Isochronous fluvial systems in Miocene deposits of Northern Pakistan

Abstract

A Palacomagnetic isochron dated at about 8.1 Myr BP and detailed lithostratigraph of a 40 m interval exposed along strike for 40 km establish the depositional patterns of two contemporaneous, interfingering fluvial systems in the upper part of the Meddle Siwalik sequence.The two systems, referred to as the buff and blue‐grey, differ in unit shape, lithofacies, bedding sequence, palaeocurrent direction and sand composition. Interfingering occurs along the south‐west‐north‐east strike of the outcrops, with the palaeodrainage directions of the two systems generally perpendicular to this line. The axis of the blue‐grey system, which deposited widespread sheet sands and silts, lay toward the south west end of the study area. The more complex axis of the buff system, which deposited shoe‐string sand bodies and lage volumes of silt and clay, lay toward the north‐east. The source area for both systems was the rising Himalyan belt to the north and noth‐east of the study area. At maximum extent the blue‐grey system occupied a channel belt at least 25 km wide. Channel belt widths and depths for the buff system are 1–3 km and 3–7 m, respectively. Current directions averge 94° for blue‐grey sands and 136° for buff sands. Blur‐grey sands contain 20% more rock fragments and are otherwise less mature than buff sands.The buff system shows a verticla pattern of avulsion, palaeosol formation and floodplain aggradation which we attribute to autocyclic processes of parallel rivers. The blue‐grey system shows phases of erosion accompaniced laterally by plaeosol formation, folowed by valley fill and overfowing of interfluve surfaces. Theis pattern may be caused by allocyclic presses affecting the source area. We interpret the blue‐grey system as a major drainage from the interior Himalayas (perhaps the ancestral Indus) and the buff system as a complex of smaller drinages along the mountain front which were probably ributaries to the bluegrey syste.Vertebrate fossils including hominoid primates from the area are almost exclusively associated with lithofacies of the buff system, and this probably refects both taphonomic and palaeoecological differences between the two systems.