12. GEOCHEMISTRY AND MINERALOGY OF SELECTED CARBONACEOUS CLAYSTONESIN THE LOWER CRETACEOUS FROM THE BLAKE-BAHAMA BASIN, NORTH ATLANTIC1

Hideo Kagami and Toshio Ishizuka, Ocean Research Institute, University of Tokyo, Tokyo, Japanand

Saburo Aoki, Natural Science Institute, Toyo University, Tokyo, Japan

ABSTRACT

Four dominant depositions of carbonaceous claystones are recognized to have occurred during the early Aptian tomiddle Albian at Site 534. There are correlations of stable isotope ratios with organic carbon content and of clay con-tent with clay mineralogy of the samples. Almost all organic carbon in these sequences has very negative terrestrial iso-tope ratios, and the clay of that age indicates predominance of aluminous montmorillonite, which is thought to be ofterrigenous origin. It is suggested that development of coastal vegetation belts and deltaic outbuilding with consequentoutpouring of land-plant detritus and terrigenous elastics into the deep basins probably led to formation of the "blackshale" facies.

INTRODUCTIONCretaceous black claystones were penetrated in DSDP

Hole 534A between 764 and 950 m sub-bottom depths.Such a thick section was encountered owing to its loca-tion near the continental margin of the United States(Fig. 1).

Black claystones from the western North Atlanticdrill sites all have broad compositional similarities, al-though there are local compositional peculiarities aswell. The claystones from Sites 101, 105, 386, 387, 391,and 534 contain an average of 50% or more clay miner-als, consisting primarily of smectite and illite, and 2 to4% feldspars. Average quartz content is 10 to 20% inclaystones at Sites 101 and 105, and it is two to threetimes greater at Sites 386 and 387 (Tucholke and Vogt,1979).

Detrital carbonaceous particles are common in EarlyCretaceous black claystones drilled in the North Atlan-tic by the Deep Sea Drilling Project. At Site 391, the Ap-tian-Albian section contains abundant tracheal facies,composed of palynomorphs and palynodebris, which arecompositionally close to those of coeval fluviodeltaicsediments on continents bordering the North Atlantic,except that marine currents sorted sporomorph speciesand admixed dinoflagellates prior to deposition (Habib,1979). It is suggested that they were deposited by tur-bidity currents from the fluviodeltaic source.

Rapid termination of euxinic black clay deposition inthe North Atlantic is indicated by the abrupt transitionfrom black claystone to variegated claystone in the Plan-tagenet Formation, deposited near the end of the Ceno-manian. Rejuvenated deep circulation could have beencaused by any one of several potential new bottom-wa-ter sources. Exchange of deep water with the South At-lantic is one of the most likely causes.

Sheridan, R. E., Gradstein, F. M., et al., Init. Repts. DSDP, 76: Washington (U.S.Govt. Printing Office).

MAJOR CYCLES OF CARBONACEOUSCLAYSTONE IN THE HATTERAS FORMATION

Downhole variation in abundance of carbonaceousclaystones (black shale), and frequency per meter ofgraded units of both calciturbidites and quartz-silt lay-ers have been examined in the Hatteras Formation; theinterval spans Cores 24 through 46. The result is shownin Figure 2. In order to plot the percentage of carbona-ceous claystone per core, careful visual and microscopicobservations of the core samples were conducted. Theclaystones containing more than 5% plant debris on thesmear slide were classified as carbonaceous in this study.Figure 2 shows four major abundance peaks of carbona-ceous claystone; these are at depths of 785 m (Core 29),830 m (Core 34), 880 m (Core 39), and 915 m (Core 43).

Core 29 contains relatively high percentages of car-bonaceous claystone, and these claystones are homoge-neous with very few silt stringers. When the cores weresplit, some of the claystones were shiny and glitteringblack (NI) and appeared to contain high percentages ofamorphous plant debris, an observation later confirmedby microscopic examination. Although the precise ageof the claystone in Core 29 is not yet known, it is tenta-tively dated as Albian (Site 534 report, this volume).

