Petrofacies and Paleotectonic Evolution of Permo-Carboniferous Gondwanan Sequences of the Bengal Basin, Bangladesh

Abstract

The Indian subcontinent, along with Australia and Antarctica, constituted Eastern Gondwananland. Permo-Carboniferous Gondwanan sequences have been reported from several isolated basins of the Peninsular India. These siliciclastic sequences were drilled in intra-cratonic basins in northwest Bengal Basin. This ~1-km-thick sequence consists primarily of massive and trough cross-bedded sandstones and laminated mudstones, with localized conglomerate and coal layers. Sandstone petrography, heavy mineral assemblage studies, heavy mineral geochemistry, and detrital geochronology were used in this study to decipher provenance history of Gondwanan sediments at two localities (Khalashpir and Barapukuria) from the Bengal Basin of Bangladesh. Petrographic studies suggest that these sequences are mostly immature and poorly sorted arkosic sandstones (Khalashpir-Qt58F30L12, Barapukuria-Qt52F31L17), with some compositions ranging from quartzarenite to litharenite. Although monocrystalline quartz is dominant, considerable polycrystalline quartz fragments have also been found. K-feldspars dominate over plagioclase feldspars. Among lithic fragments, sedimentary types are abundant. Significant amounts of chert are observed. Heavy minerals are volumetrically rare and of low diversity in sediments of northwest Bangladesh. However, samples from Khalashpir have higher heavy mineral concentrations than those from Barapukuria. Garnet geochemistry suggests a metamorphic grade in the source terranes containing amphibolite and granulite facies rocks. Tourmaline chemistry suggests that the sediments were derived from aluminous and Ca-poor metapelites, metapsammites, and quartz tourmaline rocks for Khalashpir and well DOB 2 of Barapukuria. However, tourmaline samples from well DOB 4 of Barapukuria suggest Li-poor granitoid, pegmatite and aplitic sources. Laser 40Ar/39Ar ages were determined for single crystals of detrital muscovite from Gondwanan sequences. Three samples were analyzed from Khalashpir well GDH 46 at different stratigraphic levels. The deepest sample yielded the broadest age range, with a dominant mode at circa 515 Ma and lesser clusters of ages at circa 550, 570, and 600 Ma. The other two shallower samples are dominated by ages with similar single modes at circa 495-500 Ma. The oldest samples may have been derived from the adjacent Indian craton and/or the Meghalayan craton. Younger samples were contributed from the Pinjarra orogen and proto-Himalayan orogens formed during Neoproterozoic to Early Paleozoic collision between India and Australia.