Mari A. Vice1, Richard H. Fifarek2
(1) University of Wisconsin-Platteville, Platteville, WI
(2) Southern Illinois University, Carbondale, IL
ABSTRACT: Dolomitization of The Mississippian Mission Canyon Formation, Northern Bighorn Basin, Montana And Wyoming: Petrographic And Isotopic Constraints
Four distinct dolomites are identified in the Mission Canyon Formation based on their petrographic, stable isotopic and geochemical characteristics. The dolomites are: I) finely to medium crystalline, subhedral to anhedral mosaics which replace lime mudstones (dolomitized lime mudstones); II) more coarsely crystalline, euhedral to subhedral mosaics (crystalline dolostones); III) minor medium to coarsely crystalline, euhedral to subhedral mosaics which replace ooid grainstones; and IV) dolospar cement. Dolostones I and II are typically intercalated. Dolomitization is localized in the upper part of the formation but increases with depth and is most extensive in the lowest (Big Goose) member.
Dolostone I has higher average Na (466 ppm) and S (288 ppm) contents and heavier isotopic values (d13C 1.89o/oo;d18O -0.72o/oo) than dolostone II (263 ppm; 375 ppm; d13C=1.23o/oo;d18O=-4.27o/oo). All dolostone types and the dolospar cement have non-stoichiometric Ca and Mg concentrations; Fe (310 to 2710 ppm) was detected in about half the samples analyzed. These data suggest dolostone I formed primarily from evaporated marine fluids, particularly during and shortly after Big Goose deposition. Dolostone II formed later in the subsurface by recrystallization from mixed marine-meteoric waters.
Dolospar cement consisting of saddle-shaped rhombohedra is present in breccia cavities in the top (Bull Ridge) member and in some lower dolomitized intervals. It exhibits non-undulatory extinction and a dark red cathodoluminescent zonation that coincides with variations in Fe content (280 to 8650 ppm). Dolospar geochemistry and isotopic values (d13C=0.64o/oo; d18O=-10.40o/oo) suggest precipitation from basinal fluids.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado