additional ichnofossils from middle bhuban unit ...

SPECIAL PUBLICATION OF THE PALAEONTOLOGICAL SOCIETY OF INDIA No. 5; February, 2014; ISBN: 978-81-926033-2-2; pp. 257-271

ADDITIONAL ICHNOFOSSILS FROM MIDDLE BHUBAN UNIT, BHUBAN FORMATION, SURMA GROUP (LOWER TO MIDDLE MIOCENE), MIZORAM AND THEIR ENVIRONMENTAL SIGNIFICANCE CHINMOY RAJKONWAR1*, R. P. TIWARI1, VICTOR ZOCHHUANA RALTE1 and S. J. PATEL2 1

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DEPARTMENT OF GEOLOGY, MIZORAM UNIVERSITY, AIZAWL-796 004, MIZORAM DEPARTMENT OF GEOLOGY, M. S. UNIVERSITY OF BARODA, VADODARA-390 002, GUJARAT *Corresponding author’s e-mail: [email protected]

ABSTRACT The rock succession belonging to Middle Bhuban Unit of Bhuban Formation, Surma Group (Lower to Middle Miocene) is the thickest lithostratigraphic unit in Mizoram. A ~120 m thick sequence comprising sandstone, siltstone and shale belonging to Middle Bhuban Unit of Bhuban Formation, Surma Group along Bawngkawn-Edenthar road section in Aizawl, Mizoram is rich in trace fossils. This succession has yielded 23 ichnotaxa, including vertical burrows, horizontal simple burrows and trails, horizontal branched burrows and bilobate structures. Eight ichnospecies out of these were previously described and the remaining 16 are being described for the first time. These ichnospecies belong to 6 ichno-assemblages, namely, Archaeonassa, Ophiomorpha, Palaeophycus, Phycodes, Skolithos and Thalassinoides. Occurrence of Skolithos, Cruziana and at places Skolithos-Cruziana ichnofacies suggests that the rocks of Middle Bhuban Unit exposed along Bawngkawn-Edenther section were deposited under high energy conditions and sandy shifting substrate in foreshore zone and unconsolidated, poorly sorted, soft substrate and low energy condition in the shoreface to offshore zone, respectively. Keywords: Ichnoassemblages, Bhuban Formation, Bawngkawn-Edenthar road section, Aizawl, Mizoram

INTRODUCTION The ichnofossils are important tool for interpretation of depositional environment in terms of water depth, salinity, energy level, oxygenation variation (Ekdale, 1988) and help delineate stratigraphic sequence boundaries. In the context of Mizoram, the ichnological studies though started late, there is a rapid progress in the last few years. Many researchers have made significant contribution in various aspects in this field of research. Mehrotra et al. (2001), for the first time, reported Teredolites clavatus from the Upper Bhuban Unit of Bhuban Formation (Surma Group). Recently Tiwari et al. (2011, 2013), Rajkonwar et al. (2013) and Lokho and Singh (2009, 2013) described and illustrated trace fossils from the Middle Bhuban Unit of Bhuban Formation exposed around Aizawl in Mizoram. A ~120 m thick sequence of this unit comprising sandstone, siltstone and shale has been measured along Bawngkawn-Edenthar road section in Aizawl, Mizoram (Fig. 1).

Fig.1. Geological map showing location of the study area (after Malsawma et al., 2010).

This sequence is rich in trace fossils and has yielded twenty-three ichnotaxa, including vertical

258 burrows (Arenicolites isp., Diplocraterion isp., Escape structure, Laevicyclus mongraensis, Ophiomorpha borneensis, Ophiomorpha nodosa, Polykladichnus isp., Skolithos linearis, Skolithos isp., Ichnospecies Type-A), horizontal simple burrows and trails (Archaeonassa isp., Curvolithus isp., Planolites beverleyensis, Gordia isp., Palaeophycus striatus, Palaeophycus tubularis, Cochlichnus anguineus) and horizontal branched burrows (Ophiomorpha annulata, Pholeus abomasoformis, Phycodes curvipalmatum, Phycodes isp., Thalassinoides paradoxicus, Ichnospecies Type-B).

CHINMOY RAJKONWAR and OTHERS

Out of these, 8 ichnospecies, namely, Cochlichnus anguineus Hitchcock (1858), Laevicyclus mongraensis Chiplonkar and Badve (1970), Ophiomorpha borneensis Keij (1965), Palaeophycus tubularis Hall (1847), Pholeus abomasoformis Fiege (1944), Planolites beverleyensis Billing (1862), Skolithos linearis Haldeman (1840) and Thalassinoides paradoxicus Reith (1932) were already described. The purpose of the present study is to describe sixteen additional trace fossils and decipher the depositional environment of the Middle Bhuban rocks exposed in Bawngkawn area, Aizawl, Mizoram.

