of The Clay Minerals Society

Geologic Field Trip

Fithian illite

Geologic Field Trip to the Fithian Illite, Fithian, Illinois

Guidebook for field trip held on October 10, 2013

in conjunction with the 50th Anniversary Annual Meeting of

The Clay Minerals Society held at the University of Illinois at Urbana-Champaign

Stephen Altaner1 and Shane Butler2

1Dept. of Geology 2Illinois State Geological Survey Univ. of Illinois Prairie Research Institute Urbana-Champaign Univ. of Illinois Urbana-Champaign

Table of Contents

Introduction Purpose 1 Geologic History of Illinois 1

Guide to the Route 8

Stop Descriptions 1 Morrow Plots - Univ. of Illinois 11 2 Urbana Moraine - Old Church Rd ~2 miles south of Urbana 13 3 Fithian Illite - Salt Fork of Vermilion River near Fithian 16

Acknowledgments 20 References 20

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Introduction

Purpose - The purpose of this trip is to visit several interesting and important central Illinois sites that feature clay minerals in different environments including bedrock, soil, and glacial sediment. The bedrock outcrop is particularly important to clay mineralogists because it is a type locality of illite, characterized by Grim, Bray and Bradley in 1937. We also want to understand Earth’s geologic history by examining the bedrock and the glaciated landscape. We hope the trip will be memorable and fun, and for all non-geologists, we hope that rock outcrops never again will be featureless walls, but will be picture books of Earth’s geologic history.

Figure 1 General geologic regions of North America, including orogenic belts, coastal plain, and craton. Illinois is located in the cratonic platform with Paleozoic sedimentary rocks overlying Precambrian igneous and metamorphic rock (from Marshak, 2009).

Geologic History of Illinois - Before learning about Illinois’ geologic history, we need to examine the regional geology of North America. North America has three major geologic regions including orogenic belts, coastal plain, and craton (Fig. 1). There are two major orogenic (mountain) belts in North America, the Cordilleran in western North America and the Appalachians in eastern USA. Both orogenic belts formed due to convergent plate tectonic activity; the Appalachians formed during the Paleozoic Era (~500 - 250 million years ago, Ma) and the Cordilleran formed in the Mesozoic and Cenozoic Eras (~250 Ma - present). The coastal plain is where relatively young sediment eroded from the continent has accumulated on the continental margin. The craton, which is in the interior of North America, represents a tectonically stable area that has not experienced tectonic activity, e.g., deformation, metamorphism, and igneous activity, for a long time, typically more than a billion years. The craton includes the Canadian Shield, where Precambrian age basement rock (igneous and metamorphic rocks older than ~550 Ma) occurs at the surface in central and eastern Canada. The cratonic

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platform, which includes Illinois and the US Midcontinent in general, consists of Paleozoic and sometimes Mesozoic sedimentary rocks covering Precambrian basement rocks. Rocks of the cratonic platform have experienced mild deformation including basins (downwarping of continental crust) and domes (upwarping of continental crust).

Figure 2 General geologic column showing the age and type of rocks in Illinois (from Frankie, 2005). Now, let’s review briefly the geologic history of Illinois. The first record of geologic activity in Illinois is the Precambrian basement, which involves igneous and metamorphic rock that is 1.5 - 1.4 billion years old. Precambrian basement does not outcrop at the surface anywhere in Illinois because it is buried by younger rock.

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After that, there is a ~1 billion year interval during which no rock was preserved in Illinois and there was widespread weathering and erosion of the Precambrian basement. This interval produced a major unconformity (erosional surface) on top of the Precambrian basement rock. At the end of the Precambrian (~600 - 700 Ma), a continental rift began in southern Illinois, but it later stopped and failed both to split North America and to form extensive new oceanic crust there. Soon after that, there was a broad cratonic embayment with seawater inundating Illinois and much of the North American craton throughout much of the Paleozoic Era from ~540 - 280 Ma; this is because sea level was much higher during most of the Paleozoic Era than it is today. A relatively high sea level, which is also called a marine transgression, results in deposition of sediment such as sand, mud, and calcite shells, which can become the sedimentary rocks sandstone, mudstone (or shale), and limestone after deep burial. There were also marine regressions, which represent a relative drop in sea level resulting in the continents exposed to weathering and erosion of previously formed rock. Figure 2 shows the different kinds of rock found in Illinois as well as their geologic age. The most common bedrock (solid rock that outcrops at Earth’s surface) is Pennsylvanian (Late Paleozoic) in age (Fig. 3). There is considerable economic value in Paleozoic sedimentary rocks including quarried rock for building and road construction, mineral and ore deposits, coal, and oil. During this trip we will see rocks of the Pennsylvanian (Late Paleozoic) Period.

