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Changes in sediment types (lithofacies) and sediment components (carbonate, foraminifers, calcareous nannoplankton) in the South Atlantic Ocean from Early Cretaceous through Pliocene are portrayed on a series of palinspastic maps which graphically document the sedimentological succession as this ocean basin evolves. Determination of paleolithofacies is based upon smear slide analyses of dated sediment from the base of over 300 piston cores and dredges which provides a broad regional coverage and supplements DSDP drilling data (with Leg 39 results as the primary data base). These maps graphically illustrate the development of oceanic circulation, CCD levels, terrigenous sediment influx, and biological productivity concomitant with changing sea-floor physiography, cycles of transgression and regression, and the Tertiary climatic deterioration.

Summary The distribution of sapropelic sediments in the early South Atlantic Ocean is used to test palaeocirculation models related to oxygen-deficient conditions. The data presented here demonstrate that preservation of organic matter was widespread and occurred in many phsyiographic settings. Given the silled-basin physiography of the relatively isolated South Atlantic and the benign Cretaceous climate, a preservational model of sapropel accumulation and its corollary of restricted bottom circulation successfully satisfy the constraints of the geological record. Under the environmental conditions outlined for the early and mid-Cretaceous, a generally dysoxic condition for the warm saline bottom-water is appropriate. The prevailing dysoxia, however, was punctuated by episodic bottom anoxia, which often expanded into the upper reaches of the water column. In the Cretaceous South Atlantic, the only excursion from the dysoxic condition occurred during a Cenomanian ‘ventilation event’ (represented by a widespread hiatus in the stratigraphic record) which is tied to an interval of climatic cooling and the establishment of effective circulation between the North and South Atlantic Oceans. The geological record for epicontinental regions, although modified by local physiography and other environmental parameters, suggests a relationship between sea-level positions and sapropel accumulation. No relationship, however, can be established between sea-level and sapropel preservation in the oceanic basins of the South Atlantic.