Fig.2. Generalized lithocolumn of Bawngkawn-Edenther section, showing distribution of trace fossils. Dashed lines indicate continuous occurrence.

ADDITIONAL ICHNOFOSSILS FROM MIDDLE BHUBAN UNIT, BHUBAN FORMATION, SURMA GROUP (LOWER TO MIDDLE MIOCENE), MIZORAM AND THEIR ENVIRONMENTAL SIGNIFICANCE

GEOLOGICAL SETTING The Lower to Middle Miocene succession of Mizoram mainly represented by 6000m thick Surma Group consists of repetitive succession of arenaceous and argillaceous rocks with a few intercalations of shell limestone, calcareous sandstone and intraformational conglomerate (Karunakaran, 1974; Ganju, 1975; Tiwari and Kachhara, 2003). Surma Group has been subdivided into Bhuban and Bokabil Formations. Bhuban Formation is the dominant lithostratigraphic unit in the state of Mizoram attaining a thickness 5000 m and further divisible lithostratigraphically into Lower, Middle and Upper Bhuban Units. The trace fossils described in the present paper comes from ~120 m thick rock succession belonging to Middle Bhuban Unit of Bhuban Formation exposed at Bawngkawn-Edenther road section (23° 45’ 11” N and 92° 43’ 05” E) in Aizawl comprising sandstone, siltstone, silty-sandstone and shale (Fig. 2). SYSTEMATIC DESCRIPTION The trace fossils assemblage is characterized by a moderate diversity with 23 ichnospecies belonging to 18 ichnogenera which can be clustered into four informal groups: vertical burrows, horizontal simple burrows and trails, horizontal branched burrows and bilobate structures. Among the recorded trace fossils from the Bhuban Formation of Bawngkawn area, Cochlichnus anguineus Hitchcock (1858) (Pl. I, fig. D), Laevicyclus mongraensis Chiplonkar and Badve (1970) (Pl. I, fig. K), Ophiomorpha borneensis Keij (1965) (Pl. II, fig. B), Palaeophycus tubularis Hall (1847) [Pl. II, fig. E(ii)], Pholeus abomasoformis Fiege (1944) (Pl. I, fig. L), Planolites beverleyensis Billing (1862) (Pl. II, fig. F), Skolithos linearis Haldeman (1840) (Pl. II, fig. C) and Thalassinoides paradoxicus Reith (1932) (Pl. III, fig. C and D), were extensively discussed previously (Tiwari et al., 2011). Remaining 16 trace fossils are being described for the first time from the Middle Bhuban Unit of Bhuban Formation, Mizoram. VERTICAL BURROWS Ichnogenus Arenicolites Salter, 1857 Ichnospecies: Arenicolites isp. (Pl. I, fig. B) Description: Simple, vertical, U-shaped burrow with no spreiten. Limbs of burrow are circular in cross section. Diameters of the limbs are 14-16 mm and uniform throughout the height of the burrow. The present burrow

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posses a widening upward structure. Remarks: On the basis of U-shaped vertical nature without spreiten, present specimen is assigned to ichnogenus Arenicolites. The specimen is differentiated from the other species of Arenicolites by the characteristics of its thick lining and widening upward structure U-shape structure. The possible producer of Arenicolites is suspension feeding annelids (Hakes, 1976) or crustacean like organisms (Goldring, 1962). Tiwari et al. (2011) and Rajkonwar et al. (2013) described Arenicolites from the Middle Bhuban rocks of Mizoram. Ichnogenus Diplocraterion Torell, 1870 Ichnospecies: Diplocraterion isp. (Pl. III, fig. B) Description: U-shaped burrow with spreiten, vertical, endichnial structure oriented straight and more or less parallel to each other. The width of the U varies from 3.8 to 4.2 cm; diameter of the arms 0.8 to 1.2 cm and maximum observed length is 9.5 cm. Remarks: Diplocraterion is dwelling burrow of suspension feeding organisms. It can be differ from Arenicolites by the presence of spreiten. Diplocraterion is interpreted to be made by suspension feeder in high energy condition (Fürsich, 1974 and 1975). Rajkonwar et al. (2013) reported D. helmerseni from the Bhuban Formation of Aizawl, Mizoram. Ichnogenus Ophiomorpha Lundgren, 1891 Ichnospecies: Ophiomorpha nodosa Lundgren, 1891 (Pl. II, fig. A and D) Description: Endichnial, lined, unbranched, vertical to inclined burrows. The walls of the burrows consist of regularly distributed discoid pellets. The depths of the burrows are ranges from 30 to 36 cm in observed specimens but it also penetrates to more depth. Diameter of the burrow and pellets ranges from 1.5 to 2.8 cm and 0.2 to 0.3 cm respectivly. The burrow fill is same as the host rock but pellet lined structures consist of darker material (muddy clastic sediment) than the host sediment. Remarks: The morphological characters of the present burrows are similar to O. nodosa Lundgren (1891). Different ichnospecies of Ophiomorpha are differentiated on the basis of variations in burrow configuration, shape and distribution of the pellets (Frey et al., 1978; Howard and Frey, 1984; Uchman, 2001). From North-East India, Rajkonwar et al. (2013), Singh

