Proceedings of the 47Th Forum on the Geology of Industrial Minerals

Proceedings of the 47th Forum on the Geology of Industrial Minerals

A Brief Overview of the Geology and Mineral Specimens from the Conco Mine: North Aurora, Kane County, Illinois

Jared T. Freiburg Illinois State Geological Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign

Thornton Quarry (photograph by Donald Mikulic)

To cite this article: Freiburg, J.T., 2015, A brief overview of the geology and mineral specimens from the Conco Mine: North Aurora, Kane County, Illinois, in Z. Lasemi, ed., Proceedings of the 47th Forum on the Geology of Industrial Minerals: Illinois State Geological Survey, Circular 587.

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ILLINOIS STATE GEOLOGICAL SURVEY Prairie Research Institute University of Illinois at Urbana-Champaign 615 E. Peabody Drive Champaign, Illinois 61820-6918 http://www.isgs.illinois.edu

A Brief Overview of the Geology and Mineral Specimens from the Conco Mine: North Aurora, Kane County, Illinois

Jared T. Freiburg* Illinois State Geological Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign

ABSTRACT sions. The operation was initially a of the Kankakee and Wisconsin Arches surface quarry, referred to as the Aurora (Figure 2). The northwest-trending Urban sprawl around the City of Chi- Quarry, and was owned by Conco-West- Kankakee Arch, a southeastern exten- cago has increased the demand for ern Stone Company. In 1991, Conco- sion of the Wisconsin Arch, began devel- construction aggregates in northeastern Western began underground mining opment at the close of the Canadian Illinois. Limited surface land available operations and named the underground (Early Ordovician) time period, which for quarry expansion and a cumbersome complex the Galena-Platteville Mine. In resulted in the separation of the Michi- surface permitting process have led to 2006, Lafarge North America purchased gan Basin to the northeast from the an increase in underground limestone the mine and renamed it the Conco Illinois Basin to the south (Collinson et and dolomite mines in the Chicago area. Mine. al. 1988). The region is host to local fold Since 1991, the Conco Mine in North and fault zones, including brittle faults Aurora has been mining limestone Deeper mining into the Paleozoic bed- that offset Phanerozoic sedimentary and dolomite underground. Previously rock at the Conco Mine has resulted stratigraphic contacts and monoclinal owned by Conco-Western and now in the discovery of trace amounts of folds that locally tilt the bedding (Mar- owned by Lafarge North America, the sphalerite mineralization along with shak and Paulsen 1996). Two dominant Conco Mine continues to prove that massive solution cavities lined with cal- trends of fold and fault zones throughout underground aggregate mining around cite crystals concentrically zoned with the midcontinent are recognized: (1) a large metropolis can be economical pyrite and marcasite. The discovery of east–west to northwest–southeast and and successful. An exciting discovery these mineral deposits has led to new (2) north–south to northeast–southwest. during expansion of the Conco Mine insights into Mississippi Valley-type Regionally and locally, major fault, frac- was the presence of massive solution (MVT) mineralization in northeastern ture, and joint systems trend northwest– cavities lined with large specimen- Illinois as well as the timing, composi- southeast and northeast–southwest. The quality minerals, such as calcite, pyrite, tion, and potential source of diagenetic Sandwich Fault Zone coincides with the and marcasite. The scientific impor- fluids responsible for alteration of the Kankakee Arch and follows this domi- tance of these minerals has been rec- bedrock. Along with these geologic nant regional northwest–southeast trend ognized, and the mining company has insights, spectacular mineral specimens (Figures 2 and 3). The Sandwich Fault allowed the author to extract specimens. have been preserved from the Conco Zone is approximately 84 mi (135 km) Research has been ongoing to interpret Mine. Through the generosity of the long and 0.6 to 1.9 mi (1 to 3 km) wide the diagenetic fluids responsible for car- mine operators, mineral specimens (Kolata and Graese 1983). On the north- bonate diagenesis of the limestone host have been extracted by the author for east side of the fault zone, Paleozoic rock and the precipitation of world-class scientific research and have been placed strata dip eastward into the Michigan mineral specimens. in museums and private collections. Basin. Conversely, on the southwestern The preservation of these specimens is side, the strata dip southward toward INTRODUCTION extremely important for future research, the Illinois Basin. The Conco Mine is for understanding the geologic environ- located approximately 22 mi (35 km) In northeastern Illinois, urban sprawl ment in which the third largest city in northeast of the Sandwich Fault Zone around the City of Chicago and the con- the United States is located, and for the just off the northern flank of the Aurora tinuous growth of the Chicago suburbs enjoyment of generations to come. Syncline (Figure 3). The Aurora Syncline have increased the demand for lime- trends northeastward across southern stone and dolomite aggregate. With lim- Kane County (Figure 3). It is approxi- ited land surface available for expansion GEOLOGIC SETTING mately 1.9 to 6.2 mi (3 to 10 km) wide and and a cumbersome surface permitting The Conco Mine is an underground has a maximum relief of approximately process, operators have begun convert- room-and-pillar mine in North Aurora, 98 ft (30 m; Nelson 1995). A recent mag- ing quarry operations into underground Illinois, located along the Fox River, netic anomaly map of Illinois compiled mining operations. The Conco Mine, approximately 40 mi (65 km) west of by the U.S. Geological Survey (Daniels et located in North Aurora, Illinois (Figure downtown Chicago (Figure 1). The mine al. 2008) suggests a scissored pattern of 1), is the result of one of these conver- lies immediately east of the intersection magnetic strength trending northwest–

