Lesser Himalayan Strata

During Paleozoic to Mesozoic times, the Himalaya shows two major depositional strata, which are Tethyan Himalayan Strata and Lesser Himalayan Strata. Tethyan Himalayan Strata consists of well-defined marine succession from Upper Proterozoic to Cretaceous, and it is well-developed in many basins to the north of Great Himalaya. On the contrary, the Lesser Himalayan Strata in Paleozoic times has a quite different succession. Most parts of it are sparsely fossiliferous or even devoid of any well-defined fossils. Moreover, it consists of many varied litho-facies, 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 of Lesser Himalayan Strata

Geography of Lesser Himalayan Sequence in Himalaya

The Himalayan mountain chain is a fold and thrust belt which can be divided into four units bounded by thrusts from south to north: the Sub-Himalaya, Lesser Himalaya, Greater Himalaya and Tethyan Himalaya.^[1] The Lesser Himalayan Zone has a lower relief and elevation of the mountains compared to Greater Himalaya. The Lesser Himalaya Sequence (LHS) is bounded to the south by the Main Boundary Thrust (MBT) and to the north by the Main Central Thrust (MCT).

Geographic locations of major formations discussed. Modified from N.R. McKenzie et al. (2011).

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).^[2][3] 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 the Great Lesser Himalayan Unconformity, which separates the older LHS from the overlying younger LHS that has an age of Permian to Middle Eocene.^[4]

During Paleozoic and Mesozoic, LHS starts from the basal Tal Formation, which is an Outer Lesser Himalayan sequence in Garhwal Himalaya. The Tal in fact lies between the period of Late Proterozoic to Palaeozoic Cambrian.^[5] After that, there was a great hiatus between the Middle Proterozoic rocks and the overlying Palaeocene-Eocene strata.^[6] This indicates that LHS experienced a long period of time of denudation or non-deposition in Western Himalaya like in Jumla, resulting in missing Gondwana Strata there during the Mesozoic.^[6] While in the Central and Eastern Himalaya, younger Gondwana strata with Permian to Paleocene age were preserved and unconformably overlying the older LHS.^[4] The next succession of LHS in 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 Late Palaeocene to Middle Eocene,^[5] marking the start of Cenozoic era.

Table showing the major formations that will be discussed in the following sections:

LESSER HIMALAYAN SEQUENCE
	Western Himalaya		Central Himalaya			Eastern Himalaya
	Garhwal Himalaya		Western Nepal	Central Nepal		Bhutan
Sirmur 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 (Hitus)
Mussoorie Group	Tal Formation (Cambrian)
	Krol Formation		~~~~~~~~Great Lesser Himalayan Unconformity~~~~~~~			Baxa Group
	Blaini Formation		Nawakot Unit			Daling-Shumar Group

Tal Formation

The Tal Formation belongs to the Mussorie 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.^[7]

The Tal in the Mussoorie Synform can be divided into Lower Tal and Upper Tal. For the Lower Tal, there are four subdivisions, namely the Chert, Argillaceous, Arenaceous and Calcareous Units.^[8] 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.^[9]

The Upper Tal can be subdivided into lower quartzitic sequence and upper thick calcareous sequence containing abundant fragmentary shells of bivalve, gastropod, bryozoa etc.^[10] 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 Cretaceous times.^[11]

There is an increase of energy for deposition from the Lower Tal to the Upper Tal. Because of lacking well-defined body fossils in the Tal, it is proposed that the deposits of the Tal were formed in the Late Precambrian near Precambrian-Cambrian transition,^[9] except for the uppermost Manikot Shell Limestone, which is proposed to be formed in Late Cretaceous and unconformably overlain by Subathu Formation in Tal Valley, Garhwal Himayala.^[12]

The details of lithologies and depositional environment of Tal Formation^[10] are shown in the table below:

Formation	Lithologies	Depositional Environment
Upper Tal	Manikot Shell Limestone - Grey, ooliticm sandy, current-bedded fossiliferous limestone containing fragmentary bivavles, gastropod and quartzite	High-energy shallow tidal sea
	------------------------locally unconformable------------------------
	Phulchatti Quartzite - White to purplish, felspathic, fine-grained to gritty, current-bedded, locally conglomeratic	Shoal
Lower Tal	Calcareous Unit - Ferruginous, sandy limestone or calcareous quartzite (locally developed)
	Arenaceous Unit - Siltstone, micaceous, grey to dark grey	Mixed flat and mud flat of intertidal zone
	Argillaceous Unit - Shale, micaceous, grey to dark grey, locally carbonaceous with calcareous pyritous nodules	Protected lagoon or embayment
	Chert Unit - Chert, black with intercalation of black shale, phosphate beds and nodules (locally developed)

Gondwana Strata

Geography of Gondwana Units in Western and Central Nepal

Gondwana strata are not exposed in Garhwal Himalaya after the Tal Formation owing to the great hitus, some outcrops can be found in Central and Eastern Himalaya during Late Paleozoic to Mesozoic times.

