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Controls on Fabric Development and Morphology of Tufas and Stromatolites, Uppermost Pethei Group (1.8 GA), Great Slave Lake, Northwest Canada

By
Michael C. Pope
Michael C. Pope
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.
*

Present Address: Department of Geology, Washington State University, Pullman, WA, 99164, U.S.A.

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John P. Grotzinger
John P. Grotzinger
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.
*

Present Address: Department of Geology, Washington State University, Pullman, WA, 99164, U.S.A.

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Published:
January 01, 2000

Abstract

A unique lufa and stromatolite succession, represented by the uppermost 10 m of the 1.8 Ga Hearne Formation (Pethei Group), northern Canada, developed across a large carbonate platform during a transition from normal marine to evaporitic conditions. In ascending order, the facies that document this transition consist of den-dritically branching tufa, irregularly laminated flat to domal stromatolites, and even, isopachously laminated domal stromatolites. The morphologies and textures of these tufas and stromatolites are similar to structures produced in heavily mineralized depositional environments (e.g., hot-spring and hypersaline depositional systems). Comparison with structures produced in the mineralizing systems, as well as with laboratory experiments of biological growth and abiotic mineral precipitation, provide insight into the mechanistic processes that contributed to development of the unusual facies of the uppermost Hearne Formation.

This comparison suggests that the Hearne tufa and stromatolites were formed by biotic and abiotic processes whose influence on morphology fluctuated during the deposition of these facies. The key to understanding the dominant role of abiotic processes in development of these unusual carbonate fabrics lies in recognizing that these features formed during a transition from normal marine to evaporite conditions when seawater became warmer, increasingly saline, and more conducive to in situ mineralization. The tufa facies and domal, isopachously laminated stromatolite facies are both considered to have resulted from abiotic precipitation of carbonate mud induced by progressive oversaturation of seawater associated with increasing temperature and salinity during restriction of the Pethei basin. These facies are not observed in normal marine carbonates of this age and younger, and so the presence of such extreme environmental conditions are considered essential for the development of this facies. The generic growth mechanism of diffusion-limited aggregation (or similar depositional process) is invoked here to account for growth of micritic, dendritically branching tufa as a dominantly abiotic process. Similarly, domal stromatolites with even, isopachous laminae and evidence for surface-normal growth may have been produced mainly by abiotic mineral precipitation of micrite cement at the sediment-water interface. Whether or not micrite precipitation was kinetically aided by the presence of microbes remains uncertain, because there is no preserved evidence of such structures. However, the characteristically irregular lamination of the flat to domal stromatolites is most consistent with the former presence of discontinuous microbial mats, which would have trapped and bound loose sediment. Abundant precipitation is not indicated in this facies, because no calcified sheaths are preserved.

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Contents

SEPM Special Publication

Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World

John P. Grotzinger
John P. Grotzinger
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Noel P. James
Noel P. James
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SEPM Society for Sedimentary Geology
Volume
67
ISBN electronic:
9781565761896
Publication date:
January 01, 2000

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