The paleoenvironment of anaerobic sediments in the Late Mesozoic South Atlantic Ocean

Abstract

Laminated dark calcareous oozes/chalks/limestones as well as clayey and marly mudstones/claystones with high organic carbon contents were deposited during Jurassic, Early and Late Cretaceous times in the Atlantic, Pacific and Indian Oceans. In the South Atlantic, this sedimentary facies has been encountered only in drill sites close to the American and African continental margins. The reconstructed paleogeography of the South Atlantic, the paleodepth of deposition, and the fossil content of these sediments make clear that the Late Cretaceous anaerobic paleoenvironment developed under the influence of an oceanic mid-water oxygen minimum at moderate water depths (500–2500 m), because oxygenated sediments have been observed in the deep basin elsewhere. Whether the Early Cretaceous and Jurassic anaerobic sediments were deposited in an euxinic basin or under an oceanic mid-water oxygen minimum remains an open question because the few drill sites have not sampled the deepest part of the Early Cretaceous and (?) Jurassic South Atlantic basins.