies,such as those investigated in Breton Sound, presently is unknown.

According to Robert Floyd (personal communication 1993), a recognized expert on magnetic surveyof the Outer Continental Shelf, hundreds of magnetic anomalies similar to those investigated in the projectarea occur within the Chandeleur Sound, Breton Sound, and Main Pass Lease Areas. These have beenfound to produce a magnetic signature that is virtually indistinguishable from that produced by an historicshipwreck. The data concerning these anomalies can be found in hazard survey reports for federal leaseareas within the Louisiana Shelf. In particular, the reports concerning Chandeleur Area Blocks 9, 12, 13,23, 42, and 43 contain detailed analyses of materials associated with similar magnetic anomalies (RobertFloyd, personal communication 1993). Unfortunately, because of their proprietary nature, permission toexamine and cite any of these reports has not been obtained.

A different source of anomalous magnetism has been observed by Ted Hampton (personalcommunication 1993) and John Greene (personal communication 1993) within magnetic surveys from theLouisiana Continental Shelf. This type of magnetic perturbation consists either of long meanderinganomalies associated with fluvial channels, or monopolar low-amplitude anomalies associated with deltaicdistributary channels. These anomalies may represent CRM associated with iron oxides present withineither natural levee or point bar deposits. The very common occurrence of natural gas of unspecified originwithin the channel and point bar sands (Anderson and Bryant 1990; Suter 1986) implies that methane-derived magnetic minerals such as magnetite and pyrrhotite also may have formed within these channelsand may be responsible for these anomalies.

53

CHAPTER IV

THE ARCHEOLOGICAL SIGNIFICANCE OF CHEMICAL ANDBIOLOGICAL REMNANT MAGNETISM

The Origin of Magnetic Anomalies in the Project Area

Since it was not possible to collect core samples of sediments from the magnetic contrastresponsible for creating magnetic anomalies in the project area, an absolutely precise origin for theseanomalies cannot be determined. However, their size, shape, and apparent association with 13C-depletedcarbonate-cemented sediments suggest that these magnetic anomalies were caused by Chemical RemnantMagnetization (CRM) and Bacterial-Biological Magnetization (BRM) created by localized plumes of methane.Although unknown in composition, gas actively bubbling from the seafloor presents additional evidence ofthese associations. If such seeps are present, then the most likely source for these magnetic anomalieswould be the presence of either magnetite or pyrrhotite with minor amounts of other magnetic mineralsproduced within the nearsurface portion of the methane plume (Machel and Burton 1991a, 1991b).

At this time, it is unclear whether these plumes are formed by thermogenic methane escaping fromoil or gas accumulations within the underlying Miocene strata, or are formed by biogenic methane derivedfrom the decomposition of organic matter within the underlying deltaic sediments. The presence of nearbygas fields (Figure 8) demonstrates that the Miocene strata underlying the magnetic anomalies containthermogenic methane and natural gas. However, the presence of a major deltaic lobe underlying BretonIsland and the project area implies that biogenic methane leaking from these deltaic sediments also couldbe the cause of the hydrocarbon seeps. In either case, hundreds, if not thousands, of magnetic anomaliessimilar to those investigated in the project area should exist within the Louisiana Continental Shelf.Theoretically, magnetic anomalies of similar origin also may occur within Lake Pontchartrain and elsewherewithin the boundaries of the New Orleans District.

Regional mapping of the distribution and types of magnetic anomalies, on the scale of MagneticClusters A through G, is lacking at this time. Regional maps of magnetic anomalies within the Gulf ofMexico have been produced; however, these maps reflect only magnetic contrasts related to large-scaledifferences between lithologic units and geologic structures within the thick wedge of Cenozoic andMesozoic deposits and the underlying crust. The precise distribution and types of magnetic anomalies,such as those investigated in Breton Sound, presently is unknown.

According to Robert Floyd (personal communication 1993), a recognized expert on magnetic surveyof the Outer Continental Shelf, hundreds of magnetic anomalies similar to those investigated in the projectarea occur within the Chandeleur Sound, Breton Sound, and Main Pass Lease areas. These have beenfound to produce a magnetic signature that is virtually indistinguishable from that produced by an historicshipwreck. The data concerning these anomalies can be found in hazard survey reports for federal leaseareas within the Louisiana Shelf. In particular, the reports concerning Chandeleur Area Blocks 9, 12, 13,23, 42, and 43 contain detailed analyses of materials associated with similar magnetic anomalies (RobertFloyd, personal communication 1993). Unfortunately, because of their proprietary nature, permission toexamine and cite any of these reports has not been obtained.

