20. MIDDLE CRETACEOUS BLACK SHALES AT SITE 530 IN THESOUTHEASTERN ANGOLA BASIN1

Dorrik A. V. Stow, Grant Institute of Geology, University of Edinburgh, Edinburgh, Scotland,EH9 3JW, United Kingdom

andWalter E. Dean, U.S. Geological Survey, Denver, Colorado

ABSTRACT

The middle Cretaceous black shale interval at Site 530 is 170 m thick and late Albian to Coniacian in age. The or-ganic-carbon-rich sediments occur as 260 separate beds (average 4 cm, maximum 60 cm thick) and make up less than10% of the recovered section. Associated lithologies are greenish, grayish, and reddish mudstones, marlstones, and rarelimestones. Organic-carbon contents of the black shales average about 5% (maximum 16%), and of the interbeddedsediments, less than 0.5%. Careful study of the sedimentary and biogenic structures and composition and review of pa-leoceanographic conditions in the Angola Basin indicate that a complex interplay of processes controlled black shale ac-cumulation. Relatively low oxygen concentrations in sediment and bottom waters occurred periodically, and conditionslocally may have been anoxic or near anoxic both in the basin and on the continental margin. Pelagic, hemipelagic, andturbiditic depositional processes all operated to varying degrees at different times.

INTRODUCTIONDrilling at many DSDP sites has encountered strata

containing relatively high concentrations of organic mat-ter (>1% total organic carbon). Such organic-carbon-rich strata are particularly common in Early to middleCretaceous sections of the North and South Atlanticoceans (Arthur and Natland, 1979; Weissert, 1981; Deanand Gardner, 1982) and were cored in the lower part ofHole 53OA (lithologic Unit 8, Fig. 1; see Site 530 sum-mary chapter). In this chapter we refer to these organic-carbon-rich sediments as "black shales" although notall are strictly black fissile mudstones.

The widespread occurrence of Cretaceous black shaleshas led to several generalized and simplified models toexplain their origin. Currently the three most popularmodels invoke giant stagnant basins, oceanwide expand-ed oxygen minima, or a rapid supply of organic matteralong basin margins (e.g., Demaison and Moore, 1980).However, detailed analyses of black shales from anyone area or site clearly show that a much more complexinterplay of controls was operative during accumulationof the black-shale facies.

Our primary aim in this chapter is to document thesedimentary characteristics of black shales from Site 530.We also outline briefly what is known from other sourcesabout the mid-Cretaceous Angola Basin and South At-lantic Ocean, and hence deduce some of the factors thatwe believe influence the accumulation of the black shalefacies. Other chapters in this volume deal in more detailwith the organic geochemistry (Meyers, Brassell, andHue, Deroo et al., Brassel, this volume) and inorganicgeochemistry (Dean and Parduhn, Dean, Arthur, andStow, this volume) of these deposits.

Hay, W. W., Sibuet, J.-C, et al., Init. Repts. DSDP, 75: Washington (U.S. Govt.Printing Office).

BLACK SHALE CHARACTERISTICSThe black shale facies at Site 530 makes up just less

than 10% of the 170 m of section above basement fromlate Albian to Coniacian (lithologic Unit 8). It is not athick, uniform, organic-carbon-rich interval but occursas 260 individual beds ranging in thickness from less than1 to 62 cm, and averaging 4.3 cm (Table 1, Fig. 2). Thesebeds are intercalated with reddish, greenish, and grayishmudstones, calcareous mudstones, marlstones, and rarelimestones which form a variable proportion of the sec-tion. Green and then red mudstones that contain up to40% CaCO3 are the dominant lithologies in the 65 m im-mediately above basement (Cores 99-105); black shalesreach a maximum of about 50% of the recovered sectionin the Turonian from 65 to 75 m (Cores 97 and 98); redmudstones with little carbonate are dominant throughthe remaining upper 95 m of Unit 8 (Cores 87-96)(Fig. 2). The overall sedimentation rate throughout isrelatively low—mostly less than 1 cm/1000 yr. (< 10 m/I06 yr.).

