Depositional history of the basal Barakar beds - a study from Ramgarh Coalfield, Hazaribagh District, Bihar

Abstract

The rock record of the basal Barakar Formation of the Ramgarh Coalfield, Hazaribagh District, Bihar has been interpreted as an ancient example of the sandur deposit, i.e., a glacial outwash plain deposit having two distinct units as (a) channel facies divisible with riffles and pools, and (b) interchannel bar facies. Each of the facies of the environment in its stratigraphic setup, lithological attributes, sedimentary structural assemblage and palaeocurrent pattern correspond to one in the model.

The lenticular and sheet-like gravelly and pebbly sandstones with crude and sharp parallel stratifications, graded beds and scour and fill structures characterize the bar facies whereas, coarse arkosic sandstones with ripple and megaripple bedding, lenticular to planar cross-beds, and buried megaripple trains characterize the riffles of the channel facies. Gravel lenses at regular intervals have been related to the transverse ribs of the riffle facies. The lenticular sandy shales, siltstones and carbonaceous shales within the basal Barakar beds swings between NW to NNW.

The basal Barakar beds, at the bottommost part maintain a gravel: sand ratio to the order of 40:60; the proportion of sand, however, increases upwards. The gravelly and pebbly sandstone are highly indurated with fresh feldspars in all cases. The gravels are rounded to well rounded with high sphericity values.

Depositional features exhibit an uninterrupted pattern of sedimentation from the glacial Talchir Formation, through the sandur basal Barakar beds to the true fluvial Middle and Upper Barakar beds till the end of the Ironstone Shale Formation. The sandur sedimentation model with channel and bar facies as identified from the basal Barakar beds of Ramgarh Basin has also been presented in a block diagram.