NEOGENE SHALLOW-MARINE AND FLUVIAL SEDIMENT DISPERSAL, BURIAL, AND EXHUMATION IN THE ANCESTRAL BRAHMAPUTRA DELTA: INDO-BURMAN RANGES, INDIA

摘要

The stratigraphic record of Cenozoic uplift and denudation of the Himalayas is distributed across its peripheral foreland basins, as well as in the sediments of the Ganges-Brahmaputra Delta (GBD) and the Bengal-Nicobar Fan (BNF). Recent interrogation of Miocene-Quaternary sediments of the GBD and BNF advance our knowledge of Himalayan sediment dispersal and its relationship to regional tectonics and climate, but these studies are limited to IODP boreholes from the BNF (IODP 354 and 362, 2015-16) and Quaternary sediment cores from the GBD (NSF-FIRE: Life on a tectonically active delta, 2010-18). We examine a complementary yet understudied stratigraphic record of the Miocene-Pliocene ancestral Brahmaputra Delta in outcrops of the lndo-Burman Ranges fold-thrust belt (IBR) of eastern India. We present detailed lithofacies assemblages of Neogene delta plain (Tipam Group) and intertidal to upper-shelf (Surma Group) deposits of the IBR based on two similar to 500 m stratigraphic sections. New detrital-apatite fission-track (dAFT) and (U-Th)/IIe (dAlIe) dates from the Surma Group in the IBR help to constrain maximum depositional ages (MDA), thermal histories, and sediment accumulation rates. Three fluvial facies (F1-F3) and four shallow marine to intertidal facies (M1-M4) are delineated based on analog depositional environments of the Holocene modern GBD. Unreset dAFT and dAlIe ages constrain MDA to similar to 9-11 Ma for the Surma Group, which is bracketed by intensification of turbidite deposition on the eastern BNF (similar to 13.5-6.8 Ma). Two dAlle samples yielded younger (similar to 3 Ma) reset ages that we interpret to record cooling from denudation following burial resetting due to a thicker (similar to 2.2-3.2 km) accumulation of sediments near the depocenter. Thermal modeling of the dAFT. and dAHe results using QTQt and IIeFTy suggest that late Miocene marginal marine sediment accumulation rates may have ranged from similar to 0.9 to 1.1 mm/yr near the center of the paleodelta. Thermal modeling results imply postdepositional cooling beginning at similar to 8-6.5 Ma, interpreted to record onset of exhumation associated with the advancing IBR fold belt. The timing of post-burial exhumation of the IBR strata is consistent with previously published constraints for the avulsion of the paleo-Brahmaputra to the west and a westward shift of turbidite deposition on the BNF that started similar to 6.8 Ma. Our results contextualize tectonic controls on basin history, creating a pathway for future investigations into autogenic and climatic drivers of behavior of fluvial systems that can be extracted from the stratigraphic record.