The Himalayan foreland basin is an active collisional

foreland basin A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...

system in South Asia. Uplift and loading of the

Eurasian Plate The Eurasian Plate is a tectonic plate that includes most of the continent of Eurasia (a landmass consisting of the traditional continents of Europe and Asia), with the notable exceptions of the Indian subcontinent, the Arabian subcontinent and ...

on to the

Indian Plate The Indian Plate (or India Plate) is a minor tectonic plate straddling the equator in the Eastern Hemisphere. Originally a part of the ancient continent of Gondwana, the Indian Plate broke away from the other fragments of Gondwana , began mov ...

resulted in the flexure (bending) of the Indian Plate, and the creation of a depression adjacent to the Himalayan mountain belt. This depression was filled with sediment eroded from the Himalaya, that

lithified Lithification (from the Ancient Greek word ''lithos'' meaning 'rock' and the Latin-derived suffix ''-ific'') is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithificati ...

and produced a sedimentary basin ~3 to >7 km deep. The foreland basin spans approximately in length and in width. From west to east the foreland basin stretches across five countries:

Pakistan Pakistan ( ur, ), officially the Islamic Republic of Pakistan ( ur, , label=none), is a country in South Asia. It is the world's List of countries and dependencies by population, fifth-most populous country, with a population of almost 24 ...

India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...

Nepal Nepal (; ne, नेपाल ), formerly the Federal Democratic Republic of Nepal ( ne, सङ्घीय लोकतान्त्रिक गणतन्त्र नेपाल ), is a landlocked country in South Asia. It is mai ...

Bangladesh Bangladesh (}, ), officially the People's Republic of Bangladesh, is a country in South Asia. It is the eighth-most populous country in the world, with a population exceeding 165 million people in an area of . Bangladesh is among the mos ...

, and

Bhutan Bhutan (; dz, འབྲུག་ཡུལ་, Druk Yul ), officially the Kingdom of Bhutan,), is a landlocked country in South Asia. It is situated in the Eastern Himalayas, between China in the north and India in the south. A mountainous ...

. The Himalayan foreland basin has been studied within the Himalaya (where the foreland basin succession has been uplifted and exposed in the Sub-Himalaya and

Lesser Himalaya The Lower Himalayan Range ( ne, पर्वत शृङ्खला parbat shrinkhalā) – also called the Middle Himalayas or Lesser Himalayas or Himachal – is a major east–west mountain range with elevations 3,700 to 4,500 m (12,000 to ...

), and in the subsurface (where petroleum exploration wells and seismic data are used). The foreland basin fill traces back to the onset development of the foreland basin which started during the

Paleogene The Paleogene ( ; British English, also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period, geologic period and system that spans 43 million years from the end of the Cretaceous Period million yea ...

period around 45 to 50 Ma. Deposition in the foreland basin appears to be

diachronous In geology, a diachronism (Greek language, Greek ''dia'', "through" + ''chronos'', "time" + ''-ism''), or diachronous deposit, is a sedimentary rock formation in which the material, although of a similar nature, varies in age with the place where i ...

; a lag in time exists between parts of the basin. The western extreme of the basin started developing during the

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...

around 57-54 Ma, the central portion began developing <2 Ma later, and the basin gets younger as it progresses towards the east. The stratigraphic succession of the basin is important as it preserves the sedimentary record of India-Eurasia collision.

Geologic setting

During the Late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

and early

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...

, the Indian Plate drifted northward a vast distance resulting in the closure of the Neo-Tethys Ocean. Approximately 40 to 50 million years ago remnants of the ocean vanished as India collided with the Eurasian Plate. As the continental plates are relatively low density they cannot be subducted. This results in the Eurasian Plate being thrusted up leading to the rise of the

Tibetan Plateau The Tibetan Plateau (, also known as the Qinghai–Tibet Plateau or the Qing–Zang Plateau () or as the Himalayan Plateau in India, is a vast elevated plateau located at the intersection of Central, South and East Asia covering most of the Ti ...

, bounded to the south by the collisional Himalayan mountain range. The Himalayan foreland basin is adjacent to the Himalayan mountain belt; it laps onto the Indian

Craton A craton (, , or ; from grc-gre, κράτος "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and ...

to the south and is bounded by stacked thrust sheets of the Himalayas to the north. The foreland basin, much like the Himalayan mountain range, spans approximately west to east across Pakistan, India, Nepal, Bhutan and Bangladesh. The actively subsiding foreland basin lies beneath the Punjab region of Pakistan and the Gangetic plain of India and southernmost Nepal.

