FACIES ARCHITECTURE OF THE TIDE INFLUENCED GRAVELLY SAND BAR DEPOSIT: MESOPROTEROZOIC BAYANA FORMATION, LALSAR, RAJASTHAN

Abstract

The Bayana Basin, located in the north-eastern part of the North Delhi Fold Belt (NDFB), comprises thick succession of Mesoproterozoic marine sediments. The present study deals with the meta-sedimentary succession in and around Lalsar area within Lalsot sub-basin of the main Bayana Basin. The lower part of the succession is represented by coarser clastics, viz., conglomerates and pebbly sandstone, manifesting deposition from successive cohesive sedimentgravity flow forming a basin-marginal gravelly bar/fan. Downslope subaqueous emplacement of several such flows led to intake of water, producing density stratification and resultant crossstratified pebbly sandstone in the middle part of the succession. Laterally-accreted cross-strata with mud-draped foresets, abundant reactivation surfaces, double mud drapes, lenticular bedding, etc. indicate subtidal origin of these sediments. Abundance of coarser clastics and low mud-sand ratio suggest reworking of the gravelly bar/fan sediments by open marine tidal currents. This indicates fluctuation in the relative sea level causing landward encroachment of the sea and/or changes in rate and amount of sediment supply. Such variations are common in Proterozoic intracratonic basins and can be correlated to exhumation under repeated activation of basin-marginal faults. Downslope decrease in the supply of coarser-clastics led to undisturbed, low-energy subtidal depositional system with development of herring-bone cross-strata and increased mudsand ratio.