U.S. DEPARTMENT OF THE INTERIOR Prepared in cooperation with the GEOLOGIC INVESTIGATIONS SERIES U.S. GEOLOGICAL SURVEY STATE GEOLOGICAL SURVEY MAP I–2669 (SHEET 1 OF 3)

89° 88° 89° 88° the evolution of concepts of the geologic history of the Mahomet Bedrock Valley sys- 150 M DISCUSSION r RD CO. ac 57 D CO. e WOODFO kin WOODFOR tem and the origin of the sediment cover that incrementally buried it (for example, v a Sibley i w 5 0 R MC LEAN CO. 39 R 45 MC LEAN CO. 25 150 55 ive 54 7 Horberg, 1953; Kempton and others, 1991; Herzog and others, 1995; and Larson r 5 naw k 50 INTRODUCTION ki e 1 ac e 50 0 and others, 1997). It was a primary goal of our study to build upon the findings of 51 r 0 M 74 165 C 7 This geologic mapping project was conducted cooperatively by the Illinois State g 5 1 prior investigations, using newly refined stratigraphic data to produce updated, revised Mackinaw in 00 2 7 r 1 5 5 Danvers 115 Sp Geological Survey (ISGS) and the U.S. Geological Survey (USGS) to map the 9 125 150 75 maps that could be used for various computer-aided applications such as ground-water 47 Mackinaw 1 1 00 deposits in east-central Illinois (figs. 1 and 2). This area provides an excel- Chicago Normal 15 2 modeling. 100 0 5 ° Normal ° lent geologic setting to develop and test new techniques for mapping Quaternary 40 30' B IROQUOIS CO. 49 40 30' IROQUOIS CO. Another goal was to produce these maps using digital methods because, increasing- i 12 9 9 g Bloomington 5 deposits in three dimensions (that is, mapping the thickness and distribution of geo- Bloomington Gibson City FORD CO. 75 0 FORD CO. ly, counties, planning agencies, and other entities are using geographic information Fo Paxton 20 Gibson City u 1 logic materials both at the land surface and in the subsurface), because it has diverse r 150 50 Paxton systems (GIS) to support decisionmaking and planning. These entities need digital geo- rk 9 9 50 o D Rankin F it 50 Quaternary geology and thick, regional sand and gravel aquifers within a buried logic map information and specific information or interpretations derived from geo- 122 k ch k e

r e . 75 bedrock valley system, the Mahomet Bedrock Valley (figs. 3–5). Decades ago, this val- t o r M logic maps. This derived information, combined with related scientific and socioeco- s F 55 O 1 e C 150 S i 25 12 1 12 00 e r a d 5 50 5 l n d 1 C ley commonly had been considered part of the Teays River system, a proposed west- W d a g l 50 nomic data, can help support reasoned decisions. Commonly, this information is com- d g a e N Peoria i u 74 m FORD CO. F FORD CO. o o A M S n rk ward-flowing drainage system formed during preglacial and glacial times, which was E piled and analyzed using GIS technology because of the large size and complexity of CHAMPAIGN CO. 75 CHAMPAIGN CO. V L Bloomington/Normal . R thought to extend across Illinois, Indiana, and Ohio, to West Virginia. Modern evi- 50 C iv e many map databases and the need to readily update and revise maps as new interpre- O e

54 r r M m

C dence, however, suggests the Mahomet Bedrock Valley is a local drainage system in

M M 45 tations and data become available. Because computer-based mapping of deposits in N i 2

Champaign/Urbana l C C Le Roy

5 A i

western Indiana and eastern Illinois that formed during early glaciations through alter- 57 o

E

L Danville L three dimensions is not yet a common, well-established practice, we developed GIS- n

E E Le Roy Decatur TAZEWELL CO. L TAZEWELL CO.

A A ation of the preglacial drainage patterns (Melhorn and Kempton, 1991). k C 47 R based methods to integrate point data (key stratigraphic control) and areal data (geo-

N N Heyworth ee Rantoul i r M Heyworth

Springfield LOGAN CO. v LOGAN CO.