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EXPLANATION OF PLATE I A. Archaeonassa isp., B. Arenicolites isp., C. Curvolithus isp., D. Cochlichnus anguineus Hitchcock (1858), E. Didymaulichnus lyelli Rouault, 1850, F. Didymaulichnus lyelli Rouault, 1850, G. Escape structure, H. Gordia isp., I. Phycodes isp., J. Ophiomorpha annulata, K. Laevicyclus mongraensis Chiplonkar and Bavev, 1970, L. Pholeus abomasoformis Fiege, 1944.

ADDITIONAL ICHNOFOSSILS FROM MIDDLE BHUBAN UNIT, BHUBAN FORMATION, SURMA GROUP (LOWER TO MIDDLE MIOCENE), MIZORAM AND THEIR ENVIRONMENTAL SIGNIFICANCE

et al. (2008) and Singh et al. (2010) reported O. nodosa from the Bhuban Formation of various parts of Mizoram and Manipur. It has also been reported from other parts of India by many workers (Chiplonkar and Ghare, 1975; Kundal and Sanganwar, 2000; Kundal et al. 2005; Kundal and Dharashivkar, 2006; Kundal and Mude, 2008; Patel et al. 2008 and Mude et al. 2012).

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Ichnospecies: Polykladichnus isp. (Pl. III, fig. A) Description: Y-shaped, unornamented, isolated burrow disposed perpendicular to the bedding plane. Burrow preserved as positive epirelief. Observed depth of the burrow is 46 cm and the diameter of the arms are ranges from 1.2 to 1.4 cm. Burrow fills are massive and structure-less and similar with the surrounding material.

Ichnogenus Polykladichnus Fürsich, 1981

EXPLANATION OF PLATE II A. Ophiomorpha nodosa Lundgren, 1891, B. Ophiomorpha borneensis Keij, 1965, C. Skolithos linearis Haldeman, 1840, D. Ophiomorpha nodosa Lundgren, 1891, E. (i) Phycodes curvipalmatum Hall, 1852, (ii) Palaeophycus tubularis Hall, 1847, F. Planolites beverleyensis Billing, 1862, G. Palaeophycus striatus Hall, 1852, H. Skolithos isp.

262 Remarks: This ichnospecies displayed similar morphological characters with Polykladichnus. Tiwari et al. (2011) described Polykladichnus irregularis from Bhuban Formation of Mizoram. Polykladichnus irregularis is Y-shaped burrow made by the suspension/ deposit feeder polychaetes (Fursich, 1981). The present burrow displays a much bigger size as compare with Polykladichnus irregularis described by Tiwari et al. (2011) and Rajkonwar et al. (2013), while the overall morphology is same. Polykladichnus is considered to be a mucus bound dwelling burrow of polychaetes like Nereis, Amphinome, etc. (Patel and Desai, 2009). Ichnogenus Skolithos Haldemann, 1840 Ichnospecies: Skolithos isp. (Pl. II, fig. H) Description: Burrows occur as isolated, solitary cylindrical, unbranched tubes disposed perpendicular to the bedding plane. The burrows appear as circular openings at the surface. The diameters of the burrows are ranges from 1 to 2.5 cm. Remarks: Present specimens are placed under Skolithos isp. as these exhibit isolated, unbranched, cylindrical tubes, perpendicular to bedding plane. Skolithos burrows widely recognized in the shallow water, intertidal deposits (Seilacher, 1967) and in various shallow marine environments (Fillion and Pickerill, 1990; Alpert, 1974) and the probable producers are annelids or phoronids (Alpert, 1974). Ichnospecies Type-A (Pl. III, fig. E) Description: The burrow is unbranched, curved, unornamented and disposed vertical to incline to the bedding plane. The diameter of the burrow is 1.2 to 1.5 cm and observed length is about 85 cm. The colour of the burrow is same with the of the host rock. Remarks: There is no previous record of burrow like Ichnospecies Type-A from the Cenozoic or other sedimentary successions of India. Specific identification has been deferred for the want of mote material. ESCAPE STRUCTURE (Pl. I, fig. G) Description: Funnel-shaped, vertical to steeply inclined structures. Structures are 9 to 12 mm wide and 32 cm long, with a pointed lower end. The upper end of the structure is almost indistinct with lateral laminations

