The physiographic setting of Wisconsin lies at the junction of the Superior uplands in the north, and several subsections of the Central Lowlands Province in the United States. A veneer of unconsolidated Quaternary deposits overlies bedrock throughout much of the region [
24]. Bedrock consists of a complex array of eroded Precambrian rocks, overlain by a sequence of marine rocks related to ocean transgression-regression cycles. Bedrock units dip radially away from the Wisconsin Arch, toward the Michigan basin in the east, the Illinois basin in the south, and gently toward Iowa and Minnesota in the west. Wisconsin’s geologic history is preserved in rocks and unconsolidated sediments from three distinctly different periods of time, with long intervals of erosion or nondeposition occurring between each [
20].
2.2. Paleozoic History
The second interval of Earth’s history that is recorded in Wisconsin includes sedimentary rocks deposited during the Early to Middle Paleozoic Era. Rocks deposited during this interval consist mainly of sandstone, dolostone, and shale, and form the bedrock throughout much of the southern two-thirds of the state (
Figure 2). Nearly all of these rocks are marine or marginal marine, deposited during some of the highest sea levels of the Paleozoic Era. These rocks range in age from Late Cambrian to Late Devonian, with the Devonian rocks only preserved along the Lake Michigan shoreline north of Milwaukee. Paleozoic sedimentary rocks range in thickness from 0 m thick to at least 700 meters (2300 feet) thick in parts of eastern and southeastern Wisconsin. These strata thicken significantly toward the ancestral Michigan basin, where younger Paleozoic and Mesozoic sedimentary rocks overlie them [
25,
26]. Key events in the history of development of Wisconsin’s aquifers included formation of the ancestral Michigan basin to the east (
Figure 2), as well as the Illinois basin to the south. Later fluid flow events emanating from these basins appear to have influenced the mineralogy of much of the Paleozoic bedrock, which has important implications for its present day groundwater quality [
26].
Paleozoic rocks in Wisconsin and adjacent states are only slightly deformed, with gentle folding and faulting occurring during the Paleozoic Era (e.g., [
25,
26]). Subsidence of the Michigan basin, a classic intracratonic sedimentary basin, had the most pronounced effect on the structure of these rocks, with development beginning during the Late Cambrian and occurring simultaneously with sedimentation throughout the Paleozoic Era. The subsidence of the ancestral Michigan basin is centered over a portion of the Proterozoic Midcontinent rift system [
25]. This subsidence resulted in a significant dip of the strata toward the center of the basin, which resulted in a concentric “bull’s-eye” pattern on bedrock geologic maps (
Figure 2). Paleozoic strata in eastern portions of Wisconsin typically dip eastward between about 5 and 7.5 m/km (25 and 40 ft/mi) [
20].
Figure 2.
Simplified bedrock geologic map for the western U.S. Great Lakes region showing Wisconsin and Michigan, USA. Geologic systems of rock are as follows: pϵ = Precambrian, ϵ = Cambrian, O = Ordovician, S = Silurian, D = Devonian, M = Mississippian, Pn = Pennsylvanian, J = Jurassic. Modified from [
20].
Figure 2.
Simplified bedrock geologic map for the western U.S. Great Lakes region showing Wisconsin and Michigan, USA. Geologic systems of rock are as follows: pϵ = Precambrian, ϵ = Cambrian, O = Ordovician, S = Silurian, D = Devonian, M = Mississippian, Pn = Pennsylvanian, J = Jurassic. Modified from [
20].
Figure 3 presents a generalized stratigraphy of Paleozoic sedimentary rocks in Wisconsin, with only a generalized overview of the stratigraphy presented here. The lowermost Paleozoic rocks in Wisconsin are Middle to Upper Cambrian sandstones (~520 to 485 million years ago), which were deposited as sea level gradually rose to cover most of the North American craton [
27]. They are exposed over a large portion of western and central Wisconsin, and they form the principal portion of the deep confined aquifer system in eastern and southern Wisconsin. These sandstones have average thicknesses of about 120 m [
5,
6,
28,
29]. These rocks have been extensively studied along the Mississippi River Valley in western Wisconsin (e.g., [
27]), but their poor exposure beneath glacial sediments in eastern and southeastern Wisconsin has impeded research on these rocks, despite their regional importance as aquifers. A lack of economic deposits of petroleum or natural gas in the region has further limited our understanding of these units.