136 C C C e 136 136 r Rantoul The total glacial drift succession is locally thicker than 500 ft in the Mahomet logic mapping) in three dimensions. These methods are briefly described here and in

O

O k t l . . 5 e k 0 Bedrock Valley, whereas the bedrock uplands are covered by 50 to 300 ft of glacial e a e re MC LEAN CO. ch MC LEAN CO. Soller and others (1998). r C S it 125 C D 7 sediments (fig. 4). Glacial deposits overlie Paleozoic bedrock ranging in age from Atlanta oo DE WITT CO. k ig DE WITT CO. 5 Maps constructed using GIS techniques are in some ways easier to produce than con- p r B 1 Atlanta 1 r a L o 50 0 0 a k o 25 0 1 0 to . Transmissivity and water quality vary among the bedrock c ng F 75 2 ventional, hand-drawn maps. For example, map revision and generation of color g i Po Farmer City S Farmer City 5 u K in k 2 2 t Cree p 5 5 S 5 units. Through leakage upward into the Mahomet Sand aquifer, the bedrock units have h 2 0 proofs is done more quickly in a GIS. For other needs, however, conventional map- t o 5 r o St. Louis o n 5 some effect on its water quality, and should be considered in the modeling and man- h ping can be easier and less time consuming. For example, consider an area having N 0

C 74 c 1 5 C

V 0 V 2 1

H n 0 7 H agement of ground-water resources (Panno and others, 1994). Figure 5 prominently

E 150 5 Mahomet E 55 5 . a R 0 . thin, discontinuous units. While creating a hand-drawn set of maps showing elevation A 7 A

R

51 r 49 5 R 54 O i O

M M

M M

C B v C shows the Mahomet Bedrock Valley incised into the bedrock surface; it also shows the

Lake . r . 1 e of the top of each unit, the geologist will attempt to ensure, visually, that a unit’s ele- P P

I e 0 I

L

T O T O 0 L v r A 1 1 i A

I

T I

C 7 T 1 2 Clinton 0 C

O k I I O I axis of the LaSalle Anticlinorium near the center of the map area. Across this major

R 5 5 I 5 5

e G

e Clinton T 0 T G vation contour lines do not conflict with those of overlying and underlying units (so that, 0

N W W N e k Mahomet 0 n N r e T 2 T N i

Lincoln re E A 5 E A C north-south-trending structure, the Mahomet Bedrock Valley changes course; the l C C C I Lincoln I

C D n Clinton D C for example, the elevation of a lower unit does not surpass an upper unit). In so doing, 10 48 a P O o P O

O e O r il m S . e . upvalley part (to the east) is oriented northeast-southwest whereas the downvalley part

. e M a 47 . the geologist produces an internally consistent, three-dimensional geologic model and D n g Ogden e n 74 (to the west) is oriented southeast-northwest. The map area also contains some of the T a G 72 Champaign set of maps for a region. 10 S 121 o 10 Champaign Urbana 150 Urbana o St. Joseph oldest glacial sediments identified in the region, including a complex sequence of With GIS techniques, maps are produced that are similar in appearance to hand- s e diamictons and sands and gravels associated with multiple glaciations and buried soils C drawn maps; to the eye, each elevation map may appear not to conflict with the ele- r L k k e O e e e associated with interglaciations. A diamicton is a mixture of clay, silt, sand, gravel, and e k G vation maps of other stratigraphic units. However, to develop a truly internally-con- r e

L

r A 0 C DE WITT CO. E DE WITT CO. 5

54 C 130 C O boulders that, if of glacial origin, is commonly referred to as till; although most of the C N sistent set of maps, the maps are processed into a raster (gridded) format, as described

H H m

G lt Fork C

a MACON CO. A MACON CO. A

P P A S b t diamictons in the map area are interpreted as till, we use diamicton, the more gener- s Monticello l O

M M

I a I N S below. Then, conflicts in elevation between horizons (and larger conflicts across sev- a

A d A Figure 1.—Location of the map . Monticello 1

P P n r

T T C e Homer i A r A al descriptor. However, the term “till” is retained where it is part of the stratigraphic

T

. T

O r Mount Pulaski 5 eral horizons) are easy to detect. Correcting those conflicts is not, however, a trivial

57 a 2 .

I I

F 45

G G area, east-central Illinois. O

C s C . r

Mount Pulaski O C N e N name of a unit.

