KARST SUBSIDENCE IN THE MISSISSIPPIAN AQUIFER OF AUDRAIN COUNTY, MO
KARST SUBSIDENCE IN THE MISSISSIPPIAN AQUIFER OF AUDRAIN COUNTY, MO
Figure 1: Looking out from the capped paleo-sink at the site. Ice column to the right of the entrance.

GO 571
Blake Smotherman

Spring, 2009

Contents
Abstract Regional Geology and Hydrology
Karst Features Site Hydrology
Future of the Site References

Abstract

In 2008, the Missouri Geological Survey received a report that two sinkholes had opened on a property in northern Audrain County, in northern Missouri. While sinkholes are common in the karsted areas of southern Missouri, it is rare to have active karst north of the Missouri River, in an area typically covered in glacial till and Pennsylvanian residuum. Upon investigation by geologists from the Survey, it was discovered that two paleo-sinks had opened. One compromised the clay liner of a three-acre artificial lake and de-watered it in 24 hours. A second paleo-sink that retained its overburden cap had opened nearby, possibly due to leakage from the lake traveling through the karst under the farmstead and winnowing away the fines from the bottom up. A nearby gaining stream was observed to not be losing water into the underlying karst. Other features indicating a shallow, underlying karst system were also observed during the initial investigation. Though not normally exposed at the surface in this region of Missouri, this stratigraphic unit represents a formation within the Mississippian Aquifer

The investigation of these sinkholes provided a rare glimpse into the nature of the underlying Mississippian aquifer in this region. There appears to be fault displacement in Youngs Creek to the west of the site, indicating that this area may be an uplifted horst, having brought the Mississippian-age strata closer to the surface and displacing the thick surficial material that is typical for this area. This horst and graben landscape, generated by tensional tectonic forces, is exposed in the southern Missouri, but is usually covered by glacial drift or residuum in the northern part of the state. The thin veneer of surficial material that covers this site and the close proximity of karsted Mississippian limestone to the surface indicates that this area probably has a zone of freshwater in the subsurface that is not typical of the heavily mineralized groundwater in this region. The size of this zone could depend on variables such as the extent of the underlying karst terrain, differential heads of pressure and barrier boundary conditions generated by the up-thrown section of karsted Mississippian-age limestone as well as the probable changes in hydraulic conductivity. Without thick glacial drift present, there is a lesser chance that waters moving through the vadose zone will mineralize because of long residence times in low-conductivity materials to the extent that is common in most of the rest of northern Missouri. This area also represents a zone of potential groundwater contamination, considering the karst bedrock and the location of the site to large confined animal feeding operations.

Field analysis of the materials that had filled the paleosink revealed a mixture of residual Mississippian-age clasts and glacially derived silts, clays and sands. Investigations into the inside of the paleo-sink revealed that its roof was composed of a dome of high-plasticity redoxomorphic clays and poorly sorted gravel- to boulder-size clasts. The paleo-sink was observed to have collapsed further and had developed linearity that may indicate subsidence along a karst conduit. Continued collapse along this inferred conduit within the aquifer may eventually threaten a bridge that crosses Youngs Creek.

Regional Hydrology and Geology

Figure 2 is a structure map of Audrain County, Missouri. The site is approximately 15 miles to the northwest of Mexico, Missouri (black wire frame box). While the structure map shows two major anticlines in the county, water, oil and gas well logs, as well as numerous geohydrologic projects performed in the area by the Missouri Geological Survey have indirectly inferred the existence of a horst and graben landscape as is seen in southern Missouri. This landscape is generated by tensional tectonic forces. As the continental crust is stretched during a particular geologic period, rhomboidal prisms of carbonate develop along fractures. Some of the prisms slowly sink topographically lower, while alternating prisms or ridges remain topographically higher. In part this produces the hills and valleys seen on the Ozark Plateau today. It is reasonable to assume that northern Missouri also developed a similar horst and graben landscape that was subsequently ground down or buried by multiple Quaternary glacial advances.

