2. site 76

2. SITE 76

The Shipboard Scientific Party1

MAIN RESULTS

Two holes at this site continuously cored 27 meters oflower Pliocene to Recent phillipsitic clay and cal-careous nannofossil ooze interbedded with calcareousturbidites. The fossils (calcareous nannofossils andforaminifera) contained in these turbidites range in agefrom Eocene to Pleistocene. The drill bit was stoppedby a silicified calcareous turbidite of Early Plioceneage. The average accumulation rate at this site wasbetween 6 and 7 m/m.y. Since no biogenic silica ispresent in the sediments recovered, this site deservesfurther study as a possible example of non-biogenicchert formations.

INTRODUCTION

Background and Objectives

Site 76 (Figure 1) was selected by the Chief Scientistof Leg 9 after discussions with the scientific staffs ofLegs 8 and 9. The objectives of this site were to core athick sequence of sediments north of the Tuamoturidge that had been crossed by Glomar Challenger justprior to the termination of Leg 8 in Tahiti. Thissedimentary sequence is acoustically highly stratifiedand it was assumed that the stratification is caused byinterbedding of turbidites from the Tuamotus withpelagites. Coring such a sequence, it was thought,might accomplish the following: 1) Allow a compari-son between the shallow water benthonic species foundin the turbidites and the planktonic species found inthe pelagites. This would be important because itwould permit accurate correlations between the highlyrefined planktonic stratigraphy and the less well datedbenthonic stratigraphy used on the islands. 2) Thefrequency of turbidites or lack of them might givesome clue to their history of island uplift and eventualemergence. 3) Since the site is downwind (NW) from anumber of atolls the lower part of the sequence mightcontain ash layers which would allow us to date thetime when these islands were active volcanoes. Since

J. D. Hays, Lamont-Doherty Geological Observatory, Pali-sades, New York; H. E. Cook, University of California,Riverside; D. G. Jenkins, University of Canterbury, Christ-church, New Zealand; F. M. Cook, independent; J. Fuller,Kennecott Exploration, Inc., San Diego, California; R. Goll,Lamont-Doherty Geological Observatory, Palisades, NewYork; E. D. Milow, Scripps Institution of Oceanography,La Jolla, California; W. Orr, University of Oregon, Eugene,Oregon.

this was an unscheduled site there was no surveyconducted prior to the arrival of Glomar Challenger.

OPERATIONS

Site Survey

After leaving Tahiti, the Challenger traveled over roughtopography with thin sediment cover until the smallatolls of Ake and Manihi in the Tuamotu group werepassed. Here the ocean floor became smooth and wasunderlain by a thick sedimentary sequence whichconsisted of an upper highly stratified sequence about0.2 second thick, a middle transparent layer also about0.2 second thick and a lower stratified layer ofindeterminate, but at least 0.1 second, thickness. Aprominent reflector that might be interpreted asbasement could not be located on the records. Sincethe sedimentary sequence seemed quite uniform overseveral miles of steaming, a drilling site was selectedarbitrarily. However, to assure that the spot we woulddrill was representative of the sequence we had beentraveling over, a survey was made with the Challengerwhich amounted to a one square mile area. During thesurvey the water depth did not vary more than 5fathoms from the 2440 fathoms (uncorrected) PDRdepth. The sequence of sediment layers varied onlyslightly in the relative thickness of the various acous-tical units and it was decided to drill at the northerncorner of our survey (the point farthest from theTuamotus). After leaving Site 76, we steamed duenorth for several miles and encountered the same kindof topography and sediment acoustic characteristicsthat we had noted at our drill site.