O ° ° O 1 40 0

40 C undertaking. A significant effort was spent to develop a set of maps which adhered to r v Tolono 0

i Tolono

.

e . N v C C i R T 75 R

O O In past studies, various surface and subsurface mapping techniques have been

R O 49 T our models for glaciofluvial deposition and erosional history.

i 50 C n . . v 51 o A

48 I

e m A 0 5 applied to all or parts of the map area. These include an ISGS statewide stack-unit

a P 0 2 a r Based on our experience, and considering the time needed to generate this model i 1 g M n 105 k a s map (Berg and Kempton, 1988), which shows the succession of geologic materials in La k 121 72 S 5 and set of maps, we advise that before a mapping project is begun, the planned and

ke For a 7 . k . 5 0 O their order of occurrence to a depth of 50 ft, and a small-scale (1:1,000,000) USGS Bement s 2 5 potential uses of the map products be carefully evaluated. Providing an internally con- O 75 a

LOGAN CO. C LOGAN CO. 7

C Pesotum 0 5

K 5 5 N 100 Pesotum map of thickness and character of Quaternary deposits (Soller, 1993, 1998). Detailed SANG T SA 0 sistent, three-dimensional model is essential if there is an analytical use planned, such

AMON CO. O NGAMON CO.

T C

A geographic information system (GIS) techniques (Berg and Abert, 1994, and McLean