Four common associations of black shale and relatedfacies occur repeatedly (Fig. 3). The most common, oc-curring in about 60% of the cases, is a simple alternationof green mudstone or calcareous mudstone with blackshale. The contacts between black and green are mostlybioturbated, and the black shales may be closely spaced,widely spaced, or occur as isolated beds within the greenmudstones (Fig. 3A). The other common associations,occurring in about 30% of the cases, begin with a light-colored limestone or marlstone and grade up throughlight gray calcareous mudstone and dark gray mudstoneto black shale. The black shale is overlain by gray mud-stone (Fig. 3B) or gray muddy sandstone (Fig. 3C) andthen by green mudstone. More rarely (about 5%), a thinblack shale is sandwiched by a thin "halo" of greenwithin a red mudstone sequence (Fig. 3D). In no case is

809

D. A. V. STOW, W. E. DEAN

Africa

BLithology

1000 - ~~il~^r~

-t t ^ -ir> -i A

Basalt Diatom ooze Debris-flow Clay ordeposit claystone

Sand orsandstone

Marl ormarlstone

Dolomite Limestone

Figure 1. A. Map showing location of Site 530. B. Lithostratigraphyof black-shale interval, Unit 8, Albian to Coniacian. (Revisions ofstratigraphic ages are given in Steinmetz et al., this volume.)

a black-shale bed overlain or underlain by red claystone;there is always at least a thin bed of green or gray re-duced sediment separating a black-shale bed from redoxidized sediment.

The black shales are not always laminated through-out, but show a variety of additional sedimentary struc-tures (Fig. 4, Plates 1 and 2). About 80% of the beds areat least partly laminated, and this lamination appears tobe of two basic types. Either (1) there is very fine-scalefissility marked by the parallel alignment and streaking-out of dark-colored organic and clayey material, com-monly draped around small silt lenses, or (2) there aredistinct to indistinct light gray silt laminae within thedarker-colored mudstone. Many of these silt laminaeclearly form part of very thin-bedded (commonly < 1 cm)fine-grained turbidites, and show grading, low-ampli-tude rippling, scoured bases, etc. The gray silty bases ofthe turbidites grade upward into black, massive or fissilemudstone tops. Such turbidites occur within parts of atleast 25% of the black-shale beds.

Bioturbation is common throughout the 170 m of Unit8 and is present to some degree in 40% of the black shalebeds themselves. In the transition from green mudstoneto black shale the burrows commonly become smallerand less abundant upwards. Where burrow types can berecognized, large Zoophycos in the green mudstonesgive way upward to large then small Planolites and fi-nally to very small Chondrites just below the black shale.The same sequence is reversed when passing upwardsfrom black shale to green mudstone, although some thinblack-shale beds may be partly or totally destroyed bysubsequent large-scale burrowing. The thicker beds com-monly contain thin intervals of micro-bioturbation andChondrites-type burrows, often as a slightly grayer layerwithin the black.

The interbedded multicolored facies are, for the mostpart, more thoroughly bioturbated than the black shales,so that only about 5% of the section has clear turbiditelamination and other structures remaining. There areboth significant differences and similarities in composi-tion between the black shales and associated red andgreen mudstone facies (Dean and Parduhn, Stow andMiller, this volume). There is little consistent variationin clay mineralogy between facies. Smectite ranges be-tween 10% and 60%, mixed-layer minerals 2 and 10%,illite 35 and 75%, and chlorite 1 and 15%. Chlorite ismost abundant in the lower 30 m of Unit 8, and kaolin-ite is absent or present in trace amounts only. The siltylayers throughout also have variable mineralogy that isapparently not related to any particular facies. Silt lay-ers commonly are rich in quartz and clays, sometimeswith carbonate cement, and have accessory opaque min-erals, glauconite, feldspar, micas, and organic material.Some silt layers are rich in volcanic glass and other de-bris.

Carbonate is more unequally partitioned, being verylow to absent in the black shales, up to 40% in the redand green mudstones, and 50-90% in the pale red andgreen marlstone and limestone beds. The lower half ofthe unit is more carbonate-rich than the upper. Nanno-fossils and foraminifers also are rare or absent in theblack shales.

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MIDDLE CRETACEOUS BLACK SHALES

Table 1. Black shale statistics, Hole 53OA, Angola Basin.

