Lesser Himalayan Strata
[2] The main layers of the LHS includes non-fossiliferous, low-grade, metasedimentary rocks, metavolcanic strata and augen gneiss.
[3][4] 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.
[2] 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.
[6] In the Central and Eastern Himalaya, Gondwana strata from the Permian to Paleocene were preserved unconformably onto the older LHS.
[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 the Cretaceous.
[12] The details of lithologies and depositional environment of Tal Formation[10] are shown in the table below: 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.
[14] 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.
[16] In the Jumla area of western Nepal, Gondwana strata unconformably overly the caronbate rocks of the Uppermost Nawakot Unit of Mesoproterozoic age.
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.
[6][19] The uppermost Gondwana unit is the Damudas Sub-Group, which is characterized by arenite, shale, slate and black coal beds.
[18] Abundant plant fossils like fern leaves can be found on the carbonaceous shale, characterising the Damuda coalfields and indicating a Permian age.
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] Following a sedimentary break or unconformity, the Singtali and Subathu formations were deposited as foreland basin sediments in the Garhwal Outer Lesser Himalaya during the Late Cretaceous to Middle Palaeocene.
[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.
It is a sequence of limestones, green mudrocks and subordinate sandstones, and has been paleontologically dated as from Late Palaeocene to Middle Eocene.
The limestones with normal marine fauna and thick mudstones with well preserved, burrowing-type molluscs indicate a quiet, relatively shallow shelf environment during the period of deposition.
Near the Krol and Garhwal thrusts in northeastern India, the Subathu Formation exists as a narrow and discontinuous strip, indicating that it has experienced extremely high tectonic shearing and shattering as a result of overthrusting of rocks.
[6] In the Singtali Formation times (Late Cretaceous-Palaeocene), the Indian craton submerged and stable shallow marine conditions ensued.
[1] One more possible explanation is related to extensional tectonics, such that India has drifted and detached from Gondwana, and northwards subduction of the Neotethys (Tethys Ocean) beneath Asia occurred.
[1] Therefore, the Singtali Formation has been interpreted as pre-collisional transgressive sediments, at the same time there was a global eustatic sea level rise during the Late Cretaceous.
[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.
[23] Because of the appearance of glacial diamictite and index plant fossils found in the Lower and Upper Gondwanas respectively, it has been proposed that the Lesser Himalaya had been a part of Gondwanaland during the Permian to Cretaceous.
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.
The upper Bhainskati has been found to have been deposited in a brackish or fresh water environment, indicating a gradual and minor regression period.
[23] 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.
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 the Gondwana supercontinent, Also, the Neotethys has subducted northwards beneath Asia.
[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.
