Tectonic and sedimentary evolution of the coastal basin of Tanzania during the mesozoic times
The present paper reviews and updates the stratigraphy and tectonic evolution of the coastal basin of Tanzania during the Mesozoic times based on the interpretation of existing palaeontological and stratigraphical information and supplemented by recent fieldwork investigations by the author.
Tectonic events largely controlled the evolution of the coastal basin of Tanzania and the Indian Ocean. These included the Karoo rifting during Permo-Triassic, the break up of the Gondwana Supercontinent, which started with rifting in the Triassic period, the opening of the Somali basin in the Middle Jurassic, and the Cenozoic rifting along the East African rift system. The Karoo rifting created a zone of weakness that led to the fragmentation of the Gondwana Supercontinent.
The Early Jurassic continental sediments developed in the graben-like structure with occasional marine incursions reflecting ancient seaways on the ocean side. The early Middle Jurassic oolitic limestones developed along the shoreline forming the marine carbonate platform. During these times, the area was periodically under fresh water influence. During Bajocian-Bathonian times, the whole of Tanzanian coast was completely under marine conditions though restricted to shallow sea in which coral reefs could grow. It was during these times that a stable continental shelf developed. However, the stability of the shelf was later affected by the reactivation of the Tanga Fault pattern. As a result, the sediments from the provenance including the Bajocian-Bathonian coral reefs were reworked and transported by means of gravity flow and fluvial influence, and deposited in a fault controlled shallow basin at Msolwa to form sub-marine fans. From the Callovian to Kimmeridgian, the main facies included the transgressive low energy shale and septarian marls rich in fossils, particularly ammonites. During this period, an open marine environment was predominant. This transgressive episode was succeeded by a regressive phase during Lower Cretaceous in which sandstone deposits were predominant. The Upper Cretaceous transgressive sediments (shales and marls) lie unconformably over the regressive older deposits, marking the topmost part of the Mesozoic sequence in the study area.