Core

8687888990919293949596979899

100101102103104105

Totals: 20

Corelength

recovered(cm)

935703448922446574

Disturbed918273656795607501735672995800600790800

13,170*

No. ofblackshalebeds

1125951

125

1219305825199434

16

260

Totalthicknessof black

shale beds(cm)

552252323

2

38153570

31618066794628132279

1105

Averagethickness

(cm)

54.25.02.64.6

3.23.02.93.7

10.53.12.64.25.17.04.35.55.0

4.3b

Standarddeviation

2.52.11.62.1

1.92.04.02.5

12.54.32.55.44.83.34.92.65.0

Percentof total

corethickness

17.65.62.55.2

4.05.55.38.8

52369.0

124.53.22.27.89.8

8.4

Percent orgs

Blackshale

2.05.4; 6.29.7; 2.89.6

10.5

12.31.75.88.0; 7.31.4; 8.01.7

12.04.53.52.22.89.8

5.7

Grayshale

1.31.3

1.8

0.41.95.11.2; 1.77.15.25.3

1.2

ink carbon

Other

0.2; 0.30.2; 0.30.1

0.61.2

0.9; 0.8; 0.2; 0.10.30.20.1; 0.40.20.3; 0.50.20.30.9; 1.30.4

0.4; 0.5; 0.6; 0.20.2; 0.2; 0.2

0.3

a 17,200 cm drilledb 128 beds = 1-2 cm; 34 beds > 10 cm; 6 beds >20 cm; Max. thickness 62 cm

Geochemical analyses show marked concentrationsof the trace elements As, Cd, Co, Cr, Cu, Mo, Ni, Pb,V, and Zn in the black shales relative to interbedded redand green mudstones. Concentrations of Ba and Fe gen-erally are higher in the red than in green mudstones orblack shales, whereas concentrations of Mn are higherin the green than red mudstones, and much lower inblack shales. Chemical mobilization during diagenesismay have further concentrated the most mobile ele-ments (Ba, Co, Ni, Fe) in the black shales towards thetop of the unit (Dean and Parduhn, this volume).

Organic carbon is high and variable (average 5.4%,maximum 16%) in the black shales, moderately high inthe grayish-black shales (average 1-2%), and low in theother lithologies (average 0.3%, Meyers, Brassell, andHue, this volume). The organic matter is dominantlymarine with minor terrestrial input (Deroo et al., Katz,and Meyers, Brassell, and Hue, all this volume), and itappears that a low terrestrial supply may have been con-stant through all the facies, whereas increased marinesupply characterized the black shales (see Brassell, thisvolume).

MID-CRETACEOUS ANGOLA BASINFrom previous work we know something about the

Oceanographic and sedimentary characteristics of theSouth Atlantic Ocean and Angola Basin during the mid-dle Cretaceous (LePichon et al., 1978; van Andel et al.,1977; Bolli, Ryan et al., 1978). The Angola Basin wasrelatively narrow (about 300-400 km wide), restrictedby the Walvis Ridge-Rio Grande Rise to the south andby the closure of Africa against South America to thenorth, and was a maximum of 3.5 km deep. The climatewas warm, surface and bottom waters were both warm(about 30° and 15°C, respectively, Barron et al., thisvolume) and saline (Brass et al., 1982). Sea levels world-wide were high, leading to broad shallow continentalshelves, flooded estuaries, and increased evaporation es-pecially at low latitudes.

Normal pelagic, hemipelagic, and turbiditic sedimen-tation processes all were operating in the basin, but witha reduced supply of terrestrial material and low accumu-lation rates overall. Ten other DSDP sites in the SouthAtlantic have recovered mid-Cretaceous strata (Barker,Dalziel et al., 1977; Supko, Perch-Nielsen et al., 1977;Bolli, Ryan et al., 1978; Ludwig and Krasheninnkov,1980). To the south, Sites 327, 328, 330, and 511 on theFalkland Plateau recovered mostly open-marine, oxy-genated sediments, but with older Cretaceous to Juras-sic organic-carbon-rich strata of variable lithology atSites 330 and 511. In the Cape Basin, Site 361 recoveredreddish mudstones and dark gray shales with organiccarbon contents rarely as much as 1%. Sites 356 and 357on the Sào Paulo Plateau and Rio Grande Rise are com-posed of interbedded marlstones, chalks, and limestones.On Walvis Ridge, Site 363 showed five thin black-shalehorizons within a sequence of marlstone and mudstone.The sequence at Site 364 on the Angola Basin slope isvery similar to that at Site 530 with maximum blackshale development that is approximately synchronous.Site 365 on the Angola Basin rise had indications of thinblack shale layers in a displaced "slump" unit.