Coring

On arrival at Site 76 at 0027 hours, December 8, 1969,a Burnett beacon was dropped and the drill string runto the sea floor. Coring was started at the sea floor bypunching the bit into the sediment without rotating.To our surprise, before the first core was completed itwas necessary to rotate the bit and break circulation.After recovery of our first core our general plan was todrill 50 meters and core again, continuing the practiceuntil we reached basement. The bit was stopped by ahard layer at 25 meters below the sea floor. Since thelower drilling assembly was not buried in the sedi-ments, no more than 10,000 pounds of weight wasapplied to the bit. Rotation on the hard layercontinued for about an hour. The bit was broughtabove the mud line and respudded (Hole 76A), coring

150 I4O°W

MARQUESAS ISLANDS

IO°S

20°

Figure 1. Location of Site 76.

from 9 to 18 meters and from 18 to 25.5 meters. At 25meters the bit again encountered the firm layer.Apparently just before the drill string was drawn up,the ship surged applying additional weight to the bitwhich was sufficient to snap off two drill collarscausing their loss, along with the bit, core barrel, andinner barrel assembly.

LITHOLOGY

The sediments at Site 76 are divided into five informalunits (Figures 7, 9, 11 and 13). Because this was theonly site drilled in the Tuamotu Archipelago the arealextent of these units is unknown. However, they maybe fairly extensive because many of the sedimentsconsist of allochthonous carbonate sands, which arebelieved to have been derived from the Tuamotuvolcanic-carbonate islands, about 100 kilometers south-west of Site 76.

The five units are differentiated on the basis of color,percentage of calcium carbonate, microfossil types,zeolite distribution, and percentage of clay. Alloch-thonous carbonate beds are common throughout thesection and comprise about 15 to 25 per cent of thetotal thickness. The beds of carbonate debris range

from 2 to 90 centimeters in thickness and containclasts up to 5 centimeters in maximum dimension.

The dusky brown material in these units which isreferred to as "red clay" consists, at least in part, ofamorphous iron oxides and crystallized iron oxide inthe form of goethite (Cook and Zemmels, 1971). Thezeolites phillipsite and clinoptilolite are present inthese sediments with phillipsite being the most abun-dant (Cook and Zemmels, 1971).

Fossils of various ages occur within these sediments.Displaced fossils range in age from late Eocene toPleistocene; therefore, it is possible that the entiresequence was deposited in Pleistocene time.

Unit 1 (0 to 9.1 meters)

Unit 1 consists of dusky brown (5YR2/2) phillipsite-clay mud and interbedded allodapic2 carbonate debrisbeds. About 15 to 20 per cent of this unit consists ofallochthonous carbonate sands that occur in beds

2Meischner (1964) proposed this term for shoal-water organicor inorganic carbonate sands which were transported (alloch-thonous) to a contemporaneous deeper water environmentbefore lithification.

BIOSTRATIGRAPHY

FORAMINIFERA

G. fistulosus

S. dehiβoene

NANNOFOSSILS

G. "ooeanioa"Zone?, D,browjeri Zoneto possiblyC. rugosusZone

RADIOLARIANS

IES

-S

ER

IES

u-• 31/5 a j

PL

IOC

EN

E

TE

RS

wS

25 -

5 0 -

75 -

1 0 0 -

125 -

ISO —

175 -

2 0 0 -

RE

S

U

1•1 1

Al

A2r~l

IAT

ION

OR

N

LITHOLOGIC DESCRIPTION

DUSKY BROWN Z e o l i t e

Clay Mud.

WHITE t o VERY PALEORANGE Coral-BryozoanAlgae AllochthonousCarbonate Sands.

GRAYISH ORANGE Nanno-f o s s i l Ooze.

MODERATE BROWN Chert .

OL

OG

ICL

UM

N

X Ot, <->

-Z-i-:Z^4

mA A A A

CaCO3

25 50 75

Illllii!!!!!!

ii•l

SILICEOUSBIOTA

"'

20 401 1 1 1

Figure 2. Site 76 summary.

FORAMS

40 60 80

SILICACLAY

VOLCANICGLASS

R. IR A B

.50 1.58

SEDIMENTATION RATE

m/106 yrs

10 20

POROSITY-g/cm3

GRAPE O SYRINGE SAMPLE

SOUND VELOCITYkm/sec

0

DENSITY -%GRAPE A SECTION WT.

