Geochemical characteristics of Proterozoic carbonate lithofacies of Indravati Basin, Chhatisgarh, Central India: Implication of depositional and diagenetic history

Abstract

The intra-cratonic Proterozoic Indravati Basin, Central India, located on the south eastern margin of the Bastar craton is represented by a Lower clastic and the Upper carbonate succession. The upper carbonate succession is represented by two formations namely the Kanger Limestone Formation (the non-stromatolitic platform) which is overlain by the Jagdalpur Formation (the stromatolitic carbonate platform). Present work provides new field and petrological observations and high precision selected trace and REE data from carbonates in order to interpret the depositional conditions of these rocks. The two carbonate lithofacies (A & C) identified on the basis of their field occurrences, distinctive petrographic and geochemical characters. The lithofacies A is a bedded micritic limestone, devoid of any allochems and the lithofacies-C is a dolomitized, stromatolite bearing limestone. Detailed petrographic observation and geochemical characters suggest an early to late diagenesis involving the processes of compaction, dissolution, cementation, recrystallization and replacement. The trace element geochemistry from number of samples shows that they are characteristic of shallow water deposits. Although a similar pattern of REE abundances resembling to that of modern seawater has been observed for all the carbonate samples with depleted LREE, the greatly enriched HREE and negative Eu anomaly, the positive Ce anomaly deviates from the modern seawater patterns. All the carbonate samples display a distinctive negative Eu anomaly. The La N/Yb N ratio of 3.64 for Lithofacies A and 3.31 for Lithofacies C samples indicates a relatively moderate degree of REE fractionation. The trace and rare earth geochemistry together with sedimentological data strongly support deposition of Kanger (lower bedded limestone) in subtidal condition and Jagdalpur (upper stromatolitic) carbonate Formation in shallow marine intertidal-supratidal condition. The change in depositional and diagenetic conditions is further substantiated by factor analysis of the geochemical data which clearly differentiates the two carbonate lithofacies.