Sedimentology and compositional characteristics of siliciclastic and associated sediments in Ruvu basin: implication on paleo-depositional environment, provenance, and tectonic setting

Abstract

The paleo-depositional environment of the Ruvu basin ranges from alluvial, floodplain, low- and high-energy fluvial channels, and deltaic sub-environments of Ngerengere beds, Tanga beds, and Jurassic Msata formations. Amboni limestones that overlap the Karoo sequences, Bagamoyo Formation and Sakura Formation were probably deposited in a shallow marine environment. Trace elements show that the sediments were deposited in oxic environments with a V/Cr ratio of 0.85 and Ni/Co ratio of 2.9 to anoxic environments with a V/Cr ratio >4.3. Low Cr, Ni, Co, and V, high Y/Ni and Zr/Cr ratios of up to 0.83 and 3.6, respectively, and low Ti/Zr ratios ranging from 0.004 to 0.01 indicate a contribution from a felsic source, and few ratios >1 show some contribution from a mafic source. This is supported by the diverse enrichment in light rare earth elements (LREEs), small negative Eu anomalies and modest heavy REEs reflecting a dominantly granitic source. The provenance is probably from the Usagaran mobile belt and Proterozoic Mozambique mobile belt. The cross-plot between Th and Ta from mid-Jurassic to Cretaceous sediments indicates that the basin was essentially developed as a passive continental margin; however, discriminant factor plots of the Karoo indicate the existence of a rift setting.