Remote Sensing, Structural and Rock Magnetic Analyses of the Ramgarh Structure of SE Rajasthan, Central India-Further Clues to Its Impact Origin and Time of Genesis

Abstract

The Ramgarh structure of SE Rajasthan, central India, situated within an almost undeformed, flat-lying Vindhyan Supergroup of sedimentary rocks of Meso- to Neoproterozoic age, is a potential candidate of asteroid impact crater for last many decades. A fresh observation on remote sensing images (ASTER, Landsat and Google Earth Imageries) along with structural analyses in field show that this rectangular structure has a rim-to rim diameter of ~<2.5 km with a present diameter/depth ratio of ~12, a small central conical peak (~6 m high), and quaquaversal dips of rim crest sandstones with average dips between 21° and 50°. Unlike the surrounding sedimentary rocks, which only show two sets of wide-spaced (~2 m) vertical fractures trending NE-SW and NW-SE, the country rocks within the structure show extreme brittle deformation including vertical fractures in numerous directions, moderately dipping fractures trending mostly NE-SW and NW-SE, and moderate fault planes with N-S and E-W trends. The geometry of the Ramgarh structure is very similar to those of asteroid impact craters, where the profound brittle deformation of the sedimentary country rocks within the structure could have been resulted due to sudden shock during the impact. Reactivation of fractures existing within the pre-impact country rocks inside and adjacent to the Ramgarh structure by the shock effect is also possible. Our present observation on sub-samples from a cm-sized glassy silicate piece and our previous study on mm-sized particles, recovered from this structure, show that these magnetic materials have very high Natural Remanent Magnetization (NRM) (~2–19 Am⁻¹) and NRM to saturation isothermal remanent magnetization ratio (REM) (~7–145%) indicating the presence of a high magnetic field during their formation, much higher than the ambient Earth’s magnetic field. A natural phenomenon that could generate a unique ring-shaped deformation structure on a monotonously flat-lying, undeformed sedimentary country rock as well as a high magnetic field in and around this structure is a hypervelocity asteroid impact. The rectangular shape of the Ramgarh structure, which resembles the Arizona Crater, USA, was resulted due to post-impact dextral slip along a NW-SE unnamed fault, followed by dextral NE-SW faulting and minor sinistral slip along E-W fracture. These fractures reactivated perhaps during the modification stage of evolution of the Ramgarh structure. Our remote sensing observation further confirms that the impact took place on the palaeo-channel of Parvati River, which is now displaced towards W due to impact.

Appendix

See Figs. 9 and 10.

Fig. 9

Hand specimen photograph of cm-sized silicate glassy piece recovered from within the rocky debris deposited on the outer wall of western rim of Ramgarh structure (diameter of coin shown as scale is 22 mm)

Fig. 10

a, b Sandstone sample from Kul River section N of Ramgarh structure (location R 38), the dark matters among the grains (shown by arrow in b) are manganese encrustation. c, d impure limestone from Ramgarh structure (R 18) containing numerous fine grain quartz sand particles. e, f percolation of manganese solution within the sandstone (shown in box) sample from the rim of Ramgarh structure (R 11). g, h manganese encrustation containing numerous sub-angular to sub-rounded quartz grains (R 15). GPS locations of samples: R 11: 25° 19.378′N, 76° 37.933′E; R 15: 25° 19.582′N, 76° 37.570′E; R 18: 25° 19.406′N, 76° 37.071′E; R 38: 25° 22.929′N, 76° 36.961′E. Note fineness of grain size of sandstones occurring on rim crest of Ramgarh structure