Controls on Fabric Development and Morphology of Tufas and Stromatolites, Uppermost Pethei Group (1.8 GA), Great Slave Lake, Northwest Canada
Present Address: Department of Geology, Washington State University, Pullman, WA, 99164, U.S.A.
Michael C. Pope, John P. Grotzinger, 2000. "Controls on Fabric Development and Morphology of Tufas and Stromatolites, Uppermost Pethei Group (1.8 GA), Great Slave Lake, Northwest Canada", Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World, John P. Grotzinger, Noel P. James
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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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Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World - Precambrian carbonates are usually regarded at the simple cousins of the sedimentary realm, composed of stromatolites and dolostones, texturally not challenging and commonly altered beyond recognition by the vagaries of time, diagenesis and metamorphism. However, these carbonates that formed deep in time are commonly exquisitely preserved and contain within them a record of the evolving young earth. SEPM Special Publication 67 explores these aspects. Resulting from a 1997 SEPM/CSPG symposium entitled? Precambrian Carbonates,? these 18 papers demonstrate the importance of understanding these rocks, since within them is contained a record of the early ocean, atmosphere, and biosphere.