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ABSTRACT

This one-day trip provides an overview of the hydrostratigraphic attributes of the Platteville aquitard in the Twin Cities Metropolitan area. As a shallowly buried, extensively fractured carbonate rock in an urban setting, vulnerable to contaminants, the Platteville has been the subject of a wide variety of geomechanical and hydrogeologic studies over the past few decades. This work, combined with our own borehole geophysics and outcrop observations, has led to a more comprehensive understanding of the Platteville. The field trip will provide examples of what we have learned from these many different data sources, which collectively lead to a characterization of the Platteville as a complex “hybrid” hydrogeologic unit. Under certain conditions, and from one perspective, it can serve as an important aquitard that limits vertical flow, whereas in other conditions, and from another perspective, it is best considered a karstic aquifer with bedding-plane parallel conduits of very high hydraulic conductivity that permit rapid flow of large volumes of water. One particular focus of the trip will be demonstration of what appears to be predictability in both vertical and bedding-plane fracture patterns that in turn provides some degree of predictability of flow paths in three dimensions. These relationships appear to be operative for the Platteville in other parts of the Upper Midwest where the Platteville is shallowly buried. We will demonstrate that effective management of such complex, karst, “hybrid,” hydrogeologic units requires a sophisticated, nuanced understanding of their heterogeneous behavior.

Introduction

The Paleozoic bedrock of the Twin Cities Metropolitan (TCM) area of Minnesota, USA, provides over one-half of the drinking water for its 3 million citizens. Increasing demand for groundwater and concerns about contamination of deep aquifers have led to a number of studies over the past 20 years that have significantly improved our understanding of this aquifer system, resulting in redefinition of the classic divisions of the section into regional aquifers and aquitards on the basis of hydrostratigraphic attributes (Runkel et al., 2003, 2006; Tipping et al., 2006). Strong emphasis has been placed on the stratigraphic context of macropores in an effort to improve the predictability of fracture-dominated flow.

Although aquifers in the bedrock system are now relatively well characterized, our understanding of the intervening aquitards remains poor, a non-provincial problem in hydrogeology (Bradbury et al., 2006). Methods for characterizing their physical properties are not well developed, especially the acquisition of data that provide insight into vertically oriented features. Limitations to characterizing vertical fractures are perhaps the firstorder problem that has hindered our ability to evaluate the integrity of aquitards.

This field trip provides an overview of our ongoing research on the Late Ordovician Platteville Formation in the central part of the TCM area (Figs. 1 and 2), where it is traditionally considered the middle part of the Decorah-Platteville-Glenwood Aquitard (Kanivetsky, 1978). Here, the Platteville is a shallowly buried (<30 m, 100 ft) carbonate-dominated formation that occupies a position near the top of the Paleozoic bedrock sequence of the Twin Cities basin, a broad regional depression developed in the northernmost major preserved extent of Paleozoic bedrock in the Upper Mississippi Valley region (Mossler, 1972). Anticlines, synclines, and faults in this basin are attributed to reactivation of structures from the Mesoproterozoic Midcontinent Rift. Strata were deposited in a broad, shallow, near-equatorial epeiric ramp referred to as the Upper Mississippi Valley epeiric ramp and consist mostly of sandstone, siltstone, carbonate, and shale.

Our research integrates a variety of data sets for the purpose of constructing a conceptual model (Fig. 3) that improves predictability of groundwater flow in the Platteville Formation. As a shallowly buried, fractured bedrock layer in an urban setting, the Platteville is contaminated at a large number of sites that have been comprehensively studied to develop remedial strategies. Subsurface hydrogeologic information from these sites includes results from discrete-interval packer tests, borehole geophysics (including flowmeter), larger scale aquifer tests, potentiometric surface mapping, and dye traces. The Platteville Formation also commonly plays an important role in urban engineering projects, and data acquired from underground excavations that expose bedding plane views of vertical fractures and quantify leakage through them are used to provide insight into a third dimension. Our own research has also added outcrop context, including characterizing the initiation and termination of vertical fractures in a mechanical stratigraphic analysis and providing stratigraphic context for the position of active conduits (i.e., springs).

Our results thus far on this and other Paleozoic bedrock units traditionally regarded as aquitards in this region indicate that they are best considered “hybrid” hydrogeologic units. Even intervals of strata a few meters thick can significantly limit vertical flow, serving to protect underlying groundwater from contamination, but they can also contain bedding plane conduit(s) of very high hydraulic conductivity with the potential to accommodate rapid flow of large volumes of contaminants. We informally refer to such bedrock layers as “aquitardifers.” However, despite this complex heterogeneity, we show that with sufficient data there appears to be predictability in both vertical and bedding plane fractures in relatively undeformed, layered sedimentary rock, which in turn provides some degree of predictability of flow paths in three dimensions.

Trip Summary

This field trip focuses on the hydrostratigraphy of the Platteville Formation, specifically the fracture attributes that dominate its groundwater flow system. The trip includes nine stops along the Mississippi River and distal parts of its tributaries (Fig. 1). Natural exposures of the Platteville at each of the first eight stops are used to illustrate the fundamental stratigraphy of the formation, its material properties, and vertical and bedding-plane fracture patterns, integrated in a mechanical stratigraphic analysis. The final stop, at the Minnesota Library Access Center (MLAC) underground archive building, will include a summary of our current conceptual model of the Platteville based on our observations at previous stops, combined with information gleaned from the construction and remediation at the library.

Field Trip Stops

Stop 1. Shadow Falls: Ordovician Formations of the Twin Cities Metropolitan Area

Location: Stop 1 is situated at the west end of Summit Avenue at the intersection with East River Road. UTM coordinates of Stop 1 parking area: 484,368 E/4,976,418 N. TRS location: T. 28 N, R. 23 W, sec. 5, SE, NW. All UTM coordinates are given in North American Datum 83, Zone 15.

Directions: From Minneapolis, drive east on Interstate 94. Exit on Vandalia Street/Cretin Avenue (exit 237); turn right on Cretin Avenue. Turn right on Mississippi River Boulevard and take an immediate left to stay on Mississippi River Boulevard. Turn right into the Shadow Falls parking area at Summit Avenue.

Description: This stop provides an excellent and accessible view of the Late Ordovician succession of sandstone, shale, and carbonate strata in the TCM area, including the upper St. Peter Sandstone, Glenwood Formation, Platteville Formation, and lower Decorah Shale (Fig. 2). The well-known medium- to finegrained, well-sorted, white, mature quartz sandstone of the St. Peter forms the lowest part of the exposure. The greenish-gray, phosphatic, condensed shale of the Glenwood Formation lies stratigraphically between the St. Peter Sandstone below and the carbonate-dominated Platteville Formation above and is generally 0.6–1.2 m (2–4 ft) thick.

Figure 1.

Map of the Twin Cities Metropolitan area, showing springs, boreholes with flow logs, vertical fracture traces, dye traces, major contamination sites in the area with extensive hydrogeologic data, and the Paleozoic bedrock geology. The field trip route and stops are also shown. The inset illustration shows the general outline of the Twin Cities basin.

Figure 1.

Map of the Twin Cities Metropolitan area, showing springs, boreholes with flow logs, vertical fracture traces, dye traces, major contamination sites in the area with extensive hydrogeologic data, and the Paleozoic bedrock geology. The field trip route and stops are also shown. The inset illustration shows the general outline of the Twin Cities basin.

Figure 2.

Generalized stratigraphic column, depicting the lithology, thickness, and nomenclature for the Upper Ordovician formations in the Twin Cities Metropolitan area. The spring count lists the number of springs emanating from a specific stratigraphic interval. The composite natural gamma log (units in API-G) is the log shown on the left alongside the log constructed from a handheld gamma-ray scintillometer (measurements in counts per second) shown on the right.

Figure 2.

Generalized stratigraphic column, depicting the lithology, thickness, and nomenclature for the Upper Ordovician formations in the Twin Cities Metropolitan area. The spring count lists the number of springs emanating from a specific stratigraphic interval. The composite natural gamma log (units in API-G) is the log shown on the left alongside the log constructed from a handheld gamma-ray scintillometer (measurements in counts per second) shown on the right.

Figure 3.

Conceptual model of hydrogeologic attributes of the Platteville Formation and adjacent units. All parts of the formation are known, at least locally, to have moderate to very high horizontal hydraulic conductivity. Preferential termination of vertical fractures results in one or more aquitards in the Hidden Falls Member that commonly perch a water body in the overlying heavily fractured Magnolia Member. The combined Hidden Falls, Mifflin, and Pecatonica Members, despite yielding water horizontally and having vertical fractures, serve as a significant vertical barrier to flow in areas several tens of meters away from eroded edges of the formation. Closer to eroded edges, aquitard integrity is substantially diminished. See text (especially for Stop 9) for additional discussion, including horizontal and vertical hydraulic conductivity and sources of information compiled to support the model.