D SYRINGE SAMPLE

1.6

N A T U R A L GAMMA10' counts/75 sec

PENETROM-ETER

Rgure 3. Site 76 summary.

BIOSTRATCGRAPHIC CHART FORAMINIFERA

Figure 4. Site 76 Biostratigraphic Chart Foraminifera.

26

BIOSTRATIGRAPHIC CHART NANNOFOSSILS

100

125

150

30

40

50

TAXA

+* 1K04

II

NANNOFOSSIL LEGEND: Rare to infrequent occurrence. 1 Frequent occurrence. Greater than frequent occurrence.

Figure 5. Site 76 Biostratigraphic Chart Nannofossils.

BiOSTRATIGRAPHIC COMPARISON CHART

DEPTHBELOW I W | ù

SEAFLOOR oFORAMINIFERA

ZONAL INDEX TAXA

NANNOFOSSILS" RADIOLARIA

ZONAL INDEX TAXA ZONAL INDEX TAXA

|2A

I I

I '

t 4 1

S .3

I I I

I I I '

I I IO Q.

* NANNOFOSSIL LEGEND: Rare to infrequent occuirence. Frequent occurrence.

Figure 6. Site 76 Biostratigraphic Comparison Chart.i Greater than frequent occurrence.

SERIESSUBSERIES

LU

2E

LU

C_>

CO

i

Q _

MET

ERS

l —

o

- 3 —

4 —

5 —

- 6 —

1 1

1 1

1 1

1 1

1 1

1 1

1 1

00

1 1

1 1

1 1

1 I

1 1

1 1

I 1

SECT

IONS

1

2

3

4

5

6

LITHCOLUMN

. :o.'..»σ

2

:- z:• .•ö . vo

• : Z - ~ H

2-"

L Z ~~

: ~j _

lo.vQ . i t f

:-Z -:

• -. 2

-_-_-_-_; Z;-_:

:I-_-Zl-_: Z -_-

X÷z—-z--

:- - - - - - Z-:

- z

• o o o o Ö o•• o o ββe> β

%&>%&££§ÓC>OOOODCÖ

^ ^

- 22 • -

-:z --------^ -

&£*&#

:-Z:-—-—:

SMEA

RSL

IDES

*

*

*

*

%CaCo3

25 50 75i i i

II

II

1

1

1

1

1

1

1

1

11

1

-J-

zr±

III—i

—

II II II

1k


UNIT 1

DUSKY BROWN (5YR2/2), zeo i i t i c (25%-40%) - clay (60%-75%)mud.

VERY PALE ORANGE (10YR8/2), foraminiferal (15%-25%) -calcareous nannofossil (75%-85%) ooze.

WHITE (N9) to VERY PALE ORANGE (10YR8/2), coral l ine -bryozoan - algal (?) ailodapic packstone to grainstonetu rb id i t e . Graded from sand and pebble sizes at base tos i l t and mud sizes at top. Contains micro manganese (?)nodules 2-5 mm in diameter.

Figure 7. Hole 76, Core 1 (0 to 9.1m).

m 0•n i

-1

-2

-3

-4

-5

-6

-7

o-o

-9

NATURAL GAMMAI03 counts/75 sec

0 2.0 4.0i i i i \

POROSITY SOUND VELOCITY% km/sec50 100 1.4 1.6 1.8 2.01 1 ! 1 I J

. • • /

I*

•• /

i•

•

•

j

SECTION 10cm

-25

-50

-75

100

-125

150Figure 8. Hole 76, Core 1, Sections 1-6, Physical Properties.

SERIESSUBSERIES

LU

CD

2 —

4 —

- 6 •

7 —

LUCO

LITHCOLUMN

n ' • ' • " ö

Q.O. . :

»• 9' o: :"

-Ir--I-Z Jr--ir•

^

:-_-_-_-!-: Z-i•

p?;v.cx'•

Q •*•."•'oπra

• ij^Hr-IH

". 7 r_T-r-_—^-_-

- Z ->I-I->I•

_ Z -JT--JT-T-

i '->>i->i-

5"H- LU

L U r~\%CaCo3

25 50 75

LITHOLOGY DESCRIPTION

UNIT 2

DUSKY BROWN (5YR2/2), zeolitic (4OJ5-6OS5) - clay (40%-60%) mud.

