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GEOLOGY OF THE FALL ZONE IN DELAWARE

By

NENAD SPOLJARIC

Geologist, Delaware Geological Survey

March, 1972

CONTENTS

Page

ABSTRACT . • • • 1

INTRODUCTION • • 2

ACKNOWLEDGMENTS. • • • 2

DEFINITION OF FALL ZONE. • 2

GENERAL GEOLOGY. • • 3

Piedmont Province · • 3

Coastal Plain Province. • • • • • 3

GEOMORPHOLOGY. • • • • • 6

Introduction. • • • • • 6

Piedmont Province · • • • • 6

Coastal Plain Province. • 7

METHODS OF STUDY · 7

INTERPRETATION OF SUBSURFACE GEOLOGY · 8

Crystalline Basement Complex. • • • 8

Iron and Chestnut Hill Problem. • • • 11

Weathered Basement Complex. 12

Potomac Formation · • • • 13

Columbia Formation. 17

Younger Quaternary Sediments. • 21

CONCLUSIONS. • • • • 24

Geologic History. • • 24

Applied Geology • • • 27

Future Prospects. • 27

REFERENCES CITED · 28

ILLUSTRATIONS

Page

Plate 1. Well-developed cross bedding inalluvial fan sediments. · · · · · · · · 22

2. Erosional unconformity betweenalluvial fan deposits and theColumbia Formation. · · · · · · · · · · 25

Figure 1. Area of study showing well locations. · 4

2. Surface of the crystalline basementcomplex . · · · • · · · · • · · · · 9

3. Cumulative curves of the PotomacFormation · · · · · · · · · · · · · · · 14

4. Lithofacies distribution of thePotomac Formation · · · · · • · · · 16

5. Cumulative curves of the ColumbiaFormation · · · · · · · · · · · · · · · 18

6. Lithofacies distribution of theQuaternary deposits · · · · · · · · · · 20

7. Cumulative curves of the alluvialfan sediments · · · • · · · · · · · • · 23

GEOLOGY OF THE FALL ZONE IN DELAWARE

ABSTRACT

The complex geologic framework of the Fall Zone inDelaware is primarily caused by diverse structuralfeatures present in the crystalline basement rocks thathave exerted a considerable influence on the distribu­tion of the overlying sediments of the Coastal Plain.

A lateral fault and three different sets of verticalfaults were recognized in the crystalline basement complex.The vertical faults have broken the surface of the base­ment complex into horst-graben structures. Although thesefaults were probably formed before the deposition of thePotomac Formation (Lower Cretaceous), there are indicationsthat they are still active.

The deposits of the Potomac Formation were broughtinto the area through the ancient Delaware River Valleyand through the Piedmont. Fine sediments, clay and silt,are predominant. Sands are generally confined to themain distributary valleys. The areal distribution andthickness of the Potomac Formation were greatly influencedby the structures of the underlying crystalline basementcomplex. A considerable amount of these sediments waseroded from the area before the overlying Pleistocenedeposits were laid down.

The Pleistocene Columbia Formation was alsodeposited by a complex distributary stream system.Most of these materials were brought into the areathrough the ancient Delaware River Valley, althoughconsiderable amounts were supplied directly through theadjacent Piedmont. At the close of the Pleistocene (?)extensive flooding occurred producing a set of alluvialfans at the southern margin of the Piedmont. Most of thematerial composing these fans seems to have been suppliedby nearby sources.

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The results of this study show that there are noadditional major sources of ground water in the CoastalPlain sediments in the area. Because of this it issuggested that future investigations be directed towardthe faults in the crystalline basement complex thatmay contain useful quantities of ground water. Nevertheless,it is pointed out that the mere presence of the faultsdoes not necessarily mean that economical amounts ofground water are available.

INTRODUCTION

The area between Wilmington and Newark is amongthe fastest growing portions of the State. This growthnecessitates the accelerated development of mineralresources, particularly water, to accommodate the needsof the thriving community. To secure such resources itis important to study the geologic framework of thisarea including the distribution and physical andwater-bearing characteristics of the rocks present bothat the surface and in the subsurface.

Therefore! the purpose of the present study is toinvestigate the geologic makeup of the zone betweenWilmington and Newark from the standpoint of itscapability to sustain the present rate of populationand economic growth.

ACKNOWLEDGMENTS

Discussions of various topics related to this studyheld with the staff of the Delaware Geological Survey,particularly Dr. R. R. Jordan, State Geologist, andMr. J. C. Miller, Hydrogeologist, are gratefullyacknowledged. This study was partly supported by theWater Resources Center, University of Delaware, Dr. R. D.Varrin, Director.

DEFINITION OF FALL ZONE

The term Fall Zone was adopted by Davis (1904) andwas applied to the sharp junction between the Appala­chian Piedmont Province and the Atlantic Coastal PlainProvince. This zone is marked by waterfalls and rapidson most streams crossing it. It has been suggestedthat this zone delimits the trace of an ancient

2

peneplain (Sharp, 1929); a pre-Cretaceous erosionalsurface now covered by the Coastal Plain sediments. Itwas also speculated that this peneplain left traces onthe Appalachians. Such speculations have been ques­tioned by Flint (1963).