The Ordovician Period (485 to 443 million years ago) saw variable deposition of carbonate rocks, sandstones, and shale. Rocks of this age generally mimic the outcrop pattern of Cambrian rocks (
Figure 2) and are exposed along a narrow horseshoe-shaped band stretching from the Mississippi River Valley in the west, south to the Illinois Border, and then northeastward along the eastern portion of the state where they border younger Silurian rocks. The lower half of the Ordovician section includes carbonate rocks of the Prairie du Chien Group, sandstones and minor shale of the Ancell Group, and carbonates from the Sinnipee Group (
Figure 3). The vast majority of the carbonates in Wisconsin have been transformed from limestone to dolostone, with most limestone restricted to the southwestern portion of the state [
26]. The most important aquifer within the Ordovician section is the Ancell Group, which consists mainly of the St. Peter Sandstone and a few other relatively thin units. The upper-half of the Ordovician section is dominated by the Maquoketa Shale, which is the most important regional confining unit and only present in eastern Wisconsin.
Figure 3.
Simplified stratigraphic column for Paleozoic rocks in northeastern Wisconsin. Pleistocene sedimentary and younger Silurian and Devonian strata for southeastern Wisconsin are not shown here for simplicity, but can be found elsewhere [
30,
31].
Figure 3.
Simplified stratigraphic column for Paleozoic rocks in northeastern Wisconsin. Pleistocene sedimentary and younger Silurian and Devonian strata for southeastern Wisconsin are not shown here for simplicity, but can be found elsewhere [
30,
31].
The Middle and Late Paleozoic Era is recorded mainly in eastern Wisconsin by a sequence of Silurian (443 to 419 million years old) dolostone units as much as 240 m thick that were deposited in both open and marginal marine environments. These rocks form the backbone of the Niagara cuesta that forms the uplands of eastern Wisconsin. Numerous stratigraphic and bedrock investigations have been conducted on northeastern Wisconsin’s Silurian rocks (see [
20,
31] and references therein). Younger Devonian rocks are only locally important along a narrow band near Lake Michigan and are not a focus of this article. There is a region-wide erosional disconformity throughout the Midwestern United States between the Late Paleozoic and the Jurassic Period, known as “The Lost Interval” [
32]. This interval is significant, despite the lack of strata from this period, because regional denudation and likely significant karst development occurred in certain carbonate rocks in the region during this time.
2.3. Quaternary History
The youngest of the three geologic intervals in Wisconsin was recorded during the later part of the Pleistocene Epoch of the Quaternary Period (2.6 million to 11,700 years ago). These sediments range in age from at least 780,000 years old in parts of central and northwestern Wisconsin, to about 13,000 years old, with the vast majority of these sediments deposited during the last three advances of ice lobes during the Wisconsin Glaciation (between about 32,000 and 13,000 years before present [
10,
24]). Only about three-fourths of Wisconsin was glaciated, leaving a region of the southwestern portion of the state known as “The Driftless Area.”
The absence of major mountain-building events in the region for at least 1.5 billon years, coupled with extensive Paleozoic deposition and several Pleistocene glacial advances, has resulted in a relatively gentle topography throughout most of Wisconsin. The maximum topographic relief in the state is less than 425 meters (1400 feet) [
33]. The most significant local relief results from the erosional resistance of castellated mounds of well-cemented Cambrian quartz sandstone, along with several large quartzite monadnocks that result in hills up to 150 to 200 meters high. Additional topographic relief of up to 70 meters occurs along regional glacially sculpted erosional ridges, such as the Niagara Escarpment in eastern Wisconsin, and an extensive array of end moraines from lobes of the Laurentide ice sheet that invaded Wisconsin during the Late Pleistocene.
2.4. Hydrostratigraphy
The hydrostratigraphy in the state is relatively straightforward overall, but locally complex due to significant variation in the geology of Precambrian bedrock and Pleistocene glacial sediments. In contrast, the hydrostratigraphy of the Paleozoic rocks is relatively consistent throughout the region.
Table 2 presents a generalized hydrostratigraphy for Wisconsin. In general, Precambrian basement rocks have relatively low porosity and permeability, with fracture flow providing the most significant permeability in most rocks, with the exception of some sandstones of the Midcontinent Rift System in northwestern Wisconsin [
34]. In some areas, water is extracted principally from a single hydrostratigraphic unit. However, in some regions, multiple aquifers may be accessed by wells in close proximity. In southeastern Wisconsin, at least three aquifers are used. In other regions, such as the Niagara Escarpment region of northeastern Wisconsin, as many as 3 or 4 aquifers are used by domestic wells in close proximity. The well depth varies based on a number of factors, including well driller, water quality concerns, cost, and water quantity needs.
Paleozoic sedimentary rocks supply a large proportion of the water wells in the state. The most prolific bedrock aquifers include the Cambrian sandstones and the Ordovician St. Peter Sandstone (Ancell Group), which are present across at least half of the area of the state. These sandstones form the principal portion of the deep confined aquifer system in parts of Wisconsin and are about 120 m thick on average, but can be substantially thicker in some areas (e.g., [
5,
7,
28]). Ordovician dolostones that overlie each of these sandstone aquifers (
Figure 3,
Table 2) form important aquifers for some domestic water wells, but the vast majority of high capacity wells in these areas are open to one or both of the sandstone aquifers.
