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Heavy mineral and paleocurrent direction data suggest that the ancestral Indus is an analog of the modern Indus River (Pakistan). During the past 18 m.y., the Indus has maintained a relatively stationary outlet position along the Himalayan Front. Sandstones from three sections of Siwalik strata have been sampled and their heavy mineral suites analyzed. These stratigraphic sections cover some 200 km in an east-west direction in the Potwar Plateau area. In the western and central Potwar Plateau sections, blue-green hornblende makes a conspicuous first appearance at 11 Ma, the boundary between the Chinji and Nagri Formations. Above this boundary, blue-green hornblende dominates the heavy mineral assemblage of Siwalik sand. In the Upper Siwalik beds the heavy mineral suite contains as much as 75 percent blue-green hornblende. Unroofing of the Kohistan Arc terrane is the most likely explanation for this detrital hornblende. In marked contrast, the eastern Potwar region does not show the same abundance of blue-green hornblende. Significantly, the modern rivers of the eastern Potwar do not carry abundant blue-green hornblende either; only the Indus River does. Paleocurrent measurements taken in the Chinji Village section near the center of the Potwar Plateau indicate a northwest-to-southeast flow direction, whereas those in the Trans-Indus 120 km west of Chinji Village indicate a northeast-to-southwest flow direction. These data indicate that the Siwalik sequence of northern Pakistan is configured as a large-scale alluvial fan with the ancestral Indus shifting course back and forth across the Potwar Plateau region with a frequency of 104 to 105 yr/cycle. River sinuosity varied systematically from side to side of this fan, with minimum sinuosity attained along a north-south axis. As indicated by the absence of blue-green hornblende, the ancestral Indus did not reach the eastern Potwar Plateau (Kotal Kund area, 100 km east of Chinji) during the past 11 m.y.

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