RECONSTRUCTING EASTERN GONDWANALAND: DETRITAL HISTORY OF PERMO-CARBONIFEROUS GONDWANAN SEQUENCES FROM SOUTH ASIA

The Indian subcontinent, Australia, Antarctica, and the Madagascar made up the Eastern Gondwanaland, once existing part of a supercontinent. Gondwanan sequences in the Indian subcontinent were initially deposited in one master basin. In India and Nepal, these Permo-Carboniferous siliciclastic Gondwanan sequences are exposed in several isolated graben basins and two flanks of the Himalayas respectively, whereas in Bangladesh these sequences are drilled in intra-cratonic basins in northwest Bengal Basin. These sequences of variable thicknesses consist primarily of massive and trough cross-bedded sandstones and laminated mudstones, with localized conglomerate, diamictites, and coal layers. Sandstone petrographic analysis were done for thirty five samples from different stratigraphic levels of Gondwanan sequences of Jharia basin (India) and from three different drilled wells in northwest Bangladesh to help reconstruct regional detrital history of Late Paleozoic Eastern Gondwanaland.

The sandstones from Bangladesh are angular to sub-angular and show greater immaturity compared to the coeval sandstones from India. Several tectonic provenance fields were identified for these mostly poorly sorted arkosic and quartz arenitic sequences. Sandstone modal analysis shows that Gondwanan sediments from Bangladesh area (~Qt₆₆F₂₀L₁₄) plot in the ‘recycled orogenic’ to ‘transitional continental’ provenance fields of Dickinson (1985) whereas Indian Gondwanan samples (~Qt₈₄F₃L₁₃) plot in the ‘craton interior’ to ‘recycled orogenic’ fields. Monocrystalline quartz was dominant over polycrystalline quartz in both Indian and Bangladesh samples. Although significant amount of feldspar was observed from the Bangladesh samples and the Talchir Formation from India, no feldspar was found in the samples from the Barakar Formation (India). The highest concentrations of heavy minerals were found in the Barakar Formation. Mud injections in several samples were found to be calcified.

Compositional as well as textural variations suggest different provenance of contrasting source rocks for these sediments of similar depositional age. Ongoing detrital geochronology and mineral chemistry studies will help better understand the provenance of these Permo-Carboniferous sequences.