FACIES ANALYSIS OF THE LATE PROTEROZOIC NAGTHAT FORMATION, NAINITAL HILLS, KUMAUN LESSER HIMALAYA

The Proterozoic Nagthat Formation of Nainital area is composed mainly of fine- to coarse-grained quartzarenite with subordinate amount of pe bbly sandstone, conglomerate, siltstone and silty shales. On the basis of such parameters as grain size, primary structures, palaeocurrent a nd morphology of lithounits, seven major lithofacies have been disting uished in the Nagthat sequence. These are, medium to coarse-grained gr avelly quartzarenite (Lithofacies A), bar cross-bedded medium-grained quartzarenite (Lithofacies B), horizontally laminated fine-grained qua rtzarenite (Lithofacies C), fine to medium-grained channelised quartza renite (Lithofacies D), fine-grained quartzitic sandstone (Lithofacies E), silty sandstone-shale (Lithofacies F) and matrix supported conglo merate (Lithofacies G). Lateral and vertical distribution of constitue nt lithofacies demonstrate deposition of Nagthat sediments in a progra dational (regressive) barrier island system in subtidal upper shorefac e and channels, subtidal longshore bars, foreshore beachface, tidal ch annels (inlets), intertidal sandflat-channel and mixed flat environmen ts, and occasionally as gravity flows in subtidal gravelly channels.