The peak of carbonaceous claystone abundance inCore 34 is dated as middle Albian. These carbonaceousclaystones are dark gray (N3) to greenish black (5Gy1/2). Some contain greenish gray (5GY 6/1) stringersconsisting mostly of quartz and feldspar, which tend todilute the concentration of organic carbon. The quartz-silt stringers are less than a millimeter thick, and someshow wavy lamination or Flaser-type lamination; theyoccur in both carbonaceous claystones and the interbed-ded silty claystones. The origin of the quartz stringerhas not yet been determined, but possibilities are beingconsidered. One is a "shower" of terrigenous materialsfrom very weak turbidity currents. Another possibilityinvolves bottom traction-current activity such as a con-tour current. The presence of wavy laminae less than a

429

H. KAGAMI, T. ISHIZUKA, AND S. AOKI

Hole 534A

Age

early

Miocene

lateEocene

Cenom.

Aptian

Valanginian

Tithonian

Kimm

Callovian

Description

Intraclast chalk/green mudstone

Zeolite-siliceousmudstone; sandstone;porcellanite

Var. claystone

Black greencarbonaceousclaystonevar.

Calcareous claystone;nannofossil chalk;radiolarian/nannofossillimestone

Reddish green graycalcareous claystone;micritic-bioclasticlimestone

Dark claystone ;radiolarian claystone;pelletal limestone

GreatAbaco

Member

BermudaRise

Plantagenet

Hatteras

Blake-Bahama

CatGap

Unnamed

47

130

545

I I

"950

I I I I I I

386

1635

I I I I J I I J I I i I I 2 0

45c

40c

35c

30c

25°

90° 85° 80° 75° 70° 65° 60°

Figure 1. Location of Site 534 in the Blake-Bahama Basin of the western North Atlantic, and a generalized geologic columnar section of Hole 534A.

millimeter in size in the burrowed silty claystone maysupport this contention.

Thin turbidites consisting of recrystallized calcite andquartz and showing Bouma-sequence structures also oc-cur in Core 34. Calcilutites up to 10 cm in thicknessshow normal-size grading. They occur in greatest abun-dance in Core 34, as shown in Figure 2. The source ofthe calciturbidites was probably the upper continentalslope above the CCD (calcite compensation depth),where a supply of carbonates was more likely available.

The major abundant peak of carbonaceous materialin Core 39 occurs in Albian rocks. The carbonaceousclaystones are olive black (5Y 2/1) to black (NI) andfairly homogeneous, indicating relatively high organiccarbon content. Although quartz stringers appear in theblack portions, they are more abundant in the inter-bedded silty claystones. The lower part of each carbo-naceous claystone tends to be homogeneous, whereasthe upper part is commonly laminated, with occasionalquartz stringers, which may suggest en masse input of

430

GEOCHEMISTRY AND MINERALOGY OF SELECTED CARBONACEOUS CLAYSTONES

750-

8 0 0 -

Coreno.

t25

2

29 B3 0 ZZZ

31 m,

32 ^

33 7Z.

34 ^

Carbonaceous claystone

20 40 60 80

Frequency/metergraded units

35E 850-

37

38

39

40

900- 41

43

I36 I

_ J

46 ZZ

950-

Δ Calciturbidites

© Silty layer

Figure 2. Downhole variation in abundance of carbonaceous claystonesand graded layers in the Hatteras Formation. (The recovery fre-quency in numbers per meters of graded beds of calciturbidites[dashed line] and silty layers [solid line] is shown in the right col-umn.)

carbonaceous matter, with later dilution by normalocean-bottom processes. Smear-slide observation indi-cates concentration of amorphous organic matter in thebottom part of these beds.

The carbonaceous claystone frequency peak in Core43, the lowest in the Hatteras Formation, has not beendated, but the age may be early Aptian, as in Core 44.The carbonaceous claystone is greenish black (5G 2/1)to dark gray (N3), less commonly grayish black (N2),similar to the Core-34 type with stringers. Calciturbi-dites with pelletal texture are abundant in early Aptian

rocks. These rocks are also characterized by a gradualupward decrease of laminated calcareous claystones,which shows that the site subsided gradually below theCCD. There are no radiolarian sands within the carbon-aceous claystones. Chalcedonic quartz bands in the car-bonaceous claystones have been observed only in Core48, dated Barremian and older.

