Facies Mosaic in a Fiord: Carboniferous-Permian Talchir Formation, India

Pradip K. Bose, G. Mukhopadhyay, H. N. Bhattacharya

Facies analysis of the basal 37 m of the Carboniferous-Permian Talchir Formation in a glacier-fed bedrock trough in Dudhi nala, Bihar, India, provides insight into the pattern of sedimentation of course gravels in a fiord.

Rapid transitions between 11 recognized facies, together with their complex organization, random variability in bed thickness, and differences in clast shape, size, and composition indicate coalescence of fans developed from numerous point sources bordering the elongated trough. Converging slide masses and lodgment tillites on the slopes flanking the trough give way to sediment gravity flow deposits composed of an array of conglomerates (matrix and clast supported with normal, inverse or absence of grading), attendant turbidite sands, and prodelta mud. The rheology of the in-trough flows ranged from plastic laminar to fluidal turbulent in response to flow from slope to floor of the trough.

Rapid calving of icebergs during the onset of deglaciation established a wave regime at the mouth of the trough and deposited cross-stratified sandstone replete with dripstones. The impact of large dripstones landing triggered turbidity currents. Continued rise in water level led to eventual preservation of the fan complex under onlapping wave-built shoal facies that grade into a sequence of upward-thinning hummocky cross-stratified sandstone beds virtually devoid of dripstones.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.