This article was previously presented at the 47th Forum on the Geology of Industrial Minerals, held May 15–17, 2011, in Champaign, Illinois. Page numbers shown are not final. *E-mail: [email protected]

Illinois State Geological Survey Circular 587 1 Sources: Esri, DeLorme, NAVTEQ, USGS, Intermap, iPC, NRCAN, Esri Japan, METI, Esri China (Hong Kong), Esri (Thailand), TomTom, 2013

Fox River

I88

Figure 1 Map of the Chicagoland area and the location and an aerial photograph of the Conco Mine at the intersection of I-88 and the Fox River in North Aurora, Illinois. The arrow on the aerial photograph points to the mine’s decline.

2 Circular 587 Illinois State Geological Survey 0 100 mi N 0 200 km Wisconsin Arch Michigan Basin UMV -5000 0 !

0 Kankakee 0 Arch Forest City Basin Illinois Basi

Mississippi River Arch n 0

-2500 0

-2500

-5000 0

0 0

-2500 Ozark Uplift

-2500 -2500 0 ati Arch

Mississippi Embayment Cincinn

Elevation (ft) of the top of the Upper Mississippi Valley -2500 Galena Group or equivalents Zinc-Lead Mining District (UMV)

Conco Mine, North Aurora, Major regional faults and folds ! Kane County, Illinois

Figure 2 Major basins, faults, and folds in the north-central United States, and the location of the Conco Mine and the Upper Mississippi Valley Zinc-Lead Mining District (modified from Freiburg et al. 2012). southeast. The Aurora Syncline sits in a a combination of these (Sumner 1976). be suspected (W. John Nelson, personal magnetic low and is parallel to a mag- Within the mine, small vertical faults communication, 2011). with centimeter-scale offset are evident netic high to the northwest and south- The bedrock stratigraphy northeast of within fractures, along with trace slick- east. The magnetic high to the northwest the Sandwich Fault, on the southwest ensides on horizontal clay-rich bedding is followed by a magnetic low to the margin of the Michigan Basin, consists planes. The most prominent features are north, similar to the magnetic low of the of Precambrian igneous basement rock fractures with no apparent offset inter- Aurora Syncline (Figure 3). These mag- unconformably overlain by a sequence secting massive solution cavities. With netic anomalies may reflect deep crustal of Cambrian to Silurian sedimentary little to no vertical offset apparent on rock petrology, basement structures deposits composed of sandstone, lime- these fractures yet development of mas- such as a horst and graben en echelon to stone, dolomite, and shale. Devonian sive solution structures, strike-slip fault- the southern tip of the Sandwich Fault, and younger strata covered the area but significant deposits of iron sulfides, or ing or transtensional movement might

Illinois State Geological Survey Circular 587 3 BOONE MCHENRY LAKE

COOK

KANE

DE KALB

DUPAGE

Conco Mine Sandwich Fault Zone !

