Caves and Karst of the Upper Midwest, USA

Karst in the Upper Midwest occurs within a thick sequence of mixed carbonate and siliciclastic Cambrian through Pennsylvanian sedimentary rocks, with a minor occurrence of karst in Proterozoic sandstone. Deposition of the sediments occurred on a marine epeiric ramp that spanned much of the North American continent through most of the Paleozoic. The Upper Midwest region experienced dramatic changes in sea level over geologic time, resulting in the observed sequence of interbedded carbonate and clastic rocks. The greatest degree of karst development occurs within (1) the Lower Ordovician Prairie du Chien Group below the Sauk-Tippecanoe (Knox) unconformity, (2) the Upper Ordovician Galena Group, (3) the Middle and Upper Devonian Wapsipinicon and Cedar Valley Groups, and (4) the Middle Mississippian Mammoth Cave Group and correlative formations. Uplift and exposure of the rocks likely occurred in the Permian, with some later deposition of Cretaceous terrestrial sediments atop the marine strata. Nearly all the Cenozoic sedimentary units were removed by ice sheets during the Pleistocene; however, pockets of Cretaceous sediments persist on the margins of the Driftless Area, a region of the Upper Mississippi River Valley that remained largely free of ice during the last ice age.

Cave history among the states comprising the Upper Midwest, USA, involves three periods: (1) European exploration of the region, usually before 1850; (2) Euroamerican settlement in the century from 1850 to 1950; and (3) modern exploration by cave clubs or NSS grottoes, usually following World War II. The earliest mentions of caves in this region are often tied to French explorers along rivers in the seventeenth and eighteenth centuries. Some of the most famous show caves had their beginnings in the subsequent, settlement period. Finally, the project caving of dedicated clubs and the “pulsed paradigm” of exploration of the longer cave systems have continued to the present time. Ironically—contrasting with caves like Mammoth in Kentucky—the smaller the caves the larger their history: Cave in Rock (Illinois), Carver’s Cave and Fountain Cave (Minnesota), Decorah Ice Cave (Iowa), and the caves of the Dells (Wisconsin) are good examples.

Carbonate caves and karst lands underlie most of southeastern Minnesota. The karst features are developed in relatively flat-lying lower Paleozoic limestones, dolostones, and sandstones. These rocks are major bedrock aquifers and contain about three-quarters of the groundwater resource in Minnesota. Over half of the people in Minnesota live on the karst lands and most depend on groundwater for drinking, agriculture, industrial, and recreational water supplies. Karst phenomena, aquifers, landscapes, and caves are fundamental boundary conditions, often unrecognized, on human activities in these areas. The Minneapolis/St. Paul metropolitan area contains a wide array of caves, natural and artificial, in the St. Peter Sandstone. The natural caves are developed by groundwater piping processes and are usually found near river valleys. This sandstone was mined for silica for use in glass making and for mortar and foundry sands. The resulting artificial caves served for mushroom growing, cheese ripening, entertainment, and other repurposings. Three electronically accessible databases contain information about Minnesota’s karst features. The Minnesota Karst Features Database (KFD 2020) lists the locations of sinkholes, stream sinks, stream sieves, and miscellaneous other karst features. Tipping et al. (2015) describe the development of the KFD over several decades. Information on the locations of springs and seeps was originally part of the KFD but was then used as the start of a state-wide Minnesota Spring Inventory (MSI 2020). Brick (2017c) provides the essential guidance document for MSI with extensive background information. Information on dye tracing and springshed delineation is available in the Minnesota Groundwater Tracing Database (MGTD 2020). Green et al. (2018) describe the development and use of the MGTD.

Iowa landforms have been shaped by a range of processes, including multiple glacial incursions. Many of these diverse landforms harbor a variety of karst features, including numerous examples of caves formed by both solutional and mechanical processes acting on an array of dolostones, limestones, gypsum, and sandstones. Cave development has been noted in several geologic systems, ranging from upper Cambrian to Pennsylvanian sandstones and lower Silurian to middle Mississippian carbonates. Distributional patterns of cave and karst development in Iowa are examined as a function of distinct physiographic regions, respective lithologies, hydrogeologic settings, and structural morphologies illustrated by representative sites. Karst systems have played a significant role in shaping Iowa’s unique surficial and subterranean habitats as well as archaeological, historical, and modern landscape uses. Ongoing exploration and study of Iowa caves continues to improve our collective understanding of past climate patterns and modern water quality issues as well as supporting comparative models of karst development on a regional scale. This chapter provides an overview examining principal karst-associated landscapes and controls associated with paleokarst, pseudokarst, and karst cave occurrences in Iowa.

