QUANTIFYING STRATIGRAPHIC CORRELATIONS AND PROVENANCE WITHIN THE ANCESTRAL BRAHMAPUTRA DELTA, A RECORD OF EASTERN HIMALAYAN EXHUMATION AND THE ONSET OF THE INDIAN MONSOON

The IndoBurma Ranges (IBR) peripheral basin records late Cenozoic exhumation of the eastern Himalayas (EHS) and progradation of the ancestral Brahmaputra delta. To place IBR strata in a regional tectonostratigraphic context with the Bengal and Nicobar Fans, and EHS, we present new lithofacies descriptions, high-n U-Pb detrital zircon (dzUPb), and detrital zircon and apatite fission track analyses from Oligocene–Pliocene IBR deposits. Using statistical tests (Saylor and Sundell, 2016), we correlate IBR dzUPb age-distributions with published dzUPb data from equivalent age deposits of the EHS Siwalik Group as well as the modern Ganges and Brahmaputra rivers. We characterize the proportions of Himalayan source terranes present in IBR strata using an inverse Monte Carlo dzUPb unmixing model (Sundell and Saylor, 2017). Results indicate that dzUPb age-distributions from fluvial facies of the IBR Tipam Group match those of the late Miocene–Pliocene Middle Siwalik Group and modern Brahmaputra river deposits, confirming that the Tipam Group represents the late Miocene–Pliocene Brahmaputra braid-belt. Shallow marine and intertidal deposits (IBR Surma Group) have dzUPb age-distributions that match those from the mid-late Miocene Lower–Middle Siwalik Group. Oligocene to early Miocene IBR deposits (Barail Group) are not correlative with the Siwalik Group. Barail Group dzUPb age-distributions are similar to that of sediment from the modern Ganges river system, indicating deposition prior to development of a Brahmaputra delta in the IBR. Results from dzUPb unmixing models indicate that both Lhasa and Transhimalayan arc grains are present in all of the IBR formations. The proportion of Transhimalayan arc sediment increases up section from ~3% (Barail Group) to ~30% (Tipam Group). The proportion of Tethyan and Greater Himalayan sediment generally decreases up-section from ~15-70% (Barail Group) to <10% (Tipam Group). We interpret that the middle Miocene stratigraphic transition from the Barail to the Surma group (~12 Ma) and the corresponding increase in the proportion of Transhimalayan grains at the expense of Tethyan and Greater Himalayan grains records headward expansion of the paleo-Brahmaputra watershed commensurate with increased discharge following a middle Miocene onset of the Indian Monsoon.