Paleoenvironmental reconstruction using ichnological analysis of the Barren Measures Formation from the uppermost section of Chutua Nala, West Bokaro Basin, Jharkhand, India

Abstract

Middle Permian Barren Measures Formation exposed in Chutua Nala, Northern part of West Bokaro Basin hosts sparse to densely bioturbated lithounits. The study area comprises of diverse primary sedimentary structures such as flaser, lenticular bedding, mud draped forests, herringbone cross stratification, bidirectionality of ripples which indicate an overall tidal influence. 3 sedimentary facies namely Lenticular Red Sandstone(F2), Carbonaceous Shale(F1) and Heterolith(F3) was observed. Ichnodensity as well as diversity is high in the region and 14 different ichnotaxa were identified; namely Bergaueria, Conichnus, Cylindrichnus, Diplocraterion, Macaronichnus, Ophiomorpha, Paleophycus, Planolites, Rhizocorallium, Skolithos, Taeinidium, Teichichnus, Thalassinoides and Zoophycos along with some Bivalve Resting traces, Fugichnia. These trace fossils represent the dwelling and feeding burrows of deposit as well as suspension feeders. These ichnotaxa mostly represent marine habitat (Teichichnus, Zoophycos,Cylindrichnus,Rhizocorallium) and are seen to occur periodically in the muddy substrate, therefore indicating several episodic pulses of marine incursions in the tide dominated setting. Further ichnofabric analysis was done and the section was classified under 5 ichnofabrics namely Thalassinoides Rhizocorallium (IF-1), Ophiomorpha-Planolites (IF-2), Skolithos-Macaronichnus (IF 3), Teichichnus (IF-4) and Ophiomorpha-Paleophycus (IF-5). IF-1 ichnofabric indicates deposit feeder dominance and low rate of sedimentation. IF-3 ichnofabric indicates a high energy phase associated with high rate of sedimentation, as observed from vertical burrow of Macaronichnus cutting across the foreset of sand. IF-5 ichnofabric dominantly indicates suspension feeding activity in the topmost part of the succession in a stressed environment. IF-4 ichnofabric symbolises the calm and low energy environment suitable for deposit feeders to dwell in muddy substrates experiencing variation of salinity, oxygen etc. IF-2 ichnofabric indicates assemblage of both suspension and deposit feeder. The study of the ichnofabric revealed how the nature of the substrate and feeding habits of the organism changed throughout the succession giving an idea of the sediment-organism relationship, oxygenation patterns and hydrodynamic energy fluctuation. Combining the sedimentological and ichnological aspects, it can be justified that the study area is a fluvio-marine transitional environment with high signatures of tidal activity as evident from the ichnofabric, ichnotaxa and sedimentological studies.