Himalayan Geology, Vol. 44 (1), 2023, pp. 81-96, Printed in India

Paleoclimatic signatures in shales and mudstones of the Miocene Surma Group and its equivalent Jamalganj Formation in the Bengal Basin, Bangladesh: a review

MD. MEZBAH- UL- BARI, RUMANA YEASMIN*

Department of Geological Sciences, Jahangirnagar University, Savar, Dhaka-1342, Bangladesh

*Email (Corresponding author) : rychumki@juniv.edu

Abstract: The geochemistry of shales and mudstones of the Miocene Surma Group and its equivalent Jamalganj Formation of Bengal Basin is carried out to ascertain the paleoclimatic conditions of the provenance areas. The geochemical compositions of about 141 samples from seven exploration boreholes covering three geotectonic provinces of the Bengal Basin were collected from published sources. Major and trace element compositions and their ratios can be used as proxies for determining paleoclimate of the provenance areas. The Miocene mudstones and shales of both Surma Group and Jamalganj Formation of the Bengal Basin are SiO₂ rich and interpreted as quartzose in composition. The paleoclimatic interpretation from Al₂O₃ /TiO₂, Rb/Sr, Rb/Al₂O₃, Zr/Al₂O₃ and Nb/Al₂O₃ ratios suggest that the provenance areas of the Jamalganj Formation in Stable Shelf were experienced hot and humid climatic condition which caused high rate of precipitation, severe weathering of the parent rocks and consequent high riverine fluxes into the Bengal Basin. In contrast, the Surma Group shales and mudstones of the Sylhet Trough and the Chittagong Tripura Fold Belt were produced in semihumid to humid conditions, where the weathering intensity of the parent rock was moderate to low. The weathering intensity of both Jamalganj Formation and Surma Group shales and mudstones are satisfied by the Al₂O₃ –CaO*+Na₂O–K₂O ternary plot which shows comparable results as the major and trace element ratios.

Keywords : Plaeoclimate, weathering intensity, sediment flux, Miocene, Bengal Basin