A COMPARATIVE SANDSTONE PETROGRAPHIC AND HEAVY MINERAL STUDY OF THE OLIGOCENE SEDIMENTS FROM THE BENGAL BASIN AND SOUTHEAST SHILLONG, NE INDIA: UNDERSTANDING THE TECTONICS ALONG THE NORTHERN MARGIN OF THE BENGAL BASIN

The Oligocene Barail deposit of the Bengal Basin, exposed only at the Sylhet Trough, is relatively thin (~ 800 m) and mainly composed of siltstones, silty shale, and sandstones. In contrast, the arenaceous Oligocene deposit of the southeast (SE) Shillong which is situated to the north of the Sylhet Trough is ~ 4650 m thick and divided into three formations from old to young: Laisong, Jenum, and Renji. Their depositional environments ranged from marginal marine to turbiditic at the SE Shillong and shelf dominated tidal at the Sylhet Trough. A distinct variation in lithofacies, sandstone modal analysis and heavy mineral analysis suggest a difference in source rock of these two neighboring Oligocene deposits at the foreland basin of the eastern Himalaya.

The Oligocene sandstones from both locations consist dominantly of monocrystalline and polycrystalline quartz, sedimentary and metamorphic lithic fragments with very little feldspars. Sandstones in Sylhet Trough are more quartzose which contain less feldspar and lithic fragments (indicate relatively high sediment maturity) than those of the SE Shillong. The ZTR (zircon-tourmaline-rutile) index is high in Sylhet Trough sediments (36.41%) compared to the Oligocene sediments of SE Shillong (26.44%). The high ZTR index and overall low unstable mineral assemblages indicate higher maturity and intense chemical weathering of the Oligocene sediments of the Bengal Basin, which may have derived from the neighboring Indian craton. On the other hand, the sandstone composition of the Sylhet Trough (Qt₇₈ F ₁L₂₀) and SE Shillong (Qt₆₆ F ₄L₃₀) reflect provenance from orogenic belts (Himalayas and Indo-Burman ranges). During the Oligocene, the Bengal Basin perhaps was positioned further south by the equator resulting in exposure to intense chemical weathering and Himalayan tectonism during that time was probably more distant from the Bengal Basin of present time. Ongoing study on detrital geochronology and mineral mapping using an electron probe is expected to provide additional nformation on provenance of these two neighboring Oligocene sandstones.