Figure 3.

Conceptual model of hydrogeologic attributes of the Platteville Formation and adjacent units. All parts of the formation are known, at least locally, to have moderate to very high horizontal hydraulic conductivity. Preferential termination of vertical fractures results in one or more aquitards in the Hidden Falls Member that commonly perch a water body in the overlying heavily fractured Magnolia Member. The combined Hidden Falls, Mifflin, and Pecatonica Members, despite yielding water horizontally and having vertical fractures, serve as a significant vertical barrier to flow in areas several tens of meters away from eroded edges of the formation. Closer to eroded edges, aquitard integrity is substantially diminished. See text (especially for Stop 9) for additional discussion, including horizontal and vertical hydraulic conductivity and sources of information compiled to support the model.

In the Twin Cities, the Platteville Formation ranges from ~7.0–8.8 m (26–29 ft) in thickness and is subdivided into four members, from bottom to top: Pecatonica, Mifflin, Hidden Falls, and Magnolia (Mossler, 2008). These are distinguished mainly by lithology and bedding style and correspond to major depositional facies of the formation (Fig. 2). The Pecatonica Member is a thin bed of burrowed, reworked, fossiliferous dolostone only 0.3–0.6 m (1–2 ft) thick. It commonly contains quartz sand, phosphate clasts and bored hardgrounds (centimeter-sized discontinuous surfaces of synsedimentary lithification commonly bored into by animals on the seafloor). The hardgrounds are regionally traceable bedding planes. One is present at the contact with the Mifflin Member, and the other is 5 cm below, within the Pecatonica. The Mifflin Member is a wavy-bedded, nodular, fossiliferous, heavily bioturbated limestone. Ranging from 3.5 to 3.9 m (11 to 13 ft) thick, it is the thickest member within the Platteville. Very thin, siliciclastic-rich carbonate beds are interbedded within the nodular, bioturbated limestone, giving it an alternating dark-gray and light-gray coloration pattern. The Mifflin is overlain by a dolomitic, phosphatic, argillaceous carbonate known as the Hidden Falls Member. It is massive and nonfossiliferous except for sporadic thin, fossiliferous lenses. The Hidden Falls Member ranges from 1.2 to 1.8 m (4 to 6 ft) thick and is recessive in outcrop, especially in the lower part. The top of the Hidden Falls is marked by a laterally extensive, burrowed, (Chondrites) diastemic surface. The Magnolia Member overlies the Hidden Falls. It is 2.1–3 m (7–10 ft) thick and is characterized by 2–5 cm coquina layers spaced about every 30 cm in an otherwise nonfossiliferous dolomitic mudstone. The lowermost Magnolia, directly above the contact with the Hidden Falls, is composed of several interbeds of carbonate, argillaceous carbonate, and fossiliferous carbonate, and, as we will see at subsequent field trip stops, it plays an important hydrostratigraphic role within the Platteville Formation.

Gamma logs acquired using a handheld gamma-ray scintillometer on this outcrop allows us to recognize individual members of the Platteville on the numerous borehole geophysical logs from the TCM area (Fig. 2). This was an immensely useful tool for correlating internal Platteville stratigraphy to high-resolution hydrogeologic data such as electromagnetic (EM) flowmeter logs. As we proceed throughout the day we’ll see several examples highlighting the strong correlations between stratigraphic units, fracture network development, and groundwater flow paths in this “hybrid” hydrogeologic unit.

Stop 2. St. Mary’s Spring: Platteville Springs and Their Stratigraphic Occurrence

Location: Stop 2 is along West River Parkway near the West Bank of the University of Minnesota campus beneath what was formerly called St. Mary’s Hospital. UTM coordinates of Stop 2 parking area: 481,835 E/4,979,341 N. TRS location of parking area: T. 29 N, R. 24 W, sec. 25, NE, SE. UTM coordinates of the spring: 481,560 E/4,979,414 N. TRS location of the spring: T. 29 N, R. 24 W, sec. 25, NW, SE.

Directions: From Stop 1 continue northward on Mississippi River Boulevard. Turn right following signs for westbound Lake Street. Take a right onto West River Parkway and travel northbound. We will park off of West River Parkway in a parking area for Lower Riverside Park and walk along the bike path (northwest) toward the spring.

Description: Although the Platteville Formation is composed chiefly of carbonate rock with very low matrix permeability (ranging from 10–7 to 10–4 md; hydraulic conductivity 2 × 10–10 ft/d to 2 × 10–7 ft/d) (Runkel et al., 2003), and traditionally has been classified as an aquitard, wells open to the formation across the TCM area commonly have yields adequate for domestic needs, and hydrogeologic investigations have documented at least local areas with very high hydraulic conductivity (e.g., Liesch, 1973; Barr Engineering, 1983a; Peer Environmental and Engineering Resources, Inc., 1999). Secondary pores in the Platteville accommodate flow both vertically and horizontally. Vertical secondary pores include fractures typical of stress release conditions as well as vertical joints that are part of a large-scale orthogonal system oriented northeast-northwest. Horizontal pores are represented by bedding-plane conduit networks concentrated along discrete stratigraphic intervals, which are the focus of this stop.

Bedrock along the Mississippi River Valley from the confluence with the Minnesota River at Fort Snelling and upstream to St. Anthony Falls was only relatively recently exposed. The valley was cut by a retreating waterfall, now St. Anthony Falls, beginning ~12,000 yr ago after the most recent ice age, where it existed as a tributary to Glacial River Warren. This specific portion of the valley near St. Mary’s spring was cut ~2500 yr ago (Wright, 1972). Today the valley includes a large number of springs that discharge from this incised bedrock. As part of our ongoing research, we have examined all mapped springs along this stretch of the river and documented their stratigraphic position within the Platteville and adjacent units. Of the 48 springs we’ve examined, the greatest percentage (42%) discharge at the contact between the Hidden Falls and Magnolia Members (Fig. 2). The contact between the Hidden Falls and Mifflin Members has the second greatest percentage of discharging springs (25%). The remaining springs discharged at several other stratigraphic positions but appear to be statistically insignificant (8% or less). St. Mary’s spring (Fig. 4) is one of the best examples of a high flow, 12–15 gallons per minute (gpm), spring that discharges at a discrete bedding plane conduit at the Hidden Falls–Magnolia contact. The preferential stratigraphic position of springs at this contact is consistent with observations from the deeper subsurface. Engineering and hydrogeologic investigations at a number of sites across the TCM area have documented a discrete high hydraulic conductivity interval approximating the Hidden Falls and Magnolia contact (Barr Engineering, 1983b; Peer Environmental and Engineering Resources, Inc., 1999).

In an effort to evaluate the relationship between the stratigraphic position of springs and groundwater flow paths in the deeper subsurface, away from effects of a heavily fractured bluff edge, we collected a series of borehole geophysical logs from eight monitoring wells open to the Platteville Formation on the West and East Banks of the University of Minnesota campus (Fig. 1). The logging included EM flowmeter tests under ambient and stressed conditions during which water was injected at rates between ~0.8 and 10 gpm (Fig. 5). Video, caliper, and natural gamma-ray logs were also collected to recognize bedding plane fractures and determine their precise stratigraphic positions. Six wells of the eight had a single bedding plane fracture that dominated hydraulics, all at the Hidden Falls and Magnolia contact (Fig. 6). Hydraulic conductivities calculated for the Hidden Falls and Magnolia bedding plane conduit from injection in these wells ranged from 300 ft/d to 47,000 ft/d (Fig. 6). One of these six wells had a second bedding plane fracture that was hydraulically active, at 0.6 m (2 ft) above the Hidden Falls and Magnolia contact within the lower Magnolia Member. This fracture did not appear to dominate flow when we injected, although it did have a high hydraulic conductivity value of 2000 ft/d. Two of the eight tested wells had very different results. One of the wells is cased below the top of the Hidden Falls Member, open to the middle of the Hidden Falls and the upper part of the Mifflin Members. Hydraulic conductivity is so low in this borehole that we were unable to achieve a static head during injection. The other well is open to the Hidden Falls and Magnolia boundary, but there does not appear to be a bedding plane conduit at that horizon, and injected water exited the borehole directly at the base of the casing, stratigraphically within the middle of the Magnolia Member. In this case, water may be leaving at an opening between the borehole wall and the well casing.

Figure 4.

Photograph showing a close-up view of St. Mary’s spring discharging from the Hidden Falls and Magnolia bedding plane contact. The scale object is 16 cm long.

Figure 4.

Photograph showing a close-up view of St. Mary’s spring discharging from the Hidden Falls and Magnolia bedding plane contact. The scale object is 16 cm long.