Figure 3 Bedrock geologic map of Illinois (from Frankie, 2005). The major geologic feature of Illinois is the Illinois Basin, which subsided rapidly in the Late Precambrian to Early Cambrian Period due to a failed continental rift (Reelfoot rift and Rough Creek graben) and sank more slowly during the rest of the Paleozoic Era. The early rapid subsidence gives the Illinois Basin the shape of a

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spoon in cross-section with the thickest accumulation of sedimentary rocks in southern Illinois (Fig. 4). Figure 3 shows a roughly circular pattern of different ages of bedrock with the youngest rock (Pennsylvanian) in the middle; this is characteristic of a structural basin. Figure 5 shows the extent of the Illinois Basin and other

Figure 4 Stylized north-south cross-section of the Illinois Basin showing Precambrian basement rock (Pre-C) and sedimentary rocks of the following periods: Cambrian (C), Ordovician (O), Silurian (S), Devonian (D), Mississippian (M), Pennsylvanian (P), Cretaceous (K), and Tertiary (T) (from Frankie, 2005).

Figure 5 Major structural features of the Illinois Basin area. The thick dashed line is the approximate edge of the Illinois Basin. Shaded areas are crests of arches or domes and the stippled area is the Mississippi Embayment (ME). FAF = fluorspar area faults (from Nelson and Marshak, 1996).

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geologic structures (faults and folds) in Illinois. The second largest geologic structure after the Illinois Basin itself is the La Salle Deformation Belt, which consists of a variety of faults and folds including one that approximates a monocline (a one-sided fold produced by a fault in deep Precambrian basement rocks). Uplift in the La Salle Deformation Belt occurred in the Late Mississippian and Early Pennsylvanian Periods.

Figure 6 Extent of Quaternary glaciation in the northern hemisphere (from Marshak, 2009). The next "geologic event" is another major unconformity, this time on top of the Paleozoic bedrock surface. This erosional surface represents ~300 million or more years of erosion. During this time, a rolling land surface formed over Illinois with deep bedrock valleys. Much of this topography was erased by deposition of sediment in the valleys by continental glaciers (thick sheets of moving ice) during the Quaternary Period (the last ~2.6 million years of Earth history). Glacial ice was thickest in the Hudson Bay area of Canada and spread out to cover most of Canada and much of the northern part of the US (Fig. 6). During the Quaternary Period there were ~10 major glacial advances (i.e., times of cooler temperatures and expanded coverage of continental crust by glacial ice) and ~10 major retreats, which are also called interglacial episodes (i.e., times of warmer temperatures and reduced coverage of continental crust by glacial ice). In Illinois most of glacial deposits are either from the Illinois Glacial Episode (300,000 - 125,000 years Before Present or BP), which cover nearly the entire state or from the Wisconsin Glacial Episode (75,000 - 12,500 BP), which cover only northeastern Illinois

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(Fig. 7 and 8). During interglacial episodes, the climate was relatively warm, resulting in soil formation. Glacial deposits include the following: glacial till - unsorted sediment containing a range of grain sizes from large boulders to sand to fine-grained clay that was deposited at the front edges, sides, or bottom of the glacier; outwash - sorted sediment (sand and gravel) deposited by streams formed by the melting glacier; lake sediment - layers of silt and mud deposited in ponded glacial meltwater; and loess - wind-blown silt. During this trip we will see glacial till from the Wisconsin Glacial Episode. An end moraine is a mound of glacial till created during the overall retreating stage of glaciation after the front edge of a glacier is in the same location for decades to centuries, continuing to accumulate glacial till during that time. There are ~30 well-developed end moraines of the Wisconsin Episode in northeastern Illinois (Fig. 7). Our trip will travel through a gently rolling landscape crossed by several end moraines and we will visit the Urbana moraine at Stop 2. End moraines are good places to build cities and towns in Illinois because they represent better-drained and less swamp-like areas with a “great view." Champaign-Urbana is located on two end moraines, which act as the headwaters and drainage divide for the following three rivers: the Embarras, which flows to the south; the Boneyard Creek, which flows to the east; and the Kaskasia, which flows to the west. The Great Lakes formed by glacial erosion of deep valleys in weak shale bedrock; much of the scoured-out rock from Lake Michigan ended up being sediment deposited over Illinois, giving the state great soil but unspectacular topography. Additional economic value of Quaternary deposits include sand and gravel for building and road construction and groundwater in outwash deposits of sand and gravel within deep bedrock valleys.

