CHEMISTRY OF FIRST CYCLE SEDIMENTS AS CLIMATIC AND STRATIGRAPHIC TOOLS

First-cycle continental Gondwana sediments in the intra-cratonic Raniganj Basin, India range from the Late Paleozoic to Early Mesozoic. The sediments derived from an adjacent Precambrian granitic source were deposited within a fluvio-glacial and fluvial systems when the climate was changing from an Icehouse to Greenhouse State. During sedimentation the source rock and the environment of deposition broadly remained uniform. Climate, through chemical weathering, controlled the nature of mineralogy and chemistry of sediment and generated four sedimentary facies: tillite-sandstone-shale (Facies A); sandstone-shale-coal (Facies B); red/green shale-sandstone (Facies C); sandstone-conglomerate with very little shale (Facies D). Chemical parameters of first-cycle sediments are useful as paleoclimate indicators because climate and sediment chemistry are directly related. In addition, because climate change can be geologically instantaneous, the sediment chemistry of climatically controlled facies can be used as a stratigraphic tool. Twenty-three sandstone and 26 shale samples were chemically analyzed to test this hypothesis.

A chemical maturity pattern in sandstone expressed by the parameter, SiO2/Al2O3 discriminates different facies shown with their average values as follows: Facies A (6.2), Facies B (7.8), Facies C (7.2), Facies D (28.5). In the case of shale, the best discrimination is observed based on a maturity parameter defined as Al2O3/(K2O + Na2O + CaO + MgO). Average value of this parameter also show a similar pattern as above: Facies A (2.0),Facies B (3.6), Facies C (1.6), Facies D (15.7). In both cases Facies A and Facies C seem to be immature while Facies B is relatively mature and Facies D is the most mature facies in the succession.

The observed trends due to climatically controlled chemical weathering are interpreted as follows: glacial (Facies A), temperate humid (Facies B), warm semi-arid (Facies C), warm humid (Facies D). Since the chemical parameters change at the facies boundaries, they can also be used as a stratigraphic tool.