40AR/39AR DETRITAL MUSCOVITE GEOCHRONOLOGY OF CENOZOIC STRATIGRAPHIC SEQUENCES OF THE ASSAM-BENGAL SYSTEM

The Cenozoic erosional history of the Himalayas is recorded in part within the kilometers thick clastic wedges deposited in the subsiding foreland basins of the northeast-southwest trending Assam-Bengal (A-B) system. Compositions of these predominantly siliciclastic sequences provided contrasting provenance information suggesting shifts in depocenters due to uplift of the Himalayas and subsequent changes in subsidence patterns in the foreland basin. Laser ⁴⁰Ar/³⁹Ar ages were determined in the ANIMAL facility for detrital muscovite grains from Cenozoic sequences of three depocenters: i) upper Assam, near the Schuppen belt area; ii) east of the Shillong Plateau, southeastern part of the Meghalaya and lower Assam; and iii) the Sylhet trough and Chittagong Hills of Bangladesh in the A-B system.

Muscovites from the Oligocene of upper Assam provide cooling ages ranging from Late Triassic to Eocene with dominance of Late Cretaceous grains. In the samples east of the Shillong Plateau, muscovite grains show cooling ages from Paleoproterozoic to Early Oligocene with conspicuous presence of Eocene, Late Cretaceous, and Late Cambrian-Early Ordovician assemblage.

Muscovites from the Miocene of the upper Assam range from Oligocene to Late Cretaceous, whereas the muscovites from Shillong Plateau area range from Miocene to Early Cambrian. Muscovite grains from Bangladesh also provide a similar distribution (Miocene to Early Cambrian) of cooling age.

This study suggests that sediments from upper Assam were derived mostly from the rising Himalayas and the Indo-Burman ranges. Although the area around the Shillong Plateau and the Bengal basin received sediments from the Himalayas and Indo-Burman ranges, the Indian shield also contributed sediments to these sequences as well. Absence of Late Oligocene muscovites from Oligocene sediments in Meghalaya suggest that the Shillong Plateau was not a source of the muscovites until the plateau uplifted in the Miocene. The Eocene and older detrital age modes for muscovite that are characteristic of the Oligocene from the Meghalaya are replaced by a prominent Miocene age mode in the Miocene sediments of the Meghalaya. This shift in detrital muscovite ages is interpreted to result from Miocene uplift of the Shillong Plateau and influx of sediment from the plateau to the Meghalaya section.