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.
Highlights
Our Earth preserves a long history of climate change in the stratigraphic record [1]
The cold arid climate for the Talchir Formation is well corroborated with illite, chlorite/smectite, illite/smectite (40-80% illite), and chlorite dominant assemblage and major oxide compositions (enrichment of mobile elements, depletion of alumina, high index of compositional variability (ICV) (1.12-1.28), and relatively low CIA values (52.6-65.1)) of the Talchir mudstones
The warm humid condition in the Barakar Formation is mainly represented by kaolinite, illite, and illite/smectite dominant assemblage and is further confirmed by the relatively high CIA values (72.9-88.2), low ICV (0.33-0.62), low K2O/Al2O3, and enrichment of Al2O3 in the mudstones
Summary
Our Earth preserves a long history of climate change in the stratigraphic record [1]. Clay mineralogy of sedimentary successions is believed to provide important clues for conditions of continental weathering and climate control [3,4,5,6]. The use of clay minerals as a paleoclimate indicator is somewhat constrained presumably due to limited preservation of ancient sedimentary successions that are minimally affected by postdepositional changes during diagenesis and metamorphism. In well-preserved and leastaltered rock records, clay mineral assemblages may turn out to be very useful indicators of changing continental weathering regimes and the causative climate changes [7, 8]. Some alteration of clays is prevalent during deposition and early diagenesis [9,10,11], the clay mineral assemblage
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