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Early Permian Macroinvertebrate Assemblages from the Siang and Subansiri Districts, Arunachal Pradesh: Implications on the Regio

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Early Permian Macroinvertebrate Assemblages from the Siang and Subansiri Districts, Arunachal Pradesh: Implications on the Regio J. Earth Syst. Sci. (2021) 130:87 Ó Indian Academy of Sciences https://doi.org/10.1007/s12040-021-01569-3 (0123456789().,-volV)(0123456789().,-volV) Early Permian macroinvertebrate assemblages from the Siang and Subansiri districts, Arunachal Pradesh: Implications on the regional stratigraphy, palaeoenvironment, palaeoecology, and palaeobiogeography 1,4, 2 2 SUBHRONIL MONDAL * ,DEBAHUTI MUKHERJEE ,BASHISHA IANGRAI , 3 1 ARINDAM ROY and SUBHAJIT SINHA 1Department of Geology, University of Calcutta, 35, B.C. Road, Kolkata 700 019, India. 2Palaeontology Division, Geological Survey of India, North Eastern Region, Meghalaya, Shillong 793 014, India. 3Palaeontology Division, Geological Survey of India, Central Headquarters, 15 A and B Kyd Street, Kolkata 700 016, India. 4Present address: Department of Earth Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur, West Bengal 741 246, India. *Corresponding author. e-mail: [email protected] MS received 16 July 2020; revised 2 January 2021; accepted 4 January 2021 The Early Permian marine strata exposed at the Siang and Subansiri districts, Arunachal Pradesh are known for brachiopod and molluscan fossils. A stratigraphic review and palaeoenvironmental, palaeoecological, and palaeobiogeographic interpretation of these Early Permian fossils have been dealt here on the basis of additional fossil and rock materials. Our study conBrms the following: (1) the brachiopod-dominated assemblage represents Early Permian, most likely the Sakmarian and younger, age; (2) all marine fossils were suspension feeding groups, representing normal marine condition. Based on the associated sediments and ecologies of taxa, the autecology of the assemblage represents an oxic, soft substrate condition, typical of shallow shelf environment; the plant remains of the upper Khelong/Bhareli Formation, however, indicate a coastal environment; (3) the depositional paleoenvironment indicates the waning phase of a marine glacial episode; and (4) this Early Permian bivalve- brachiopod assemblage show Gondwanan aDnity, having strong faunal similarity with the East and West Australia, Nepal and North India (Kashmir) which were part of the Indoralian Province. Keywords. Permian palaeobiogeography; Gondwana; Indoralian Province; Northeast India; marine fossils. 1. Introduction During this interval, the northern part of India, Tibet, Pamir, and Karakorum (Pakistan) were The Permian was an interval of some major drifting apart from the Gondwana land (Angiolini tectonic changes including the breakup of the peri- et al. 2007; Shen et al. 2013). The remnants of the Gondwana and spreading of the Neotethys ocean. marine habitats of the faunal assemblage of these Supplementary material pertaining to this article is available on the Journal of Earth System Science website (http:// www.ias.ac.in/Journals/Journal˙of˙Earth˙System˙Science) 87 Page 2 of 23 J. Earth Syst. Sci. (2021) 130:87 areas have been the focus of much research eAorts biostratigraphic frameworks, which has been fol- (Nakazawa et al. 1975; Singh 1988; Singh and lowed in the present study. Associated sedimento- Archbold 1993; Shen et al. 2003; Angiolini et al. logical aspects were also analyzed to further conBrm 2005). In the north-eastern part of India, the Early the litho- and biostratigraphic characteristics of Permian fossil-bearing marine sediments of Aru- these marine units; (2) recently, during several nachal Pradesh occur in an east–west strip and are Beldworks in the west of Siang river of Arunachal associated with the coal-bearing Gondwana Pradesh, we collected several marine fossils (mostly (Acharyya et al. 1975; Kesari 2010), although brachiopods) from these formations. Integrating detailed works on these rocks are mostly lacking. lithofacies and fossil distributions, we provide a These aforementioned sediments of Early Permian detailed palaeoecological, palaeoenvironmental, age, exposed along Siang, Kameng, Subansiri, and and palaeobiogeographic accounts of these Early Papum Pare districts in Arunachal Pradesh are Permian fossils to identify their global aDnities. lithologically quite diverse (Roy Chowdhury and Sinha 1984). From the Darjeeling foothills to the West Siang district, the Gondwana sequence of 2. Material and methods Eastern Himalaya has been mapped (Jain and Tha- kur 1975), but the continuity of the sediments in the 2.1 Stratigraphic and palaeontological accounts direction of the strike is truncated east of Siang river by the Siang Fracture Zone (Nandi 1975). Laskar Since there is no consensus regarding a generalized (1959) proposed that the northern boundary