THE SANDSTONE KARST OF PINE COUNTY, MINNESOTA

Central Pine County, located in east central Minnesota, has approximately 250 mapped karst sinkholes. The sinkholes feed a complex groundwater flow system, which in turn feeds a number of springs. This system is characterized not only by the high matrix porosity of a well-sorted arenite, but also by fracture flow in visibly enlarged joints and conduit flow along some bedding planes.

Sinkholes occur over the Hinckley Sandstone, where Superior Lobe glacial sediments dominate the landscape. Bedrock is a major control on the distribution on sinkholes. All sinkholes lie northwest of the Hinckley Fault, where thick Hinckley Sandstone has well-developed fracture systems. Southeast of this fault, the Hinckley is a thin layer above the less pure Fon du Lac Sandstone, where fractures are not as well developed. The type of preexisting fracture network is important to karst development. Fracture control on sinkhole formation is supported by the spatial distribution of sinkholes and caves, as well as the location of springs. Development of sinkholes is also controlled by shallow depth to bedrock and heterogeneity in the glacial sediments.

There are a number of springs in this area, which display a bimodal distribution of waters. One group is well oxygenated, often with nitrate and coliform bacteria and has short tritium residence times. The second group has long tritium and 14C residence times, is anoxic, and has high dissolved iron. The former group of waters resurges in clear springs, while the latter group is characterized by bright orange springs, due to the presence of iron-oxidizing bacteria.