Clay Mineral and Geochemical Proxies for Intense Climate Change in the Permian Gondwana Rock Record from Eastern India

Abstract

The clay mineral assemblages and geochemical compositions of the Permian Talchir and Barakar mudstones of the Raniganj basin, India, have been used to interpret terrestrial paleoclimate. The Talchir Formation presents unequivocal evidences of the Permian global glacial climate, and the overlying Barakar Formation with braided fluvial deposits immediately follows the glacial amelioration stage to a humid warm climate. Sediments unaffected by burial diagenesis and originated from a similar source under contrasting climates are ideal for developing proxies for substantial climate shift. Illite (28.4-63.8%), illite/smectite (0-58.6%, 40-80% illite), chlorite (0-53.9%), and chlorite/smectite (5.6-29.8%) constitute the clay mineral assemblage in the Talchir Formation whereas illite (5.3-78.2%), illite/smectite (trace-34.1%, mostly 60-90% illite), and kaolinite (36.1-86.8%) dominate the clay mineral assemblage in the Barakar Formation. The Talchir mudrocks are enriched in mobile elements and depleted in alumina w.r.t. PAAS, have relatively higher K2O/Al2O3 ratios (~0.3), high ICV (1.12-1.28), and lower CIA values (52.6-65.1) compared to those of the younger Barakar mudstones. The Barakar mudstones are depleted in mobile elements w.r.t. PAAS, have relatively low ICV (0.33-0.62) and K2O/Al2O3 values (0.11-0.16), and higher CIA values (72.9-88.2). Textural, mineralogical immaturity, and rock fragments of different components of the basement seen in the Talchir sandstones show these sediments being a first-cycle sedimentary deposit. The distinctive clay mineral assemblage and major oxide composition of the Talchir mudrocks attest to a unique low intensity chemical weathering in cold arid climate. Significant presence of kaolinite as well as distinctive geochemical characters of the Barakar mudrocks marks a shift in the paleoclimate from cold arid to humid. This climatic shift is further supported by the proportion and composition of illite/smectite across the formations. The relative proportion of chlorite and kaolinite and composition of illite/smectite therefore closely corroborate the significant climate shift, and such proxies, therefore, are useful indicators of climate extremes in the geological record.