The northeastern corner of the Indian Ocean is contained in the Andaman basin which has an area of 900,000 square kilometers. The northern and eastern third of the Andaman basin is composed of the shallow Irrawaddy delta shelf and Malay shelf, respectively 200 and 170 kilometers broad from the coasts to the 200-meter deep-shelf break. Along this terrace the bottom drops to the topographically complex basin floor with a maximum depth of 4,400 meters east of the Andaman-Nicobar Ridge.

Principal sediment source for the basin is the load of the Irrawaddy River, estimated during the last century at 265,000,000 metric tons per year, a figure which may be in error by a large amount. The subaerial delta is accumulating very little of the sediment, which reaches the sea and is displaced eastward by monsoon-driven currents. A 12,000-square-kilometer area southeast of the subaerial delta is the main depositional site. Marked shoaling of this area during the past century is too great to be explained by sediment accumulation alone, and it is suggested that localized tectonism is distorting the delta shelf. Deposition also is localized in the disturbed area, as sedimentation strives to re-establish and maintain a stable delta-shelf gradient of less than 0°01^prime.

A radiocarbon date from a basin core and foraminiferal data indicate a deep-basin depositional rate of 15 centimeters per 1,000 years. Sediment carbonate contents provide a rough comparison of relative depositional rates for the basin floor and the delta, indicating that sediment accumulates on the delta shelf at least 10 times faster than in the deep basin, or at a rate of from 100 to 200 centimeters per 1,000 years. Approximately 90 per cent of the Irrawaddy's load is deposited on the delta and only 10 per cent reaches depositional sites beyond the delta.

End_of_Article - Last_Page 479------------