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Origin of varve-type lamination, graded claystones and limestone-shale ‘couplets’ in the lower Cretaceous of the western North Atlantic

Summary The petrography and sedimentary structures of pelagic and hemipelagic sediments in the early Cretaceous interval of the Blake-Bahama Basin reveal a subtle interplay between terrigenous source materials, plankton productivity and diagenesis. Three specific sediment types drilled at Site 534A (Leg 76) of the DSDP are: (1) persistent, fine parallel lamination resembling varves; (2) thin, size-graded nannofossil claystones and ‘black shales’, and (3) ‘couplets’, composed of pale burrowed and dark laminated nannofossil radiolarian carbonates. The varve-type lamination reflects some combination of fluctuations of terrigenous plant material plus fine clastic input and plankton productivity. Both possibly reflect short periodicity (10s to 100s of years) climatic variations. Lithification took longer in the clay-and organic-rich laminae. Slow compaction then accentuated the lamination. By contrast, purer nannofossil oozes lithified more quickly and now show less compaction. The graded claystones and ‘black shales’ were mostly redeposited by turbidites from within, or near, the oxygen-minimum zone on the upper continental slope. Turbiditic input reduces the need for bottom water anoxicity. The dark-light couplets reflect wetter periods on land averaging 20–60 000 years, during which abundant plant material entered the Atlantic. Contrary to some recent suggestions, the early Cretaceous Atlantic need not have been poorly circulated and weakly oxygenated. Instead, both short and longer periodicity climatic fluctuations controlled input of mostly land-derived material. High rates of organic matter input, partly turbiditic, produced sub-surface sediment anoxia below a circulating relatively fertile early Cretaceous Atlantic Ocean.