Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

D. S. Chauhan ¹, K. M. Mathur ², Narayan Ram ¹

Affiliations
1 Department of Geology, Jai Narain University, Jodhpur - 342001, IN
2 Ground Water Department, Govt. of Rajasthan, Jodhpur, IN

Abstract

The Pokaran Boulder Bed forms the basal lithofacies of Neoproterozoic-Cambrian Marwar Supergroup. The boulder bed denotes specific sedimentary milieus and palaeoclimatic settings during the terminal Proterozoic period and occurs in three distinct forms, viz., massive conglomerate, boulder spread and stratified conglomerate. These three forms of the boulder bed independently rest over the basement of Malani rhyolite and/or granite (>698 Ma) and mark an erosional unconformity at the base of the Neoproterozoic Marwar Supergroup (1000-540 Ma). It is suggested that the massive conglomerate is a product of weathering and disintegration of the basement rocks operating under semi-arid to arid conditions. The boulder spread is envisaged to be glaciogenic, whereas the stratified conglomerate is a product of glaciofluvial action. The stratigraphy of Marwar Supergroup is believed to be linked to the global Neoproterozoic glaciation. The base of Marwar Supergroup is interpreted to be late Neoproterozoic (<610 Ma). The occurrence in the same basin of climatically incompatible glaciogenic Pokaran Boulder Bed and Hanseran Evaporite vis-a-vis diamictiteevaporite fill is visualized to be the effect of ice rings orbiting the earth. This has implications for stratigraphic correlation of such unique sequences.

Keywords

Pokaran Boulder Bed, Malani Rhyolite, Glaciofluvial Sediment, Evaporite, Marwar Supergroup, Neoproterozoic Glaciation.

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D. S. Chauhan ¹, Bhanwara Ram ², Narayan Ram ¹

Affiliations
1 Department of Geology, J.N. Vyas University, Jodhpur, IN
2 Department of Mines and Geology, Govt, of Rajasthan, Jodhpur, IN

Abstract

The middle horizon of the latest Proterozoic Jodhpur sandstone is extensively quarried, yielding best quality of building stone as well as much needed valuable geological information. This sandstone shows prolific development of wave ripples and other related structures. It has been observed that the sandstone sequences show ripples developed in a hierarchical order. There are straight crest ripples at the base, followed upward by undulatory ripples, sinuous ripples, interference ripples and adhesion ripples/adhesion warts at the top. Formation of ripples in general and ripple hierarchy in particular is discussed, in the light of wave-generated structures in modern environment. It is deduced that the straight crest ripples have formed under relatively deep-water conditions; whereas undulatory ripples, sinuous ripples and interference ripples developed under gradually shallowing water conditions. The adhesion ripples/adhesion warts indicate subaerial condition of sedimentation. It is envisaged that the hierarchically ripple bedded Jodhpur sandstone sequences represent shallowing upward cycles developed under nearshore-beach environment. These cyclic sequences are critically analyzed. It is suggested that they must have evolved within the confines of shoreface-backshore settings of beach environment. The overail evolution of these ancientbeach sandstone sequences in terms of periodic transgression and sediment aggradation is stressed. Concurrently, the Jodhpur sandstone, as building stone, is considered as a gift of ancient beaches.

Keywords

Jodhpur Sandstone, Building Stone,’Ripple Hierarchy, Ancient Beaches, Rajasthan.

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