25 I

Lake A as development of a ground-water flow model. However, if adequate high-quality data P Decatur Decatur M Decatur and others, 1997) have also been developed for the region. This part of east-central are not available, these maps should not be developed, but more conventional, vector- Illinois provides an excellent opportunity to test and develop new concepts and proce- based methods for preparing maps of each surface should be used to provide a gen- Refer to figure 17 map on sheet 3 for base information sources SCALE 1:500 000 Base from ISGS data that was digitized from USGS SCALE 1:500 000 dures for portraying geologic materials in three dimensions, for possible use in future eral, visual depiction of the geologic framework. 1:24,000- and 1:62,500-scale topographic maps Universal Transverse Mercator projection 10 0 10 20 MILES 10 0 10 20 MILES state and national mapping programs. Universal Transverse Mercator projection This study was conducted in a ground-water-rich area of the State, where delineation Creating a vector map 10 0 10 20 KILOMETERS 10 0 10 20 KILOMETERS of sand and gravel aquifers is essential to better understand resource potentials, to Figures 13–16 (sheet 2) present for each of five primary Quaternary stratigraphic resolve conflicts over ground-water use, to support planning for ground-water protec- units and two minor sand layers a block diagram (three-dimensional perspective view) tion strategies, and to support regional economic growth. Previous studies by Horberg showing topography of the upper surface, an elevation map of the upper surface, and (1945, 1953), and Kempton and others (1991) increased our understanding of the a thickness map. A block diagram and an elevation map of the bedrock surface are Figure 2.—Selected natural and cultural features in the map area. Figure 8.—Thickness of the Wedron and Mason Groups (Wisconsin Episode); includes minor thickness of the Quaternary deposits. However, since then, available subsurface data have improved, also shown. Our mapping of each stratigraphic unit was an iterative process that, Cahokia Formation (Wisconsin and Hudson Episodes) as alluvial fill in the valleys and the Peoria Silt as loess on the models of regional geologic history have been refined, and the public’s need for more through re-examination of stratigraphic data and maps, gradually refined our under- uplands. Arcuate bands of thicker sediment correspond to end moraines (see fig. 3), where glacial ice stagnated. precise information has increased. Consequently, existing maps have become outdat- standing of the vertical and lateral distribution of each unit. To map a unit, we first Sinuous areas of thin sediment correspond to modern river valleys, where the Wedron and Mason Group deposits ed. New maps can supply societal benefits not realized by pre-existing ones [for exam- plotted the stratigraphic control data, then prepared a hand-contoured map based on have been eroded (for example, the valleys of the Sangamon River and Salt Creek). The southwestern part of the ple, see Bernknopf and others (1993) and Soller and Bernknopf (1994)]. A signifi- the data and on an understanding of the regional distribution of the materials and geo- cantly updated database of subsurface information and newly developed methods that map area lies beyond the limit of Wedron Group deposits. There, only thin, discontinuous Cahokia Formation and logic history (for example, the middle Banner Formation had a glaciofluvial origin and incorporate digital mapping techniques have allowed us to provide updated maps Mason Group deposits occur, mostly as alluvium, outwash, and loess; to permit them and the underlying units to was confined to bedrock valleys). The map was then scanned and a vector map of the based on an improved understanding of the regional geology. be mapped efficiently by computer, a uniform thickness of 15 ft was assumed (see discussion under “An internally linework was created. consistent geologic model and set of maps,” fourth paragraph). This report comprises three sheets. Figures 1–12 are shown on sheet 1, figures 13–16 are on sheet 2, and figures 17–19 are on sheet 3. Converting to raster format ACKNOWLEDGMENTS A vector map generally is a faithful rendition of a hand-drawn contour map. For example, each vector, or line, on an elevation map of the upper surface of the middle These maps are the product of a cooperative project supported by the ISGS and the Banner Formation has an elevation value (for example, the 475- or 500-foot elevation USGS. We thank Bill Shilts (ISGS Chief and State Geologist) and John Pallister contour). Areas between contour lines possess a range of elevation (for example, (Program Coordinator, USGS National Cooperative Geologic Mapping Program) for between 475 and 500 ft), and the elevation at any location on the map (other than on fostering the cooperative environment needed to conduct the project. We particularly a contour line itself) cannot be more precisely defined. Although such values may be wish to thank Mary Mushrush (ISGS) for many weeks of work in helping to develop the EXPLANATION inferred by interpolation, they are not explicitly defined. A raster map, however, database for the mapping. We also thank Pius Weibel (ISGS) for providing the map of depicts information at each of many regularly spaced grid cells. It contains more infor- bedrock geology, and James Estabrook and Paul Mathieux (both USGS) for their skill- Cahokia Formation and Mason Group, mation than a vector map, because it also provides an estimated or interpolated value Henry Formation—Recent, coarse- to ful and timely work on map editing and graphic design. The quality of the maps and between data points and contour lines. Computer-generated cross-sections, three- fine-grained alluvial deposits and glacial text were improved by thoughtful reviews provided by Ardith Hansel, Robert Krumm, dimensional visualizations, and many modeling routines (for example, for ground-water outwash and Don McKay (all ISGS), and Carl Koteff (USGS). flow) require raster data. Mason Group, Equality Formation—Clay, Data on the vector map were processed to a raster format. Although useful for silt, and sand deposits of glacial lakes analysis, raster maps can appear somewhat different from vector maps, because they Mason and Wedron Groups—Undiffer- tend to show the map information with a blocky or jagged appearance rather than the entiated Mason and Wedron; predomi- 89° 88° smoothly drawn boundaries to which we are accustomed. For example, refer to the nantly diamictons (tills) of Wedron Group northeast corner of figure 15B (sheet 2), specifically the areas classified as 25–50 and overlain and underlain by formations, WOODFORD CO. 25 50–75 ft thick. There, a jagged contact is displayed because each raster data point, 0 members, and tongues of the Mason MC LEAN CO. 100 5 1 which varies by as little as one foot from its neighbor, has been assigned to an eleva- Group 75 5 00 25 7 tion category. This has been done purely to aid in visual comprehension—the contin- 50 End moraine uous data on a raster map are force-fit into an interval classification, which is more 0 appropriate to vector maps. Jagged contacts, while they may appear to signify errors Mackinaw 15 Ground moraine 125 Normal 50 or inconsistencies on our maps, actually connote very small changes in elevation (for ° 100 75 40 30' 100 UOIS CO. example, values on either side of the jagged 50-foot contour in the northeast corner 75 IROQ 50 Bloomington Upper Glasford Formation, Radnor Till 1 FORD CO. of figure 15B vary by only a foot or so). In contrast, values on either side of that con- 50 50 Gibson City 25 75 Member—Mainly diamicton (till) with Paxton tour on a vector map may only be inferred to differ by somewhat less than the contour 5 10 some associated deposits 7 75 0 interval, here 50 ft.

