0016-7622/2019-93-4-471/$ 1.00 © GEOL. SOC. INDIA | DOI: 10.1007/s12594-019-1202-0 JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.93, April 2019, pp.471-481 Ichnological Significance of Upper Disang Formation and Lower Barail Formation (Late Eocene to Early Oligocene) of Nagaland, Northeast India, in the Indo-Myanmar Ranges H. S. Rajkumar 1,* , I. Soibam 2 , K. S. Khaidem 3 , S. S. Sanasam 2 and Ch. M. Khuman 4 1 Department of Geology, United College Lambung, Chandel 795 127, India 2 Department of Earth Sciences, Manipur University, Canchipur 795 003, India 3 Department of Geology, D.M. College of Science, Imphal 795 001, India. 4 Department of Geology, Nagaland University, Meriema 797004, India *E-mail: mohnarooprajkumar@gmail.com ABSTRACT The Upper Disang-Lower Barail Transition Sequences of Nagaland in northeast India, Indo-Myanmar Ranges (IMR) are found to preserve reasonably diverse biological sedimentary structures. Three ichnofacies were observed, Skolithos ichnofacies consisting of Bergaueria hemispherica, Gyrochorte comosa, Ophiomorpha nodosa, Skolithos linearis and Thalassinoides paradoxicus; Cruziana ichnofacies – Curvolithus simplex, Scolicia plana, Scolicia strozzii, Scolicia vertebralis, Treptichnus pedum, Taenidium diesingi and Palaeophycus tubularis; and Nereites ichnofacies – Chondrites targionii and Paleomeandron elegans. Presence of Skolithos and Cruziana ichnofacies suggest well oxidising, substrate rich depositional environment in the fore- shoreface to offshore setting in general; and an overall possible fluctuation in the depositional bathymetry which is supported by the presence of Nereites ichnofacies in horizons alternating with Skolithos and Cruziana ichnofacies. The depositional system is proposed to be proximal to distal hyperpycnal-delta-fed turbidite system. Observation of sand spherules also suggests possibility of shoreline as one of the depositional environments. INTRODUCTION Nagaland is located in the northern extension of the Arakan Yoma Ranges representing orogenic upheavals during Cretaceous and Tertiary periods. The state is largely a hilly region and forms part of the northern continuation of the Disang-Barail Groups of sediments, which are mainly developed in the state of Manipur. The Naga Hill sediments, especially the Barail Group contain quite a lot of biogenic sedimentary structures, which are widely appraised as one of the most vital tools for evaluation of sedimentary environments in terms of hydrodynamic conditions during deposition, substrate stability, salinity variations, oxygenation levels, bathymetry, etc., (Seilacher, 1967; Bromley, 1996; Howard and Frey, 1984; Pemberton et al. 2001; Buatois and Mángano, 2011; Zhang and Zhao, 2015 and the references therein). In addition, the Upper Disang Formation is also found to contain fairly abundant trace fossils. So far, two research papers on trace fossils of the Naga hills have been published (Lokho et al. 2017; Khalo and Pandey 2018). The former studied the trace fossils of Yikhum and Sechu- Zubza section (Laisong Formation) and suggested a proximal hyperpycnal-delta-fed coarse grained turbidite in shallow marine setting based on three ichnofabrics, namely, Ophiomorpha - Palaeophycus, Chondrites- Planolites, and Thalassinoides and associated sedimentary structures. The later indicated a shallow nearshore to offshore marine environment with fluctuating energy conditions on the basis of two ichnofacies namely Skolithos and Cruziana, along the Pfutsero-Mesulumi road section in Phek district (the Disang-Barail Transition Sequences, Srivastava et al. 2004). However, the present research paper may certainly be able to make further addition to the palaeoenvironmental significance of trace fossils of the Naga hills, mainly the Disang-Barail Transition Sequences, since, the research outcomes discussed here were from locations which have not been reported earlier. GEOLOGY Based on the geo-tectonic and morpho-tectonic features, the Naga hills can be divided longitudinally into a number of belts. These are, from east to west; the Naga Metamorphics, the Ophiolite Belt, the Inner Fold Belt, the Kohima Synclinorium and the Schuppen Belt (Pandey, 2005). The Inner Fold Belt occupies the central part of Naga hills and consists of two major Synclinoria, namely the Kohima Synclinorium to the south and the Patkai Synclinorium to the north. It marks the margin of the Molásse basin which extends toward the Bengal basin indicating that the basin had been receding towards southwest as the uplift of the Indo-Myanmar Ranges (IMR) began gradually. A large spread of Disang Group of sediments with isolated covers of sediments of Barail Group and the Disang-Barail Transition Sequences characterize the geological setting of this belt and confined within two major tectonic zones, the Haflong-Disang Thrust in the west and the Ophiolite-Disang Thrust in the east. The Disang Group is characterised by thick succession of dark grey splintery shales. Carbonaceous materials are found in small lenses. Rao (1983) suggested brackish water, tidal flat environment of deposition for some of the members of the Disang Group. Its sedimentary record shows a continuity of shelf environment, where rapid subsidence favoured accumulation of a thick pile of Palaeogene rocks. On the other hand, the Barail Group is represented by hard, compact and well bedded sandstones which show different shades of colour and herringbone cross beddings indicating periodic variation of flow direction of the transporting medium (Gaur and Chakradhar, 1985). The Barails in the Zubza area is characterised by a thick argillaceous bluish grey shale, carbonaceous shale and a few centimetre scale coal seams. The Disang-Barail Transition Sequences in Nagaland is characterized by 80–100 m thick heterogeneous succession of sand- mud lithology that overlies gradationally the repetitious argillaceous Disang sediments and passes upward into dominantly arenaceous Barail Group of rocks. The sand-silt units in the succession exhibit numerous alternations of thin, flaggy, well cemented siltstone, fine- grained, occasionally medium-grained sandstones and sandy or silty shales. Separation of the Disang Group from the Barail Group is presently based on the first appearance of multi-storeyed sandstones