MODERATE YELLOWISH BROWN (10YR5/4), zeolitic (2%-5%) -clay (10%-15%) - calcareous nannofossil (80%-85%) ooze.

WHITE (N9), foraminiferal (40%-60%) - calcareous nanno-fossil (40%-60%) packstone; probable turbidite.

Base - UNIT 2

Top - UNIT 3

MODERATE YELLOWISH BROWN (10YR5/4), clay (<1%) -foraminiferal (15%-25%) - calcareous nannofossil

ooze.

Micro manganese (?) nodules (5%-10%) in an unidentifiedlime chalk matrix.

Base - UNIT 3

Top - UNIT 4

VERY PALE ORANGE (10YR8/2), calcareous nannofossil(>99%) ooze.

Figure 9. Hole 76A, Core 1 (9.1 to 18.2m).

m 00

L

-1

-2

-3

-5

-6

-7

-8

-9

NATURAL GAMMA

I03 counts/75 sec

0 2.0 4.0

SECTION 1 2rOcm

i i i

POROSITY SOUND VELOCITY% km/sec

50 100 1.4 1.6 1.8 2.0j.

. J

-25

-50

-75

100

-125

-1501

*

CDfi<DPπ

O

o

CDfiCDP H

o+->o

1 3CDfiCD

i • • ; f

!** ,'"β | ;

• * •».

• ; , ,

•

-*:

;

-*3j

Figure 10. Hole 76A, Core 1, Sections 1-6, Physical Properties.

SERIESSUBSERIES

1 —

- 3 •

4 —

5 —

- 6 •

oooh-oLJU

oo

LITHCOLUMN

Qó.ö ,°00 •V

'.*„.'.

^ff*.O»».'Q°J'

%CaCo325 50 75


UNIT 4

DUSKY BROWN (5YR2/2), calcareous nannofossil (10%-zeolitic (20%-30%) - clay (5035-75%) mud.

Base - UNIT 4

Top - UNIT 5

VERY PALE ORANGE (10YR8/2), clay (1%£-30%) - calcareous nannofossil

GRAYISH ORANGE (10YR7/4), clay (<1%)fossil (>98%) ooze.

- foraminiferalooze.

- calcareous nanno-

Figure 11. Hole 76A, Core 2(18.2 to 27.3m).

-1

-2

-3

-5

-6

-7

-8

-9

NATURAL GAMMAI03 counts/75 sec

0 2.0 4.0

SECTION 1rOcm

POROSITY SOUND VELOCITYkm/sec

m 0 50 100 1.4 1.6 1 .8 2 . 0- 0 I 1 1 I 1 1 1

atl /;

-25

-50

h75

-100

-125

^150Figure 12. //o/e 76̂ 4, Core 2, Sections 1-6, Physical Properties.

. .

SERIESSUBSERIES

LU

LU

O

1

MET

ERS

|

1 —

i i

i i

| i i

i i

i i

i i

iCM

<

4 —

1 1

1 I

1 1

1 1

1 1

1 1

1 1

1 1

1 1

1 1

1 1

1 1

1 1

1 1

1

CO

1 1

1 1 1

1 1 1

1 I 1

1 1

1

SECT

IONS

1

2

3

4

5

6

LITHCOLUMN

NOTCORED

III

SMEA

RSL

IDES

%CaCo3

25 50 75

•

LITHOLOGY DESCRIPTION

UNIT 5

DARK YELLOWISH ORANGE (10YR6/6) and MODERATE YELLOWISHBROWN (10YR5/4), part ly s i l i c i f i e d foraminiferal (?)(l5%-25%) - calcareous nannofossil (75%-85%) packstoneto grainstone.

