SEEPS AND SPRINGS AT A PLATTEVILLE “OBSERVATORY” ON THE RIVER BLUFFS

BJ Bonin Independent Contract Geologist, 284 Macalester Street, St. Paul, MN, 55105 USA [email protected]

Greg Brick Department of Earth Sciences, University of Minnesota, , MN, 55455 USA [email protected]

Julia Steenberg Minnesota Geological Survey, 2609 Territorial Road, St. Paul, MN, 55114 USA [email protected]

Abstract teville–Glenwood Shale contact. Overall, the hydro- Residential building construction along the Mississip- stratigraphic attributes of this site are consistent with pi River bluffs in the 1970s created a unique enclosed how the Platteville has been recently characterized in the outcrop of the Late Platteville Formation TCM area in a fractured bluff edge setting (Anderson et at Lilydale, Minnesota. This outcrop was examined in al., 2011). early 2013 after a newly-formed spring flooded an el- evator shaft the previous year, drawing attention to the The enclosed outcrop features many seep- and spring- foundation conditions. related mineral deposits. Most notable were the iron- stained flowstone and microgours near the seeps and The Lexington Riverside property is a six story condo- springs along fractures in the , and calcite rafts minium complex constructed within the top of the bluff. on the surfaces of the pools. At some damp locations a A two-level underground parking garage was built into fungal ecosystem has developed. Gypsum beards have the bluff. Bedrock was mechanically excavated to ac- grown in dry portions of the cavern. commodate the construction of the building, creating This man-made cavern, and others nearby, present an unweathered rock surface. The space between the unique opportunities to research groundwater flow in structure and the excavated rock face, running for 150 fractured bedrock settings. Studying the spring locations meters, was roofed over, and is used as a utility space. relative to joints and bedding, changes in spring flow At least three dominantly carbonate members of the rate over time, and mineral deposition rates, are possible Platteville Formation are visible: Mifflin, Hidden Falls, in this accessible location without the complication of and Magnolia, in ascending order. The foundation of the surface water inputs or instrumental interference from structure was constructed on the lowermost Platteville the general public. limestone and and is tile-drained to the nearby river gorge. References Anderson, JR, Runkel, AC, Tipping, RG, Barr, KDL, Most of the seeps and springs on the property, both in- and Alexander, EC, Jr. 2011. Hydrostratigraphy of side and on the grounds, belong to the three Platteville a fractured, urban aquitard. Geological Society of spring-lines identified for the Twin Cities Metropolitan America Field Guides, v. 24: 457-475. (TCM) area by Brick (1997). Groundwater emanates Brick, G. 1997. Along the Great Wall: Mapping the from both vertical joints and horizontal bedding plane Springs of the Twin Cities. Minnesota Ground partings within the Platteville Limestone and at the Plat- Water Association Newsletter 16(1): 1-7.

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