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 Rajasthan

GEOLOGY

Rajasthan is endowed with a continuous Geological sequence of rocks from the oldest Archaean, Metamorphites, represented by Bhilwara Super Group (more than 2,500 million years old) to sub-recent, alluvium and wind blown sand.  The western and north-western parts of the state are covered by vast blanket of young unconsolidated deposits including the blown sand of the Thar Desert (Marusthal) of western Rajasthan.  The remaining area exposes wide variety of hard rocks, which include various types of metamorphic schists, quartzites, marbles and gneisses of Pre-Cambrian age with associated acid, and basic intrusive rocks.  The sedimentaries include the rocks of Aravalli Super group, Delhi Super group, upper Precambrian Vindhyan Super group and of Cambrian to Jurassic, Cretaceous and Tertiary ages.  The southeastern extremity of the state is occupied by a pile of basaltic flows of Deccan Traps of Cretaceous age.  Several mineral deposits of economic importance occur in association with the above rock units.

The geological sequence of the state is highly varied and complex, revealing the co-existence of the most ancient rocks of Pre-Cambrian age and the most recent alluvium as well as wind- blown sand.  The Aravallis, one of the most ancient mountains in the world, have the oldest granitic and gneissic rocks at their base, overlain by the rocks of the Aravalli Super group, Delhi Super group, the Vindhyan Super group and younger rocks.  These rocks are highly metamorphosed at certain places and show rich occurrences of minerals of great commercial importance.

The characteristic feature of the geology of Rajasthan is the presence of several groups of rocks belonging to Archaean and Pre-Cambrian ages.  They form the Aravalli mountain system, which runs across the state from the north of Delhi in the north-east to the Gulf of Cambay in the south-west.  The central part of the Aravalli ranges is occupied by a great synlinorium composed of Aravalli and Delhi rocks.  Because of the thin deposits of sand in this region, the rock exposures are good but in the west and the south-west, they are often engulfed in sandy alluvium and desert sands.

The Archaeans consist of the Bhilwara Super group (Bundelkhand Gneiss and the Banded Gneissic complex).  The Aravallis, an enormously thick series of argillaceous rocks, came into existence at the close of the Archean era when the sediments which were deposited in the seas of that age, underwent an upheaval by orogenic activities. These vast mountains were peneplaned in later ages. The Aravalli super group is a vast formation composed of basal quartzites, shales, conglomerates, composite gneisses and slates.

The Delhi Super group overlies the Aravallies.Delhi super group is divided into lower Ralio group,middle Alwar group and upper Ajabgarh group. Ralio group is rich in crystalline limestones, grits, schistose rocks and quartzites.  The famous marble of Makrana (Nagaur district) belongs to this group.  Alwar group and Ajabgarh group consist mostly of calc-silicates, quartzites, grits and schistose rocks.

The other important lithological formations consist of a thick series of sedimentary rocks comprising sandstone, limestone and shales.  These have been classified as upper and lower Vindhyans in the east and Marwar in the west.  The deposition of these rocks in western Rajasthan was preceded by igneous activity, which included a thick pile of lava, mostly of an acidic nature.  The plutonic equivalent of these lava are seen in the form of granite bosses and sills in Jalor, Siwana, Mokalsar and Jodhpur areas.  Rocks of the above mentioned igneous activity have been designated as Erinpura granite and Malani igneous suit.

There was an encroachment of an arm of the sea from the south-westernly direction into western Rajasthan during the Jurassic period. Jurassic formations are distinctly noticeable in a vast area around Jaisalmer and some of the fossils of this age are found in these rocks.  The outcrops of these rocks are, partly, covered by wind-blown desert sands.  Of special interest are the Bap (Jodhpur district) and Pokran (Jaisaimer district) beds of upper Carboniferous age, which have now been exploited for ground water.  They are composed of boulders of Malani rhyolites showing effects of glaciations.  Violent volcanic activity in the form of fissure eruptions marked the close of Mesozoic era in the lower Cretaceous age.  The main characteristic of this was a stupendous outburst of covered volcanic energy, resulting into the eruption of thick streams of lava over vast areas.  These rocks, known as Deccan traps, are found in southern and south-eastern Rajasthan.  The Deccan trap, extending over a vast area in southern Jhalawar and in the eastern parts of Chittaurgarh and Banswara districts, are notable formations of Upper Cretaceous to Lower Eocene age when a large area of peninsular India was also covered with fissure eruptions of black lava.

During Eocene times, marine transgression seems to have inundated a large part of western Rajasthan with the deposition of thick beds of fossilliferous limestone.  To the north of Jaisalmer, the Jurassics are overlapped by nummulitic limestone.

Pleistocene sand alluvium, blown sand, kankar(calcium nodules), carbonate beds and evaporite deposites of recent and sub- recent age are found over a large area of western and eastern Rajasthan.

The great Boundary Fault, through which the River Chambal has carved its course, passes through south-eastern parts of the state. This fault is visible in Begun(Chittaurgarh district) and northern parts of kota. It reappears again in Sawai Madhopur and Dhaulpur districts. Besides this, several mega lineaments also traverse in the state.


 
Nodal Officer : Er Inderjeet Bishnoi, Email : dditjpr.wr@rajasthan.gov.in, Phone No. : 0141-2702672 Ext. 229