ABSTRACT: Sequence Stratigraphy of Mississippian Peritidal and Shallow Marine Carbonates, Mission Canyon Formation, Williston Basin, North Dakota

WITTER, DAVID N., Shell Western Exploration and Production, Inc., Houston, TX, and KEITH W. SHANLEY, Shell Development Company, Box 481, Houston, TX

Upper Mission Canyon strata (Glenburn-Rival intervals) are composed of parasequences that define seaward-stepping, upward-thinning fourth-order sequences that in turn define the highstand portion of a third-order composite sequence. Sequence boundaries are recognized in the subsurface via prominent gamma-ray marker beds that reflect an increased potassium content that may be due to the concentration of eolian transported feldspars. Sequence boundaries underlying the Mohall, Bluell, Coteau, and Dale intervals are characterized by a basinward shift in facies tracts and a change from aggradational to highly progradational parasequence stacking patterns. These boundaries are interpreted to reflect a significant decrease in the rate of relative sea level rise. The sequence boundary underly ng the Sherwood is also represented by a basinward shift in facies tracts; however, incision in excess of 50 ft into underlying peritidal carbonates of the Mohall sequence suggest that a significant fall in relative sea level occurred. Sandstones that overlie the Sherwood sequence boundary were deposited within incised valleys during the subsequent rise in relative sea level.

Critical to resolving these high frequency stratigraphic relationships are the vertical succession of facies that define parasequence stacking patterns on a low-relief carbonate platform. Twenty facies deposited in open marine, restricted marine, and evaporite facies tracts have been recognized. Within the upward-shoaling succession of facies commonly found in parasequences, certain facies are found to substitute for one another owing to their bathymetric "equivalence." Skeletal packstones and crinoidal wackestones, for example, may substitute for each other within a shallowing-up succession of facies. Recognition of the substitution of facies within parasequences is critical and may be expressed in terms of relative probability

distribution curves. This allows facies changes caused by the migration of depositional environments, an autocyclic phenomena, to be more accurately distinguished from those changes caused by a basinward or landward shift in depositional environments due to regionally significant allocyclic events. An appreciation of these complex stratigraphic relationships is necessary for continued exploitation of reservoir and seal packages.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)