QUATERNARY BREAK-OUT FLOOD SEDIMENTS IN THE PESHAWAR BASIN OF NORTHERN PAKISTAN

The presence of horizontally bedded, fining upward sequences of sands, silts, and clays throughout the Peshawar basin of northern Pakistan h as previously been ascribed to Pleistocene lakes within the basin. Clo se inspection of these sediments, however, show sedimentological prope rties that suggest these stacked sequences are more characteristic of deposits of periodic break-out flood deposits and not lacustrine depos its. Exposures near the villages of Piran, Nowshera, Jehingira, Jalala and along the Kabul River show evidence of subaerial exposure (mudcra cks and bioturbation zones), lateral continuity and fracture sets. At least three distinct cycles of deposition are apparent in the basin, e ach exhibiting slightly different structural and sedimentological prop erties. The groupings and dimensions of the rhythmite sequences indica te that within each cycle multiple episodes of flooding occurred. The absence of significant erosional and depositional features, as well as the preservation of mud rip-up clasts and bioturbation zones further suggests that the periods between episodes of flooding were probably o n the order of 10(1) to 10(2) years. Evidence of ice-dams and lakes in the Indus drainage system is abundant. Cross-valley moraines and lacu strine deposits that stretch many kilometers upstream are visible in t he middle Indus valley between the villages of Sazin and Shatial and S kardu. Calculated ratios of sediment to water suggest that an upstream reservoir at least 32 km(3) and perhaps as large as 128 km(3) would h ave been required to produce the extent of rhythmites observed in the Peshawar basin. Generating a reservoir of this size in the Indus drain age system was accomplished near the villages of Sazin-Shatial where l arge cross-valley moraines and upstream lacustrine sediments indicate large ice-dams as do the moraines and lacustrine sediments near Gilgit and Skardu. The origin of the graded sediments in the Peshawar basin, however, has not been correlated to the failure of a specific ice-dam . The temporal relationship of the graded sediments is unclear. Limite d thermoluminescence (TL) analysis of the youngest set of graded sedim ents (referred to as 'lake beds' by previous investigators) in the Pes hawar basin suggest an age of at least 130,000 years BP. This time cor relates to the Yunz stage of glaciation along the middle Indus valley which spans from the mid to late Pleistocene to about 130,000 years BP . (C) 1998 Elsevier Science B.V. All rights reserved.