Basin subdivisions

The Himalayan foreland basin has been divided on the basis of modern

drainage divide A drainage divide, water divide, ridgeline, watershed, water parting or height of land is elevated terrain that separates neighboring drainage basins. On rugged land, the divide lies along topographical ridges, and may be in the form of a singl ...

s, and subsurface topography. Subdivisions based on drainage divides are most commonly used, with the Indus Basin reflecting the drainage area of the

Indus River The Indus ( ) is a transboundary river of Asia and a trans-Himalayan river of South and Central Asia. The river rises in mountain springs northeast of Mount Kailash in Western Tibet, flows northwest through the disputed region of Kashmir, ...

, and the Ganga Basin representing the drainage area of the

Ganges River The Ganges ( ) (in India: Ganga ( ); in Bangladesh: Padma ( )). "The Ganges Basin, known in India as the Ganga and in Bangladesh as the Padma, is an international river to which India, Bangladesh, Nepal and China are the riparian states." is ...

. The basin overlies a series of

depressions and ridges, that have also been used to subdivide the

. The rock units that make up the Indian Plate vary dramatically along the length of the basin, changing from

Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...

Mobile Belt rocks, to

Archean The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth Earth ...

, and Proterozoic Vindhyan Supergroup sedimentary rocks. These Indian Plate rock units extend under the foreland basin and have been correlated to a series of depressions and ridges underneath the foreland basin. As the Indian Plate has flexed (bent) under the Eurasian Plate, the depressions and ridges have acted as flexible and stiff areas respectively, influencing the thickness of the foreland basin fill.

Stratigraphy

The Himalayan foreland basin has been divided into different rock units in different parts of the basin. The earliest deposits of the foreland basin are marine mudstones, which are unconformably overlain by continental deposits.

Neogene The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period Mya. ...

and

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...

continental deposits make up the vast majority of the foreland basin fill. The stratigraphy of the foreland basin is best known from studies of uplifted strata in the

Lesser Lesser, from Eliezer (, "Help/Court of my God"), is a surname. Notable people with the surname include: * Adolf Lesser (1851–1926), German physician * Aleksander Lesser (1814–1884), Polish painter and art critic * Anton Lesser (born 1952), Bri ...

and Sub-Himalaya, supplemented by data from the small amount of wells that test the hydrocarbon potential of the foreland basin drilled in India and Nepal.

Subathu/Bhainskati/Kohat formations—earliest foreland basin fill

The

Subathu Formation of India (correlative to the Bhainskati Formation of Nepal, and the Kohat Formation of Pakistan) represents the oldest known foreland basin deposits, and

unconformably An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

overlies older strata. The Subathu Formation and equivalents are relatively thin intervals (<150 m) predominantly composed of fossiliferous, organic-rich black shale. These units are interpreted as shallow marine deposits. Marine to shallow marine

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...

in the form of shales and minor amounts of sand also consists of predominantly green mudstone with minor red facies and were dated back to the upper paleocene to lower mid eocene period based on the existence of Nummulites gathered from biostratigraphical data. petrographic interpretation of the green Subathu Formation were shown to be predominantly sedimentary with minor traces of serpentine schist input. Although the red facies shows a more felsitic and of volcanic origins, it entails that it originates from continental flood basalts of the Indian craton. The Subathu Formation was interpreted to be a preservation of the intense collision between the two plates in the western part of the foreland basin that leads to thrusting. An evidence of a silicified chert breccia strata existing just on top of the rigid precambrian basement was interpreted as a growth fault that developed as a result of compressional tectonics. The thrust slices in the sub-himalayan ranges now preserves some of the Subathu Formation. Although a debate has arise, it is inferred that some locations where the Subathu Formation is exposed are now considered to be the

forebulge In geology, a forebulge is a flexural bulge in front as a result of a load on the lithosphere, often caused by tectonic interactions and glaciations. An example of forebulge can be seen in the Himalayan foreland basin, a result of the Indian-Euras ...

of the foreland basin as it is overlain by a much younger formation where a time hiatus or an

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

has occurred. A time hiatus of an approximately 10 MA are inferred based on

thermochronology Thermochronology is the study of the thermal evolution of a region of a planet. Thermochronologists use radiometric dating along with the closure temperatures that represent the temperature of the mineral being studied at the time given by the dat ...

and

magnetostratigraphy Magnetostratigraphy is a geophysical correlation technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout the section. The samples are analyzed to determine their '' ...

between the Subathu and the overlying formation, but it is highly controversial.

