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The paleoenvironment of anaerobic sediments in the Late Mesozoic South Atlantic Ocean
Earth and Planetary Science Letters, 33 (1977) 301-309 ©Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
301
[11
THE PALEOENVIRONMENT OF ANAEROBIC SEDIMENTS IN THE LATE MESOZOIC SOUTH ATLANTIC OCEAN J()RN THIEDE
School of Oceanography, Oregon State University, Corvallis, Ore. 97331 (USA) and TJEERD H. VAN ANDEL
Department of Geology, School o f Earth Sciences, Stanford University, Stanford, Calif. 94305 {USA) Received September 3, 1976 Revised version received November 1, 1976
Laminated dark calcareous oozes/chalks/limestones as well as clayey and marly mudstones/claystones with high organic carbon contents were deposited during Jurassic, Early and Late Cretaceous times in the Atlantic, Pacific and Indian Oceans. In the South Atlantic, this sedimentary facies has been encountered only in drill sites close to the American and African continental margins. The reconstructed paleogeography of the South Atlantic, the paleodepth of deposition, and the fossil content of these sediments make clear that the Late Cretaceous anaerobic paleoenvironment developed under the influence of an oceanic mid-water oxygen minimum at moderate water depths (500-2500 m), because oxygenated sediments have been observed in the deep basin elsewhere. Whether the Early Cretaceous and Jurassic anaerobic sediments were deposited in an euxinic basin or under an oceanic mid-water oxygen minimum remains an open question because the few drill sites have not sampled the deepest part of the Early Cretaceous and (?) Jurassic South Atlantic basins.
1. Introduction Anaerobic sediments indicative of a reducing depositional environment are not extensive in the open ocean. Today they are restricted either to isolated basins whose bottom waters are not, or are only very slowly, renewed, or to substrates beneath the oceanic mid-water oxygen m i n i m u m developed under highly fertile and productive surface-water masses along continental margins [I ]. This fact has been well known for several decades [2], but these two separate depositional environments are hard to distinguish because laminated anaerobic sediments which have a high or. ganic carbon content occur in both [3]. Two modern environments which can probably be assumed to represent analogs of any fossil record for a depositional environment of laminated marine sediments rich in organic matter are shown in Fig. 1. The Black Sea represents the modern present example
of an anoxic basin (Fig. 1). A marked pycnocline at 1 5 0 - 2 0 0 m water depth separates the well-oxygenated, low-salinity and warm surface water from the anoxic deep-water masses which are of higher salinity and contain large quantities of hydrogen sulfide [4,5]. Rich zoobenthic faunas populate the region shallower than 1 5 0 - 1 7 5 m [6], but are absent from the fine-grained laminated clayey and marly muds of the anaerobic depths [7] because of high concentrations of dissolved hydrogen sulfide. Organic carbon contents of the sediments range from <1% in the shallow-water regions to >5% in the basin deep [8]. Sediments deposited under anaerobic conditions also occur in the open ocean where a oxygen minimum impinges on the bottom under highly fertile and productive surface water. The reduction of the dissolved oxygen content mainly results from the oxidation of organic matter settling through the water column. A typical case of this depositional environment occurs in
302 0
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Fig. 1. Schematic representation of the aerobic/anaerobic water masses and their impact upon the distribution of sediments in the Black Sea and in the northern Indian Ocean. Data for the Black Sea have been taken from Sverdrup et al. [4] and Degens and Ross [45]; for the northern Indian Ocean from Wyrtki [9] and van Stackelberg [10].
the northwestern Indian Ocean where the dissolved oxygen content approaches zero between water depths ranging from 150-200 to 800 m [9]. Where this midwater oxygen minimum intersects the continental margin (open upper continental slope), laminated finegrained muds with high organic carbon contents cover the sea floor (Fig. 1). Below the oxygen minimum zone, a layer of oxygenated sediment, increasing in thickness with increasing depth and dissolved oxygen content in the bottom waters, was observed along the continental margin off India and Pakistan [10]. Though the dissolved oxygen values approach zero in the center of the oxygen minimum layer, the sediments which appear to be anaerobic yield indigenous benthic foraminiferal faunas [11]. Similar observations have also been made along the western North and South American continental margin where a well-developed midwater oxygen minimum occurs at present [ 12].
2. Anoxic sediments in the Late Mesozoic South Atlantic Ocean Neither of these two environmental settings produces regionally widespread anoxic sedimentary facies. Thus, it was unexpected when the Deep Sea Drilling
Project recovered Late Mesozoic sediments from drill sites in the Indian and the Atlantic, as well as from the Pacific Ocean which had obviously been deposited under anoxic conditions and which therefore had many of their primary sedimentary structures preserved. Most of the sites in the Atlantic and Indian Oceans are located close to continental margins, but Late Mesozoic anoxic sediments in the Pacific Ocean occur on the flanks of seamount chains (or plateaus) in the open ocean [ 13,14], where well-oxidized paleoenvironments of similar age existed in deep water [15]. This coincidence points to the existence of a strong midwater oxygen minimum (at approximately 500-1500 m water depth) in the Pacific Ocean during mid-Cretaceous [16]. However, the Atlantic and Indian Oceans were no more than narrow, young basins during the Late Mesozoic and might well have turned totally stagnant as the Black Sea and the Cariaco Trench are today, or as the eastern Mediterranean has been during certain time spans of the Late Pleistocene. In this paper we discuss the paleoenvironment of laminated, dark mudstones, chalks and limestones of Early and Late Cretaceous age that have been penetrated in South and Central Atlantic drill sites during DSDP Legs 14, 36, 39 and 40 [17-20]. We shall therefore present a reconstruction of the paleogeography of the
303 South Atlantic Ocean during the Early and Late Cretaceous (Fig. 2), reconstruct the paleodepth of deposition of all occurrences of anaerobic sediments in the South Atlantic Ocean during this time span (Fig. 3), and try to compare their sedimentary facies with modern analogs (Fig. 4), such as those found in the Gulf of California, a young and narrow ocean basin [21 ], which could have much in common with the narrow and young early Atlantic Ocean. Due to the limited nature of our data we are not going to discuss reasons why the anaerobic sediments occur simultaneously during relatively short, but well-defined, periods in widely distant regions of the Late Mesozoic oceans; also we do not consider the possibility that the organic carbon contents of these sediments might be entirely detrital in origin.
