Lithology of Mesozoic and Cenozoic Sediments of the Eastern Indian Ocean, Leg 27, Deep Sea Drilling Project

Abstract

During Leg 27 five sites were drilled in the eastern Indian Ocean-four in abyssal plains near the western margin of Australia and one in the Timor Trough. Abyssal plain sediments are divided into two major units: (1) a lower, acoustically transparent layer of relatively uniform thickness draped over basaltic basement and (2) a horizontally layered sequence of highly variable thickness filling low areas on the surface of the transparent layer. The transparent layer, ranging in age from Upper Jurassic to Upper Cretaceous, consists chiefly of dark gray, siliceous clays and claystones with minor zeolitic clay and nannofossil ooze. Sedimentation rates are between 5 and 30 m/m.y. being highest in the Lower Cretaceous. Upper Cre-taceous sediment are sparse or absent. The relatively high sedi-mentation rates for pelagic clay are probably due to the proximity of the Australian continent. The layered unit consists of calcareous oozes and lesser zeolitic clay and radiolarian ooze, all of Cenozoic age. Many of the oozes contain shallow-water foraminifers and graded sequences suggesting deposition by mass transport. Calculated sedimentation rates for the upper Cenozoic range from 5 to 15 m/m.y. Very few lower Tertiary sediments were recovered because drilling sites were selected near crests of highs in the transparent layer where Cenozoic sediments are thin.