The Sandstone Karst of Pine County, Minnesota

The glaciated, forested landscape of central Pine County in east-central Minnesota contains a series of sinkholes, stream sinks, springs and caves. The features are formed in Precambrian Hinckley Sandstone and overlying unconsolidated glacial deposits. This is a sandstone karst. The features serve the same function as in carbonate karst terrains: sinkholes and caves focus recharge into a heterogeneous subterranean flow system that discharges into springs. The Hinckley Sandstone is a quartz arenite. No carbonate grains or cements have been found in sandstone samples from the sinkhole area, nor is there evidence that calcite solution controls bedrock permeability. Three parameters appear to control the distribution of sinkholes: depth to bedrock, type of underlying bedrock, and meter-scale heterogeneity of surface sediments. The permeability structure of the Hinckley Sandstone appears to be controlled by fractures and depositional features at centimeter to meter scale. Field mapping in the area has revealed 309 karst features: 237 sinkholes, 25 stream sinks, 32 springs and 15 caves. Recent LiDAR coverage indicates that there are many more sinkholes and other karst features than the original mapping was able to locate. Interpretation of the LiDAR images is challenging because karst processes, glacial processes and human activity have all produced natural and anthropogenic closed depressions of a variety of sizes and shapes in this landscape