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Palaeomagnetism of Mesoproterozoic limestone and shale successions of some Purana basins in southern India
- MICHIEL O. DE KOCK, NICOLAS J. BEUKES, JOYDIP MUKHOPADHYAY
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- Journal:
- Geological Magazine / Volume 152 / Issue 4 / July 2015
- Published online by Cambridge University Press:
- 02 January 2015, pp. 728-750
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The ‘Purana’ basins were long considered Neoproterozoic basins until geochronology and palaeomagnestism showed parts of the Chattisgarth and lower Vindhyan basins to be a billion years older. Historically, the successions in the Chattisgarth Basin are correlated with similar successions in the Pranhita–Godavari and Indravati basins. In India, differentiating between early–late Mesoproterozoic rocks and those spanning the Mesoproterozoic–Neoproterozoic boundary is possible by comparing magnetic declination and inclination; palaeomagnetism is therefore a very useful correlation tool. Here we report a new Stenian-aged palaeopole (50.1°N, 67.4°E, radius of cone of 95% confidence A95 = 12.4°, precision K = 30.1) from carbonate and shale successions of the Pranhita–Godavari and Chattisgarth basins (the C+/– magnetization). In addition, an early diagenetic remagnetization (component A) was identified. No primary or early diagenetic magnetizations were identified from the Indravati Basin. Here, as well as in stratigraphically higher parts of the other two successions, widespread younger magnetic overprints were identified (B+ and B– magnetic components). Our C+/– palaeopole is constrained by palaeomagnetic stability field tests, is different from known 1.4 Ga and 1.0 Ga Indian palaeopoles, but similar to a 1.19 Ga palaeopole. Penganga Group (Pranhita–Godavari Basin) deposition was probably initiated at around 1.2 Ga. A similar palaeomagnetic signature confirms its correlation with the Raipur Group (Chattisgarth Basin), of which the deposition spans most of the Stenian period (c. 1.2–1.0 Ga). Sedimentation in these groups began significantly later than c. 1.4 and c. 1.6 Ga, as suggested by ages reported from below the Raipur and Penganga groups, respectively.
2 - Geological Evolution of the Proterozoic Earth
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- By Donald R. Lowe, Stanford University, Nicolas J. Beukes, Rand Afrikaans University, John P. Grotzinger, Massachusetts Institute of Technology, Raymond V. Ingersoll, University of California, Joseph L. Kirschvink, Institute of Technology, Cornelis Klein, University of New Mexico, Ian B. Lambert, Resource Assessment Commission, Australia, Ján Veizer, University of Ottawa
- Edited by J. William Schopf, University of California, Los Angeles, Cornelis Klein, University of New Mexico
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- Book:
- The Proterozoic Biosphere
- Published online:
- 04 April 2011
- Print publication:
- 26 June 1992, pp 43-80
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Summary
The Proterozoic Eon extends from 2500 to 550 Ma, from the close of the Archean to the beginning of the Cambrian. It includes three principal geochronologic divisions: Lower or Proterozoic I (2500 to 1600 Ma), Middle or Proterozoic II (1600 to 900 Ma), and Upper or Proterozoic III (900 to 550 Ma). These definitions are consistent with previous usage (Schopf 1983a) and with recommendations of the Subcommission on Precambrian Stratigraphy of the International Union of Geological Sciences (Plumb and James 1986). Although some criticism has been voiced at the use of absolute ages rather than stratigraphic or paleontologic events for subdividing Precambrian time (Cloud 1987), we find that the lack of well-developed, widespread, narrowly constrained, isochronous Precambrian biostratigraphic markers, equivalent to Phanerozoic faunal successions, presents an as yet insurmountable barrier to the establishment of globally useful Precambrian biostratigraphic subdivisions.
Systematic treatment of the geological evolution of the Proterozoic earth and similar long-term or large-scale aspects of Proterozoic history is complicated at present by our incomplete knowledge of existing Proterozoic rocks, selective preservation/obliteration of certain types of terranes, and uncertain geochronology and correlation. Many Proterozoic sequences remain essentially unstudied, especially in parts of central and northern Africa, South America, and Asia, and their stratigraphies, ages, and tectonic settings are unresolved. Information from these sequences is essential to evaluation of global patterns of Proterozoic geologic evolution, sediment recycling, and tectonics.
4 - Proterozoic Atmosphere and Ocean
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- By Cornelis Klein, University of New Mexico, Nicolas J. Beukes, Rand Afrikaans University, Heinrich D. Holland, Harvard University, James F. Kasting, Pennsylvania State University, Lee R. Kump, Pennsylvania State University, Donald R. Lowe, Stanford University
- Edited by J. William Schopf, University of California, Los Angeles, Cornelis Klein, University of New Mexico
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- Book:
- The Proterozoic Biosphere
- Published online:
- 04 April 2011
- Print publication:
- 26 June 1992, pp 135-174
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Summary
In this Chapter an overview is developed of aspects of the Proterozoic atmosphere and oceans based as much as possible on geologic evidence, but supplemented by theory, whenever such evidence is indirect, incomplete, or lacking. Much of the theoretical treatment is rather oversimplified and speculative. Several biologically important aspects of the Proterozoic environment are addressed, namely, the partial pressures of oxygen and carbon dioxide in the atmosphere and possible changes in their partial pressures as a function of Precambrian time. Aspects of the chemistry and evolution of the Proterozoic ocean are discussed as well.
Banded iron-formations (BIFs) are the most abundant chemical sediments found throughout much of Precambrian time. Because they are generally devoid of clastic components, their chemistry, their oxidation state, and their temporal distribution provide important clues about the chemistry and the chemical evolution of the Precambrian ocean and atmosphere. Section 4.2 provides a synopsis of the average major element chemistry of banded iron-formations throughout the Precambrian; all iron-formations older than about 1.9 Ga represent very similar chemical systems. Iron-formations formed between 0.8 and 0.6 Ga are distinctly different and are more highly oxidized. Few iron-formations are younger than about 1.8 Ga; a minor resurgence in BIF deposition occurred between 0.8 and 0.6 Ga. After about 1.85 Ga, the atmosphere and oceans became rather highly oxygenated and the oceans as a whole became depleted in iron.