OUTCROP CHARACTERIZATION OF KARST DEVELOPMENT ON A SHALLOW CARBONATE PLATFORM: MISSISSIPPIAN MADISON FORMATION, BIGHORN BASIN, WY, USA

Although karsted carbonates are considered among the most attractive hydrocarbon reservoirs, their heterogeneous nature also makes them among the most complex. The karsted surface that caps the Mississippian Madison Formation is well-exposed in outcrops in the northeastern Bighorn Basin, WY, USA, providing an opportunity to examine lateral variation in key reservoir properties along a major karsted surface. Five closely spaced stratigraphic sections through extensive canyon exposures of the Mission Canyon Member (uppermost member of the Madison Formation) were studied to characterize lateral variation in karst development at the top of the unit. This member comprises eight lithofacies, arranged in a shallowing upward succession, which are interpreted to represent open to restricted platform interior settings. The unconformable surface is highly irregular and represents at least 10 million years of exposure and non-deposition. Karst features include large solution-enhanced fractures, vertical dissolution pipes, and cave systems. A laterally persistent stromatolite horizon, which occurs near the top of the Madison Limestone, provides a datum against which the depth of karsting can be measured. Pipes and caves, which measure as much as 12 m wide and 20-30 m deep, often contain a collapse breccia consisting of cobble-to-boulder-sized limestone blocks with a red, fine-grained sandstone matrix. Solution features are separated by unaffected peritidal facies. The topography of the unconformable surface is filled in by the Darwin Sandstone Member of the Amsden Formation, a fine-grained, cross-bedded reddened quartzite. This study provides an outcrop analog for understanding karsted reservoirs that will aid in the assessment of examples in the subsurface.