Sedimentation,Tectonics and Palaeoenviment in Eastern Kachchh,gujarat

Wagad Formation of eastern Kachchh, partly equivalent to the Bhuj Formation (Mesozoic) of the Mainland, exhibits a continuous sedimentation starting at the base with ferruginous sandstone interlaminated with shale (lower member) grading upward into an essentially subarkosic sandstone with interbedded kaolinitic shale (upper member). The top of the upper member is partly lateritised. The sandstone - shale interbedded sequence is found to repeat in several cycles and each of the cycle is capped by thick ferruginous sandstone.

Cross- bedded sandstone and subarkose with silty shale indicates a deltaic basin with distributaries flowing northwest and southwest. Thickening of sediments towards centre and west from other three directions indicates gradual deepening of the basin westward. Transitional sedimentation from ferruginous to subarkosic has experienced storm wave action for a short period as evident from hummocky stratification at a few places. Sedimentary structures in the area reflect initiation of Mesozoic sequence with a transgressive phase of deposition in a near shore tidal environment. Seasonal fluxes of sediment supply and maximum evaporation at times, as evident from gypsum in the sediment, have also been recorded. Sediments, its structures and freshwater plant remains denote a deltaic nonrnarine environment of deposition during upper Gondwana (Late Jurassic to lower Cretaceous (?)) and at least a part of the upper member sediments represent laterally migrating channel bar deposits.

Culmination of Mesozoic sedimentation is indicated by vertical tectonic movement in the entire Kachchh. This is responsible for formation of anticlinal fold,with broad crest that closes near Adesar (23°33'N : 70°59'E) in regional scale and a number of domes, half-domes and doubIy plunging anticlines intimately associated with high angle normal faults in local scale in the area. The Adesar anticline plunges at a low angle towards 60°and the axial trace of the fold trends NESW to ENE-WSW. The area is traversed by a number of high angle normal faults and transverse faults. One volcanic plug of alkali olivine basalt occurs within lower member of the Wagad Formation near Khandek. Field relations and petrology of this rock indicate a hot-spot or mantle-plume type origin of this post-Mesozoic alkali basalt. Other intrusives are either olivine basalt or olivine gabbro. The marine water transgresses after a long period of non-deposition under a calm environmental condition of a shallow basin at the time of lower Miocene. Bivalve fossils are found within rnicaceous sandstone and mottled claystone of Khari Nadi Formation. Following another hiatus pelletoid calcareous sandstone, equivalent to miliolitic limestone of western Kachchh of Miliolite Formation gets deposited. Long tabular current bedding shows constant eastward wind direction. Pelletoid sands, deposited in the area now forming the Great Rann wherc a near shore environment was present during the Pleistocene period, are blown eastward and get precipitated wherever any natural barrier was encountered.