3. The paleogeography and paleobathymetry of the Late Mesozoic South Atlantic Ocean We have adopted a paleogeographic and paleobathymetric reconstruction of the South Atlantic [22] based on plate rotations and sea-floor subsidence with age [23]. Fig. 2 shows maps for 80 and 110 m.y.B.P. representing approximately the anaerobic sediment ranges of 7 5 - 9 5 and 100-110 m.y.B.P., respectively. In addition, we note that the drill sites on the Falkland Plateau penetrated anoxic sediments of Jurassic age [18]. During Aptian/Albian times the South Atlantic was a narrow, elongated ocean consisting of a number of basins some of which may have been more than 3 km, but probably less than 4 km deep, and separated from each other by other ridges rising close to or even above sea level [24]. The connection of the Albian/Aptian South Atlantic to the Southern Ocean was broad and relatively shallow except for a narrow, deep channel just south of Africa; the North and South Atlantic Oceans may have been connected through a narrow basin which was probably less than 1 - 2 km deep and which did not permit an exchange of the deep-water masses. Aptian/Albian sediments have been encountered only in six drill sites; five drill sites in the South Atlantic [18,20] penetrated sapropels [25-27] and black mudstones while extensively burrowed and mottled calcareous mudstones were encountered at site 144 in the southern North Atlantic [17]. Most sites
with Aptian/Albian sediments in the South Atlantic are situated close to the continental margin and only site 361 sampled a location close to the former deepest portion of the Early Cretaceous Cape Basin. Twenty million years later the South Atlantic Ocean (Fig. 2) had widened to about twice its former size; the main basins had deepened to more than 4 km water depth and the ocean was connected to the North Atlantic as well as to the southern oceans through wide gaps allowing for the exchange of deep as well as surface water. Sites where late Cretaceous sediments have been found are scattered throughout the South Atlantic; they are located on the continental margins, the aseismic ridges and in the deep parts of the former ocean basin. Laminated very dark or even black anaerobic sediments have been found on the Sao Paulo Plateau and on the Rio Grande Rise [28], and along the southwest African continental margin [29]. Sediments of Late Cretaceous age deposited under oxygenated conditions have been recovered from the Brazilian Basin during DSDP Legs 4 [30] and 39 [28], and from the Argentinian Basin during DSDP Legs 36 [31] and 39 [28].
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i Fig. 2. Distribution of anoxic sediments in the CampanianCenomanian and Aptian-Albian deposits of the South Atlantic. Paleogeographyand paleobathymetry after Baumgartner and van Andel [38] and Sclater et al. [22] for 80 and 110 m.y.B.P., respectively. Numbers are DSDP drill site identifications; the bathymetric contours are in kilometers. Ages on maps represent time spans for which sediments are shown.
304 It is interesting to note that the sapropelic sediments seem to have a wider distribution in the drill sites off the eastern rather than off the western South Atlantic continental margins though this result might be biased by the location of the drill sites. Most highly fertile and productive regions of the major ocean basins today as well as probably in former times are part of the eastern boundary currents with their upwelling systems. Accordingly, the oceanic mid-water oxygen minima are developed most strongly in the eastern part of the oceans while their intensity decreases westwards [32].
363
144 sapropelic and black shale "
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4. The paleodepth of deposition of anoxic sediments To reconstruct the paleodepth of deposition of anoxic sediments from the South Atlantic (Fig. 3) we have used the method of Berger [33] for those drill sites where the necessary data (sediment ages and thicknesses, age and depth of basaltic basement) were available. The subsidence of aseismic ridges seems to follow a curve similar to the subsidence of oceanic basaltic basement [34], though these structural highs rise to several thousand meters above the surrounding normal sea floor (the subsidence curve therefore has to be raised vertically by this difference). The occurrences o f shallow-water sedimentary components in several drill sites on the flank of Rio Grande Rise [24] and of Walvis Ridge [28] allow us to monitor the subsidence of these aseismic ridges with time in considerable detail. The subsidence curve of sites where we have had to extrapolate the age and depth of the basaltic base. ment are evidently much less reliable. The subsidence of sites on continental basement (sites 327 and 330) cannot be reconstructed at present, since we do not know enough about the vertical tectonic movements of stable continental margins after the initial rifting [35]. However, the occurrences of macrofossils (lnoceramus sp. [31,36]) seem to indicate that these sites have also subsided a few thousand meters since Late Mesozoic times. Four alternatives exist for the subsidence of site 364 which terminated just above 3 km thick evaporite sequence on basement of about 115 m.y.B.P. (anomaly M-4) judging from geophysical data [29]. If the salt is autochthonous, it must have been deposited in a water depth of about 2.5 km or in a > 2 km deep subaerial depression [37]. If the salt had been injected
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Fig. 3. Variation of lithology of South Atlantic drill sites with paleodepth and age. Heavy black lines: anoxic sediments interbedded in sequence; thin lines: sediments poor in organic matter; arrow heads: basaltic basement. Two subsidence curves are given for site 364: upper one assumes salt flow during Turonian-Cenomanian, lower one during Oligocene [381. Subsidence for sites 327 and 330 is unknown; the symbols are located at present depth, although the macrofossils seem to indicate that this site was at considerably shallower water depth during Late Mesozoic times [36]. Lithology directly from core descriptions. into the area later by salt tectonics analogous to that of the Gulf of Mexico, two periods can be considered. There is evidence for considerable salt deformation in the Oligocene [38], but migration at this time yields an early subsidence history not in accord with the paleodepth distribution of calcareous sediments [39].
305 Hence, we have assumed an early salt migration about 9 0 - 1 0 0 m.y. ago, which yields the upper of the two subsidence curves in Fig. 3. The reconstructed depths of deposition o f the Late Mesozoic anoxic sediments of the South Atlantic occupy a wide range between 500 and 3000 m (Fig. 3). The Early Cretaceous (Albian/Aptian) sapropelic claystones of sites 327 and 330 on the Falkland Plateau are now in water depths of 2500 to 3000 m but were certainly much shallower at the time of deposition. Site 361 shows that Early Cretaceous anoxic sediments also occurred on the western (mid-ocean ridge) flank of the Cape Basin (Fig. 2) in the same water depth. However, since we do not have samples from the deepest part of this basin nor of the Argentine Basin, it is impossible to ascertain whether the basin centers were also anoxic or if the sapropelic deposits in shallower water belonged to an oxygen minimum like those of the Late Cretaceous. The Late Cretaceous sediments seem to have been deposited at considerably shallower water depths, and we possess sedimentary records (Figs. 2 and 3) which monitor the oxygenated deep-water paleoenvironment during the deposition of the anoxic laminated marls and claystones.
