Arenicolites helixus isp. nov. and associated ichnofossils from the Bhuban Formation, Surma Group (Lower-Middle Miocene) of Aizawl, Mizoram, India

A well preserved and diversified trace fossil assemblage has been identified from the Middle Bhuban unit of the Bhuban Formation, Surma Group (Lower to Middle Miocene) exposed along the Bawngkawn - Durtlang road section, Aizawl, Mizoram. A 615 meter thick succession comprising sandstone, siltstone, shale, mudstone and their admixtures in various proportions is exposed along this section. Trace fossils mostly occurred in fine to medium grained sandstone. These are ethologically diverse and represent predominantly infaunal structures of both suspension and deposit feeders. A total of 30 ichnospecies belonging to 19 ichnogenera have been identified from the collection. These are Arenicolites carbonarius, A. tenuis, A. helixus isp. nov., Conichnus conicus, Didymaulichnus lyelli, Diplocraterion helmerseni, Gordia marina, Gyrolithes isp., Katbergia isp, Laevicyclus mongraensis, Lanicoidichna metulata, Lockeiasiliquaria, Monomorphichnus bilinearis, Ophiomorpha irregulaire, O. nodosa, Palaeophycus annulatus, P. tubularis, P. heberti, P. striatus, Planolites beverleyensis, Polykladichnus irregularis, Psilonichnus lutimuratus, P. upsilon, Skolithos linearis, S. verticalis, Teichichnus flexuosus, T. rectus, Thalassionoides horizontalis, T. paradoxicus, T. suevicus, and a large complex burrow system. Vertical burrows of Arenicolites, Gyrolithes, Psilonichnus, Skolithos and Ophiomorpha are infaunal colonization of the suspension feeders under high energy and shifting substrate conditions. Horizontal burrows like Planolites, Palaeophycus, Teichichnus and Thalassinoides are the infaunal deposit feeder structures, which usually live at sediment-sediment interface in low to moderate energy conditions. The behavioural nature and distribution pattern of the ichnofossils as well as sedimentological attributes suggests that Middle Bhuban succession was deposited under fluctuating energy conditions in foreshore to shoreface/offshore zones of shallow marine environment.