Intervals between the four major abundance peaks ofcarbonaceous claystone represent silty claystone deposi-tion, including deposition of variegated claystones. Siltylayers with current laminations occur only in the varie-gated intervals without calciturbidites. Because the siltylayers and the variegated claystones have the same min-eralogical composition, their deposition is likely linked.Several hypotheses are considered with respect to envi-ronment of deposition of the silty layer: (1) Calciturbi-dites were deposited in this stratigraphic interval but dis-solved completely to leave the silty layers during or afterdeposition. (2) The CCD falls considerably in the watercolumn so that calcium carbonate deposition decreasedon the slope, with none on the basin floor; therefore,overburden was insufficient to cause slumping of car-bonate sediments on the slope. (3) Silty turbidites weredeposited by relatively weak turbidity currents flowingalong the bottoms of canyons, so that no contaminationwith carbonates took place. (4) Oxygenated bottom cur-rents winnowed bottom sediment to form the silty layer.The oxygen-rich waters could also have caused plant de-bris to be destroyed or diluted. Owing to an equable cli-mate with the smooth surface temperature gradient inthe ocean during the middle Cretaceous, sluggish deepcirculation was suggested by Fischer and Arthur (1977).Therefore, another current mechanism such as a Rossbycurrent could be expected to have existed in this portionof the continental margin.

ORGANIC CARBON STABLE ISOTOPE RATIOS

Stable isotope ratios (δC13) of total organic carbonwere measured in closely spaced samples from the Hat-teras Formation at Hole 534A. Figures 3 and 4 show theposition within the cores of 23 samples studied. Sampleswere thawed and treated with 0.57V hydrochloric acid toremove carbonate carbon, then dried in air at room tem-perature. They were burned in a stream of pure oxygenat a temperature of 1000°C for 120 min. to yield CO2.The evolved gas was passed through hot copper oxide,manganese oxide, and silver grains to remove impuri-ties. The organic carbon content was determined bycombusting a weighed residual sample and measuringmanometrically the volume of purified gas. The carbonisotopic composition of the CO2 collected was analyzedin a Nier-Mackinney-type mass spectrometer (Varian Mat250) (Ishizuka, 1978).

The results are given in parts per thousand from thePDB Chicago standard used by Craig (1957). The resultsindicate strongly terrestrial values of δC13, in the rangeof-26.8 to -23.8‰ (Table 1). At the present time, ter-restrially derived organic matter appears to play only asmall part in determining the δC13 content of the totalorganic carbon in continental margin sediments. Off-shore from areas of heavy runoff and sediment input,

431

H. KAGAMI, T. ISHIZUKA, AND S. AOKI

Chemical composition(%)

20 40 60

Acid soluble and δ C 1 3 (PDB)ignition loss (%) (%0)

10 30 -24 -26

1 0 0 -

Figure 3. Major element analytical data and organic carbon stable iso-tope ratios in Core 29, Section 2, Hole 534A. (In the right column,the δC13 ratio is shown by the dashed line, and organic carbon con-tent by the solid line.)

such as the Mississippi and Niger deltas, the δC13 valuesrange from terrigenous-influenced (around — 24%0) totypically marine (— 20%0) within a few tens of kilometersfrom shore (Gearing et al., 1977). The only locationswhere very negative, terrestrial isotope ratios (around— 26%0) were recorded were in rivers, such as the Am-azon river, of hot and humid equatorial regions andin a 100 km inland canal along the Gulf of Mexico.Even here the sediment δC13 changed from -26.6‰ to- 23.9%o while still in fresh water in the canal (Gearinget al., 1977).

From these observations, it is concluded that the pa-leoclimate during the Early Cretaceous was very differ-ent from the present climate and that a much greater

Cm

Chemical composition

20 40 60

Acid soluble and δC^3 (PDB)ignition loss (%) (%0)

10 30 -24 -26

4 0 -

5 0 -

Carbonaceous clay

A I 2 O 3

F e 2 ° 3 K 2 O × 1 0 SiO2

MgO2 x 10

^Laminated

CaCO

\

I I I I I I I I

Figure 4. Major element analytical data and organic carbon stable iso-tope ratios in Core 39, Section 5, Hole 534A.

amount of terrestrial organic matter was incorporatedinto the continental margin sediments of that time.Assuming that tropical conditions prevailed during theEarly Cretaceous favoring dense forests along the coastand that there was a heavy influence of land runoff to-ward the ocean basins seems fully justified.

MAJOR ELEMENT CONTENTAfter grinding samples to a fine powder, we heated

the samples at a temperature of 1000°C for 90 min.Therefore the analyzed data are given on a carbonate-and water-free basis.

