Transition from foreland‐ to piggyback‐basin deposition, Plio‐Pleistocene Upper Siwalik Group, Shinghar Range, NW Pakistan

Abstract

ABSTRACT Plio‐Pleistocene synorogenic deposits of the Upper Siwalik Group in the Shinghar Range (Trans‐Indus Salt Ranges) of north‐western Pakistan record the transition from foreland‐basin to piggyback‐basin deposition on the hangingwall of the Salt Range thrust. The Siwalik and Upper Siwalik Groups are over 4 km thick in the Shinghar Range. The lower 3 km consists of the Miocene Siwalik Group, which was deposited by a south‐flowing foreland trunk stream, the palaeo‐Indus River. The upper 1·5 km consists of the Upper Siwalik Group, which is herein divided into three members. The lowest member includes deposits of the south‐flowing palaeo‐Indus River and is distinguished from the underlying Siwalik Group by the first appearance of conglomerate. The transition from the lower member to the middle member is interpreted as recording uplift on the Salt Range thrust. As the Salt Range thrust was active, the palaeo‐Indus River was bifurcated to the east and west around the embryonic Shinghar Range and overbank and lacustrine deposition occurred, represented by the middle member. When the Shinghar Range achieved significant topography, the upper member was deposited by streams transporting gravel and sand that flowed north and west out of the range and into a piggyback basin that formed on the hangingwall of the Salt Range thrust. New and previously published palaeomagnetic stratigraphy and fission‐track ages from volcaniclastic deposits within the Upper Siwalik Group provide tight constraints on the chronology of sedimentary‐facies transitions and timing of uplift of the Shinghar Range. The integration of sedimentological and geochronological data indicates that motion on the Salt Range thrust and repositioning of the Indus River began at ∼1·0 Ma.