THE CURIOUS RING STRUCTURE OF RAMGARH IN VINDHYANS 0F RAJASTHAN
Within the vast stretch of almost horizontally-bedded Neoproterozoic Vindhyan sediments an elevated circular physiographic structure (25° 20” 00’: 76° 37” 30”) occurs near Ramgarh village located about 12 km east of Mangrol in Baran district, Rajasthan. For its discrete elevation it can be spotted from a distance of 40 to 50 km. This unusual rocky structure attracted the attention of geologists since its discovery. It is called by different names by different workers viz. ‘Ramgarh structure’, ‘Ramgarh meteoritic structure’, ‘Ramgarh ring structure’, ‘Ramgarh dome’, ‘Ramgarh dome structure’, ‘Ramgarh astrobleme’, etc. It falls in Survey of India toposheet 54 C/ 11 and is situated about 110 km ENE of Kota. It can be approached via Jaipur-Tonk-Indergarh-Mangrol, Alwar-Gangapur-Sawai Madhopur-Sheopur-Mangrol and Kota-Baran-Mangrol road routes.
Satellite image showing regional disposition of Ramgarh ring structure (Google Earth, 2010)
On regional scale the area forms part of Kota plateau. The Ramgarh ring structure forms a pronounced 150 to 200 m geomorphic high in the form of circular hill with an elevation of 440 m above MSL. It has raised rim and a circular depression in its centre forming a plain with average elevation of 260 m above MSL. The surrounding plain is 240 m above MSL. The diameter of the ring structure is 3.2 km. A small river, located in the southwestern opening of the hill, forms part of the drainage of south to north flowing Paarvati River located 4 km in the west. A number of small rills and gulleys with radial/ centripetal drainage towards the centre are developed in the area.
Mallet (1869) of the Geological Survey of India was the first to visit the area. Geologically it was first mapped by Kishan Singh on 1: 63,360 scale during 1882-83. Coulson (1927-28) also mentioned it while describing the geology of Bundi State. Ground geophysical surveys were conducted in the area by Sharma & Singh and Jaganathan & Rao in 1969-70. During 1971-72 Rakshit (1973) prepared a photogeological map of Ramgarh and adjoining areas on 1: 60,000 scale; and ground geological map on 1: 31,680 scale. After Crawford (1972) mentioning the Ramgarh structure as a possible impact structure, the curiosity of geoscientific community arose more (Sharma, 1973; Balasundaram and Dube, 1973; Ahmad et al., 1974). Since then many GSI geologists visited the structure and conducted various field investigations so as to know the actual cause of its formation. During 1979-80 Ramasamy (1981) brought out further details of the area by mapping on 1: 12,500 scale. Vimal Kumar and Reddy (1984) investigated the structure by drilling during 1981-82. Since then the area is being visited and studied by geoscientists of various organizations (Nayak, 1997; Master and Pandit, 1999; Sisodiya et al., 2006). The efforts have resulted in better understanding of the origin of this ring structure. Brief detail of the work carried out and conclusions drawn by different workers is presented here for an overview.
Closer satellite view of the Ramgarh ring structure (Google Earth, 2010)
On a regional scale the area around Ramgarh ring structure comprises rocks belonging to Bhander Group of Vindhyan Supergroup. Detailed geological mapping of 9 sq km area on 1: 12,500 scale was carried out by Ramasamy (1981), who gave the following litho-stratigraphic succession of the rocks present in Ramgarh area:
Lower Bhander sandstone
Lower Bhander limestone
Upper Rewa sandstone and Jhiri shale
The older most lithounits i.e. the Upper Rewa sandstone and Jhiri shale of Rewa Group are exposed intermittently in the nala flowing just southeast of Bandewara temple, and forming a folded outcrop. A thin band of dolomite and reddish brown sandstone occurring southeast of temple belong to this sequence. These rocks form an open upright antiformal structure near the temple. Jhiri shales are finely laminated and calcareous in nature and have thin greenish siltstone bands. These are overlained by a band of finely laminated purplish sandstone (Upper Rewa), whose bedding is defined by ferruginous laminations, alongwith grains of glauconite, similar to the Upper Rewa sandstone of other areas. It is overlained by thin interlayered sequence of dolomite and shaly limestone. The Ganurgarh shale forming the lowermost lithounit of Bhander Group occurs in the central, almost flat, circular depression of the structure. It is dark green to purplish in colour, soft, splintery and very finely-laminated. It is traversed by calcite veins near the entrance opening of the structure. It shows a gradational contact with Lower Bhander limestone near the staircase leading to Krishnamataji temple where it is interlayered with arenaceous limestone. South of this temple the shale is highly contorted probably due to fault. Lower Bhander limestone forms an unmistakable marker horizon of Upper Vindhyans showing conformable relationship with lower and upper lithounits. It is exposed in nala sections and occurs forming a thin rim all along the periphery