Monday, June 25, 2012

Minnesota Geology Monday - Karst

Carbonate rock, like limestone and dolostone, are the primary bedrock components of most of the southeastern corner of Minnesota.  These carbonate rocks in southeastern Minnesota are Ordovician in age (488 - 443 million years old).  Because the glacial drift is relatively thin, or nonexistent in this part of the state, these limestones and dolostones are at or near the surface.  Karst easily develop in ten counties in the area as the carbonates are dissolved by water.

The name karst comes from the German word Kras, a region in Yugoslavia.  Landscape features common to karst include caves, sinkholes, and disappearing streams, among others.  Fillmore County in southeastern Minnesota has more of these karst features (including over 10,000 sinkholes) than all of the other counties in the state combined.

There are numerous caves in the area that have formed as water has dissolved carbonate minerals over time.  Cave formations (commonly called speleotherms), like stalagmites, stalactities, and flowstone, radiometric dating has determined the ages of these structures to  between recent to 350,000 years old.  Two caves are open to the public via guided tours, Mystery Cave (http://www.dnr.state.mn.us/state_parks/forestville_mystery_cave/index.html) and Niagara Cave (http://www.niagaracave.com/default.asp).

Disappearing streams is another feature common to karst landscapes.  About a half mile downstream from the visitor center at Mystery cave is example of a dissappearing stream.  The South Branch of the Root River is flowing over fractured bedrock and dissappearing into Mystery Cave.  The picture below demonstrates the view upstream from a county bridge.




Looking downstream from the same bridge and the river is gone.  The river cuts approximately four miles off of its surface path by escaping through fractured carbonate rock and flowing through Mystery Cave.



The most common karst feature in Minnesota are sinkholes.  Sinkholes develop when overlying sediments collapes into cavities in carbonate rock.  Many sinkholes form above joints in the bedrock.  As demonstrated by the picture below, the city of Fountain claims to be 'The Sinkhole Capital of the U.S.A.'.



The easiest way to see sinkholes in this area is to drive the county roads past the local farm fields.  As sinkholes develop, farmers can no longer farm that location, so trees and other vegetation begin to grow out of the sinkhole.  The farmer continues to plant crops around the sinkhole.



A Google Earth image below demonstrates how many sinkholes have developed in the area over time.  The image is one local field and shows numerous sinkholes, the farmer continues to farm the area around the sinkholes.




Karst can be linked with pollution of ground and surface waters.  Because water quickly drains from the surface via fractures and joints in the bedrock, any chemicals or wastes are quickly transported to other locations.  The city of Lewiston discovered just how quickly water can drain away from the surface via a sinkhole.



The grown over pictures below show Lewiston's three sewage lagoons that are no longer in service.  On February 20, 1991, public works employees found a sinkhole had developed in lagoon number 2.  Over a period of a few hours to a day, and estimated 7.7 million gallons of partially-treated water drained into the groundwater system.