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Geological Society, London, Special Publications; 1987; v. 26; p. 271-286;
DOI: 10.1144/GSL.SP.1987.026.01.19
© 1987 Geological Society of London

Part III: The Stratigraphic Record

Carbonaceous sediments and palaeoenvironment of the Cretaceous South Atlantic Ocean

H. B. Zimmerman

Union College, Schenectady, N.Y. USA
Division of Polar Programs, National Science Foundation, Washington, D.C. 20550, USA

A. Boersma & F. W. McCoy

, 404 Gate Hill Road, Stony Point, N.Y. USA
Lamont-Doherty Geological Observatory, Palisades, N.Y. USA

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.





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