About this webinar
The Mandawa Basin of Tanzania is one of several coastal basins that developed in response to the breakup of Gondwana during the Early Mesozoic. It is underexplored, with only seven deep wells drilled and whilst these wells have proven that all the elements of a working petroleum system exist within the Mandawa Basin, commercial discoveries remain elusive. In the wake of a series of large deep water gas discoveries in Tanzania, there has been a renewed interest in understanding the geology of the onshore basins, including a review of the subsurface stratigraphy and source to sink relationships of the detrital sediments.
This presentation focuses on the sedimentary rocks of the Kilwa Group deposited between the Early Cretaceous (Albian) to Late Paleogene (Oligocene) Epochs in the Mandawa Basin. The Kilwa Group currently comprises five clay-dominated lithostratigraphic formations deposited in a mid to outer shelf setting across a passive margin and has been the subject of detailed scientific research by the Tanzanian Drilling Project (TDP) for nearly 20 years. Approximately 1500 core samples have been subjected to inductively coupled plasma (ICP) optical emission spectroscopy (OES) and mass spectrometry (MS) analysis, with high-resolution data being acquired for fifty elements. These data have been employed to develop a chemostratigraphic framework for the Kilwa Group and a subset of samples have also been subjected to heavy mineral analysis by Raman spectroscopy and U-Pb geochronology in order to refine the provenance interpretations made from the elemental data.
Using the inorganic geochemical data, the Kilwa Group is divided into four chemostratigraphic sequences and six chemostratigraphic packages that broadly support the pre-existing lithostratigraphy. The key elements employed in this study are interpreted to model variations in the abundance of heavy minerals, feldspars, clay minerals and authigenic minerals, which themselves reflect temporal changes in climate, environment and sediment provenance within the Mandawa Basin. The combined geochemical and heavy mineral data highlight a change in provenance at the Cretaceous-Tertiary boundary, with the zircon, garnet and apatite-dominated sediments of the Late Cretaceous derived almost exclusively from the Neoproterozoic Mozambique Belt, whereas a portion of the epidote, kyanite and titanite-rich sediments of the Paleogene are derived from the Palaeoproterozoic Ubendian Belt further west. These results highlight that drainage networks extended deeper into the Tanzanian hinterland during the Paleogene, with at least some of the detrital sediment being routed through the Palaeozoic-aged Selous Basin.