Figure 7 Generalized map of Quaternary glacial deposits in Illinois (from Frankie, 2005).

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Figure 8 Time table of Quaternary events in Illinois (from ISGS GeoNote 3).

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Guide to the Route Stop 3 (Fithian illite) is on private property. The owners have graciously given us permission to visit on the day of the field trip only. Please conduct yourselves as courteous guests and follow instructions from the field trip leaders. Figure 9 shows the field trip route. Walking Directions We will begin our trip by walking from the Illini Union (1401 W. Green St., Urbana, IL) of University of Illinois at Urbana-Champaign to Stop 1 (Morrow Plots). Outside of the Natural History Building we will view a plaque, which commemorates the pioneering work of Ralph Grim, who many consider to be the founding father of clay mineralogy. Ralph Grim was a professor at the Dept. of Geology of the University of Illinois from 1948 - 1967. After that, we will walk along the Main Quad of the University of Illinois to the Morrow Plots. Stop 1 - Morrow Plots At this stop we will view the oldest continually used experimental agricultural fields in the US. Miles to next point

Miles from start

0.0 0.0 Begin road log at south end of Mathews St. near the Morrow Plots. TURN RIGHT (east) onto Nevada St.

0.3 0.3 TURN RIGHT (south) onto Lincoln Ave.

1.6 1.9 TURN LEFT (east) onto Windsor Rd.

0.5 2.4 TURN RIGHT (south) onto Race St (CR 1350 E). 2.0 4.4 TURN LEFT (east) onto Old Church Rd (CR 1200 N).

1.3 5.7 Pull over onto right side of road for Urbana Moraine. Stop 2 - Urbana Moraine At this stop we will marvel at the “breathtaking” view from the summit of the Urbana moraine and discuss the origin and significance of end moraines. We will also view drill core of glacial till from the Wisconsin Episode of glaciation, which represents the earth material comprising the Urbana moraine. 0.0 5.7 Continue straight ahead on Old Church Rd (CR 1200 N).

1.2 6.9 TURN RIGHT (south) onto IL Rte 130 (CR 1600 E).

2.0 8.9 TURN LEFT (east) onto Sidney-Homer Road (CR 1000 N).

10.9 19.8 TURN LEFT (north) onto N Main St. (CR 2700 E) in center of town of Homer.

0.05 19.9 TURN RIGHT (east) onto Catlin-Homer Road (E 2nd St./CR 1100 N).

4.3 24.2 TURN LEFT (north) onto CR 300 E. The piles of rock on the right (south) are from the

Fairmount Quarry, where Material Service Corp. mines Pennsylvanian age limestone (Millersville Limestone Member of Bond Formation) for construction aggregate. The

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limestone, which is 6 m thick, is overlain by 9 m of glacial till and 1 m of loess. The limestone has a well-developed polished and striated (parallel scratch marks) top surface, created by rock grains embedded in the bottom of the moving glacier. There are even some deep grooves in the bedrock surface with large grains from the till concentrated within the grooves.

1.1 25.3 Pull over onto the right side of the road just before a bridge over the Salt Fork of the Vermilion River. Carefully cross the road and walk ~200 m upstream (west) along the river to the Fithian Illite.

Stop 3 - Fithian Illite At this outcrop next to the Salt Fork of the Vermilion River we will view a type locality of illite (Fithian illite), which is found in underclay (gray mudstone below a coal seam) and is part of a package of sedimentary rock called a cyclothem. We will also see sandstone, paleosol in the underclay, coal, black shale, and limestone. 0.0 25.3 Continue straight ahead on CR 300 E.

1.7 27.0 TURN LEFT (west) onto Lincoln Trail Rd. (CR 1550 N).

1.1 28.1 TURN RIGHT (north) onto (CR 200 E).

3.0 31.1 TURN LEFT (west) onto I-74 West.

