Ichnology of the Laisong Formation (late Eocene-early Oligocene) of the Naga Hills, Indo-Burma range (IBR): Paleoenvironmental significance

The late Eocene-early Oligocene Laisong Formation, basal Barail Group from the Naga Hills, Indo-Burma Range has yielded a low diversity of trace fossils i.e. Chondrites, ?Gyrolithes, Ophiomorpha, Palaeophycus, Planolites and Thalassinoides. Three ichnofabrics namely Ophiomorpha-Palaeophycus, Thalassinoides and Chondrites-Planolites have been identified. The Ophiomorpha-Palaeophycus ichnofabric grades upsection into Chondrites-Planolites ichnofabric in a fine to medium grained sandstone unit through a succession of alternating thinly bedded sandstone and mudstone containing Thalassinoides ichnofabric. The ichnofabric distribution is primarily controlled by the change in depositional energy levels from low to high. A low diversity trace fossils and lack of graphoglyptids in the Laisong Formation of Naga Hills supports the presence of a proximal part of the hyperpycnal lobes of delta-fed marine coarse-grained turbidite system.