PALEOGENE STRATIGRAPHY OF THE SOUTHEAST SHILLONG PLATEAU AND NORTHERN SYLHET TROUGH AREA OF THE EASTERN HIMALAYAS

Paleogene sediments deposited in subsiding foreland troughs of lower Assam, southeast of the Shillong Plateau and the Bengal Basin, decreases in thickness toward southwest. Abrupt change in thickness is observed from southeast of the Shillong Plateau (~5.5 km) in India to the northern Sylhet Trough (~1.8 km) of the Bengal Basin. Such a change in ~100 km may be attributed to variation in depositional facies developed from Paleogene pre-shortened topography of the region.

The lowermost unit of the sequence is the pre-Himalayan calcareous sandstone, and limestone of the Therria/Tura Formation of the Jaintia Group, which is regarded as upper Paleocene to lower Eocene shelf facies. This unit is conformably overlain by the mid-Eocene Sylhet Limestone and upper Eocene Kopili Formation. In addition to nummulitic limestone in Sylhet Trough, the Shillong section also has alternating sand layers, reflecting changes in depositional environment and differential clastic influx over time. The Kopili Formation hosts an alternating sequence of sandstone, siltstone, and black shale with marl beds in Shillong, but comprises mostly dark gray mudrock containing subordinate marl beds in the Sylhet Trough, representing prodeltaic to shallow marine facies. The Kopili in Shillong contains more organic carbon compared to the Sylhet Trough, suggesting anoxic and deep basinal conditions in the Shillong area. The Kopili Formation is conformably overlain by fining-upward Oligocene Barail Group, which consists predominantly of sandstones with subordinate shales deposited in a lowstand fluvial to estuarine environments.

The Paleogene sequences of the Shillong Plateau and Sylhet Trough represent downstream and somewhat younger asymmetric orogenic depocenters, which received clastic wedges from the rising Himalayas and the Indo-Burman ranges during the Oligocene. The depocenters shifted south and westward since the Oligocene resulting from lithospheric flexure proximal to the orogenic fronts.