of these regional stratigraphy of the Permian rocks exposed rocks exposed at Siang and Subansiri districts show in Siang and Subansiri districts, Arunachal Pra- thrusted nature of contact. Later, Laskar and Roy desh (Bgure 1), we Brst attempted to provide a Chowdhury (1977) added that these units are basi- general, concise stratigraphic review. Some of these cally bounded by two regional thrusts – the northern units have rich marine fossils. Sedimentological part upthrusted against the Siwaliks by the Main details of the fossiliferous units and representative Boundary Thrust (MBT), and the southern part rock samples were collected for thin section studies associated with older metamorphosed and migma- to ascertain the stratigraphic position and for the tized rocks (Roy Chowdhury and Sinha 1984). palaeoenvironmental studies. Extensive tectonic disturbances make it difBcult Several Beld investigations were conducted to to identify the continuity of the lithofacies later- study the Permian rocks exposed in and around ally. The Early Permian sediments are con- Rilu, Tatamori, Littemori, and Kimin, all in the formably lying above the Miri Formation, a west of Siang river, in between 2016 and 2018 sequence of cross bedded-ripple marked, sandstone (Bgure 2). Fossils have been collected from well- intercalated with Bne shale but the stratigraphic deBned stratigraphic units of the Bomte Member of status of this unit is still debatable as it has been the Bichom Formation, and the Khelong/Bhareli correlated with numerous other formations (Ku- Formation (Bgure 3). These fossils are hosted in a mar 1997; Kesari 2010). Above this formation, black carbonaceous, grey shale unit intercalated Early Permian fossil-bearing units are present, with thin sandstone bands with calcareous, fer- which has been regionally correlated with Bijni ruginous cherty nodules (Bgure 4). Other than this, Formation in Uttarakhand, Sisne Formation in a few nodules host brachiopod or other types of Nepal, Rangit Pebble Bed in Darjeeling, and Diuri fossils inside it as a nucleus (Bgure 5). These nod- Formation in Bhutan (Nandi 1975; Laskar and Roy ules are similar to the ones recorded by Laskar in Chowdhury 1977; Bhargava 2008). The basic Ranga valley, conBrming that the Early Permian character of these correlatable formations is their brachiopod and molluscan fossils of the Siang and overall arenaceous composition, frequently associ- Subansiri districts belong to the Bomte Formation. ated with coaly units, and discontinuous horizons In comparison, in Littemori, the fractured car- with rich near-coastal and marine macro-inverte- bonaceous nodules have numerous mollusc fossils, brates. However, although many attempts have dominantly gastropods, which are otherwise rare. been made to raise a generalized stratigraphy of In all cases, the state of preservation of fossils, this region, no consensus has been obtained. whether they are present within the concretion or In this context, the present study attempts to within the sediment, are always poor – the original perform the following objectives: (1) provide a shell materials have been removed during diagen- concise review of the regional lithostratigraphic and esis and only the casts and some moulds are J. Earth Syst. Sci. (2021) 130:87 Page 3 of 23 87 Figure 1. Comparative stratigraphic schemes of Arunachal Pradesh proposed by different researchers. Vertical lines temporally differentiating different members and groups are somewhat arbitrary. Figure 2. Distribution of Early Permian rocks along the Siang and Subansiri districts in Arunachal Pradesh. The map also shows fossil locations (star marked). preserved (Bgures 5 and 6). Thin section study also traces of chamber walls that differentiates it from indicates that concretions are fossiliferous with concretions. Because of the poor preservation and fossil shells partially or fully dissolved with faint patchy distribution of fossils, instead of bulk 87 Page 4 of 23 J. Earth Syst. Sci. (2021) 130:87 Figure 3. Stratigraphic column used in the present study. For details, see Kesari (2010) and the stratigraphic review discussed in the text. samples, all identiBable shells or macroinverte- Gondwanan and Palaeoequatorial realms. Global brates were collected. For the same reason, it was occurrences of the Early Permian genera have impossible to identify these up to species level and primarily been obtained from the Paleobiology hence, we restricted our study up to the genus and database supplemented with other publications family levels. (Runnegar 1969; Waterhouse 1980, 1987; Shen The taxonomic validity of all previously reported 2018). Since the Indian record is poorly represented taxa was checked using the Paleobiology database in the Paleobiology database, we used few addi- (www.paleobiobd.org/). Genera recorded in tional publications to improve this database, the palaeontological literature with
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