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

The Oligocene deposits of the Bengal Basin exposed only at the Sylhet Trough, as the Barail Group, is relatively thin (~ 800 m) and mainly composed of siltstone, silty shale, and sandstone. In contrast, the arenaceous Oligocene deposits 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 formations. The 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 Himalayas.

Oligocene sandstones from both locations consists dominantly of monocrystalline and polycrystalline quartz, sedimentary and metamorphic lithic fragments with very little feldspar. Sandstones in Sylhet Trough are more quartzose and contain less feldspar and lithic fragments (indicate relatively high sediment maturity) than those of the SE Shillong. The high ZTR (zircon-tourmaline-rutile) index (36.41%) and overall low mineral assemblages also resembles higher maturity and intense chemical weathering and suggesting that the Indian craton toward the west might have been a possible source area for the Oligocene sediments of the Bengal Basin. On the other hand, the sandstone composition of Sylhet Trough (Qt₇₈ F ₁L₂₀) and SE Shillong (Qt₆₆ F ₄L₃₀) reflect a recycled orogenic provenance. During the 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 than at present.