Tertiary and Cretaceous paleoenvironments in the southwest Atlantic Ocean: Preliminary results of Deep Sea Drilling Project Leg 71

Abstract

DSDP Leg 71 in the South Atlantic confirmed that the early opening (Neocomian-Aptian) of the South Atlantic was marked with restrictive circulation in which shale units high in organic carbon (1.7% to 4.1%) were deposited. Ratios of gaseous hydrocarbons and pyrolysis-fluorescence analyses suggest a fairly high degree of maturity of the black shale. A comparatively complete Cretaceous section provides biostratigraphic reference for the South Atlantic. Major erosion occurred at or near the Tertiary-Cretaceous boundary prior to the opening of the Drake Passage (Oligocene–middle Miocene). The early Tertiary was marked by mild climatic conditions and periods of exceptionally rapid sediment accumulation (as much as 44 m/m.y.) separated by hiatuses or condensed intervals. Paleomagnetic measurements recognize the Brunhes and Matuyama (with Jaramillo and Olduvai events), Gauss (with Kaena and Mammoth events), and Gilbert (with Cochiti event) Epochs. Correlation of this paleomagnetic scale with siliceous microfossil zonations was accomplished. Siliceous and calcareous microfossils reveal pronounced fluctuations of the Polar Front in the Pliocene and Pleistocene. Warm intervals occurred in the late Gilbert and middle Guass Epochs; cooler conditions were prevalent in the late Gilbert-early Gauss. The late Pliocene was marked by climatic deterioration with brief warmings in the uppermost Matuyama and upper Brunhes. Sedimentation rates dropped markedly from 180 m/m.y. in the early Pliocene (Gilbert Epoch) to 2.3 m/m.y. in the Pleistocene (early Brunhes).