Central Himalaya - Central and Western Nepal

Gondwana Strata of LHS in Western and Central Nepal

In Nepal, the older LHS with age ranging from Paleoproterozoic to uppermost Precambrian are separated from the younger LHS by the Great Lesser Himalayan Unconformity.^[4] Missing Early Proterozoic strata of LHS suggests that the rocks might be eroded before the deposition of Gondwana strata. Younger continental facies Gondwana strata are first deposited after the unconformity. They are then unconformably overlain by marine facies Tertiary Unit. Gondwana strata are exposed in form of isolated tectonic patches across the LHS, but mainly developed within two zones in Nepal in central Himalaya. First is in central Nepal, where several outcrops of Tansen Group can be found. Second is in western Nepal, where Gondwana strata are exposed in Jumla-Humla basins near the Tethyan Himalaya.^[13]

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 called Lower Gondwanas) and younger Taltung and Amile Formations (or called Upper Gondwanas). The Sisne Formation is dominated by glacial diamictite and fluvial deposits.^[14] In the upper part of the formation, shales are found to contain fenstellid bryozoan fossils, indicating that the Lower Gondwanas in central Nepal has an age from Late Carboniferous to Permian time.^[14]

For the Upper Gondwanas, it is further subdivided into Taltung and Amile Formations. Taltung Formation is characterised by coarse-grained, volcaniclastic conglomerates, sandstones and silty shales. They are deposited by northwestward-flowing fluvial channels.^[15] Abundant plant fossils are found in the Taltung Formation, and they are dated to have an age within Late Jurassic to Early Cretaceous.^[16] 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.^[13][14] 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 Upper Amile Formation and the overlying Bhainskati Formation of Tertiary Unit.^[13] For the age of the Amile Formation, it is dated to within Early Cretaceous to Early Paleocene, while the age of Bhainskati Formation is biostratigraphically dated as from Middle to Late Eocene.^[16]

In Jumla area of western Nepal, Gondwana strata are unconformably overlying the caronbate rocks of Uppermost Nawakot Unit of Mesoproterozoic age.^[4] The Gondwanas here are characterized by quartzose sandstones, black shales, quartz pebble conglomerates as well as coal and lignite. They are dated as an age of Jurassic to Paleocene.^[16] It can be seen that the lithology of Gondwanas here is quite similar to that of Amile Formation in central Nepal. Also, the Gondwana Unit is overlain by Bhainskati Formation of Tertiary Unit, similar to the situation in central Nepal.

However, in fact, the Gondwana Unit is not very well-developed in Jumla area.^[4][14] The strata with lithology similar to that of Taltung Formation and Lower Gondwanas are missing here. In other words, LHS in 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 Jumla area than in Tansen area.^[4]

Gondwana Units of LHS in Southeastern Bhutan

Eastern Himalaya - Bhutan

Compared to Nepal, the Gondwana strata is exposed in a relatively smaller area in Bhutan. In southeastern Bhutan, the basal LHS begins with the metamorphic Daling-Shumar Group, followed by Baxa Group that is characterized by quartzite, phyllite and dolomite succession of Neoproterozoic to probably Cambrian age.^[17][18] The bottom of 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 Diuri Formation with Proterozoic to Permian age.^[19] It consists of conglomerate, quartzite, phyllite as well as diamictite with interbedded slates. At the base of this formation, pebbles composed of quartzite and siliceous dolomite are embedded in a fine sericitic quartzite matrix. The conglomerate layers are overlain by thicker beds of slate and phyllite. Clear schistosity can be observed too.^[19] The diamictite found in fact implies a source of glacial origin. It is probably correlated to the glaciation event of Gondwana supercontinent during the Late Paleozoic.^[18]

Diuri Formation is then overlain by Setikhola Formation, which is characterized by feldspathic sandstone, shale, graywacke, coal lenses and plant fossils. One sequence consists of interbedding of sandstone and shale and is intensely bioturbated with flames structures observed as well.^[6] This indicates a depositional environment of beach or mudflat. Also, another sequence of interbedding calcareous greywacke and carbonaceous shale is found. Ripples and cross-laminae can be observed on the greywacke, while small-scaled sun cracks and slump folds can be seen on shale.^[6] It is proposed that this sequence has a depositional environment of semi-isolated basin. The marine fossils contained here indicate a Permian age of Setikhola Formation.^[6][19]

