Ichnology of shallow marine argillaceous limestone, Miocene, Western Kachchh, India

Miocene sequence of the northwestern part of the Kachchh has been studied for their trace fossils content. This sequence is ~80 m thick and chiefly comprises of highly fossiliferous and bioturbated argillaceous limestone and shales. Organic components are mainly represented by bivalves, gastropods, echinoderms, foraminifers, corals and bryozoans while the inorganic components are mainly carbonate e.g. micrite, microspar, sparite with moderate occurrence of rounded to subrounded quartz and glauconite. Total 46 ichnospecies of 32 ichnogenera were identified which are shows highly diversified behavioral activities and displayed abundance of dwelling and feeding structures of the infaunal organisms, which are found to be at the same level in the beds. Apart from distinct forms, succession also comprises of a large vertical and horizontal burrow, annulated burrow, polychaete burrow, abundant incomplete and indistinct forms, and boring structures. The whole sequence is mainly dominated by Arenicolites, Ophiomorpha, Palaeophycus, Planolites, Polykladichnus, Skolithos, Rhizocorallium, Thalassinoides; the Arenicolites, Ophiomorpha, Polykladichnus and Skolithos are the member of the Skolithos ichnofacies while Palaeophycus, Planolites, Rhizocorallium and Thalassinoides are the member of the Cruziana ichnofacies, which are very commonly found to occur in same beds. The intensely bored rocks indicate semiconsolidated substrate for development of Glossifungites ichnofacies type conditions. The presence of Skolithos-Cruziana ichnofacies indicates unconsolidated, shifting, poorly sorted soft substrate in moderate to low wave and current energy conditions while Glossifungites ichnofacies indicate firm but unlithified sediments. Sedimentological and ichnological evidences indicate that the Miocene argillaceous limestones were deposited in foreshore-shoreface environments.