|
The Rock Elm Structure (RES) is a geologically anomalous, nearly circular region 6.3 km. in diameter in west-central Wisconsin, USA. It is interpreted as formed by an extraterrestrial impact during the middle Ordovician 430-505 Ma. The RES consists of a ring boundary discontinuity, a central uplift with Cambrian rocks raised over 300 meters from their normal stratigraphic position, deformed blocks of lower Ordovician dolostones, and a post-impact ring-basin filled with lower to middle Ordovician strata. Quartz grains from central uplift rocks contain two types of micro-structures identical to known planar fractures typical of shock metamorphism. These allowed tentative estimates of shock pressures of at least 5 and perhaps greater than 10 GPa. (French et. al. 2004). The basin fill consists of two units not found outside the RES: the Rock Elm shale and overlying Washington Road sandstone. An angular unconformity separates them from underlying deformed Cambro-Ordovician rocks. The Rock Elm shale is a minimum of 35 meters thick and consists of non-calcareous kaolinitic shale interbedded with silty feldspathic wacke. The conformable Washington Road sandstone is at least 15 meters thick and consists of fine to medium grained feldspathic wacke.
Archeogastropods and conodont fossils narrow the basin fill's age to 430 - 470 Ma and establish a marine origin. Both gold and diamonds are found in the alluvium of streams draining the RES. During placer gold mining in the late 1880's, at least 10 small diamonds, one 2 kt. in size, were found. (Kunz, 1891). In 1987 Ex-Min explored the area for its diamond potential, but abandoned the effort after finding no bedrock sources. In 1983-1985 Superior Oil explored the area for gold, but found no bedrock gold mineralization. A SEM study showed that the placer gold was spongy with silver-depleted rims, suggesting that the gold had traveled a substantial distance, hence was likely derived from glacial till. The heavy minerals such as gold and diamonds were preferentially trapped in the RES because the interlayered sandstone and shale of ring basin fill created natural riffles in the area's riverbeds. The RES is relevant to the local community in many ways. The Washington Road sandstone has been quarried for flagstone. The Rock Elm shale has potential for ceramics. The shale weathers to a heavy, clay-rich soil that has restricted land use to grazing and forestry and presents slope stability problems. The quality of water from the shale is marginal. As the region suburbanizes, bedrock distribution must be included in land use decisions. Good outcrops of the deformed rock are present in a county park and provide a draw for geo-tourism. Local university geology classes routinely use these outcrops as teaching labs. Recreational panners travel to the area to look for placer gold and diamonds. The RES is featured in a popular geology CD tour for the county developed by the local tourism bureau.
|