GENERAL GEOLOGY

Piedmont Province

In the study area (Figure 1) this province iscomposed of crystalline rocks. The oldest rocksexposed belong to the Glenarm Series (Knopf and Jonas,1922) and they are represented by micaceous schists,gneisses, and migmatites of the Wissahickon Formation.Amphibolites and hornblende gneisses, which are indirect contact with the Wissahickon. Formation, are apart of the Wilmington Complex and were studied indetail by Ward (1959). The Port Deposit Granodiorite,a quite extensive unit in nearby Maryland, occupies asmall area west of Newark.

Most of the crystalline rocks of. the PiedmontProvince were severely metamorphosed during thePaleozoic orogenies. For this reason their agerelationships are obscured. The age determinations onsimilar rocks in Maryland suggest a late Precambrian(?)or early Paleozic age for the Wissahickon Formation(Wetherill et al., 1966).

Coastal Plain Province

The Coastal Plain sediments in the study area(Figure 1) are represented by the Early CretaceousPotomac Formation and the Quaternary sediments of theColumbia Formation and younger, unnamed deposits.

The Potomac Formation was studied in detail byGroot (1955) who was able to subdivide it into astaurolite-zircon-tourmaline-kyanite zone and zircon­rutile zone. Only the lower, staurolite-zircon­tourmaline-kyanite zone, is present in the area of study.

Jordan (1968) showed that the lower part of thePotomac Formation is sandier than its upper part. Thissandier part of the Potomac was singled out by Sundstromet al. (1967) as the lower hydrologic zone; it is a goodaquifer.

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Spoljaric (1967a) made a detailed study of thesesediments in a small area west of Delaware City andconcluded: "The deposition of the Potomac sedimentsappears to have been continuous throughout the time oftheir formation ••• The geometry of the sand bodiesresembles the shoestring channel deposits formed byunidirectional currents ••• " Although geologistsagree that the Potomac sediments were deposited by acomplex stream system, their stratigraphic subdivisionis still a subject of controversy (Groot, 1955; Grootand Penny, 1960).

The age of the Potomac Formation was determinedprimarily on the basis of paleobotanical evidences.Berry (1911), Dorf (1952), Groot and Penny (1960),Groot, Penny, and Groot (1961), Brenner (1963), andDoyle (1969) all seem to agree that this formation isnot older than the Barremian and not younger than theTuronian.

The Potomac sediments are unconformably overlainby the Quaternary deposits. The statewide study of theQuaternary Columbia Formation by Jordan (1964) revealedthat the streams which deposited these materialsentered the area from the north, mainly through thepresent Delaware River Valley. In the northern andcentral parts of Delaware these sediments were laiddown in fluvial environments. In the southern partthey form a shoreline-lagoonal complex.

Investigation of the Columbia Formation in northernDelaware showed that the Pleistocene streams formed acomplex braided system in the area south of the Chesa­peake and Delaware Canal while north of the Canal thestreams were confined to straight and meanderingchannels (Spoljaric, 1967b).

A detailed study carried out in the Middletown­Odessa area (Spoljaric, 1970; Spoljaric and Woodruff,1970) disclosed complex depositional processes operatingat the time of the Columbia deposition. Frequent changesin water and sediment discharges and flooding were quitecommon. Flooding was usually accompanied by the influxof coarse gravelly materials and changes in the positionof stream channels. The streams seem to have beenshallow and relatively wide, although they only rarelyexceeded half a mile in width.

The age of the Columbia Formation is a subject ofsome contention. Basically there are two opposing views:

5

one (Jordan, 1964) favors, at least, Sangamon-earlyWisconsin age for these sediments; the other (Spoljaric,1970), a younger, perhaps even late Wisconsin, age. Forthe details of the arguments set forth by these twoproponents the reader is referred to the correspondingpublications. However, it should be stressed that theevidence available at present is inadequate to corrob­orate either view.

Younger, unnamed, Quaternary sediments unconform­ably overlie the Columbia Formation in the area ofstudy. They have been discovered during the course ofthis study and are discussed in detail elsewhere inthis report.

GEOMORPHOLOGY

Introduction

The development of land forms in the study area wasmainly controlled by the underlying geologic structuresand rock types. However, recent advancements in tech­nology make it now possible for man to modify the landforms according to his needs. Although these artificialalterations of natural features of the land surface areat present relatively unimportant, they should not beignored. Nature reacts to such alterations by attempt­ing to reestablish the natural equilibrium which canprove to be disastrous to the people living in aparticular area. History has recorded many such disas­ters. The most common responses of the nature aremanifested as soil creep, sliding and slumping ofportions of the land surface, collapse of river dams,flooding, erosion, and many others.

