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The Irrawaddy Delta: Tertiary Setting and Modern Offshore Sedimentation
The Irrawaddy River, one of the world's largest, is the modern descendant of Tertiary predecessors which have filled the 1500 km long Burmese Tertiary Trough progressively from the north, depositing a deltaic and shallow marine sequence with a cumulative stratigraphic thickness greater than 23,000 meters. A wedge-shaped subaerial delta with an area of about 35,000 km2 is continuing to encroach southward into the northern end of the Andaman Sea; however, the fastest growth of the delta is offshore in the Gulf of Martaban, southeast of the main subaerial delta lobe. Approximately 87 percent of the 334 x 106 tons of sediment discharged annually by the Irrawaddy is brought to sea during the summer rainy season of the southwest monsoon. Most of the Irrawaddy sand load, deposited nearshore, contributes to the growth of the subaerial delta, which advances its shoreline at an average rate of 2.5 km/100 years. Most of the Irrawaddy sediment load is suspended silt and clay. Prevailing currents generated by the southwest monsoonal winds sweep this turbid discharge eastward into the Gulf of Martaban, where suspended sediment greater than 300 mg/L were observed over 100 kilometers from shore at the height of the 1967 monsoon.
Rapid sedimentation in and south of the Gulf of Martaban has modified the shelf topography by partially burying a relict sand surface. The western half of the shelf, south of the subaerial delta is not rceiving Irrawaddy sediment; here the entire shelf slopes 0°2' from coast to shelf break, and the sediments are sands with significant amounts of autochthonous calcium carbonate and very little organic matter. In contrast, the floor of the Gulf of Martaban is flat, and the offshore portion of the shelf south of the Gulf slopes about 0°4'. This area is accumulating carbonate-poor silty clay at rates of about 200 cm/1000 years. A tongue of these sediments extends seaward in a zone of shallow delta-shelf channels to the head of a large submarine canyon incised into the continental slope. Along this avenue approximately 10 percent of the Irrawaddy sediment slowly flows as turbid near-bottom water along any available gradient into the deep offshore basin.
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