The uppermost Gondwana unit is Damudas Sub-Group, which is characterized by arenite, shale, slate and black coal beds. In fact, Setikhola Formation and Damudas Sub-Group are together termed as the Gondwana succession.^[17] The bedrock of Damudas Sub-Group is made up of gritty, micaceous and cross-laminated sandstones. These fricable sandstone layers are interbedded with coal beds that have been sheared and crushed.^[18] Abundant plant fossils like fern leaves can be found on the carbonaceous shale, characterising the Damuda coalfields and indicating an Permian age.^[6][19] Generally, the strata here are lenticular and display a fining-upwards sequence. In tectonic context, it is proposed that the Gondwana strata here have experienced post—Gondwana orogenic movements, resulting in folded rocks followed by overturned beddings.^[17]

Singtali Formation and Subathu Formation

After the sedimentary break or unconformity, Singtali and Subathu Formations as foreland basin sediments are deposited in Garhwal Outer Lesser Himalaya in northern India during Late Cretaceous to Middle Palaeocene. With extreme complex structural setting there, including isoclinal overturned folding and multiple thrusting, both formations can be found overlying the Tal Formation.^[4][6] In addition, with the similarities of the lithology as well as depositional environment, it is sometimes quite difficult to distinguish between Singtali and Subathu Formations. The main difference discussed in the following is related to the tectonic events during their deposition.

Singtali Formation

The Singtali Formation belongs to the Sirmur Group of Outer Lesser Himalaya of Garhwal. It is also called "Upper Tal" as the uppermost Manikot Shell Limestone,^[20][21] however, this Formation is distinct from the basement Tal Formation upon which it rests unconformably. Its main lithology is dominated by sandy, oolitic and shelly limestones with subordinate quartz arenites. Medium-bedded massive strata are predominant and no sedimentary structures are visible.^[1] Singtali Formation has been assigned as a Late Cretaceous-Palaeocene age based on faunal evidence.^[9] In terms of depositional environment, the dominance of limestone in the Singtali Formation and sparse fauna would imply shallow marine conditions at that time. A high-energy, agitated environment can be inferred from the presence of ooids.^[1]

Subathu Formation

The Subathu Formation also belongs to the Sirmur Group of Outer Lesser Himalaya of Garhwal. It is a sequence of limestones, green mudrocks and subordinate sandstones, and has been paleontologically dated as from Late Palaeocene to Middle Eocene.^[20] The rocks are rich in fossils, such that limestones with normal marine fauna and thick mudstones with well preserved, burrowing-type molluscs, indicating a quite, relatively shallow shelf environment during the period of deposition, and this depositional environment is similar to that of Singtali Formation.^[20] In fact, it contains the oldest Himalayan foreland basin rocks. Near the Krol and Garhwal thrusts in northeastern India, Subathu Formation displays a very narrow and discontinuous strip, indicating that it has experienced extremely high tectonic shearing and shattering as a result of overthrusting of rocks. Consequently, Subathu Formation is partially preserved in Krol nappe and under the Garhwal thrust, and is unconformably overlying the Tal Formation.^[6]

Distinctions between the Singtali and Subathu formations

In the Singtali Formation times (Late Cretaceous-Palaeocene), the Indian craton submerged and stable shallow marine conditions ensued. This event is possible related to flexure, such that the Spontang ophiolite was obducted onto the Northern Indian plate margin.^[1] One more possible explanation is related to extensional tectonics, such that India has drifted and detached from Gondwana, and northwards subduction of Neotethys (or called Tethys Ocean) beneath Asia.^[1] Therefore, Singtali Formation has been interpreted as pre-collisional transgressive sediments, at the same time the Late Cretaceous global eustatic sea-level high stand appeared.^[22]

For the Subathu Formation, its tectonic setting is different from that of Singtali Formation. It was deposited during the suturing of India and Eurasia, between initial and terminal continental collision.^[1] The inferred pattern of northward shallowing and reduced sedimentation conflicts with classic foreland basin models. However, these depositional patterns may reflect basement fault reactivation, giving rise to paleohighs, rather than simply crustal loading following on from the collision.

