Origin and geochemistry of Cretaceous deep-sea black shales and multicolored claystones, with emphasis on Deep Sea Drilling Project Site 530, southern Angola Basin.

Detailed trace-element profiles from organic-carbon-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 shales 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. Evidence from several DSDP sites in the Atlantic 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 bottom-water mass. -from Authors