Obruchevella and other terminal proterozoic (Vendian) organic-walled microfossils from the bhander group (Vindhyan Supergroup), Madhya Pradesh

Abundant specimens of Vendian marker species of Obruchevella, a helically coiled filamentous microfossil, are recorded from the sediments of Bhander Group of western Son Valley and DMH-A well, in Madhya Pradesh. Various species of Obruchevella appear in the Ganurgarh Shale, become abundant in the overlying Nagod Limestone and the Sirbu Shale, and are associated with other characteristic Terminal Proterozoic organic-walled microfossils. The appearance of Obruchevella parva and O. valdaica in the Ganurgarh Shale, with associated occurrence of Vandalosphaeridium reticulatum and Trachysphaeridium laufeldi, suggests Late Cryogenian-Early Vendian (ca. 650 - 590 Ma) age. The abundance of above species of Obruchevella in the Nagod Limestone, along with O. delicata, is broadly indicative of an Early Vendian (ca. 690 - 570 Ma) age. Yet, the appearance of O. delicata, Lophospharidium rarum and Cymatiosphaera sp. in the Nagod Limestone, having their earliest occurrences from Late Vendian, suggests that the age of the Nagod Limestone extends into the Late Vendian (ca. 570 - 560 Ma) also. The persistence of the above species of Obruchevella, and the occurrence of Cristallinium sp., Dictyolidium sp. and O. parvissima in the Sirbu Shale which generally show their appearances in Early Cambrian, is suggestive of a Late Vendian-Early Cambrian age. However, the abundance of Vendian marker species of Obruchevella and the absence of distinctive Early Cambrian acritarchs in the Sirbu Shale, favours a Late Vendian age. The recovered microfossils provide a strong evidence for a latest Cryogenian to Late Vendian (ca. 650 - 544 Ma) age-range for the Bhander Group. The presence of Vendian marker species of Obruchevella and other Terminal Proterozoic age-potential organic-walled microfossils in the Bhander Group brackets the upper age limits of the Vindhyan Supergroup to the Late Vendian, and seems that it does not encompass the Lower Paleozoic (Cambrian).