Petrology and Sedimentary Provenance of Mesozoic and Cenozoic sequences in the Mandawa Basin

Mandawa Basin in coastal Tanzania was formed as a result of tensional forces related to the Gondwana break-up and the opening of the Indian Ocean. Mandawa Basin has been influenced by transgressive and regressive phases which have led to deposition of various sedimentary sequences from Late Triassic to recent age. These sedimentary sequences have been deposited in different depositional environments, from alluvial fans proximal to source area to deep marine, distal environments. The sedimentary sequences display various mineralogical and textural compositions and heavy mineral contents. Offshore sediments from Statoil, Block 2, display comparable petrography and heavy mineral content to corresponding onshore sediments. Conventional heavy mineral analysis, heavy mineral ratios and electron microprobe analysis are all tools for reconstructing the provenance of a sedimentary sequence. The heavy mineral assemblages in Mandawa Basin are influenced by several different processes, such as transport, weathering and diagenesis, which can alter the compositions of the heavy minerals assemblages to the point where they no longer reflect the true composition of the source area. The composition of heavy minerals then indicate the stability of different heavy minerals and the different degrading processes acting on the sediment, thus complicating the interpretation and identification of provenance. Heavy mineral ratios and single grain analysis are tools which provide a better reflection of the true source rock mineralogy and are to a less degree affected by processes that alter the heavy mineral assemblages. Heavy mineral assemblages and the electron microscope analysis of garnet point towards a sedimentary input from the Mozambique Belt, west of Mandawa Basin. To provide a link between the sedimentary sequences and the source areas, information regarding source rock mineralogy is needed. Samples from different basement lithologies as well as sediments from rivers draining specific source areas are necessary if such a link is to be made. Large river systems in Tanzania which drain and transport sediments from several source areas, and the possibility of reworked sediments, provide a further challenge when trying to reconstruct the sedimentary provenance