Results from correlating spring positions with stratigraphic intervals is consistent with the borehole flowmeter logging results, both demonstrating preferential development of bedding plane conduits at the Hidden Falls–Magnolia contact. However, the outcrop results do appear to differ in that a relatively large percentage (25%) of springs emanate from the base of the Hidden Falls Member, a stratigraphic position for which subsurface conduits have not been recognized in our flowmeter logging, and have only rarely been recognized in other borehole hydrogeologic tests. Our current hypothesis to account for this discrepancy is that springs emanating below the top of the Hidden Falls Member may be attributed to a step-down effect at the bluff edge, where connectivity of vertical and bedding plane fractures are enhanced. It is possible that groundwater that travels along the Hidden Falls and Magnolia conduit across most of the subsurface extent of the Platteville steps stratigraphically down within meters of the bluff edges. We will see an example of a possible “step-down spring” at Stop 8, and will discuss support for this hypothesis later during the field trip.

Stop 3. Mendota Bike Trail: Mechanical Stratigraphy of the Platteville Formation

Location: Stop 3 is in Mendota Heights along the Mendota bike trail. We will park off Highway 13 where the trail intersects the highway. UTM coordinates of Stop 3 parking area: 487,417 E/4,970,516 N. TRS location of the parking area: T. 28 N, R. 23 W, sec. 27, NE, NW. UTM coordinates of the outcrop: 478,900E/4,970,964N. TRS location of the outcrop: T. 28 N, R. 23 W, sec. 22, SE, SE.

Directions: From Stop 2, turn around traveling south on West River Parkway and veer right, up the hill, to E Franklin Avenue. Take a right on Riverside Avenue and turn right onto eastbound Interstate 94. Exit onto southbound I-35 E/Kellogg Boulevard and follow signs for southbound I-35 E. From I-35 E, take the Highway 13 exit, turning right. We will park off of Highway 13 at the trail crossing and walk northbound on the trail to the exposure.

Description: Vertical fracture patterns in layered, undeformed rocks such as the Platteville are controlled by stratigraphic features. Vertical fractures typically initiate within a stratigraphic unit, spanning its entire thickness, referred to as a mechanical unit, and terminate at distinct stratigraphic horizons, known as mechanical interfaces. Stratigraphic features such as unit thickness, bedding thickness, and strength all play a key role in determining the trace length and spacing of vertical fractures. Ductile beds such as shale and other types of weak beds and interfaces commonly are preferential termination horizons for vertical fractures. Defining these units and interfaces is known as mechanical stratigraphy, which can provide insight into fracture network development and improved prediction of fluid flow pathways (Underwood et al., 2003; Cooke et al., 2006).

Figure 5.

Example of the geophysical and hydraulic data collected from a monitoring well on the East Bank of the University of Minnesota campus (well no. 664362). There was no measurable ambient flow in this well. Water was injected in this hole at a rate of 10 gpm, and all water exited the borehole at a depth corresponding to a caliper deflection and bedding plane fracture in the video log at about 14 m (46 ft). Gray lines indicate measurements from ambient conditions, and black lines reflect injection conditions. Inset photograph is a still shot from the video log of the fracture that accepted water.

Figure 5.

Example of the geophysical and hydraulic data collected from a monitoring well on the East Bank of the University of Minnesota campus (well no. 664362). There was no measurable ambient flow in this well. Water was injected in this hole at a rate of 10 gpm, and all water exited the borehole at a depth corresponding to a caliper deflection and bedding plane fracture in the video log at about 14 m (46 ft). Gray lines indicate measurements from ambient conditions, and black lines reflect injection conditions. Inset photograph is a still shot from the video log of the fracture that accepted water.

Figure 6.

Correlated gamma logs of the six wells on the West and East Banks of the University of Minnesota campus, with a bedding plane fracture that dominated hydraulics in the borehole at the Hidden Falls–Magnolia contact, and their hydraulic conductivity values, ranging from 300 to 47,000 ft/d. H—fracture height, which represents the range of possible fracture apertures estimated from the video and caliper logs.

Figure 6.

Correlated gamma logs of the six wells on the West and East Banks of the University of Minnesota campus, with a bedding plane fracture that dominated hydraulics in the borehole at the Hidden Falls–Magnolia contact, and their hydraulic conductivity values, ranging from 300 to 47,000 ft/d. H—fracture height, which represents the range of possible fracture apertures estimated from the video and caliper logs.

Mapping vertical fractures on exposure surfaces is a technique used in mechanical stratigraphy to characterize fracture density and termination horizons. Stop 3 is the most accessible, well-exposed outcrop of three in the TCM area (Fig. 7), where we have mapped vertical fractures in the Platteville Formation. Results at this locality show that the members of the Platteville each have a distinct style of vertical fracture density and spacing. The Magnolia Member has a high density of vertical to subvertical fractures with a wide range in trace length, creating a “blocky” texture. The Hidden Falls Member has a very high density of vertical to subvertical, straight to curvilinear fractures that are most abundant in the upper part of the unit. The Mifflin Member has a relatively wide spacing of vertical, straight fractures with long traces that typically extend through the entire thickness of the member. The Pecatonica has a narrower spacing of vertical, straight fractures with traces that span the thin bed. The Pecatonica is covered in the Figure 7 area but is exposed in other areas along this trail and at Stop 4. The members of the Platteville act as mechanical units, and the contacts between the members typically terminate vertical fractures, acting as mechanical interfaces. Individual vertical fractures extending through the entire formation are rarely present at some highly weathered outcrops.

Vertical fractures preferentially terminate at the Hidden Falls and Magnolia contact as well as near the base of the Hidden Falls. The Hidden Falls Member has a number of material properties differing from members above and below that may account for preferential termination of vertical fractures. Internally it is massive, without individual centimeter-scale beds characteristic of the Magnolia and Mifflin, as indicated by the conchoidal fractures characteristic of the member. It contains a higher percentage of siliciclastic material, largely clay and silt (especially in the middle part of the member), and on weathered exposures the densely fractured, small, blocky texture of the Hidden Falls is more similar to outcrops of relatively ductile shale, such as the Glenwood, than to brittle carbonate rock. Tensile- and compressive-strength values for the Hidden Falls, based on samples from a number of sites in the TCM area, average ~12,000 psi, ~40% lower than values for the Mifflin and Magnolia (CSC Joint Venture, 1985). The coincidence of preferential termination of vertical fractures with preferential development of bedding plane conduit(s) at the top of the Hidden Falls Member may also be indicative of a cause and effect relationship whereby bedding plane conduits at this position served as a weak mechanical interface where vertical fractures terminated.

Stop 4. Shepard Road: A Large Cliff-Top View of Vertical Fractures in the Platteville

Location: Stop 4 is a short stop along Shepard Road; we will park off the road to view the large exposure. UTM coordinates of Stop 4: 494,938 E/4,977,071 N. TRS location: T. 28 N, R. 22 W, sec. 4, NW, NW.

Directions: From Stop 3 continue back onto Highway 13 toward I-35 E. Head north on I-35 E. Exit on Shepard Road and take a right. Exposure is ~5 mi down Shepard Road on the north side of the road.

Description: This stop provides the most expansive view of the Platteville, Glenwood, and upper St. Peter formations in the area (Fig. 8A). Fracture characteristics here are similar to those at the Mendota locality (see Stop 3), where vertical fractures preferentially terminate at the top and near the base of the Hidden Falls. All three sites at which we have traced fractures in detail, and casual observations at several other, less well exposed sites in the TCM area, display preferential termination at these two stratigraphic intervals, suggesting that this is a regional and predictable phenomenon.

The spring, borehole, and fracture data we’ve compiled so far identify the Magnolia–Hidden Falls contact as a key stratigraphic position for groundwater flow. From a horizontal perspective it corresponds to the most prevalent high hydraulic-conductivity conduit. It also represents a mechanical interface or bed that has inhibited propagation of vertical fractures, which results in the strata directly below having potential to serve as an aquitard in a vertical direction. The lowermost part of the Hidden Falls Member, also an interval of preferential joint terminations, may similarly serve as a stratigraphically discrete aquitard. The outcrop and subsurface hydrogeologic expressions of this phenomenon is the presence of springs and water table aquifers perched preferentially in this relatively thin part of the Platteville Formation. Nested monitor wells similarly reflect vertical resistance to flow across the Hidden Falls, with heads above and below known to differ by as much as ~3 m (Fig. 3) (Braun Intertec Corporation, 2011). We have recently started collection of head data from Platteville wells, and preliminary results reveal an abrupt change in head corresponding to the Magnolia–Hidden Falls contact (Fig. 8B).

Stop 5. Minnehaha Falls Park Area: Lunch at Sea Salt with Discussion

Location: Stop 5 is in Minnehaha Falls Park. UTM coordinates of Stop 5: 483,396 E/4,973,646 N. TRS location: T. 28 N, R. 23 W, sec. 18, SE, NE.

