COMPOSITIONAL STUDY OF PERMO-CARBONIFEROUS SILICICLASTIC GONDWANAN SEQUENCES FROM CENTRAL-EASTERN INDIAN PLATFORM

The Late Carboniferous-Permian deglaciation and subsequent marine encroachment triggered deposition of Gondwanan sequences in the intracratonic basins of the Indian Platform, part of the eastern Gondwanaland. These sedimentary sequences are distributed in multiple sub-basins of present-day central-eastern India and northwestern Bangladesh which shows variability in both petrography and mineralogy providing evidences of changes in depositional environments and outline possible source areas for sediments.

Here we present, petrographical and mineralogical analysis of 15 Gondwanan sandstone samples collected during previous and last field seasons from Jharia basin in India and Barapukuria, Khalashpir, Jamalganj and Naogaon basins in Bangladesh. Two samples from the lower Gondwanan Talchir Formation in India show an average modal composition of Qt₆₉F₁₉L₁₂ that varies significantly with the three quartzose samples from upper Gondwanan Barakar Formation with an average modal composition of Qt₈₇F₁L₁₂. In contrast sediments from Bangladesh are mostly arkosic to subarkosic with average modal composition of two samples from Barapukuria showing Qt₆₈F₂₀L₁₂, three samples from Khalashpir showing Qt₇₀F₁₃L₁₇, three samples from Jamalganj showing Qt₇₄F₁₁L₁₅ and two samples from Naogaon showing Qt₇₁F₁₈L₁₁. Heavy mineral analysis shows mixed assemblages of minerals including garnet, apatite, rutile, tourmaline, zircon and opaques that are mostly magnetite, hematite and possible limonite and ilmenite. Minor amount of epidote, sphene and staurolite was also observed. Sandstones from the Barakar Formation show significant increase in stable minerals (ZTR) percentage of ~34% whereas all the other samples have an average of less than ~10%.

Modal analysis indicates presence of multiple source terrane for the sediments in India and Bangladesh. Possible source areas in support of heavy mineral assemblages includes the Prydz Bay Belt and the Kuunga orogen in northeastern Antarctica, the Pinjarra and the Albany-Fraser systems in southwestern Australia as well as the Indian cratons. Planned microprobe and detrital geochronological analysis are expected to narrow down possible source terranes and provide critical information related to the tectonic reconstruction of the eastern Gondwanaland.