Turbidites in a Glacial Sequence: A Study from the Talchir Formation, Raniganj Coalfield, India

Ancient turbidites have mostly been described from flysch sequences. This paper deals with an occurrence of non-flysch turbidites from ancient glacial-lake sediments belonging to the Talchir formation (upper Carboniferous) of the Raniganj coalfield, India. The turbidites occur, punctuating a long sequence of glaciolacustrine shales and siltstones, as sandy inter-beds, usually 40-50 cm. thick, having a graded base (often sole-marked) and laminated top with slump structures and parallel, current-ripple, and convolute laminations. The glaciolacustrine sediments also contain varve-like rhythmites containing current-generated structures like current-ripple laminations and sole marks in the sand laminae. These are also believed to have been deposited by turbidity currents. A number of pebbly mudstones, interbedded with the turbidites and the rhythmites, have been interpreted as deposits made by high-density turbidity currents. Available evidence shows that the oversteepened and overloaded front of an outwash delta built within the glacial lake caused slumps. These in their turn generated turbidity currents, which spread over the lake bottom causing deposition of the turbidites. Petrographic study of the turbidites and the rhythmites shows that these are very fine-grained, moderately to poorly sorted arkosic wackes with unimodal, nearly lognormal grain-size distributions.