Figure 2: Structural geology map of Audrain County. The site is within the box located in the northwest corner.(McCracken, 1971)

Figure 3 is a site map showing the proximities of the karst collapses and the area of headward erosion that exposed an area of carbonate float and residuum above the gaining stream. Though not clearly shown on the map, sudden bends in the channel of Youngs Creek seem to indicate the existence of structural controls on the stream bed. These abrupt angles are often indicative of normal faulting, which is common in Missouri, but can also indicate patterns of joints and fractures that may by the product of solution-weathering in the uppermost strata of the carbonate, or may be related to tectonically produced lineaments in the basement granite that underlies all of the continental crust.

Figure 3: Map showing karst features of the site in proximity to Youngs Creek, a gaining stream.

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Figure 4: The extent of the Mississippian Aquifer in Missouri. (Miller and Appel, 1997)

The Mississippian aquifer (Figure 4) is the uppermost aquifer in Paleozoic rocks in northern Missouri. The aquifer extends over all of Missouri north of the Missouri River and into southern Iowa and southwestern Illinois (Miller and Appel, 1997). It is bounded by the Missouri River on the west and south, and by the Mississippi River and the Lincoln Fold (a structural anticline) on the east. Lithostratigraphically, the aquifer is composed of Mississippian-age (360-325 mya), Osagean-sequence rocks. The Burlington-Keokuk Limestone, the Fern Glen Formation, the Sedalia Formation, and the Chouteau Formations compose the aquifer; of these formations, the Burlington-Keokuk is the principal water-yielding formation. The aquifer can be overlain by glacial drift, reworked loess or residuum derived from the breakdown of Pennsylvanian-age cyclothemic deposits such as sandstone, shale, limestone and coal. The thick overburden usually serve as a low permeability confining unit over northern Missouri, contributing to and preserving the highly mineralized state of the groundwater in this area. With the Mississippian aquifer so close to the land surface at the site, there should be a zone of fresh groundwater perched above the saline groundwater.

Recharge to the aquifer is from precipitation. Typically in this part of northern Missouri, the precipitation would have to travel through the normally thick, low permeability overburden, becoming mineralized before it infiltrates the carbonate matrix of the Mississippian Aquifer. At the site, with the Mississippian rocks exposed at or near the surface, it is reasonable to assume that comparatively fresh water in the form of direct precipitation is moving immediately into the carbonate, creating a zone of fresh water that is not typical of areas north of the freshwater-saline water transition line in Missouri.

The uppermost bedrock exposed at the site is Burlington-Keokuk Limestone. The carbonate exposures at the site were coarse-grain, crinoidal packstone/grainstone with discontinuous amorphous blebs of white chert. The blebs displayed horizontality indicating that the diffuse silica had precipitated and concentrated above and along discontinuous bedding planes or some other low permeability areas.

The surface residuum on the property was identified as a mix of inorganic clayey silts and inorganic silty clays (ML/CL) with underlying inorganic clays of high plasticity (CH) in the clay gravel (GC) residuum (Figure 5). The gravels were composed primarily of angular Warsaw and Burlington-Keokuk chert. The uppermost bedrock is the Mississippian-age Burlington-Keokuk Limestone, which exhibits high permeability at this site. This formation is composed of the Burlington Limestone, a characteristically white to gray, medium to coarsely crystalline, medium to coarsely crinoidal, medium to thick-bedded, often cross-stratified, chert-free to sparsely cherty limestone. The conformably underlying Keokuk Limestone is a bluish-gray, medium- to coarsely crystalline, medium-bedded limestone which contains abundant light-gray chert in the form of layers and nodules. The contact between the two formations is transitional and often difficult or impossible to identify.(Thompson, 1995) Underlying the Burlington-Keokuk Limestone is the Mississippian-age Chouteau Group. This formation consists of two limestone facies. The lower facies is a very finely crystalline to sublithographic, dark-gray mudstone, usually appearing as wavy-bedded units with thin undulating shale partings. The upper facies is a finely to medium-crystalline, light-gray limestone.(Thompson, 1995)

Figure 5: Loess deposit overlying paleo-sink residuum.

