Organic-walled microfossils from the Proterozoic Vindhyan Supergroup of Son Valley, Madhya Pradesh, India

Well-preserved and diversified organic-walled microfossil assemblages are recorded from the Vindhyan sediments of Son Valley and DMH-A well, in Madhya Pradesh. The microfossils include acritarchs, coccoid and filamentous taxa that suggest a Meso-Neoproterozoic age for the Vindhyan Supergroup which, hitherto, was assigned a Late Paleoproterozoic to Early Paleozoic age, based on fossil evidences and radiometric datings. The Kajrahat Limestone, within the basal Semri Group, recorded abundant filamentous microfossils, viz. Polythrichoides, Karamia, Arctacellularia and Siphonophycus along with simple unornamented sphaeromorph acritarchs (Leiosphaeridia spp.), suggesting ca. 1500-1450 Ma age of Early Mesoproterozoic. The microfossil assemblage of Deonar Formation also includes the above taxa; however, Satka, Eomicrocystis and acanthomorph acritarchs, Tappania spp. , appear within this formation along with abundant polygonomorph acritarchs referable to Octoedryxium. The Deonar microfossil assemblage resembles the assemblage of Roper Group (northern Australia) and suggests ca. 1450-1350 Ma age of Early to Middle Mesoproterozoic. The sediments of Kheinjua Subgroup are marked by the appearance of various species of Navifusa, Simia and Pterospermopsimorpha in the Koldaha Shale with overall abundance of Tappania, Satka, Eomicrocystis, Kildinosphaera and Leiosphaeridia. The presence of Middle to Late Mesoproterozoic marker taxa, viz. Tappania plana, T. tubata and Navifusa segmentata helps to correlate the Koldaha Shale and Salkhan Limestone assemblages with the assemblage of the Ruyang Group (China), suggesting an Ectasian-Stenian (ca.1350-1050 Ma) age. In additon to the above taxa, the appearance of Early Neoproterozoic marker taxa, such as, Vandalosphaeridium, Bavlinella, Melanocyrillium and budding leiosphaerids in the Rampur Formation indicates a Late Stenian-Tonian age (ca. 1050-850 Ma) for this formation. The microfossil assemblage of the Rohtas Subgroup is quite distinct as the marker taxa of the Kheinjua Subgroup, viz. Tappania spp.and Navifusa spp. disappear. The presence of Trachysphaeridium laufeldi, Vandalosphaeridium reticulatum, Bavlinella faveolata and Stictosphaeridium spp. , and the disappearance of Eomicrocystis, Satka and budding leiosphaerids within this formation allow its correlation with Middle Neoproterozoic Miroyedikha (Siberia) and Husar-Kanpa (central Australia) assemblages, suggesting an Early Cryogenian (ca.850-750 Ma) age for the Rohtas Subgroup. The microfossil assemblages from the Kaimur and Rewa groups are represented by the species of Symplassosphaeridium, Synsphaeridium and Leiosphaeridia. The occurrence of B. faveolata, T laufeldi and Octoedryxium truncatum in their assemblages suggest a Middle to Late Neoproterozoic (Late Cryogenian; ca.750-650 Ma) age. The sediments of the Bhander Group also include the above taxa. However, the appearance of Ediacaran (Vendian) marker species of Obruchevella, viz. O. parva and O. valdaica in the Ganurgarh Shale, and their abundance in the overlying Nagod Limestone and Sirbu Shale, suggests a Late Cryogenian-Early Ediacaran (ca. 650-570 Ma) age for the Bhander Group. Yet, the appearance of Obruchevella delicata, Lophosphaeridium tentativum and Cymatiosphaera sp. in the Nagod Limestone, and Obruchevella parvissima, Cristallinium sp. and Dictyotidium sp. in the Sirbu Shale suggest that the age of the Bhander Group extends into the Late Ediacaran (ca.570-544 Ma). The record of Calymmian (ca. 1500 Ma) to Late Ediacaran (ca. 544 Ma) organic-walled microfossil assemblages categorically suggests an Early Mesoproterozoic to Terminal Proterozoic age-range for the Vindhyan Supergroup. The occurrence of well-developed Middle and Late Mesoproterozoic microfossil assemblages in the Deonar Formation and Kheinjua Subgroup negates the Late Paleoproterozoic (ca. 1630 Ma) age assignment to them, based on radiometric datings. The presence of Ediacaran (Vendian) marker species of Obruchevella and the absence of distinctive Early Cambrian acritarchs in the Bhander Group brackets the upper age limits of Vindhyan Supergroup to the Late Ediacaran, and does not encompass the Lower Paleozoic.