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The organic carbon contents of these sediments taken from the site reports, have been plotted against the projected water depth (Fig. 4). The anaerobic sediments have organic carbon contents of up to more than 10%. Otherwise, the sediments which have been deposited in the Late Mesozoic South Atlantic in an anaerobic paleoenvironment, span a wide range of different lithologies (Table 1). The dominant sedimentary facies in the near-continent sites (especially on the southwest African side) are dark olive gray to black sandy-silty claystones or black shales, while sites further offshore or into the flanks of structural highs contain marly chalks and limestones. Common to all occurrences, however, are the dark, if not black, hues and frequent laminations in the millimeter scale. At many sites, the uppermost limit of the anoxic facies is not a sharp one but laminated and intensively burrowed portions of the sediment column alternate in a cyclic manner, the thickness and the frequency of the laminated layers decreasing upwards while more and
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Fig. 4. Variation of the organic carbon content of sediments with paleodepth for the 70-95 and the 95-110 m.y.B.P. periods in the South Atlantic (data from DSDP site reports). Dashed line is an envelope to facilitate comparison with the inserted curve representing the relationship between organic carbon contents of the surface sediments and water depth of deposition (Holocene) in the Gulf of California [21 ]; the laminated, diatomaceous continental slope sediments in the Gulf of California have been described in detail [40] and can be related to the oceanic mid-water oxygen minimum [41 ] in this region.
more of the sediment column is burrowed. Simultaneously, the sediment colors change to light brownish and reddish gray. The increased burrowing as well as the color changes seem to be indicative o f better oxygenated bottom-water masses and of a paleoenvironment more favorable to benthic life than the anoxic sediments. The cyclic nature of these frequent changes (for example, well developed in sites 356 and 357) represent an interesting monitoring device for the
TABLE 1 Occurrences of anoxic sediments in the Late Mesozoic South Atlantic Ocean (lithology and ages from shipboard core descriptions) DSDP site nos. (water depths)
Type of anoxic sediment
Age
Nature of underlying sediment
Nature of overlying sediment
Remarks and sources of information
144 Top Demerara Rise (2957 m)
zeolitic black and olive marl, high Corg contents
Late Aptian to Early Turonian
Late Albian to Early Aptian olive gray marls and shelly limestone
zeolitic graygreenish Late Campanian to Early Maestrichtian marls
benthic foraminifers more abundant than planktonic ones [ 17 ]
327 Falkland Plateau (2411 m)
Nanno-rich olive gray to black sapropelic clay-
Aptian?
olive gray Neocomian claystones
Early to Middle Albian gray nanno claystones
1311
Large molluscs: l n o c e r a m u s and belemnite
stones
330 Falkland Plateau (2636 m)
olive black, laminated sapropelic clay/claystones
?Middle-Late Jurassic (Callovian, Oxfordian) to Neocomian
Middle(?)-Late Jurassic silts/ clays with benthic and nektonic molluscs, underlain by Precambrain gneisses and granites
Early to Middle Albian light brown zeoliterich nanno clays
356 Sao Paulo Plateau (3203 m)
laminated dark, marly dolomitic limestone
Late Albian
not penetrated
Late Turonian calcareous mudstones
dark, calcareous mudstones
Late Turonian
Late Albian marly dark gray, moddolomitic limeerately burrowed stones Coniacian calcareous mudstones with I n o c e r a m u s
357 Rio Grande Rise (2086 m) 361 SW African continental margin Cape Basin (4549 m)
siliceous marly limestone
Santonian
not penetrated
Santonian marly limestones
frequent I n o c e r a m u s [281
laminated black shales and sandy mudstones
Early Aptian (?) to Albian
not penetrated
dark gray shales alternating with grayish red claystones (Cretaceous?)
[29]
363 Walvis Ridge (2247 m)
yellowish gray laminated limestones to carbonaceous mudstones
Late Aptian to Albian, Turonian to Santonian
Early Cretaceous limestones with calcarenite layers
Campanian gray to grayish brown marly nannofossil chalk
uppermost part cyclically intensively burrowed [291
364 SW African continental margin Angola Basin (2449 m)
alternating burrowed dolomitic limestones and black laminated shales
Late Aptian to Early Albian
heavily burrowed dolomites of unknown age
?Middle Albian olive to brownish gray limestones
uppermost part cyclically intensively burrowed [291
black sapropelic shales
Late(?) Albian to ConiacianSantonian
Late Albian greenish gray to reddish brown burrowed nannofossil chalk
Coniacian-Santonian greenish gray to grayish brown nannofossil chalk to claystone
[291
-
-
[31]
[28]
307 oxygen contents of the oceanic bottom water masses. Benthic fossils from the Cretaceous anoxic sediment are only rarely mentioned. However, benthic foraminifers and large molluscs (bivalves, cephalopods) are mentioned here and there in the core descriptions (see Table 1). Large fragments of the thick prismatic layer of Inoceramus [36] which seem to have inhabited soft mud bottoms of the outer shelf and upper continental slope as epibenthos are a conspicuous faunal element. At present, we have no means of evaluating whether these fossils might be displaced from shallower water depths.
6. Similarities and dissimilarities to analogous modem sediments The anoxic sediments of the Late Mesozoic South Atlantic Ocean resemble in many ways deposits which are found today in regions of well-developed oceanic mid-water oxygen minima, such as in the Gulf of California, off Peru, off southwest Africa, and off the Indian subcontinent [10,21,40-43]. The similarities consist of the generally fine grain size of the sediments, the primary sedimentary structures (laminations), the colors of the sediments (dark olive/dark greenish gray to black), the presence of indigenous benthic faunas (foraminifers [11,12]), the high organic contents of the sediments (Fig. 4), and of a depositional environment in intermediate to shallow water depth along the continental margins (Figs. 2 and 3). The dissimilarities consist of compositional and faunal/floral differences; modern oceanic anaerobic sediments are usually rich in diatoms which virtuelly did not exist in the Late Mesozoic (though Late Cretaceous diatomites have been observed in fossil upper continental slope sediments in California [44]), whereas the Late Mesozoic anaerobic sediments contain remains of large epibenthic bivalves (lnoceramus) which are now extinct. It also seems that anoxic depositional environments, which are restricted in today's ocean, were much more widespread in the South Atlantic during the Late Mesozoic. This wider distribution might well result from the fact that the South and North Atlantic Oceans were linked only through a relatively narrow and possibly intermittent connection during the Early Cretaceous (Fig. 2) and that the whole Atlantic Ocean was closed to the north throughout the Late Mesozoic and
Early Cenozoic thus allowing the existence of a midwater oxygen minimum as strong as known today from the northern Indian Ocean [9]. Consequently, all evidence obtained from the anoxic sediments of the Late Mesozoic South Atlantic points to a depositional paleoenvironment such as we observe under today's oceanic mid-water oxygen minima. Although this evidence satisfactorily explains the Late Cretaceous occurrences of anoxic sediments, the euxinic model cannot be totally discarded when discussing the Early Cretaceous and Jurassic anoxic sediments. The main deficiency of the older data is the lack of drill sites in the center of the former ocean basins that would prove which of the two models (Fig. 1) for the deposition of anoxic sediments applies to the Early Cretaceous South Atlantic and to the Jurassic deposits of the Falkland Plateau.