A different source of anomalous magnetism has been observed by Ted Hampton (personalcommunication 1993) and John Greene (personal communication 1993) within magnetic surveys from theLouisiana Continental Shelf. This type of magnetic perturbation consists of either long meanderinganomalies associated with fluvial channels or monopolar, low amplitude anomalies associated with deltaic

54

distributary channels. These anomalies may represent CRM associated with iron oxides present withineither natural levee or point bar deposits. The very common occurrence of natural gas of unspecified originwithin the channel and point bar sands, e.g., Anderson and Bryant (1990) and Suter (1987), implies thatmethane-derived magnetic minerals, such as magnetite and pyrrhotite, also may have formed within thesechannels and might be responsible for these anomalies.

Archeological Significance

The available evidence indicates that Magnetic Clusters A, C, D, F, and G are caused by localizednatural magnetic contrasts within the seafloor bottom. The presence of friable calcite-cemented sandstoneassociated with these anomalies strongly implies that the magnetic contrasts were associated with plumesof methane or natural gas seeping from the underlying sediments. Without further tests involving coresampling, it is not clear precisely whether the methane is thermogenic or biogenic in origin. Within thesandy sediments of the near surface seafloor, microbial oxidation of the methane produces a geochemicalenvironment rich in bicarbonate and reprecipitated calcite. Magnetic minerals, such as magnetic andpyrrhotite, formed by chemical and biological processes related to the methane seep a few meters belowthe surface, have created most of the magnetic anomalies recorded during Phase I survey. Theseformations have created pockets of anomalous localized magnetism that closely mimic the localizedmagnetic perturbations of historic shipwrecks in amplitude, duration, type, and extent.

If the magnetic anomalies comprising Clusters A, C, D, F, and G are related to hydrocarbon seeps,two criteria might be used to differentiate natural magnetic anomalies from those produced by shipwrecks.First, either subbottom profiler, minisparker, or high frequency seismic data might be used to detect thepresence of gas seeps associated with these types of magnetic anomalies. For example, bubble plumesrising from seafloor seeps show up as strong backscattering regions in these types of data. Typically, theseismic record from any of these techniques should show gas-charged sediments associated with this typeof magnetic natural anomaly as acoustic wipeout zones and zones of acoustic turbidity, also called "chaoticfacies." These techniques also may show gas-charged sediments as velocity pull-downs or as multiplereservations called "ringing" on their records (Andersen and Bryant 1990). Diver investigations performedat the Phase I level also could help to characterize the bottom conditions and ascertain the presence ofcarbonate sandstone in suspected areas.

It will be difficult to predict the distribution of these magnetic anomalies. Thermogenic methane,natural gas, and biogenic methane underlie almost the entire Louisiana Continental Shelf (Andersen andBryant 1990). Also, although general rules about the association between hydrocarbon seeps and eithergeomorphic or geologic features, such as faults, can be generated, such rules will fail to predict the locationof many seeps because of insufficient data concerning many of the factors governing their location.

Since it is virtually impossible to predict the occurrence of pockets of anomalous magnetismresulting from chemical and biological remnant magnetism, it is recommended to the New Orleans Districtthat two changes be made in the methods employed to survey for submerged cultural resources within theDistrict. First, it is recommended that continuous seismic profiling records be obtained of the upper 10 to15 m (32.8 to 49.2 1f) of the sub-bottom materials. The system should offer the ability to produce very highresolution records without transducer ringing that commonly acts to obscure the first 4.5 to 9 m (15 to 30it) of conventional pinger-type sub-bottom profiling records.

Secondly, characterization of bottom conditions should be made at selected anomalies that appearto possess significant duration to determine the potential presence of carbonate-cemented sandstone. Thepresence of this material coincides with the presence of methane seeps.

55

The mere presence of naturally occurring pockets of anomalous magnetization does not obviatethe need for diver groundtruthing. While it is hoped that implementation of the recommendations discussedabove will help to eliminate some non-anthropogenic anomalies from further consideration, no means ofremote sensing offers a panacea to the cultural resource manager. The troubling fact remains that ashipwreck could well occur in the same area as methane seeps and contribute its anomalous magnetismto that occurring naturally.

