Rajmahal Traps on:
[Wikipedia]
[Google]
[Amazon]
The Rajmahal Traps is a volcanic igneous province in Eastern India, covering the parts of
The Rajmahal Traps is a volcanic igneous province in Eastern India, covering the parts of Jharkhand
Jharkhand (; ) is a States and union territories of India, state in East India, 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 ...
, West Bengal
West Bengal (; Bengali language, Bengali: , , abbr. WB) is a States and union territories of India, state in the East India, eastern portion of India. It is situated along the Bay of Bengal, along with a population of over 91 million inhabi ...
and Meghalaya
Meghalaya (; "the abode of clouds") is a states and union territories of India, state in northeast India. Its capital is Shillong. Meghalaya was formed on 21 January 1972 by carving out two districts from the Assam: the United Khasi Hills an ...
. The Rajmahal Hills of Jharkhand
Jharkhand (; ) is a States and union territories of India, state in East India, 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 ...
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 earlyCretaceous
The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
. 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 confirms this. According to plate tectonics
Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...
, the Indian subcontinent
The Indian subcontinent is a physiographic region of Asia below the Himalayas which projects into the Indian Ocean between the Bay of Bengal to the east and the Arabian Sea to the west. It is now divided between Bangladesh, India, and Pakista ...
was over this hot spot during the Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is 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, doi-access=free
Lithology
The Rajmahal volcanics are predominantly tholeiitic basalt, quartz tholeiite,olivine
The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...
tholeiite and alkali basalt. The Intertrappean Beds are composed of sedimentary rock
Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...
such as siltstone, claystone and shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...
.
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 Gondwanagraben
In geology, a graben () is a depression (geology), depressed block of the Crust (geology), crust of a planet or moon, bordered by parallel normal faults.
Etymology
''Graben'' is a loan word from German language, German, meaning 'ditch' or 't ...
, form a triple junction at the mouth of the Bengal basin. The traps evolved along the then eastern continental margin of India, following rifting of Gondwanaland. Over the epochs, the upper part of the lava deformed in a cold, brittle fashion and formed graben
In geology, a graben () is a depression (geology), depressed block of the Crust (geology), crust of a planet or moon, bordered by parallel normal faults.
Etymology
''Graben'' is a loan word from German language, German, meaning 'ditch' or 't ...
structures.
Fossils
The Intertrappean Beds contain an assemblage of LowerCretaceous
The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
plant fossils. The assemblage includes ''Cladophlebis indica'', ''Dictyozamites indicus'', ''Taeniopteris spatulata'', and ''Brachyphyllum rhombium''.
See also
* Deccan Traps * RajmahalReferences