7. Conclusions (1) Laminated dark calcareous oozes/chalks/limestones and marly mudstones with high contents of organic carbon were deposited in the South Atlantic during Jurassic, Early and Late Cretaceous times. (2) All drill sites where the older Jurassic and Aptian/Albian anoxic sediments have been penetrated in the South Atlantic Ocean, occupied originally positions close to the southwest African continental margin (Fig. 2). However, this result may be biased due to the scarce sample coverage. (3) Sites with Late Cretaceous anoxic sediments have been drilled along the southwest African continental margin as well as along the South American continental margin and on Rio Grande Rise. In contrast, an oxygenated facies was deposited in the Brasil and Argentine Basins as well as on the Falkland Plateau (Fig. 2) during this same period. (4) The paleodepths of deposition of Early Cretaceous anoxic sediments range from 2500 to 3000 m (probably to much shallower water depths at the continent near locations such as Falkland Plateau drill sites); the paleodepth of deposition of the Late Cretaceous ones range from <500 m to approximately 2500 m (Fig. 3). (5) Sediments which were deposited in deep water during Late Cretaceous time simultaneously with the anoxic sediments prove that this anaerobic paleoen-
308 v i r o n m e n t d e v e l o p e d u n d e r t h e i n f l u e n c e o f a n oceanic mid-water oxygen minimum.
Acknowledgements J. T h i e d e was o n e o f the l u c k y s e d i m e n t o l o g i s t s o n Leg 39 o f t h e Deep-Sea Drilling Project to t h e s o u t h west A t l a n t i c Ocean. B o t h a u t h o r s a c k n o w l e d g e w i t h g r a t i t u d e t h e h e l p a n d assistance received f r o m various s h i p b o a r d scientists o f t h e S o u t h A t l a n t i c D S D P legs a n d f r o m t h e s t a f f m e m b e r s o f t h e Deep Sea Drilling Project. J. T h i e d e ' s p a r t o f this s t u d y was s u p p o r t e d b y t h e Office o f Naval R e s e a r c h t h r o u g h c o n t r a c t n u m ber N 0 0 0 1 4 - 7 6 - C - 0 0 6 7 u n d e r p r o j e c t N R 0 8 3 - 1 0 2 ; T.H. v a n A n d e l ' s p a r t h a s b e e n c o m p l e t e d u n d e r the N a t i o n a l Science F o u n d a t i o n G r a n t n u m b e r D E S 710 0 5 7 3 AO2.
References 1 W.G. Deuser, Reducing environments, in: J.P. Riley and G. Skirrow, eds., Chemical Oceanography, 3 (Academic Press, London, 1975) 2nd ed., p. 1. 2 K.M. StrCm, Land-locked waters and the deposition of black muds, in: P.D. Trask, ed., Recent Marine Sediments (American Association of Petroleum Geologists, Tulsa, Okla., 1939) 356. 3 P.D. Trask, Organic content af Recent marine sediments, in: P.D. Trask, ed., Recent Marine Sediments (American Association of Petroleum Geologists, Tulsa, Okla., 1939) 428. 4 H.U. Sverdrup, M.W. Johnson and R.H. Fleming, The Oceans, their Physics, Chemistry and General Biology (Prentice Hall, New York, N.Y., 1946) 1087 pp. 5 H.G. ()stlund, Expedition "Odysseus 65": radiocarbon age of Black Sea deep water, Am. Assoc. Pet. Geol. Mem. 20 (1974) 127. 6 E.S. Trimonis, Some characteristics of carbonate sedimentation in Black Sea, Am. Assoc. Pet. Geol. Mem. 20 (1974) 279. 7 G. Miiller and P. Stoffers, Mineralogy and petrology of Black Sea basin sediments, Am. Assoc. Pet. Geol. Mem. 20 (1974) 200. 8 K.M. Shimkus and E.S. Trimonis, Modern sedimentation in Black Sea, Am. Assoc. Pet. Geol. Mem. 20 (1974) 249. 9 K. Wyrtki, Oceanographic Atlas of the International Indian Ocean Expedition (National Science Foundation, Washington, D.C., 1971) 531 pp. 10 U. von Stackelberg, Faziesverteilung in Sedimenten des indisch-pakistanischen Kontinentalrandes (Arabisches Meer), "Meteor" Forsch.-Ergebn. C9 (1972) 1.
11 B. Zobel, Biostratigraphische Untersuchungen an Sedimenten des indisch-pakistanischen Kontinentalrandes (Arabisches Meer), "Meteor" Forsch.-Ergebn. C12 (1973) 9. 12 F.B. Phleger and A. Soutar, Production of benthic foraminifera in three east Pacific oxygen minima, Micropaleontology 19 (1973) 110. 13 E.D. Jackson, S.O. Schlanger et al., Initial Reports, DeepSea Drilling Project 33 (1976) 161 (site 317). 14 R.L. Larson, R. Moberly et al., Initial Reports, Deep-Sea Drilling Project 32 (1976) in press (site 305); 32 (1976) in press (site 306). 15 E.L. Winterer, J.I. Ewing et al., Initial Reports, Deep-Sea Drilling Project 17 (1973) 17 (site 164); 17 (1973) 103 (site 166); 17 (1973) 145 (site 167); 17 (1973) 263 (site 170). 16 E.L. Winterer, Carbonate sediments on the flanks of seamount chains, Proc. ONR Symp. The Fate of Fossil Fuel CO2, Honolulu, January, 1976, in press. 17 D.E. Hayes, A.C. Pimm et al., Initial Reports, Deep-Sea Drilling Project 14 (1972) 283 (site 143, site 144). 18 P.F. Barker, I.D.W. Dalziel et al., Leg 36, Geotimes 10, No. 11 (1974) 16. 19 K. Perch-Nielsen, P.R. Supko et al., Leg 39 examines facies changes in South Atlantic, Geotimes 20, No. 3 (1975) 26. 20 H.M. Bolli, W.B.F. Ryen et al., Basins and margins of the eastern South Atlantic, Geotimes 20, No. 6 (1975) 22. 21 T.H. van Andel, Recent marine sediments of Gulf of California, Am. Assoc. Pet. Geol. Mem. 3 (1964) 216. 22 J.G. Sclater, S. Hellinger and C. Tapscott, The paleobathymetry of the Atlantic Ocean from the Jurassic to Present, J. Geol. (1976) in press. 23 J.G. Sclater and D.P. McKenzie, Paleobathymetry of the South Atlantic, Geol. Soc. Am. Bull. 84 (1973) 3203. 24 J. Thiede, The subsidence of aseismic ridges: evidence from sediments on Rio Grande Rise (SW Atlantic Ocean), submitted to Assoc. Pet. Geol. Bull. 25 The term sapropel is today widely used for black, organic carbon-rich, laminated marine mudstones which have been encountered in many oceanic drill sites. This use does not conform with the original definition of sapropel [26] and with the definition listed in the AGI Glossary of Geology [27]: ... an unconsolidated, jelly-like ooze or sludge composed of plant remains, most often algae, macerating and putrifying in an anaerobic environment on the shallow bottoms of lakes and seas...". 26 H. Potoni6, Die rezenten Kaustobiolithe und ihre Lagerst~/tten,Abh. Preuss, Geol. L.A. 55, No. I - I I I (1908-1915) 980 pp. 27 M.R. Gary, R. McAfee and C.L. Wolf, eds., Glossary of Geology (American Geological Institute, Washington, D.C., 1972) 805 pp. 28 P.R. Supko, K. Perch-Nielsen et al., Initial Reports, DeepSea Drilling Project 39 (1977) in press. 29 H.M. Bolli, W.B.F. Ryan et al., Initial Reports, Deep-Sea Drilling Project 40 (1976) in press. 30 R.G. Bader, R.D. Gerard et al., Initial Reports, Deep-Sea Drilling Project 4 (1970) 753 pp.
