One of the major depositional strata in the Himalaya is the Lesser Himalayan Strata from the Paleozoic to Mesozoic eras. It had a quite different marine succession during the Paleozoic, as most parts of it are sparsely fossiliferous or even devoid of any well-defined fossils. Moreover, it consists of many varied lithofacies, making correlation work more difficult. This article describes the major formations of the Paleozoic – Mesozoic Lesser Himalayan Strata, including the Tal Formation, Gondwana Strata, Singtali Formation and Subathu Formation.
Geological background The Himalayan mountain chain is a fold and thrust belt that can be divided into four units bounded by thrusts from south to north: the Sub-Himalaya, Lesser Himalaya, Greater Himalaya and Tethyan Himalaya. The Lesser Himalayan Zone has a lower relief and elevation of the mountains compared to Greater Himalaya. The Lesser Himalaya Sequence (LHS) is bounded by two main thrusts: the Main Central Thrust (MCT) in the north and the Main Boundary Thrust (MBT) in the south.
The main layers of the LHS includes non-fossiliferous, low-grade, metasedimentary rocks, metavolcanic strata and augen gneiss. These have been dated as an age ranging from 1870 Ma to 520 Ma (i.e. Proterozoic to Cambrian). Near the end of the Early Cambrian, there was a regional diastrophism (i.e. deformation of the Earth's crust) or crustal movement that heaved up the Indian subcontinent, interrupting the sedimentation in the Lesser Himalaya and causing a widespread unconformity in Nepal. This is known as the Great Lesser Himalayan Unconformity, which separates the older LHS from the overlying younger LHS that has an age of Permian to Middle Eocene.
During the Paleozoic and Mesozoic, the LHS starts from the basal Tal Formation, which is part of the Outer Lesser Himalayan sequence in the Garhwal Himalaya. The Tal was deposited between the period of the Late Proterozoic to Palaeozoic Cambrian. After that, there was a great hiatus between the Middle Proterozoic rocks and the overlying Palaeocene-Eocene strata. This indicates that the LHS experienced a long period of denudation or non-deposition in the Western Himalaya (e.g. in the Jumla area), resulting in missing Gondwana Strata in Jumla during the Mesozoic (see more in the table of "Gondwana Strata of LHS in Western and Central Nepal").Dhital, M. R. (2015). Geology of the Nepal Himalaya: Regional Perspective of the Classic Collided Orogen. Springer. In the Central and Eastern Himalaya, Gondwana strata from the Permian to Paleocene were preserved unconformably onto the older LHS. The next succession of the LHS in the Garhwal Himalaya comes to the Singtali Formation, which was deposited from the Late Cretaceous to Palaeocene, followed by the Subathu Formation, which was deposited from the Late Palaeocene to Middle Eocene,Dubey, A. (2014). Understanding an Orogenic Belt: Structural Evolution of the Himalaya. Springer. marking the start of Cenozoic Era.
Table showing the major formations discussed in the following sections: LESSER HIMALAYAN SEQUENCEWestern HimalayaCentral HimalayaEastern HimalayaGarhwal HimalayaWestern NepalCentral NepalBhutanSirmur Group Subathu Formation (Late Paleocene – Middle Eocene)Tertiary Units (Bhainskati Formation) Gondwana Units (Late Carboniferous – Permian) Singtali Formation (Late Cretaceous – Paleocene)Gondwanas (Jurassic – Paleocene) Tansen Group - Upper and Lower Gondwanas
(Late Permo-Carboniferous – Paleocene)Lacking Gondwana Units (Hiatus)Mussoorie Group Tal Formation (Cambrian)Krol Formation~~~~~~~~Great Lesser Himalayan Unconformity~~~~~~~Baxa GroupBlaini FormationNawakot UnitDaling-Shumar Group
Tal Formation The Tal Formation belongs to the Mussoorie Group of Outer Lesser Himalaya of Garhwal in northwestern India. It is well exposed along the Krol Belt, and is overlying the Precambrian Krol Group.
The Tal in the Mussoorie Synform can be divided into the Lower Tal and Upper Tal. For the Lower Tal, there are four subdivisions: the Chert, Argillaceous, Arenaceous and Calcareous Units. The basal black shale succession with sandy limestone represents a depositional environment of a protected lagoon or embayment, while the overlying siltstone is deposited in a mud flat of an intertidal zone.
The Upper Tal can be subdivided into lower quartzitic sequence and upper thick calcareous sequence containing abundant fragmentary shells of bivalves, gastropods, bryozoa, etc. The Phulchatti quartzite succession represents the deposits of a shoal environment, while the uppermost shell limestone sequence indicates an increasing energy of the shallow tidal sea, and a marine transgression in the Cretaceous.Srivastava, R. V. K., & Tewari, V. C. (1986). The nature of glauconite in the Upper Tal Formation of the Lesser Himalayan belt and its role in resolving the problem of unconformity between upper Tal Quartzite and Shell Limestone.
There is an increase of energy for deposition from the Lower Tal to the Upper Tal. Because of the lack of well-defined body fossils in the Tal, it has been proposed that the deposits of the Tal were formed in the Late Precambrian near Precambrian-Cambrian transition, except for the uppermost Manikot Shell Limestone, which has been proposed to have been formed in the Late Cretaceous and unconformably overlain by the Subathu Formation in the Tal Valley, Garhwal Himayala.PRASAD, V., & SARKAR, S. (2002). FOSSIL SC YTONEMA (NOSTOCALES) FROM THE SUBATHU FORMATION OF TAL VALLEY, GARHWAL HIMALAYA, INDIA. coordinates, 134(x13), 5.