Figure 6 displays the typical stratigraphic succession and its hydrologic characteristics for this area (Miller and Van Dike, 1997). In northern Missouri, much of the Pennsylvanian lithology has been eroded into a thick overburden of sands, silts and clays, covered with glacial till deposits, or has been scoured away by Quaternary glacial advances. The thin overburden at the site represents layers of Mississippian residual chert clasts, a small amount of Pennsylvanian sands or Quaternary sand tills and reworked loess. The Mississippian-age rocks are the overlying bedrock at the site and at areas to the east along the Lincoln Fold, a large structural anticline that dominates the geology and hydrology of northeastern Missouri.

Figure 6: Lithologic and hydrologic stratigraphy of northeastern Missouri, (Miller and Van Dike, 1997)

Figure 7 demonstrates the averages of total dissolved solids in different regions of the state. Notice that the least potable groundwater supplies are concentrated in northern Missouri. This is due to the introduction of minerals such as iron, chlorides and sulfates, mostly from the breakdown of Pennsylvanian-age cyclothems. Many of the cyclothems have coal-bearing strata that are also rich in iron sulfides. The sulfuric acid produced by these ores liberate iron, magnesium and manganese that is picked up by slow-moving groundwater. Carbonates from beds of dolomites and limestones, as well as the iron-rich (ferruginous) cement that binds many of the sandstones also contributes to the mineralization of groundwater in these areas.

Figure 7:Total dissolved solids in groundwater in Missouri (Missouri Groundwater Atlas, 1986)

Figure 8 displays the major aquifers of the state in the areas that they are the most used. Keep in mind that the older, underlying aquifers, such as the Cambrian-age St. Francois aquifer, underly the younger aquifer systems. For instance, in Audrain County around the site of interest, usable groundwater can be found in some glacial till deposits, Pennsylvanian-age sandstones, Mississippian-age limestones and Ordovician and Cambrian-age formations at the base of the continental carbonate sequence.

Figure 8: Primary regional aquifers in Missouri. Audrain County in the northeast is north of the freshwater-saline water transition line. (Missouri Geological Survey Fact Sheet No. 4).

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Karst Features

The initial subsidence that caused concern is a sinkhole that opened below a 3 acre lake, draining it in within a 24-hour period (Figure 9). The lake was in its last stages of draining during the site visit. The clay at the surface of the lake was observed to have roughly hexagonal cracks in it that appeared to penetrate several inches into the subsurface, indicating substantial drying in the past that may have helped compromise the structural integrity of the clay liner of the lake (Figure 10). At the mouth of the sinkhole where water was streaming in, a bright yellow redoxomorphic clay (Figure 11) was observed under the compacted clay pad of the lake, along with residual chert clasts. These clays are indicative of karst underlying the site. Many paleosinks both collect clays brought into them and generate clays by their own dissolution. These clays become illuviated (concentrated) at various depths with groundwater of different geochemical make-ups percolating through them. These waters and their constituent ions react with metals such as iron, manganese and aluminum in the clays to produce the variable colors of oxidation and reduction that can be seen in the clays at the site. According to a relative of the landowner, they had constructed the lagoon 25 years ago and that it had never appeared to leak before the sinkhole opened up.

The stream that continues to feed into the basin of the former lake originates from a spring to the north. Along the way, the stream is becoming contaminated with livestock excrement and is providing a constant flow of effluent into the paleokarst under the site. Assuming that this paleokarst is on a topographically higher area of the aquifer, the resulting head coupled with the extant karst conduit system could force the contaminated groundwater far beyond the site.

Figure 9: Paleo-sink that reactivated beneath the three-acre lake.

Figure 10: Horizontal and vertical fracturing in the clay liner of the lake.

Figure 11: Eye of the paleo-sink, displaying redoxomorphic clays that occur in the karst at the site.

The second subsidence is a large rotational collapse that has opened up an underground void to the south of the house on the property (Figure 12). A stair-step scarp of surficial material leads down into the void. Upon investigation, the void was discovered to have a residual cap with no obvious bedrock support. The cap contained thin layers of gray, inorganic, highly plastic (CH) clays and thick strata of clay gravel (GC) residuum. The floor appeared to be Mississippian-age Burlington-Keokuk Limestone bedrock with large white chert blebs similar to some of the chert material in the residual fill. These characteristics can be indicative of a filled paleo-sink deposit. This feature is approximately 100 feet to the north of Youngs Creek, a gaining stream with substantial flow. Yet the bottom of the sink is approximately 20 feet below the bed of the stream.