Directions: From Stop 4 continue back on Shepard Road heading west. Continue on Shepard Road ~9 mi and follow signs toward Mississippi River Boulevard. Take a right onto Ford Parkway and head west over the Mississippi River. Turn left onto 46th Avenue S and right onto Godfrey Road; Minnehaha Falls Park is on the left.

Description: Here we will be eating lunch at Sea Salt Eatery in the park. There will be time to discuss what we have viewed so far on the field trip and explore Minnehaha Falls. The stream flows over the Platteville limestone, capping the Glenwood shale and St. Peter Sandstone.

Stop 6. Fort Snelling Bike Trail: Platteville Conduit Development

Location: Following lunch we will drive to the south end of the park area and walk down the Fort Snelling bike trail to a small Platteville exposure. UTM coordinates of the exposure: 484,473 E/4,972,271 N. TRS location: T. 28 N, R. 23 W, sec. 18, SE, NE.

Figure 7.

Vertical fracture map of the Mendota locality with and without the outcrop photograph. Vertical fractures are shown as black lines. The gray lines represent the upper and lower contacts of the Hidden Falls and correspond to the vertical-fracture termination horizons. Each member of the Platteville Formation has a distinct style of vertical fracture spacing and density and act as mechanical units with vertical fractures terminating at member contacts specifically near the top and bottom of the Hidden Falls Member. Exposure is ~7.6 m (25 ft) high.

Figure 7.

Vertical fracture map of the Mendota locality with and without the outcrop photograph. Vertical fractures are shown as black lines. The gray lines represent the upper and lower contacts of the Hidden Falls and correspond to the vertical-fracture termination horizons. Each member of the Platteville Formation has a distinct style of vertical fracture spacing and density and act as mechanical units with vertical fractures terminating at member contacts specifically near the top and bottom of the Hidden Falls Member. Exposure is ~7.6 m (25 ft) high.

Figure 8.

(A) Vertical fracture map of the Shepard Road locality with and without the outcrop photograph. Vertical black lines represent the vertical fractures. Gray lines depict the vertical-fracture termination horizons near the top and bottom of the Hidden Falls Member similar to Figure 7. Exposure is ~14 m (45 ft) high. (B) Vertical differences in static head conditions within the Platteville Formation from three monitor wells on the campus of the University of Minnesota. These preliminary packer-test results suggest poor vertical hydraulic connectivity between the Magnolia Member and underlying members, indicated by the abrupt change in head differences above and below the packer across the Magnolia–Hidden Falls contact strata. This contact corresponds to the point at which vertical fractures preferentially terminate in outcrop.

Figure 8.

(A) Vertical fracture map of the Shepard Road locality with and without the outcrop photograph. Vertical black lines represent the vertical fractures. Gray lines depict the vertical-fracture termination horizons near the top and bottom of the Hidden Falls Member similar to Figure 7. Exposure is ~14 m (45 ft) high. (B) Vertical differences in static head conditions within the Platteville Formation from three monitor wells on the campus of the University of Minnesota. These preliminary packer-test results suggest poor vertical hydraulic connectivity between the Magnolia Member and underlying members, indicated by the abrupt change in head differences above and below the packer across the Magnolia–Hidden Falls contact strata. This contact corresponds to the point at which vertical fractures preferentially terminate in outcrop.

Discussion: This outcrop is one of the most accessible outcrops of the Hidden Falls and Magnolia contact strata. The laterally continuous recessive bedding plane (Figs. 9A and 9B) corresponds to the position of the bedding plane conduit recognized in many boreholes and at springs across the TCM area. As we’ve discussed at previous stops, it is also a mechanical interface, where vertical fractures preferentially terminate in outcrop. This stop is an opportunity to get an up-close look at this hydraulically important interval of strata again, fully describe its features, and briefly discuss modes of origin of bedding plane conduits.

As mentioned at Stop 1, the contact between the Hidden Falls and Magnolia is transitional compared with the other member contacts in the Platteville Formation. The top of the Hidden Falls is placed at a regionally traceable burrowed surface separating a dolomitic mudstone below from a fossiliferous argillaceous limestone above. The lowermost half-meter of the Magnolia Member consists of lenticular, centimeter-scale interbeds of fossiliferous, burrowed, and argillaceous limestone that contrasts to the cleaner, dolomitic mudstone beds with thin fossiliferous lenses characteristic of the Magnolia higher in the section. The thickness of this transitional interval ranges from ~0.15 to 0.4 m (0.5 to 1.5 ft). At this stop the recessive, weathered, bedding plane lies at the base of this transitional interval. Other exposures around the TCM area can be seen where this recessive bedding plane is either within the middle or at the top of this transitional interval, suggesting that the conduit developed at this interval is not along a single, precise stratigraphic position at centimeter scale everywhere in the TCM area.

At this stop the recessively weathered, transitional interval contains two limestone beds (Fig. 9B). The lower bed is a fos-siliferous limestone ~2–5 cm thick, and the upper bed is a burrowed mudstone ~5 cm thick. Partings between these beds at this and other outcrops are filled with a brownish-orange to gray clay. Engineering borings elsewhere in the TCM area describe a similar clay-rich material at this stratigraphic position, indicating that it is present in the subsurface away from the bluff edge. Although referred to as a bentonite by some previous workers, a sample of this material from Shadow Falls (Stop 1) was analyzed by Mossler (1985), who reported that the clay was illite. A water-washed and sieved sample collected from Shadow Falls contained very fine sand- to silt-sized calcareous crystals that may represent carbonate residuum from dissolution. Furthermore, some of the fine-grained unconsolidated material along these partings is laterally transitional with indurated limestone lenses, also consistent with production of the fine material via solution weathering.

Our tentative interpretation is that the fine-grained material filling bed partings at this and other exposures is largely the relatively insoluble product of solution weathering, perhaps including both material derived from in situ weathering as well as material transported in suspension via turbulent conduit flow. Understanding the origin and timing of emplacement of this material may help us further understand the development of this bedding plane conduit system. Our working hypothesis is that the incipient development of the bedding parallel conduit system in this discrete interval may be the result of very early, preferential dissolution of the relatively coarse limestone beds in an otherwise largely finer grained and dolomitic Platteville Formation above and below.

Figure 9.

(A) Photograph of the Magnolia and Hidden Falls contact on the Fort Snelling bike trail, showing termination of vertical fractures at the top of the Hidden Falls Member. Exposure is ~3 m (10 ft) high. (B) Close-up view, showing the bedding plane interval that corresponds to the conduit recognized in many boreholes and at springs across the Twin Cities Metropolitan area and the clay-rich material that is filling the bed partings. Head of hammer for scale is ~18 cm long.

Figure 9.

(A) Photograph of the Magnolia and Hidden Falls contact on the Fort Snelling bike trail, showing termination of vertical fractures at the top of the Hidden Falls Member. Exposure is ~3 m (10 ft) high. (B) Close-up view, showing the bedding plane interval that corresponds to the conduit recognized in many boreholes and at springs across the Twin Cities Metropolitan area and the clay-rich material that is filling the bed partings. Head of hammer for scale is ~18 cm long.

Stop 7. Camp Coldwater Spring: Dye Tracing in the Platteville

Location: From Stop 6, continue south down the Minnehaha Park Road. Follow signs for “Camp Cold Water Spring.” The spring is situated on the grounds of the former U.S. Bureau of Mines, directly northeast of the intersection of Highways 55 and 62. UTM coordinates of the spring: 484,490E, 4,971,800N. TRS location: T. 28 N, R. 23 W, sec. 20, SW, NE.

Discussion: Camp Coldwater Spring emerges from the west side of a small masonry pond and adjoining tower (Fig. 10). The spring itself is a cultural and historical landmark. It figures into the Dakotah creation story and is still considered a sacred spot by Native Americans. It also served as the water source and campsite for the men constructing Fort Snelling in 1820; because it was a notable supply of cold water, it was given its name. The spring continued serving as the water supply of the fort until the start of the twentieth century, using the structure in front of the spring for that function.

Camp Coldwater Spring emerges from an outcrop of the Platteville; monitoring of the spring’s discharge in recent years has found it fluctuating from 50 to 125 gpm, with flows typically ranging from 70 to 90 gpm. This makes Camp Coldwater Spring the largest remaining spring in Minneapolis. Because of concern for the effects of the construction of State Highway 55 would have on Camp Coldwater Spring and other springs along the Minnehaha Creek gorge in Minnehaha Park, several hydro geologic studies were carried out from Minnehaha Park to Camp Coldwater Spring.

Figure 10.

Photograph of Camp Coldwater Spring, discharging near the base of the tower at the Hidden Falls and Magnolia contact.

Figure 10.

Photograph of Camp Coldwater Spring, discharging near the base of the tower at the Hidden Falls and Magnolia contact.

