Sandstone-Shale Geochemistry of Miocene Surma Group in Bandarban Anticline, SE Bangladesh: Implications for Provenance, Weathering, and Tectonic Setting

Abstract

The present study analyzes the geochemical composition of sandstone and shale of the Miocene Surma Group to decipher the provenance, tectonic settings and paleoweathering condition of source area in the Bandarban Anticline which is at the western margin of Indo-Burmese Hill Ranges. Statistical empirical index of chemical weathering of the sediments that have been extracted by the Principal Component Analysis (PCA) is used to understand the weathering profile of the sediments of the study area. The PCA of the geochemical composition yields three principal components (PC–1, PC–2, and PC–3), which capture total variance 52.83%, 17.58% and 6.94%, respectively. The PC–1 shows the loss of SiO2 during weathering of preexisting source rocks; PC–2 reveals the enrichment of Na2O, CaO, and P2O5 due to leeching and carried by groundwater during weathering; highest loadings with MnO and Cr shows in PC–3 due to redox environment during early diagenetic of marine sediments. The MFW and A–CN–K diagrams show an intense weathering trend, and backward trend of the MFW diagram and the major elements provenance discriminant diagram refers to the mature polycyclic quartzes provenance and originated dominantly from felsic to intermediate igneous rocks. The trend of the SiO2/Al2O3–Na2O/K2O shows the hydraulic sorting effect and sediments were originated primarily from a recycled sedimentary provenance. The CIA (67.68–80.89), ICV (0.60–1.29, avg. 0.83) and K2O/Na2O ratios show a moderate to high maturity of the sediments and is derived from both weak and intensively weathered source rocks. Discriminate diagrams related to tectonic provenance refer to the deposit of the sediment dominantly under the influence of collision (active continental collision, compression) and mature sediment derived to the depositional basin after upliftment of the source areas after that collision.