Piedmont Province

The area underlain by the Wissahickon Formation ischaracterized by relatively steep slopes and deeplyincised stream valleys, gently rolling hills, and thegeneral absence of upland plains. Main streams (WhiteClay, Red Clay, Mill, and Pike creeks) flow southward,roughly perpendicular to the strike of the geologicstructures. Their tributary streams, however, floweither toward the southwest or northeast, followingthat strike. The stream valleys generally lack well­developed flood plains and there is little sediment

6

deposited in the stream channels. This is primarilydue to the relatively high stream gradients which favordegradation over aggradation.

The portion of the area of study occupied by theoutcropping Wilmington Complex is not very extensiveand land forms here are not significantly differentfrom those observed in the Wissahickon Formation.

Coastal Plain Province

Geomorphologic features of the Coastal Plain aredistinctly different from those in the Piedmont Province.The Coastal Plain portion of the study area is charac­terized by a low, flat land surface lacking distinct anddiverse features observed in the Piedmont. Thestream valleys are shallow and wide with well-developedflood plains. The surface elevations are low, rarelyexceeding 100 feet (SLD), and many streams draining intothe Delaware Bay are tidal for a considerable distanceinland. A typical example of such tidal streams is theChristina River.

Because of the low gradients, and because of thetidal influence, these streams contribute little sedi­ment to the Delaware Bay. Most of their load isdeposited shortly after they enter the Coastal Plainfrom the Piedmont, and only very fine sediment (siltand clay) remains in suspension. Nevertheless, most ofthese fine sediments settle out before reaching the Bay.

Peculiar geomorphologic features of the CoastalPlain portion of the study area are Iron and Chestnuthills. These hills rise to an elevation of more than300 feet and are composed of basic igneous rocks,predominantly gabbro. The relationship of these rocksboth to the Piedmont and the Coastal Plain is unclear.

METHODS OF STUDY

The descriptive and electric logs from 140 wellswere utilized in the study. Although the quality ofthe well logs varies somewhat, only those consideredreliable were used. In addition, subsurface and surfacesamples were investigated and several laboratoryanalyses of the samples performed.

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The parameters of the lithofacies maps were com­puted on the basis of both electric and descriptivelogs. Even though the descriptive logs are notordinarily employed in the computation of such param­eters, the limited availability of electric logs madethis necessary.

The density of the control points on the maps· isvariable and ranges from high in the Newark area toscarce in the central and eastern parts of the studyarea. However, because the area is so small, the biasin the distribution of the control points is not con­sidered very significant.

INTERPRETATION OF SUBSURFACE GEOLOGY

Crystalline Basement Complex

The crystalline rocks of the Piedmont Provinceextend southward under the Coastal Plain and form thebasement upon which the younger sedimentary rocks weredeposited.

Not much is known about the composition of thisbas~ment complex. The rocks composing this complex aredense, hard, and were thought to be of little impor­tance as a possible source of ground water; as a resultthey were not drilled. In addition, drilling throughsuch rocks requires specialized equipment that isusually not readily available.

The surface of the basement complex was believedto be continuous although the existence of faults hadbeen suspected by Groot and Rasmussen (1954). Thepresent study has shown that this surface is indeedbroken by faults into small horst-graben structures.

The main structural feature of the basement is theright-lateral fault (Figure 2) running from the northernpart of Newark toward the east-northeast approximatelythrough the central portion of the study area. Althoughthe displacement along this fault is unknown, the con­figuration and smooth transition of the structuralcontours across the fault line suggest that the movementof several hundred yards could account for the relation­ships on both sides of the fault.

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The area north of this lateral fault is charac­terized by a set of roughly parallel faults runningnorth-northwest to south-southeast. They haveproduced a small horst, northeast of Newark, with avertical displacement of about 40-50 feet along itswestern boundary, and 10-30 feet along its easternboundary. The area east of this horst is broken by twostep faults with the vertical displacement of about 10and 20 feet respectively. Most of these parallel faultsextend into the Piedmont and appear on the aerial photo­graphs as lineations (J. C. Miller, personal communica­tion).

In the area south of the lateral fault threedistinct structural trends, manifested by the faultsets, are observed: east-west, northwest-southeast, andnorth-south.

The east-west running fault is located in the south­central part of the area. The maximum vertical displace­ment of its southern, downwarped block (graben) exceeds60 feet. The vertical displacement along the faultsthat bound this graben to the east and west is about 130feet and 70 feet respectively.

A set of northwest-southeast trending faults isroughly parallel to the faults observed in the northernpart of the study area. In the Newark area thenorthwest-southeast trending faults surround a structuraldepression, a graben, with vertical displacements rangingfrom about 40 feet to more than 100 feet. The relation­ship of the fault along the southwestern boundary ofthis graben to Chestnut and Iron hills is somewhatobscured by a thick cover of the weathered material thatslumped and slid down the sides of these hills. However,the consistent trend of this fault with that of theother northwest-southeast faults suggests that its originwas independent of the crystalline rocks of Chestnut andIron hills.