The details of lithologies and depositional environment of Tal Formation are shown in the table below: FormationLithologiesDepositional EnvironmentUpper TalManikot Shell Limestone – Grey, oolitic, sandy, current-bedded fossiliferous limestone containing fragmentary bivalves, gastropod and quartziteHigh-energy shallow tidal sea ------------------------locally unconformable------------------------Phulchatti Quartzite – White to purplish, felspathic, fine-grained to gritty, current-bedded, locally conglomeraticShoalLower TalCalcareous Unit – Ferruginous, sandy limestone or calcareous quartzite (locally developed)Arenaceous Unit – Siltstone, micaceous, grey to dark greyMixed flat and mud flat of intertidal zoneArgillaceous Unit – Shale, micaceous, grey to dark grey, locally carbonaceous with calcareous pyritous nodulesProtected lagoon or embaymentChert Unit – Chert, black with intercalation of black shale, phosphate beds and nodules (locally developed)
Gondwana strata
Gondwana strata are not exposed in the Garhwal Himalaya after the Tal Formation owing to the great hiatus; some outcrops can be found in Central and Eastern Himalaya during Late Paleozoic to Mesozoic times.
Central Himalaya – Central and Western Nepal
In Nepal, the older LHS with age ranging from the Paleoproterozoic to uppermost Precambrian are separated from the younger LHS by the Great Lesser Himalayan Unconformity. Missing Early Proterozoic strata of the LHS suggest that the rocks may have been eroded before the deposition of the Gondwana strata. Younger continental facies Gondwana strata were first deposited after the unconformity. They are then unconformably overlain by a marine facies Tertiary Unit. The Gondwana strata are mainly developed within two zones in Nepal in the central Himalaya. The first is in central Nepal, where several outcrops of the Tansen Group can be found. The second is in western Nepal, where the Gondwana strata are exposed in the Jumla–Humla basins near the Tethyan Himalaya.
The Tansen area contains Gondwana sediments that are identified by the presence of fossils. Two major formation can be found there. They are older Sisne Formation (or the Lower Gondwanas) and younger Taltung and Amile formations (or the Upper Gondwanas). The Sisne Formation is dominated by glacial diamictite and fluvial deposits. In the upper part of the formation, shales are found to contain fenstellid bryozoan fossils, indicating that the Lower Gondwanas in central Nepal dates from the Late Carboniferous to the Permian.
The Upper Gondwanas are further subdivided into the Taltung and Amile formations. The Taltung Formation is characterised by coarse-grained, volcaniclastic conglomerates, sandstones and silty shales. They were deposited by northwestward-flowing fluvial channels.Sakai, H. (1991). The Gondwanas in the Nepal Himalaya. Sedimentary Basins of India: Tectonic Context, Gyanodaya Prakashan, Nainital, 202–217. Abundant plant fossils are found in the Taltung Formation, and they are dated to the Late Jurassic to the Early Cretaceous. The Amile Formation is unconformably overlying the Taltung Formation. It is dominated by white quartz arenites, quartz pebble sandstones, carbonaceous shales and limestones with coral, mollusc and vertebrate fossils. In the upper part of the formation, an abrupt change of lithology from thick, coarse-grained quartzose sandstones to interbedded layers of black marine shales and fine-grained quartzose sandstones are observed. This is probably the contact between the Upper Amile Formation and the overlying Bhainskati Formation of the Tertiary Unit. The Amile Formation is dated to the Early Cretaceous to Early Paleocene, while the Bhainskati Formation is biostratigraphically dated as from the Middle to Late Eocene.
In the Jumla area of western Nepal, Gondwana strata unconformably overly the caronbate rocks of the Uppermost Nawakot Unit of Mesoproterozoic age. The Gondwanas here are characterized by quartzose sandstones, black shales, quartz pebble conglomerates as well as coal and lignite. They are dated as Jurassic to Paleocene. The lithology of Gondwanas here is quite similar to that of the Amile Formation in central Nepal. Also, the Gondwana Unit is overlain by the Bhainskati Formation of the Tertiary Unit, similar to the situation in central Nepal.
However, in fact, the Gondwana Unit is not very well developed in the Jumla area. The strata with lithology similar to that of the Taltung Formation and Lower Gondwanas are missing here. In other words, the LHS in the Jumla area is lacking a part of the Gondwana Unit of age ranging from Late Carboniferous-Permian to Early Cretaceous. This is probably due to a greater effect by the Great Lesser Himalayan Unconformity in the Jumla area than in the Tansen area.
Eastern Himalaya – Bhutan Compared with Nepal, the Gondwana strata are exposed in a relatively smaller area in Bhutan. In southeastern Bhutan, the basal LHS begins with the metamorphic Daling-Shumar Group, followed by the Baxa Group that is characterized by quartzite, phyllite and dolomite succession of Neoproterozoic to probably Cambrian age. The bottom of the Gondwana Units (Diuri Formation) is then directly overlying the Baxa Group.
Generally, there are three main formations of Gondwana Units exposed in southeastern Bhutan. The bottom one is the Diuri Formation with Proterozoic to Permian ages.Gansser, A. (1983). Geology of the Bhutan Himalaya, 181 pp. Denkschr. Schweiz. Naturforsch. Ges, 96. It consists of conglomerate, quartzite, phyllite as well as diamictite with interbedded slates. At the base of this formation, pebbles are composed of quartzite and siliceous dolomite. They are embedded in a fine quartzite matrix. The thicker beds of slate and phyllite are overlying the conglomerate layers. Clear schistosity can be observed. The diamictite found implies a source of glacial origin. It is probably correlated to the glaciation event of the Gondwana supercontinent during the Late Paleozoic.