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Local Facies Variability in the Mission Canyon Limestone, West Flank, Bighorn Mountains, Wyoming

VICE, MARI ANN, and JOHN E. UTGAARD, Southern Illinois University, Carbondale, IL

Comparison of two sections of the Mission Canyon Limestone (Madison Group, Mississippian) located approximately one-half mile (850-900 m) apart and at opposite ends of a single flatiron reveals significant lateral facies variations. The southern section, Dry Fork of Horse Creek, is 83 ft (25 m) thicker than the northern Horse Creek section. This substantial difference in thickness cannot be attributed solely to pre-Amsden erosion and solution collapse: all three members are thicker and more variable lithologically at Dry Fork.

The lower (Big Goose) member is composed mostly of cherty, dolomitized lime mudstones at both locations. At Dry Fork, it contains numerous skeletal facies, particularly in the upper part. Skeletal facies are insignificant at Horse Creek. The two upper members are composed mainly of skeletal limestones; however, grain-supported facies are much more abundant at Dry Fork. Dolomitized mudstones predominate in the major breccia at the base of the middle (Little Tongue) member at Horse Creek, and lime mudstones predominate at Dry Fork. Additional breccias occur at other horizons: five at Dry Fork, four at Horse Creek.

Conclusions drawn from the initial study of these two outcrops follow: (1) the local extent of grainstones and packstones suggests that the shoals were geographically less extensive than the subtidal muddy bottoms and intertidal-supratidal mud flats. (2) The limited geographic and vertical extent of some breccias suggests evaporite deposition in localized muddy facies. (3) Conditions favoring dolomitization were limited mainly to the mudstones and occurred during and shortly after deposition of the Big Goose Member.

 

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)