358 283 Aurora Syncline 234 195 er iv 162 R s 133 e n 105 i a l 81 P

s 58 KENDALL e 35 D 13 –9 –30 –52 –74 WILL –97 Sandwich Fault Zone –120 LA SALLE –145 –172 –201 –234 GRUNDY –273 –322 0 10 20 mi –397 N 0 10 20 30 km nanoteslas KANKAKEE

Figure 3 Magnetic anomaly map of structures in northeastern Illinois (Nelson 1995; Daniels et al. 2008) and the location of the Conco Mine. Map courtesy of the U.S. Geological Survey. were eroded after Pennsylvanian times in river valleys, where bedrock is typi- 1978) and the uplift of the Kankakee (Kolata et al. 1978). Quaternary-age gla- cally exposed. Arch. The Galena Group comprises an cial drift directly overlies the Silurian almost continuous succession of car- bedrock (Figure 4) and generally ranges The mine is currently operating in the bonate rocks (dolomite and limestone) from approximately 49 to 295 ft (15 to 90 Wise Lake and Dunleith Formations of deposited on a platform in northeast- m) thick (Kempton et al. 1985); however, the Ordovician Galena Group (Figure ern Illinois. Rocks of the Galena Group at the mine, drift thickness is less than 4). The distribution of the Galena Group in northern Illinois are dominated by approximately 49 ft (15 m). This thick- rocks in northern Illinois reflects the lime mudstone and wackestone inter- ness difference is occasionally the case truncation of the southeast-projecting bedded with thin beds of grainstone Wisconsin Arch (Willman and Kolata

4 Circular 587 Illinois State Geological Survey GRAPHIC FORMATION COLUMN DESCRIPTION THICKNESS (feet) (not to scale) GROU P SERIE S SYSTEM HOLO- Cahokia Fm (0–15) Alluvium, sand, silt, and clay CENE Grayslake Peat (0–5) Peat and muck Equality Fm (0–35) Silt loam, massive, Henry Fm (0–70) Sand, silt, and clay, laminated Sand and gravel, stratified Peddicord Tongue Till, sand, and gravel; laminated sand, silt, and clay (0–35) Sand, silt, and clay; laminated Robein Silt Fm (0–28) Organic-rich silty clay

QUATERNARY Glasford-Banner Till, sand, and gravel; laminated sand, silt, and clay PLEISTOCEN E Fm (0–375)

Wedron Fm (0–250) Joliet-Kankakee Dol (0–50) Dolomite, fine grained Elmwood Dol Dolomite, fine grained, cherty (0–50) ALEXAN - DRIAN SILURIAN Wilhelmi Fm (0–20) Dolomite; fine grained, argillaceous; shale, dolomitic a Shale, dolomitic; dolomite, fine to coarse grained, argillaceous

Maquoket (undiff.) (0–210) Wise Lake Fm CINCINNATIA N (120–150) Dolomitic limestone, fine to medium grained Dygerts K-bentonite Dolomite; some limestone, fine to medium grained,

Galena (“Trenton” ) Dunleith Fm- cherty, interbedded bentonites Guttenberg Dol (35–55) Dolomite, fine to medium grained, with red brown Quimby’s Mill- shaly laminae Nachusa Fm (50) Dolomite, fine to medium grained, slightly cherty Grand Detour- Dolomite, fine to medium grained, cherty

ORDOVICIAN Mifflin Fm (43)

Platteville Pecatonica Fm (38) Dolomite, fine to medium grained, cherty, sandy at base

CHAMPLAINIAN Glenwood Fm Sandstone, poorly sorted; silty dolomite and green shale St. Peter Ss Sandstone, white, fine to medium grained, well sorted

Ancell (60–400) Shakopee Dol Dolomite, fine grained New Richmond Ss Sandstone, fine to medium grained Oneota Dol Dolomite, fine to coarse grained Prairie du Chien (undiff.)

CANAD- IA N (0–400) Eminence Dol Dolomite, fine to medium grained, trace sand (20–150) and glauconite Potosi Dol (90–225) Dolomite, fine grained, trace sand and glauconite

Franconia Fm Sandstone, fine grained, glauconitic; green and red shale (75–150)

Ironton-Galesville Ss (155–220) Sandstone, fine to medium grained, dolomitic CAMBRIAN ST. CROIXAN Eau Claire Fm Sandstone, fine grained, glauconitic; siltstone, shale, (350–450) and dolomite

Mt. Simon Ss Sandstone, white, coarse grained, poorly sorted (1,400–2,600)

PRECAMBRIAN Granite, red

Figure 4 Stratigraphic column of northeastern Illinois (Bauer et al. 1991; Graese 1991; Freiburg et al. 2012). Units in bold indicate rocks encountered at the Conco Mine. Devonian and younger strata were eroded in post-Pennsylvanian times. Column is not to scale.

