Evidence of Quaternary break-out floods along the middle Indus Valley and in the Peshawar Basin of northern Pakistan

Catastrophic flood sediments, consisting of stacked sequences of fining upward sediments (rhythmites) occur throughout the Peshawar Basin of northern Pakistan. The flood sediments are the depositional remnants of large floods, emanating from the Indus River drainage basin, that were temporarily ponded in the Peshawar Basin. The stratigraphic arrangement of rhythmites in the basin shows at least three distinct episodes of deposition, each separated by an unknown quantity of time. Sedimentological characteristics of individual rhythmite strata further suggests that individual rhythmites were likely deposited from separate floods. The apparent periodic nature of rhythmite deposition is characteristic of similar sediments in North America that resulted from the periodic bursting of Pleistocene ice-dams. A comparable mechanism is proposed for the Peshawar Basin rhythmites. Estimated volumes calculated from the maximal distribution of rhythmite deposits in the Peshawar Basin suggest a reservoir ranging from approximately 32 to approximately 128 km$\sp3$ would be required to flood the basin. The presence of a large Pleistocene ice-dam along the middle Indus valley near the villages of Sazin and Shatial in northern Pakistan is inferred by local and distal geomorphic evidence. Lacustrine sediments perched along the Indus valley walls 20 to 60 meters above the valley floor offer the strongest evidence of the lacustrine setting. Large glacial till deposits along this stretch of the Indus valley are oriented transverse to the river and suggest that glacial ice emanating from the Darel valley likely dammed the Indus River in this area. Mega-ripple features and large boulder beds on downstream terraces and floodplains of the Indus offer supporting evidence of large floods, however the timing of these deposits is not clear. Reconstructing a reservoir in the middle Indus valley capable of accounting for the floodwater volumes apparent in the Peshawar Basin suggests a lake approximately 600 meters deep that extends upstream as far as 140 kilometers. Maximum discharge estimates of the resulting floods range from 5.2 $\*$ 10$\sp4$ to 4.8 $\*$ 10$\sp5$ m$\sp3\sp\*$sec$\sp{-1}$