Geological Timeline/Sequence of Rocks – Rajasthan
Rajasthan is endowed with a continuous geological sequence of rocks from the oldest Archaean Metamorphic, represented by Bhilwara Super-group (>2500 m. y.) to sub-recent alluvium & windblown sand. The geological sequence of the state is highly varied and complex, revealing the co-existence of the most ancient rocks of the Pre-Cambrian age and the most recent alluvium as well as windblown sand.
Rajasthan forms north-western part of the Indian Shield. The State exposes a variety of litho logical and tectonic units ranging in age from Archaean to Recent times. Before going into details of Geology of Rajasthan, let us first see, geology time in general to make sense of terms in geology.
The basement rocks – the Sandmata Complex, Mangalwar Complex and Hindoli Group of Bhilwara Super group – occupy central and south-eastern plains. They are Archaean in age and comprise in general, granulite-gneiss; amphibolites, metapelite, paragneiss, calc-silicate rocks and greywacke (the older granite-greenstone belt) and metavolcanic, met greywacke (the younger granite- greenstone belt) respectively.
The Lower Proterozoic supracrustal rocks of the Jahajpur, Rajpura-Dariba, Pur-Banera and Sawar Groups of Bhilwara Super-group rest on the basement rocks of the Mangalwar Complex and host a number of lead, zinc and copper deposits.
The Proterozoic fold belts, viz., the Aravalli fold belt (the Aravalli Super-group) and the Delhi fold belt (the Delhi Super-group) occupies the southern and south- eastern, and south-western and north-eastern Rajasthan respectively. The Aravalli Super-group is represented by metamorphosed and complexly folded clastic sediments with minor chemogenic and organogenic assemblages with interlay red basic volcanic, whereas the Delhi Super group comprises mainly carbonates, metavolcanics, metasammites and metapelite, intruded by magmatic rock of Phulad Ophiolite Suite and syn-orogenic granites of Sendra- Ambaji, Baraith, Dadikar, Harsora, etc.
A number of base metal deposits are located in these belts as also other minerals.
The isolated hillocks of western Rajasthan constitute the Upper Proterozoic Malani Igneous Suite and the Erinpura Granite pluton. Eastern Rajasthan is characterised by the vast sedimentary stretch constituting the Vindhyan, which is juxtaposed against the rocks of the Bhilwara Super-group along the Great Boundary Fault
The northern and north-western parts of the State exhibit Upper Proterozoic-Early Cambrian rocks of the Marwar Super group which are overlain by sedimentary rocks of different ages of Paleozoic and Mesozoic Era. Many industrial mineral deposits are found in these rocks. The Deccan Traps are restricted to the south-eastern part of the State in Chittorgarh- Banswara area.
The Cenozoic rocks are manifested in Barmer and Jaisalmer basins in the west and Ganganagar-Palana shelf in the north.
The Quaternary sediments of Aeolian and fluvial origin constitute the Thar Desert of Rajasthan.
The geological history of the northwest Indian craton in Rajasthan region covers a wide span of time from ca. 3.5 Ga to 0.5 Ga. This craton incorporates a wide variety of lithological and tectonic units representing the basement rocks (Banded Gneissic Complex of Heron, 1953), Proterozoic fold belts (Aravalli and Delhi), and Late Proterozoic igneous suites (Malani, Jalore, and Siwana). Basement rocks, comprising the gneissic terrain of the Sandmata Complex (SC), the Mangalwar Complex (MC), and the Hindoli Group (HG) forming the Bhilwara Super group (BSG), are essentially Archaean in age. This is evident from ca. 3.5 Ga age of some mafic inclusions within the BGC and from the age of the intrusive granites (2.9 Ga, until and Ganglia granites). Gneisses, comprising the SC and the MC, represent the reclassified Banded Gneissic Complex of Heron (1953). Rock types of the SC are composed largely of granulite fancies gneisses of diverse composition, such as metapelite and samiti gneisses, basic intrusive, and calcareous, clastic and chemogenic met sediments which occur as xenoliths, streaks and patches, and thrust bound large bodies in the gneissic terrain. Although Gupta et al. (1981) have shown in their map of the Aravalli region that the SC forms a distinct litho unit which lies to the west of the MC, the SC occurs as tectonic wedges within the basement gneisses of the MC.
