A PLANKTONIC MARKER AND CALLOVIAN OXFORDIAN FRAGMENTATION OF GONDWANA- DATA FROM OGADEN BASIN, ETHIOPIA

In this paper we outline the Triassic to Jurassic history of the easte rn Ogaden Basin as obtained from micropalaeontological studies. Contin ental sediments bearing Upper Triassic spores and pollen are overlain by Tethyan shallow water limestones yielding foraminifera and dinoflag ellates of Callovian aspect. Flooding of the carbonate platform close to the Callovian Oxfordian boundary brought about replacement of large r foraminiferid assemblages (adapted to warm, clear, oligotrophic wate rs) by smaller benthic foraminiferid assemblages (adapted to deeper, m uddier substrates). A brief influx of early planktonic foraminifera (t he first recorded from east Africa) provides evidence for a connection with oceanic waters of the western Tethys. These changes are attribut ed to a series of major transgressions (c. basal Callovian and Callovi an/Oxfordian boundary) which, in turn, coincided with the initiation o f sea floor spreading during the opening of the Somalia Basin of the w estern Indian Ocean. The spread of early planktonic foraminifera may h ave been enhanced by the breakup of Gondwana and the formation of new ocean basins.