Sediments of the Andaman Basin, northeastern Indian Ocean

The Andaman Basin is an area of 800,000 km2 separated from the Bay of Bengal by the Andaman-Nicobar Ridge. The basin's principal sediment source is the Irrawaddy River's annual load of about 265 · 106 metric tons of silty clay. Climate and physiography trap most of this sediment in the basin; over 90% of Burmese rainfall, 80% of runoff and 87% of sediment discharge result from the southwest monsoon. Prevailing winds of this season drive westerly currents which displace the turbid Irrawaddy discharge eastward from the river's mouths. The eastern and inner Irrawaddy delta-shelf is accumulating 90% of this sediment at a rate of 200 cm100 year. Sediments of the outer western delta-shelf and Malay continental margin are shelly, muddy sands, relict from a low Quaternary sea-stand. These areas and the Andaman-Nicobar Ridge are neither accumulating much sediment nor contributing to the basin's central trough. Contribution from the Irrawaddy constitutes 86% of the central trough's sediment, the difference being foraminiferal carbonate. Areal patterns of sediment parameters indicate that basinward transport of Irrawaddy sediment is controlled by bottom topography. Depositional rates average 15 cm1,000 year in the central trough, which has a total fill of about 1.5 km. These data indicate that the basin is young. The beveled Malay continental margin and the absence of any other logical provenance for thick flysch-like Tertiary sediments in the Andaman-Nicobar Ridge suggest that the basin began to form in the Late Miocene as Malaya rifted away from the Andaman-Nicobar Ridge.