of the central depression. The limestone is purplish and ash to blue grey in colour, finely-laminated, arenaceous, moderately massive and well-jointed. It contains shale partings, intra-formational conglomerate and breccia, typical of this formation. The well-rounded calcilutite pebbles are embedded in calcareous matrix. Semria shale occurs in the prominent inner slopes of the structure and is covered by debris of Lower Bhander sandstone. It is greenish and purplish in colour, finely-laminated, soft and friable. The purplish shale (57 m) occurring at the bottom is followed upward by greenish shale (58 m), pale yellow, soft and vuggy dolomite (15 m) and purplish shale (50 m) as seen in the entrance part of the structure. The dolomite is traversed by randomly-oriented calcite veins and showed presence of micro-faults. Lower Bhander sandstone is the topmost bedded horizon continuously exposed and forming top and outer slopes of the ring structure. The sandstone is white to pale yellow and pinkish in colour, fine to medium-grained, hard, coarse-grained and massive at places. At the southeastern inner slope, at places at the contact with Semria shale, it shows bouldery nature, shale pellet conglomerate and breccia like structures. It shows well-developed joints, but at places, it has irregular and step-like fractures filled with ferruginous material. At the southeastern outer slope area, it shows yellow ochrous infillings, which on weathering give rise to pieces and pellets of iron material. A block of white, hard, massive ortho-quartzitic, sheared sandstone with slickenside occurs as rootless mass west of Bandewara temple indicating it to be a fall-back material.
Detailed geological map (A) and broad open warps (B) of Ramgarh area (after Ramasamy, 1981)
The sedimentary structures noted in the area include bedding, cross-bedding, intra-formational conglomerate/ breccia, shale pellet conglomerate/ breccia and occasional ripple-marks. All the rock formations i.e. from Ganurgarh shale to Lower Bhander sandstone displayed qua-qua-versal dips indicating towards the ring structure to be a domal structure or dome. The amount of dips vary in different formations viz. from 30° to 39° in Ganurgarh shale, 20° to 30° in Lower Bhander limestone, 20° to 40° in Semria shale at the contact with Lower Bhander sandstone. The amount of dip is much higher in close proximity to faults. The deformational structures developed in the lithounits are open broad warps, disharmonic folds, joints, fractures and faults. The joints developed in these rocks are generally open, vertically disposed and without any development of silicification or slickenside. Several faults have been mapped in the area, which trend mostly in NW-SE and NE-SW directions displaying a radial pattern. The entrance opening of the ring structure has been created by NE-SW trending fault-followed drainage. The broad open warps (antiformal as well as synformal) are well shown by Lower Bhander sandstone with shallow radial outward plunges of 3° to 30°. The Jhiri-Rewa sequence exposed in the central part forms an antiformal structure with one of its limb displaying complex/ disharmonic folding with steep plunges, overturning at hinges and reclined geometry at places. These folds also show abrupt changes from layer to layer from bottom to upwards. No axial plane cleavages are seen related to folds.
Vimal Kumar and Reddy (1984) investigated the structure by drilling during 1981-82. A total of ten vertical boreholes (nine shallow and one deep) were drilled in the area to know the sub-surface behavior of rocks. The deepest borehole drilled in the central part of the structure reached upto 452 m depth. Drill core study showed the presence of an un-interrupted sedimentary sequence belonging to Rewa and Kaimur Groups. No intrusive or volcanic rock was intersected in any borehole. Analytical results of core samples did not show chemical composition akin to known meteorite. The cores of the borehole drilled near Bandewara temple, however, displayed tight isoclinal folds with vertical axial plane surfaces, flow-folds, micro-faults and micro-thrust (low-angle) faults in Jhiri shale-sandstone sequence.
Fold, faults and form surfaces as seen in borehole cores (after Ramasamy, 1987)
Extra-terrestrial impact craters, more than 200 in numbers, have been identified from different parts of the earth. Meteorites and comets were known to mankind since ancient past as is evidenced from the iron-meteorite beads placed in an Egyptian pyramid dated 3000 BC (Iyengar, 2004). The Ancient Classics of India also referred such activities. The well-recognized impact crater (age 50,000 years) in India is that of Lonar in Maharashtra, where glass fragments resulting from shock melting, shocked plagioclase (maskelyinite), etc. have been identified (Radhakrishna, 2004). The other impact structures identified in the country include Dhala in Bundelkhand craton of central India; Luna in Banni plains of Kachchh region, Gujarat; and Shiva in off shore region of Mumbai (Karanth, 2006; Rao, 2007, Pati et al., 2008).