CHINMOY RAJKONWAR and OTHERS

opening upward into the overlying rock beds. Remarks: According to the origin there are two groups of conical structures developed in the laminated or cross bedded sandstone or siltstone (Hofmann et al., 2012) i.e. escape or upward movement of the animals due to the increasing sedimentation rate and liquid or gas escape structures. Buck and Goldring (2003) suggested that, the structures of biogenic origin displaying steplike lamina displacements defining a series of arcuate shear planes propagating from lower point are attributed to collapse structures. Morphological analysis of these structures indicates that rapid upward locomotion of a small animal produces downward advection and deformation of the loose sediments (Buck and Goldring, 2003). HORIZONTAL SIMPLE BURROWS AND TRAILS Ichnogenus Archaeonassa Fenton and Fenton, 1937 Ichnospecies: Archaeonassa isp. (Pl. I, fig. A) Description: Concave axial grooves locally flanked by marginal positive ridges preserved as epireliefs. Course is curved, irregularly winding, rarely meandering. Overcrossing is common among specimens, selfovercrossing is rarely. Axial groove is 3 to 4 mm wide and the trail width is 9 to 12 mm. Remarks: The Ichnogenus Archaeonassa has been extensively used for describing simple trails displaying a median groove flanked by levees (Buatois and Ma´ngano, 2002; Jensen, 2003; Jensen et al., 2006). Earlier interpretations suggest gastropods as a trace maker for Archaeonassa (Fenton and Fenton, 1937). Buckman (1994) also suggested echinoids and trilobites as a probable trace maker. Jensen (2003) inferred that Ediacaran and lower Paleozoic Archaeonassa might have been created by ‘‘mollusc type’’ animals. Ichnogenus Curvolithus Fritsch, 1908 Ichnospecies: Curvolithus isp. (Pl. I, fig. C) Description: Straight to slightly curved, horizontal to sub-horizontal burrows with a positive trilobate epirelief. The trail consists of a broad median ridge and two narrower lateral ridges, the width of the median ridge is 3 to 4 mm and the lateral ridges are 1-2 mm. Remarks: Curvolithus has been recorded from several parts of the Middle and Upper Jurassic shelf deposits of East Greenland (Heinberg, 1970, 1973; Fürsich and

ADDITIONAL ICHNOFOSSILS FROM MIDDLE BHUBAN UNIT, BHUBAN FORMATION, SURMA GROUP (LOWER TO MIDDLE MIOCENE), MIZORAM AND THEIR ENVIRONMENTAL SIGNIFICANCE

Heinberg, 1983; Heinberg and Birkelund, 1984; Surlyk and Clemmensen, 1983). Heinberg (1973) inferred the carnivorous gastropod as a probable trace maker for Curvolithus and classified as a repichnion (Heinberg and Birkelund, 1984) on the basis interpretation. Sanganwar and Kundal (1997) reported this species from Bagh Group, Dhar district, M.P. Ichnogenus Gordia Emmons, 1844 Ichnospecies: Gordia isp. (Pl. I, fig. H) Description: Smooth, horizontal trails on parting surfaces forming irregular loops. The trails are 1to 2 mm wide, and loops are about 3–8 mm in diameter. Sediments in the trails are identical to the host rock. Remarks: Gordia is a facies-breaking trace fossil known from both marine and non-marine settings (Pickerill et al., 1984). It has been inferred that in terrestrial environments, loop-like trails are produced by the millipede Julus and are caused by a dragging body in wet mud (Boy, 1976; Rolfe, 1980). Pulmonate gastropods also left looped trails (Abel, 1935). In fresh waters environments, similar traces can be interpreted as locomotion trails (repichnia) or feeding traces (pascichnia), produced by insect larvae (Gibbard and Dreimanis, 1978) or gastropods (Gibbard and Stuart, 1974; Merta, 1980). Ichnogenus Palaeophycus Hall, 1847 Ichnospecies: Palaeophycus striatus Hall, 1852 (Pl. II, fig. G) Description: Horizontal, straight, unbranched, full relief, thinly lined burrow with faint striations. The length of the burrow is 9 cm and diameter is 1.2 cm. The burrow is fill is identical to the host rock. Remarks: This ichnospecies can differ from the other ichnospecies of Palaeophycus on the basis of having striations. Although striations are not clearly visible on the burrow due to erosion, the specimen is assigned to P. striatus considering the gross morphology. This ichnospecies has been recorded for the first time from the Surma succession of Northeast India by Rajkonwar et al. (2011). Kundal and Sanganwar (2000) reported this species from Bagh Group, Dhar district, M.P. HORIZONTAL BRANCHED BURROWS Ichnogenus Ophiomorpha Lundgren, 1891 Ichnospecies: Ophiomorpha annulata Ksiazkiewicz, 1977 (Plate-1, fig. J)