Figure 12: Looking into collapse of the second subsidence, a paleo-sink with its cap intact over the karst conduit.

Figure 13 shows a column of icicles that formed from water perolating through the wall of the sink. This formation was approximately 2 feet in diameter. Water could be seen dripping along its surface at a steady rate. This demonstrates that the conduits in the carbonate bedrock give this part of the aquifer characteristics more typical of karst aquifers in southern Missouri, such as higher rates of discharge and transmissivity, with low storativity due to the size and interconnectedness of the conduit system. This could also accelerate the intrusion of contaminated surface waters into the subsurface. The Burlington-Keokuk Limestone in this area is usually underlain by Mississippian-age, Kinderhookian sequence Chouteau Group strata, such as Compton or Chouteau Limestones. While these lithologies generally have lower permeabilities due to their depth and multiple shale partings, the head and steady stream of water could potentially start to move through these underlying rocks as well, spreading groundwater contamination even farther. While most residential and municipal wells in this area penetrate to deeper Cambrian- and Ordovician-age aquifers, improper or compromised well casings could allow contaminated groundwater to spread fecal coliform and Girardea lambdia into drinking water wells.

Figure 13: Ice stalagmite formed by groundwater percolating into paleo-sink.

Figure 14 shows the matrix of the aquifer and the size that the conduits an attain in this near-surface part of it. The aquifer is composed of recrystallized grains of crinoids with some brachiopod and bryozoan debris. These grains have large surface areas that allow the weakly acidic precipitation moving into the conduit system to further dissolve the calcite crystals that make up most of the aquifer material, increasing its permeability and ability to transmit large amounts of groundwater at comparatively high velocities. The chert nodules and blebs that make up a small part of the aquifer act to divert groundwater around them, potentially further spreading groundwater around them and allowing the carbonic acid that the water carries to attack more of the aquifer skeleton.

Figure 14: Burlington-Keokuk Limestone forms the carbonate matrix of the karst topography and the Mississippian aquifer.

As can be seen from the roots and branches near the conduit at the base of the paleo-sink (Figure 15), the discrete flow moving into this structure can move large amounts of material. Though overburden collapse is also transporting material into the hole, water movement is concentrating it towards the swallet at the base.

Figure 15: Karst conduit at bottom of the paleo-sink. (approximately 30 feet deep from cavern entrance to conduit at the bottom)

Figure 16 shows the sharp contact between the underlying Burlington-Keokuk Limestone and the overlying residuum. As phreatic water moves slowly through the residuum matrix, it will come into contact with the limestone and both begin to precipitate some of the ionic load in solution and also begin to move laterally and dissolve and redistribute some of the carbonate. Note the heavily oxidized iron-rich clays between the chert clasts near the contact.

Figure 16: Residuum and carbonate contact.

Figure 17 is the roof of the paleo-sink. As the fine materials were winnowed away into the karst conduit system, movement of clay formed a medial cross-section through the residual material filling the paleo-sink. This allowed a unique perspective in being able to observe the middle of the structure, seeing how the residuum contacted and interacted with the limestone. Though not easily discernable in Figure 17, some of the carbonate float in the roof had begun to form small stalactites, indicating that this void had been opened for some time, with a thin cap of clay and loess holding up the overlying land surface.

Figure 17: Roof of the paleo-sink, formed by chert and carbonate clasts in a redoxomorphic clay matrix.

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Site Hydrology

The present issue for groundwater contamination is the amount of livestock excrement that is being carried unimpeded into the paleo-sink in the former lake basin. The water moving into it has a very strong smell of sewage that betrays its contamination. The depth and extent of the buried karst topography in the area of the site is unknown at present. It could be a very localized phenomenon, or it could connect with other buried zones of karst that could spread contaminated groundwater much farther away.

It is notable that on the south side of Youngs Creek, across from the site, a large Confined Animal Feeding Operation (CAFO) lagoon exists that did not appear to be leaking at the time of the site visit. Two other ponds were observed on-site that also appeared to be holding the designed amount of water. While the lagoon is stable now, if underlying karst is connected with the paleo-sinks across Youngs Creek, this lagoon may be compromised in the coming years. Again, at this time, there is no way to know.