Dagshai/Dharamsala/Dumre formations - earliest continental deposits

The Dagshai Formation has been dated back to the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

Epoch where it consists of primarily fine grained material of

alluvial Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...

origins. The Dagshai is distinctive by its red color consisting of mostly red mudstone, siltstone and grey sandstone. It is the oldest continental deposits and overlies the Subathu formation. A debate has arise of whether the Dagshai Formation overlies conformably or unconformably on top of the Subathu formation. recent research by dating detrital micas and fission track dating of detrital zircons shows conclusively that there is an unconformable nature between the Subathu and Dagshai Formation. Magnetostratigraphic data suggests that the Daghsai formation was deposited at approximately 27 Ma with 2 Ma uncertainty. Previously, multiple studies of facies interpretation of the Dagshai formation has been conducted with varying results regarding the past depositional environment; the presence of quartzitic sandstones was thought to be leftovers of an extensive and prolonged weathering in alluvial plains in which time tropical weathering was intensifying. The study by Yani Najman et al. interpreted that the Dagshai is a result of crevasse play and

overbank An overbank is an alluvial geological deposit consisting of sediment that has been deposited on the floodplain of a river or stream by flood waters that have broken through or overtopped the banks. The sediment is carried in suspension, and be ...

floodplain facies because of an abundance of fine grained material; The general setting of the Dagshai Formation was concluded to be a past alluvial environment.

Siwalik Group - the thickest record of Himalayan detritus

The Siwalik Group is a coarsening upward

siliciclastic Siliciclastic (or ''siliclastic'') rocks are clastic noncarbonate sedimentary rocks that are composed primarily of silicate minerals, such as quartz or clay minerals. Siliciclasic rock types include mudrock, sandstone Sandstone is a clastic ...

succession that makes up the thickest accumulation of detritus derived from the Himalaya in the foreland basin. The deposits are continental, and largely reflect deposition in

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...

meandering river A meander is one of a series of regular sinuous curves in the channel of a river or other watercourse. It is produced as a watercourse erodes the sediments of an outer, concave bank (cut bank) and deposits sediments on an inner, convex bank w ...

, and

braided river A braided river, or braided channel, consists of a network of river channels separated by small, often temporary, islands called braid bars or, in English usage, ''aits'' or ''eyots''. Braided streams tend to occur in rivers with high sediment l ...

environments. The Siwalik Group has been informally divided into the Upper, Middle, and Lower Siwalik since the early 1900s based on

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

fossil markers. Numerous researchers have broken the Siwalik Group into formations at different locations along the Himalaya, but these formations are local-scale, and cannot be correlated regionally. Boundaries of the Siwalik Group are

, as they have been constrained to different times at different locations along the Himalaya. Because these boundaries are diachronous, it would be incorrect to assign a single specific age to the top or bottom of each subdivision.

Lower Siwalik

The Lower Siwalik is the base of the Siwalik Group. Deposition of the Lower Siwalik started in the Middle Miocene. The Lower Siwalik is characterized by alternating

of sandstone and mudstone, deposited in fluvial and

environments.

Paleosol In the geosciences, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The precise definition of the term in geology and paleontology is slightly different from its use in soil science. In geolo ...

deposits are commonly interlayered with sandstone

lenses A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

on the scale of >1 m to 10 m. The Lower Siwalik is conformably overlain by the Middle Siwalik.

Middle Siwalik

The Middle Siwalik was deposited from the upper

to the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58 This unit is dominated by sandstone beds, punctuated by thin mudstone to siltstone horizons. The sediment supply for the Middle Siwalik originated from the main provenance of the rising Himalayas. With vertical facies variation from sandstone-mudstone to sandstone-mudstone-conglomerate with a thickness approximately . The unique part of this multistorey sand complex is the fact that it is underlain by a major erosional surface which extends laterally for hundreds of meters. individual stories vary in thickness and are recognized by the presence of intra and extra-formational clasts along the base of each storey. These facies assemblages suggests a deposition by sheet floods in a braided channel environment. The vertical stacking of this multistorey sandstone complex further indicates of a channel bar that migrates with channels existing predominantly as depositional sites. The Middle Siwalik is conformably overlain by the Upper Siwalik.

Upper Siwalik

The

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...

Upper Siwalik is interpreted as the sedimentary record of the last phase of Himalayan orogeny. The Upper Siwalik predominantly consists of conglomerate facies in its upper layers and alternations of sandstone, mudstone and conglomerate in the lower part with a maximum thickness of . The Upper Siwalik is overlain by unlithified Quaternary sediments such as the Neogal conglomerate and red clays. The basal part of the Upper Siwalik shows predominantly of crudely stratified conglomerate, sandstone and massive mudstone which suggests a deposition of high energy conditions; this facies assemblage and characteristics are commonly found in gravel transport fans and suggests a deposition by gravelly braided rivers in medial to distal alluvial fan settings.