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Description: Y-shaped horizontal branched burrow. The length of the branches are up to 10 cm, with a diameter of about 1.5 cm and diameter of the central shaft is 0.5 cm. Branches are slightly winding without enlargement at the bifurcation point. The burrow material is similar with the surrounding sediments. Remarks: The basic morphological arrangement of the present specimen described herein resembles the ichnogenus Ophiomorpha. The ichnospecies of Ophiomorpha can be differentiate on the basis of shape, nature of the lining and the distribution pellets (Frey et al., 1978; Howard and Frey, 1984; Uchman, 2001). Therefore, the wall morphology and horizontal Y-shape structure of the pr esent burrow is similar to Ophiomorpha annulata Ksiazkiewicz (1977). Tiwari et al. (2011) and Rajkonwar et al. (2013) reported Ophiomorpha borneensis, O. irregular and O. nodosa from the Middle Bhuban Unit of Bhuban Formation (Surma Group) from Aizawl, Mizoram. Ichnogenus Phycodes Richter, 1850 Ichnospecies: Phycodes curvipalmatum Hall, 1852 [Pl. II, fig. E (i)] (Pl. III, fig. F) Description: Hypichnial, horizontal structures, consisting of two or three branches originated from the same point of a thick, slightly curved single stem. The branches are oval in cross section, with burrow diameters of 8 to12 mm, while the main tube is 14 mm and 16 mm in diameter. Burrows filled with very fine-grained sand which is identical with the host rock. The burrow occurs associated with Palaeophycus tubularis. Remarks: Phycodes reflects a variety of behavioural activities by the trace maker (Han and Pickerill, 1994), but two basic interpretations are: (i) a fodinichnion produced by an organism that systematically mining a nutrient-rich layer along a silt-mud surface (Seilacher, 1955) or (ii) a structure performed by an organism that burrowed outwards from a single point and then withdrew to a ‘home-case’ only to re-burrow outwards again in part the previously excavated tunnel (Marintsch and Finks, 1982; Singh et al., 2008). The compound burrow systems P. curvipalmatum have been interpreted as probable combination dwelling-deposit feeding structures produced by endobenthic crustaceans occupying and operating the systems for relatively long time intervals (Miller, 2001). Phycodes are the characteristic trace fossil of the Cruziana ichnofacies and is mainly related with shallow water environments (Singh et al., 2008). It is commonly present at the base

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EXPLANATION OF PLATE III A. Polykladichnus isp., B. Diplocraterion isp., C. Thalassinoides paradoxicus Woodward, 1830, D. Thalassinoides paradoxicus Woodward, 1830, E. Ichnospecies Type-A, F. Phycodes curvipalmatum Hall, 1852, G. Ichnospecies Type-B

ADDITIONAL ICHNOFOSSILS FROM MIDDLE BHUBAN UNIT, BHUBAN FORMATION, SURMA GROUP (LOWER TO MIDDLE MIOCENE), MIZORAM AND THEIR ENVIRONMENTAL SIGNIFICANCE

of centimeter-thick siltstone or silty sandtone beds within shales (Seilacher, 2000; Mangano et al., 2005). Ichnospecies: Phycodes isp. (Pl. I, fig. I) Description: Broom like structure of the burrow, consisting of horizontal tunnels originating nearly from same point. The diameter of the burrows varies from 0.8 to 1.8 cm and preserved as positive epirelief. The proximal part of the main tunnels unbranched while distal tunnels divide into several cylindrical/ subcylindrical tunnels. Remarks: The present burrows displayed similar characteristics of Phycodes Seilacher (1955). Singh et al. (2010) described this ichnospecies from the Bhuban and Boka Bil Formations of Western Hill Manipur, North-East India. Ichnospecies Type-B (Pl. III, fig. G) Description: Burrows horizontal to the bedding plane, meandering and branched. The burrow fill is different from the host rock. The diameter of the burrow ranges from 1.5 to 2.5 cm. Remarks: The horizontal meandering pattern of the present burrow similar with ichnogenus Cochlichnus, but its branching nature refuses the category. There is no previous record of burrow like Ichnospecies Type-B from the Cenozoic or other sedimentary successions of India. Specific identification has been deferred for the want of mote material. BILOBATE TRACE FOSSIL Ichnogenus Didymaulichnus Young, 1972 Ichnospecies: Didymaulichnus lyelli Rouault, 1850 (Pl. I, fig. E and F) Description: Straight to gently curved, smooth, bilobate trails, horizontal to the bedding plane. The lobes are separated centrally by a very narrow median furrow. The traces are about 3.5 to 6 cm in length and 0.6 to 0.8 cm wide and preserved in convex hyporelief. Remarks: Didymaulichnus is generally described as the crawling tr ails pr obably of molluscan or igin (Hantzschel, 1975; Hakes, 1976). Patel et al. (2012) described this ichnospecies from the Jurassic rocks of Gangta Bet, Western India. Rajkonwar et al. (2013) first recorded the ichnospecies from the Surma succession of Northeast India.