Illinois State Geological Survey Circular 587 5 and packstone, and they primarily an excellent impermeable seal over the line the cavity walls. In this case, cavity represent a warm, shallow marine envi- Galena Group. The portal to the Conco walls are lined with a thick [up to 3.3-ft ronment characterized by low depo- Mine is a decline through the base of the (1-m)] sparry calcite cement before the sitional energy (Willman and Kolata Silurian (Figure 1), through the entire precipitation of euhedral calcite crys- 1978). Regionally, the Galena Group is Maquoketa, and down to the Galena tals. In the one cavity where this thick extensively to completely dolomitized; Group. calcite cement occurs, euhedral calcite however, within the Aurora Syncline, crystals are modified rhombohedron– limestone facies have remained only oscalenohedron combinations. All other partially dolomitized (Graese 1991). SOLUTION CAVITIES cavities encountered are predominantly Dolomitization of the Galena Group In the Conco Mine, solution structures scalenohedrons, with occasional rhobo- or Trenton Limestone in the Michigan are exclusively located along fractures, hedron–scalenohedron combinations Basin has been widely studied and with the majority of meter-scale solu- precipitated on the sucrosic dolomite is reviewed by Freiburg (2010) and tion cavities found along northwest– cavity wall. Freiburg et al. (2011). southeast-trending fractures. The first occurrence of meter-scale solution During the deposition of the Ordovi- structures is found in the first level of MINERALS cian carbonates, a convergent plate the mine, which has a roof elevation Minerals found in solution cavities at boundary developed along the eastern of approximately 430 ft (131 m) and a the Conco Mine include calcite, marca- coast of proto-North America, forming bench level (floor) elevation of approxi- site, pyrite, and rare sphalerite. These continent-wide volcanoes situated in mately 381 ft (116 m). Solution structures are common gangue and ore minerals an island arc system (Kolata et al. 1986). have also been found in the second level found in MVT deposits, suggesting that The volcanoes deposited widespread of the mine but are currently inacces- hydrothermal fluids migrated out of a ash beds throughout the midcontinent sible because of mining operations. Two nearby sedimentary basin (likely the and intermittently throughout deposi- general types of solution structures have Michigan Basin), altering the rocks of tion of the Galena Group. Over time, been identified throughout the mine: northeastern Illinois and depositing diagenetic alteration of the ash beds (1) open-spaced cavities, and (2) closed- minerals (Freiburg 2010; Freiburg et al. formed K-bentonite beds (Kolata et al. spaced solution structures. Open- 2011). Massive MVT gangue mineral 1996) or illite-smectite clay-rich beds. In spaced cavities are host to the fine min- deposits, such as those found at the the Conco Mine, Dygerts K-bentonite is eral specimens depicted in this paper. Conco Mine, have not been recorded encountered in the Wise Lake and Dun- Many closed-spaced solution structures previously in northeastern Illinois, thus leith Formations of the Galena Group are typically collapsed breccias with making preserved mineral specimens (Figure 4). clasts of centimeter-scale calcite spar from the Conco Mine significant geo- The Galena Group unconformably over- clasts; sucrosic dolomite clasts; and logic markers of the region. calcite, pyrite, and marcasite cements. lies the Platteville Group (Kolata et al. Calcite is the most common mineral 1998), which comprises mostly dolomite Other closed-space solution structures appear to be primarily sucrosic dolo- found at the Conco Mine. As previously and cherty dolomite. The Galena and noted, calcite is found as rhombohe- Platteville Groups unconformably over- mite focused along fractures. Fracture- focused sucrosic dolomite is likely the drons (Figure 6) and more commonly as lie the Ordovician Glenwood Formation, scalenohedrons (Figure 7). Calcite twin- a shaley dolomitic sandstone, and the St. result of diagenetic alteration caused by fluid migration along fractures. ning is rare, and few specimens have Peter Sandstone, a significant regional been encountered (Figure 8). All calcite aquifer (Figure 4). The St. Peter repre- Open-spaced solution cavities com- is concentrically zoned, with multiple sents a probable pathway for regional monly form below a bentonite bed. stages of calcite, marcasite, and pyrite fluid flow and may have played an Dygerts K-bentonite (Figure 4) is identi- precipitation (Freiburg 2010; Freiburg important role in the transport of diage- fied in level 1, and centimeter- to meter- et al. 2011) creating a phantom effect netic fluids and the development of solu- scale cavities are commonly found in euhedral calcite crystals (Figure 9). tion structures and the massive mineral directly below this bed. Typically, a Many scalenohedrons have a rhombo- deposits at the Conco Mine. distinct vertical fracture penetrates the hedron phantom showing early calcite In northeastern Illinois, the Galena bentonite, and the cavity is found along growth habits (Figure 10). Large speci- Group is overlain by the Maquoketa the fracture and below the bentonite mens of pyrite and marcasite are rarely Group, which is primarily composed of (Figure 5). The original limestone host- encountered, but when found, they are a thick carbonate-rich shale sequence ing the cavity is commonly altered to a typically massive clusters of euhedral (Figure 4). The lowermost formation of sucrosic dolomite that is cemented with crystal growth (Figure 11) or are sta- the Maquoketa Shale group is the Scales marcasite, pyrite, and calcite. Other lactitic (Figure 12). The mineralogy at Shale. The Scales Shale is a mostly dark cavities have little early dolomitization the Conco Mine is reviewed in detail in gray to dark brown shale that creates before marcasite, pyrite, and calcite Freiburg and Rakovan (2012).