309 31 P.F. Barker, I.D.W. Dalziel et al., Initial Reports, Deep-Sea Drilling Project 36 (1976) in press. 32 G. Dietrich, K. Kalle, W. Krauss and G. Siedler, Allgemeine Meereskunde (Gebr. Borntraeger, Berlin-Stuttgart, 1975) 593 pp. 33 W.H. Berger, Deep-sea carbonates: dissolution facies and age-depth constancy, Nature 236 (1972) 392. 34 R.S. Detrick, J.G. Sclater and J. Thiede, Subsidence of aseismic ridges, submitted to Earth Planet. Sci. kerr. 35 D.J.J. Kinsman, Rift valley basin and sedimentary history of trailing continental margins, in: A.G. Fischer and S. Sheldon, eds., Petroleum and Global Tectonics (Princeton Univ. Press, Princeton, N.J., 1975) 83. 36 J. Thiede and M.G. Dinkehnan, The occurrence o f l n o c e r a m u s remains in Late Mesozoic pelagic and hemipelagic sediments, Initial Reports, Deep-Sea Drilling Project 39 (1976) in press. 37 K. Burke, Atlantic evaporities formed by evaporation of water spilled from Pacific, Tethyan, and Southern Oceans, Geology 4 (1975) 613. 38 T.R. Baumgartner and T.H. van Andel, Diapirs of the con-
39
40
41 42
43
44 45
tinental margin of Angola, Africa, Geol. Soc. Am. Bull. 82 (1971) 793. T.H. van Andel, J. Thiede, J.G. Sclater and W.W. Hay, Depositional history and paleo-oceanography of the South Atlantic Ocean during the last 125 million years, J. Geol. (1976) in press. S.E. Calvert, Factors affecting distribution of laminated diatomaceous sediments in Gulf of California, Am. Assoc. Pet. Geol. Mem. 3 (1964) 216. G.I. Roden, Oceanographic aspects of Gulf of California, Am. Assoc. Pet. Geol. Mem. 3 (1964) 3 1 . H.H. Veeh, W.C. Burnett and A. Soutar, Contemporary phosphorites on continental margin of Peru, Science 181 (1973) 844. H.H. Veeh, S.E. Calvert and N.B. Price, Accumulation of uranium in sediments and phosphorites on the south west African shelf, Mar. Chem. 2 (1964) 189. Personal communication, Dr. J.C. Ingle, Department of Geology, Stanford University, August 1976. E.T. Degens and D.A. Ross, eds., The Black Sea - Geology, Chemistry and Biology, Am. Assoc. Pet. Geol. Mem. 20 (1974) 633 pp.
301
[11
THE PALEOENVIRONMENT OF ANAEROBIC SEDIMENTS IN THE LATE MESOZOIC SOUTH ATLANTIC OCEAN J()RN THIEDE
School of Oceanography, Oregon State University, Corvallis, Ore. 97331 (USA) and TJEERD H. VAN ANDEL
Department of Geology, School o f Earth Sciences, Stanford University, Stanford, Calif. 94305 {USA) Received September 3, 1976 Revised version received November 1, 1976
Laminated dark calcareous oozes/chalks/limestones as well as clayey and marly mudstones/claystones with high organic carbon contents were deposited during Jurassic, Early and Late Cretaceous times in the Atlantic, Pacific and Indian Oceans. In the South Atlantic, this sedimentary facies has been encountered only in drill sites close to the American and African continental margins. The reconstructed paleogeography of the South Atlantic, the paleodepth of deposition, and the fossil content of these sediments make clear that the Late Cretaceous anaerobic paleoenvironment developed under the influence of an oceanic mid-water oxygen minimum at moderate water depths (500-2500 m), because oxygenated sediments have been observed in the deep basin elsewhere. Whether the Early Cretaceous and Jurassic anaerobic sediments were deposited in an euxinic basin or under an oceanic mid-water oxygen minimum remains an open question because the few drill sites have not sampled the deepest part of the Early Cretaceous and (?) Jurassic South Atlantic basins.
1. Introduction Anaerobic sediments indicative of a reducing depositional environment are not extensive in the open ocean. Today they are restricted either to isolated basins whose bottom waters are not, or are only very slowly, renewed, or to substrates beneath the oceanic mid-water oxygen m i n i m u m developed under highly fertile and productive surface-water masses along continental margins [I ]. This fact has been well known for several decades [2], but these two separate depositional environments are hard to distinguish because laminated anaerobic sediments which have a high or. ganic carbon content occur in both [3]. Two modern environments which can probably be assumed to represent analogs of any fossil record for a depositional environment of laminated marine sediments rich in organic matter are shown in Fig. 1. The Black Sea represents the modern present example
of an anoxic basin (Fig. 1). A marked pycnocline at 1 5 0 - 2 0 0 m water depth separates the well-oxygenated, low-salinity and warm surface water from the anoxic deep-water masses which are of higher salinity and contain large quantities of hydrogen sulfide [4,5]. Rich zoobenthic faunas populate the region shallower than 1 5 0 - 1 7 5 m [6], but are absent from the fine-grained laminated clayey and marly muds of the anaerobic depths [7] because of high concentrations of dissolved hydrogen sulfide. Organic carbon contents of the sediments range from <1% in the shallow-water regions to >5% in the basin deep [8]. Sediments deposited under anaerobic conditions also occur in the open ocean where a oxygen minimum impinges on the bottom under highly fertile and productive surface water. The reduction of the dissolved oxygen content mainly results from the oxidation of organic matter settling through the water column. A typical case of this depositional environment occurs in
302 0
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4
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e
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EROB, ~5
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E
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--" ~
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6
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.to F
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AEROBIC / ' Rich zoobenthos above 1 5 0 - 1 7 5 m . Laminated clayey and marly muds in the anoxic depth; no benthic life.