Figure 13. Hole 76, Core Catcher (27.3 to 27.6m).

Figure 14. Hole 76, Core 1, Section 1.

37

Figure 15. Hole 76, Core 1, Section 5.

0—i

25—

50—I

75—

100—

125-

150—

y

K-rtr

Description

«-GRAYISH ORANGE(10YR7/4), z e o l i t i c(<U)-c lay (<1%)-calcareous nannofossil(>98%) ooze.

GRAYISH ORANGE.(10YR7/4), z e o l i t i c(<l%)-clay (15-20%)-calcareous nanno-foss i l (80-85%) ooze.

BROWN (5YR2/2)calcareous nanno-foss i l (5-10%)-clay(40-50%)-zeolit ic(40-50%) mud.

WHITE(n() clay (5-15%)-coral (40-50%)-foraminifera (40-50%)

.allodapic packstone."Graded from lime sandand pebbles sizes upto 4x4x1.5 cm at thebase to sand sizesat the top; bothupper and lower con-tacts are sharp.

Figure 16. Hole 76A, Core 2, Section 6.

which range from 5 to 90 centimeters in thickness.These beds generally are very low in their mud contentand vary from white to very pale orange in generalcoloration. Carbonate rock fragments and sands com-prise the allochthonous constituents—these consist offoraminiferal grainstone clasts up to 5 centimeters inmaximum diameter and individual fragments of coral,bryozoa, red algae (?), foraminifera, and manganesemicronodules (Figure 14). Mixed age assemblages ofcalcareous nannofossils also occur. One of these beds,90 centimeters thick, is graded from sand and pebblesizes at the base to silt sizes at the top with sharp lowerand upper contacts (Figure 15).

About 75 to 85 per cent of unit 1 is a dusky brownclay mud, very rich in phillipsite. Phillipsite occurs inpercentages ranging from 25 to 50 per cent, and "redclay" ranges between 30 and 60 per cent. Thesephülipsitic "red clay" muds are massive with no visiblelaminations.

Unit 2 (9.1 to 13.6 meters)

Unit 2 is a dark yellowish-brown (10YR2/2) clay-phillipsite—calcareous nannofossil ooze and inter-bedded allodapic carbonates. Allochthonous carbonatedebris beds in this unit range from 35 to 50 centi-meters in thickness and comprise about 15 to 25 percent of the total thickness of the unit. In contrast tomost of the debris beds in unit 1, debris beds in unit 2have a higher percentage of mud in their matrix. Thismud is probably a primary depositional feature, and isnot due to mixing during drilling as suggested by theirrelatively undisturbed basal and upper contacts.

The coarse-grained fraction of the debris is similar tothat in unit 1; however, light colored foraminiferalgrainstone fragments are more numerous and larger inunit 2. The mud matrix in these allochthonous depositsis a dark yellowish-brown (10YR2/2) clay (5 to 10 percent)-foraminiferal (10 to 20 per cent)-calcareousnannofossil (70 to 80 per cent) ooze.


Moderate yellowish-brown (10YR5/4) foraminiferal-calcareous nannofossil ooze constitutes unit 3. Itappears to have been very badly disturbed duringcoring, and no laminations are apparent. Much of theooze is probably allochthonous, based on the mixedages of the microfossils in these sediments, but theoriginal textures and the thickness of the beds are notknown.


This unit consists of grayish-orange (10YR7/4) cal-careous nannofossil ooze and allodapic carbonatedebris beds. Allodapic carbonate beds in this unit rangefrom 5 to 35 centimeters in thickness, have a high mudcontent, no visible grading, and a high percentage of

pebble-sized fragments; thus, they are very similar tothe debris beds in unit 2. The interbedded pelagic mudsare dusky brown (5YR2/2) calcareous nannofossil (10to 25 per cent)—phillipsitic (20 to 30 per cent)—clay(50 to 75 per cent) muds that have well defined,relatively undisturbed, horizontal beds 2 to 5 centi-meters in thickness.


