Late Neoproterozoic (Ediacaran)-Early Paleozoic (Cambrian) Acritarchs from the Marwar Supergroup, Bikaner-Nagaur Basin, Rajasthan

Abstract Characteristic latest Neoproterozoic and Early Paleozoic acritarchs and associated organic-walled microfossils are recorded from the sediments of Marwar Supergroup encountered in BGW-A well (Bikaner-Nagaur Basin) from 1123-481 m depth. Six distinct acritarch assemblages, broadly comparable with globally known Ediacaran (Vendian) and Cambrian assemblages are recognised. The recovered microfossils provide precise age for different units of the Marwar Supergroup whose ages, till now, were poorly understood due to absence or paucity of invertebrate and other mega and microfossils. Jodhpur Group (1123-1105 m), the basal unit of Marwar Supergroup, records abundant ornamented sphaeromorphs (Lophosphaeridium spp.) alongwith various species of Leiosphaeridia, suggesting Late Ediacaran age. Occurrence of small micrhystrids (Asteridium spp.) and appearance of Dictyotidium birvetense, Pterospermella solida and Annulum squamaceum in lower part of Bilara Group (1105-1081 m), suggests latest Ediacaran to early Early Cambrian age. Hanseran Evaporite Group (1068-907m) is marked by abundant Retisphaeridium dichamerum, Dictyotidium birvetense, Cristallinium cambriense, Comasphaeridium sp. cf. C. strigosum and Archaeodiscina umbonulata indicating late Early Cambrian to early Middle Cambrian age. Microfossil contents in Nagaur Group (907-727m) are very poor. The succeeding Upper Carbonate Sequence (727-481 m) shows abundant Cristallinium randomense, Cymatiosphaera crameri and Asteridium spp., along with other species of Cymatiosphaera and Cristallinium, and also includes the early Late Cambrian marker forms, such as Striatotheca loculifera and Dorsenidium (Veryhachium) minutum that suggest late Middle Cambrian to early Late Cambrian age. The recorded acritarch assemblages suggest Late Ediacaran to early Late Cambrian (ca 570-500 Ma) age for the Marwar Supergroup, with demarcation of Precambrian-Cambrian boundary within the lower part of Bilara Group. Associated microfossils indicate intertidal/foreshore depositional environment for Jodhpur and Nagaur groups, whereas inner neritic to subtidal for the Bilara, Hanseran and Upper Carbonate units. Occurrence of an Early Permian palynoflora in the succeeding sequence (481-427 m depth) indicates a major hiatus of ca 200Ma between the Marwar and overlying Bap/Badhaura sequences which involves the absence of middle Late Cambrian to Late Carboniferous sediments in this basin. Latest biostratigraphic data from Ujhani (Ganga Basin) and Bhander (Vindhyan Basin) sediments suggest that the above two sequences are closely comparable with the Marwar Supergroup, and show the prospective exploration challenges as do the Marwar Supergroup.