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Abstract

The deposits of the Potwar Plateau in northern Pakistan show large-scale changes in fluvial deposition during the period between 14 and 6 my BP in three superimposed formations (Chinji, Nagri, Dhok Pathan) approximately 2700 m in total thickness. Five cross sections spaced throughout this sequence document intervals 30-80 m thick over lateral distances of 3-5 km. These show the architectural relationships of sand bodies, fine-grained facies, pedogenic horizons, and vertebrate fossil occurrences within floodplain deposits between major channel belt sandstones. Vertebrate fossil localities are particularly frequent within the fine-grained fill of different scales of abandoned floodplain channels (crevasse-splay and/or tributary channels). Such channels are larger and more common in the lower part of the sequence where average rates of sediment accumulation were 0.14-0.32 m/1000 yrs. as determined from the Siwalik magnetostratigraphic time framework. Higher average accumulation rates of 0.46-0.48 m/1000 yrs for the upper part of the sequence correspond to fewer fine-grained channel fills and lower fossil abundance. Changes in type and frequency of floodplain channels are attributed to a combination of differences in the fluvial regimen and in sediment accumulation rates. The quality of the vertebrate record is linked to the frequency and/or magnitude of abandoned channels on the sub-Himalayan alluvial plain and thus to large-scale tectonic and climatic controls on the fluvial systems.

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