Major element content was determined using fuseddisc samples by an automatic X-ray fluorescence spec-trometer (Rigaku 1KF 3064) connected to a microcom-puter (Nova 01) at the Geological Institute, Universityof Tokyo (Matsumoto and Urabe, 1980). Analyses made

432

GEOCHEMISTRY AND MINERALOGY OF SELECTED CARBONACEOUS CLAYSTONES

Table 1. Organic carbon stable isotope ratios.

Depth fromtop of section

(cm)

Core 29, Section

75.0-76.077.5-78.581.0-81.582.0-83.084.5-85.086.5-87.589.0-90.091.5-92.093.0-94.096.5-97.5

100.0-100.5103.0-103.5106.5-107.0

Core 39, Section

37.0-38.038.0-40.043.0-45.045.0-47.050.0-51.551.5-53.553.5-55.055.2-56.758.5-60.062.0-63.763.7-64.864.8-66.566.5-68.5

C organic(«/o)

2

0.025 ± 0.020.069 ± 0.02

0.45 ± 0.020.87 ± 0.030.52 ± 0.010.56 ± 0.020.33 ± 0.011.37 ± 0.011.61 ± 0.040.73 ± 0.020.97 ± 0.031.74 ± 0.061.65 ± 0.01

5

1.03 ± 0.112.13 ± 0.358.41 ± 0.646.49 ± 0.094.33 ± 0.461.66 ± 0.083.29 ± 0.042.58 ± 0.053.55 ± 0.005.05 ± 0.031.86 ± 0.213.67 ± 0.075.55 ± 1.86

δ C 1 3

(%

-23.9-24.7-23.8-24.2-24.4-24.5-24.0-24.9-24.5-24.7-24.9-25.6-25.7

-24.4-25.8-26.6-26.8-27.1-24.8-25.6-25.3-25.8-26.2-25.3-25.6-25.3

PDBo)

± 0.1± 0.2± 0.1

± 0.2± 0.2± 0.1

± 0.3± 0.0± 0.3± 0.0± 0.1

± 0.1± 0.1± 0.1± 0.1

± 0.1± 0.1± 0.1± 0.1± 0.1± 0.0± 0.1

0.5NHCl-soluble(CaCθ3 %)

3.56.24.16.23.53.03.34.06.7

13.44.43.65.2

6.55.99.96.3

11.46.97.16.76.9

14.24.67.37.1

Watercontent

(%)

19.921.120.222.523.422.318.225.423.021.925.28.427.9

16.115.919.218.618.417.219.618.119.220.222.217.418.4

on a total of 15 black shales from Site 534 are given inTable 2 and Figures 3 and 4.

Analyzed data show substantial differences in blackshales between the Hatteras and Blake-Bahama forma-tions. Black shales in the Blake-Bahama Formation(Core 48, Section 5 in Table 2) are intercalated in nanno-fossil chalk composed dominantly of calcium carbon-ate. Interbedded black shales have more detrital frac-tions composed of kaolinite and quartz, and show rela-tive abundance of silica and alumina. The black shales

could have been deposited by turbidity currents result-ing from the erosional activity on the continent or eu-static change in sea level (de Graciansky et al., 1979). Orintercalation of calcium carbonate might have been con-trolled by periodical change in biogenic productivity inthe sea (Mèliéres, 1979). This type of black shale withcalcium carbonate cyclicity has been reported from theBay of Biscay (Mèliéres, 1979). . .

Black shales in the Hatteras Formation show, on theother hand, significant concentration of silica and alu-mina and no obvious cyclicity of calcium carbonates orother elements. There are remarkable concentrations inorganic carbon associated with smectite, which is ob-served in Core 39, Section 5 (Fig. 5). This type of blackshale is characterized by high concentration of organiccarbon and very fine grains of detrital matter.

In Core 29, Section 2, it is observed that silica con-centrates significantly in the pelagic clay interval (Fig.3). This characteristic is interpreted to mean that deposi-tion of terrigenous organic carbon was interrupted byrenewed sea-bottom circulation, and as a result win-nowed residuals of quartz-silt stringers were deposited.Organic carbon is not significantly concentrated in thistype of black shale, and observed relative abundances ofillite and kaolinite are indicative of terrigenous originsfor this interval.