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TRACE FOSSIL ASSEMBLAGES The trace fossil assemblage from the Middle Bhuban Unit of Bhuban Formation, Surma Group, Mizoram is composed of 23 ichnospecies from 18 ichnogenera which include, Archaeonassa isp., Arenicolites isp., Curvolithus isp., Cochlichnus anguineus, Didymaulichnus lyelli, Diplocraterion isp., Escape structure, Gordia isp., Laevicyclus mongraensis, Ophiomorpha borneensis, Ophiomorpha annulata, Ophiomorpha nodosa, Palaeophycus striatus, Palaeophycus tubularis, Pholeus abomasoformis, Phycodes curvipalmatum, Phycodes isp., Planolites beverleyensis, Polykladichnus isp., Skolithos linearis, Skolithos isp., Thalassinoides paradoxicus, Ichnospecies Type-A and Ichnospecies Type-B. The trace fossil assemblage throughout the succession is typical of Skolithos, Cruziana and Skolithos/Cruziana ichnofacies. This assemblage is significant in terms of ichnotaxonomic composition, mode of preservation, ethology, tiering structure and do not necessarily represent communities (Hofmann et al., 2012). The Middle Bhuban rocks of Bhuban Formation exposed at Bawngkawn area shows ecologically related group of trace fossils. Six ichno-assemblages, namely, Archaeonassa, Ophiomorpha, Palaeophycus, Phycodes, Skolithos and Thalassinoides are recognized in the collection. These ichno-assemblages are characterized by a particular trace fossil association, named after a dominant ichnogenus and some of these assemblages may record the superimposition or more than one community whereas others may represent variations related to lateral heterogeneity within an individual community (Fig. 3). Archaeonassa assemblage This assemblage is characterized by simple grazing trails of Archaeonassa isp., Curvolithus isp., Didymaulichnus lyelli, Planolites beverleyensis and Gordia isp. representing superficial grazing structures of detritus feeders and deposit feeding activities of vermiform organisms. This assemblage occurs in sandstone characterized by wavy structure. Archaenassa assemblage represents break in sedimentation and lowenergy conditions, characterizing prodelta environment subjected to fresh water discharge (Hofmann et al., 2012).

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Fig. 3. Generalized lithocolumn of Bawngkawn-Edenther section, showing the distribution of trace fossils assemblages

ADDITIONAL ICHNOFOSSILS FROM MIDDLE BHUBAN UNIT, BHUBAN FORMATION, SURMA GROUP (LOWER TO MIDDLE MIOCENE), MIZORAM AND THEIR ENVIRONMENTAL SIGNIFICANCE

Ophiomorpha assemblage This ichno-assemblage consist of Ophiomorpha nodosa and Ophiomorpha borneensis with Arenicolites isp., Skolithos isp., Phycodes curvipalmatum, laevicyclus mongraensis, Polykldichnus isp. and Ichnospecies TypeA. The assemblage is observed in fine to medium grained grey sandstone, tide dominated wavy, cross-laminated silty-sandstone and alternate beds of sandstone-shale. The ichno-assemblage shows mixing of suspension feeders and deposit feeders. Crustacean like organisms are the most probable trace maker of this assemblage (Joseph et al., 2012). Ophiomorpha assemblage is dominated by vertical shafts in high density suggesting high energy shallow marine condition and the dominance of suspension feeders as well as the absence of iron oxide rim at its burrow walls representing high oxygen content in the sediments (Joseph et al., 2012). Palaeophycus assemblage Palaeophycus assemblage consist of Palaeophycus striatus, Palaeophycus tubularis and other associated trace fossils are Ophiomorpha borneensis, Phycodes curvipalmatum, Skolithos isp. and Cochlichnus anguineus. This assemblage occurs in alternating sandstone-shale lithology of Bhuban Formation of Bawngkawn area characterized by current and wave ripples and cross lamination. The horizontal burrows of Palaeophycus assemblage appears to have been produced by feeding and grazing activities of deposit feeders like polychaetes (Joseph et al., 2012), in well oxygenated environment with abundant subsurface food sources. The high abundance of feeding and grazing traces suggests extremely quiet water condition or lowest energy level (Fursich and Heinberg, 1983; Patel et al., 2012) and less abrupt shifting of sediments, less change in temperature and salinity (Joseph et al., 2012). Phycodes assemblage Phycodes assemblage is dominated by Phycodes isp. and Phycodes curvipalmatum. Ophiomorpha borneensis, Palaeophycus tubularis, Thalassinoides paradoxicus and Ichnospecies Type-B are subordinate components. This assemblage occurs in cross laminated silty-sandstone. It shows predominance of deposit feeding activities of probable trace makers like vermiform annelids and crustaceans (Joseph et al., 2012). The assemblage is mainly related with shallow water environments and less frequently recorded in

