The Indo-Myanmar Ranges (IMR) is a broadly N-S trending arcuate fold-thrust belt which extend from the East Himalayan Syntaxis (EHS) southwards along the eastern side of the Bay of Bengal to the Andaman Sea. It is connected with the Himalayan belt and Lhasa terrain through Gangdese-Myanmar arc, formed as a result of the subduction of the Neo-Tethys slab in the Cretaceous and the subsequent collision between the Indian and Asian continents at ca. 50 Ma (Arboit et al 2021). The range is subdivided into Nagaland-Manipur Hills, Chin Hills and Arakan Yoma from north to south. The Nagaland-Manipur-Chin hills are flanked to the west by the low hill tracts of N-S trending fold belt of the Neogene molasse sediments (Khin et al, 2017; Acharyya, 2010). The chemical compositions of these rocks are significantly controlled by plate tectonic settings of their provenances and depositional basins, and as a result, the siliciclastic rocks from different tectonic settings signify terrain-specific geochemical signatures (Bhatia and Crook, 1986; Roser and Korsch, 1986). The classification of rocks formed in such setting and their studies on provenance, weathering and depositional condition have been done through geochemical composition studies (Bhatia, 1983 and Mc Lennan et al., 1993). Ravikant et al (2015) proposed derivation of the Paleogene-Neogene sediments of Assam basin by erosion from adjacent crustal and orogenic source based on zircon U-Pb geochronology and Hf isotopic studies. Allen et al 2008 have also studied provenance of the sedimentary sequence of IMR in Myanmar part. However, detail study of geological and geochemical characteristics, provenance and tectonic set up and paleogeographic layout at the time of such deposition in the Manipur-Nagaland section of IMR are yet to be studied. In the present study, field observation, geochemical studies of different formations of the Barail Group and detrital geochronology of a sandstone sample from Laisong Formation, the basal formation of the group exposed in Dimapur-Kohima-Chakhabama-Peletkie Road section of Nagaland, NE India,are used to understand the geology, provenance and tectonic setting and paleogeography prevailing at the time of their deposition.
Regional Geology
The geology of Manipur and Nagaland (Figure 1) is divided into (i) Assam Shelf, (ii) Schuppen Belt, (iii) Palaeogene - Neogene sedimentarysequence and (iv) Ophiolite belt towards east near to Indo-Myanmar Border (GSI, 2011; Aitchison et al 2019). Assam Shelf is the peripheral foreland basin sediments deposited uncomformably over pre-Tertiary granitic basement (Ravikant et al, 2015). Itconsists of a relatively thin sequence of sediments of Barail Group, Surma Group and Tipam Group exposed mainly in Dhansiri valley of Nagaland. Part of Assam Shelf sediments has been thrust over by tectonic slices of Schuppen Belt. The schuppen belt is a zone of imbricate thrusting involving the Paleogene-Neogene foreland sediments (Ravikant et al 2015). The belt comprises sedimentary rocks of Barail, Surma, Tipam groups along with Namsang and Dihing formations. The Assam shelf and Schuppen belt are present in Nagaland. Paleogene –Neogene sedimentary sequence arepost-Upper Cretaceous sequence commencing from Disang Group onwards to Surma Group. Theyare deposited over by Recent and Pleistocene sediments. This sequence is folded and thrusted (GSI, 2011; Aitchison et al 2019). They are thrusted over by Ophiolite belt. The Ophiolite belt comprises zones of heavily disrupted and strongly dismembered serpentinite‐ and mud‐matrix mélanges (Aitchison et al 2016). The eastern part of the ophiolite is thrusted by unit of schist and gneiss of Naga Metamophics (Aitchison et al 2019). The sedimentary units of Neogene and Paleogene, and ophiolite belt are exposed in both Manipur and Nagaland states of India.
Table 1
Lithostratigraphic position of Barail Group in the study area.
Period
|
Group
|
Stratigraphic Unit
|
Lithology
|
Early Pliocene to Middle
Miocene
|
Surma
|
Bhuban Formation
|
Buff to light brown, friable, fine to medium grained, even to lenticular, thin to thickly bedded sandstone interbedded with subordinate khaki green shale and siltstone.
Dark grey shale interbedded with lenticular light brown siltstones, coarsely interlayered with sub-ordinate fine grained, compact, medium grained, thin bedded sandstone.
