New Record of Ediacaran Biota from the Jodhpur Sandstone of Marwar Supergroup, western Rajasthan, India
Nuevo registro de la biota ediacárica en la Arenisca de Jodhpur (Supergrupo de Marwar), Rajasthan occidental, India

V.S. Parihar¹

¹Department of Geology, Jai Narain Vyas University, Jodhpur -342005, Rajasthan, India. Email: geoparihar@gmail.com; ORCID ID: https://orcid.org/0000-0002-6055-8378

IntroductionTOP

The Marwar Supergroup (MSG), earlier known as Trans-Aravalli Vindhyans, occupies an area of about 51,000 km² in northwestern Rajasthan (Pareek, 1981). It consists of about 1000 m of undisturbed and unmetamorphosed sedimentary successions represented by sandstone, conglomerate, siltstone, dolostone, limestones and claystone indicative of various marine, shallow marine to sub-tidal and non-marine environments (Pareek, 1984; Pandey & Bahadur, 2009). The sediments of the Marwar Supergroup (MSG) uncomformably overlies the uneven and ragged basement of rocks of the Malani Igneous suite (Pareek, 1984), which have been dated at 771± 5 Ma by U-Pb methods (Gregory et al., 2009) and overlain by Permo-Carboniferous Bap Boulders beds (Pareek, 1984).

ResultsTOP

The Marwar Supergroup is divided into three groups, in ascending order, the Jodhpu, Bilara and Nagaur groups. The Jodhpur and Nagaur groups are argillaceous-arenaceous while the Bilara Group is calcareous in nature. Evaporitic deposits are included in the Hanseran Group, which is found only in sub-surface (Kumar & Chandra, 2005). The Jodhpur Group has been further subdivided into the Pokaran Boulder bed, the Sonia Sandstone and theGirbhakar Sandstone. Both the Sonia Sandstone and the Girbhakar Sandstone were unified into the Jodhpur Sandstone by Chauhan et al. (2004). In the last few years, the Jodhpur Sandstone of the Marwar Supergroup is globally known for the best repositories of Ediacaran biota (Raghav et al., 2005; Sarkar et al., 2008; Kumar & Pandey, 2009; Parihar et al., 2012; Srivastava, 2012c). This study documents here the discovery of macroscopic well-preserved specimens of Aspidella, Hiemalora, large Ediacaran discs (diameter from 22 to 84 cm), Tirasiana disciformis, Medusinites asteroides, Anfesta-like Ediacaran body fossils and microbial mat structures (Arumberia banksi, Kinneyia mat structures, wrinkle mat structures and other mat structures) from the Jodhpur Sandstone of Marwar Supergroup in Sursagar and Bhopalgarh area, Jodhpur, western Rajasthan (Figs. 1–2). There, the Tirasiana disciformis, Medusinites asteroides and Anfesta-like Ediacaran body fossils are reported here for the first time. In Sursagar area, the Jodhpur Sandstone is known as Sonia Sandstone and the lithology is represented by about 16–18 m thick yellowish brown, purple and pinkish brown medium- to fine-grained sandstone, siltstone and brown shale and few conglomeratic beds having laminations, ripple-marks, cross-bedding, graded bedding and rain-prints sedimentary structures (Chauhan et al., 2001). In Bhopalgah area, the Jodhpur Sandstone is known as Girbhakar Sandstone and the lithology is represented by about 100 m thick sandstones, divided into eastern and western blocks, comprising sandstone, siltstone, claystone, and chert with ripple marks, flute casts, trough and planar cross beddings sedimentary structures (Ram & Chauhan, 2007).

Aspidella sp. (Billings, 1872) is a flat, circular to subcircular or oval shaped disc with very sharp inner circular body, and occurs on yellowish brown to pinkish brown medium- to fine-grained sandstone bedding surfaces, with positive relief in Sursagar area; it is considered as a buried holdfast of frond-like organisms (Fig. 1A-C) (Gehling et al., 2000). Hiemalora sp. (Fedonkin, 1980) is a circular to sub-circular, flat disc surrounded by numerous faint radiating arms resembling either tentacles or rootlets comparable to those of sea-anemones or jelly fishes (Fedonkin, 1980); it is recorded from the yellowish brown to pinkish brown medium- to fine-grained sandstone in the Sursagar area (Fig. 1D-E). Tirasiana disciformis (Palij, 1976) is circular or sub-circular to elongated or oval-shaped, having three concentric rings surrounding a prominent round central tubercle preserved in yellowish brown to pinkish brown medium- to fine-grained sandstone bedding surfaces with positive relief in Sursagar area; no radial structures or grooves are seen in our specimens; the diameters of the complete discs are 10 cm (Fig. 1F) to 15 cm (Fig. 1G) and their central tubercle is about 1.5 cm in diameter. Medusinites asteroides (Sprigg, 1949) is a circular to sub-circular, bipartite disc-shaped impression found in yellowish brown to pinkish brown medium- to fine-grained sandstone bedding surfaces in Susagar area; the outer raised disc is separated from the inner disc by a deep surrounding depression and inner disc relatively smaller; no radial structures or any ornament are founded in our specimen; the total diameter of the Medusinites disc is 9–10 cm (Fig. 1H) and is considered to be a medusoid cnidarian (McCall, 2006). Anfesta like (Fedonkin, 1987) body fossils are triradial-shaped discs belonging to the Class Trilobozoa (trilobal medusa), and are preserved in yellowish brown and purple medium- to fine-grained sandstone of the Jodhpur Sandstone in Sursagar area; no radial structures or grooves are noted (Fig. 1I-K); the diameter of theses triradial-shaped discs ranges from 8 to 12 cm, and the remains considered as radial cnidarians (Fedonkin, 1987, 1992). The Ediacaran discs (Fig. 2a-c) are large (diameters range from 22 to 84 cm), circular to sub-circular or sheath-like structures preserved as discoidal forms on the top of yellowish brown to pinkish brown medium- to fine-grained sandstone bedding surfaces in Sursagar area, as three dimensional body fossils with morphological comparable to largest Vendian medusoid sedentary forms (Wade, 1972; Fedonkin, 1987; Hagadorn et al., 2000; McCall, 2006). Arumberia banksi (Glaessner & Walter, 1975) (Fig. 2D-E), Kinneyia mat structures (Walcott, 1916) (Fig. 2F-G) and algal mat structures (Fig. 2H) occur on top and sole of light brown to pinkish brown, brownish medium- to fine-grained sandstone bedding surfaces of the Jodhpur Sandstone in the Bhopalgarh area, whereas the various types of well-preserved sinuous, curved to meandering and straight marks known as “wrinkle mat structures” (Fig. 2I-J) are preserved on yellowish brown to pinkish brown medium- to fine-grained sandstone bedding surface of the Jodhpur Sandstone in Sursagar area. These mat structures were created through the interplay of Cyanobacteria or other microbial mat-forming groups of bacteria and physical sedimentary processes (Gehling, 1999; Noffke et al., 2001; Noffke, 2010).

ConclusionsTOP

This study suggests that the aforementioned Ediacaran fossil assemblages consist mainly of benthic organisms and indicated shallow-marine environments under moderate energy conditions for the Jodhpur Sandstone of the Marwar Supergroup. Based on the above mentioned Ediacaran fossil assemblages, the Jodhpur Sandstone of the Marwar Supergroup is broadly correlated with the Fermuse Formation, Newfoundland, South Australia, White Sea of Russia and Norway, and regionally correlated with the Bhander Group of Vindhyan Supergroup and Krol Group of Lesser Himalaya.

ReferencesTOP