Nagthat Formation: An example of a progradational, tide-dominated Proterozoic succession in Kumaun Lesser Himalaya, India

The Proterozoic Nagthat Formation of the Krol-belt succession, in the Naini tal area, is composed mainly of fine- to coarse-grained quartzarenite with a subordinate amount of purple to grey sandstone, siltstone-shale and congl omerate horizons. The association with spilitic lava flows, variable palaeo current trends and the restricted lateral extent of the Nagthat Formation w ithin the Krol-belt succession imply an active role for tectonism in the ba sin of deposition. In the upward coarsening succession of the Nagthat Forma tion, six major lithofacies have been identified: medium- to coarse-grained gravelly quartzarenite (Lithofacies A), planar cross-bedded, medium-graine d quartzarenite (Lithofacies B), horizontally laminated, fine-grained quart zarenite (Lithofacies D), interbedded sandstone-shale (Lithofacies E) and m atrix-supported conglomerate (Lithofacies F). The constituent lithofacies a re repetitive in nature. forming upward fining unit cycles and interpreted to reflect deposition as upper shore-face, shoals and bars: barrier-beachfa ce, tidal channels (inlets), intertidal-sandflat-mixedflat environments and , occasionally, in the form of gravity flows in subtidal channels. The gene ral upward coarsening succession of the Nagthat Formation represents deposi tion in a progradational (regressive) barrier island system. The palaeocurr ent pattern in the Nagthat Formation is distinctly polymodal and indicates sediment distribution across the roughly NW-SE trending shoreline, in respo nse to a dominating flood tidal current system. The palaeocurrent pattern s hows higher variability in the upper shore-face deposits than in the tidalf lat domain. A recycled metasedimentary terrain served as the source for the Nagthat Formation, probably supplying the sediments from E, NE and S direc tions. (C) 1999 Elsevier Science Ltd. All rights reserved.