. For presentation, we considered creating a smoothed, vector version of each raster O 125 C map. However, the time and expense involved, and, more important, our desire to

N FORD CO. 75 A

E emphasize the analytic uses of digital geologic maps, led us to retain the raster maps CHAMPAIGN CO. 100 L C 100 in this report. To aid visual aesthetics, we chose a small raster grid size (100 meters), 75 M

M thereby minimizing the characteristic blockiness of raster maps. If only the key strati-

C Le Roy

L 0 graphic control data were considered in the gridding, this grid size would be inappro- E 10 TAZEWELL CO. Figure 3.—Perspective view, looking north, showing surficial geology and buried Quaternary units in the map area. A 12 125 5 N priately small. However, for each stratigraphic unit a general interpretation of deposi- Heyworth LOGAN CO. Surficial geology is from Lineback (1979) and Stiff (1996); the latter is a digital version of the map by Hansel and C 1 Rantoul O 00 tional and erosional history was developed (a conceptual geologic process model), pro- 10 Johnson (1996). Subsurface geology is from this study, and is shown in more detail on sheet 2; subsurface units . 0 Local school students visiting USGS drill rig at the Gifford site in Champaign County. viding a basis for assumptions about each unit’s three-dimensional distribution. Our MC LEAN CO. 75 ISGS geologists are explaining sediment cores from stratigraphic test hole and dis- are the same as those shown in figure 7. Bedrock is shown in gray. In the southwestern part of the map area, DE WITT CO. grid size was selected to maintain the traditional, vector-like appearance of the maps Atlanta cussing area’s geologic history and resources. The Gifford site is shown in figure 11 the Upper Glasford Formation is covered in places by younger deposits, which for modeling purposes are assumed while creating a digital map product that could be adapted to more analytical purpos- Farmer City (locality 774, section 1, T. 21 N., R. 10 E.). to be 15 ft thick (see discussion under “An internally consistent geologic model and set of maps,” fourth paragraph). es. For an application such as ground-water modeling, the grid cells may be aggre- 125 7 Black line shows the southwesternmost limit of Wedron Group deposits. Topography is from edited, digital ver- 5 100 gated to provide a spatial framework more realistic to the needs of that application. C

V

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sions of USGS 1:100,000-scale topographic maps. Image is vertically exaggerated approximately 30x. Mahomet E .

A

R O REGIONAL GROUND-WATER RESOURCES

M C M . An internally consistent geologic model and set of maps

P

I

T O L

A

T C I

I O Ground water within sand and gravel of the buried Mahomet Bedrock Valley aquifer I T 125 G

W N

T N After each elevation map was rasterized, it was compared to the stratigraphic con- 5 E A system has been estimated by the Illinois State Water Survey (Visocky and Schicht,

C Lincoln 0 I

Clinton D C

P O 25 O trol data and to the elevation maps of stratigraphic units above and below it. This was . 1969) to be able to provide about 445 million gallons per day (mgd) to large munici- 10 . 75 palities, industry, and private residences. Figure 6 shows representative data from the first stage of an iterative process of re-evaluating stratigraphic interpretations in the 0 Champaign Urbana water-well records. Even during the drought of 1988, however, only about 85 mgd database and refining the maps. In many cases, interpreting the stratigraphy was dif- ficult because units of distinctly different ages and different depths looked the same. 150 were used, which was the maximum yearly usage (Illinois State Water Plan Task Force

L 0 O For example, in test borings that sample multiple diamictons, upper Banner Formation 0 5 [ISWPTF], 1997). Therefore, the resource potential of the aquifer system is quite

G 0 125 1 A DE WITT CO. 75 diamictons can be misidentified as lower Glasford Formation due to their similar C 5 large. N 2

H

C

MACON CO. A P The Mahomet Sand, which fills the deepest parts of the bedrock valley, is the thick- appearance, especially if intervening soils are not present. If the elevation of a strati-

O

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A

. 0 Monticello 10 P T 0 graphic unit at a particular point was anomalously higher than appropriate, based on A 1 est and most widespread aquifer in the system. In addition, overlying the Mahomet

Mount Pulaski . T . ° I ° 88

G 89 O 25 C

O 1 the regional geologic map trend, it was re-examined for a potentially better fit with an C N Sand are sand and gravel units intercalated with fine-grained sediment. Where the

° O

40 C

Tolono

. N

C 0 T 75 overlying map unit. In some cases, the lithologic characteristics of the sample were

O Mahomet Sand is absent, these aquifers are important sources of water for farmsteads, O