CLAY MINERALOGYForty-one samples from Cores 29, 39, and 48 were

collected for clay mineral investigation. Core 29, Sec-tion 2 and Core 39, Section 5 are composed of carbona-ceous claystone, and Core 48, Section 5 is calcareousclaystone or nannofossil chalk. The core samples rangein age from middle to Early Cretaceous; samples fromCore 29, Section 2 are Albian, those from Core 39, Sec-tion 5 range from the Aptian to the Albian, and samplesfrom Core 48, Section 5 are Barremian.

Each sample was first soaked in distilled water tosoften, then ground in an agate mortar, and again dis-persed in distilled water. Then the clay-size fractionsmaller than 2 µm was collected by sedimentation. Quan-

Table 2. Major element analyses.

Depth fromtop of section

(cm) Siθ2 Tiθ2 AI2O3 MnO MgO CaO K2O P2O5 Total

Core 29, Section 2

86.5-87.589.0-91.096.5-97.5

103.5-106.5106.5-107.0

Core 39, Section 5

50.0-51.553.5-55.258.5-60.062.0-63.763.7-64.8

Core 48, Section 5

70.8-72.575.5-78.080.0-81.583.0-83.585.5-87.0

66.21171.54763.64362.51465.679

63.18364.54762.97262.57065.372

19.38557.41539.33763.793

4.191

0.8840.9000.9480.9120.941

0.8940.8990.8920.8890.896

0.1510.4080.2840.1080.092

17.18015.52619.31218.93019.399

17.75017.56918.06817.88518.114

4.60510.3327.8512.3682.598

7.5785.6607.6067.9307.984

8.1747.8458.0178.7677.276

1.6616.6742.2710.9571.544

0.0060.0040.0260.0120.007

0.0220.0250.0290.0180.020

0.0550.0090.0170.0380.065

2.5231.9113.0022.7792.775

3.0342.9203.2603.1943.253

0.9571.9031.5690.4710.699

0.5680.6010.5450.6310.645

0.5960.6660.6660.7970.589

56.30816.35338.85231.96065.962

2.2752.5082.3532.1252.117

1.9642.0121.8822.0562.253

0.5021.2531.2060.5500.238

2.6182.3872.9502.8672.888

2.9903.0433.1893.1703.178

0.2911.6550.6560.3390.144

0.0450.0690.0390.0480.056

0.0860.0840.0870.0840.048

0.1020.0550.1660.0590.101

99.888101.113100.42498.748

102.491

98.69399.61099.06299.430

100.999

(84.017)96.05792.209

100.643(75.634)

433

H. KAGAMI, T. ISHIZUKA, AND S. AOKI

786.0-i

884.0-

965.8-

Age

Alb

ian

Ap

tia

n t

o m

idd

le A

lbia

nB

arr

em

ian

Core-section

2 9 2

39-5

48-5

Interval (cm) -

75-7677.5-78.5 -81-8282-8383-8586-87.589 9191.5-92.5 -93-9496.5-97.5 -

100-100.5 -103-103.5 "106.5-107 -37 3838-4043-4545-4750-51.551.5-53.5 -53.5-55.2 -55.2-56.7 -58.5-6062-63.763.7-64.8 -64.8-66.5 -66.5-68.5 -

* 68-69.3*69.3-70.8 -|*70.8 72.5 -

74-75.575.5-7880-81.581.5-8383-83.583.5-84.284.2-85.5 -

* 85.5-87*89.5-92*92-94*94-96*96 96.5 -

Clay minerals « 2 µm

Sm 50(%) | K

?

?

/

•

/

\K

/

;

?)

?

1

> 1? ?

I I 1 l l 1 1 1 l \

OthersQ

5 <%)

!

)

F

OO

OO

OO

OO

OO

O

I O

oo

oo

oo

oo

oo

o.

oo

I 1

1 I

I I

1 1

I I

1 I

I 1

I

Ca

1 I

1 1

1 1

1 1

1 1

1 1

1

I 1

1 1

1 1

1 1

O

1 1

I 1

•

OO

O

1 O

OO

OO

O

O

O

Figure 5. Composition of clay minerals in the clay fractions (<2 µm).(Sm = smectite, K = kaolinite, F = feldspar, I = illite, Q = quartz,Ca = calcite, * = no quantification of clay minerals, — = absent,O = present, and = very abundant.)

titative and qualitative estimations of clay minerals weremade following the method reported by Sudo et al.(1961) and Oinuma (1968).

