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Diagenesis and dolomitization of the Ouray Formation, San Juan Mountains, Colorado
Carver, Franki J.
Carver, Franki J.
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2014
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2014
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Abstract
The Devonian/Mississippian Ouray Formation covers an area that includes Utah, Colorado, New Mexico and Arizona. It extends from the subsurface of the Paradox and San Juan Basins to become minimally exposed along the uplifted portions of the southern and western flanks of the San Juan Mountains in southwestern Colorado. The Ouray Formation is observed to have an abundance of dolomite in exposed outcrops that also extends to the subsurface. The predominantly dolomitic formation outcrops at Molas Lake have a localized pod of limestone with overlying silicified dolomite within the same section. Observations in other regional outcrops also demonstrate a similar limestone-dolomite relationship in various isolated sections within the formation. In addition to early pervasive dolomitization, late hydrothermal dolomitization also affected this formation. The likely source for late-stage alteration is fluids generated and circulated during Eocene volcanic activity of the San Juan Mountains, nearly 300 million years after Ouray Fm. deposition. The goal of this thesis was to investigate this variability in diagenesis and dolomitization within the Ouray Formation. Timing of these events is critical; dolomitization was both early (essentially syndepositional) and late. Field work and lab work consisted of measured sections, correlations, petrographic analysis, [delta superscript 18]O and [delta superscript 13]C isotope analysis, and Paradox Basin core inspection. The results of the data give strong evidence for a shallow carbonate platform depositional setting with meter-scale cycles of shallow subtidal to peritidal strata. The common appearance of silica-replaced evaporites indicates an arid depositional environment. Massive early dolomitization resulted from reflux of hypersaline brines in the near-surface and shallow burial realms. Additional diagenetic responses are to deep burial and late hydrothermal fluids of the San Juan volcanoes.
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