Q: bJ 3.0. I~l: 3.5.
muds
02 (milL) - -
s
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---
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Fig. 1. Schematic representation of the aerobic/anaerobic water masses and their impact upon the distribution of sediments in the Black Sea and in the northern Indian Ocean. Data for the Black Sea have been taken from Sverdrup et al. [4] and Degens and Ross [45]; for the northern Indian Ocean from Wyrtki [9] and van Stackelberg [10].
the northwestern Indian Ocean where the dissolved oxygen content approaches zero between water depths ranging from 150-200 to 800 m [9]. Where this midwater oxygen minimum intersects the continental margin (open upper continental slope), laminated finegrained muds with high organic carbon contents cover the sea floor (Fig. 1). Below the oxygen minimum zone, a layer of oxygenated sediment, increasing in thickness with increasing depth and dissolved oxygen content in the bottom waters, was observed along the continental margin off India and Pakistan [10]. Though the dissolved oxygen values approach zero in the center of the oxygen minimum layer, the sediments which appear to be anaerobic yield indigenous benthic foraminiferal faunas [11]. Similar observations have also been made along the western North and South American continental margin where a well-developed midwater oxygen minimum occurs at present [ 12].
2. Anoxic sediments in the Late Mesozoic South Atlantic Ocean Neither of these two environmental settings produces regionally widespread anoxic sedimentary facies. Thus, it was unexpected when the Deep Sea Drilling
Project recovered Late Mesozoic sediments from drill sites in the Indian and the Atlantic, as well as from the Pacific Ocean which had obviously been deposited under anoxic conditions and which therefore had many of their primary sedimentary structures preserved. Most of the sites in the Atlantic and Indian Oceans are located close to continental margins, but Late Mesozoic anoxic sediments in the Pacific Ocean occur on the flanks of seamount chains (or plateaus) in the open ocean [ 13,14], where well-oxidized paleoenvironments of similar age existed in deep water [15]. This coincidence points to the existence of a strong midwater oxygen minimum (at approximately 500-1500 m water depth) in the Pacific Ocean during mid-Cretaceous [16]. However, the Atlantic and Indian Oceans were no more than narrow, young basins during the Late Mesozoic and might well have turned totally stagnant as the Black Sea and the Cariaco Trench are today, or as the eastern Mediterranean has been during certain time spans of the Late Pleistocene. In this paper we discuss the paleoenvironment of laminated, dark mudstones, chalks and limestones of Early and Late Cretaceous age that have been penetrated in South and Central Atlantic drill sites during DSDP Legs 14, 36, 39 and 40 [17-20]. We shall therefore present a reconstruction of the paleogeography of the
303 South Atlantic Ocean during the Early and Late Cretaceous (Fig. 2), reconstruct the paleodepth of deposition of all occurrences of anaerobic sediments in the South Atlantic Ocean during this time span (Fig. 3), and try to compare their sedimentary facies with modern analogs (Fig. 4), such as those found in the Gulf of California, a young and narrow ocean basin [21 ], which could have much in common with the narrow and young early Atlantic Ocean. Due to the limited nature of our data we are not going to discuss reasons why the anaerobic sediments occur simultaneously during relatively short, but well-defined, periods in widely distant regions of the Late Mesozoic oceans; also we do not consider the possibility that the organic carbon contents of these sediments might be entirely detrital in origin.
3. The paleogeography and paleobathymetry of the Late Mesozoic South Atlantic Ocean We have adopted a paleogeographic and paleobathymetric reconstruction of the South Atlantic [22] based on plate rotations and sea-floor subsidence with age [23]. Fig. 2 shows maps for 80 and 110 m.y.B.P. representing approximately the anaerobic sediment ranges of 7 5 - 9 5 and 100-110 m.y.B.P., respectively. In addition, we note that the drill sites on the Falkland Plateau penetrated anoxic sediments of Jurassic age [18]. During Aptian/Albian times the South Atlantic was a narrow, elongated ocean consisting of a number of basins some of which may have been more than 3 km, but probably less than 4 km deep, and separated from each other by other ridges rising close to or even above sea level [24]. The connection of the Albian/Aptian South Atlantic to the Southern Ocean was broad and relatively shallow except for a narrow, deep channel just south of Africa; the North and South Atlantic Oceans may have been connected through a narrow basin which was probably less than 1 - 2 km deep and which did not permit an exchange of the deep-water masses. Aptian/Albian sediments have been encountered only in six drill sites; five drill sites in the South Atlantic [18,20] penetrated sapropels [25-27] and black mudstones while extensively burrowed and mottled calcareous mudstones were encountered at site 144 in the southern North Atlantic [17]. Most sites
with Aptian/Albian sediments in the South Atlantic are situated close to the continental margin and only site 361 sampled a location close to the former deepest portion of the Early Cretaceous Cape Basin. Twenty million years later the South Atlantic Ocean (Fig. 2) had widened to about twice its former size; the main basins had deepened to more than 4 km water depth and the ocean was connected to the North Atlantic as well as to the southern oceans through wide gaps allowing for the exchange of deep as well as surface water. Sites where late Cretaceous sediments have been found are scattered throughout the South Atlantic; they are located on the continental margins, the aseismic ridges and in the deep parts of the former ocean basin. Laminated very dark or even black anaerobic sediments have been found on the Sao Paulo Plateau and on the Rio Grande Rise [28], and along the southwest African continental margin [29]. Sediments of Late Cretaceous age deposited under oxygenated conditions have been recovered from the Brazilian Basin during DSDP Legs 4 [30] and 39 [28], and from the Argentinian Basin during DSDP Legs 36 [31] and 39 [28].
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i Fig. 2. Distribution of anoxic sediments in the CampanianCenomanian and Aptian-Albian deposits of the South Atlantic. Paleogeographyand paleobathymetry after Baumgartner and van Andel [38] and Sclater et al. [22] for 80 and 110 m.y.B.P., respectively. Numbers are DSDP drill site identifications; the bathymetric contours are in kilometers. Ages on maps represent time spans for which sediments are shown.
304 It is interesting to note that the sapropelic sediments seem to have a wider distribution in the drill sites off the eastern rather than off the western South Atlantic continental margins though this result might be biased by the location of the drill sites. Most highly fertile and productive regions of the major ocean basins today as well as probably in former times are part of the eastern boundary currents with their upwelling systems. Accordingly, the oceanic mid-water oxygen minima are developed most strongly in the eastern part of the oceans while their intensity decreases westwards [32].
