Tracing the Sources and Depositional Pathways for the Oligocene Sediments in the Andaman Forearc

Finding provenance of the siliciclastic turbidites deposited in the Andaman Trench-Forearc Basin had remained contentious with most indicators pointing towards major sediment sourcing either from eastern Myanmar or from nascent Himalayan mountain belt. Whereas the palaeo-Irrawaddy River was considered as the main transporting agent for sediments originating in Myanmar, the Himalayan sediments were believed to have been recycled from the earliest Bengal Fan. Here, we examine existing Sr–Nd isotopic data for Oligocene sediments deposited in the basin and exposed today on the Andaman Accretionary Prism (AAP) to determine their provenance and transport pathways. We also present new data for sediments in the Barail Group, Bengal Basin and use them to test the sediment recycling hypothesis. Results of our study confirm that material from the Himalayan sources, transported through the submarine Bengal Fan, contributed significantly to the Andaman basin during Oligocene. Sr–Nd isotopic compositions considered along with published U–Pb detrital zircon age data reveal that both the Tethys Himalaya and Higher Himalaya were the major sediment suppliers to the basin, with a minor contribution coming from juvenile source(s) in the Indo-Myanmar ophiolite belt and/or the central Myanmar volcanic arc. We propose that the large Himalayan contribution was a result of rapid exhumation and high rate of erosion in the north-eastern Himalaya and Tibet during Late Eocene and Oligocene and sediment transport by the palaeo-channels of the Yarlung-Siang-Brahmaputra river system via the Bengal Basin/Fan.