Abstract: Mississippian Subaerial Exposure Surfaces within the Sherwood Subinterval, Mission Canyon Formation, Lucky Mound Field, North Dakota

Randolph B. Burke

Subaerial exposure surfaces seen in cores record a wide range of sedimentary characteristics that are highly variable geographically. The best developed exposure surfaces are interpreted to have developed on islands. These surfaces include decimeter scale solution channels and vertical fractures lined with laminated micritic cements and containing geopetal sediment. Vadose pisolites with meniscus and gravity cements extend downward for decimeters. Less developed surfaces are recognized by micritization of grains commonly associated with oxidized reddish coloration and containing black coated pisoids.

Porosity associated with subaerial exposure surfaces has been locally enhanced in some areas and occluded elsewhere. Some secondary porosity formed by vadose processes of dissolution, results in solution collapse breccia and enlargement of interparticle voids. Dissolution void size ranges from millimeter-scale vugs to decimeter cavernous where core ends have coarse spar calcite coatings. Destruction of porosity results from cementation within the vadose and phreatic zones. Vadose cements are micritic, occurring as layers of very thin laminations. Phreatic cements are primarily anhydrite and calcite with lesser dolomite and celestite. The difference in the magnitude and type of porosity between the upper and lower Sherwood shows that the subaerial exposure surface not only contributed o the heterogeneity of the reservoir, but also may partition the reservoir.

The best developed subaerial surface here defines the boundary between the upper and lower Sherwood. This boundary is recognized on geophysical logs around the margins of the Williston Basin, but the character of the geophysical response is highly variable. Variability in log response probably reflects the highly variable exposure surface.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994