56

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64

PERSONAL COMMUNICATIONS

Paul Aharon 1993

Whitney J. Autin 1992

Fu Baoshun 1993

Robert Floyd 1993

John Greene 1993

Ted Hampton 1993

Chad McCabe 1993

65

APPENDIX A

SCOPE OF SERVICES

8 February 1993

REVISED SCOPE OF SERVICESCONTRACT DACW29-D-001 1

Delivery Order 03Investigation of Anomalies located within the Breton Sound Disposal Area,

Plaquemines Parish, Louisiana.

1. Introduction. The Breton Sound Disposal Area is located at Mile 0.0 to -2.5 justsouth of the Mississippi River Gulf Outlet (MRGO). The proposed COE project calls forthe construction of a pilot berm consisting of material routinely dredged from theMississippi River Gulf Outlet. This pilot berm will be constructed on the right descendingside of the channel, southeast of Breton Island. A submerged cultural resources surveywas conducted of the area selected for the berm in 1992. Five anomaly clusters werelocated which may have the potential to be significant resources. This investigationconsists of testing four of the anomalies in the impact area to determine if any aresignificant historic properties (Anomaly Clusters A, C, D, and G.

2. Study Area. The study area consists of four anomaly clusters located within the areaselected for berm construction. The fifth anomaly cluster will be evaluated if possible,within the time allotted for this project.(see attached map).

3. Background Information. The study area was surveyed by Goodwin and Associatesin 1992. Approximately 1,850 acres were surveyed using a EG&G 260 sidescan sonar,a Geometrics G866 proton precession magnetometer, an Odom Echotrack surveyfathometer and a real time positioning system consisting of a Magnavox MX200differential GPS receiver, a Micronet terrestrial MF data link, and a Macintosh computerwith Navigate software. The complete equipment array was deployed over the projectarea at track intervals of 600 feet for collecting acoustic data and at 150 ft. intervals formagnetic data (as was specified in the scope of services). A total of 79 magneticanomalies were located of which a portion were attributed to the same magnetic eventon adjacent lines. Additional investigations were recommended at five anomaly clustersA, C, D, F, and G. The State Historic Preservation Officer. concurs with therecommendation to further investigate these four anomalies.

4. General Nature of the Work. The study consists of relocating the four anomalyclusters and exposing, delineating, and recording the most aerially significant magnetictarget associated with each anomaly cluster to determine significance for the NationalRegister. Each contributing anomaly in the cluster need not be tested based on thetheory that any significant shipwreck would produce a single wreck focus with a scatteringof associated debris. Testing of the focus should be sufficient to determine significance.

5. Study Requirements. The study will be conducted utilizing current professional

standards and guidelines including, but not limited to:*the National Register Bulletin 15 entitled, "How to Apply the National RegisterCriteria for Evaluation";

*the Secretary of the Interior's Standards and Guidelines for Archeology and

Historic Preservation as published in the Federal Register on September 29,1983;

*the Louisiana Division of Archeology's Comprehensive Archeological Plan datedOctober 1, 1983 and the Cultural Resources Code of Louisiana, dated June 1980;

*the Advisory Council on Historic Preservation's regulation 36 CFR Part 800entitled, "Protection of Historic Properties."

The study will be conducted in two phases: investigation of anomaly clusters and dataanalysis \report preparation.

A. Phase 1. Investigation of Anomaly Clusters. Phase 1 will begin with relocating the fouranomaly clusters A, C, D and G. Once located each cluster will be subjected to a closeinterval magnetic survey. The focus of the cluster will be examined to determine thenature of the anomaly. The search grid will include the 50,000 square meters to definethe cluster (224 m to a side). Survey tracks will be spaced 15 m. apart. A magneticcontour plot of the survey area will be produced in the field to further define the areaexhibiting the highest magnetic reading. The results of the contour plot will then be usedto guide underwater subsurface testing at the area. Previous work on shipwrecks hasindicated that the focus normally represents the greatest concentration of submergedwreckage.

The only anomaly cluster to produce an acoustic reading, anomaly cluster G, will beexamined primarily by diving on the objects. A Marine gradiometer will be used to furtherdefine the area of anomalies A, C, and F with the greatest magnetic reading which thenwill be the focus for diving, probing, and excavation.

