PALEO LANDSLIDES IN SE MN KARST LANDS

Paleo landslides/bluff collapses are evident in Shaded Relief DEM LiDAR images of Southeast Minnesota’s Karst Lands. The features occur in bluffs along incised bedrock meanders along the Root, Zumbro and Cannon Rivers. The meanders have eroded down through the Upper Ordovician Galena Group carbonates into or through the underlying Decorah Shale. The landslides formed where the rivers have eroded the bases of the valley walls to form near vertical bluffs more than about 30 m high. The landslide scarps range from 100 m to over 1.4 km long subparallel to the rivers. The collapses are linear to concave curvilinear toward the rivers in plan view. Most formed along the outside walls of incised meander loops. The slide movement was apparently downward into the rivers only 50 to 200 m from the head-wall scarps. The collapsed areas are longer along the river than they moved downward toward the rivers. In the LiDAR images the landslides appear as broken, chaotic areas, often with river parallel trenches compared to the relatively smoothly sloping adjacent areas along the valley walls. Dense sinkhole arrays occur on the ridge tops above some, but not all, of the landslides. Only a few of the landslides have been field checked, but most of those checked have springs or seeps emerging from the faces of the landslides. Water lubrication of sliding on the Decorah Shale is apparently involved in the movement of landslides.

The landslides clearly predate the 1850s conversion of SE MN to farm land by European immigrants. The landslides are typically covered by well-developed forests. The toes of many of the landslides are being actively eroded by the current river flows. These landslides are older than a few hundred years, are probably thousands of years old and may have formed at the end of the Pleistocene. Visual scanning of the LiDAR images identified 36 landslides in Fillmore County, 23 in Olmsted County, 3 in Goodhue County and 1 in Dodge County. The landslide locations are recorded as “landslides” in the “X, miscellaneous features” class in the electronically accessible Minnesota Karst Features Database (KFD).

https://gisdata.mn.gov/dataset/geos-karst-feature-inventory-pts