363
144 sapropelic and black shale "
9 A E
354
4. The paleodepth of deposition of anoxic sediments To reconstruct the paleodepth of deposition of anoxic sediments from the South Atlantic (Fig. 3) we have used the method of Berger [33] for those drill sites where the necessary data (sediment ages and thicknesses, age and depth of basaltic basement) were available. The subsidence of aseismic ridges seems to follow a curve similar to the subsidence of oceanic basaltic basement [34], though these structural highs rise to several thousand meters above the surrounding normal sea floor (the subsidence curve therefore has to be raised vertically by this difference). The occurrences o f shallow-water sedimentary components in several drill sites on the flank of Rio Grande Rise [24] and of Walvis Ridge [28] allow us to monitor the subsidence of these aseismic ridges with time in considerable detail. The subsidence curve of sites where we have had to extrapolate the age and depth of the basaltic base. ment are evidently much less reliable. The subsidence of sites on continental basement (sites 327 and 330) cannot be reconstructed at present, since we do not know enough about the vertical tectonic movements of stable continental margins after the initial rifting [35]. However, the occurrences of macrofossils (lnoceramus sp. [31,36]) seem to indicate that these sites have also subsided a few thousand meters since Late Mesozoic times. Four alternatives exist for the subsidence of site 364 which terminated just above 3 km thick evaporite sequence on basement of about 115 m.y.B.P. (anomaly M-4) judging from geophysical data [29]. If the salt is autochthonous, it must have been deposited in a water depth of about 2.5 km or in a > 2 km deep subaerial depression [37]. If the salt had been injected
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Fig. 3. Variation of lithology of South Atlantic drill sites with paleodepth and age. Heavy black lines: anoxic sediments interbedded in sequence; thin lines: sediments poor in organic matter; arrow heads: basaltic basement. Two subsidence curves are given for site 364: upper one assumes salt flow during Turonian-Cenomanian, lower one during Oligocene [381. Subsidence for sites 327 and 330 is unknown; the symbols are located at present depth, although the macrofossils seem to indicate that this site was at considerably shallower water depth during Late Mesozoic times [36]. Lithology directly from core descriptions. into the area later by salt tectonics analogous to that of the Gulf of Mexico, two periods can be considered. There is evidence for considerable salt deformation in the Oligocene [38], but migration at this time yields an early subsidence history not in accord with the paleodepth distribution of calcareous sediments [39].
305 Hence, we have assumed an early salt migration about 9 0 - 1 0 0 m.y. ago, which yields the upper of the two subsidence curves in Fig. 3. The reconstructed depths of deposition o f the Late Mesozoic anoxic sediments of the South Atlantic occupy a wide range between 500 and 3000 m (Fig. 3). The Early Cretaceous (Albian/Aptian) sapropelic claystones of sites 327 and 330 on the Falkland Plateau are now in water depths of 2500 to 3000 m but were certainly much shallower at the time of deposition. Site 361 shows that Early Cretaceous anoxic sediments also occurred on the western (mid-ocean ridge) flank of the Cape Basin (Fig. 2) in the same water depth. However, since we do not have samples from the deepest part of this basin nor of the Argentine Basin, it is impossible to ascertain whether the basin centers were also anoxic or if the sapropelic deposits in shallower water belonged to an oxygen minimum like those of the Late Cretaceous. The Late Cretaceous sediments seem to have been deposited at considerably shallower water depths, and we possess sedimentary records (Figs. 2 and 3) which monitor the oxygenated deep-water paleoenvironment during the deposition of the anoxic laminated marls and claystones.
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The organic carbon contents of these sediments taken from the site reports, have been plotted against the projected water depth (Fig. 4). The anaerobic sediments have organic carbon contents of up to more than 10%. Otherwise, the sediments which have been deposited in the Late Mesozoic South Atlantic in an anaerobic paleoenvironment, span a wide range of different lithologies (Table 1). The dominant sedimentary facies in the near-continent sites (especially on the southwest African side) are dark olive gray to black sandy-silty claystones or black shales, while sites further offshore or into the flanks of structural highs contain marly chalks and limestones. Common to all occurrences, however, are the dark, if not black, hues and frequent laminations in the millimeter scale. At many sites, the uppermost limit of the anoxic facies is not a sharp one but laminated and intensively burrowed portions of the sediment column alternate in a cyclic manner, the thickness and the frequency of the laminated layers decreasing upwards while more and
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Fig. 4. Variation of the organic carbon content of sediments with paleodepth for the 70-95 and the 95-110 m.y.B.P. periods in the South Atlantic (data from DSDP site reports). Dashed line is an envelope to facilitate comparison with the inserted curve representing the relationship between organic carbon contents of the surface sediments and water depth of deposition (Holocene) in the Gulf of California [21 ]; the laminated, diatomaceous continental slope sediments in the Gulf of California have been described in detail [40] and can be related to the oceanic mid-water oxygen minimum [41 ] in this region.
more of the sediment column is burrowed. Simultaneously, the sediment colors change to light brownish and reddish gray. The increased burrowing as well as the color changes seem to be indicative o f better oxygenated bottom-water masses and of a paleoenvironment more favorable to benthic life than the anoxic sediments. The cyclic nature of these frequent changes (for example, well developed in sites 356 and 357) represent an interesting monitoring device for the
TABLE 1 Occurrences of anoxic sediments in the Late Mesozoic South Atlantic Ocean (lithology and ages from shipboard core descriptions) DSDP site nos. (water depths)
Type of anoxic sediment
Age
Nature of underlying sediment
Nature of overlying sediment
Remarks and sources of information
144 Top Demerara Rise (2957 m)
zeolitic black and olive marl, high Corg contents
Late Aptian to Early Turonian
Late Albian to Early Aptian olive gray marls and shelly limestone
zeolitic graygreenish Late Campanian to Early Maestrichtian marls
benthic foraminifers more abundant than planktonic ones [ 17 ]
327 Falkland Plateau (2411 m)
Nanno-rich olive gray to black sapropelic clay-
Aptian?
olive gray Neocomian claystones
Early to Middle Albian gray nanno claystones
1311
Large molluscs: l n o c e r a m u s and belemnite
stones
330 Falkland Plateau (2636 m)
olive black, laminated sapropelic clay/claystones
?Middle-Late Jurassic (Callovian, Oxfordian) to Neocomian
Middle(?)-Late Jurassic silts/ clays with benthic and nektonic molluscs, underlain by Precambrain gneisses and granites
Early to Middle Albian light brown zeoliterich nanno clays
356 Sao Paulo Plateau (3203 m)
laminated dark, marly dolomitic limestone
Late Albian
not penetrated
Late Turonian calcareous mudstones
dark, calcareous mudstones
Late Turonian
Late Albian marly dark gray, moddolomitic limeerately burrowed stones Coniacian calcareous mudstones with I n o c e r a m u s
357 Rio Grande Rise (2086 m) 361 SW African continental margin Cape Basin (4549 m)
siliceous marly limestone
Santonian
not penetrated
Santonian marly limestones
frequent I n o c e r a m u s [281
laminated black shales and sandy mudstones
Early Aptian (?) to Albian
not penetrated
dark gray shales alternating with grayish red claystones (Cretaceous?)
