PETROFACIES EVOLUTION OF EARLY TERTIARY SEQUENCES FROM SOUTHEAST SHILLONG PLATEAU, NORTHEAST INDIA

A ~12-km thick sequence of primarily siliciclastic Tertiary deposits outcrop in a continuous section in the south-southeastern Shillong Plateau of northeast India. This area is at the junction between the Assam and Bengal basins, two large depocenters south of the eastern Himalayas. Sandstone modal analysis on these Tertiary sequences document the detrital history of the Assam-Bengal system.

The Eocene Jaintia Group is one-kilometer of deltaic to shallow-marine strata ranging from coaliferous clastic deposits and fossiliferous limestones to black shale. The Tura Sandstone, the basal part of the Jaintia Group, is compact, and composed of silica- cemented quartz arenite, indicating a cratonic source. The Kopili Formation in the upper part of the Jaintia Group is matrix-rich fine-grained quartz wacke. This sequence closely resembles the Eocene passive margin sequence of the Sylhet trough of the Bengal Basin. In contrast, the Eocene section in Assam (Margherita-Changlang area) is clastic (deep- water Disang Group), which represents an active margin deposit. The Oligocene Barail Group is thick (4.65 km), conformably overlies the Jaintia Group, and is composed of deep-marine clastic deposits. These submature litharenites are composed of abundant sedimentary lithic clasts and sparse glauconite grains. A lithic arenite sandstone member from the upper Oligocene formation contains chert fragments and abundant detrital muscovite. Coeval Oligocene strata in both Assam (~5 km) and the northeast Bengal Basin (~ 1.5 km) are deltaic to marginal-marine deposits as evidenced by presence of coal seams in the Assam Basin. The Miocene Surma Group (~2.5 km) represents a deep-marine sequence in the southeast Shillong Plateau, consisting of matrix-rich, fine-grained lithic wacke with a dominance of low-grade metamorphic clasts and subordinate sedimentary and chert fragments. The Surma Group is overlain by upper Miocene to Pliocene quartz arenites of the Tipam Sandstone (1+ km).

Early Paleogene clastic units in the study area around the Shillong Plateau were derived initially from a cratonic source with a shift in sediment source to an orogenic belt beginning in the late Eocene.