Organic Geochemical Comparison of Cretaceous Green and Black Claystones from Hole 530A in the Angola Basin

Abstract

Three pairs of Upper Cretaceous black shales and adjacent green claystones from Hole 53OA were analyzed to compare types and amounts of organic matter and lipids and to seek information about their environments of deposition. The organic-carbon-rich black shales have C/N ratios nearly seven times those of the organic-carbon-lean green claystones. The lipid content of organic matter in the black shales is about ten times less than in adjacent green layers. Organic matter in both types of rocks is thermally immature, and distributions of alkanoic acids, alkanols, sterols, and alkanes contain large amounts of terrigenous components. Pristane/phytane ratios of less than one suggest that younger Turonian sediments were laid down under anoxic conditions, but ratios greater than one suggest that older Turonian Cenomanian deposits accumulated in a more oxic environment. Closely bedded green and black layers have very similar types of lipid distributions and differ primarily in concentrations, although black shales contain somewhat larger amounts of terrigenous lipid components. Geochemical and stratigraphic evidence suggests much of the organic matter in these samples originated on the African continental margin and was transported to the Angola Basin by turbidity flow. Rapid reburial of organic-carbon-rich sediments led to formation of the black shales.

Astronomically Driven Variations in Depositional Environments in the South Atlantic During the Early Cretaceous

“…Although our results are specific to Site 364 and the margins of northern South Atlantic, periodic black shales of similar age (Early Cretaceous) have been reported elsewhere in the proto‐Atlantic, for example, at Site 530 in the Angola basin (e.g., Deroo et al, ; Katz, ; Meyers et al, ; Rullkötter et al, ; Stow & Dean, ); Site 511 in the Falkland Plateau (e.g., Deroo et al, ), and Site 367 in Gambia Abyssal Plain (e.g., Hofmann et al, ; Wagner et al, ). There are also numerous publications on the astronomically controlled formation of black shales in the Late Cretaceous, for example, from Site 959 in the Ivorian Basin (Beckmann et al, ; Holbourn et al, ), Leg 207 (Sites 1257–1261) at Demerara Rise (Hofmann & Wagner, ; Meyers et al, ), Site 530 in the Angola basin (e.g., Arthur et al, ; Deroo et al, ; A. Forster et al, ; Stow & Dean, ), the Western Interior Seaway (Eldrett et al, ), and ODP Site 1138 in the Indian Ocean (Dickson et al, ).…”

Organic-Carbon Content of Sediments and Rocks from Deep Sea Drilling Project Sites 603, 604, and 605, Western Margin of the North Atlantic

“…Atomic C/ N ratios of the black claystones average 33, whereas the values of interbedded organic-carbon-lean strata average about 12 (Table 2). Similar contrasts in organic-carbon contents and C/N ratios have been observed in Cenomanian black shales and adjacent green claystones (Meyers, Trull, et al, 1984) and have been interpreted as resulting from enhanced (but only partial) preservation of organic matter by rapid burial (the black shales) in a normally oxic depositional environment (the green claystones).…”

Organic Geochemical Character and Hydrocarbon Source Potential of Selected Black Shales, Deep Sea Drilling Project Hole 603B

“…The black shale sequences are associated with quartz silt maxima and are included in fining-upward sequences, both of which are consistent with such a mechanism of emplacement. Transport by turbidity currents has been proposed for other Atlantic Ocean Cretaceous black shales at several locations (e.g., Cornford, 1979;Summerhayes, 1981;Graciansky et al, 1982;Dean et al, 1984;Meyers et al, 1984). Rapid sedimentation, as associated with turbidity currents, would also be consistent with the general lack of evidence for massive bioturbation.…”