6 Circular 587 Illinois State Geological Survey B

Fracture

Cavity

Bentonite

25 cm

Figure 5 Photograph of the edge of a solution cavity, a vertical fracture, and Dygerts K-bentonite (overlying the solution cavity; Freiburg et al. 2012). The fracture was the conduit for diagenetic fluids, and the bentonite was an impermeable seal.

Figure 6 Modified rhombohedron from the Conco Mine, 12 × 5 cm (Freiburg 2010; Freiburg et al. 2011). Specimen is from the collection of Jared Freiburg. Photograph by Jared Freiburg; used by permission.

Illinois State Geological Survey Circular 587 7 Figure 7 Calcite scalenohedron with inclusions of marcasite Figure 8 Twinned calcite with marcasite and pyrite, 8 and pyrite, 13.5 × 9 cm. Specimen is from the collection of Matt × 8.5 cm. Specimen is from the collection of Mike and Zukowski. Photograph by Jeff Scovil from the collection of Jared Sally Bergmann. Photograph by Jeff Scovil from the Freiburg; used by permission. collection of Jared Freiburg; used by permission.

Figure 9 Calcite scalenohedron zoned with marcasite and pyrite, Figure 10 Calcite scalenohedron zoned with marcasite 18 × 15 cm. Specimen is from the collection of Jared Freiburg. and pyrite, 3 × 2 cm. Notice the early rhombohedron Photograph by Jeff Scovil from the collection of Jared Freiburg; habit outline by marcasite and pyrite. Specimen is from used by permission. the collection of Jared Freiburg. Photograph by Jared Freiburg; used by permission.

8 Circular 587 Illinois State Geological Survey Figure 11 Cubic pyrite cluster, 10 × 6.5 cm. Specimen is from the col- Figure 12 Marcasite stalactite cluster, 15 × 10 cm. lection of Jared Freiburg. Photograph by Jeff Scovil from the collection of Specimen is from the collection of Jared Freiburg. Jared Freiburg; used by permission Photograph by Jeff Scovil from the collection of Jared Freiburg; used by permission.