Basin evolution

Initial collision and onset of the foreland basin development

The

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided by ...

marked the initial time of the India-Eurasia collision. Based on paleomagnetic records, around the time of 55 to 50 Ma the velocity of the Indian Plate decreased rapidly and is followed by a sequence of thrusts and compressional tectonics between the two plates which then triggered the development of the Himalayan Mountain belt. It is believed that the initial collision occurred close to the equator where deposits of

bauxite Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO(O ...

is found within the stratigraphy of the foreland basin overlying a silicified

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...

strata existing on the pre-existing basement of the basin. The chert breccia strata was interpreted as a growth fault in the fold thrust belt as a result of compressional tectonics. With the collision being an active ongoing process, it progressively generates weight which resulted a downward flexing of the subducting Indian Plate and created an accommodation space to be filled with sediments. The flexural subsidence of the basin is slow as a result of the hard and rigid precambrian basement making a relatively shallow foreland basin.

Active convergence

During the

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

epoch the ongoing process of an active convergence between the two plates increased crustal thickening and further increased the load that originated from the Himalayan mountain belt. From Eocene to early Miocene time, thrusts and uplifts are occurring at the Himalayan zone. The initial development of the foreland basin indicated from the oldest marine deposits in the existing stratigraphy shows the basinal subsidence was shallow as a result of the rigid precambrian basement coupled with a slow rate of sedimentation and sediment starvation. The Eocene period also marked the onset of a shift from marine facies deposits on to fluvial deposits stratigraphically. Continued motion of the Indian Plate after the India-Asia collision further led the focus of the deformation of the Indian Plate across of the marginal area of the Indian Shield. This type of event has resulted in intracontinental shortening. Major intracontinental shear associated with the Central Crystalline Zone has been also been recognized.

Climate change and erosion

The Oligocene-Miocene boundary has an important effect on the structure of the basin.

Carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

Isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

data and

Pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...

analysis indicates of a climatic change around SE Asia which significantly increases humidity of the region. From this, reconstruction of monsoonal records was established and it is inferred that around 24-20 Ma is the time when the monsoon intensified. The intensification of the monsoon also resulted in intensification of erosion around the Himalayas. This erosion then caused a reduction in mass of the Himalaya which made the foreland basin partly invert, rebound, and essentially uplift. This is supported by the discovery of the older marine deposition of the Subathu formation in parts of the thrust slices in sub-Himalayas, at a high elevation than it would normally be found.

Hydrocarbon potential

Despite the presence of gas seeps along the Himalaya, and over 70 years of hydrocarbon exploration, no commercially viable hydrocarbon fields have produced from the foreland basin succession. Exploration wells have been drilled in the Indian, Pakistani, and Nepalese portions of the basin. Most of the drilled wells intersecting the Siwalik Group (Upper, Middle, and Lower Siwalik) indicate poor source rock potential. Organically rich Subathu Formation samples in the form of coal show total organic content levels up to 80%, suggesting the Subathu Formation (and correlative units) may have source rock potential. Despite the high TOC levels, the coals exhibit low

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

index Index (or its plural form indices) may refer to: Arts, entertainment, and media Fictional entities * Index (''A Certain Magical Index''), a character in the light novel series ''A Certain Magical Index'' * The Index, an item on a Halo megastru ...

which indicates the potential to only form gaseous hydrocarbons. Furthermore, it is assumed that the gaseous hydrocarbon potential by them may have been lost by tectonic events that occurred in the basin. This is evident by palynological rock study that further indicates of more than 96% of the samples taken from the Subathu Formation was organically lean. Exploratory wells for hydrocarbon exploration were also drilled in the town

Jawalamukhi Jawalamukhi, also Jawalaji, is a Shakti Pitha town and a nagar parishad in Kangra district in the Indian state of Himachal Pradesh. Hindu genealogy registers are kept here like that of Haridwar. The Hindi word 'Jwalamukhi' literally means 'Volc ...

. Although the discovery of gas seepages around this area was recorded, currently it does not hold any commercial value to be produced.

Active deformation of the foreland basin

Himalayan deformation was previously thought to stop at the foot of the Himalaya, or the northern boundary of the foreland basin (the Main Frontal Thrust). The Himalayan foreland basin was thus thought to be undeformed. Himalayan deformation has been shown to extend into the subsurface of the foreland basin, in the form of blind thrust faults, and

strike-slip faults In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

. These faults reach over 37 km south of the Main Frontal Thrust, and are responsible for several modern-day topographic highs. Active deformation of the foreland basin has only been mapped in one area of Nepal, but may be present in other regions.

References

{{Reflist Landforms of Asia