TRANSGRESSIVE-REGRESSIVE CYCLES IN PERMIAN BARREN MEASURES FORMATION, PRANHITAGODAVARI VALLEY, INDIA – IMPLICATIONS ON PALAEOGEOGRAPHY

Abstract

Gondwana basins in peninsular India are riftogenic, half-graben basins preserving thick succession of syn-rift and post-rift sedimentary deposits. The rocks of the Gondwana Supergroup (Late Carboniferous – Early Cretaceous) are scattered across several isolated basins in palaeorift valleys in Peninsular India. One such rift valley is the Pranhita-Godavari Valley, where a thick repository of the entire Lower and Upper Gondwana succession is preserved. The Middle Permian Barren Measures Formation of the Lower Gondwana Group is relatively poorly understood from a sedimentological point of view. An integrated approach using facies analysis, geochemical analysis and sequence stratigraphic analysis of the Barren Measures Formation can potentially provide significant insights into the palaeoenvironment, palaeogeography, and palaeoclimate existing during Middle Permian time. The Barren Measures Formation is traditionally interpreted as fluvio-lacustrine deposit at its type area, Jharia coalfield, and other Gondwana basins in peninsular India. The present research focuses on outcrop based sedimentological and sequence stratigraphic analysis supplemented by laboratory-based investigations of diagenesis. The indepth facies analysis from selected outcrops, recorded eight distinct facies types, categorized into three facies associations, namely, (i) conglomerate-sandstone facies association (BM-CS); (ii) sandstone-mudstone facies association (BM-SM); and (iii) mudstone facies association (BM-M). The BM-CS facies association comprises pebbly, coarse-grained trough cross-stratified sandstone facies (BM-CS1), planar tabular cross-stratified sandstone facies (BM-CS2), pebbly coarse-grained plane-bedded sandstone facies (BM-CS3), red, ferruginous pebbly cross-stratified sandstone facies (BM-CS4) and lensoidal conglomerate facies (BM-CS5). The sandstones of the BM-CS facies association show lenticular geometry, concave-up basal contacts, large scale cross-stratifications with pebbles concentrated near the base and that are vertically stacked in nature. The BM-SM facies association comprises fine-grained yellow sandstone facies (BMSM1) and fine-grained cross-stratified heterolithic facies (BM-SM2). Each facies preserves feature such as: (i) tidal bundles characterized by laterally accreting foresets draped with mud and separated by reactivation surfaces; (ii) foresets with laterally variable thickness occurring within a single cross-strata set; (iii) mutually opposite foreset bundles in vertically adjacent beds; (iv) sigmoidal strata bundles; and (v) abundant flaser beddings. The BM-M facies association is composed of ferruginous siltstone/mudstone facies (BM-M1) and calcareous mudstone facies (BM-M2). The facies types bear signatures of pedogenic overprinting in the form of rhizocretions, root tubules and colour mottling effects. Petrographically, the sandstones and mudstones are dominantly composed of quartz and orthoclase, remnant plagioclase, heavy minerals, biotite, muscovite, clay minerals, matrix, ferruginous, silica and calcite cement. The entire succession of the Barren Measures Formation is characterized by multiple vertically stacked, fining-upward cycles. Each fining-up cycle is defined by a coarse-grained pebbly trough cross-stratified sandstone near the base and fine-grained, heterolithic sandstone-mudstone or a mudstone near the top. The overall facies architecture reveals deposition under high energy unidirectional currents. The vertically stacked, multi-storeyed nature of the cross-stratified sandstones of the BM-CS facies association in the lower part of the successions with paucity of fine-grained sediments of the BM-M facies association points to a high-energy river deposit. The occurrence of tidal bundles, reactivation surfaces, spring-neap-spring tidal cyclicity, and systematic changeover of different types of foresets, oppositely directed cross-strata sets, and flaser bedding signify tidally influenced fluvial channels and overbanks. The mudstone facies association (BMM) indicates deposition in a low energy calm environment, which underwent post-depositional pedogenic modifications resulting in the formation of hardpan calcrete horizons. A nearby lowgrade metamorphic source rock with a short transport path is inferred from the petrographic study of the sandstones. The pebbly coarse-grained sandstone beds of the BM-CS present near the base of most of the fining-upward cycles record various soft-sediment deformation structures (SSDS). SSDS such as: (i) complexly deformed layers; (ii) pseudonodules; (iii) load and flame structures; (iv) various water escape structures including sedimentary dikes; and (v) syn-sedimentary faults are generally sandwiched between two undeformed beds. The deformed beds are present in the close vicinity of several intrabasinal faults. The SSDS are unique because of their (i) extremely large size, (ii) complex pattern with normal grading and (iii) predominance in coarse-grained sandstone. In the absence of triggering agents like storms/pounding waves, rapid dumping (massive beds), slumps, or sediment gravity flows in the study area, frequent palaeo-earthquake shocks are possibly the only trigger responsible for liquefaction of the sediments. The extensive liquefaction along the sediment-water interface is evidenced by the complex character and large size of the deformation structures. The coarse-grained sandstone beds bearing large SSDS, characteristically present at the basal part of each fining-up cycle, are thus identified as seismites, indicating possible liquefaction by frequent palaeo-seismic events.