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

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 semi -humid 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.