The Illinois Basin occupies much of Illinois and contains Paleozoic sedimentary rocks consisting of Cambrian through Pennsylvanian strata. Most of the formations are dominated by limestone and dolomite. The carbonate rocks range from Ordovician through Mississippian in age, with lesser amounts occupying Cambrian and Pennsylvanian strata. Most of these carbonate rocks are exposed in the northern, western, and southern margins of Illinois and adjacent states. All bedrock has been extensively fractured by tectonic forces, and were exposed beneath a thin cover of unconsolidated sediment, recharge has dissolved the carbonate rocks along fracture and bedding planes, creating caves, solution-enlarged crevices, and cover-collapse sinkholes. Illinois has five major karst areas. The northernmost karst region is the Driftless Area in northwestern Illinois, where dolomites of the Ordovician Galena and Silurian formations are exposed in outcrops or near the surface and form cover-collapse sinkholes, short network-type caves, sinuous conduits, and small karst springs. Some of the caves and conduits were created and mineralized by Mississippian-type ore-forming solutions during the Permian Period, which were mined for lead and zinc sulfides from the 1700s to the mid-1900s. The North-Central karst region is similarly dominated by dolomites of the Ordovician Galena Group that form small sinkholes and short network-type caves. Bedrock in both areas has abundant fractures and solution-enlarged crevices exposed in road cuts and quarries. Situated in western Illinois, the Lincoln Hills karst region contains rocks that range from the Silurian to Mississippian Periods that form cover-collapse sinkholes and caves. Most of the caves are network-type caves that are relatively short owing to infilling of fine-grained sediment. Located in southwestern Illinois, the Salem Plateau karst area is composed of the St. Louis and Ste. Genevieve Limestone formations, forming the largest cover-collapse sinkholes, the largest springs, and the longest caves in the state, most of which are branchwork type. The Shawnee Hills karst area in southern Illinois has abundant sinkholes and network-type caves in a variety of formations that range from the Silurian through Mississippian Periods. This area is home to Cave-In-Rock, a well-known historic remnant cave.

Wisconsin’s caves and karst landscape are more widespread than is commonly appreciated and they merit greater attention. Roughly, 20% of the state exhibits karst landscape and carbonate-cemented sandstones contribute to carbonate-dominated groundwater underlying nearly 45% of the state. The caves and karst are formed primarily in Ordovician and Silurian dolostones and in underlying and interbedded Cambrian and Ordovician sandstones. Pleistocene glaciation has obscured much of the eastern, central, and south-central karst, leaving two main areas of exposure, in the Door Peninsula and adjacent counties of east-central Wisconsin and in the southwestern Driftless Area. In eastern, Wisconsin surface karst is essentially limited to Door County, where there are extensive open joints, sinkholes, sinking streams, karren, caves, and springs, plus glaciokarstic benches and steps. The caves are of dissolutional, littoral, and mechanical origin. Rapid karstic hydrologic connections between the surface and shallow groundwater pose serious threats of contamination, necessitating appropriate land use regulation. The karst hydrology of the Silurian aquifer south of Door County continues to be underappreciated but became manifest during construction of a deep tunnel stormwater management network in Milwaukee in the 1980s and 1990s. The southwestern karst is better known and escaped extensive Pleistocene glaciation. It may have evolved more-or-less continuously since the Paleozoic, but hypogenic contributions, long-term environmental episodicity and recent Pleistocene influences may all be involved. The karst is a fluviokarst, integrated into the fluvial landscape with a wide array of dry valleys, sinkholes, caves, and springs. Dissolution remains active, but the karst hydrogeology deserves greater study, as do the springs and caves, which have probably been influenced by hydrothermal processes, hypogenic dissolution, and possibly by dissolution under saline–freshwater mixing conditions. Lead ores were mined extensively from the Platteville and Galena formations during the nineteenth century and quarrying remains a significant industry. Land use has been dominated by small-scale agriculture, but intensive agriculture and sand mining have emphasized the potential for land-use conflicts and detrimental environmental impacts. Regional karst science has yet to be incorporated adequately into planning mechanisms, but the karst is becoming a central theme for NGOs and the regional press. There are also caves and karst developed in Paleozoic sandstones in southwestern and central Wisconsin, including pavements, natural bridges, and other fragile rock formations.