BLACK SHALE MODELAny model for black shale deposition at Site 530 needs

to explain the 260 separate organic-carbon-rich layersthat form less than 10% of a dominantly organic-carbon-poor bioturbated, oxidized facies, that occur with irreg-ular periodicity (9,600 to 393,000 yr.), and that showvariable structures, thicknesses, and organic-carbon con-tents. The black shales do not appear to have been de-posited by any single process but show evidence for bothturbiditic and pelagic deposition, so that the black shale"events" were superimposed on normal basinal sedi-mentation.

In order to explain this complexity and variability ofmid-Cretaceous organic-carbon-rich facies at Site 530,

811

0

1

2-

— 3

ε

εo5 5

87

!

6H

8 -

9

Coniacian-Santonian

89 90 91

-Turonian-

95 96 97 98

-Cenomanian

99

Red Green Black CaCOg Basalt

0 10 20

ian late Albian

100 101 102

m

Wk

i

2522

103 104 105

n

1

- 0

- 1

- 2

4 &•oEo

° o

- 7

8

9

Figure 2. Distribution of black shales and associated facies, Cores 87 to 105, Hole 53OA.

MIDDLE CRETACEOUS BLACK SHALES

Black shale

Gray mudstoπe

Green mudstone

Red mudstone

Pale green marlstone

Greenish-white limestone

Muddy sandstone

Pyrite

B

LJLI j I

1 1 1 !I I \\

I 30

2 0

10

Figure 3. Typical black-shale sequences from Site 530: A. Occursabout 60% of the time. B. and C. together, about 30%. D. lessthan 5%.

we need to consider a range of factors that were prob-ably operating in the Angola Basin at this period.

1) Factors acting to reduce seawater oxygen content(see discussions by Arthur and Natland, 1979; Demaisonand Moore, 1980; Tissot et al., 1980): (a) The relativelysmall, silled basin would restrict circulation and tend toproduce density stratification of the water column; (b)The high temperature and salinity of the bottom waterwould result in lower dissolved oxygen content; (c) Thehigh sea levels would result in transgression of low-lying

land areas and possibly increase the supply of terrestri-al plant material seawards, and also stimulate marineplankton productivity over broad shelves, both factorstending to produce poorly oxygenated shelf waters andan increased supply of organic matter; (d) Increasedevaporation over wide shelves would produce saline wa-ters (also poor in oxygen) that would tend to spreaddense, oxygen-depleted waters to the basin floor, in-creasing stratification.

2) Factors acting to increase seawater oxygen content(Brass et al., 1982): (a) The normal wind-forced advec-tion of water masses; (b) Normal geothermal heating ofthe basin floor; (c) Circulation and overturn via salinewaters, turbidity currents, and spillover from the CapeBasin to the south.

3) We are less certain about factors acting to increasethe supply of organic material to the sediments (see dis-cussion by Schlanger and Jenkyns, 1976; Cornford, 1979;Welte et al., 1979): (a) Surface productivity in the openocean or over shelves probably was subject to temporalvariations; (b) Similarly, the supply of terrestrial or-ganic matter would be temporally and spatially variable,dependent on climate, rivers, etc,; (c) Turbidity currentswould serve to transport both organic-carbon-rich andorganic-carbon-poor sediments from the shelf and slopeto the basin; (d) The rate of sedimentation of both or-ganics and inorganics would affect supply, burial, andpreservation of organic carbon—in the Angola Basin,sedimentation rates were low on average, but clearlyvariable.

