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VICE, MARI A., University of Wisconsin-Platteville, Platteville, WI; and RICHARD H. FIFAREK, Southern Illinois University, Carbondale, IL

Abstract: Cementation History of the Mission Canyon Formation, Montana and Wyoming

Petrography of samples from the Mission Canyon Formation from the northern Bighorn Basin region of southern Montana and northern Wyoming reveals a complex diagenetic history that extended from deposition in the Mississippian to the present. Calcite cements predominate and several stages of calcite cementation are observed. Chert, dolospar and kaolinite cements are also present. Preliminary isotopic and minor element data suggest fluid types that are consistent with the proposed paragenetic sequence.

Early diagenesis started soon after deposition and included precipitation of several calcite cements. In addition, silicification was common, affecting both micritic and grain-supported microfacies. Lime mud-supported microfacies were preferentially dolomitized. Most of the evaporite solution-collapse breccias formed during this stage.

At least three cements precipitated during burial. Dolospar cements lined breccia pores in the upper member and occluded some porosity in the dolomitized facies in the basal member. Iron-rich calcite spars precipitated in open pores and coated the dolospars. Coarse anhydrite cements precipitated but are observed only in the subsurface.

Laramide uplift produced joints along which coarse calcite crystals precipitated when surface exposures were subjected to meteoric waters. Dissolution produced vuggy porosity in which botryoidal calcite and coarse dog-tooth and rhombic calcites precipitated. Chalcedonic and kaolinite cements precipitated in some of the pores, and thin crusts of chalcedony, carbonate and gypsum formed on some outcrop surfaces. These later cements are clearly post-Laramide and may be modern in age.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana