Petrofacies Analysis and Detrital Geochronology of Oligocene Sediments from the Bengal Basin and Southeast Shillong, Northeast India

Abstract

Oligocene sedimentation at the Assam-Bengal system provides a record of shifting sediment source areas and unroofing events in the adjacent Himalayan mountain and accretionary wedges of Indo-Burman ranges during the early stages of India-Eurasia collision. The present study is focused on the Oligocene sediments known as the Barail Group that exposed in the Sylhet Trough of the Bengal Basin (800-1600 m) and in the southeast of the Shillong Plateau (~4600 m) also known as lower Assam Basin of NE India. Mineralogical, geochemical, and geochronological studies on samples collected from the above locations provide critical information for the evolution of the Oligocene sequences of these basins. In the Bengal Basin, the Oligocene Barail sandstones show a mixed provenance bearing the signature of both stable craton and orogenic belt. Modal analyses show that Oligocene sandstone in the Sylhet Trough is mostly sublitharenite with a mean composition of Qt79F1L20. The lower Oligocene units in this location are quartz-rich, poor in feldspar and lithic fragments, which indicate the Indian Craton as a potential source area. The upper Barail units are mostly litharenites and contain abundant sedimentary and metasedimentary lithic fragments suggesting derivation from adjacent orogenic belts. In southeast Shillong, the modal composition of Barail sediments shows similar nature (dominantly litharenite with a mean composition of Qt68F3L29) with that of the upper Barail Group of Sylhet Trough and points to the adjacent orogens as provenance. Heavy mineral assemblages in both Sylhet Trough and southeast Shillong are rich in opaque and ultra-stable minerals including zircon, tourmaline, and rutile (ZTR). In Sylhet Trough, the heavy mineral data exhibit mixed source which includes both cratonic (lower Barail samples with high ZTR index) and orogenic provenance (upper Barail samples with relatively low ZTR index and increasing unstable heavy mineral assemblages). In southeast Shillong, the increased concentration of unstable heavy minerals (such as chrome spinels, amphibole, and serpentine) and relatively low ZTR index (30.25%) of non-opaque minerals suggests the orogenic source. Chrome spinel data in Oligocene samples from the Sylhet Trough are rich in chromium with low Fe3+ and might be derived from Himalayan Alpine-type ophiolites and those in southeast Shillong were mostly derived from the Indo-Burman Ranges. Tourmaline data from the Bengal Basin and southeast Shillong also indicate a dominantly low-grade metasedimentary provenance with a minor contribution from granitoid and pegmatitic rocks. In Sylhet Trough, laser 40Ar/39Ar analyses on detrital muscovites of Barail sandstone show a wide range of values that point to contribution from a diverse source. A conspicuous difference in dominant age mode is observed between the lower and upper units of the Barail Group. The lower Barail sediments yield mainly Cambro-Ordovician ages with dominant mode range from ~537 Ma to 447 Ma. Muscovites from the lower Barail Group of the Bengal Basin were likely derived from nearby Indian shield areas particularly the Chotanagpur gneissic complex to the west. In contrast, muscovites from the upper Barail Group dominated by Cenozoic ages (66 to 23 Ma). Notably, lower Barail has only one muscovite grain that falls in the Cenozoic age range. Age data from the upper Barail Group of the Sylhet Trough of the Bengal Basin are interpreted to have probable Himalayan source terrane(s). Oligocene Barail sandstones from the southeast Shillong contain muscovites of a wide range of ages with the dominant mode at Paleogene (~28 to 81 Ma). During the lower Oligocene, the Bengal Basin perhaps was positioned further south by the equator resulting in exposure to intense chemical weathering, and the Himalayan tectonism during that time was probably more distant from the Bengal Basin. Considering the similarly (in sediments composition and detrital muscovite ages) of detritus of upper Oligocene with that of southeast Shillong, this can be suggested that the Bengal Basin was brought closer to the southeast Shillong to receive similar sediments from the Himalayas, the Tibetan Plateau, and the Indo-Burman Ranges.