| Sandstone compositions in Tertiary stratigraphic sequences of the Bengal basin have been studied in order to constrain sediment-source rock types and to document temporal variation in detrital composition. This was complemented by lithofacies analysis of subsurface well logs and cores, focusing on hydrocarbon-rich Miocene strata, to constrain paleogeography.; Eo-Oligocene sandstones are dominated by sub-angular monocrystalline quartz grains with only minor feldspar grains and lithic fragments {dollar}rm (Qtsb{lcub}94{rcub}Fsb3Lsb3).{dollar} These sandstones contain sparse heavy minerals {dollar}({lcub}sim{rcub}0.2%),{dollar} dominated by opaques and stable heavies, especially tourmaline, garnet, rutile, and zircon. Sandstones of the Miocene Surma Group are rich in feldspar grains and in argillite and low-grade metamorphic lithic fragments relative to Oligocene sandstones. Heavy minerals are abundant and diverse, with abundant blue-green amphibole and sparse chromite in the Boka Bil Formation. Maps compiled of subsurface sand thicknesses and sand/shale ratios of the Surma Group describe a large-scale meander extending westward from NE Bengal basin and turning southward toward the Bengal fan.; Sandstones in younger units contain almost all of the mineral species present in the Miocene Surma Group. These younger sandstones are rich in K-feldspars relative to the plagioclase-rich Surma sandstones. Low- to intermediate-grade metamorphic lithic fragments are more common in these units, with very little or no volcanic lithic fragments. These units show appreciable orthopyroxene, sillimanite, and andalusite.; These petrographic analyses suggest that tectonism began considerably later in the eastern Himalaya than has been documented to the west, with major unroofing beginning in the earliest Miocene. Whereas Eo-Oligocene sandstones appear to be derived largely from the Indian craton, Miocene and younger sandstones show orogenic provenance, with lithic populations indicating progressive unroofing through time. In particular, the abundance of K-feldspar in the younger (Plio-Pleistocene) sandstones, relative to plagioclase-rich Surma sandstones, indicates a granitic source, probably Miocene leucogranites of the High Himalayan Crystalline terrane. Lithofacies analysis suggests that deltaic deposits of the Miocene Surma Group filled the Sylhet Trough of the northeast Bengal basin from the east, and that the source terrane of this sediment included the north-trending Indo-Burman ranges immediately adjacent to the east. |