[29]
363 Walvis Ridge (2247 m)
yellowish gray laminated limestones to carbonaceous mudstones
Late Aptian to Albian, Turonian to Santonian
Early Cretaceous limestones with calcarenite layers
Campanian gray to grayish brown marly nannofossil chalk
uppermost part cyclically intensively burrowed [291
364 SW African continental margin Angola Basin (2449 m)
alternating burrowed dolomitic limestones and black laminated shales
Late Aptian to Early Albian
heavily burrowed dolomites of unknown age
?Middle Albian olive to brownish gray limestones
uppermost part cyclically intensively burrowed [291
black sapropelic shales
Late(?) Albian to ConiacianSantonian
Late Albian greenish gray to reddish brown burrowed nannofossil chalk
Coniacian-Santonian greenish gray to grayish brown nannofossil chalk to claystone
[291
-
-
[31]
[28]
307 oxygen contents of the oceanic bottom water masses. Benthic fossils from the Cretaceous anoxic sediment are only rarely mentioned. However, benthic foraminifers and large molluscs (bivalves, cephalopods) are mentioned here and there in the core descriptions (see Table 1). Large fragments of the thick prismatic layer of Inoceramus [36] which seem to have inhabited soft mud bottoms of the outer shelf and upper continental slope as epibenthos are a conspicuous faunal element. At present, we have no means of evaluating whether these fossils might be displaced from shallower water depths.
6. Similarities and dissimilarities to analogous modem sediments The anoxic sediments of the Late Mesozoic South Atlantic Ocean resemble in many ways deposits which are found today in regions of well-developed oceanic mid-water oxygen minima, such as in the Gulf of California, off Peru, off southwest Africa, and off the Indian subcontinent [10,21,40-43]. The similarities consist of the generally fine grain size of the sediments, the primary sedimentary structures (laminations), the colors of the sediments (dark olive/dark greenish gray to black), the presence of indigenous benthic faunas (foraminifers [11,12]), the high organic contents of the sediments (Fig. 4), and of a depositional environment in intermediate to shallow water depth along the continental margins (Figs. 2 and 3). The dissimilarities consist of compositional and faunal/floral differences; modern oceanic anaerobic sediments are usually rich in diatoms which virtuelly did not exist in the Late Mesozoic (though Late Cretaceous diatomites have been observed in fossil upper continental slope sediments in California [44]), whereas the Late Mesozoic anaerobic sediments contain remains of large epibenthic bivalves (lnoceramus) which are now extinct. It also seems that anoxic depositional environments, which are restricted in today's ocean, were much more widespread in the South Atlantic during the Late Mesozoic. This wider distribution might well result from the fact that the South and North Atlantic Oceans were linked only through a relatively narrow and possibly intermittent connection during the Early Cretaceous (Fig. 2) and that the whole Atlantic Ocean was closed to the north throughout the Late Mesozoic and
Early Cenozoic thus allowing the existence of a midwater oxygen minimum as strong as known today from the northern Indian Ocean [9]. Consequently, all evidence obtained from the anoxic sediments of the Late Mesozoic South Atlantic points to a depositional paleoenvironment such as we observe under today's oceanic mid-water oxygen minima. Although this evidence satisfactorily explains the Late Cretaceous occurrences of anoxic sediments, the euxinic model cannot be totally discarded when discussing the Early Cretaceous and Jurassic anoxic sediments. The main deficiency of the older data is the lack of drill sites in the center of the former ocean basins that would prove which of the two models (Fig. 1) for the deposition of anoxic sediments applies to the Early Cretaceous South Atlantic and to the Jurassic deposits of the Falkland Plateau.
7. Conclusions (1) Laminated dark calcareous oozes/chalks/limestones and marly mudstones with high contents of organic carbon were deposited in the South Atlantic during Jurassic, Early and Late Cretaceous times. (2) All drill sites where the older Jurassic and Aptian/Albian anoxic sediments have been penetrated in the South Atlantic Ocean, occupied originally positions close to the southwest African continental margin (Fig. 2). However, this result may be biased due to the scarce sample coverage. (3) Sites with Late Cretaceous anoxic sediments have been drilled along the southwest African continental margin as well as along the South American continental margin and on Rio Grande Rise. In contrast, an oxygenated facies was deposited in the Brasil and Argentine Basins as well as on the Falkland Plateau (Fig. 2) during this same period. (4) The paleodepths of deposition of Early Cretaceous anoxic sediments range from 2500 to 3000 m (probably to much shallower water depths at the continent near locations such as Falkland Plateau drill sites); the paleodepth of deposition of the Late Cretaceous ones range from <500 m to approximately 2500 m (Fig. 3). (5) Sediments which were deposited in deep water during Late Cretaceous time simultaneously with the anoxic sediments prove that this anaerobic paleoen-
308 v i r o n m e n t d e v e l o p e d u n d e r t h e i n f l u e n c e o f a n oceanic mid-water oxygen minimum.
Acknowledgements J. T h i e d e was o n e o f the l u c k y s e d i m e n t o l o g i s t s o n Leg 39 o f t h e Deep-Sea Drilling Project to t h e s o u t h west A t l a n t i c Ocean. B o t h a u t h o r s a c k n o w l e d g e w i t h g r a t i t u d e t h e h e l p a n d assistance received f r o m various s h i p b o a r d scientists o f t h e S o u t h A t l a n t i c D S D P legs a n d f r o m t h e s t a f f m e m b e r s o f t h e Deep Sea Drilling Project. J. T h i e d e ' s p a r t o f this s t u d y was s u p p o r t e d b y t h e Office o f Naval R e s e a r c h t h r o u g h c o n t r a c t n u m ber N 0 0 0 1 4 - 7 6 - C - 0 0 6 7 u n d e r p r o j e c t N R 0 8 3 - 1 0 2 ; T.H. v a n A n d e l ' s p a r t h a s b e e n c o m p l e t e d u n d e r the N a t i o n a l Science F o u n d a t i o n G r a n t n u m b e r D E S 710 0 5 7 3 AO2.
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tinental margin of Angola, Africa, Geol. Soc. Am. Bull. 82 (1971) 793. T.H. van Andel, J. Thiede, J.G. Sclater and W.W. Hay, Depositional history and paleo-oceanography of the South Atlantic Ocean during the last 125 million years, J. Geol. (1976) in press. S.E. Calvert, Factors affecting distribution of laminated diatomaceous sediments in Gulf of California, Am. Assoc. Pet. Geol. Mem. 3 (1964) 216. G.I. Roden, Oceanographic aspects of Gulf of California, Am. Assoc. Pet. Geol. Mem. 3 (1964) 3 1 . H.H. Veeh, W.C. Burnett and A. Soutar, Contemporary phosphorites on continental margin of Peru, Science 181 (1973) 844. H.H. Veeh, S.E. Calvert and N.B. Price, Accumulation of uranium in sediments and phosphorites on the south west African shelf, Mar. Chem. 2 (1964) 189. Personal communication, Dr. J.C. Ingle, Department of Geology, Stanford University, August 1976. E.T. Degens and D.A. Ross, eds., The Black Sea - Geology, Chemistry and Biology, Am. Assoc. Pet. Geol. Mem. 20 (1974) 633 pp.