The MC of the BSG terrain contains varied lithological assemblages and tectonic units of a ‘greenstone like’ sequence, and comprises ultramafic bodies and mafic igneous bodies of volcanic and plutonic precursors, now represented by amphibolites, highly diverse met sediments such as metapelite and aluminous paragneiss, fuchsite bearing quartzite and low Mg marble and calc silicate gneiss, coarse clastic such as greywacke and tuffaceous sediments represented by graphitic schist. Granodioritic and tonalitic gneisses (Until, Ganglia) represent the consolidation of the early crust at ca. 2.9 Ga and the end Archaean craton isation is indicated by the Berach Granite (ca 2.6 Ga). Although Archaean craton isation event is well documented in the BSG rocks of Rajasthan, some authors put controvertible arguments against this end Archaean event and believe that the HG is not a part of the Archaean basement on the equivocal premise that the Berach Granite (2.6 Ga) forms the basement for the HG in SE Rajasthan. Nevertheless, the Archaean Proterozoic boundary can be constrained to a slot of 2.5 to 2.6 Ga in Rajasthan from the available field geochronology and thematic data base. It may, however, mentioned that the stratigraphic relationships of the different lithological assemblages of the BSG are not clear as the different rock units are usually demarcated by prominent ductile shear zones (DSZ) running for kilometers. Thus, during the Proterozoic period, the Archaean crust (BSG) was extensively reworked through the development of DSZs and granitic activity. The MC presumed to represent an Archaean primary granite greenstone belt whereas the HG is suggested to represent a secondary granite greenstone belt in NW Indian shield.
The HG occurs in an actuate belt containing felsic and mafic metavolcanics and met greywacke forming a turbidity sequence. The end-Archean Berach Granite intrudes the HG.
Proterozoic Fold Belts
The Proterozoic geologic history of south central Rajasthan is contained in a number of distinct fold belts. The principal fold belts are lower to Middle Proterozoic Aravalli Fold Belt (AFB) and Middle to Upper Proterozoic South Delhi Fold Belt (SDFB). The basement to these fold belts is the BSG or the BGC. Other time equivalent early Proterozoic cover sequences in this region include Jahajpur Group, Pur Banera Rajpur Dariba-Bhinder Groups, Sawar Group, and others,
The Proterozoic history is marked by significant secular, changes in terms of basin development, lithocharacters, and mineralisation types. Some of these cover rock sequences contain Pb Zn whilst others Pb Zn Cu or Cu Ag Au mineralisations. The early Proterozoic Aravalli and the equivalent rock, sequences, including the Jahajpur, Pur Banera Bhinder, and Rajpur Dariba Bethanie belts, are considered to have developed in ensile rift basins which except for the main AFB, failed. The isolated basins possibly developed as pull apart basins. These rift basins representing different belts, such as Rajpur Dariba, are strongly tectonised and physical continuity with other belts such as Sawar and Agucha belts is lost in many places. The main AFB evolved as an aulacogen widening southward from a possible triple junction located near Nathdwara Delwara area where rift volcanics are well documented. The fractured arm continued NNE through the SC and was destined to open in Middle Proterozoic time as the south Delhi rift.