Their individual tectonic significance related to foreland basin evolution will be discussed more detailed in the next section. The general similarities and differences between the Singtali and Subathu Formations are shown in the table below:

Formation	Period	Lithologies	Depositional environment	Tectonic event(s)
Subathu	Late Palaeocene - Middle Eocene	Limestone, shale, green mudrock	Shallow marine environment	Indian-Eurasia continental collision
Singtali	Late Cretaceous - Palaeocene	Limestone, sandstone	Shallow marine environment	Obduction of Spontang ophiolite onto the Northern Indian plate margin; Neotethyan subduction beneath Asia; Continued northward drift of India after detachment from Gondwana

Geological Significance during Paleozoic to Mesozoic times

Gondwana Strata

In the Nepal Himalaya, the Lower Gondwanas of glacial diamictite is unconformably overlain by fluvial Taltung Formation (Upper Gondwanas), which contains lots of plant fossils distributed widely within the Tansen area. Alkali basalt lave flows are interbedded with the fluvial beds in Lower Taltung. Gravelly braided river facies are shown in the Lower Taltung while silty meandering river facies are displayed in the Upper Taltung, as a result, the sequence is fining upwards. The strata were deposited in a terrestrial basin on the Gondwanaland.^[23]

Because of the appearance of glacial diamictite and index plant fossils found in Lower and Upper Gondwanas respectively, it has been proposed that the Lesser Himalaya had been a part of the Gondwanaland during the Permian to Cretaceous.^[14] Later on, the basaltic lava flows found in fact indicate a tectonic setting related to basaltic volcanism as the volcanic clasts were derived from the underlying lava and transported through the river from Gondwanaland. The interbedding layers of fluvial sediments and basaltic lava bands imply that there was repeated occurrence of basaltic eruption and erosion and sedimentation of fluvial deposits alternatively. These events were probably caused by breaking up and rifting of Gondwanaland during the Late Jurassic to Early Cretaceous.^[23]

The whole sequence of Upper Gondwanas (including both Taltung and Amile Formations) represents a non-marine deposition. Data from the paleocurrent direction show that the sediments were derived from the south, because the Indian subcontinent was drifting northwards towards the Lesser Himalaya.^[23] After that, it comes to the deposition of Bhainskati Formation in shallow marine environment. The upper Bhainskati is even found to be deposited in brackish or fresh water environment, indicating a gradual and minor regression period. The regression phase was probably initiated by the sea level change in northern Neotethys.^[24] However, overall there were no significant changes in tectonic setting during the Early Cretaceous to Early Palaeocene. In fact, the Bhainskati Formation is correlated to the Subathu Formation in Garhwal Himalaya. The deposition of these marine facies in a shallow marine environment is associated with the foreland basin development.^[23]

Downward displacement and flexure of Indian Plate as a result of obduction of ophiolites onto the Indian Plate.

Tectonic events related to Singtali Formation

The Early Tertiary geology of the Indian Lesser Himalaya conforms well with the classic foreland basin model. In Late Cretaceous times, this area of the northern region of the Indian plate finally became submerged after a long period when sub-aerial conditions had dominated. This resulted in deposition of the marine Singtali Formation. A possible explanation for this event is that ophiolites such as the Spontang ophiolite, were obducted onto the Indian plate Zanskar continental shelf in the Campanian^[25] or Maastrichtian,^[26] resulting in downward displacement and flexure of the Indian plate hundreds of kilometres to the south. Moreover, it has been proposed that the marine transgression is related to extensional tectonic setting, such that the Late Albian has detached from India and has started to drift from Gondwana supercontinent, Also, the Neotethys has subducted northwards beneath Asia.^[27] This event is accompanied with the Late Cretaceous global eustatic sea-level high stand as well.^[22]

Tectonic events related to Subathu Formation

The initial contact between India and Eurasia have taken place at 62 - 60 Ma in the northwestern Himalaya, with terminal collision culminating by 55 Ma in the east.^[28] The Subathu Formation rocks were deposited during the suturing and initial collision of India and Eurasia.

However, the western intermediate structural level localities show a much thinner Subathu marine sequence compared to the eastern intermediate structural level localities and the lowest structural level. The thickness variations between the west and the east could be explained by the progressive suturing of India and Eurasia from northwest to east,^[28] with later suturing in the east allowing a longer period where marine conditions could predominate.

This progressive suturing, however, would not explain the thickness difference between the lowest and intermediate structural levels. As the intermediate structural level restores further to the north than the lower structural level, northward shallowing of the basin (i.e. towards the load) is implied. This is different from the theoretical model, where the depocentre is close to the load and shallows towards the craton.^[29] In the Lesser Himalayan early foreland basin, palaeohighs, which are resulted from basement fault reactivation, may have been located in the west between the load to the north and the marine Subathu basin to the south.^[1] This would result in shallowing towards, and reduced sedimentation on the palaeohigh, which coupled with the probable distal nature of the basin, therefore, explaining the thin sequences of the western intermediate structural level localities.

After suturing, fluvial facies are overlying the marine Subathu Formation. It is associated with the uplift of HImalaya and regression of sea in Late Eocene to Cenozoic times.^[5]

See also

• Geology of the Himalaya
• Geology of Nepal
• Himalayan foreland basin

References

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