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The sea floor of the upper Indus fan is characterized by gradients less than 1/500, channels with levees approximately 100 m (330 ft) high, distinct echos with several continuous subbottom reflectors on 3.5-kHz records, and generally fine-grained sediments (Tc-e sequences) except in channels where coarse-grained sediments (Ta-e) are present. On multichannel and sparker seismic lines, the levee complexes are characterized by overlapping wedge-shaped reflection packages, and channel axes by high amplitude discontinuous reflections. Since the Oligocene and Miocene, several major episodes of extensive migration of the channels on the upper fan (with sediments more than 3 km, or 10,000 ft, thick) and of the feeder canyons on the Pakistan-India shelf are recognized. During the atest episode (Plio-Pleistocene), 3 distinct regions, each with numerous channel-levee complexes fed by the sediments of both the Indus and the now-extinct Hakra-Nara rivers on the Pakistan-India shelf, are identified.
The middle fan has gradients of 1/500 to 1/1,000, numerous channels with levees approximately 20 m (66-ft) high, and convex morphology due to extensive channel-levee buildup. Gradients less than 1/1,000, channels with levees 8-20 m (26-66 ft) high, prolonged echos with few or no subbottom reflectors on 3.5-kHz records, smooth continuous reflections on seismic records, and the highest sand content in the sediments (Ta-e) of any fan region characterize the lower fan. Although unchannelized sheet-flow turbidite deposition was the dominant mode, channelized and overbank deposition also played a significant role in the sedimentation of the lower fan. On a gross scale, 2 extensive Quaternary sand-rich deposits (lobes), primarily laid down by sheet flows, are mapped on the lower fan.
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