The north-south structural trend is represented bythe vertical fault located south of Newark. The dis­placement along this fault locally exceeds 100 feet.Structurally this is the most complex portion of thestudy area because' all three structural trends intersecthere. This complexity is well displayed by a perplexedconfiguration of the structural contours. Apparently aconsiderable fracturing and crushing of the crystallinerocks occurred here thus intensifying indirectly theweathering, which has resulted in more than 100 feetof weathered material.

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The age of the structures in the study area isunknown. However, the fact that the faults in thebasement complex controlled to a considerable extentthe thickness and lithologic distribution of the over­lying Potomac Formation suggests that they may haveoriginated a relatively short time before thedeposition of these sediments (see discussion on thePotomac Formation, p. 13). The mutual relationship ofthe fault sets shows that they did not form at the sametime. It can be further concluded that in the areasouth of the lateral fault, the oldest structural trendis north-south. In the northern portion of the area,the vertical faulting was subsequent to the lateralmovement. Although it is impossible to determine whichstructural set is the oldest, there is little doubt thatthe northwest-southeast trend is the youngest.

Are the movements along these faults still active?According to the Earthquake Information Bulletin (1971)the only earthquake to center in Delaware occurred onthe 9th of October, 1871; its intensity on the Richterscale was 5 and it caused considerable damage inWilmington. Other nearby towns also sustained someinjury: New Castle and Newport in Delaware, and Oxfordin Pennsylvania. In other areas the earthquake wasaccompanied by various noises described as rumbling andexplosions. Thus it seems that the faults are indeedstill active, albeit the vigor and intensity of thisactivity is very low. Small tremors which have beenreported from time to time in recent years seem tosupport this.

Iron and Chestnut Hill Problem

The rocks composing these hills are mainly basicintrusives: gabbro, norite, and pyroxenite (Scott,Stafford, and Woodruff, personal communication), formedby direct crystallization from molten magma in thesubsurface.

A magnetic map (Henderson et al., 1963) and amagnetic survey done recently in this area show thatthese igneous rocks do not extend into the subsurfacebasement complex (R. E. Sheridan, University of Delaware,personal communication); thus, both Iron and Chestnuthills may be considered rootless, and therefore theirorigin does not seem to be related to the nearby Piedmont.It is hypothesized that these rock masses were broughtinto the present locations by gravity gliding from the

11

south (Sheridan, personal communication). The magneticmap (Henderson et a1., 1963) suggests the existence ofsimilar blocks in the subsurface south and southeast ofIron Hill.

If this hypothesis is correct, then the glidingmust have occurred sometime between the metamorphism ofthe basement rocks and the deposition of the PotomacFormation in the Early Cretaceous.

Although this is an interesting and intriguinginterpretation of the origin of Iron and Chestnut hills,there is no proof to substantiate it. Nevertheless,the evidence available at present warrants a seriousconsideration of this hypothesis.

Weathered Basement Complex

Crystalline rocks in the study area are almosteverywhere overlain by their weathered products. Thesematerials are composed of tight, varicolored clays andvery clayey, poorly sorted silts and sands, and locallyexceed 100 feet in thickness. Fragments and bouldersof partly decomposed original rocks are frequentlyencountered.

The thickness of the weathered material varies fromplace to place and this variability if probably due tothe original rock type, geochemical conditions ofweathering, tectonic fracturing and crushing, and,possibly, to slumping of such materials on steep slopes,such as along the northern side of Chestnut and Ironhills, for example. For all these reasons no consistentrelationship between the location of the faults and thethickness of the weathered material has been observed.The only exception to this was discussed on p. 10.

The age of the weathered material is unknown.Although no data are presently available to solve thisproblem, the following hypotheses are proposed: thematerial formed before, contemporaneously with, orafter the deposition of the Coastal Plain sediments.

The first hypothesis seems most likely because asufficient time was available (Early Paleozoic-EarlyCretaceous) for the weathering of the crystalline rocksto proceed to depths sometimes exceeding 100 feet.The deposition of the fluvial Potomac sediments inEarly Cretaceous time may have been accompanied, inearly stages, by erosion of the underlying weathered

12

material. However, this erosion may not have beeneffective because the Coastal Plain area was con­tinuously subsiding at that time, thus promotingdeposition rather than degradation. Therefore, most ofthe original weathered material probably would havebeen preserved and covered by the Potomac sediments.

The second hypothesis could have been important inthe early stages of the Potomac deposition allowingdirect influx of rainwater into the basement rocksthrough a thin veneer of the Potomac sediments thuspromoting the breakdown of the unstable components inthe crystalline rocks. It is, however, difficult tovisualize how such a process could have produced morethan 100 feet of the weathered material in a relativelyshort period of time.

The third hypothesis can account for a portion ofthe weathered material, but it seem$ very unlikely thatthe subsurface weathering could have been so effectivesince the commencement of the Potomac deposition toproduce such a large volume of the weathered material,particularly because the crystalline rocks are quitehard, dense, and do not allow for an easy penetrationof ground water. However, the subsurface weatheringcan be quite intensive in the presence of hydrothermalsolutions, as shown by Eroshchev-Shak (1970}1 but thereis no known evidence that such solutions were presentin the study area.