X-ray diffraction analysis revealed the presence ofsmectite, illite, and kaolinite, but chlorite, which is wide-ly prevalent in oceanic sediments (Griffin et al., 1968;Rateev et al., 1969; and Aoki et al., 1974), was not iden-tified. Nonclay minerals such as quartz, feldspar, andcalcite were also identified in the clay fraction. X-raydiffraction patterns are shown in Figure 6. Quartz isfound in all samples, and feldspar occurs in most sam-ples from Core 29, Section 2 and Core 39, Section 5, butnot in Core 48, Section 5 samples. Calcite occurs only insamples of Core 48, Section 5. Clay mineral composi-tion of samples from the three cores appears in Figure 5.

In Core 29, Section 2 (Albian) smectite is the predom-inant clay mineral. The content of smectite ranges from45 to 79%, with an average of 69%. Change in smectiteconcentration vertically in the cores is variable. The con-tent of illite ranges from 14 to 42%, and is 23% on theaverage. The change in illite abundance vertically is alsovariable. The content of kaolinite ranges from 4 to 12%,with an average of 8%. Kaolinite is widely distributed,

IUT \J

10.0A

1-EG

3.3

3.2

s

2-UT

2-EG

3-UT

3-EG

10 20

C u K α (20)

30

Figure 6. X-ray diffraction patterns of selected samples. (1. Sample534A-29-2, 81-82 cm; 2. Sample 534A-39-5, 37-38 cm; 3. Sample534A-48-5, 80-81.5 cm; UT = untreated sample; and EG = ethyl-ene glycol.)

434

GEOCHEMISTRY AND MINERALOGY OF SELECTED CARBONACEOUS CLAYSTONES

though in small amounts. The content of quartz rangesfrom 2 to 4%, and is 2% on the average.

In Core 39, Section 5 (Aptian-Albian) smectite is byfar the dominant clay mineral. The content of smectiteranges from 92 to 95%; it is 93% on the average. Thecontent of illite ranges from 4 to 7%, with an average of6%. Kaolinite occurs in trace amounts. The averagecontent of quartz is 2%.

Core 48, Section 5 (Barremian) is characterized bycalcareous claystone. Seven of 15 available samples havesize fractions coarser than 2 µm. Because of the dilutioneffect of a great quantity of calcite in these coarse sizefractions, clay minerals in these samples have not beenquantitatively analyzed. Core 48, Section 5 is also char-acterized by the predominance of smectite, which rangesfrom 54 to 100% and is 75% on the average. The con-tent of illite ranges from 0 to 33% and is 21 % on the av-erage. Kaolinite concentration ranges from 0 to 13%,with an average of 4%. The average content of quartz is3%. Calcite is present in all but one sample.

In summary, the clay mineralogy of 41 sediment sam-ples from Hole 534A, ranging in age from the Barre-mian to the Albian, was determined by X-ray diffrac-tion. Smectite is the dominant clay mineral throughoutthe cores, particularly in Core 39, Section 5 (Aptian-Albian), in which it makes up more than 90% of theclay minerals in the samples. The smectite in these sam-ples is not an iron-rich montmorillonite, such as occurswidely in the Pacific deep-sea sediments (Aoki et al.,1974 and 1979), but an aluminous montmorillonite likethat prevailing on the continent. This interpretation isconfirmed by analyses in which the smectite remained asa phase of hydrogen montmorillonite at 12 to 13 Å inthe diffractogram after being heated at 95 °C for 30min. with 6iVHCL. Terrigenous origin of the smectite isalso suggested by the occurrence of associated terrige-nous minerals. On the basis of many analyses of Atlan-tic DSDP samples, Chamley (1979) reported that Creta-ceous smectite is mostly of terrigenous origin.

DISCUSSIONInformation derived from palynology, vitrinite re-

flectance, pyrolysis, and other studies of kerogen andextractable organic matter have shown that nearly all ofthe organic matter in Lower Cretaceous, dark-coloredsediment was derived from terrestrial plants with somecontribution of second-cycle organic carbon, possiblyderived from eroded coal deposits (Habib, 1979; Tissotet al., 1979). Only in Cenomanian dark-colored clay-stones does more hydrogen-rich organic matter of ma-rine derivation make up a significant part of the totalorganic material (Tissot et al., 1979).

