The Mianwali and Tredian Formations: An Example of the Triassic Progradational Deltaic System in the Low-Latitude Western Salt Range, Pakistan

Abstract

The Nammal Gorge, Salt Range, Pakistan, preserves a record of the interacting Triassic open-marine carbonate sedimentation, cyclic siliciclastics progradation and fluctuating relative sea level. The sediments at the bottom in the Mianwali Formation, show bivalves and ammonoids rich wacke- to packestone lithofacies (i.e. ML 1 and ML 2), which indicates an open-marine depositional setting. The overlying shale of the Mianwali Formation (i.e. ML 3), has ammonoids and gastropods, the lower part of which indicates a relatively deep-marine depositional setting. Upwards, the ML 3 grades into the sandstones and shales (i.e. ML 4), which show ripple marks, mudcracks, bioturbation, graded, and cross bedding and syndepositional deformation. These features indicate deposition in delta lobe/shoreline sub-environments, with a high siliciclastic input. The lower part of the overlying Tredian Formation contains interbedded shale and sandstone (i.e. TL 1), having large scale slumps, which indicate deposition in cyclic, proximally located varied sub-environments of the delta topsets/channels and in the flood plain environments. The overlying thick, coarse, conglomeratic sandstone (i.e. TL 2) has planar, trough cross bedding and lenticular channelized bedding, which indicates a fluvial to continental environment. Further up, in the Tredian Formation, the presence of dolomite (i.e. TL 3) represents marine transgression. The facies association in the Mianwali and Tredian formation indicates many coarsening upward cycles representing the progradation of individual delta lobes. This study therefore proposes that, subsequent to the Permian-Triassic paraconformity, an open-marine depositional setting established in this area that gradually built up into a progradational deltaic system.