So the problem of the age and origin of theweathered material in the area remains. The hypothesesset forth above, perhaps, suggest the possible avenuesof future investigations.

Potomac Formation

The composition of the Potomac sediments in thestudy area is dominated by silts and varicolored clays.The sands are fine- to medium-grained, cross-bedded,often poorly sorted, gray to light yellow, and contain,on the average, about 5 percent clayey and silty matrix(Figure 3). They are concentrated in the southern andwestern parts of the area and in places reach more than100 feet in thickness. However, in such areas the sandsequence is not continuous but, rather, it is composedof sand beds separated by thin clay layers.

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GRAIN SIZE (in I' units)

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Figure 3. Cumulative curves of the Potomac Formation.Sample no. 23023 taken from the hole Db12-42at the depth of -90 feet (SLD). Sample no.23021 taken from the hole Db12-42 at thedepth of -80 feet (SLD).

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The form and areal distribution of the sand bodies(Figure 4) suggest that the streams that transportedand deposited these materials entered the area from thenorth, northwest, and east. Main flow directions ofthese ancient streams were inferred on the basis of thedip of the underlying structures in the crystallinebasement complex, and also from the lithofacies distri­butions in the Potomac sediments. The multitude ofdirections clearly shows that these sediments weredeposited by a complex stream system.

A detailed study of the Potomac Formation atDelaware City (Spoljaric, 1967a) has revealed that thedeposition of these sediments was initially stronglycontrolled by the configuration of the crystallinebasement surface; this is also evident in the presentstudy area. For example, the depression located in thenorthern portion of Newark acted as a catchment for athick accumulation of the Potomac clays. The influenceof the basement surface is also apparent in thesouthern part of the area along the east-west trendingfault; the Potomac thickness on the downdropped side ofthe fault is more than 130 feet larger than on the up­lifted, northern side. Similar relationships aredisplayed to various degrees along the other structuralfeatures in the area.

Slumping of the weathered material may have playedan important role on the distribution of the Potomacsedimentary bodies in some portions of the area. Directevidence of such slumping was observed on the north­eastern sides of Iron and Chestnut hills. In addition,hole Db21-35, drilled just east of these hills, hasrevealed possible intermittent layering of the weatheredmaterial and the Potomac sediments that could have beenproduced by recurrent slumping of the weathered materialinto the depositional environments of the Potomac sedi­ments. An unusually thick sequence of such material inthe synclinal depression in the east-central part ofNewark may have been formed by slumping from theelevated sides of the depression.

The Potomac sediments are separated from the over­lying Quaternary deposits by a major erosional uncon­formity (Early Cretaceous-Pleistocene) in the studyarea. The surface of the Potomac sequence in theCoastal Plain portion of the area does not exceed 100feet above sea level. However, in the hole Da24-3located on Chestnut Hill at the elevation of 270 feet(SLD) the Potomac sediments were found at this same

15

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elevation; the total thickness of these sediments is91 feet. This "anomalous" elevation is a strong indica­tion that a considerable amount of the Potomac depositswas eroded and removed from this area before the deposi­tion of the Quaternary units. Similarly, intensiveerosion preceding the deposition of the QuaternaryColumbia Formation was recognized in the Middletown­Odessa area (Spoljaric, 1971) where a large amount ofthe underlying greensands of the Rancocas Group wasremoved.

This reduction in the Potomac thickness posesseveral questions. First, what was the original north­ward extent of these sediments, and second, where isthe material that was eroded and removed from this area?

Unfortunately, there is no evidence to answer thefirst question. It seems, however, that the PotomacFormation must have occupied at least the southernmostpart of the Piedmont, but no remnants of these depositsare known from there.

There is some evidence to answer, at least partially,the second question. Recent study of the Magothy Forma­tion in the area west of Delaware City (Spoljaric, inpress) has shown that this unit is composed mainly of thereworked Potomac sediments. The Magothy sediments weredeposited in a small delta at the margin of a northwardtransgressing sea and here they directly overlie theolder Potomac units. However, this formation is rarelymore than 50 or 60 feet thick and thus, probably, cannotaccount for all of the eroded Potomac. The location of theremaining portion of the Potomac sediments is unknown.One may speculate that it was laid down farther south inthe Coastal Plain area, possibly as the younger Potomacdeposits.

Columbia Formation

In the present study area this formation is composedof gravels, sands, silts, and some clays. The sands aremoderately to poorly sorted, cross-bedded, yellow tobrownish-yellow in color, and contain on the averageabout five percent clayey and silty matrix (Figure 5).Fine sediments, silts, are locally abundant and gravelsare subordinate.

The Columbia sediments were deposited in streamchannels, flood plains, and associated environments. A

17

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Figure 5. Cumulative curves of the Columbia Formation.Sample no. 37524 taken from the hole Dbll-5lat the depth of 20-25 feet. Sample no. 37526taken from the hole Dbll-5l at the depth of30-35 feet.