0.0 31.1 California Ridge Wind Farm is on the right (north) in the distance. California Ridge Wind Farm California Ridge Wind Farm was completed at the end of 2012 by Chicago-based Invenergy. Spreading over an area of 113 million m2 in Vermilion and Champaign Counties, it contains 134 wind turbines and generates 200 MW (enough power to provide electricity to ~65,000 average American homes), making it the seventh largest wind farm in Illinois. Each wind turbine stands 100 m high and has three 50 m long blades. It is located on the Gifford Moraine (an arcuate, east-west trending ridge of glacial till), which is a relatively large moraine. Immediately south of the Gifford Moraine is the older Newtown Moraine, which is less prominent. Illinois ranks #4 among US states for installed wind power capacity (3.6 GW, after Texas, California, and Iowa) but wind power generates only 3.9% of Illinois’ electricity, which is #16 among the states. 13.9 45.0 Take the University Ave./IL Rte 130 exit, EXIT 185.

3.1 48.1 TURN LEFT (south) onto Lincoln Ave.

0.4 48.5 TURN RIGHT (west) onto Green St.

0.4 48.9 Illini Union (1401 W. Green St., Urbana) is on the LEFT (south).

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Figure 9 Map of field trip route; Stop 1 = A, Stop 2 = B, Stop 3 = C (from Google Maps).

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Stop Descriptions

STOP 1: Morrow Plots - Univ. of Illinois (Crop Experiments, Mollisol) The Morrow Plots are the oldest continually used experimental agricultural fields in the US and the second oldest in the world, after the research station at Rothamsted, founded in 1843 in England. Begun in 1876, the Morrow Plots have provided important data on the impact of crop rotation, soil nutrient depletion, and fertilizer on crop yield (Fig. 10). In 1968, the Morrow Plots were designated a National Historic Landmark but unlike most historic landmarks, the Morrow Plots continues as a working research location providing important data to address current and future societal issues. Located near the center of campus at the Univ. of Illinois, the Morrow Plots also remind us that this university was established as a land-grant university when it was founded in 1867.

Figure 10 The Morrow Plots are the oldest experimental agricultural fields in the US (photo from The UI Histories Project: Virtual Tour at the University of Illinois)

The US was celebrating its 100th birthday when the Morrow Plots were begun in 1876. Illinois farmers earned $180 per year and corn sold for 30 cents per bushel, compared to ~$5 per bushel today. Illinois’ average corn yield was 30 bushels per acre, whereas today it is 5 - 6 times that value. Over two-thirds of the 46 million people in the US lived on farms. Only 388 students attended Illinois Industrial University, whose name changed to Univ. of Illinois in 1885. The visionaries behind the Morrow Plots were George Morrow, who became the first dean of the College of Agriculture, and Manley Miles, a professor of agriculture. Illinois was already famous for its deep, black prairie soils that were well suited to growing corn. Morrow and Miles wanted to discover whether this productivity was sustainable and how different cropping-systems would affect crop yield and soil properties. Some of their questions were: If corn were grown every year on the same field, would yields decline? If so, how soon and by how much? If other crops alternated with corn, would that help to maintain the soil's productivity?

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Originally there were 10 half-acre plots. Today only three plots remain, totaling six-tenths of an acre. The Observatory (to the north) and Gregory Street and Mumford Hall (to the south) now occupy part of the original plot area. The north plot, Number 3, has been planted with corn every year since 1876. The middle plot, Number 4, was in a two-year rotation of corn-oats until 1968, when soybeans replaced oats, which better reflected common farming practices in Illinois. The south plot, Number 5, originally was in a six-year rotation of corn-corn-oats-meadow-meadow-meadow, but this was changed in 1901 to a three-year rotation of corn-oats-clover. In 1953, the south plot's rotation was changed again, to corn-oats-alfalfa. Underground tiles were added to help with soil drainage, as was done throughout Illinois. Some general results of agricultural experiments at the Morrow Plots include: 1) continuous corn plantings lowered soil organic matter and soil concentration of key plant nutrients, which resulted in substantially lower yields over time; 2) crop rotation delayed soil depletions; 3) adding fertilizer and lime as well as introduction of hybrid corn caused large jumps in crop yields for all styles of plantings, especially for those involving crop rotations. It is popularly believed that the University built the Undergraduate Library with three stories underground, so that the Library would not block the sun onto the Morrow Plots. However, the University master plan at the time called for a large open plaza on that end of campus, which apparently was a more important reason why the Library was built underground. The soil on the Morrow Plots is classified as Flanagan silt loam, which is in the soil order mollisol, or prairie soil. This nearly level, dark-colored soil is developed on loess (wind blown silt), which is on top of glacial till; it has somewhat poor natural drainage without the added tiles. Illinois soil is extremely fertile because the parent material of the soil (loess and glacial till) is rich in plant nutrients and the area was a swamp for thousands of years, which means that organic matter was naturally preserved in the soil. Velde and Peck (2002) studied clay minerals in soil from the Morrow Plots over time and as a function of cropping method. Based on curve decomposition methods, they conclude that mica, illite, and two mixed-layer illite/smectite (I/S) phases occur in all soil samples. Continuous corn plantings show a loss of K-bearing clay minerals (illite and illite in I/S) compared to the section with corn-oats-hay rotation (Fig. 11). They conclude that K is supplied by these minerals for plant growth when the soil becomes depleted in K due to single crop plantings.