Grayish-orange (10YR7/4) calcareous nannofossil oozeand allodapic debris beds comprise unit 5. In this unitcarbonate debris beds are of two basic types: mud-richallodapics and mud-poor allodapics. Mud-poor typesare least disturbed and exhibit excellent, sharp basaland upper contacts with the enclosing zeolitic-clays.One mud-poor debris bed is 45 centimeters thick andshows excellent grading from sand- and pebble-sizeddebris at the base to sand sizes at the top (Figure 16).

About 75 per cent of this unit consists of grayish-orange (10YR7/4) calcareous nannofossil ooze withsome of the ooze containing up to 5 per cent clay. Thecalcareous nannofossil oozes with the highest percent-age of clay seem to be more indurated than themudfree oozes.

The top one-third of unit 5 contains minor amounts ofmoderate yellowish-brown (10YR5/4) phillipsitic (10to 15 per cent)—foraminiferal (10 to 20 per cent)—calcareous nannofossil (65 to 80 per cent) ooze andmicromanganese nodules in a white chalk.

This hole was terminated in a dark yellowish-orange(10YR6/6) partially silicified limestone. Silica occursas a replacement and an in te rp article, void-fillingchalcedony. The original rock was composed domi-nantly of sand-sized spheroidal grains whose deposi-tional texture was that of a packstone to grainstone. Itis most likely that these grains were either foram-inifera, oolites, radiolarians, or a combination of theseconstituents which have been completely replaced bychalcedony. No obvious indication of the original graintype(s) remains. However, immediately above thischert is an allodapic mud-poor debris bed that containsabundant foraminifera and clasts of foraminiferalgrainstones. This chert may represent a partiallysilicified portion of an allochthonous foraminiferadebris bed.

PHYSICAL PROPERTIES

Natural Gamma

At Site 76 the natural gamma radiation readings rangefrom 1087 to 1756 counts/75 sec. Pale orange anddark brown sediments at Site 76 could be differen-tiated on the basis of natural gamma radiation emis-sion.

In Hole 76, Core 1, Section 1, the natural gammareadings range from 1349 to 1756 counts. The duskybrown sediments gave off higher levels (1600 to 1756counts) of gamma radiation than the interbedded lightorange and white sediments (Figure 14). The highercounts in the dark brown sediments are probably dueto the high content of the potassic zeolites phillipsiteand clinoptilolite, and montmorillonite and mica. Theinterbedded pale orange to white foraminiferal-calcareous nannofossil oozes and foraminiferal grain-stone turbidites record lower natural gamma readings(1349 to 1700 counts). The smaller clay and zeolitecontent of the light colored sediments is the probablecause of the lower readings.

In Hole 76A, Core 1, Section 4 the moderate yellowforaminiferal-calcareous nannofossil oozes, as seenthrough the core liner and in the core catcher, recordlower natural gamma radiation readings than the darkphillipsitic clay mud in other sections. These lowerreadings are probably the result of less clay and zeolitecontent in the lighter sediments.

In Hole 76A, Core 2, Section 2 the readings range from1178 to 1368 counts. This section was not opened butthe sediments, as seen through the core liner and in thecore catcher, appeared to be dusky brown phillipsiticclay interbedded with orange calcareous nannofossilooze and turbidite material.

The remaining cores were not tested for natural gammaradiation emission because of recorder malfunction.

Porosity

Porosity at Site 76 ranges from 52 to 86 per cent.There is no correlation between porosity and depth atthis site. This is a shallow 27.6-meter hole andapparently was not deep enough to show evidence ofcompaction. The lowest porosity was in a moderateyellow brown foraminiferal-calcareous nannofossilooze. The highest porosity was in a dusky brownphillipsitic clay mud. However, there is no consistentvariation between lithology and porosity.