Aravalli Fold Belt
The stratigraphic succession of the Aravalli Fold Belt (AFB) is given by many workers. Most workers have proposed two major stratigraphic sequences within the AFB, a shelf fancies with basic lavas and coarse clastic at the base (Delwara and Debari Groups) in the east and a deep sea turbidity fancies (Jharola Group) with ultramafic slivers in the west, the latter occurring in a zone approximately marking the contact between the two fancies. This zone (Rakhabdeo Lineament) has been considered a suture in the AFB. Unconformable relation between the Archaean basement and the overlying grit arkoses chert horizon below the Delwara Group signifies a first order erosional unconformity, which in Rajasthan, represents the Archaean Proterozoic boundary. However, at many places, the contact of the Proterozoic sequence and basement is highly tectonised.
Delhi Fold Belt
The Middle to Upper Proterozoic Delhi Super group rocks occur in two belts, namely, (i) the North Delhi Fold Belt (NDFB) in NE Rajasthan, developed in three sub basins in Alwar, Bayana and Khetri areas, and, (ii) the South Delhi Fold Belt (SDFB) along the Aravalli hill range in central Rajasthan. The two belts are separated by a migmatitic gneiss tract around Ajmer in the south, poorly exposed rock assemblage in the middle and by stratigraphically problematic rock sequence extending northward possibly up to Khetri.
The Raialo, the Alwar and the Ajabgarh Groups of NE Rajasthan forming the NDFB do not perhaps extend beyond south of the Sambhar Jaipur Dausa wrench fault. The rocks of the SDFB are deposited, in two sub-basins flanking a median basement inlier, west of Bhim. The eastern sub-basin contains pelitic and samiti rocks of the Rajgarh Group signifying a continental slope fancies; and a platformal pelite carbonate sequence of the Bhim Group. The western sub-basin contains basic and felsic volcanics with shallow-water clastic forming the Bharatiya and the Sendra Groups. The contacts between the different sequences are defined by prominent ductile shear zones and thrusts. Several conglomerate horizons, such as Shringaar Ki-shangarh and Bar are developed within the SDFB which signify erosional unconformities. The status of the Bar conglomerate is problematic.
An important attribute of the SDFB is the Phulad ophiolite which is best developed in the southern part of the fold belt. The ophiolite zone is interpreted as dismembered fragments of oceanic crust developed within the South Delhi rift basin and is considered to represent a suture zone where high P low T metamorphic imprints are recognizable. From tectonic development and presence of prominent DSZs and basement slivers, the SDFB appears to represent a mélange zone.
The Late Proterozoic event in Rajasthan is marked by the opening of the Sirohi basin within the Pre Delhi terrain to the west of the SDFB in the Trans Aravalli region. Two lithological units, namely, Punagarh and Sandesh Groups are identified by recent workers. These two groups occur in isolated basins, unconformable overlying the Sirohi Group met sediments and the migmatitic of the Erinpura Granite affinity. The Sirohi Group itself occurs as isolated inliers within the unclassified sequences of the Erinpura craton. However, the Sirohi Group has been tentatively correlated on litho structural grounds with the Jharola Group of the Aravalli Super group. The Punagarh and the Sandesh Groups are represented in the trans-Aravalli range primarily by bimodal acid basic volcanics, volcaniclastics and terrigenous sediments, and are presently placed at the top of the SDFB sequence.
In SE Rajasthan, the Great Boundary Fault (GBF), a prominent dislocation zone, marks the boundary between the HG basement rocks and the paratectonic Vindhyan Super group rocks of, Proterozoic age. The Vindhyan equivalent sediments also developed in NW Rajasthan in a separate basin and are grouped within the Marwar Super group composed of molasse type sediments and evaporate. After the Delhi orogeny, the NW Indian craton across the Aravalli range witnessed large scale magmatic activity represented by felsic and mafic Malani volcanics, Jalore and Siwana plutons of Late Proterozoic age.
