REGRESSIVE TRANSGRESSIVE SEDIMENTATION IN THE ORDOVICIAN SEQUENCE OF THE SPITI (TETHYS) BASIN, HIMACHAL-PRADESH, INDIA

The Spiti basin comprises about 10,000 m of more or less fossiliferous Cambrian to Cretaceous sedimentary succession in the Tethyan-Tibetan Himalaya. The Ordovician strata in this basin consist of dominant sandstone with minor mudstone-siltstone facies and locally developed basal conglomerate and persistent topmost minor carbonates. This sequence has been regarded variously as continental and/or marine. To stretch out the environment of deposition, a detailed study on field characteristics, palaeocurrents, petrography and reported marine fauna and trace fossils was carried out, and suggests three major depositional environments, viz., fluvial, transitional and marine. The fluvial facies assemblage is characterised by fine- to coarse-grained multistoried sheets of sublithic arenite with abundant planar cross-stratification and low mudstone content and is interpreted as having formed in a braided river environment. The generalised palaeoslope is towards NE and NW. The transitional facies comprises fine- to medium-grained sublithic to subarkosic arenites having sheet and lenticular sandstones with small- to medium-scale trough and planar cross-stratification and subordinate herringbone cross-stratification, sigmoidal reactivation surfaces, flaser and tidal bedding, etc. These features indicate the influence of both non-marine (fluvial) and marine (tidal flat) environments. The marine facies consists of fine- to medium-grained subarkose, sublithic arenite and quartz wacke as well as sandy dolomicrite in the upper part. The sandstone facies are generally multistoried, having a sheet geometry, and are interstratified with mudstone. The important sedimentary structures include trough, planar and hummocky cross-stratifications and parallel lamination. The palaeoflow direction is generally towards NE and NW with some minor fluctuations toward the south and west (landward side). These characteristic features reveal deposition in the nearshore-shore zone. Three different stages of regression and transgression are recognised. Stage I resulted in the deposition of fluvial facies on the earlier marine domain (Cambrian) representing a regressive phase caused by differential uplift and subsidence probably triggered by magmatic intrusion and/or regional tectonics. Stage II represents channelising of the fluvial system on tidal flats marking an initiation of transgression. Finally Stage III, denoting the development of shore zone facies caused by basin subsidence and a change in sea level, is interpreted as a transgressive phase.