The Rajmahal Traps is a volcanic igneous province in

Eastern India East India is a region of India consisting of the Indian states of Bihar, Jharkhand, Odisha and West Bengal and also the union territory of the Andaman and Nicobar Islands. The region roughly corresponds to the historical region of Magadha fr ...

, covering the parts of

Jharkhand Jharkhand (; ; ) is a state in eastern India. The state shares its border with the states of West Bengal to the east, Chhattisgarh to the west, Uttar Pradesh to the northwest, Bihar to the north and Odisha to the south. It has an area of . It ...

West Bengal West Bengal (, Bengali: ''Poshchim Bongo'', , abbr. WB) is a state in the eastern portion of India. It is situated along the Bay of Bengal, along with a population of over 91 million inhabitants within an area of . West Bengal is the fourt ...

and

Meghalaya Meghalaya (, or , meaning "abode of clouds"; from Sanskrit , "cloud" + , "abode") is a states and union territories of India, state in northeastern India. Meghalaya was formed on 21 January 1972 by carving out two districts from the state of As ...

. The

Rajmahal Hills The Rajmahal Hills are located in the Santhal Pargana division of Jharkhand, India. They were located on the northern margin of the Gondwana supercontinent, and its hills are today inhabited by the Sauria Paharia people whilst its valleys are dom ...

is the type area of this province. Multiple layers of solidified lava made the Rajmahal Traps which are dipping 2–5° towards the north-east. Individual layers vary in thickness from less than to more than .

Genesis

These volcanic rocks were formed from the eruptions over the Kerguelen hotspot in the early

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 ...

. Courtillot, Vincent. ''Evolutionary Catastrophes: The Science of Mass Extinctions.'' Cambridge, Cambridge University Press, 1999; p. 95. The similarity between the geochemical data of Rajmahal volcanos and lavas of the

Kerguelen Plateau The Kerguelen Plateau (, ), also known as the Kerguelen–Heard Plateau, is an oceanic plateau and a large igneous province (LIP) located on the Antarctic Plate, in the southern Indian Ocean. It is about to the southwest of Australia and is ...

confirms this. According to

plate tectonics Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...

, the Indian subcontinent was over this hot spot during the

Period. The original lava flow covered an area of nearly . Below the Bengal basin the flows cover .{{cite journal, last=Singh, first=A. P., author2=Niraj Kumar , author3=Bijendra Singh, title=Magmatic underplating beneath the Rajmahal Traps: Gravity signature and derived 3-D configuration, journal=Proc. Indian Acad. Sci. (Earth Planet. Sci.), date=December 2004, volume=113, issue= 4, page=759, doi=10.1007/BF02704035, bibcode=2004InEPS.113..759S, s2cid=129952630, url=http://www.ias.ac.in/article/fulltext/jess/113/04/0759-0769, accessdate=8 March 2017

Lithology

The Rajmahal volcanics are predominantly tholeiitic basalt, quartz tholeiite,

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

tholeiite and alkali basalt. The

Intertrappean Beds The Intertrappean Beds are a Late Cretaceous geologic formation in India. The beds are found as interbeds between Deccan Traps layers. Indeterminate theropod and pterosaur remains have been recovered from the formation, as well as dinosaur eggs ...

are composed of

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

such as

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

claystone Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too sm ...

and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

Structural evolution

The western boundary of the Rajmahal Traps is faulted and down-thrown towards the east. The eastern boundary of this trap has a North-South trending, fault-controlled basement. This basement connects the Purnea basin of the Ganga valley with the Bengal basin. These faulted contacts, along with the Damodar Gondwana

graben In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic contex ...

, form a

triple junction A triple junction is the point where the boundaries of three tectonic plates meet. At the triple junction each of the three boundaries will be one of three types – a ridge (R), trench (T) or transform fault (F) – and triple junctions can b ...

at the mouth of the Bengal basin. The traps evolved along the then eastern continental margin of India, following rifting of

Gondwanaland Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...

. Over the epochs, the upper part of the lava deformed in a cold, brittle fashion and formed

structures.

Fossils

The

contain an assemblage of Lower

plant fossil Paleobotany, which is also spelled as palaeobotany, is the branch of botany dealing with the recovery and identification of plant remains from geological contexts, and their use for the biological reconstruction of past environments (paleogeogr ...

s. The assemblage includes ''Cladophlebis indica'', ''Dictyozamites indicus'', ''Taeniopteris spatulata'', and ''Brachyphyllum rhombium''.

References

Cretaceous volcanism Extinction events Geography of Jharkhand Volcanism of India

Genesis

Lithology

Structural evolution

Fossils

See also

References