Silcrete in the Uppermost Cambrian and Lower Ordovician of the Wisconsin Arch and Michigan Basin--Implications for Subaerial Exposure

SMITH, GEORGE L., University of Wisconsin, Madison, WI

Silcrete within uppermost Cambrian strata and the Lower Ordovician Prairie du Chien Group delineates the basinward extent of subaerial exposure during three relative sea-level lowstands that exposed portions of the Wisconsin arch and Michigan basin before, during, and after Prairie du Chien deposition. Shallow-water Prairie du Chien carbonates and Cambrian sandstones were deposited in tropical, epeiric seas during relative sea-level highstands.

Fabrics identical to Cenozoic silcretes consist of cryptocrystalline microquartz (chert), microquartz (<0.02 mm), megaquartz (>0.02 mm), and length-slow and length-fast chalcedony. Silicified crusts of oolitic grainstones and/or anhydrite nodules typically coat uppermost Cambrian and Prairie du Chien unconformities. Silicification associated with intra- and post-Prairie du Chien unconformities is generally restricted to within 5 m of the overlying unconformity. Highly

permeable lithologies, such as oolitic grainstones, boundstones, and paleokarst breccias, were preferentially silicified during intra- and post-Prairie du Chien exposure. Reworked silcrete clasts typically overlie unconformities. Silcrete is a practical exposure indicator as it is easily identified in outcrop and well cuttings.

Silcrete within uppermost Cambrian strata is generally restricted to the Wisconsin arch axis, but occurs as far east as western Michigan. Silcrete below the intra-Prairie du Chien unconformity indicates subaerial exposure of the entire arch and basin. In contrast, silcrete below the post-Prairie du Chien, pre-St. Peter unconformity indicates that exposure was restricted to the Wisconsin arch and western margin of the Michigan basin in eastern Wisconsin. Relatively rapid basin subsidence may have limited the geographic extent of post-Prairie du Chien, pre-St. Peter exposure.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)