PETROFACIES AND PALEOTECTONIC EVOLUTION OF PERMO-CARBONIFEROUS GONDWANAN SEQUENCES OF THE BENGAL BASIN, BANGLADESH

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, adjacent to Indian craton. This ~1-km-thick sequence consists primarily of massive and trough cross-bedded sandstones and laminated mudstones, with localized conglomerate and coal layers.

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 contents are dominant, considerable polycrystalline quartz fragments have also been found. Plagioclase feldspars dominate over orthoclase feldspars. Among lithic fragments, sedimentary types are abundant. Significant amount of chert are observed. Inhomogeneous distribution of sedimentary features between Khalashpir and Barapukuria suggest different tectonic and/or sedimentological influences in the study area. Heavy minerals are volumetrically rare and of low diversity the sediments of northwest Bangladesh. Garnets are the dominant non-opaque heavy mineral types. Garnet geochemistry suggests a metamorphic grade in the source terranes representing amphibolite and granulite facies.

Laser ⁴⁰Ar/³⁹Ar ages for single crystals of detrital muscovite from Gondwanan sequences revealed Neoproterozoic to Late Ordovician crystallization time. The oldest samples may have been derived from the adjacent Indian craton and/or the Shillong Meghalayan craton. Younger samples were contributed from the Neoproterozoic to Early Cambrian Pinjarra orogen and proto-Himalayan orogens formed during Neoproterozoic to Early Paleozoic due to the collision between India and Australia.