The Paleozoic marine sedimentation in Rajasthan was restricted to three major basins in Jaisalmer, Barmer and Nagaur and Jalore districts. During the Permo Carboniferous period the Bap boulder bed and Bhadura terrigenous sediments were deposited over the rocks of the Marwar Super group. There is no record of any Triassic sedimentation in this region. However, the Jurassic period is marked by large scale marine deposition and proliferation of flora and fauna. The culmination of the Mesozoic witnessed large scale outpouring of basaltic lavas in southeastern Rajasthan. These volcanic rocks form the northern extension of the Deccan Traps. This volcanic activity, however, did not affect the sedimentation in the northwestern Mesozoic basins where deposition of marine and continental sequences continued uninterrupted during the Cenozoic period.
Major Terrains of Rajasthan
From the distribution and inter relation of important tectonic and stratigraphic units in Rajasthan, four major terrains have been identified. These are (i) the Marwar terrain, (ii) the fold belt terrain which includes the Aravalli and Delhi fold belts, (iii) the basement terrain and (iv) the Vindhyan terrain.
The rejuvenation of the pre existing faults during the post-Neocene to Recent times facilitated Quaternary sedimentation in marginal fault troughs, graben and lakes formed by disorganized river courses. A major part of the Quaternary sediments also occurs in the western semi arid zone spreading over 36,000 km2 in Rajasthan. The Quaternary sediments comprising sequence of fluvial, fluvial lacustrine and successive Aeolian sediments correspond to different phases of pale climatic conditions. At least three arid phases during ca. 200 ka, 18 14 ka and 6 3 ka continuing to Recent are identifiable with intermittent three wet phases during older than 200 ka, 120 ka and 10 6 ka periods. The present day morphology of the Thar Desert is the outcome of the Aeolian processes operating since the Late Pleistocene.
The stratigraphic succession established by GSI in Rajasthan is given below.
South central Rajasthan contains a mosaic of Proterozoic fold belts within a reworked Archaean basement complex. These fold belts, characterised by resurgent tectonics, are demarcated by, prominent crustal dislocations, and marked by ductile shear zones. Moreover, there are prominent shear zones and thrusts within the fold belts which are either vestiges of Proterozoic sutures or Ophiolite mélange zones.
Aravalli Fold Belt
The Aravalli Fold Belt (AFB) has a tectonic contact with the basement gneisses which, in fact, is a tectonised unconformity, marked prominently in the north where the Delwara sequence of the basal Aravallis is tectonically juxtaposed against the BGC along a reworked unconformity. This tectonic zone, the Delwara Dislocation Zone (DDZ), extends south and southeast along the Aravalli basement interface, and truncates a number of litho units. Another important tectonic feature of the AFB is the Rakhabdeo Suture Zone (RSZ) which divides the AFB into two contra sting segments, namely, the platform sequence in the east and deep sea fancies (Jharola) in the west. The RSZ, marked by tectonised serpentinite bodies and minor metagabbro and amphibolites is considered the Aravalli suture along which the dismembered Aravalli oceanic/ transitional crust was obducted and the eastern and the western domains were sutured.
Delhi Fold Belt
The South Delhi Fold Belt (SDFB) has developed on an intracratonic rift basin which was floored by an oceanic /transtional crust. The remnants of this crust are preserved as dismembered Ophiolite mélange (Phulad Ophiolite) Apart from the DSZs related to the Ophiolite zone, the SDFB contains an up thrust basement wedge, flanked by two prominent thrust zones. All these tectonic features make the SDFB an imbricate thrust zone which should have deeper crustal significance. The western boundary of the SDFB is defined by a dislocation zone, the Phulad Dislocation Zone (PDZ), against the basement rocks, while the eastern boundary is a prominent thrust zone (Kaliguman Dislocation Zone, KDZ) which separates the Delhi rocks from the Sandrnata Mangalwar Complex rocks in the north and the Aravalli sequence in the south.