Lead-zinc mineralized crevice caves developed within the Galena Group (Ordovician) of the Upper Mississippi Valley lead-zinc district (UMVLZD) result from both hypogene and epigene processes. Far-field tectonic movements occurring in the late Paleozoic resulted in extensive fracturing and consequent hydrothermal circulation in the late Paleozoic between the Proterozoic basement and the carbonate strata of the Galena Group. This caused significant coeval solution of the limestone, dolomitization, and sulfide mineral precipitation, especially where fractures intersected strata of a specific lithofacies characterized by the trace fossil Thalassinoides. At these intersections pod-shaped masses of sulfide minerals formed. During the Cenozoic, erosion of strata overlying the Galena Group resulted in the oxidation of the pod-shaped sulfide mineral deposits with resultant production of sulfuric acid and consequent solution of surrounding dolostone forming in some cases substantial caves and cave systems. Historically these caves were a major source of the mineral galena (PbS) with a utilization time line reaching back to Archaic (3500–1000 BCE).

Ten fields of cave science in the Midwestern USA are examined from the standpoint of the dominant paradigms in those fields: Karst, geology, hydrogeology, sedimentology, meteorology, biology, spring ecology, paleontology, archeology, and rock art. The paradigm chosen for examination in each field generally has roots in the nineteenth century, so as to give it some historical depth, but not so remote as to have little relevancy to ongoing, contemporary research. It was found that the dominant paradigms arose within the Midwest itself in some cases, diffusing to the outside, while in others it was imported from surrounding areas.

Over 300 non-accidental animal species are recorded from subterranean (cave and groundwater) habitats in the Upper Mississippi River Valley (UMV) region. Most of these are trogloxenes or troglophiles and these are not restricted to nor morphologically specialized for subterranean habitats. Conspicuous examples are bats, fish, moths, camel crickets, and many kinds of flies. A rich fauna of some 56 species are seemingly obligatory inhabitants of subterranean habitats. Twenty-two species of these are terrestrial, termed troglobites, and 34 species are aquatic, called stygobites. The troglobites include pseudoscorpions, mites, spiders, millipeds, collembolans, bristletails, and a beetle. These generally have smaller distributional ranges, but some do occur in states outside of the UMV region. The stygobites include flatworms, snails, amphipods, and isopods. These generally have larger distributional ranges and most occur in southwestern Illinois, which was near to, but not covered by Pleistocene glacial ice. A subset of stygobites are known only from groundwaters in non-cavernous glaciated regions. Among the vertebrates, ten species of trogloxenic cave-inhabiting bats are known from the UMV. Other vertebrates include the Spring cavefish (Forbesichthys agassizii), three species of plethodontid salamanders, one frog, and a bird that nests in cave entrances. Included among the mammals, other than bats, are the White-footed mouse (Peromyscus leucopus), Eastern woodrat Neotoma floridana, and Raccoon (Procyon lotor).

Agriculture in Iowa, Illinois, Minnesota, and Wisconsin (the Upper Mississippi Valley region, or UMV) is an important component of each state’s economy. Karst landscapes though in the UMV can be locally common and can complicate agricultural activities. Complications include thin and rocky soil, nutrient and fertilizer management concerns, and effects on groundwater resources. Best Management Practices (BMPs) for agricultural activities have historically been written for surface water concerns, with minimal consideration for the interconnection between surface and groundwater in karst areas. This chapter documents the concerns of higher livestock concentrations in karst landscapes and presents examples (and case studies) of approaches and innovative techniques to minimize agriculture’s environmental impacts in karstic areas. The goal of this chapter is to provide helpful information that will facilitate changes in agricultural practices in karstic landscapes of the UMV that will aid in protecting groundwater and soil resources.

Caves of the Midwestern U.S.A. have a history of paleoclimate studies going back more than 50 years. The most valued paleoclimate archives are speleothems, especially stalagmites, which commonly contain a straightforward internal stratigraphy, grow over many thousands of years in some cases, and can be readily dated using uranium-series techniques. Some of the more unique contributions that have come from Midwestern caves include: using speleothem carbon isotopes as a vegetation proxy, reconstructing the episodic nature of cave flooding from detrital layers in speleothems, and using broken speleothems and altered speleothem growth to identify the timing of past seismic events. Because speleothems contain high-quality paleoclimate information, the pace of cave paleoclimate studies has intensified in recent years. This increased demand on a fragile resource highlights the need for a strong conservation ethic among scientists working in caves.