4) We are also uncertain about temporal and spatialvariability of factors and conditions within the AngolaBasin during the mid-Cretaceous (see regional synthesesby Bolli, Ryan et al., 1978; LePichon et al., 1978): (a)Some climatic cyclicity probably occurred and wouldhave affected a number of the factors outlined above;(b) Restricted shelf basins or coastal lagoons may haveprovided localized temporary sinks for organic matter;(c) Redeposition, largely by turbidity-currents, wouldalso be localized.

It appears, then, that the bottom waters and sedimentsat Site 530 in the Angola Basin were sufficiently oxygen-ated during most of the middle Cretaceous to supportan active infauna and to remove most of the organic car-bon supplied so that red, oxidized sediment accumulat-ed. However, for much of the time, the bottom waterswere relatively depleted in oxygen (see factors 1 a-dabove) and/or the rate of supply of organic matter wasgreater so that the underlying sediments were reducedand green sediments were produced. During these peri-ods of oxygen-deficient water and/or sediment, severalof the factors outlined above periodically combined toproduce a diagenetic environment within the sediment(and probably for some distance above) that was (just)anoxic. This allowed preservation of organic carbonand accumulation of gray or black mud within a greenmud sequence. With subsequent diagenesis and compac-tion many of these developed the fissility and color oftrue "black shales." Although such anoxic or near-an-oxic conditions occurred periodically throughout the en-tire period of deposition of lithologic Unit 8 (late Albian

813

D. A. V. STOW, W. E. DEAN

Black shale

Gray mudstone

Green mudstone

Silt laminae

Fissile lamination

^ S Irregular lamination

~Z ~•~^. Chondrites burrows

Planolites burrows

Zoophycos burrows

Irregular burrows

Microturbidite

Figure 4. Details of sedimentary structures that are typical of black shales and adjacent layers from Site530.

to Coniacian), the depositional system within the An-gola Basin, and probably the entire mid-Cretaceous At-lantic Ocean, was more delicately poised with respect tosupply of organic matter and oxygen concentrations inwater masses during the early Turonian, so that moreorganic matter accumulated and was preserved, as indi-cated by the increased frequency of black-shale beds inCores 97 and 98 (Fig. 2).

Even with detailed inspection of the black shales atSite 530, it is not possible to isolate the factors that weremost influential in their formation and diagenesis. Wesuspect different factors may have had more weight atdifferent times and for different beds. In Figure 5A wesummarize the range of conditions that may have con-tributed to anoxic sedimentation. At times, turbiditycurrents transported organic-carbon-rich sediments fromthe shelf and slope to the basin; at other times, hemipe-lagic material rich in organic matter settled through ananoxic or near-anoxic water column and was preservedin anoxic sediments. In Figure 5B the delicate balancehas been tipped (very slightly) in favor of oxic sedimen-tation, through bioturbation and accumulation of greenor red muds, depending upon dissolved oxygen concen-trations in sediments and/or bottom waters. Finally,Figure 5C shows the importance of lateral variability onsediment types and processes within the Angola Basinand surrounding areas at any one time during the mid-dle Cretaceous. Preliminary comparison with other Cre-taceous black shales in the North and South Atlantic

suggest that this more variable and complex model ofsedimentation is more realistic than the several simpli-fied models that have been proposed previously.

ACKNOWLEDGMENTSDAVS acknowledges personal support from the Natural Environ-

ment Research Council, U.K. He would also like to thank the secre-tarial and technical staff at the Grant Institute of Geology, and AlanE. Kemp and Chris Cornford for reviewing the manuscript.

REFERENCES

Arthur, M. A., and Natland, J. H., 1979. Carbonaceous sediments inthe North and South Atlantic: The role of salinity in stable stratifi-cation of Early Cretaceous basins. In Talwani, M., Hay, W. W.,and Ryan, W. B. F. (Eds.), Deep Drilling Results in the AtlanticOcean: Continental Margins and Paleoenvironment: Washington(Am. Geophys. Union, Maurice Ewing Series), 3:375-401.

Barker, P. F., Dalziel, I. W. D., et al., 1977. Init. Repts. DSDP, 36:Washington (U.S. Govt. Printing Office).

Bolli, H. M., Ryan, W. B. F., et al., 1978. Init. Repts. DSDP, 40:Washington (U.S. Govt. Printing Office).

