LAUSR

Abstract Trace fossils, being almost always in situ, yield information about the organism's behaviour, in response to its ambient environment, largely controlled by the hydrodynamic conditions, salinity and bathymetry. The Neogene Siwalik succession, exposed in the Tista valley, the southern part of the Darjeeling-Sikkim Himalaya, contains fifteen ichnogenera sporadically distributed, generally in suites of low abundance, although at places the bioturbation index is high. Three major ichnofacies, namely Scoyenia, Skolithos and Cruziana, have been recognised, based on the association of different kinds of ichnogenera preserved in dominantly wave-reworked delta-front, open marine bay-fill and prodelta deposits. The overall characters of the Siwalik trace fossils are: i) low diversity; ii) low abundance; iii) diminutive size; iv) paucity of suspension-feeding structures; v) co-occurrence of both vertical and horizontal trace fossils, and vi) predominance of simple structures formed by trophic generalists. A predominance of drifted plant litter, synsedimentary deformation structures, hyperpycnites and rare structures of suspension-feeding organisms is consistent with a deltaic depositional environment. The overall diminutive size of trace fossils, their low diversities and low abundances are attributed to a stressful environment caused by freshwater influx. These characteristics of Siwalik ichnogenera probably mark the distinctions likely to be observed between trace fossils of deltaic and non-deltaic shoreline deposits. It thus appears that the riverine discharge during floods followed by post-flood recovery of normal marine conditions might have been a key controlling factor in shaping the ichnological characteristics in the river-wave interactive deltaic system of the Siwalik sea. The findings of ichnogenera typical of marine affinity unequivocally establishes a marine deltaic depositional environment, negates the exclusively continent-interior alluvial fan-fluvial interpretation for this succession of earlier workers.