Although the similarities between depositional processes and products as well as the analogous controls on basinfilling and evolution appear to have enjoyed great uniformity throughout the sedimentary rock record, a noticeable distinction exists in the rates and intensities of a broad range of geological processes in the Precambrian epoch. This paper searches for distinctiveness in the Precambrian sedimentary record, both siliciclastic and carbonate, through an extensive, though not exhaustive, review of the relevant literature augmented by new observations. While differences in Precambrian deltaic, aeolian, glacial and possibly also lacustrine deposits and settings appear to have been small, their large-scale development was controlled largely by a combination of temporal and geodynamic influences, essentially of global compass. In this regard the onset of the supercontinent cycle and major perturbations in palaeo-atmospheric composition appear to have been significant. Marine environments provide a 2 poor platform for Precambrian -Phanerozoic comparisons of sedimentation patterns, as those from the former period are preserved almost exclusively in epeiric settings, an environment essentially lacking on modern Earth. For the shallow marine carbonates, biological mediation of chemical sediment deposition changed radically from dominance by microbial biota in the Precambrian to a combination of metazons, protozoans and algae for the skeletal carbonates of the Phanerozoic. Despite it being widely recognized that Precambrian channel systems were braided in all environments (deltaic, tidal, alluvial, fluvial) as a consequence of the lack of vegetation and poor development of soils, the fluvial setting has some enigmatic aspects. Amongst these is evidence for ponding of muddy detritus in apparently sandstone bed-load dominated braided systems, with effects on local palaeoslopes which have resulted in unusual palaeohydraulic parameters for Precambrian fluvial systems. This is perhaps a field of research which holds greater promise when investigating sedimentation patterns prior to the Phanerozoic.

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This paper aims to unveil neotectonic imprints in topography, drainage and sediments in the 46.25 km long course of the River Chel from its source down to its alluvial fan at the base of the Himalayan Mountain Front in the DarjeelingeJalpaiguri districts of India. A semi-circular ridge delimits its primary catchment. Within confinement of this watershed basin the drainage pattern is composite being convergent along the periphery and divergent on a butte inside. All these geomorphic neotectonic imprints are accompanied by ramp and flat structures and spectacular mylonitization of rocks.High hypsometric index and convex shape of the hypsometric curve derived from the central near-straight course of the river between the primary catchment and the Main Frontal Thrust (MFT) also reflects tectonic youthfulness of the river course. It is well manifested also in widely variable stream index and stream gradient index ratios (SL/K) often exceeding 2. In response to neotectonism, this river course as a whole shifted westward between 1962 and 2007. Maximum reduction of the stream gradient on top of the MFT is eloquent enough about recent uplift of the thrust ridge. The high average slope gradient of canyon wall about 45.68 is well consistent with this uplift. Very low channel-width/valley-height ratio along the river further corroborates the uplift. The alluvial fan system of the River Chel is comprised of five morphogenetic fans stacked one above another with a tendency to shrink and shift progressively upslope. They differ from each other in terms of tilt, axial orientation, primary depositional surface gradient and convexity in transverse section and thus present a writ of ongoing tectonism. Progressive upward increase in the share of distal crystalline rocks in clast composition within alluvial fan package is a clear proxy for southerly advancement of the MFT. Concomitant increase in maximum clast size is in good agreement with sediment source uplift. All the five fans are, however, dormant now. Present-day River Chel deeply incises through all of them and suggests further basement uplift in the context of frequent evidences of neotectonism all around, although the role of climate remains uncertain in absence of adequate data.

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