Sedimentology of the Narmada alluvial fan, western India

Abstract

The Narmada alluvial fan is one of the world's largest, with an axial length of 23 km. The architecture is dominated by debris-flow deposits (Gms facies). Matrix support, a clay content of 3% and clast contact indicate that the clast-support mechanism resulted from a combination of buoyancy and dispersive pressure. The other faci facies (GSh), planar cross-stratified gravel facies (Gp 1 and Gp 2), sand-sheet facies (Sm), and trough cross-stratified sand facies (St). Gms, GSh and Sm facies are debris-flow and sheet-flow deposits that aggraded the fan, whereas Gp 1 and St are channel bars and channel fills that dominated the fan between major flood events. The fan is characterised by subrounded to rounded clasts. The rounding is due to the elongated catchment area upstream of the fan apex, as clasts are rounded during prolonged bed load transport and are temporarily arrested upstream of the fan apex as channel bars. These clasts are remobilized and entrained in debris-flows on the fan during events of anomalous discharge (storm events). The basalt clasts show a progressive fall in maximum clast size from 150 cm to 10 cm away from the fan apex. The Narmada river exhibits discharges of up to 60,000 m 3/s, but, due to reconfinement of the feeder channel resulting from tectonic reactivation of pre-existing lineaments during the Late Pleistocene, this does not aggrade the fan. Tectonism has influenced the location of the depositional site, has provided the necessary physiographic contrast, and has played an important role in the erosion of the fan, whereas climate-controlled primary and secondary processes have determined the nature of alluvial architecture.