Aside from the observed sinkholes, there was a feature to the east of the subsidences that was also indicative of active karst formation at the site. The feature appeared to be an area of headward erosion with large residual chert and carbonate clasts in its bed (Figures 18 and 19). The carbonate was Burlington-Keokuk Limestone. Fine-grained surficial material appeared to be washing out when the dam forming the lake was overtopping during flood events, leaving behind the chert and non-weathered limestone. Several pinnacles of similar limestone were observed along the north side of Youngs Creek. While not good for the quality of the surface waters of Youngs Creek, the overtopping did expose a larger area of the buried karst topography. Since this feature was not holding any water, it is also losing into the subsurface, possibly connecting to the karst conduits of the two paleo-sinks.

Figure 18: Overtopping of the lake dam has laid the grass down in the direction of flow.

Figure 19: Water flowing over dam has resulted in headward erosion that has washed away fine silts and clays, leaving behind limestone float and residual chert clasts.

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Future of the Site

Based on the observations made during the site investigation, these paleo-sinks will likely continue to grow unless engineering remediation and wiser water-use practices are used on the property. At this time, the landowners are attempting to secure funding to rebuild the lake on the same property. This would seem to only generate more head of pressure and hold more water to wait for the next sinkhole collapse. While the karst is not affecting the landowner's home directly, some evidence of the formation of rotational slides like the one that opened the capped paleo-sink were observed. The uppermost land surface, either loess or landscaping, is setting on a glide plane between that material and the residual clays in the karst topography. When moisture collects and starts traveling along the glide plane, it lubricates the uppermost materials and causes them to move as well, potentially cracking foundations and torquing the house structure to the point that windows and doors stick and cracks form along the walls. This glide plane movement, coupled with the potential for other paleo-sinks to open or new ones to form, would seem to add a sense of urgency to developing more effective surface and subsurface water control on the propoerty. Karst in this area is very rare, and no other similar features are known of within several miles of the site. These features may be part of a connected, regional paleo-karst landscape that has been filled with residual material and covered with glacial drift and reworked loess. This site is being impacted by the transport of large amounts of fine materials such as silt and clay into a deeper aquifer system through solution-weathered joints and conduits. As this material is transported away, a void space is left into which the surficial materials are collapsing. The void space may be defined by the geometry of high-plasticity clay layers in the subsurface. Increased rainfalls from this year, including the passage of Hurricane Ike, may have also accelerated the process of sinkhole collapse at this site. Future heavy rain events will surely continue to affect the development of karst in this area of northern Missouri.

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References

Coduto, D.P., 1999, Geotechnical Engineering: Principles and Practices, pgs.24, 207-210

Cleary, R.W., Cherry, J.A., Nielson, D.M., et.al, 2009. Pinceton Groundwater Pollution and Hydrology Coursebook, pgs.2-110 - 2-118,

Fetter, C.W., 2001. Applied Hydrogeology, pgs. 314-318, 356-357, 362-363

McCracken, M.H., 1971. Structural Features of Missouri. pg. 65

Miller, D.E, Vandike, J. E., 1997. Groundwater Resources of Missouri. pg. 154

Miller, James A., and Appeal, Cynthia L., 1997, GROUND WATER ATLAS of the UNITED STATES, Kansas, Missouri, and Nebraska, HA 730-D. http://www.capp.water.usgs.gov/gwa/ch_d/-text5.html

MISSOURI WATER ATLAS, 1986, SP-002.

Ritter, D.F., Kochel, R.C., Miller, J.R., 2002. Process Geomorphology, pgs. 414-420.

Sutch, S., Dirth, L., 2006. ASBOG Study Manual, pgs. 8-9 - 8-14.

Thompson, T.L., 1986. Paleozoic Succession in Missouri, Part 4 Mississippian System RI-70, pgs. 90-96, 101-106

Thompson, T.L., 1995. The Stratigraphic Succession in Missouri,Volume 40 (Second Series), pgs. 79-83

White, W.B., Crawford, N., 2008. Karst Hydrology Field Studies Manual, Lecture 9

Websites and web pages

http://mo.water.usgs.gov/fact_sheets/gndwat.htm