Investigations at buried anomaly clusters will attempt to delineate, expose, record andidentify the nature of the anomaly. The targets will be characterized as much as possiblein the time allotted but may not be identified positively. Either an eight foot solid steelprobe or an 15 ft hollow core hydraulic probe will be utilized to determine if the anomalycan be uncovered by standard excavation techniques. If an anomaly cluster is coveredwith 3 feet or less of overburden, a small test trench will be excavated using a hydraulicdredge. Each anomaly cluster will be examined and evaluated for its potential historicalsignificance.In the event that the anomaly cluster is buried with more sediment than is practical toutilize hand excavation techniques, then other mechanical means will be employed, suchas prop washes.

The fifth anomaly cluster, Anomaly Cluster F, which was recommended for testing in the

survey report will be investigated if possible, within the time allotted for this phase. Thefull time allotted (including travel and possible weather days) will be utilized to examineto examine this anomaly cluster.At a minimum, a measured map incorporating horizontal and vertical controls should bemade of each potentially significant site. This will be accompanied by measured drawingsof significant features that may contribute to a determination of significance. If visibilitypermits, all anomaly clusters will be photographed. Two copies of a brief managementsummary which presents the results of the fieldwork will be submitted to the COR within6 weeks after delivery order award. The management summary will include a briefdescription of each anomaly located during the survey and recommendations for furtherwork if necessary.

B. Phase 2: Data Analysis and Report Preparation. All data will be analyzed usingcurrently acceptable scientific methodology. The data analyses and report presentationwill include as a minimum:

(1) post-plots of survey transects, data points and bathymetry;(2) same as above with magnetic data included;(3) plan views of all potentially significant anomalies showing transects, datapoints, and contours;(4) correlation of magnetic, sonar, and fathometer data, where appropriate.

The interpretation of identified magnetic anomalies will rely on expectations of thecharacter (i.e. signature) of shipwreck magnetics derived from the available literature.The potential for each anomaly cluster to contribute to archeological or historicalknowledge will be assessed. Thus, the Contractor will classify each anomaly as eithereligible for inclusion in the National Register, or not eligible. The Contractor shall fullysupport his recommendations regarding site significance. The report will include asummary table listing all anomalies, the assessment of potential significance, andrecommendations for further work.

If determined necessary by the COR, the final report will not include detailed site locationdescriptions, state plane or UTM coordinates. The decision on whether to remove suchdata from the final report will be based upon the results of the survey. If removed fromthe final report, such data will be provided in a separate appendix. The analyses will befully documented. Methodologies and assumptions employed will be explained andjustified. Inferential statements and conclusions will be supported by statistics wherepossible. Additional requirements for the draft report are contained in Section 6 of thisScope of Services.

6. Reports:Management Summary Two copies of a brief management summary will be submittedto the COR within 6 weeks after delivery order award.

Draft and Final Reports (Phase 1-3). Eight copies of the draft report integrating allphases of this investigation will be submitted to the COR for review and comment within9 weeks after work item award. As an appendix to the draft report, the Contractor shall

submit the state site forms. The written report shall follow the format set forth in MIL-STD-847A with the following exceptions:

(1) separate, soft, durable, wrap-around covers will be used instead of self covers;(2) page size shall be 8-1/2 x 11 inches with 1-inch margins;(3) the reference format of American Antiquity will be used. Spelling shall be inaccordance with the U.S. Government Printing Office Style Manual dated January1973.

The COR will provide all review comments to the Contractor within 6 weeks after receiptof the draft reports (15 weeks after work item award). Upon receipt of the reviewcomments on the draft report, the Contractor shall incorporate or resolve all commentsand submit one preliminary copy of the final report to the COR within 3 weeks (18 weeksafter work item award). Upon approval of the preliminary final report by the COR, theContractor will submit 30 copies and one reproducible master copy of the final report tothe COR within 22 weeks after work item award. The Contractor will also providecomputer disk(s) of the text of the final report in Microsoft Word or other formatacceptable to the COR.

7. Weather Contingencies. The potential for weather-related delays during theinvestigation necessitates provision of weather contingency days in the delivery order.Two weather contingency days have been added to the fieldwork. The Contractorassumes the risk for any additional costs associated with weather delays in excess of twodays. If the Contractor experiences unusual weather conditions, he will be allowedadditional time on the delivery schedule but no cost adjustment.