The imbricate tectonic signatures of the SDFB are also reflected in the tectonics of the adjacent Sandmata/Mangalwar Complex terraine where the granulite fancies Sandmata rocks are bound by imbricate DSZs within the amphibolites fancies Mangalwar rocks. From tectonic relations it is suggested that the lower crustal rocks of the Sandmata Complex have been emplaced as tectonic wedges at various structural levels, and the gravity high over the Sandmata belt is a probable indication of this feature. The Mangalwar Complex to the east of the Sandmata Complex contains a number of subs parallel DSZs which are probably the expression of subsurface imbricate thrust zones between the lower and the upper crustal rocks.
Great Boundary Fault (GBF)
The GBF is an important dislocation zone in southeastern Rajasthan. The geological and structural setting indicates that the GBF, a reverse fault, is also an imbricate fault zone which sliced the Vindhyan and pre Vindhyan rocks. The trace of the GBF is curvilinear, and it appears to be a rotational fault with the hinge located nearly 20 km south of Chittaurgarh. These features indicate that the GBF is related to indentation tectonics caused by impingement of the Bundelkhand massif with the Vindhyan cover rocks on the Precambrian terrain of Rajasthan, as a result of regional crustal deformation of the Indian plate after the collision and jamming of the Indian and the Asian plates. In this context, the GBF has a regional crustal significance in the tectonic development of the NW Indian plate segment in Late Tertiary and Quaternary times, and it is likely that this fault Zone is still active.
The Mesozoic and Cenozoic sediments of Western Rajasthan are contained in structurally controlled basins of the Rajasthan shelf which formed a part of the Indus Shelf. This shelf region is divided into several segments by sub surface basement ridges. The Delhi Lahore ridge separates the Indus Shelf from the Punjab Shelf and marks the northern boundary of the Indus Shelf. The Jaisalmer Mari High differentiates the Jaisalmer Basin. The Devikot Pokaran Nachna High is a prominent NW SE trending gravity high. This basement ridge and the Fatehgarh Fault to the south of this ridge delimit the Jaisalmer Basin, in the southeast and the Barmer Basin in the northwest. The structural style of the Jaisalmer Basin is controlled by major fault from the western edge of the outcropping belt up to Dangwada-Lang areas in the northwest and to the south of Lunar Miajlar area. These faults exhibit “flower structure” and are the result of wrench fault tectonics. Another basement ridge passes through Lakhanpur Barmer. Structurally, the Cambay, Graben is connected with the Rajasthan Shelf through the Sanchor and Barmer Graben.
In Marwar Craton several lineaments have been identified. The major lineaments are the Luni Sukri Lineament (LSL) and the Jaisalmer Barwani Lineament (JBL). The LSL trends NE SW and is aligned along Luni and Sukri rivers forming a significant linear/curvilinear feature in the desert tract of Rajasthan. It extends for 750 km from Great Rann of Kachchh in the southwest to Sambhar Lake in the northeast with an ENE WSW to NE SW trend. The northeastern extension of the LSL between Bakhasar and Sambhar Lake is controlling the Luni and Sukri rivers in the desert country. This lineament might, represent the northwestern limit of the Delhi basin. The absence of Cambrian or Eocambrian sediments belonging to the Marwar Super group beyond southeast of this lineament would have limited the Late Proterozoic to Early Paleozoic basin in the, Southeast during the evolution of the Marwar basin. The alignment of earthquake epicenters of varying intensities from 1819 to 1976 A.D. in Kachchh area and adjoining areas indicate neotectonism along this lineament.
The JBL is 1000 km long lineament extending from Barwani in the southeast to Jaisalmer in the northwest in a NNW SSE to NW SE direction, the major segment of which passes through the desert tract. It delimits the southwestern boundary of the Aravalli basin near Godhra and abruptly cuts the Delhi rocks in their southwestern extension in Vadnagar and Palanpur areas. North of Barmer, it delimits the western boundary of the Tertiary basin and crops out as a well defined fault within the Mesozoics of Jaisalmer area. A few circular features, located along this lineament in Jaisalmer area, are interpreted as sub surface domes and basins associated with tectonism of this lineament, and these may be promising zones for hydrocarbon accumulation.