The spring discharge emerges from the convergence of several vertical joints and a bedding plane fracture at the Hidden Falls–Magnolia contact (Fig. 11). A seismic study by Bison Service Co. (2000a, 2000b) found that the two joints intersecting at the spring had orientations of N 36° W and N 54° E. The former joint was described as a more prominent feature and extended to the northwest to the portion of Minnehaha Park west of State Highway 55.

Several pumping-test investigations have been carried out in this portion of Minnehaha Park. Liesch (1973) carried out several pumping tests using dozens of wells installed in the overburden and Platteville. The estimated transmissivities from these tests are summarized in Table 1 and range from 1400 to 1,600,000 gallons per day per foot (gpd/ft). Another pumping test was conducted in 2000 and analyzed by Kelton Barr Consulting (2000). The test revealed a drawdown distribution distended in two principal directions that inferred the location and influence of two hydraulically significant joints, trending approximately N 45° W and N 50° E; these orientations closely correspond to the two major joint orientations measured by Olsen (1999, personal commun.) on nearby outcrops along the Mississippi River and Minnehaha Creek bluffs. The former joint also was in very close proximity to the wells that yielded the three transmissivity values >500,000 gpd/ft in Liesch (1973). A quarter-mile southeast of the location of the pumping tests, the extrapolation of the former joint passed very closely by the east side of an excavation into the Platteville for a grit chamber as part of the Highway 55 construction. Groundwater flooded into the east side of the excavation via a bedding plane fracture at the Hidden Falls–Magnolia contact; the excavation was dewatered at a rate of 500 gpm for several months (Kelton Barr Consulting, 2000).

Additional geophysical investigation using ground-penetrating radar was carried out in Minnehaha Park upland directly south of Minnehaha Falls. The results found a network of northeast- and northwest-trending joints. The locations and intersections of these joints corresponded closely with the occurrence of minor springs along the bluff between Minnehaha Falls and Camp Coldwater Spring.

Dye tracing studies were carried out in 2001 in the intersection area of State Highways 55 and 62 west of Camp Coldwater Spring (Fig. 1). Eosin Y fluorescent dye was added to a trench excavated to the Platteville surface 125 m west of Camp Coldwater Spring. The dye began to arrive at the spring in <1.5 h, migrating at an apparent velocity of 2 km/d (1.24 mi/d). A subsequent dye-trace test was carried out at a dewatering sump at the center of the present-day intersection, ~290 m (950 ft) west-southwest of the spring. Within hours of a court-ordered halt to pumping, fluorescein dye was added to the sump. The dye was detected at Camp Coldwater Spring 16 d later. The travel path was more complex for this test, as the sump was excavated into the glacio-fluvial sediments filling a buried east-west valley eroded through the Platteville a short distance south of the spring. The apparent flow velocity of 19 m/d (63 ft/d) is a combination of flow through the alluvial porous media and the Platteville fractures (Alexander et al., 2001).

Figure 11.

Geology of the Camp Coldwater Spring vicinity, showing the location and orientation of vertical joints (black lines) that converge toward the emergence of the spring. The dark-gray background color represents a full Platteville subcrop; the medium-gray color represents a partial Platteville subcrop where only the Hidden Falls, Mifflin, and Pecatonica Members are present; and the lightest gray color is where all the Platteville is absent. Compiled from Sunderman and Kurtz (2000) and Bison Service Company (2000a, 2000b).

Figure 11.

Geology of the Camp Coldwater Spring vicinity, showing the location and orientation of vertical joints (black lines) that converge toward the emergence of the spring. The dark-gray background color represents a full Platteville subcrop; the medium-gray color represents a partial Platteville subcrop where only the Hidden Falls, Mifflin, and Pecatonica Members are present; and the lightest gray color is where all the Platteville is absent. Compiled from Sunderman and Kurtz (2000) and Bison Service Company (2000a, 2000b).

TABLE 1.

SUMMARY OF TRANSMISSIVITY AND STORAGE COEFFICIENT VALUES FROM PUMPING TESTS NEAR MINNEHAHA FALLS PARK CARRIED OUT BY LIESCH (1973) AND KELTON BARR CONSULTING (2000)

Well No.Pumping Rate (gpm)TestDuration(hr)Maximum Drawdown(ft)Transmissivity(T)(gpd/ft)Storage Coefficient(S)Parameters calculated from:
T1d-wa904817.540,000Recovery in pumping well
T1d-wa904817.536,5542.60E-03Time-drawdown curve at t1f-Pa (6”)
T1d-wa904817.5530,0008.80E-03Distance-drawdown curve from T9-wa to T10-wa
T2d-wa824.51,400Time-drawdown curve at pumping well
T2d-wa824.53,8404.00E-05Time-drawdown curve at t2b-Pa (6”)
T2d-wa824.55,0001.20E-04Distance-drawdown curve
T3d-wa30247.55,2004.20E-05Time-drawdown curve at t3b-Pa (6”)
T4b-Pa (6”)814.94,0004.00E-03Time-drawdown durve at t4d-wa
T4d-wa811.94,2004.00E-03Time-drawdown curve at t4b-Pa (6”)
T5b-Pa (6”)81.50.57,0001.80E-05Time-drawdown curve at t4b-Pa (6”)
T6-wa812.95,700Time-drawdown curve at pumping well
T7-wa101.253.72,900Time-drawdown curve at pumping well
T9-wa6020.14700,000Time-drawdown curve at pumping well
T10-wa6010.071,600,0009.00E-03Distance-drawdown curve at observation wells
T2d-Wb1032.736,8001.60E-01Drawdown at T2C-Pb
T4b-Wb3012.1517,6005.80E-02Drawdown at pumping well t4b-Wb
T4b-Wb3012.1526,000
Average =176,2472.24E-02
Median =6,8004.00E-03
Mean =16,8811.53E-03
Well No.Pumping Rate (gpm)TestDuration(hr)Maximum Drawdown(ft)Transmissivity(T)(gpd/ft)Storage Coefficient(S)Parameters calculated from:
T1d-wa904817.540,000Recovery in pumping well
T1d-wa904817.536,5542.60E-03Time-drawdown curve at t1f-Pa (6”)
T1d-wa904817.5530,0008.80E-03Distance-drawdown curve from T9-wa to T10-wa
T2d-wa824.51,400Time-drawdown curve at pumping well
T2d-wa824.53,8404.00E-05Time-drawdown curve at t2b-Pa (6”)
T2d-wa824.55,0001.20E-04Distance-drawdown curve
T3d-wa30247.55,2004.20E-05Time-drawdown curve at t3b-Pa (6”)
T4b-Pa (6”)814.94,0004.00E-03Time-drawdown durve at t4d-wa
T4d-wa811.94,2004.00E-03Time-drawdown curve at t4b-Pa (6”)
T5b-Pa (6”)81.50.57,0001.80E-05Time-drawdown curve at t4b-Pa (6”)
T6-wa812.95,700Time-drawdown curve at pumping well
T7-wa101.253.72,900Time-drawdown curve at pumping well
T9-wa6020.14700,000Time-drawdown curve at pumping well
T10-wa6010.071,600,0009.00E-03Distance-drawdown curve at observation wells
T2d-Wb1032.736,8001.60E-01Drawdown at T2C-Pb
T4b-Wb3012.1517,6005.80E-02Drawdown at pumping well t4b-Wb
T4b-Wb3012.1526,000
Average =176,2472.24E-02
Median =6,8004.00E-03
Mean =16,8811.53E-03

The karstic and fractured flow patterns within the Platteville in the vicinity of Minnehaha Park and Camp Coldwater Spring are not thought to have been significantly altered and thus still reflect those flow patterns established prior to the creation of the Mississippi River and Minnehaha Creek gorges, created 12,700–10,000 yr ago (Wright, 1972). These investigations reveal a predominance of flow generally through the Hidden Falls–Magnolia contact, augmented and redistributed by hydraulically significant joints intersecting that contact.

Stop 8. Bird Dropping Spring: Hydrocarbon Contamination in the Platteville

Location: Parking UTM coordinates: 479,970 E/4,980,608 N. TRS location: T. 29 N, R. 24 W, sec. 23, SW, SE. Walk down bike path southeast toward the spring. Spring UTM coordinates: 480,340 E/4,980,559 N. TRS location: T. 29 N, R. 24 W, sec. 23, SE, SE.

Directions: From Stop 7 take a right on to Hiawatha Avenue (Highway 55) heading north. Keep right at the fork, follow signs for 3rd Street and merge onto 3rd Street S. Turn right onto Chicago Avenue and left onto S 2nd Street. Take a right on Portland Avenue S and veer right under the Stone Arch Bridge to parking area.