Brass, G. W., Southam, J. R., and Peterson, W. H., 1982. Warm sa-line bottom water in the ancient ocean. Nature, 296:620-623.

Cornford, C , 1979. Organic deposition at a continental rise: Organicgeochemical interpretation and synthesis at DSDP Site 397, east-ern North Atlantic. In von Rad, U., Ryan, W. B. F., et al., Init.Repts. DSDP, 47, Pt. 1: Washington (U.S. Govt. Printing Office),503-510.

Dean, W. E., and Gardner, J. V., 1982. Origin and geochemistry ofredox cycles of Jurassic to Eocene age, Cape Verde Basin (DSDPSite 367), continental margin of northwest Africa. In Schlanger, S.O., and Cita, M. B. (Eds.), Nature and Origin of Cretaceous Or-ganic Carbon-Rich Fades: London (Academic Press), pp. 55-78.

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MIDDLE CRETACEOUS BLACK SHALES

Demaison, G. J., and Moore, G. T., 1980. Anoxic environments and oilsource bed genesis. Am. Assoc. Petrol. Geol. Bull., 64:1179-1209.

LePichon, X., Melguen, M., and Sibuet, J.-C, 1978. A schematicmodel of the evolution of the South Atlantic. In Charnock, H.,and Deacon G. (Eds.), Advances in Oceanography: New York(Plenum), pp. 1-48.

Ludwig, W. J., and Krasheninnikov, V., 1980. Tertiary and Cretaceouspaleoenvironments in the southwest Atlantic Ocean: Preliminaryresults of DSDP Leg 71. Geol. Soc. Am. Bull., 91(l):655-664.

Schlanger, S. O., and Jenkyns, H. C , 1976. Cretaceous oceanic anox-ic events: Causes and consequences. Geol. Mijnbouw, 55:179-184.

Supko, P. R., Perch-Nielsen, K., et al., 1977. Init. Repts. DSDP, 39:Washington (U.S. Govt. Printing Office).

Tissot, B., Demaison, G. J., and Masson, P. 1980. Paleoenvironmentand petroleum potential of the mid-Cretaceous black shales in theAtlantic basins. Am. Assoc. Petrol. Geol. Bull., 64:2051-2063.

van Andel, Tj. H., Thiede, J., Slater, J. G., and Hay, W. W., 1977.Depositional history of the South Atlantic Ocean during the last125 million years. J. Geol., 85:651-697.

Weissert, H., 1981. The environment of deposition of black shales inthe Early Cretaceous: An ongoing controversy. Soc. Econ. Pale-ontol. Mineral., Spec. Publ., 32:547-560.

Welte, D. H., Cornford, C , and Rullkötter, J., 1979. Hydrocarbonsource rocks in deep-sea sediments. Proc. 11th Ann. OffshoreTech. Conf. (Houston), 1:457-464.

Date of Initial Receipt: October 12, 1982

Increased evaporation over broad shelf

RestrictedSite 5 3 0 Zone of b a s j n Lagoon Marsh

• local upwelling

^ _ _ Bottom-water stagnation .1

o > : Estuary '•''/•'

Figure 5. Schematic cross-sections and map through part of middle Cretaceous Angola Basin showing: A.Range of conditions favoring locally anoxic sedimentation. B. Range of conditions favoring oxic sedi-mentation. C. Regional variability within basin. Stippled areas indicate areas where organic matter ispreserved in the sediment.

815

D. A. V. STOW, W. E. DEAN

— 80

— 75

—I 70

Plate 1. Details of black shale layers from Site 530. 1. Section 105-4. 2. Section 96-4. 3. Section 96-5. Shaded box at side indicates black shale,oblique hachure is gray shale, and the unshaded parts are greenish mudstones-marlstones. Arrows show locations of thin turbidites.

816

MIDDLE CRETACEOUS BLACK SHALES

Plate 2. Details of black shale layers from Site 530. 1. Section 98-3 and 2. Section 100-1. Shaded box represents black shale, stipple representsgray muddy sandstone. Zoophycos burrow and pyritic siltstone shown in 1; locations of two thin turbidite layers are shown by arrows in 2.

817