ELSEVIER zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Sedimentary zyxwvut Geology Sedimentary Geology 107 (1997) 263-279 Sedimentology of the Narmada alluvial fan, western India L.S. Chamyal *, A.S. Khadkikar, J.N. Malik, D.M. Maurya Department of Geology, Fuculr) of Science, MS. University of Baroda, Baroda, 390 002, India Received 20 June 1995; accepted 29 April 1996 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ 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 facies include gravel/sand-couplet facies (GSh), planar cross-stratified gravel facies (Gp, and Gpz), 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, 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 m3/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. Keywords: alluvial fan; river; Quatemary; sedimentology; India 1. Introduction Alluvial fans owe their existence to several si- multaneously acting processes. Of prime importance is an abrupt change in the regional physiographic setting where the river becomes unconfined (Bull, 1977; Blair and McPherson, 1994a,b). This abrupt change is commonly at a fault that separates a moun- tainous hinterland from an alluvial plain. The rapid high surface run-off responsible for fan aggradation * Corresponding author. occurs through a large number of low-order tribu- taries connected to the feeder channel of the fan. Fan deposits are built by rock avalanches, debris- flows, sheet-flows and so forth, and commonly con- tain subangular to angular clasts (Larsen and Steel, 1978; Pierson, 1981; Ballance, 1984; Blair, 1987; McArthur, 1987; Beaty, 1990; DeCelles et al., 1991; Evans, 1991; Blair and McPherson, 1992, 1994a,b; Brierley et al., 1993; Abrahms and Chadwick, 1994; Kumar et al., 1994). Most fans studied are a few kilometres long. The relative roles of climate and tectonic reju- 0037-0738/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII SOO37-0738(96)00030-9