PETROFACIES AND PALEOTECTONIC EVOLUTION OF PERMO-CARBONIFEROUS GONDWANAN SEQUENCES OF NORTHWESTERN BENGAL BASIN

The Indian subcontinent, along with Australia and Antarctica, constituted the Eastern Gondwanaland. Permo-carbonifeorus Gondwanan sequences have been reported from several isolated basins of Peninsular India. At the eastern limit these sequences, represented by ~1-km thick Kuchma and Paharpur formations, have been encountered at northwestern Bengal basin from different exploratory wells. Depth of these Gondwanan sequences increase towards southeast, indicating subsidence during Gondwanan rifting. Sandstone modal analysis and heavy mineral studies from drilled-core samples have been carried out to obtain petrofacies and paleotectonic evolution history of Permo-carboniferous Gondwanan sequences of northwestern Bangladesh.

Petrographic studies suggest that these sequences are mostly immature and poorly sorted arkosic sandstones (~Qt₈₁F₁₄L₅), with some compositions ranging from quartz-arenite to litharanite. 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. Higher concentrations of heavy minerals are found in Barapukuria (0.021gm to 1.35gm; toward north) than Khalashpir wells (0.053gm to 0.98gm; southeast). The late-Paleozoic Gondwanan sequences are well known for coal deposits, which is also evident by the abundance of organic matters in the core samples. Several mud injections were observed in the samples from different stratigraphic levels.

Sandstone composition, sedimentary features and textural properties suggest that these sequences were derived from a short distance, most likely the shield areas of eastern India and east Antarctica, and possibly from the adjacent Kuunga orogen existed between India and Antarctica during the late Paleozoic.