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detailed study of these deposits in the Middletown-Odessaarea (Spoljaric, 1970; Spoljaric and Woodruff, 1970)revealed that the ancient streams that deposited thesematerials were shallow, usually less than half a milewide, and frequently shifted their courses within abraided stream system. It was suggested (Spoljaric,1970) that the great lithologic variability of theColumbia sediments is indicative of fluctuating waterand sediment discharges of the Pleistocene streamscaused by short term climatic changes (seasonal, forexample) and is not related to the Pleistocene glacialand interglacial phases.

In the present study area, however, the Pleistocenestream system seems to have been confined to straightvalleys, as suggested by Spoljaric (1967). This isconfirmed by the geometry of the sedimentary bodies; forexample, the sand percent distribution shows that thesand bodies have a shoestring form characteristic ofsuch valleys.

The lithofacies map (Figure 6) of the Quaternarydeposits indicates that these ancient streams enteredthe area from the north and northeast. This is ingeneral agreement with Jordan's conclu~ion (1964).Although there is little doubt that the main streamsystem was located in the ancient Delaware River Valley,a considerable amount of these materials were broughtinto the Coastal Plain also directly through thePiedmont. However, the evidence for this is somewhatobscured by the overlying younger Quaternary sediments.

The areal distribution of the Columbia lithologiesseems to have been controlled by the topography andcomposition of the underlying Potomac surface. Thevalleys in this surface were developed in the areascomposed of fine Potomac deposits, such as clays andsilts. For this reason these areas were the locationof main Pleistocene stream system and the transportroutes of the overlying Columbia sands. Thus, theColumbia sands are usually directly underlain by thePotomac fine sediments, although there are a few exceptionsto this, for example, the Newark area and the southeasternportion of the study area.

A similar relationship has been also observed betweenthe Pleistocene surface and the locations of the recentstreams elsewhere in New Castle County (Spoljaric, 1967).

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20

Younger Quaternary Sediments

Recent construction work at the University ofDelaware and nearby areas has revealed the existence ofthe Quaternary deposits younger than the Columbia Forma­tion. Reinspection of the subsurface samples anddescriptive logs obtained by drilling shows that thesesediments are indeed widespread in the study area; theyare mostly brown to yellow-brown sands, poorly sorted,cross-bedded (Plate 1), coarse to fine, contain largemica flakes, and on the average have about two percentclay matrix (Figure 7). The sand grains are angular andof poor sphericity. The heavy mineral suite is dominatedby magnetite (P. B. Leavens, University of Delaware,personal communication) which comprises more than 45percent of the assemblage.

The southward extent of these sediments is tenta­tively delineated on the lithofacies map (Figure 6); theprecise boundary is unknown. The geometry and the formof the sedimentary bodies, and their relationship to thestream valleys of the Piedmont are strong indicationsthat these sediments were deposited as alluvial fans.Alluvial fans generally form in the areas where heavilysediment-loaded streams emerge from mountains onto alowland and are accompanied by a marked change ingradient. "The building of fans takes place largelyduring flood times, when great volumes of water withaccompanying alluvium debouch onto them. During much ofthe time the stream channels across the fans are dry, or,if permanent streams occupy the channels, much of theirwaters is likely to sink into coarse alluvium near theapices of the fans" (Thornbury, 1969, p. 173).

Although streams heavily sediment-loaded do notexist in the Piedmont at present, the presence ofalluvial fans along the Fall Zone in the area suggeststhat such conditions prevailed in the streams in thenot too distant past.

The fresh appearance of these sediments, their texture,and their composition imply that they were derived from anearby source in the piedmont. However, the occurrence ofrelatively large amounts of magnetite in these depositsdoes not support this conclusion, because only traces ofthis mineral are present in the metamorphic rocks of thePiedmont. There are several possible explanations offeredhere to define this inconsistency.

Large concentration of this mineral may have beenthe result of the drop of stream gradients upon entering

21

Plate 1. A well-developed cross-bedding in alluvialfan sediments (Smith Hall, University ofDelaware).

22 I. I

III

I:

'1111 111111111111'

I lilli/III 111111111111

lil

III

I:

I

III

III

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..I:

....

..I:

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-2 -1 o +1 +2 +3GRAIN SIZE (in _ unita)

+4

Figure 7. Cumulative curves of the alluvial fansediments. Sample no. 1 taken at the founda­tion of Smith Hall, University of Delaware.Sample no. 2 collected at Polly Drummond HillRoad, about 1/2 mile south of the contactbetween the Piedmont rocks and the sedimentsof the Coastal Plain.

23

the Coastal Plain7 magnetite is dense and it could havebeen deposited with the coarse sediments. Although thisis a possibility, it is somewhat contradicted by theresults of a study carried out on the Columbia Formation(Spoljaric, 1970). These results show that the heavyminerals found in the Columbia Formation were transportedwith fine suspended sediments, and not with the coarsematerials. The fact that there is hardly any silt andclay in the alluvial fan deposits indicates that thesuspended load went completely through the system andwas deposited elsewhere.

