Short communicationStratigraphic position of the ∼1000 Ma Sukhda Tuff (Chhattisgarh Supergroup, India) and the 500 Ma question
Introduction
Rhyolitic tuffs near Sukhda and Sapos villages (Mukherjee and Sahoo, 2003) in the Proterozoic Chhattisgarh (var. Chattisgarh) Basin (Fig. 1), in Janjgir District of Chhattisgarh State in central India, erupted ∼1000 Ma ago as determined from U-Pb SHRIMP ages of magmatic zircons in the tuff (Patranabis-Deb et al., 2007). Mapping and stratigraphic considerations strongly suggest that the lithostratigraphic position of the tuff horizons (Sukhda Tuff for nomenclatural convenience) is near the top of the sedimentary succession of the Chhattisgarh Supergroup (Patranabis-Deb, 2001, Patranabis-Deb, 2004, Patranabis-Deb and Chaudhuri, in press). This implies that most of the Chhattisgarh Supergroup and its equivalents, such as the virtually unmetamorphosed sedimentary successions in the Vindhyan (Chakraborty and Paul, 2008, Malone et al., 2008), Kurnool (part of Cuddapah), Kaladgi–Badami–Bhima, Khariar, and Indravati basins in peninsular India (Kumar et al., 2005, Naqvi, 2005, Maheshwari et al., 2005), are also older than ∼1000 Ma. Based on detrital zircon ages and paleomagnetic data, Malone et al. (2008) have shown that the maximum age of the top of the Vindhyan Supergroup is ∼1000 Ma.
If so, inferences about the Neoproterozoic history of Earth, as deduced from sedimentary rocks found in peninsular India are rendered irrelevant. It is no puzzle that glacial deposits, such as those in the Cryogenian Snowball Earth, are absent in these basins (Williams and Schmidt, 1996, Chaudhuri et al., 1999, Kumar et al., 2005). The absolute age of the tuffs implies that these basins opened and closed before the complete assembly of Rodinia. Therefore, reconstructions of Rodinia with India in it (e.g., Dalziel, 1997) cannot draw from the sedimentary tectonics of these basins. In fact, there is a growing body of convincing evidence that India was not a part of Rodinia any way (Malone et al., 2008, Cawood et al., 2007, Kröner and Cordani, 2003, Meert and Torsvik, 2003, Torsvik et al., 2001). Hence, the impetus to put India in Rodinia based on the assumed Neoproterozic age of these basins is based on false assumptions. Finally, the new absolute ages also demand that the life forms, including metazoans and small shelly fossils (SSF) that have been reported from the strata in these basins are all much older. Because this implies that metazoan life started and evolved in deep time (Bengtson et al., 2007, Basu, 2008), the exact stratigraphic placement of these fossils needs to be verified through careful resampling of in-place material.
The Sukhda Tuff and its enclosing sedimentary package have been placed by some in the lower part of the Chhattisgarh Supergroup (Subba Rao et al., 2006, Mukherjee and Ray, 2008, GSI, 2005a, GSI, 2005b). If so, there arises a 500 Ma problem in Indian Proterozoic stratigraphy. This is a matter of much verbal public discussion (e.g., International conferences at ISI, Kolkata, January, 2008, and, at IIT, Mumbai, December, 2007 and February, 2008; see also Mukherjee and Ray, 2008). The purpose of this short note is to present arguments from our formation-mapping, facies-mapping and petrologic observations (optical and SEM-BSE-CL) to show that the Sukhda Tuff (Fig. 2) indeed is located near the top of the Chhattisgarh Supergroup. This finding requires a bold re-assessment of the Proterozoic geology of peninsular India.
Section snippets
Lithostratigraphy
There is no dispute over gross lithologies and geographic locations of outcrops in the Chhattisgarh Basin, the eastern part of which is sliced by a number of faults. We, however, interpret the relative stratigraphic positions of these lithologies differently from what is depicted on currently available maps published by the Geological Survey of India, in which the sandstone outcrop NE of Sukhda is listed as Lohardih (GSI, 2005a, GSI, 2005b). Traditionally, the Chhattisgarh Supergroup has been
Discussion
The basal conglomeritic sandstone of the Chandarpur Group (∼900 m), overlying Archean-Proterozoic granite, gneiss, schist and greenstones, named the Lohardih Formation virtually by all authors, is characterized by abundant rock fragments of granitic composition and occurs along the periphery of the basin (Fig. 1). The principal provenance of this sandstone is plutonic. Primary sedimentary structures and sedimentary architecture indicate that the sandstone was deposited in fan/braid-deltas in an
Conclusion
The sandstones in the Sarnadih, Lohardih and Kansapathar Formations are petrographically distinct (Table 2). The Kansapathar sandstones are very mature quartz arenites that were deposited in shoreface bars. The Sarnadih and Lohardih sandstones are both immature and were deposited primarily in continental and near-shore environments respectively. The Sarnadih sandstone rests on the Sukhda Tuff and the Lohardih sandstone rests on the Archean-Proterozoic basement. The provenance of the Sarnadih
Acknowledgements
This research has been supported by grants from DST-Government of India, Indian Statistical Institute, and Indiana University Foundation. An NSF equipment grant to Juergen Schieber (EAR-0318769) provided funds for the purchase of the analytical SEM that was used for acquiring the images used in this report. We thank Dr. A.K. Chaudhuri for guiding SPD's facies-mapping in the Chhattisgarh Bain including the Sukhda area. We are grateful to Joe Meert, an anonymous reviewer, and to Peter Cawood for
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