Origin and Geochemistry of Cretaceous Deep-Sea Black Shales and Multicolored Claystones, with Emphasis on Deep Sea Drilling Project Site 530, Southern Angola Basin

Abstract

Deep-water sedimentary sequences of mid-Cretaceous age, rich in organic carbon, have been recovered at many DSDP sites in the Atlantic Ocean. Most of these sequences have a marked cyclicity in amount of organic carbon resulting in interbedded multicolored shale, marlstone, and (or) limestone that have cycle periods of 20,000 to 100,000 years and average 40,000 to 50,000 years. These cycles may be related to some climatic control on influx of terrigenous organic matter and sediment, rates of upwelling and sea-surface production of organic matter, and preservation of organic matter related to deeper-water dissolved oxygen concentration. These variations in supply of organic matter had pronounced effects on the potential of the sediment for subsequent diagenetic changes and geochemical partitioning in adjacent beds.Many trace elements are enriched in organic-carbon-rich lithologies relative to interbedded organic-carbon-poor lithologies. Elements that are most commonly enriched are Cr, Ni, V, Cu, Zn, and Mo. The association of high traceelement concentrations with organic matter may be the result of concentration of these elements by organisms or by chemical sorption and precipitation processes under anoxic conditions. Detailed trace-element profiles from organiccarbon-rich strata at Site 530 suggest that there may be differential mobility of trace elements, with diffusion of some elements over distances of at least tens of meters. The sequence of trace-element mobility, from highest to lowest, is approximately Ba, Mn, Pb, Ni, Co, Cr, Cu, Zn, V, Cd, and Mo. Slowly deposited, oxidized clays directly overlying some black shale sequences are enriched in some metals, particularly Fe, Mn, Zn, and Cu, relative to normal pelagic clays, and this enrichment may be the result of upward migration of metals in pore waters during compaction or diffusion from the underlying black shale.Most depositional models that have been used to explain the accumulation of the organic-carbon-rich strata imply that reducing conditions in the sediments (and therefore the increased degree of preservation of organic matter) were the result of anoxic or near-anoxic conditions in oceanic bottom waters, or in a midwater oxygen-minimum zone. Evidence from several DSDP sites in the Atlantic, however, indicate that some of these middle Cretaceous "black shale" beds may be the result of variations in rate of supply of organic matter that produced anoxia or near-anoxia within midwater oxygen-minimum zones and possibly, under extreme conditions, throughout much of the bottomwater mass. Although bottom-water anoxia may have occurred during periods of organic-carbon-rich strata, it was not necessarily the only cause for accumulation of these strata. The main reason for the accumulation of organic-carbonrich strata was an increase in the relative amount of organic debris being deposited. Some of this organic debris was derived from continental-margin areas of increased production, accumulation, and preservation of organic matter from marine, terrest...

Synthesis of Organic Geochemical Studies, Deep Sea Drilling Project Leg 93, North American Continental Margin

“…This scenario of downslope transport and redeposition is essentially the same as earlier proposed by Dean and Gardner (1982) for Site 367 in the Cape Verde Basin, by Dean et al (1984) for Site 530 in the Angola Basin, and by Robertson and Bliefnick (1983) for Site 534 in the Blake-Bahama Basin. Also, this scenario helps explain the variability in organic character present in the sediments of the western North Atlantic.…”

“…Where a midwater anoxic layer intercepts the ocean bottom, sediments rich in organic matter can accumulate. Downslope movement of such sediments can result in formation of black shales within deep-ocean turbiditic sequences, as suggested by Dean et al (1984) for Site 530 in the Angola Basin, if reburial is sufficiently rapid to preserve the organic matter. This scenario points out another factor important to preservation of organic materials-the sedimentation rate.…”

The Cenomanian/Turonian Boundary Event (CTBE) at Hole 641A, ODP Leg 103 (Compared with the CTBE Interval at Site 398)

“…Black shale layers with chemical characteristics similar to the CTBE interval at Site 641 seem to be widespread, at least in the Cretaceous Atlantic and Tethys oceans (Brumsack, 1980;Dean et al, 1984;Brumsack and Thurow, 1986). But do these specific trace metal enrichments in black shales provide information with respect to the paleoenvironmental conditions during black shale deposition, or are these enrichments only diagenetic in origin?…”

“…Cretaceous black shales from the Atlantic Ocean are known for their unusual trace metal content Brum- sack, 1980; Dean et al, 1984). The accumulation of specific trace metals in organic-carbon-rich sediments may be under stood only in terms of the relatively well-known behavior of these elements in the marine environment.…”

Inorganic Major, Minor, and Trace Element Geochemistry and Clay Mineralogy of Sediments from the Deep Sea Drilling Project Leg 96, Gulf of Mexico

“…Three possible models could explain the inorganic geochemical variations and relative minor-and trace-element enrichment in the analyzed sediment samples: (1) bioconcentration in which marine plankton concentrate minor and trace elements as, for example, organometallic compounds (Leinen and Stakes, 1979); (2) postdepositional geochemical mobility, where migrating pore fluids dissolve and reprecipitate relatively mobile oxides, elements, and other element compounds/phases (Dean et al, 1984), and (3) clay mineral sorption (Tourtelot, 1964). Since DSDP Leg 96 samples do not show high concentrations of organic matter, nor is there any reason to believe that marine plankton or bacteria preferentially concentrated minor and trace elements that have remained after decomposition of such organisms, the bioconcentration model is not favored.…”