Ramgarh ring structure has been a subject of controversy for a long period for its origin. Several views exist to explain whether it is a product of extra-terrestrial or intra-terrestrial forces. These include intrusions such as kimberlite, carbonatite or diapir (intrusion in which more mobile and deformable material is forced into brittle overlying rocks) and related subsidence, tectonism, mechanism of centripetal rheid flow of kaolin-rich shales, a combination of magmatism and tectonism, meteorite impact, etc.
Based on the presence of shatter cone structure noticed in the colluvium near the centre Crawford (1972) was probably the first to suggest the Ramgarh ring structure to a possible impact structure.
Rakshit (1973) discussed several possible theories for its origin viz. (i) presence of intrusive rock at depth, (ii) volcanic crater, (iii) folding/ faulting, (iv) subsidence, (v) diapiric intrusion at depth and (vi) impact theory. In absence of favorable evidences he himself negated the first five theories for the origin of Ramgarh structure; and opined that the Ramgarh structure strongly resembles with an ‘impact crater’. He also suggested a ‘near surface explosion of a meteorite’ for the formation of this structure. The field evidences he found in support include (i) gradual increase of dips (70° to 80°) near centre, (ii) shattered nature of sandstone, (iii) shatter-cone like structure in small outcrop of sandstone in the central part and in sandstone of the peripheral ridge at places, (iv) radial to sub-radial pattern of fractures and faults; sometimes displaying crudely-defined circular or conical pattern and, (v) partial recrystllisation, granulation, deformation of lamellae and brecciation of quartz grains showing wavy and uniform extinction.
Balasundaram and Dube (1973) based on shear fracturing, granulation and anomalous birefringence in quartz grains concorded with Crawford's suggestion of impact origin of Ramgarh structure.
Sharma (1973) suggested Ramgarh ring structure is formed due to combined phenomenon of magmatism and tectonism.
Based on aerial view and map published in Survey of India 1961 School Atlas alongwith some field observations i.e. magnetic pieces from central part and magnetic spherules in clay from outside the structure Ahmad et al. (1974) considered the Ramgarh structure as a ‘meteoritic crater’.
Ramasamy (1981, 1987), based on detailed geological mapping and structural analysis of the area, however, considered the evolution of this curious geomorphic structure as a product of structural or tectonic deformation and called it as ‘Ramgarh dome’. Prasad (1984) in his memoir on Vindhyan Supergroup also considered it as an oblong anticline.
Murali and Lulla (1992) identified the structure using IRS-1A images and considered it a potential impact crater that was not studied so far, particularly in relation to Deccan volcanism.
Nayak (1997) opined that definitive meteoritic impact signatures are lacking and at present the structure should be considered as 'Ramgarh astrobleme'.
Master and Pandit (1999) gathered more evidences in favour of its being an ‘impact crater’. These include identification of closely spaced-fractures and multiple-joint striated surfaces (MSJS) in quartzites; undulose extinction of quartz grains and deformation twin lamellae, planar deformation structures (PDFs) in association with planar fluid inclusion trails under high magnification - all indicative of intense shock metamorphism.
Sisodiya et al. (2006) have identified Ni-Fe rich rounded micro glassy objects, severely fractured quartz grains (with corroded margins possibly after high silica polymorphs) and occasional glassy material in a siliceous brecciated matrix, sand-sized glistening black and transparent glassy (microtektites) spherules with surface dotted with vesicles or transparent glassy patches, many spherules showing digenetic alteration (glass being replaced by iron oxide), development of off-centered crystalline K-feldspar and/ or quartz spots in the spherules, crystalline silica grins showing spherulites and mosaic of spherulites characteristic of diaplectic glass formed under high pressure and planar deformation structures - evidences in favour of its being an ‘impact crater’. Reimold et al. (2006), however, do not agree with evidences cited and conclusions drawn by Sisodiya et al. (2006) in favour of meteoritic crater; and suggested more detailed analytical studies for identification of shock metamorphism in the central part of the area.
The field and laboratory investigations carried out so far suggest that the ring structure could have been formed by various processes. More evidences, however, are in favour of an extra-terrestrial or meteoritic impact of its formation; the ring structure, therefore, may be called as Ramgarh crater.
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Reimold, W.U., Trepmann, C. and Simonson, B. (2006): Discussion: comment/ reply on: ‘Impact origin of the Ramgarh structure, Rajasthan: some new evidences’ by Sisodiya et al. (2006) Jour. Geol. Soc. Ind. V. 68 No. 3 pp 561-565
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Sisodiya, M.S., Lashkari, G.L. and Bhandari, N. (2006): Impact origin of Ramgarh structure, Rajasthan: some new evidences. Jour. Geol. Soc. Ind. V. 67 No. 4, pp 423-431
Vimal Kumar and Reddy, B.V.R. (1984): Report on the investigation of Ramgarh dome structure, Kota district, Rajasthan. Unpub. Geol. Surv. Ind. report for FS 1981-82
Compiled by A.K. Grover, Director Geological Survey of India, Jaipur
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