Discussion: Bird Dropping Spring (Fig. 12) emanates from two closely spaced vertical fractures in the lower Hidden Falls and uppermost Mifflin that terminate in the middle part of the Mifflin Member. This is the only spring we observed in the TCM area that discharges at this stratigraphic position, and we believe it represents an example of the outcrop-edge, step-down effect. We hypothesize that groundwater in the nearby subsurface is primarily traveling along the Hidden Falls and Magnolia contact, based on nearby borehole flowmeter and other hydraulic tests. Groundwater steps downward as the bluff edge is approached, where vertical fractures are more densely developed and better connected vertically across the Magnolia, Hidden Falls, and uppermost Mifflin Members.

This spring is called Bird Dropping Spring in reference to the white, cottage-cheese-like, microbial masses that adhere to the Platteville Formation within the spring discharge. These masses appear here and at other sites where the groundwater is contaminated with hydrocarbons. A strong odor of hydrocarbons can be detected at this spring. The contamination in the groundwater at this location is from obsolete gasoline storage sites south of the site. Shallowly buried and fractured carbonate rock in an urban setting, such as the Platteville Formation, is highly susceptible to contamination, as pollutants can travel quickly from the surface and into a network of bedding plane and vertical fractures. Better understanding of the development of these fracture pathways and groundwater flow paths in the Platteville limestone will aid in the implementation of groundwater remediation efforts in the TCM area and in other parts of the Upper Midwest with similar geologic settings.

Stop 9. Andersen Library: Platteville Hydrostratigraphy, Putting Everything Together

Location: This stop is at the entrance to the University of Minnesota Andersen Library underground archive building. UTM coordinates: 480,895 E/4,979,991 N. TRS location: T. 29 N, R. 24 W, sec. 25, SE, NW.

Directions: From Stop 8, travel back toward Portland Avenue and take a left onto W River Parkway. Take a right onto 22nd Avenue S and a quick left into the MLAC parking area.

Discussion: Underground excavations in the TCM area are especially useful for the rare three-dimensional perspectives they provide for fractures and groundwater flow when combined with cores and outcrops. At this stop, we will show and discuss information collected during construction of an underground library archive building for the University of Minnesota, the MLAC. Outside the entrance to this facility we will highlight insights gained during excavation and remediation of contaminated leakage through the bottom of the Platteville Formation, which serves as the rock ceiling for this facility. We will also provide a summary of the field trip in the context of our current conceptual model of Platteville hydrostratigraphy (Fig. 3).

Figure 12.

Photograph of Bird Dropping Spring discharging at the base of two vertical fractures in the upper Mifflin Member. Bluff exposure in the photograph is ~2 m (7 ft) high.

Figure 12.

Photograph of Bird Dropping Spring discharging at the base of two vertical fractures in the upper Mifflin Member. Bluff exposure in the photograph is ~2 m (7 ft) high.

Excavation for the underground part of the MLAC began in 1997 and included removal of the upper ~6 m (20 ft) of the St. Peter Sandstone. When the overlying Glenwood shale was subsequently stripped, immediate and significant leakage of groundwater contaminated with coal tar derivatives occurred through a network of vertical joints exposed at the bottom of the Platteville Formation. Subsequent remedial work included installation of a pan system to capture water from leaking joints, which are restricted to the ~80 m of the excavation closest to the eroded edge of the Platteville at the river bluff (Fig. 13). In 2002, a horizontal pump-out well was installed along the Hidden Falls–Magnolia contact strata, and the system reduced leakage into the panning system by ~65% (Peer Environmental and Engineering Resources, Inc., 2003). Continued maintenance of the system includes periodic removal of microbial masses from the pan and well system that are similar in appearance to those at Bird Dropping Spring.

Combined data from MLAC pre-excavation (CNA, 1997) and remediation-related (Peer Environmental and Engineering Resources, Inc., 1999, 2003) reports result in a hydrogeologic characterization consistent with many other metro sites (Fig. 3). Surface excavations penetrating about a meter into bedrock, vertical shafts, cores, and other bedrock borings indicate that the secondary porosity in the Magnolia Member in subsurface conditions is similar to its appearance at the outcrops at Stops 3 and 4. As uppermost bedrock, it is highly fractured, with abundant and closely spaced vertical fractures and a number of bedding plane fractures, resulting in a blocky fracture pattern. As at many other metro subsurface sites, it contains a body of water that is perched on top of the Hidden Falls Member. A large number of discrete interval packer, slug, and larger scale aquifer tests of the Magnolia Member show a range in horizontal hydraulic conductivity that varies from a few feet per day to hundreds of feet per day. A bulk average horizontal hydraulic conductivity for individual sites is typically calculated at several tens to as much as ~200 ft/d. Multi-well aquifer tests, dye traces, and mapped contamination plumes across the TCM area indicate that fractures are typically well connected vertically. Vertical hydraulic conductivity for the Magnolia Member at MLAC was calculated to be as high as 4 ft/d, although lower values were obtained with the same aquifer test data using other methods of analysis (Peer Environmental and Engineering Resources, Inc., 1999).

Figure 13.

Map of excavation at the Minnesota Library Access Center underground library storage facility at the University of Minnesota, highlighting mapped vertical joints on the ceiling of the excavation, which is the bottom of the Platteville Formation. Monitoring of flow collected from pans installed beneath leaking joints provides quantification of leakage rates. The site lies beneath an ~2.1 m (7 ft) thick body of water perched on top of the Hidden Falls Member. Modified from Peer Environmental and Engineering Resources, Inc. (2001).

Figure 13.

Map of excavation at the Minnesota Library Access Center underground library storage facility at the University of Minnesota, highlighting mapped vertical joints on the ceiling of the excavation, which is the bottom of the Platteville Formation. Monitoring of flow collected from pans installed beneath leaking joints provides quantification of leakage rates. The site lies beneath an ~2.1 m (7 ft) thick body of water perched on top of the Hidden Falls Member. Modified from Peer Environmental and Engineering Resources, Inc. (2001).

The lowermost ~10–50 cm of the Magnolia, the “transitional” Hidden Falls–Magnolia contact strata described at Stop 6, is especially conductive in a horizontal direction, containing a discrete bedding-plane fracture network with hydraulic conductivity measured as high as tens of thousands of feet per day in individual boreholes (Fig. 6). Borehole geophysical logs (including EM flowmeter logs), core, and underground excavation data collected near the University of Minnesota and at other TCM sites, tens to hundreds of meters away from bluff and subcrop edges, demonstrate that this discrete interval of bedding plane conduits at the Hidden Falls–Magnolia contact is widespread across the subsurface extent of the Platteville. Hydraulic properties of the Magnolia–Hidden Falls transition interval in a vertical direction are poorly understood, but limited data indicate that they may be greatly variable. At MLAC and other sites this transitional interval has been shown to be vertically well connected to the heavily fractured Magnolia Member higher in the section, serving as a lowermost “water main” that collects and transports horizontally large volumes of water recharged vertically through uppermost bedrock. However, in a setting more deeply buried by younger bedrock, flowmeter logging has revealed head differentials across this thin transitional interval driving ambient borehole flow, an indication that discrete intervals within the lowermost Magnolia at least locally may be resistant to through-going vertical fractures and therefore serving as aquitards.

The Hidden Falls, Mifflin, and Pecatonica Members typically have markedly lower horizontal hydraulic conductivity than the Magnolia Member. Hydraulic conductivity values from packer tests typically range from 10–1 to a few feet per day (e.g., CNA, 1997). Packed intervals commonly are unable to produce water at a sustained rate above a minimum pumping threshold. Values as high as a few tens of feet per day are relatively uncommon and likely indicate intersection or proximity to bedding plane fractures or vertical joints (e.g., Barr Engineering, 1987). In a vertical sense, parts of these Platteville members (possibly along with the lowermost Magnolia transitional strata, described above) serve as aquitards. Preferential termination of vertical fractures, as described at earlier stops, along with a systematic lateral change in fracture apertures and likely connectivity, appear to determine the stratigraphic position and relative integrity of these aquitards. Leakage through the ceiling of the excavation at MLAC provides some insight into the relative bulk vertical conductivity of the combined Hidden Falls, Mifflin, and Pecatonica Members (Fig. 13). Within ~80 m of the bluff edge, relatively high leakage rates (total of ~5000 gpd; Peer Environmental and Engineering Resources, Inc., 2001, 2003) into the panning system indicate a vertical hydraulic conductivity varying from 10–1 to 10–3 ft/d across individual parts of the ceiling. Beyond ~80 m from the bluff edge the ceiling is not panned, because leakage is negligible. Using a total leakage for the unpanned area of <10 gpd (Steve Jansen, Peer Engineering, 2011, personal commun.) vertical hydraulic conductivity is <10–4 ft/d. The decreasing hydraulic conductivity deeper into the excavation reflects diminishing aperture width, and likely connectivity and trace length of vertical fractures with increasing distance from the bluff edge and subcrop surface.