Figure 11 X-ray diffraction patterns of Sr-saturated, air-dried clay size fractions of Morrow Plots samples collected in 1913 and 1996; XRD patterns are shown for continuous corn plantings and a rotation of corn-oats-hay (from Velde and Peck, 2002).

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STOP 2: Urbana Moraine-Old Church Rd ~2 miles south of Urbana (Moraine, Glacial Till)

Figure 12 The Urbana Moraine, a low relief ridge comprised of glacial till (photo by Rachel Vinsel). Because the Urbana Moraine (Fig. 12) is an end moraine formed by a continental glacier, let’s consider how end moraines form. Continental glaciers are large (>50,000 km2) thick sheets of ice that are constantly moving toward their front edge, carrying ground up rock debris of all sizes like a vast conveyor belt. The ice is thin at the edge of a glacier due to melting, and the sediment grains that were carried by the glacier are deposited there. This unsorted sediment is called glacial till. The size of a glacier depends on the amount of snowfall minus the amount of ice lost, which is mainly due to melting. If the snowfall amount exceeds melting, then the glacier enlarges and covers more land area; this is called a glacial advance. If the amount of melting exceeds snowfall, then the glacier shrinks in size and covers less land area; this is called a glacial retreat. It is important to realize that even during a glacial retreat, the glacial ice and sediment carried along with it are constantly moving forward to the front edge. If the snowfall amount is ~balanced by melting, then the glacier stays the same size and the ice margin remains in the same place. When the ice margin remains in the same place for a relatively long time (tens of hundreds of years), enough debris is carried to the glacier’s leading edge, where it piles up to form a thick mound of glacial till called an end moraine. End moraines usually form during the overall retreating stage of glaciation because during glacial advances, the expanding ice will tend to erode and destroy previously formed end moraines. In Illinois there are ~30 well-developed end moraines of the Wisconsin Episode in northeastern Illinois (Fig. 13). Either end moraines did not form in earlier glacial episodes in Illinois or they were eroded over the longer time intervals involved.

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Indian trails in Illinois tended to follow end moraines because their higher elevation meant they were better-drained and not so swampy. Similarly, highways tend to follow moraines if possible and Illinois towns, including Champaign-Urbana, tend to be located on moraines. In addition, radio and TV towers and wind farms all tend to be located on the “mountains of east central Illinois.” The Urbana Moraine, known locally as Yankee Ridge, is arc-shaped with a north-south orientation here and curving to an east-west orientation farther south of here. It forms a drainage divide between the Embarras River to the west of here and the Boneyard Creek to the east of here.

Figure 13 End moraines in Illinois of the Wisconsin Glacial Episode (from Willman and Frey, 1970).

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We will view several boxes of drill core of Wisconsin Episode glacial till from the backyard of the Natural Resources Bldg. (where the Illinois State Geological Survey is located) on the Univ. of Illinois campus. This till is similar to the till that comprises the Urbana Moraine. XRD analysis indicates that this till contains abundant illite and chlorite with small amounts of kaolinite and an expandable mineral that may be mixed-layer illite/smectite (Fig. 14). There are also non-clay minerals such as quartz, feldspars, and carbonates in the clay size fraction. The Illinois State Geological Survey has studied extensively the clay minerals in glacial till (e.g., Willman et al., 1963) and weathered glacial till (e.g., Willman et al., 1966) in Illinois. Because clay minerals in glacial till are detrital in origin, the abundance of illite has been used to distinguish different tills (e.g., Wisconsinan vs. Illinoian) and glacial flow paths (Glass and Killey, 1987).

