Resumen de Indigenous siliciclastic and extraneous polygenetic carbonate facies in the Albian-Turonian Karai Shale, Cauvery Basin, India

Nivedita Chakraborty, Anudeb Mandal, Adrita Choudhuri, Sunipa Mandal, Subir Sarkar

• The Albian-Turonian marine Karai Shale, a mixed siliciclastic–carbonate formation in the pericratonic Cauvery rift basin, India, arouses interest as its evenly laminated siliciclastic beds appear indigenous, settling from suspension, while the calcarenite beds are made of materials that were deposited elsewhere and then shifted by agents that occasionally invaded the present depositional site. Three calcarenite facies recognized differ in body geometry, sedimentary structures, recurrence and in association. One of the two calcarenite facies enclosed by grey shale is most widely distributed, characterized by tabular geometry, internal wave-cum current structures, overall graded nature and gutters at base, It is inferred as product of meteorological storms, The other calcarenite facies overlapping in occurrence constitutes a single amalgamated bed package that is characterized by highest degree of dilution by siliciclastics in coarsest fraction, selectively incorporating pebble-sized fragments of the granitic basement and wood. Its beds are internally massive or cross-stratified and interpreted as products of successive pulses of ebb of a very strong event may be a megastorm or tsunami. The third calcarenite facies is represented by a couple of trough cross-stratified sheets embedded within greenish grey shale enriched in phosphate nodules and glauconite pellets and completely devoid of storm beds. Near-orthogonal diversion of palaeocurrent between the two associations invokes ocean bottom current to impinge upon the shelf-margin during maximum transgression for the third calcarenite faces. The δ13C ratio in the TST part is consistent with deposition on open marine shelf, but in the HST part they indicate influx of meteoric water.