Identification of discrete intervals that might serve as key aquitards within the combined Hidden Falls, Mifflin, and Pecatonica Members is an ongoing focus of our research. Hydraulic data compiled thus far suggest that the Hidden Falls Member plays a key role. Heads above and below the Hidden Falls are known to differ by as much as 3 m (10 ft) on the basis of nested well measurements (Braun Intertec Corporation, 2011), and our packer-derived head measurements also show abrupt head changes across the Magnolia–Hidden Falls contact (Fig. 8B). Perched water on top of the Hidden Falls, recognized at a number of subsurface sites and expressed also by springs, likewise suggests significant vertical resistance across the member. Collectively, the mechanical stratigraphy (Stops 3 and 4) and hydraulic data support a model whereby compartmentalization of vertical fractures from the lowermost Magnolia to the upper Mifflin leads to Hidden Falls and directly adjacent strata containing one or more key aquitards. The integrity of these discrete aquitards likely increases with increasing distance from outcrop or subcrop edges. Intact, unfractured cores of the Hidden Falls indicate that the pervasive, closely spaced but relatively short trace-length curvilinear to vertical fractures characteristic of outcrops are not present in nearly the same abundance in the deeper subsurface. This provides some support for our spring “step-down” hypothesis, whereby water approaching the bluff edge steps down to the next lowest mechanical interface at which joints preferentially terminate.

The combined information from MLAC, other subsurface sites, and our outcrop observations supports our preliminary conceptual model of the Platteville as a hybrid hydrogeologic unit, with some parts acting as aquifers and some as aquitards. Like other hybrid units identified in the Paleozoic bedrock of this area, even though matrix permeability is very low, secondary pore networks accommodate moderate to very high horizontal hydraulic conductivity sufficient to yield economic quantities of water to wells, and to supply springs with flow rates >10 gpm. Dye tracing, pump tests, and secondary pore observations demonstrate that the Platteville is consistent with the definition of a karstic aquifer that includes fast-flow conduits. Data from the same collection of sites also support the traditional classification of the Platteville Formation as a confining unit, when considered from a vertical perspective, with discrete intervals such as the upper and lowermost Hidden Falls Member serving as key, relatively high integrity aquitards. Relatively thin stratigraphic intervals of 2 m or less appear to contain both the highest hydraulic conductivity bedding-plane conduits as well as the key aquitards. Despite this complexity, our ongoing work thus far appears to show a strong connection between stratigraphic units and the development of both horizontal and vertical secondary pore networks, and thus the potential for a strong degree of predictability in the flow path geometries.

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G.B.
, eds.,
Geology of Minnesota: A Centennial Volume: Minnesota Geological Survey
 , p.
515
547
.

ACKNOWLEDGMENTS

Our work in the Twin Cities Metropolitan area has been supported by the Metropolitan Council. Special thanks to Greg Brick for sharing his years of experience on Platteville springs in the area and Steve Jansen from Peer Engineering for his insights into construction of the Minnesota Library Access Center. We also thank Janet Dalgleish with the University of Minnesota Department of Environmental Health and Safety for helping us access water wells and Pat Mosites with the Metropolitan Airports Commission.

Figures & Tables

Figure 1.

Map of the Twin Cities Metropolitan area, showing springs, boreholes with flow logs, vertical fracture traces, dye traces, major contamination sites in the area with extensive hydrogeologic data, and the Paleozoic bedrock geology. The field trip route and stops are also shown. The inset illustration shows the general outline of the Twin Cities basin.

Figure 1.

Map of the Twin Cities Metropolitan area, showing springs, boreholes with flow logs, vertical fracture traces, dye traces, major contamination sites in the area with extensive hydrogeologic data, and the Paleozoic bedrock geology. The field trip route and stops are also shown. The inset illustration shows the general outline of the Twin Cities basin.

Figure 2.

Generalized stratigraphic column, depicting the lithology, thickness, and nomenclature for the Upper Ordovician formations in the Twin Cities Metropolitan area. The spring count lists the number of springs emanating from a specific stratigraphic interval. The composite natural gamma log (units in API-G) is the log shown on the left alongside the log constructed from a handheld gamma-ray scintillometer (measurements in counts per second) shown on the right.

Figure 2.

Generalized stratigraphic column, depicting the lithology, thickness, and nomenclature for the Upper Ordovician formations in the Twin Cities Metropolitan area. The spring count lists the number of springs emanating from a specific stratigraphic interval. The composite natural gamma log (units in API-G) is the log shown on the left alongside the log constructed from a handheld gamma-ray scintillometer (measurements in counts per second) shown on the right.

Figure 3.

Conceptual model of hydrogeologic attributes of the Platteville Formation and adjacent units. All parts of the formation are known, at least locally, to have moderate to very high horizontal hydraulic conductivity. Preferential termination of vertical fractures results in one or more aquitards in the Hidden Falls Member that commonly perch a water body in the overlying heavily fractured Magnolia Member. The combined Hidden Falls, Mifflin, and Pecatonica Members, despite yielding water horizontally and having vertical fractures, serve as a significant vertical barrier to flow in areas several tens of meters away from eroded edges of the formation. Closer to eroded edges, aquitard integrity is substantially diminished. See text (especially for Stop 9) for additional discussion, including horizontal and vertical hydraulic conductivity and sources of information compiled to support the model.

Figure 3.

Conceptual model of hydrogeologic attributes of the Platteville Formation and adjacent units. All parts of the formation are known, at least locally, to have moderate to very high horizontal hydraulic conductivity. Preferential termination of vertical fractures results in one or more aquitards in the Hidden Falls Member that commonly perch a water body in the overlying heavily fractured Magnolia Member. The combined Hidden Falls, Mifflin, and Pecatonica Members, despite yielding water horizontally and having vertical fractures, serve as a significant vertical barrier to flow in areas several tens of meters away from eroded edges of the formation. Closer to eroded edges, aquitard integrity is substantially diminished. See text (especially for Stop 9) for additional discussion, including horizontal and vertical hydraulic conductivity and sources of information compiled to support the model.

Figure 4.

Photograph showing a close-up view of St. Mary’s spring discharging from the Hidden Falls and Magnolia bedding plane contact. The scale object is 16 cm long.

Figure 4.

Photograph showing a close-up view of St. Mary’s spring discharging from the Hidden Falls and Magnolia bedding plane contact. The scale object is 16 cm long.

Figure 5.

Example of the geophysical and hydraulic data collected from a monitoring well on the East Bank of the University of Minnesota campus (well no. 664362). There was no measurable ambient flow in this well. Water was injected in this hole at a rate of 10 gpm, and all water exited the borehole at a depth corresponding to a caliper deflection and bedding plane fracture in the video log at about 14 m (46 ft). Gray lines indicate measurements from ambient conditions, and black lines reflect injection conditions. Inset photograph is a still shot from the video log of the fracture that accepted water.

Figure 5.

Example of the geophysical and hydraulic data collected from a monitoring well on the East Bank of the University of Minnesota campus (well no. 664362). There was no measurable ambient flow in this well. Water was injected in this hole at a rate of 10 gpm, and all water exited the borehole at a depth corresponding to a caliper deflection and bedding plane fracture in the video log at about 14 m (46 ft). Gray lines indicate measurements from ambient conditions, and black lines reflect injection conditions. Inset photograph is a still shot from the video log of the fracture that accepted water.

Figure 6.

Correlated gamma logs of the six wells on the West and East Banks of the University of Minnesota campus, with a bedding plane fracture that dominated hydraulics in the borehole at the Hidden Falls–Magnolia contact, and their hydraulic conductivity values, ranging from 300 to 47,000 ft/d. H—fracture height, which represents the range of possible fracture apertures estimated from the video and caliper logs.

Figure 6.

Correlated gamma logs of the six wells on the West and East Banks of the University of Minnesota campus, with a bedding plane fracture that dominated hydraulics in the borehole at the Hidden Falls–Magnolia contact, and their hydraulic conductivity values, ranging from 300 to 47,000 ft/d. H—fracture height, which represents the range of possible fracture apertures estimated from the video and caliper logs.

Figure 7.

Vertical fracture map of the Mendota locality with and without the outcrop photograph. Vertical fractures are shown as black lines. The gray lines represent the upper and lower contacts of the Hidden Falls and correspond to the vertical-fracture termination horizons. Each member of the Platteville Formation has a distinct style of vertical fracture spacing and density and act as mechanical units with vertical fractures terminating at member contacts specifically near the top and bottom of the Hidden Falls Member. Exposure is ~7.6 m (25 ft) high.

Figure 7.