Figure 14 X-ray diffraction pattern of Wisconsin Episode glacial till (glycol treated, <2µm size fraction) from drill core located on south side of the Natural Resources Bldg.; sample from depth of 22 m; I/S = mixed-layer illite/smectite.

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STOP 3: Fithian Illite - Salt Fork of Vermilion River near Fithian, IL (Cyclothem, Type Locality of Illite, Mudstone)

Receive a magnifying glass and acid bottle from the field trip leaders at the beginning of this stop. From the bridge, descend to the Salt Fork of the Vermilion River and walk upstream (west) ~200 m to a river outcrop. Field trip leaders will dig a trench to expose underclay (mudstone) below the coal seam. Because there are Pennsylvanian age rocks at this location we need to discuss the geologic conditions of that time period. During the Pennsylvanian Period, Illinois was located near the equator and it had a river system, which flowed to the southwest across a swampy low area and carried mud and sand from a mountainous area off to the northeast. The rivers created a flat, wide delta that spread into a shallow sea. Because the swampy lowland was only a meter or so above sea level, small changes in sea level caused large shifts in the position of the coastline. There were regular advances and retreats of large continental glaciers elsewhere that caused the rise and fall of global sea level, which in turn caused marine regressions and transgressions of shallow epicontinental seas over Illinois and other parts of the North American craton. These environmental variations resulted in a striking repetitive pattern of sedimentary rocks on the North American craton called cyclothems, which include alternating sequences of non-marine and marine sedimentary rocks (Fig. 15). An idealized cyclothem includes an unconformity (which reflects the lowest sea level and favors weathering and erosional conditions on the continent), sandstone or conglomerate (with geologic indicators of a river channel origin), mudstone, also called underclay if coal is above it (with geologic indicators of a freshwater floodplain and delta origin), coal (with geologic indicators of a tropical jungle-like swamp), black shale (with geologic indicators of quiet marine conditions), and limestone (with geologic indicators of marine conditions where calcite shelly organisms thrived in the absence of much sand or mud deposition). Some of the sedimentary layers in cyclothems can be correlated over 1,000 km or more. The term, cyclothem was coined by Harold Wanless, a former professor in the Univ. of Illinois Dept. of Geology (Wanless and Weller, 1932).

Figure 15 Ideal Pennsylvanian cyclothem (left) and typical Pennsylvanian cyclothem (right) (from ISGS GeoNote 2)

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This locality exposes the type locality of the Fithian Cyclothem (Bond Formation, Late Pennsylvanian, ~290 Ma) as well as a type locality of illite, described by Grim, Bray, and Bradley in the 1937 publication “Mica in Argillaceous Sediments.” Starting from the bottom of the section at river level, there is sandstone (with geologic indicators of a river channel origin), massive gray mudstone or underclay (Fig. 16, with geologic indicators of a freshwater floodplain and delta origin) with a paleosol at the top, coal (called the Flannigan Coal with geologic indicators of a tropical jungle-like swamp), thinly bedded black shale (with geologic indicators of quiet marine oxygen-depleted conditions), limestone (with geologic indicators of marine conditions where calcite shelly organisms thrived in the absence of much sand or mud deposition), and another sandstone at the very top (this layer is part of the Witt Cyclothem). Surficial coatings of iron oxide indicate modern weathering of pyrite (an iron sulfide mineral) in the coal and shale. Buried iron oxide zones in the underclay near the coal indicate ancient weathering of iron minerals (Fig. 17). The lower sandstone, mudstone, and coal layers represent the freshwater part of the cyclothem, associated with a lower sea level. The black shale and limestone layers represent the marine part of the cyclothem, associated with a higher sea level.

Figure 16 Massive gray Pennsylvanian mudstone (underclay) at Stop 3 containing the Fithian illite.

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Figure 17 Pennsylvanian coal (20 cm thick) with iron oxide (orange) coatings and underlying paleosol in gray underclay containing the Fithian illite. Photo is by Rachel Vinsel.

Figure 18 X-ray diffraction patterns of Fithian illite (<2µm size fraction) ~60 cm below coal seam; I/S = mixed-layer illite/smectite.

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Figure 19 X-ray diffraction patterns of Fithian illite (<2µm size fraction) ~1 m below coal seam; I/S = mixed-layer illite/smectite.

