Significance of Trace Fossils of the Jhuran Formation (Upper Jurassic) from Southwest of Bhuj, Kachchh, Western India: An Ichnocoenoses Approach

The Jhuran Formation inhabits more than half of the total area of the Mesozoic outcrops in the Kachchh Mainland and ranges in age from Kimmeridgian to Tithonian. This paper aims to recognize the trace fossil assemblages at different stratigraphic levels and interpret the palaeoenvironment based on the ichnocoenoses of the Jhuran Formation. The formation is predominantly argillaceous in the lower and middle parts, and arenaceous in the upper part. A total of 11 ichnogenera and 14 ichnospecies have been identified from the formation. Cochlichnus, Didymaulichnus, Palaeophycus and Teichichnus are found in the Lower Member of the Jhuran Formation. The maximum density and diversity of trace fossils are reported from the intercalated sequence of shale, siltstone and sandstone of the Middle Member of the Jhuran Formation. Arenicolites, Didymaulichnus, Gyrochorte, Oldhamia, Palaeophycus, Pilichnus, Sublorenzinia, Taenidium, Teichichnus and Thalassinoides are represented from the Middle Member. Didymaulichnus, Gyrochorte and Oldhamia are the only three trace fossils reported from the Upper Member. Four ichnocoenoses are described from the study area based on the dominant ichnogenus such as Gyrochorte, Palaeophycus, Teichichnus and Thalassinoides. Paleoenvironmental conditions of the study area fluctuated from offshore to middle shoreface with many temporary intervening transgressive-regressive events as depicted by these ichnocoenoses. Overall, the regressive cycle can be interpreted based on the ichnocoenoses that occurred at different stratigraphic levels.