PALAEOECOLOGICAL SIGNIFICANCE OF TRACE FOSSILS OF CHORAR ISLAND, EASTERN KACHCHH BASIN, WESTERN INDIA

The Middle Jurassic (Bajocian-Callovian) of Chorar Island in eastern Kachchh comprises of similar to 109 m thick sequence of mixed siliciclastic-carbonate sediments, limestone, ferruginous sandstone and shales; divided into Khadir and Gadhada formations. The sequence is further analysed and nine lithofacies namely, micritic sandstone, allochemic sandstone, sandy allochemic limestone, sandy micrite, cross-bedded white sandstone, ferruginous sandstone, shales, coralline limestone and mudstone are identified. The whole sequence is bioturbated in varying intensity and comprises of 19 identifiable ichnospecies (Arenicolites carbonarius, Asterosoma isp., Curvolithus isp., Didymaulichnus lyelli, Diplocraterion cf parallelum, Gyrochorte comosa, Halopoa imbricata, Hillichnus lobosensis, Lockeia silliquaria, Megagrapton irregulare, Palaeophycus tubularis, Planolites beverleyensis, Protovirgularia rugosa, Rhizocorallium commune var irregulare, Skolithos linearis, S. verticalis, Skolithos isp., Thalassinoides horizontalis and T. paradoxicus.) of 16 ichnogenera which are further analysed for ichnodisparity. The sequence is characterised by ethologically less diverse groups, mainly of dwelling and feeding structures. Presence of Hillichnus lobosensis in sandy allochemic limestone facies shows different feeding strategies adopted by tellinacean bivalves indicating a complex organism-sediment interaction. The characteristic set of environmentally related group of trace fossil reveals five ichnoassemblages (Gyrochorte, Hillichnus,Rhizocorallium, Skolithos and Thalassinoides) which represent Skolithos and Cruziana Ichnofacies. The sedimentological and ichnological analysis of the Middle Jurassic sequence of Chorar Island reveals that the sediments were deposited in middle shoreface to offshore zone in a fluctuating sea-level.