Undaturbidite in the Talchir Group, Talchir Gondwana Basin, Orissa

Abstract A sedimentary sequence constituted of interbedded sheet sandstones and shales and lenticular channel sandstones exhibits a number of sedimentary structures common in turbidites. These are various types of sole marks, graded bedding, ripple-drift cross laminations, parallel laminations and asymmetric current ripples. The sheet sandstones. show characteristic internal vertical arrangement of structures identical to turbidite sequences while the shales are laminated. Besides, a few rolled boulders and cobbles are found within the sequence. The combination of sedimentary structures suggests origin by a turbidity current mechanism. The association of lenticular channel sandstones, rolled boulders and cobbles and the grain-size characteristics, on the other hand, suggest it to be undaturbidite, having the characteristics intermediate between tractive and turbidity current deposits. The probable depositional environment of the sequence is discussed.