Department of Geological Sciences, Gauhati University, Guwahati - 781014 (Assam) (INDIA)

The different types of lithofacies have been studied from Barail Group of rocks occurring in and around Sonapur, South Jaintia Hills District, Meghalaya.

The overall generalised litholog of the area has been constructed after detailed investigation. The lithofacies consists of Sm, Sh, St, Sr, Fl Fm etc. The lithofacies successions are characterised by the presence of a couple of fining upward sedimentation cycles.

The present study gives a picture of the distribution of various facies as well as the water level conditions throughout the area. A hypothetical sedimentation model constructed for the generalised sedimentary sequences suggest rapid sedimentation and high energy conditions for the basal-stratification, through cross-strata, rippled strata, finely laminated silts, muds etc. indicating deposition in the wanning stages of a flood.

Facies observations indicate that majority of the sequences towards the base may be a product of braided river condition. The abundance of massive and horizontally stratified sands suggest the presence of an upper flow regime. The laminated fine sands along with  muddy layers may also be interpreted as sediments filling abandoned chute channels. These chute channels are active only at the flood stage and these were the sediments deposited mainly from suspension mainly as overbank facies.

The sedimentary characteristics and presence of both wave and current formed structures like ripple-marks and current beddings and suggestive of intertidal sandflat origin, where bedload transport under tidal action dominated over slack water (suspension) depositions.