The second possibility is that the magnetite wasassociated with serpentine bodies, only remnants ofwhich are still present in the Piedmont. Magnetite is acommon alteration product of minerals containing ferrousoxide, as veins in serpentine, that have resulted fromthe alteration of chrysolite.

The spreading of these sediments southward, awayfrom the Piedmont, and the locations of the apices ofthe fans suggest that their origin is related to thepresent stream valleys of the Piedmont. This, inaddition to the fact that they unconformably overliethe Columbia Formation in the study area (Plate 2),shows clearly that they are here post-Columbia in age.However, the deposition of the alluvial fans may haveoccurred during the last extensive Pleistocene floodingwhen a large volume of the sediments was brought intothis area by the Piedmont streams. This flooding mayhave had a regional significance7 an episode of floodingof a similar magnitude was detected farther south, inthe vicinity of Middletown and Odessa (Spoljaric, 19707Spoljaric and Woodruff, 1970). If the flooding in bothareas was indeed the result of the same event, then thealluvial fan sediments, in the present study area, andthe Columbia flood deposits in the Middletown-Odessaarea, were laid down contemporaneously.

CONCLUSIONS

Geologic History

The origin and composition of the crystalline base­ment complex is very poorly known. This complex probablyformed during the late Precambrian - early Paleozoic andthe original rocks were later, during the Paleozoic,severely metamorphosed. The metamorphism did not only

24

Plate 2. Erosional unconformity between the alluvialfan deposits and the Columbia Formation.About 1/2 mile north of the intersectionbetween Harmony Road and Chestnut Hill Road.

25

involve the recrystallization of the minerals composingthese rocks but was also accompanied by intensivefolding and faulting thus greatly obliterating themutual relationships among the rock units of thiscomplex.

The events that occurred between the main phasesof metamorphism and the commencement of the Potomacdeposition in the Early Cretaceous are an enigma.Iron and Chestnut hills must have been formed sometimein that period. Three phases of faulting must havealso occurred during that time. In the Early Cretaceousa complex stream system developed in the' area. Thelocations of the streams seem to have been stronglycontrolled by the topography of the underlying crystal­line basement surface. The sediments deposited duringthe Early Cretaceous reached considerable thicknessesleaving Iron and Chestnut hills as small protrusions inthe Potomac surface. The deposition of the PotomacFormation appears to have been continuous suggestingthat the area was persistently subsiding.

The events that took place between the end of thePotomac deposition and the beginning of the Pleistocenesedimentation (Columbia Formation) are another puzzle.It seems, however, that the subsidence of the areaended and erosion of the Potomac sediments from thisarea started. A portion of the materials removed fromhere was deposited farther south, in a small delta atthe margin of the northward transgressing sea, formingthe Magothy Formation. What happened to the remainderof the eroded Potomac is unknown. The degradation musthave reached its maximum in Pleistocene time whenthe deposition of the Columbia Formation started.

During the Pleistocene, another complex streamsystem developed. The main streams seem to have enteredthe area from the direction of the Delaware River Valley,although a considerable quantity of the Columbia depositswas also brought through the Piedmont. Close to the endof the Pleistocene sedimentation a surge of heavilysediment-loaded flood waters entered the area of studythrough the Piedmont and produced a set of alluvial fansat the south margin of the Piedmont.

Although the description of the geologic history ofthe study area presented here is very sketchy, it,nevertheless, shows that the area has gone through asequence of intricate geologic processes. The complex

26

geologic framework is primarily caused by diversestructural features present in the crystalline basementrocks that have exerted a considerable influence on thedistribution of the overlying, younger, sediments.

Applied Geology

The main sources of ground water in the study areaare the sands of the Potomac Formation and Quaternarydeposits. To be economically suitable as an aquifer,sands must have good permeability and porosity, and asufficient saturated thickness. The sands of both thePotomac and the Quaternary generally have these qualities,but only in certain parts of the study area. Unfortu­nately, most such areas are already utilized as waterwellfields: for example, the City of Newark fields.Although the sand units are also present in other partsof the study area, their saturated thicknesses are usuallytoo small for the development of economical supplies ofground water.

A recent study carried out by the Delaware GeologicalSurvey (1972) in a small portion of the Piedmont hasrevealed that ground water in considerable quantities maybe present along fauits and fractures in the crystallinerocks. However, it should be pointed out that quantitiesof water in such rocks are unpredictable and the merepresence of faults and fractures does not necessarilymean that a sufficient amount of ground water isavailable.

Other mineral resources, such as sand, gravel, andcrushed rock, used in construction, are available in thestudy area but are not exploited here.

Future Prospects

Based on the estimated population increase projec­tions for the study area, the demand for ground waterwill reach the available supply in about fifteen years.As the present study showed, there are no hidden sourcespresent in the Potomac and Quaternary sediments in thearea.