CONCLUSION ACKNOWLEDGMENTS D-2: Boulder, Colorado, Geological Society of America, p. 383–426. The demand for industrial minerals is The author thanks Lafarge North increasing around highly populated America and all the employees at the Daniels, D.L., R.P. Kucks, and P.L. Hill, areas such as the Chicago metropoli- Conco Mine for allowing the research 2008, Illinois, Indiana, and Ohio tan area. This has led to the expansion on, collection of, and preservation of magnetic and gravity maps and data: of a quarry operation in northeastern the mineral specimens. The author also A website for distribution of data, Illinois into an underground mining gratefully thanks Brian Washkowiak for United States (USA): Reston, Virginia, operation. These underground mining his vast cooperation and interest in the U.S. Geological Survey, Data Series operations are giving geologists a much geology at the Conco Mine. In addition, 0321. more concise view of the local geology the author is grateful to John Rakovan and a much greater understanding of the and F. Brett Denny for their reviews of Freiburg, J.T., 2010, The timing, composi- regional geology and geologic processes. this paper. tion, and source of subsurface diage- The mineral specimens recovered at the netic waters responsible for sulfide Conco Mine contribute significantly to and carbonate mineralization in solu- this understanding, and they assist the REFERENCES tion cavities of the Ordovician galena mining company in its mining model Bauer, R.A., B.B. Curry, A.M. Graese, R.C. group limestone, North Aurora, and in assessing the economic value of Vaiden, W.J. Su, and M.J. Hasek, 1991, Illinois, USA: University of Illinois at the rock. Mineral specimens from the Geotechnical properties of selected Urbana-Champaign, M.S. thesis, 86 p. Conco Mine are by-products of under- Pleistocene, Silurian, and Ordovi- Freiburg, J.T., B.W. Fouke, and Z. Lasemi, ground industrial mining around Chi- cian deposits of northeastern Illinois: 2012, New insights on Upper Missis- cago and are now preserved so future Illinois State Geological Survey, Envi- sippi Valley mineralization based on generations can study and enjoy these ronmental Geology 139, 92 p. solution cavities in the Ordovician geologic artifacts. The preservation Collinson, C., M.L. Sargent, and J.R. Jen- Galena Group at the Conco Mine, of such specimens will bolster public North Aurora, Illinois, USA: Illinois support for the inevitable development nings, 1988, Illinois Basin region, in L.L. Sloss, ed., Sedimentary cover— State Geological Survey, Circular 581, of urban mining to meet consumer 45 p. demand. North American craton: U.S.: The geology of North America, volume

Illinois State Geological Survey Circular 587 9 Freiburg, J.T., and J. Rakovan, 2012, The Kolata, D.R., J.K. Frost, and W.D. Huff, North American midcontinent: Geo- Conco Mine, North Aurora, Kane 1986, K-Bentonites of the Ordovician logical Society of America Bulletin, v. County, Illinois: Rocks and Minerals, Decorah Subgroup, Upper Mississippi 110, no. 6, p. 723–739. v. 87, no. 2, p. 116–125. Valley: Correlation by chemical fin- gerprinting: Illinois State Geological Marshak, S., and T. Paulsen, 1996, Mid- Graese, A.M., 1991, Facies analysis of Survey, Circular 537, 30 p. continent U.S. fault and fold zones: the Ordovician Maquoketa Group A legacy of Proterozoic intracratonic and adjacent strata in Kane County, Kolata, D.R., and A.M. Graese, 1983, extensional tectonism? Geology, v. 24, northeastern Illinois: Illinois State Lithostratigraphy and depositional no. 2, p. 151–154. Geological Survey, Circular 547, 36 p. environments of the Maquoketa Group (Ordovician) in northern Illi- Nelson, W.J., 1995, Structural features Kempton, J.P., R.C. Vaiden, D.R. Kolata, nois: Illinois State Geological Survey, of Illinois: Illinois State Geological P.B. DuMontelle, M.M. Killey, and Circular 528, 49 p. Survey, Bulletin 100, 144 p. R.A. Bauer, 1985, Geological-geotechnical studies for siting the Supercon- Kolata, D.R., W.D. Huff, and S.M. Berg- Sumner, J.S., 1976, Principles of induced ducting Super Collider in Illinois: Pre- strom, 1996, Ordovician K-bentonites polarization for geophysical explo- liminary geological feasibility report: of eastern North America: Boulder, ration: Developments in economic Illinois State Geological Survey, Envi- Colorado, Geological Society of geology, volume 5: Amsterdam, Neth- ronmental Geology Notes 111, 63 p. America, Special Paper 313, 84 p. erlands, Elsevier Scientific Publishing Co., 277 p. Kolata, D.R., T.C. Buschbach, and J.D. Kolata, D.R., W.D. Huff, and S.M. Berg- Treworgy, 1978, The Sandwich Fault strom, 1998, Nature and regional Willman, H.B., and D.R. Kolata, 1978, Zone of northern Illinois: Illinois significance of unconformities asso- The Platteville and Galena Groups in State Geological Survey, Circular 505, ciated with the Middle Ordovician northern Illinois: Illinois State Geo- 26 p. Hagan K-bentonite complex in the logical Survey, Circular 502, 75 p.

10 Circular 587 Illinois State Geological Survey