There are one or more possible reasons for insignifi-cant preservation of more hydrogen-rich organic matterunder anoxic conditions that might have existed at leastperiodically elsewhere in the North Atlantic in the Bar-remian to middle Albian: (1) overall sea-surface produc-tivity in this part of the North Atlantic was very low; (2)conditions were not anoxic in bottom waters in this por-tion of the North Atlantic, and more reactive organicmatter was oxidized on the seafloor before and during

the early phases of burial; and occurrence of siderite isevidence of consumption of organic matter during re-duction of sulfate; or (3) the high rate of influx of terrig-enous organic matter led to diminished expression of thepelagic marine contribution (Arthur, 1979).

An interesting aspect of the paleoenvironment at Site534 is the apparent subsidence of the seafloor at the sitebelow the CCD level (or rise of the CCD) during the ear-ly Aptian (Shipboard Party, 1981). Sedimentation of pe-lagic carbonate occurred before deposition of the Blake-Bahama Formation. Terrigenous sediment dilution be-ginning from the early Aptian is one important facet ofthe CCD problem. It is also possible that the paucity ofcarbonate in North Atlantic sites is due, in addition todilution and diagenetic factors, to low sea-surface fer-tility, fluctuating salinities, and increased turbidity ow-ing to major deltaic progradation into a somewhat re-stricted basin. By the late Albian to the early Cenomani-an, marine transgression and possibly climatic changeshad resulted in cessation of most deltaic outbuilding andtrapping of terrigenous materials on shelves and in inte-rior basins (Jansa and Wade, 1975).

Deposition of carbonaceous claystones in marine en-vironments occurred dominantly during the early Apti-an to middle Albian at Hole 534A. Almost all organiccarbon in these sequences has very negative terrestrialisotope ratios. Development of coastal vegetation beltswith consequent large outpourings of terrigenous elas-tics and land-plant detritus in a hot, humid, equatorialclimate probably led to formation of the "black shale"facies. There is very good correlation between the δC13

and the organic carbon content of samples, as shown inFigures 3 and 4, which is additional evidence of terrige-nous influx. Clay mineral analysis of rocks of that age(Core 39, Section 5) indicates predominance of an alu-minous montmorillonite-type smectite thought to be ofterrigenous origin. Abundance of smectite associatedwith kaolinite, and the absence of a mixed-layer claymineral of illite-smectite might indicate sufficient hy-drolysis to permit the development of smectite and kao-linite on the continent.

Quartz-silt layers were discovered by the major ele-ment analyses (89.0-91.0 cm in Core 29, Section 2—Fig.3), in which SiO2 content reaches 71.5%. Other thanthese silt layers, Aptian-Albian black shales do notshow any remarkable variations in chemical composi-tion. On the contrary, Barremian black shales show re-markable cyclicity, owing to interbedded carbonates.Black shales have up to 7% quartz in the clay fraction.No feldspar was observed in this core (Core 48, Section5), suggesting that the quartz may be transformed frombiogenic silica. The presence of radiolarian micrites inthin sections of the core samples (Site 534 report) sup-ports this suggestion.

ACKNOWLEDGMENTS

We thank the Japanese IPOD committee for their efforts in joiningLeg 76. We gratefully acknowledge the support of Professor NoriyukiNasu (Ocean Research Institute, University of Tokyo), Professor Ka-oru Oinuma (Toyo University), Dr. Ryo Matsumoto (Geological Insti-tute, University of Tokyo), and Dr. Masato Nohara and Dr. YukihiroMatsuhisa (Geological Survey of Japan) during preparation of the

435

H. KAGAMI, T. ISHIZUKA, AND S. AOKI

manuscript. Professor Noriyuki Nasu, and Professor Sam Boggs, Jr.(Department of Geology, University of Oregon) read the manuscript.We thank Mr. K. Sugi for making sample glass holders. Ms. Chi taniHarayama kindly prepared the manuscript.

REFERENCES

Aoki, S., Koyama, N., and Sudo, T., 1974. An iron-rich montmoril-lonite in a sediment core from the northeastern Pacific. Deep-SeaRes., 21:865-875.

, 1979. Mineralogical and chemical properties in a sedimentcore from the southeastern Pacific. Deep-Sea Res., 26(8A):893-902.

Arthur, M. A., 1979. North Atlantic Cretaceous black shales. In Sibuet,J.-C, Ryan, W. B. R, et al., Init. Repts. DSDP, 47, Pt. 2: Wash-ington (U.S. Govt. Printing Office), 719-752.