Vertical fracture map of the Mendota locality with and without the outcrop photograph. Vertical fractures are shown as black lines. The gray lines represent the upper and lower contacts of the Hidden Falls and correspond to the vertical-fracture termination horizons. Each member of the Platteville Formation has a distinct style of vertical fracture spacing and density and act as mechanical units with vertical fractures terminating at member contacts specifically near the top and bottom of the Hidden Falls Member. Exposure is ~7.6 m (25 ft) high.

Figure 8.

(A) Vertical fracture map of the Shepard Road locality with and without the outcrop photograph. Vertical black lines represent the vertical fractures. Gray lines depict the vertical-fracture termination horizons near the top and bottom of the Hidden Falls Member similar to Figure 7. Exposure is ~14 m (45 ft) high. (B) Vertical differences in static head conditions within the Platteville Formation from three monitor wells on the campus of the University of Minnesota. These preliminary packer-test results suggest poor vertical hydraulic connectivity between the Magnolia Member and underlying members, indicated by the abrupt change in head differences above and below the packer across the Magnolia–Hidden Falls contact strata. This contact corresponds to the point at which vertical fractures preferentially terminate in outcrop.

Figure 8.

(A) Vertical fracture map of the Shepard Road locality with and without the outcrop photograph. Vertical black lines represent the vertical fractures. Gray lines depict the vertical-fracture termination horizons near the top and bottom of the Hidden Falls Member similar to Figure 7. Exposure is ~14 m (45 ft) high. (B) Vertical differences in static head conditions within the Platteville Formation from three monitor wells on the campus of the University of Minnesota. These preliminary packer-test results suggest poor vertical hydraulic connectivity between the Magnolia Member and underlying members, indicated by the abrupt change in head differences above and below the packer across the Magnolia–Hidden Falls contact strata. This contact corresponds to the point at which vertical fractures preferentially terminate in outcrop.

Figure 9.

(A) Photograph of the Magnolia and Hidden Falls contact on the Fort Snelling bike trail, showing termination of vertical fractures at the top of the Hidden Falls Member. Exposure is ~3 m (10 ft) high. (B) Close-up view, showing the bedding plane interval that corresponds to the conduit recognized in many boreholes and at springs across the Twin Cities Metropolitan area and the clay-rich material that is filling the bed partings. Head of hammer for scale is ~18 cm long.

Figure 9.

(A) Photograph of the Magnolia and Hidden Falls contact on the Fort Snelling bike trail, showing termination of vertical fractures at the top of the Hidden Falls Member. Exposure is ~3 m (10 ft) high. (B) Close-up view, showing the bedding plane interval that corresponds to the conduit recognized in many boreholes and at springs across the Twin Cities Metropolitan area and the clay-rich material that is filling the bed partings. Head of hammer for scale is ~18 cm long.

Figure 10.

Photograph of Camp Coldwater Spring, discharging near the base of the tower at the Hidden Falls and Magnolia contact.

Figure 10.

Photograph of Camp Coldwater Spring, discharging near the base of the tower at the Hidden Falls and Magnolia contact.

Figure 11.

Geology of the Camp Coldwater Spring vicinity, showing the location and orientation of vertical joints (black lines) that converge toward the emergence of the spring. The dark-gray background color represents a full Platteville subcrop; the medium-gray color represents a partial Platteville subcrop where only the Hidden Falls, Mifflin, and Pecatonica Members are present; and the lightest gray color is where all the Platteville is absent. Compiled from Sunderman and Kurtz (2000) and Bison Service Company (2000a, 2000b).

Figure 11.

Geology of the Camp Coldwater Spring vicinity, showing the location and orientation of vertical joints (black lines) that converge toward the emergence of the spring. The dark-gray background color represents a full Platteville subcrop; the medium-gray color represents a partial Platteville subcrop where only the Hidden Falls, Mifflin, and Pecatonica Members are present; and the lightest gray color is where all the Platteville is absent. Compiled from Sunderman and Kurtz (2000) and Bison Service Company (2000a, 2000b).

Figure 12.

Photograph of Bird Dropping Spring discharging at the base of two vertical fractures in the upper Mifflin Member. Bluff exposure in the photograph is ~2 m (7 ft) high.

Figure 12.

Photograph of Bird Dropping Spring discharging at the base of two vertical fractures in the upper Mifflin Member. Bluff exposure in the photograph is ~2 m (7 ft) high.

Figure 13.

Map of excavation at the Minnesota Library Access Center underground library storage facility at the University of Minnesota, highlighting mapped vertical joints on the ceiling of the excavation, which is the bottom of the Platteville Formation. Monitoring of flow collected from pans installed beneath leaking joints provides quantification of leakage rates. The site lies beneath an ~2.1 m (7 ft) thick body of water perched on top of the Hidden Falls Member. Modified from Peer Environmental and Engineering Resources, Inc. (2001).

Figure 13.

Map of excavation at the Minnesota Library Access Center underground library storage facility at the University of Minnesota, highlighting mapped vertical joints on the ceiling of the excavation, which is the bottom of the Platteville Formation. Monitoring of flow collected from pans installed beneath leaking joints provides quantification of leakage rates. The site lies beneath an ~2.1 m (7 ft) thick body of water perched on top of the Hidden Falls Member. Modified from Peer Environmental and Engineering Resources, Inc. (2001).

TABLE 1.

SUMMARY OF TRANSMISSIVITY AND STORAGE COEFFICIENT VALUES FROM PUMPING TESTS NEAR MINNEHAHA FALLS PARK CARRIED OUT BY LIESCH (1973) AND KELTON BARR CONSULTING (2000)

Well No.Pumping Rate (gpm)TestDuration(hr)Maximum Drawdown(ft)Transmissivity(T)(gpd/ft)Storage Coefficient(S)Parameters calculated from:
T1d-wa904817.540,000Recovery in pumping well
T1d-wa904817.536,5542.60E-03Time-drawdown curve at t1f-Pa (6”)
T1d-wa904817.5530,0008.80E-03Distance-drawdown curve from T9-wa to T10-wa
T2d-wa824.51,400Time-drawdown curve at pumping well
T2d-wa824.53,8404.00E-05Time-drawdown curve at t2b-Pa (6”)
T2d-wa824.55,0001.20E-04Distance-drawdown curve
T3d-wa30247.55,2004.20E-05Time-drawdown curve at t3b-Pa (6”)
T4b-Pa (6”)814.94,0004.00E-03Time-drawdown durve at t4d-wa
T4d-wa811.94,2004.00E-03Time-drawdown curve at t4b-Pa (6”)
T5b-Pa (6”)81.50.57,0001.80E-05Time-drawdown curve at t4b-Pa (6”)
T6-wa812.95,700Time-drawdown curve at pumping well
T7-wa101.253.72,900Time-drawdown curve at pumping well
T9-wa6020.14700,000Time-drawdown curve at pumping well
T10-wa6010.071,600,0009.00E-03Distance-drawdown curve at observation wells
T2d-Wb1032.736,8001.60E-01Drawdown at T2C-Pb
T4b-Wb3012.1517,6005.80E-02Drawdown at pumping well t4b-Wb
T4b-Wb3012.1526,000
Average =176,2472.24E-02
Median =6,8004.00E-03
Mean =16,8811.53E-03
Well No.Pumping Rate (gpm)TestDuration(hr)Maximum Drawdown(ft)Transmissivity(T)(gpd/ft)Storage Coefficient(S)Parameters calculated from:
T1d-wa904817.540,000Recovery in pumping well
T1d-wa904817.536,5542.60E-03Time-drawdown curve at t1f-Pa (6”)
T1d-wa904817.5530,0008.80E-03Distance-drawdown curve from T9-wa to T10-wa
T2d-wa824.51,400Time-drawdown curve at pumping well
T2d-wa824.53,8404.00E-05Time-drawdown curve at t2b-Pa (6”)
T2d-wa824.55,0001.20E-04Distance-drawdown curve
T3d-wa30247.55,2004.20E-05Time-drawdown curve at t3b-Pa (6”)
T4b-Pa (6”)814.94,0004.00E-03Time-drawdown durve at t4d-wa
T4d-wa811.94,2004.00E-03Time-drawdown curve at t4b-Pa (6”)
T5b-Pa (6”)81.50.57,0001.80E-05Time-drawdown curve at t4b-Pa (6”)
T6-wa812.95,700Time-drawdown curve at pumping well
T7-wa101.253.72,900Time-drawdown curve at pumping well
T9-wa6020.14700,000Time-drawdown curve at pumping well
T10-wa6010.071,600,0009.00E-03Distance-drawdown curve at observation wells
T2d-Wb1032.736,8001.60E-01Drawdown at T2C-Pb
T4b-Wb3012.1517,6005.80E-02Drawdown at pumping well t4b-Wb
T4b-Wb3012.1526,000
Average =176,2472.24E-02
Median =6,8004.00E-03
Mean =16,8811.53E-03

Contents

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