MIXED SIGNALS: EXPLORING VARYING CONNECTIONS BETWEEN THE MIOCENE-PLIOCENE PALEO-BRAHMAPUTRA DELTA AND THE BENGAL AND NICOBAR FANS

Miocene-Pliocene facies within the Indo-Burman Ranges (IBR) indicate sedimentary environments equivalent to modern analogs on the Ganges-Brahmaputra-Meghna delta (GBMD). Depositional settings ranging from large fluvial, intertidal/upper shelf, and lower shelf/slope environments are recognized within the Tipam, Upper Surma, and Middle-lower Surma Formations, respectively. We use detrital zircon (DZ) U-Pb LA-ICPMS analyses to link changing provenance with syn-depositional tectonics and delta processes. Detrital zircon spectra indicating Himalayan provenance appear in all of the DZ results. Significant proportions (≥ 25%) of < 200 Ma grains are present within the Miocene upper Surma (shallow marine) and Tipam (fluvial) sediments and are interpreted to record a Gangdese Batholith source. In contrast, the lower Surma (shelf to slope) and older deposits consist of Tethyan and Greater Himalayan-sourced sediments and contain a small (≤ 12%) proportion of grains < 200 Ma. These results indicate that the upper Surma and Tipam Groups represent an influx of deeply exhumed Himalayan source terrane sediments. Preliminary multidimensional scaling analyses show that Tipam and Upper Surma grains cluster with sediments from the modern Brahmaputra River and (to a lesser extent) the Miocene-Pliocene Nicobar Fan, whereas the middle-lower Surma shows associations with the Miocene-Pliocene Bengal Fan and contains a mixed Ganges/Brahmaputra signal. We present two possible hypotheses to explain mixed sediment source signatures within IBR outcrops: 1.) alongshore mixing of paleo-Ganges (west) and paleo-Brahmaputra (east) rivers, or 2.) a confluent early Miocene paleo-Ganges-Brahmaputra that separated by the late Miocene/early Pliocene. Alongshore sediment transport along hundreds of kms is observed across the modern GBMD, indicating that mixing across long distances is possible in this system. It has also been suggested that the Shillong massif progressively uplifted from east to west, which could account for a late Miocene-Pliocene separation of a once confluent paleo-Ganges-Brahmaputra river. Our results demonstrate, for the first time, linkage between Miocene fluvial deltaic deposits in the IBR and deep marine sedimentation in the Bengal and Nicobar fans.