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<p>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.</p><p>Keywords: Heavy mineral; Detrital zircon U-Pb ages; Paleo-Brahmaputra; Assam-Arakan Basin</p>

Heavy mineral assemblages of sedimentary units are used as an essential provenance constraint. The sedimentary successions of the southern Assam and Siwalik foreland basins were analyzed for their heavy mineral suits and provenance determination. Heavy minerals were identified using the petrological microscope, X-ray Diffraction (XRD) and Electron Probe Micro Analyzer (EPMA) analyses. The study demonstrates that heavy minerals weight percentage in the southern Assam basin is lower than that of the northeast (NE) Siwalik basin. In the Assam basin, the weight percentage of dense minerals varies from 0.08% to 1.31%; however, in the Siwalik sediments, it varies from 2.27% to 5.78%. The relative dominance of opaque minerals over transparent heavy minerals is observed throughout the Cenozoic rocks of Assam basin, except for the Tipam unit. At the same time, the Siwalik basin successions show a low percentage of opaque minerals and high amount of non-opaque heavies. Detritus in the Assam basin reveals a distinct change in the occurrence of heavy mineral assemblages (amphibole, aluminosilicates, staurolite and a high percentage of epidote, pyroxene and sphene) in the Mio-Pliocene unit (Tipam sandstone) compared to Barail and Surma older units, implying involvement of an additional orogenic source. This study suggests that NE Siwalik sediments were primarily derived from Himalayan rocks of the Siang window, while heavies in the detritus of the southern Assam basin exhibits a mixed provenance both from the eastern Himalaya, chiefly from Dibang and Lohit valley and Indo-Burman-Ranges (IBR).

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