The Permo-Triassic Karoo-equivalent organic-rich deposits of the Tanga Basin, Tanzania: sedimentary structures, depositional processes, and deposit types

The Middle Karoo deposits of the onshore Tanga Basin of northern coastal Tanzania contain dark grey-black, fossiliferous organic-rich rocks outcropping at Kakindu and Pangarawe areas. Deposit types, depositional and post-depositional processes, conditions and features characterising these rocks are poorly understood. This work used core logging, outcrop descriptions and analysis of elemental proxies to identify deposit types and reconstruct conditions and processes controlling the deposition of the Middle Karoo rocks in the study area. Outcrop descriptions and core logging results show that the studied stratigraphy is dominated by low-density turbidites and black shales. Other identified deposit types include slides, slumps, and debris flow deposits, indicating downslope sediment remobilisation triggered by episodic fault activity and rapid sedimentation. The distribution of elemental proxies shows that detrital inputs from the same provenance influenced the deposition of the Middle Karoo rocks. Distribution trends of elemental proxies for biogenic and terrestrial inputs suggest sediment reworking by turbidity currents influenced the studied rocks' organic richness. This conforms to outcrop description results whereby significant coal fragments suggest the reworking of previous sediments by gravity flows. This aspect may be used to indicate varying source rock potential between the Middle Karoo rocks in the region. Further geochemical studies are needed to confirm this observation, which is essential while assessing the source rock potential of the Karoo rocks in the Tanga Basin.