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.