Sonic Velocity

The sound velocity readings range from 1565 m/sec to1736 m/sec. Velocity values are higher in the corescontaining much phillipsitic mud and lower in corescontaining mostly foraminiferal-calcareous nannofossilooze or where the core is significantly disturbed.

Bulk Density

Only one reading of 1.245 g/cc was taken at this site.

39

Penetrometer

Although this site was only about 30 meters deep thereis a marked contrast between the penetrometer read-ings at this site and other Leg 9 sites for thisstratigraphic interval. Readings at Site 76 were verylow and ranged from only 0.1 to 0.3 centimeters. Inthe rest of the sites, sediments with this degree ofinduration came from depths of 100 meters or more.The possible influence of the very high percentage of"red clay" at this site on the physical properties isdiscussed in the Synthesis section.

One anomalously high reading was taken at about 20meters in a zone of core disturbance.

BIOSTRATIGRAPHY

Foraminifera

Although Site 76 was the shallowest hole drilled on theentire leg, the foraminiferal faunas recovered from thethree cores yielded considerable information on theenvironment and Cenozoic history of the Tuamotuarchipelago.

A Pliocene history of successive turbidity flows fromshallow to deep water is reflected at this site bylithology and the substantial reworking of foram-iniferal and other invertebrate faunas.

The cored interval penetrated what appears to be acontinuous section including the Globigerinoides fistu-losus Zone and the Sphaeroidinella dehiscens Zone ofthe upper and lower Pliocene.

The indigenous planktonic foraminifera are abundantand well preserved except in occasional samples fromthe uppermost core in which some secondary solutionof the foraminiferal tests was evident.

Reworked foraminiferal material includes in most caseseither shallow water Amphistegina sp. or much olderupper-middle Miocene (such as, Globorotalia fohsi);lowermost Miocene (such as, Globorotalia kugleri);upper Eocene (such as, Hantkenina alabamensis) andlower Eocene (such as, Globorotalia aequa). (Seedistribution chart, Figure 4.)

This was the only hole that terminated in chert.Although foraminiferal samples from the chert itself(recorded at 30 meters) were too hard to process ormake an age assignment on, the lowermost sample(Hole 76A, Core 2, core catcher) is from the Sphaeroi-dinella dehiscens Zone and contains chert-infilledforaminifera.

Radiolaria

At Site 76, Radiolaria were not observed in smearslides by the geologists or in the calcareous microfossilsamples. Consequently, radiolarian samples were nottaken.

DISCUSSION AND INTERPRETATION

Although the bit failed to penetrate a resistant layer ata depth of only 30 meters below the sea floor, thesediments recovered at this site provide some informa-tion on a number of interesting problems that will bediscussed below.

Seismic Reflectors

The chert layer which stopped our drilling probablyoccupies the top of a thick series of interbedded cherts.These chert beds serve as strong reflectors which arerecognizable as the upper stratified layer on theseismic-reflection profiles at this site. Because the topof the strong reflectors lies at a depth of 0.05 secondon the profiles, sound velocities in the soft sedimentabove the chert must be approximately 1.5 km/secrather than the usually accepted value of 2.0 km/sec.This is borne out by the direct acoustical measure-ments on the cores. The prominent seismic reflectorswithin the upper stratified layer were followed formiles during the pre-site survey and after we left Site76.

