Trace fossils of hypersaline environment and its implication in identifying tidal inundation boundary in Great Rann of Kachchh, Western India

Abstract

Extreme hypersaline environments are known to be challenging ecosystems for the survival of organisms. These extreme environments are characterized by high evaporation, resulting in a drastic reduction in the ichnological and bioturbational diversity. Moreover, such environments are poorly understood on account of limited documentation in rock records. The Great Rann of Kachchh forms an exclusive salt-encrusted geomorphic facet and is India’s most extensive exposure to hypersaline deposits. The area is inundated by tidal ingress from the Arabian Sea during the monsoon, followed by an evaporative period. Moreover, tidal ingress in the Rann of Kachchh and its effect on the ecology ispoorly documented. Ichnological investigations of the hypersaline Great Rann of Kachchh revealed four recurring trace fossils, Arenicolites, Palaeophycus, Polykladichnus, and Skolithos, with annelids, and crustaceans as the most likely producers. Furthermore, the existence of their juvenile forms is indicated by the presence of small and tiny subsurface burrows. Hence, in the present research, we used ichnology to document the limit and mechanism of tidal ingress to demarcate the tidal inundation boundary up to the east of Khadir Island. We propose a hypothesis identifying the tidal inundation limit by larval recruits burrows and present an ichnological model for a hypersaline environment.