Multiproxy records on the fluvial successions of Assam-Arakan Basin: Implication for paleo-drainage evolution

Assam-Arakan Basin comprises Cenozoic sedimentary successions, located in northeastern India is juxtaposed to both the Himalaya and Indo-Burman Ranges (IBR). The Upper Miocene-Pliocene (Tipam sandstone) and the overlying younger Upper Pliocene-Pleistocene units (Dupi-Tila/Namsang/Dihing) of this foreland basin are fluvial successions. Heavy mineral as detritus provenance indicator has been used as one of the multiproxy records on the fluvial sequences of Assam-Arakan Basin to unravel the drainage system that deposited the same in this basin. Previous workers have advocated that the paleo-Brahmaputra river had initially flowed east of Shillong Plateau before being deflected northwesterly taking the present-day course parallel to the Plateau. However, unequivocal evidence of paleo-Brahmaputra remains enigmatic. The study demonstrates the provenance for the fluvial sedimentary units of the above foreland basin using petrography and heavy mineral distributions. X-ray Diffraction (XRD) and Electron Probe Micro Analyzer (EPMA) analyses were employed to correctly identify the heavy mineral species and support the semi-quantitative analysis of heavy minerals in the basin. The outcome of the study provides new insights towards the paleo-drainage evolution of the river course accountable for the fluvial sedimentation in the Assam-Arakan Basin. Clast petrography and heavy mineral observations indicate the probable source from Lohit- Dibang valley. Initial analysis of detrital zircon U-Pb ages from studied samples reveals major age peaks at around 500 Ma and 1025 Ma with young ages between 16 Ma and ~140 Ma. These samples do not provide ages < 10 Ma, signifying the sediments not derived from Namche Barwa massif, eroded by the Tsangpo-Siang-Brahmaputra river system. It is in contrast to similar sediments in the Siwaliks of NE Himalaya. The data supports our observation that the paleo-Brahmaputra seems not the cause for these deposits, at least during the Pleistocene. If Paleo-Brahmaputra got diverted during this period, it requires scanning the detritus from Tipam units and additional samples from Dupi-Tila/Namsang/Dihing units across the entire Assam-Arakan range to infer source and drainage system for these deposits. We tentatively propose that the Tipam and the younger Dupi-Tila/Namsang/Dihing units in the Assam-Arakan Basin were deposited by drainage flowing from Dibang-Tezu valley, that was initially linked to the Irrawaddy river system. The uplift along Naga thrust caused drainage migration, eventually meeting the present-day Brahmaputra course.

Keywords: Heavy mineral; Detrital zircon U-Pb ages; Paleo-Brahmaputra; Assam-Arakan Basin