Organic-rich Cretaceous sediments from the North Atlantic

Abstract

Summary Organic-rich sediments are abundant in the early and middle Cretaceous of the deep North Atlantic, in the calcareous Blake-Bahama Formation (Valanginian-Barremian), in the siliceous Hatteras Formation (Aptian-Cenomanian), and, in some places, in the Plantagenet Formation (Turonian-Santonian). The regional patterns of organic facies and organic enrichment reflect: (1) the rate of supply of marine versus terrestrial organic matter, (2) the rate of burial of organic matter, (3) the oxygenation of the bottom water, and (4) the oceanographic history of the North Atlantic basin. Rates of supply of marine organic matter (most now present as amorphous matter) seem to have been similar in the eastern and western North Atlantic, though they became higher in the east in Cenomanian and younger times, when coastal upwelling intensified there. Rates of supply of terrestrial organic matter were higher in the west, and off Iberia and in Biscay (except in the Cenomanian-Turonian). Organic matter is most abundant where rates of sedimentation were highest, presumably because organic matter was rapidly removed from the zone of intensive recycling near the sediment water interface. Some of the organic rich deposits were introduced by downslope displacement from the oxygen minimum zone on the continental margins. The evidence suggests that bottom waters were intermittently suboxic (0.5–0.2 ml/l O 2 ) or anoxic (<0.2 ml/l O 2 ), encouraging preservation of organic matter on the deep sea floor. A more or less permanently anoxic expanded oxygen minimum zone is also implied. While the global Cretaceous ocean seems to have been generally poorly oxygenated, local events within the North Atlantic give it a somewhat different history of accumulation of organic matter from other ocean basins. Nevertheless, the global organic enrichment event at the Cenomanian-Turonian boundary is clearly visible in the North Atlantic.