Pelagic and Turbidite Sediments

All three cores from this site contained coarse-grainedsand beds, many with sharp upper and lower contacts,alternating with either dark chocolate brown phillip-sitic clay (Hole 76, Core 1) or brown clay intermixedwith nannofossil ooze (Hole 76A, Cores 1 and 2).Because of the water depth (4590 meters) and thenature of the upper 10 meters of sediment, we haveassumed that the pelagic sediments at this site arephillipsitic clays. Consequently, the fine- and coarse-grained calcareous debris has been swept to thislocation from the nearby Tuamotu Islands (60 miles tothe south) by turbidity and bottom currents. Thismodel is not difficult to defend, because the coarse-grained layers contain genera of shallow water ben-thonic foraminifera, such as, Amphistegina and Cal-carina, as well as calcareous algae, coral sclerites,mollusc fragments and abraded reefal debris. Theseconstituents clearly indicate that the source for someor all of the coarse layers was in very shallow waternear the surf line. Planktonic foraminifera are presentalso in the coarse layers, and they are probably derivedfrom deeper water. They range in age from latePaleocene to Pliocene. Because the planktonic foram-inifera are not indigenous to the abyssal phillipsiticclays, we can safely assume that they have been carrieddown slope from shallower regions on the flanks of theTuamotu island chain. If the Tuamotu atolls were builton the top of extinct volcanoes, then the volcanoesmust have been in existence during early Eocene time(see below). The coarse turbidites also contain manga-nese micronodules in greater abundance than in thephillipsitic clays, suggesting that these were entrainedin the turbidity current before it reached the depth ofour site.

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The majority of the coccoliths and diseoasters inter-mixed with the clays of Cores 1 and 2 of Hole 76Ahave stratigraphic ranges from early Eocene to earlyMiocene. The overlap of the oldest planktonic foram-inifera and oldest calcareous nannofossils suggests thatthe oldest material entrained by the turbidity currentsis of early Eocene age. The turbidite occurring at baseof Core 2, Hole 76A (25.5 meters) contains a suite ofearly Pliocene and older planktonic foraminifera. Forthis reason, we place the age of the bottom of the holeas no older than early Pliocene but possibly younger.This provides additional evidence that not only are thecoarse layers reworked but so are the fine nannofossilsfound so abundantly between the coarse layers ofCores 1 and 2 from Hole 76A. It is interesting to notethat the age of the most common reworked nanno-fossils is early Miocene.

The small piece of chert that was present in the corecatcher of Core 2, Hole 76A, appears to be a coarsegrain-supported silicified sediment. It immediatelyunderlies a turbidite, and we conclude that the chert isthe lower silicified portion of the turbidite. If this istrue, then the chert is early Pliocene or younger in ageand is the youngest deep-sea chert on record. Chert wasnot observed in the pelagic sediment between theturbidites, and we conclude from this that the turbiditeis important in the formation of chert.

Many of the graded turbidites cored at this site containvery little mud, and their sharp upper contacts suggestthat bottom currents moving over this area havewinnowed out the fine material. Another explanationof these features is that they are channel deposits. Thelatter explanation is supported by the presence on ourprofiler records of strong vertically-arrayed reflectorsthat have short horizontal extent. These reflectorscould represent discrete channel fillings. The lack of

fine-grained material results in high permeability forthe turbidites, which may facilitate flow of silicamineralizing solutions resulting in the deposition ofchalcedony.

The origin of chert remains problematic. PreviousDSDP legs have encountered Eocene cherts inter-bedded with sediments containing large concentrationsof biogenous opal. Yet, the sediments overlying thechert at this site are devoid of this material. Becauseopaline sediments of late Miocene-early Pliocene ageare not common in this region, the sediments under-lying the cherts are also probably barren of opalinesilica. An alternative explanation must be sought. Evenin late Tertiary time, the Tuamotu Ridge may havebeen a source of hydrothermal emanations which werechanneled by the permeable coarse-grained layers.However, our preferred interpretation as the source ofthe silica is the clay minerals. The presence of largeamounts of phillipsite implies that the clays haveundergone alteration to this authigenic mineral. Theindex of refraction of this phillipsite indicates that thesilica content is relatively low (approximately 45 percent).

REFERENCES

Cook, H. E. and Zemmels, I., 1971. X-ray mineralogystudies—Leg 9. In Hays, J. D. et al, Initial Reportsof the Deep Sea Drilling Project, Volume IX.Washington (U. S. Government Printing Office), inpress.

Meischner, K. D., 1964. Allodapische kalke, turbiditein riffe-naheu sedimentations-becken. In A. H.Bouma and A. Brouwer (Eds.) Developments inSedimentology 3, Turbidites. Amsterdam (Elsevier),156.

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