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deep-marine conditions (Han and Pickerill, 1994). Phycodes a characteristic trace fossil of the Cruziana ichnofacies is commonly present at the base of centimeter thick siltstone or silty-sandtone beds within shales (Seilacher, 2000; Mángano et al., 2005). Therefore the assemblage indicates low energy conditions of the offshore to transition-shoreface environment (Joseph et al., 2012). Skolithos assemblage Skolithos assemblage consist of Skolithos linearis, Skolithos isp., Cochlichnus anguineus, Ophiomorpha annulata, Ophiomorpha borneensis and Thalassioides paradoxicus. This assemblage is observed in parallel laminated silty-sandstone and shale of Bhuban Formation exposed at Bawngkawn area. The dominant dwelling burrows of suspension feeders most probably made by the vermiform annelids (Joseph et al., 2012), are the member of Skolithos ichnofacies which indicates sudden change in the envir onmental condition (Pemberton et al., 2001), relatively moderate to high energy conditions and shifting substrate exploited by the opportunistic animals in the intertidal-subtidal environments (Mángano and Buatois, 2004; Patel et al., 2012). Thalassinoides assemblage This assemblage is dominated by trace fossils of Thalassinoides paradoxicus commonly associated with Diplocraterion isp., Pholeus abomasoformis and Skolithos isp. The Thalassinoides assemblage shows feeding and dwelling activities of crustacean and polychaete like organisms. These are normally considered typical of littoral environment with maximum water depth of ~20 m and frequently related to oxygenated environment in soft but fairly cohesive substrates (Bromley and Frey, 1974; Kern and Warme, 1974; Ekdale et al., 1984; Bromley, 1990). The assemblage occurs in sandstone and shale of Bhuban Formation. The dominance of horizontal feeding burrows of deposit feeders suggests low to moderate energy conditions, unstable and unconsolidated substrate of the shallow marine environment. DISCUSSION Distribution of the Trace fossils Trace fossil composition, diversity and abundances fluctuate throughout the studied section.

268 Some of the trace fossils occur throughout the section but less frequently and less continuously. Upper part of the section is significantly less in trace fossils as compared to the lower part. The lower part of the section shows a moderate diversity and abundance of postdepositional trace fossils. Most abundant among these are Archaeonassa isp., Ophiomorpha borneensis, Skolithos isp. and Thalassinoides paradoxicus, occurs within the grey sandstone and silty-sandstone. Diplocraterion isp., Laevicyclus mongraensis, Ophiomorpha annulata, Palaephycus striatus, Pholeus abomasoformis, Polykladichnus isp., Ichnospecies TypeA and Type-B occurring as monospecific suites. Ophiomorpha nodosa has been encountered more or less continuously in grey sandstone and alternate beds of sandstone and shale exposed in the middle part of the section associated with Arenicolites, Skolithos linearis, Thalassinoides paradoxicus and Escape structure recorded from the upper most silty-sandstone beds overlying the grey massive sandstone bed. Although the underlying sandstone beds are devoid of trace fossils, sedimentary structures like cross bedding and wave ripples are present in it. The vertical distribution of trace fossils show the shifting of ichnofacies from bottom to top of the section. Morphological variations can be seen in the trace fossils i.e. simple (Skolithos) and spreite (Diplocraterion) forms together with the branched, winding and meandering forms. Palaeoenvironmental significance of trace fossil assemblages The sandstone, silt-stone, shale and their admixtures of Middle Bhuban Unit of Bhuban Formation (Surma Group) consist of abundant and diverse group of trace fossils. The alternate sandstoneshale facies and silty-sandstone facies in the studied section are rich in trace fossils with varying density at different levels. This association represents Archaenassa assemblage, Ophiomorpha assemblage, Palaeophycus assemblage, Phycodes assemblage, Skolithos assemblage and Thalassinoides assemblage. The Archaenassa assemblage occurs in the wavy grey colour sandstone unit of lower part of the section. The trace fossils associated with these assemblages belong to Skolithos ichnofacies, Cruziana ichnofacies and at places mixing of both Skoliths-Cruziana ichnofacies. The trace fossil diversity and abundance show fluctuations of sea level and energy condition throughout the section. The continuous and abundant record of trace