Chamley, H., 1979. North Atlantic clay sedimentation and paleoenvi-ronment since the late Jurassic. In Talwani, M., Hay, W., andRyan, W. B. F. (Eds.), Deep Drilling Results in the Atlantic Ocean.Am. Geophys. Union, Third Maurice Ewing Conference Volume,pp. 342-361.

Craig, H., 1957. Isotopic standards for carbon and oxygen and correc-tion factors for mass spectrometric analysis of carbon dioxide.Geochim. Cosmochim. Acta, 12:133-149.

de Graciansky, P. C , Auffret, G. A, Dupeuble, P., Montadert, L.,and Müller, C , 1979. Interpretation of depositional environmentsof the Aptian/Albian black shales on the north margin of the Bayof Biscay. In Montadert, L., Roberts, D. G., et al., Init. Repts.DSDP, 48: Washington (U.S. Govt. Printing Office), 855-875.

Fischer, A. G., and Arthur, M. A., 1977. Secular variations in the pe-lagic realm. In Cook, H. E., and Enos, P. (Eds.), Deep Water Car-bonate Environments, Soc. Econ. Paleontol. Mineral. Spec. Publ.,25:19-50.

Gearing, P., Plucker, F. E., and Parker, P. L., 1977. Organic carbonstable isotope ratios of continental margin sediments. Mar. Chem.,5:251-266.

Griffin, J. J., Windom, H., and Goldberg, E. D., 1968. The distribu-tion of clay minerals in the world ocean. Deep-Sea Res.,15:433-459.

Habib, D., 1979. Origin and distribution of palynofacies in Cretaceouscarbonaceous sediments of the North Atlantic. In Talwani, M.,

Hay, W., and Ryan, W. B. F. (Eds.), Deep Drilling Results in theAtlantic Ocean. Am. Geophys. Union, Third Maurice Ewing Con-ference Volume, pp. 420-437.

Ishizuka, T., 1978. Stable carbon isotope composition of organic ma-terial and carbonaceous in sediments of a swamp and lakes inHonshu Island, Japan. J. Earth Sci. Nagoya Univ., 25:11-21.

Jansa, L. F., and Wade, J. A., 1975. Geology of the continental mar-gin off Nova Scotia and New Foundland. In van der Linden, W. J.M., and Wade, J. A. (Eds.), Offshore Geology of Eastern Canada,Geol. Surv. Can. Pap. 74-30, 2:51-105.

Matsumoto, R., and Urabe, T., 1980. An automatic analysis of ma-jor elements in silicate rocks with X-ray fluorescence spectrometerusing fused disc samples. /. Jpn. Assoc. Mineral. Petrol. Econ.Geol., 75:272-278.

Mèliéres, F., 1979. Mineralogy and geochemistry of selected Albiansediments from the Bay of Biscay, DSDP Leg 48. In Montadert,L., Roberts, D. G, et al., Init. Repts. DSDP, 48: Washington (U.S.Govt. Printing Office), 855-875.

Oinuma, K., 1968. Method of quantitative estimation of clay miner-als in the sediments by Z-ray diffraction analysis. /. Tokyo Univ.,General Education Nat. Sci., 10:1-15.

Rateev, M. A., Gorbunova, Z. N., Lisitzn, A. P., and Nosov, G. L.,1969. The distribution of minerals in the oceans. Sedimentology,13:21-43.

Shipboard Party, Leg 76, 1981. Challenger drills at sites off EastCoast. Geotimes, Sept. 1981:23-25.

Sudo, T., Oinuma, K., and Kobayashi, K., 1961. Mineralogical prob-lems concerning rapid clay mineral analysis of sedimentary rocks.Acta Univ. Carol. Geol. Suppl., 1:189-219.

Tissot, B., Deroo, B., and Hervin, J. P., 1979. Organic matter in Cre-taceous sediments of the North Atlantic. In Talwani, M., Hay, W.,and Ryan, W. B. F. (Eds.) Deep Drilling Results in the AtlanticOcean. Am. Geophys. Union, Third Maurice Ewing ConferenceVolume, pp. 362-374.

Tucholke, B. E., and Vogt, P. R., 1979. Western North Atlantic: sedi-mentary evolution and aspects of tectonic history. In Tucholke, B.E., Vogt, P. R., et al., Init. Repts. DSDP, 43: Washington (U.S.Govt. Printing Office), 791-825.

Date of Initial Receipt: June 25, 1982

436