Future investigations for ground water may bedirected toward the crystalline rocks both in thePiedmont and in the Coastal Plain basement complex inthe subsurface. Before any drilling into the faults in

27

the crystalline basement complex is attempted, it isnecessary to determine accurate locations and inclina­tions of the fault planes. For this reason the areawould have to be geophysica11y surveyed, preferably byseismic methods. The weathered material, which coversthe crystalline basement rocks almost everywhere in thearea, could pose some serious problems to a possibleexploitation of ground water from the faults. Thismaterial is usually impermeable, due to its clayey com­position, and may impede the recharge of ground water fromthe Coastal Plain sediments above into the fractured zonesof the crystalline basement complex below.

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Berry, E. W., 1911, The flora of the Raritan Formation:New Jersey Geo1. Survey Bull. 3, 233 p.

Brenner, G. J., 1963, The spores and pollen of thePotomac Group of Maryland: Maryland Dept. of Geo1.,Mines, and Water Resources Bull. 27, 215 p.

Davis, W. M., 1904, Elements of Physical Geography:Boston, Ginn, 401 p.

Delaware Geological Survey, 1972, Ground-water resourcesof the Newark area: Delaware Geo1. Survey Rpt.Invest. No. 18, in preparation.

Dorf, E., 1952, Critical analysis of Cretaceous stratig­raphy and paleobotany of Atlantic Coastal Plain:Am. Assoc. Petroleum Geologists Bull., v. 36,p. 2161-2184.

Doyle, J. A., 1969, Cretaceous angiosperm pollen of theAtlantic Coastal Plain and its evolutionary signif­icance: Jour. Arnold Arboretum, v. 50, p. 1-35.

NOAA, 1971, Earthquake history of Delaware: EarthquakeInformation Bull., v. 3, p. 28-29.

Eroshchev-Shak, V. A., 1970, Mixed-layer biotite-chloriteformed in the course of local epigenesis in theweathered ,crust of a biotite-gneiss: Sedimentology,v. 15, p. 115-121.

28

Flint, R. F., 1963, Altitude, lithology, and the FallZone in Connecticut: Jour. Geol., v. 71, p. 683­697.

Groot, J. J., 1955, Sedimentary petrology of theCretaceous sediments of northern Delaware in rela­tion to paleogeographic problems: Delaware Geol.Survey Bull. 5, 157 p.

Groot, J. J., and Penny, J. S., 1960, Plant microfossilsand age of nonmarine Cretaceous sediments ofMaryland and Delaware: Micropaleontology, v. 6,p. 225-236.

Groot, J. J., Penny, J. S., and Groot, C. R., 1961,Plant microfossils and age of the Raritan,Tuscaloosa, and Magothy Formations of the easternUnited states: paleontographica, B, v. 108,p. 121-140.

Groot, J. J., and Rasmussen, W. C., 1954, Geology andground-water resources of the Newark area,Delaware: Delaware Geol. Survey Bull. 2, 133 p.

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, 1968, Observations on the distribution of------s~a-n-d~s--w-l~ithin the Potomac Formation of northern

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Knopf, E. B., and Jonas, A. I., 1922, The Glenarm Series:Geol. Soc. America Bull., v. 33, p. 110.

Sharp, H. S., 1929, The Fall Zone peneplane: Science,v. 69, p. 544-545.

Spoljaric, Nenad, 1967a, Quantitative lithofacies analysisof the Potomac Formation, Delaware: Delaware Geol.Survey Rept. Invest. No. 12, p. 26.

, 1967b, Pleistocene channels of New----~C~a-s~t~l~e--C~o--un~ty, Delaware: Delaware Geol. Survey

Rept. Invest. No. 10, p. 15.

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Spo1jaric, Nenad, 1970, Pleistocene sedimentology:Middletown-Odessa area: Ph.D. thesis, Bryn MawrCollege, 193 p.

, 1971, Origin of colors and ironstone----~b-a-n-d~s--~-·n--~t~he Columbia Formation, Delaware: South-

eastern Geology, v. 12, no. 4, p. 253-266.

, in press, Upper Cretaceous marine----~t~r~a~n~s~g~r~e~s~s~ion in northern Delaware: Southeastern

Geology.

Spo1jaric, Nenad, and Woodruff, K. D., 1970, Geology,hydrology, and geophysics of Columbia sediments inthe Middletown-Odessa area, Delaware: DelawareGeo1. Survey Bull. 13, 156 p.

Sundstrom, R. W. and others, 1967, The availability ofground water from the Potomac Formation in theChesapeake and Delaware Canal area, Delaware:Univ. of Delaware Water Resources Center, 95 p.

Thornbury, W. D., 1969, principles of geomorphology:John Wiley and Sons, Inc., New York, 594 p.

Ward, R. F., 1959, Petrology and metamorphism of theWilmington Complex, Delaware, pennsylvania, andMaryland: Geo1. Soc. America Bull., v. 70,p. 1425-1458.

Wetherill, G. W., Tilton, G. R., Davis, G. L., Hart, S. R.,and Hopson, C. A., 1966, Age measurements in theMaryland Piedmont: Jour. Geoph. Research, v. 71,no. 8, p. 2139-2155.

30