Examples of Hypogenic Karst Collapse Structures, Twin Cities Metropolitan Area, Minnesota
One old feature is in eastern Minneapolis near the Mississippi River gorge. This closed depression in the land surface was incorporated into a park when Minneapolis was platted. This depression extends through the subcropping Platteville Formation to a large void in the underlying St. Peter Sandstone. A 250 meter long cave in the St. Peter extends between the collapse and the Mississippi River with typical passages 15 meters wide and 6 meters high.
The second and third collapses developed recently adjacent to river valleys buried under glacial sediments. The second collapse occurred in 1989 in Mahtomedi, MN. A hole 12.8 m wide and 11 m deep opened suddenly. The third collapse occurred in 2005 in Woodbury, MN in a newly constructed infiltration pond excavated into the subcropping St. Peter. The collapse event created several sinkholes on the sides and bottom of the pond. Excavation of three of twelve sinkholes revealed a major collapse structure comprised of several breccia pipes. These pipes are at least 10 to 15 m in diameter and extend up through almost 20 m of friable St. Peter from the underlying Prairie du Chien Group.
Recent investigations of the Prairie du Chien have found extensive solution enlargement features on a regional scale; these features have several characteristics diagnostic of hypogenic speleogenesis. The possibility of other breccia pipes in the St. Peter within the metropolitan area is problematic.