T163 The Evolution of Karst Landscapes through Time in Response to Changing Hydrologic, Geomorphic, and Tectonic Conditions (Posters)

Tuesday, 6 November 2012
9:00 AM - 6:00 PM
Hall B

MAPPING KARST SPRINGSHEDS IN FILLMORE COUNTY, MINNESOTA: INCREASINGLY NUANCED INTERPRETATIONS

Paper number: T163-7
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Fillmore County, Minnesota contains more karst features (sinkholes, caves, sinking streams and rivers and large karst springs) than there are in the rest of the Minnesota Counties combined. Springshed, karst feature, and cave mapping projects have been conducted in the Ordovician Galena Group carbonates in Fillmore County since the 1970s. The mapping projects have been prompted by growing evidence of anthropogenic contamination of the karst aquifer and the springs that are the source waters of trout streams. The main tool of the tracing work remains fluorescent dye tracing – supplemented by chemical, isotopic, and thermal tracing work. Recently, preliminary results of new, more accurate stratigraphic and structural contour mapping work by the Minnesota Geological Survey staff (Tony Runkel, private communication, 2012) are allowing an increasingly nuanced understanding of karst groundwater flow directions and spring locations.

Fillmore County is experiencing a very dry summer and spring flows are at or near record low levels. Two ongoing dye traces are providing new information on area springsheds. The Wykoff NW traces are in a previously untraced area. Rhodamine WT and eosin were injected into separate sinkholes about 0.75 km apart on a low-relief upland between Spring Valley Creek and Carter Creek. The Rhodamine WT traveled ~2 km southwest to Erdman’s Pasture Spring in hours to a few days. The eosin has not yet been detected in any of the monitoring points.

The South Branch of the Root River (SBRR) is the largest sinking river in Minnesota. The SBRR sinks starting at Mystery cave, which acts as a subsurface cutoff of two entrenched bedrock meanders. The SBRR is totally dry for several miles under most flow conditions and then rises in the Seven/Crayfish/Saxifrage Springs complex to form the head of the trout stream portion of the SBRR. The resurgent spring complex is near the middle of the Galena Group – well above the Decorah Shale regional aquitard – allowing the hypothesis that the SBRR might again lose water to bedrock meander cutoffs. Fluorescein and eosin have been injected into the SBRR downstream of the resurgent springs. Those dyes were found in Meyers Spring further downstream near the bottom of the Cummingsville Formation above the Decorah Shale.