Throughout a narrow band of eastern Wisconsin, and in parts of adjacent states, the overlying Late Ordovician Maquoketa Shale acts as a regional aquitard (e.g., [
28,
35,
36]). East of this boundary, which is well defined in places by the topographic expression of the Niagara Escarpment [
20], dolostones of Silurian age form the majority of the remaining bedrock stratigraphic section in eastern Wisconsin. In southeastern Wisconsin, the Silurian bedrock aquifer is particularly important in regions not served by municipal supplies [
7]. These rocks typically contain well-developed karst features, especially in parts of east-central and northeastern Wisconsin where the thickness of unconsolidated sediments is low. Karst development in these rocks began as early as the Devonian Period, but the timing of most karst features is not well known. Recent radiometric age dates on bones and charcoal from caves in the region shows that sediment fills are at least 6000 to 8000 years old, suggesting that the karst development likely occurred prior to the Holocene [
37,
38]. The karst-enhanced fractured Silurian dolostones provide an important regional aquifer for residents east of the Niagara Escarpment, and many high-capacity wells are open to this aquifer system.
In eastern Wisconsin, the Cambrian-Ordovician aquifer is unconfined to the west of the Maquoketa boundary throughout eastern Wisconsin and into northeastern Illinois, with the primary recharge area to the west of the boundary [
7,
28,
39,
40]. Radiocarbon dating, stable isotope analysis and noble gas data for deep aquifer waters in southeastern and northeastern Wisconsin reveal
14C ages for groundwater between 5000 and 26,000 years old [
36,
41].
The Pleistocene sediments in the region are divided into four major classes, including glacial till (ground and end moraines), glaciolacustrine (lake) sediments, outwash (sands and gravels), and pitted outwash with local ice contact deposits (e.g., [
24]). The complex regional and stratigraphic variation of these deposits is beyond the scope of this article, but they are an important aquifer system, and they play a crucial role in some of Wisconsin’s major groundwater quantity and quality problems. One portion of the Pleistocene sediments that is particularly important to describe is known as the “central sand plain” region, which covers a multi-county region in the central portion of the state. This region consists of thick glacial outwash overlying Precambrian and Paleozoic bedrock. This region has seen extensive development of groundwater resources for irrigation, and significant groundwater quality impacts have occurred there.
Table 2.
Simplified hydrostratigraphy for Wisconsin (after [
5,
6,
7,
28]).
Table 2.
Simplified hydrostratigraphy for Wisconsin (after [5,6,7,28]).
Geologic Age | Geologic Unit (Thickness, Meters) | Lithology | Hydrostratigraphic Unit |
---|
Cenozoic | Quaternary (Pleistocene) | Unconsolidated deposits (0–60 meters) Locally >150 m | Lacustrine silt and clay, glacial till, fluvial sand and gravel, and other deposits. | Local unconfined aquifer (sand and gravel) or regional confining unit (lacustrine clays and tills). |
Paleozoic | Devonian | | Black shale locally over limestone and dolostone. | Upper Aquifer; only present in southeastern Wisconsin along Lake Michigan Shoreline. |
Silurian | Undifferentiated (0–240 m) | Dolostone; fractured and karsted in many locations. | Upper Aquifer; Karsted in many locations of northeastern Wisconsin. |
Ordovician | Maquoketa Formation (0–150 meters) | Shale, dolomitic shale, and dolomite. | Confining Units; Sinnipee Group Carbonates are locally used as aquifers for domestic use. |
Sinnipee Group (120 meters) | Dolostone with some shale. Limestone in portions of southwestern Wisconsin. |
Ancell Group (0–90 meters) | Silty sandstone, fine- to medium-grained sandstone, sandy shale. | Confined Deep Aquifer. |
Prairie du Chien Group (0–60 meters) | Dolostone with varying amounts of oolitic chert and minor interbedded sandstone. | Generally an aquitard relative to the adjacent sandstones in eastern Wisconsin; effective aquifer in western Wisconsin. |
Cambrian | Trempealeau Group (0–15 m) | Fine- to medium-grained sandstone with some silty glauconitic dolomite. | Confined Deep Aquifer. |
Tunnel City Group (30–46 m) | Fine- to medium-grained sandstone, silty sandstone to sandy dolomite. Abundant glauconite commonly observed. |
Elk Mound Group (75–90 m) | Very-fine to coarse-grained sandstone. |
Precambrian | Precambrian | Undifferentiated | Crystalline rock, predominantly red granite, contains igneous and metamorphic rock. Limited sedimentary rocks (sandstones, dolostones). | Yields little to no water in many cases. Local aquifers, especially in Midcontinent Rift System rocks of northwestern Wisconsin. |