Sedimentological–ichnofabric architecture of a crevasse splay deposit near an estuary mouth: A study from Permian Barren Measures Formation, West Bokaro Basin, India

Fluvial crevasse splay deposits and their traditional facies models are often modified significantly due to marine influence, viz., incoming flood tidal currents and/or wave encroachments, particularly in downstream-controlled river setting near to delta-estuary mouth areas. An integrated sedimentological-ichnofabric analysis, often typically representing such episodic marine encroachments, may properly represent the depositional models. In the present study, a ~7-8 m-thick siliciclastic interval comprising amalgamated bioturbated sandstones-heteroliths-shale is studied in terms of sedimentological-ichnofabric properties to assess a possible crevasse splay deposit. The succession is a part of the Permian Barren Measures Formation, West Bokaro Basin, India, which is broadly interpreted as a fluvio-marine estuary-delta depositional system. Seven fining-upward cycles represent tide-dominated crevasse-channel, characterized by lenticular and horizontally laminated sandstone, followed by crevasse-fan lobes, ranging from proximal organic-rich, wave-modified bioturbated sandy-heteroliths to medial-distal muddy-heteroliths. Eight diminutive, low-diversity brackish-water ichnogenera are recorded, grouped into six ichnofabrics: Planolites (IF1), Macaronichnus (IF2), Rosselia (IF3), Palaeophycus (IF4), Cylindrichnus-Planolites (IF5), and Teichichnus (IF6) ichnofabrics; reflecting sensitive ecological responses to shifting sub-environmental controls within individual cycles. Salinity fluctuations governed colonization, with euryhaline taxa dominating tidally influenced channels and stenohaline fauna typifying wave-modified lobes. Up-section increase in ichnodiversity and wave influence suggest more frequent marine-influenced flood surges, followed by post-flood quiescent sedimentation, and episodic colonization windows in the low-lying overbank areas. Variations in sedimentation rate across flood and interflood periods regulated colonization strategies, tiering patterns, and the transition from softground to firmground ichnofabric. Collectively, these results demonstrate that interactions between fluvio-marine processes and associated benthic colonization uniquely characterize a crevasse-splay architecture near the tide-wave influenced estuarine mouth system.