Turbidites, the principal mechanism yielding black shales in the early deep Atlantic Ocean

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Abstract
A series of organic and inorganic geochemical parameters and facies indicators were studied in Late Jurassic and Cretaceous black shales and organic rich sediment sections from basins offshore Angola, Ghana and North Carolina, and from the western Wealden Basin in NW-Germany. Based on these data the sediments can be grouped into: (i) intracratonic (Bueckeberg Formation, NW Germany), (ii) shelf (Cretaceous basins offshore Ghana), (iii) continental rise turbidite (Hatteras Formation of Leg 93, Hole 603B, Section IV), and (iv) deep basin (Leg 75, Hole 530B, Section VIII) black shales. Trends of geochemical evolution are interpreted as the effect of multiple reworking and equilibration, when organicrich sediments from shelfbound basins are transferred into oceanic depocentres as turbidites. It is suggested: (i) that during turbidite phases, organic-rich suspensates originating from these primary basins on continental margins are advected along density boundaries within the water column overlying oceanic basins, and (ii) that these substrates are introduced into zones where responding microbial populations create expanded zones of oxygen depletion. Microbial degradation affects predominantly marine organic compounds contained in these recycled sediments, leaving a residual kerogen composed of structureless organic matter with pronounced terrigenous signals.