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Sedimentation and Early Diagenesis on the Mississippian Mission Canyon Platform in the Northern Bighorn Basin Region
Eleven microfacies in the Mission Canyon Limestone are grouped into three major facies associations: tidal flat, lower intertidal-subtidal, and platform-interior shoal. Widespread dolomitized tidal-flat deposits characterize the Big Goose Member of the Mission Canyon Limestone in the Big Horn Mountains, Wyoming. Correlative deposits in the Beartooth Mountains are thin grainstones separated by thick intervals of peritidal dolomitized and lime mudstones. Widespread emergence probably occurred early during deposition of the Bull Ridge Member of the Mission Canyon Limestone. This is suggested by the presence of evaporite solution-collapse breccias near the base of the Bull Ridge Member as far northwest as the north-central Beartooth Mountains and by the occurrence of siliciclastic materials (windblown continental detritus?) in the Bull Ridge Member in Big Horn Mountains and at Elk Basin. The greater abundance of subtidal and shoal deposits and smaller, discrete tidalflat deposits in all sections suggest a return to more open marine conditions during deposition of the upper Little Tongue. Extensive tidal-flat deposition and development of a second major breccia in the lower part of the Bull Ridge Member was followed by a return to open marine deposition in the upper part of this member.
Fourteen transgressive-regressive cycles are recognized in the Mission Canyon Limestone in the north-central Beartooth Mountains. Several episodes of mixed-water dolomitization of grainstone shoals, formation of evaporite solution collapse breccias, and recrystallization of sabkha dolomites are recognized. Their stratigraphic positions suggest that they formed during periods of exposure. As many as five episodes of widespread flooding of the platform (relative sea level rise) and five episodes of relative sea level fall are suggested by the juxtapositions of platform-interior shoal deposits and tidal-flat facies.
Post-Madison emergence produced a widespread karsted surface, but not all of the breccias in the Mission Canyon were developed during this episode.
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