Our XRD analysis of the Fithian illite (underclay) about 60 cm below the coal seam indicates that the <2µm size fraction consists mainly of illite along with minor amounts of mixed-layer illite/smectite (I/S) with R1 order, kaolinite, and quartz (Fig. 18). Previous sampling and XRD analysis by the Illinois State Geological Survey of a Fithian illite sample ~1 m below the coal seam indicates a much more heterogeneous assemblage of illite, mixed-layer illite/smectite (I/S) with R1 and perhaps R0 order, kaolinite, chlorite, quartz, and jarosite (Fig. 19). Jarosite (KFe3+

3(OH)6(SO4)2) probably represents an alteration product from pyrite weathering in the presence of illite, which provides the source of K. Rimmer and Eberl (1980) found large variations in mixed-layer I/S mineralogy in Pennsylvanian underclay from southwestern Illinois. In that location, there was a large and systematic increase in the smectite content of I/S towards a 2 m thick coal seam ranging from 20% smectite in I/S 1 m away to 80% smectite in I/S 50 cm away. They also found a decreasing amount of discrete illite as well as a loss of calcite and chlorite in samples closest to the coal. They interpreted these trends as due to in-situ acid leaching of the underclay caused by the coal, although the exact timing of the leaching, e.g., during the swamp stage or during burial, could not be specifically defined. Similar to their work on the clay mineralogy of Quaternary deposits, the Illinois State Geological Survey extensively studied the clay minerals in Pennsylvanian shale, mudstone, and underclay in Illinois (Hughes et al., 1987) with the goal of understanding the origins of the clay minerals and their role in delineating the geologic history of the Illinois Basin including sediment provenance, depositional environment, and conditions of diagenesis. This outcrop contains coal, which is a major economic and energy resource for Illinois. Illinois coal is bituminous rank (~80% fixed carbon). With 2.3 billion tons, Illinois has the largest recoverable bituminous coal

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reserves of any US state. The major coal seams in Illinois are both numbered and named, where No. 1 is the oldest mineable coal seam and No. 8 is the youngest. In this area, the coal seams that have been mined are the Danville (No. 7) and Herrin (No. 6) coal. Illinois coal usually has a relatively high sulfur content of 2 - 3 wt. %, which can contribute to acid rain when burned and acid mine drainage when piles of waste rock from coal strip mines undergo weathering. Much of the sulfur in coal occurs as fine-grained pyrite, which is unstable at Earth’s surface and weathers quickly to Fe-oxides and sulfuric acid. Underclay in this area also has been mined and used for brick manufacturing. The Salt Fork of the Vermilion River gets its name from saline seeps along the river a few km downstream of here. Pioneers discovered saline seeps in this area in 1819 and shallow wells were dug to obtain saline brine, which was boiled in large iron kettles to concentrate the salt. Then, the salt was sold to settlers traveling westward. Saline seeps represent salty groundwater located in bedrock that here is close to the surface. Acknowledgments We thank Evan Gragg for X-ray diffraction analysis of samples and for help with figure preparation. We thank Zak Lasemi for a helpful review. For the Morrow Plots section, we have relied on information provided in the online resource, The Morrow Plots: A Century of Learning, found at the Web site of the Department of Crop Sciences at the University of Illinois. References Berggren DJ, Killey MM (2008) Quaternary glaciations in Illinois: Illinois State Geological Survey, GeoNote 3,

available online at: http://www.isgs.uiuc.edu/maps-data-pub/publications/geonotes/geonote3.shtml (accessed Sept. 2013).

Department of Crop Sciences at the University of Illinois, The Morrow Plots: A Century of Learning: available

online at: http://cropsci.illinois.edu/research/morrow (accessed Sept. 2013). Frankie WT (2005) Guide to the Geology of the Kickapoo State Park and Surrounding Area, Vermilion County,

Illinois: Field Trip Guidebook 2005A, Illinois State Geological Survey, 36 p. Glass HD, Killey MM (1987) Principles and applications of clay mineral composition in Quaternary

stratigraphy; examples from Illinois, USA: in Tills and Glaciotectonics, Van der Meer JJM, ed., Balkema, 117-125.

Grim RE, Bray RH, Bradley WF (1937) The mica in argillaceous sediments: American Mineralogist, 22, 813-

829. Hughes RE, DeMaris PJ, White WA, Cowin DK (1987) Origin of clay minerals associated with Pennsylvanian

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