Ancient river systems in the Himalayan foredeep, Chinji Village area, northern Pakistan

Abstract

The Miocene Chinji and Nagri Formations (Siwalik Group) of northern Pakistan record ancient fluvial environments in the Himalayan foredeep basin. Excellent exposures on the Potwar Plateau (Chinji Village area) allowed detailed documentation of the geometry and stacking of sediment bodies that comprise these strata, and of variation of large-scale bedding geometry, grainsize, sedimentary structures and paleocurrent orientations within such bodies. Major sandstone bodies are tens of meters thick and are continuous along strike for many kilometers. They are composed internally of interconnected channel belt deposits, each of which contain several storeys (channel bar and fill deposits). Individual storeys, defined by inclined bedsets dipping down to a major basal erosion surface at up to 11°, are only 5 to 15 m thick within the Chinji Formation but can be up to 30 m thick within the Nagri Formation. Bedsets within storeys reflect sediment accreted during individual flood events. Along-strike variation of bedsets within storeys and the stacking patterns of storeys within channel belt deposits reflect changes in the channel bed through time due to the growth of bars, migration of channels, and channel cutoff. Generally, braided channel patterns are indicated by the relatively large number of storeys within individual channel belt deposits exposed perpendicular to paleoflow, abundant evidence for channel bar superposition due to channel switching, local evidence for mid-channel bars, dominance of coarse-grained channel fills, and low paleocurrent variations. Paleochannel reconstructions from storeys exposed within the Chinji Formation indicate that individual channel segments generally had widths of 80–200 m, maximum depths of 4–13 m, wavelengths of 1.6–2 km and discharges of 400–800 m 3/s. Full channel belt widths (1–2 km) estimated from exposures perpendicular to paleoflow, and evidence for 2–3 coeval channels within channel belts, indicate full channel discharges of 1500–2000 m 3/s. Larger channel segments reconstructed from the Nagri Formation had widths of 200–400 m, maximum depths of 15–30 m, wavelengths of 3–5 km, and discharges of 3000–5000 m 3/s. As Nagri Formation channel systems were also clearly braided, full channel dimensions and discharge estimates are probably at least a factor of two greater than for individual channel segments (i.e. order of 10,000 m 3/s). Strata between major sandstone bodies are dominated by lobate and wedge-shaped bodies (crevasse splay and levee deposits), minor channel-form bodies (deposits of minor floodplain channels), laminated mudstone bodies (lake deposits) and paleosols. These strata are arranged into meters to 10 m thick stratified sequences that were rapidly deposited, bounded by well developed paleosols recording periods when deposition rates were low. The thickness and grainsize of such paleosol bounded sequences are not directly related to the proximity of a major channel deposit along strike. Instead, such sequences within overbank deposits appear to reflect local progradation of splays and levees into low areas on the floodplain.