CHINMOY RAJKONWAR and OTHERS

fossils from lower to middle part of the section indicates good oxygenation at sea floor with high organic nutrients. The Phycodes assemblage and Thalassinoides assemblage occur at the lower most silty-sandstone bed with association of Ophiomorpha borneensis, Palaeophycus tubularis, Laevicyclus mongraensis, Skolithos isp. and Ichnospecies Type-B are being characteristic trace fossils of Cruziana and SkolithosCruziana ichnofacies which indicate deposition under low to moderate energy conditions, unstable and unconsolidated substrate of the offshore to transitionshoreface environment. The Skolithos assemblage is present in the upper most parallel laminated siltysandstone beds associated with Thalassinodes paradoxicus and escape structure and in the lower shale unit association with Cochlichnus anguineus, Ophiomorpha annulata and Thalassinoides paradoxicus suggests relatively moderate to high energy conditions and shifting substrate, while the occurrence of escape structure in the upper part indicates rapid sedimentation due to the fluvially derived clastics in a delta front (Hofmann et al., 2012). The Ophiomorpha assemblage is more or less continuously occurring in bottom to middle sandstone, silty-sandstone and alternate sandstone-shale beds of the section, indicating high energy condition, good oxygenation at sea floor with high organic nutrients in the middle-shoreface to foreshore environment (Joseph et al., 2012). Archaenassa assemblage present in the wavy sandstone bed overlying the Skolithos dominated lower shale bed, associated with other typical members of Cruziana ichnofacies (e.g. Curvolithus isp., Didymaulichnus lyelli, Gordia isp. and Planolites beverleyensis) represents break in sedimentation and low energy condition in prodelta environments (Hofmann et al., 2012). The Palaeophycus assemblage occurs in the overlying alternate sandstone-shale beds and characterized by semi vagile and vagile, middle level deposit feeder structures present in oxygenated conditions (Bromley, 1990), which is also indicative of less shifting of sediments and low energy condition in lower shoreface to somewhat quieter offshore environment (Joseph et al., 2012) under almost stable temperature and salinity. CONCLUSIONS The studied ~120 m thick Middle Bhuban rocks of Bhuban Formation, Surma Group exposed in Bawngkawn-Edenther section have yielded 23 ichnospecies belonging to six ichno-assemblages

ADDITIONAL ICHNOFOSSILS FROM MIDDLE BHUBAN UNIT, BHUBAN FORMATION, SURMA GROUP (LOWER TO MIDDLE MIOCENE), MIZORAM AND THEIR ENVIRONMENTAL SIGNIFICANCE

namely, Archaeonassa, Ophiomorpha, Palaeophycus, Phycodes, Skolithos and Thalassinoides. Trace fossils associated with these ichno-assemblages belong to Skolithos ichnofacies, Cruziana ichnofacies and at places mixing of both Skoliths-Cruziana ichnofacies. These ichnofacies suggest that the Middle Bhuban succession exposed at Bawngkawn-Edenther section, Aizawl, Mizoram was deposited under high energy conditions and sandy shifting substrate in foreshore zone and unconsolidated, poorly sorted soft substrate and low energy condition in the shoreface to offshore zone, respectively. ACKNOWLEDGEMENTS Authors are thankful to Prof. P. Kundal for reviewing the manuscript for its improvement. We had fruitful discussion with Dr. J. Malsawma, Department of Geology, Mizoram University during field work and in the preparation of litho-column. REFERENCES Abel, O. 1935. Vorzeitliche Lebensspuren. Gutav Fischer (Jena), 644 p. Alpert, S. P. 1974. Systematic review of the genus Skolithos. Journal of Paleontology, 48: 661-669. Billings, E. 1862. New species of fossils from different parts of the Lower, Middle and Upper Silurian rocks of Canada, p. 96168. In: Paleozoic Fossils, Geological Survey of Canada, 1: 1861-1865. Boy, J. A. 1976. Überblick über die Fauna des saarspfalzischen Rotliegenden (Unter-Perm). Mainzer Geowissenschftliche Mitteilungen, 5: 3-85. Bromley, R. G. 1990. Trace fossils. In: Hayman, U. (Eds.), Biology and Taphonomy, Chapman and Hall, London, 280p. Bromley, R. G. and Frey, R. W. 1974. Redescription of the trace fossil Gyrolithes and taxonomic evaluation of Thalassinoides, Ophiomorpha and Spongeliomorpha. Bulletin Geological Society of Denmark, 23: 311-335. Buatois, L. A. and Mángano, M. G. 2002. Trace fossils from Carboniferous floodplain deposites in Western Argentina: implications for ichnofacies models of continental environments. Plalaeogeography, Palaeoclimatology, Palaeoecology, 183: 71-86. Buck, S. G. and Goldring, R. 2003. Conical sedimentary structures, trace fossils or not? Observations, experiments and reviews. Journal of Sedimentary Research, 73: 338-353. Buckmann, J.O. 1994. Arechaeonassa Fenton and Fenton 1937 reviewed. Ichnos, 3: 185-192. Chiplonkar, G. W. and Badve, R. M. 1970. Trace fossils from the Bagh beds. Journal of the Palaeontological Society of India, 11: 1-10. Chiplonkar, G. W. and Ghare, M. A. 1975. Some additional trace fossils from the Bagh Beds. Bulletin Indian Geological Association, 8: 71-84. Ekdale, A. A., Bromley, R.G. and Pemberton, G. S. 1984.

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