Sediment provenance and Karoo rift basin evolution in the Kilombero Rift Valley, Tanzania

Abstract

In East Africa, several major taphrogenic events gave rise to the formation of extensive Late Palaeozoic to Mesozoic intracratonic rift basins. Deposition of fluvio-lacustrine beds took place in the tectonically controlled basins during the Permian and Early Triassic, and in Tanzania they constitute an extensive Karoo-aged sequence. The Kilombero Rift Valley occupies a minor, narrow N-S trending basin in Tanzania. The present rift valley is bounded by Precambrian basement and appears today as a Cenozoic rift modification of an earlier Permian (Karoo) rift basin. Within the present rift lies a Karoo succession with a total thickness of approx. 6000 m. The Kilombero Karoo is made up of mainly siliciclastic sediments, dominated by shales and feldspathic sandstones. Conglomeratic and calcareous units are found at various levels in the commonly calcite cemented formations. The Kilombero Karoo succession began with the deposition of debris flows and braided streams on alluvial fans adjacent to a westward facing rift escarpment. Petrographical and mineralogical analyses of clasts and heavy mineral fractions of these lowermost Karoo successions reveal a close similarity to the immediate adjacent basement lithologies. The clast population and matrix minerals support deposition close to a major (concealed) boundary fault of Karoo (or pre-Karoo) age. This initial (alluvial fan) depositional stage was succeeded by lacustrine sedimentation, before a final rift filling phase, characterized by flood plain deposition. The different stages in the rift development have been recorded by changing sedimentary facies, shifts in sediment transportation directions and changes in sediment composition. Hence, a marked transition between the lower 3500 m of the succession and the upper 2500 m is revealed in a reversal in sediment transport directions: from NW → SE to SE → NW. We interpret this shift to reflect a change in rift architecture, possibly triggered by a tectonic event. This is supported by a subtle change in the pebble/clast lithologies as well as in the amphibole compositions. Our sedimentological, mineralogical and geochemical investigations support a western, more granitoid provenance of the upper fluviolacustrine and deltaic to fan deltaic sediments. Comparisons of the heavy mineral populations in the units investigated also reflect different mineral stability during the different climatic and sedimentological environments.