Late Jurassic and Early Cretaceous sedimentation in the Mandawa Basin, coastal Tanzania

Abstract

This paper concerns the sedimentary successions deposited in the Mandawa Basin after the separation of East and West Gondwana during the subsequent southwards drift of Madagascar in the Late Jurassic to Early Cretaceous times. The aim of this study was to provide more specific details on the Late Jurassic and Early Cretaceous sedimentation and to report mineralogical and petrographical characteristics on the less well documented successions, namely the Kipatimu, Mitole and Nalwehe formations, in the central and northern parts of the Mandawa Basin.The Late Jurassic and Early Cretaceous depositional environments are reviewed, based on the sedimentological, mineralogical and petrographical results presented here, supplemented by previously published work. The Late Jurassic to Early Cretaceous depositional setting mainly reflects a shallow, tidally influenced, mixed carbonate-silisiclastic coastal ramp. The succession displays cyclical sedimentation best described as a series of transgressive-regressive sequences with limestones overlain by siliciclastics. During transgressions microbial activity was high and sedimentation rates low, resulting in micro-oncoid deposition in the late Kimmeridgian – early Tithonian (Mitole Limestone Member), and oncoids and stromatolites in the Early Cretaceous (Nalwehe Limestone Member). During the regressive phases siliciclastic marginal marine sediments were deposited over the limestones. The regressive sandstones are mainly unfossiliferous and display evidence of being deposited in a tide-dominated, marginal marine environment.