Very Thick bedded, massive, medium to coarse grained, hard, grey sandstone with thin intercalation of shale
|
-----------------------------------Unconformity-------------------------------------------------------------
|
Early
Miocene to Late Eocene
|
Barail
|
Renji Formation
|
Grey, fine to medium grained, hard, medium bedded ferruginous sandstone with thin horizons of lenticular siltstone interbedded with grey shale.
|
-----------------------------Gradational contact-------------------------------
|
Jenam Formation
|
Dark grey to bluish grey shale when fresh and dark brown when weathered with intercalations of a few thinly bedded brown, medium grained sandstone and concretions.
|
-----------------------------Gradational contact------------------------------
|
Laisong Formation
|
Light grey, fine to medium grained, hard and medium bedded sandstone with occasional plant imprints, burrows with intercalation of shale.
|
--------------------------------------------------Gradational contact---------------------------------------
|
Palaeocene to Eocene
|
Disang
|
Upper Disang Formation
|
Dark grey to brownish, laminated shales interbedded with subordinate fine grained, thin bedded sandstone and intercalation of siltstone. On weathering, the shales become platy and splintery. Load cast and asymmetric ripples are present in medium bedded sandstone interbedded with shales.
|
Geology of the study area
In Dimapur-Kohima-Chakhabama-Peletkie Road section (Figure 2), different units of Disang, Barail, Surma groups and Namsang Formation are exposed. In the present study, focus is given on the Barail Group as it records the signatures of transition from marine to deltaic depositional phase. The group belongs to Late Eocene - Oligocene (Acharya, 2010; GSI, 2011). Lithostratigraphic position of the different units of Barail Group is furnished in Table 1. The different formations of the group i.e. Laisong, Jenam and Renji formations are exposed in the Paleogene – Neogene belt, Schuppen belt and Assam Shelf. Rocks of Barail Group conformably overlie the Upper Disang Formation of Disang Group of rocks. The contact is marked by the first appearance of a thickly bedded (about 50 cm) sandstone bed (Figure 3a). The detail description of the different formations of this group is explained in the subsequent sub sections.
Laisong Formation
It comprises buff to light grey and sometimes pinkish, coarse to fine grained, massive or thin to medium bedded sandstone intercalated with dark grey shale (Fig. 4b) in the ratio 80:20 (Figure 3a). Load casts are commonly seen in the hard sandstone beds overlying shale horizons. Flattened worm burrows parallel to the bedding plane are usually filled with fine grained sand. They are ellipsoidal in shape and are of 0.3 to 5 cm diameter. Coal streaks, and imprints of leaves and stems are also observed in Laisong sandstone (Figure 3b). Petrographic studies reveal the presence of quartz, lithic fragments, feldspars, with minor hematite with siliceous cementing material (Fig. 4a). Texturally, the grains are angular to sub angular and poorly sorted. There is a noticeable overgrowth of the quartz grains. Overall description suggests the rock is coarse grained arkosic – wacke (Figure 4a).
Jenam Formation
This formation comprises predominantly of well bedded, dark grey to bluish grey carbonaceous shale when fresh, which becomes dark brown when weathered. These shales are highly fissile in character (Figure 3c). Shale beds are intercalated with well bedded, buff, medium grained sandstone. Jenam Formation, being an argillaceous dominant unit, can be differentiated from the other formations of Barail Group. Another distinguishable feature of this formation is the occurrence of coal streaks/lenses/seams. Remains of leaves and twigs/stems are observed in Jenam sandstone. Ferruginous nodules (Figure 3d) and calcite viens (Figure 3e) are occasionally observed in the shales of this formation. Petrographic study of shale reveals presence of quartz and feldspars with siliceous cementing material. Texturally, the grains are angular to sub angular and poorly sorted (Figure 4c).
Renji Formation
This formation is the uppermost formation of Barail Group of rocks. It gradationally overlies the underlying Jenam Formation and is distinguished from the remaining formations by its bands of very thick bedded to massive bedded, grey, fine to medium grained, hard, ferruginous cross-bedded sandstone with thin horizons of lenticular siltstone interlayering grey shale (Figure 3f). Sandstone, siltstone and shale occur in the ratio 60:20:20. The thickness of sandstone beds ranges from 10 cm to 7 m, siltstone from 3 cm to 15 cm and shale from 5 cm to 15 cm. It comprises quartz and other fragments with siliceous cementing material. Texturally, the grains are sub-angular to sub-rounded and poorly sorted. Petrographically, the sandstone can be classified as quartz arenite (Figure 4d).