Cretaceous and Cenozoic Sediments from the Atlantic Ocean

Abstract

Leg 14 of the Deep Sea Drilling Project completed 9 holes; 7 were located off NW Africa, 1 in Ceara Abyssal Plain, and 1 on Demerara Rise. Quaternary and Pliocene sediments consist of nanno chalk oozes and brown clay. The facies boundary (CCD) is near 5000 meters. Sedimentation rates are between 10 and 30 meters/million years (m/My) for chalk ooze and 2 to 4 m/My for pelagic clay. Redeposition off the Amazon produces terrigenous and bioclastic turbidite sequences with rates increasing, on the average, from 50 to 150 m/My. During the Miocene the CCD is shallow, with restricted calcareous ooze and extended brown clay domain. Terrigenous and siliceous deposits are abundant off NW Africa. The Oligocene is characterized by a major hiatus; where present (Demerara Rise, foot of continental rise), it appears similar to the Miocene. Off NW Africa, Eocene sediments consist of brown clays, in part zeolitic, and greenish clays which, though rich in quartz and siliceous fossils, are less rich than Miocene sediments. Chert is rare. On Demerara Rise, the Eocene is represented by siliceous foram-nanno chalk. The Paleocene rests unconformably on Maestrichtian at Site 144, where the Cretaceous-Tertiary boundary was cored. Off NW Africa, brown and green pelagic clay and dolomite-clay (chert) cycles apparently belong to this period. The major sediment types of the Post-Cenomanian Cretaceous are zeolitic radiolarian, brown clay, black shale, limestone beds and chert layers. Redeposition is thought to play a major role in the origin of mestones and cherts. Cenomanian carbonates consist mainly of nanno marl ooze, presumably deposited on an ancestral Mid-Atlantic Ridge. Calcareous carbonaceous sediments intercalated with limestone beds occur on Demerara Rise. Below the CCD, black shales, limestones and chert accumulated off NW Africa. Near the presumed boundary of ancestral ridge flank and continental rise, cyclic sequences of black shale and dolomite silts (in part ankerite) were deposited. Aptian and Albian sediments consist of black nanno marl off NW Africa and of shelly and quartzose calcarenite, and carbonaceous clay on Demerara Rise.