DFORD CO. 0 200 T C

WOO 3 . A

I inconclusive and the stratigraphic interval was assigned to the younger age, whereas in 1 A communities, and industries. They also hold water for gradual recharge into the under- O. 350 50 0 P MC LEAN C M 0 2 0 50 40 00 lying Mahomet Sand Aquifer (ISWPTF, 1997). other cases the stratigraphy was found to be correct and diagnostic of the lower unit. 0 2 7 7 45 5 5 1 In the latter case, a shortcoming of the regional mapping is indicated: the anomalously 00 The Mahomet Bedrock Valley was defined first by Leland Horberg in 1945. Since 2 LOGAN CO. 50 7 50 5 high data point was correct and represented some local relief that was not mappable 300 THICKNESS (feet) Pesotum then, the bedrock valley and its sand and gravel aquifers have been the subject of con- SANGAMON CO. 25 50 12 2 2 at our scale. Those map data were retained, and the resulting local “spike” in the map Mackinaw 4 5 For figures 4 5 5 siderable interest by scientists, planners, State and local regulatory agencies, the agri- 4 50 3 1 00 Normal 150 00 0 250 cultural and industrial community, and the general public. The ISWPTF (1997) report- surface indicates a need to gather more information for that area. This situation occurs 40° 30' and 8–10 Decatur 350 IROQUOIS CO. near the southeast corner of the map area shown in figures 13D and E (sheet 2); there, Bloomington 200 ed that increased water use and recent droughts have caused concern about long-term FORD CO. 35 250 200 0 Base from ISGS data that was digitized from USGS SCALE 1:500 000 use of the region’s aquifers. They noted that the drought of 1988 significantly affect- a small topographic high corresponds to a key stratigraphic control point. Only with 3 Paxton 0 4 450 1:24,000- and 1:62,500-scale topographic maps 0 Gibson City 0 10 0 10 20 MILES ed surface-water reservoirs that supply water for the cities of Decatur, Danville, and extraordinary efforts to collect significantly more data could the fine detail around the 0 0 5 Universal Transverse Mercator projection data point be mapped. 2 300 Bloomington (fig. 1). Potential additional development of the Mahomet Sand Aquifer

. 400

O Y 10 0 10 20 KILOMETERS to supplement these surface water supplies could amount to about 20 to 30 mgd. Of Discontinuous units are particularly difficult to map because gridding algorithms com-

300 C E

N L 00 FORD CO. 350 concern is the continued growth in the Champaign-Urbana area (whose wells are the pute cell values by interpolation methods. [We used the Arc/Info Topogrid algorithm; 250 3 A L 200 E CHAMPAIGN CO. A largest system tapping the aquifers) and dependence on the aquifers as the primary for these data, we found that other algorithms supplied in Arc/Info and in other soft- 0 0 L 0 5 C V 300 3 3 ware provided results less appropriate to our needs.] No algorithm can produce a real- M supply to many smaller communities. Other users of the ground-water supply have

M istic map where data are absent across areas of relatively high relief. Consider, for C Le Roy 250 Figure 9.—Thickness of the Glasford Formation (Illinois Episode). The pattern of sediment deposition was affected increased in number and in demand and are expected to continue to increase. The

L example, the middle Banner Formation, which is confined to valleys separated by TAZEWELL CO. E K by the Mahomet Bedrock Valley, which had been partly filled by older sediment. Areas of thicker sediment most- number of irrigation systems has expanded and there have been increased industrial A M N Heyworth C 200 activities such as the production of ethanol, which requires large volumes of water. expanses of upland. A gridding algorithm must compute a value for every cell, includ- LOGAN CO. A ly correspond to this ancient valley. Sinuous areas of thinner sediment correspond to modern river valleys, where C Rantoul O 350 H O the Glasford Formation has been partly or completely eroded (for example, the valleys of the Sangamon River and Because resource development is expected to increase, many communities have ing those far removed from data points, and each cell’s value depends in some mea- . 0 0 300 MC LEAN CO. 3 150 O R Salt Creek). sought to control ground-water resource development near their wells and well fields, sure on adjacent cells. Unrealistic cell values that greatly departed from values on the DE WITT CO. D 250 Atlanta M 125 and local water authorities have been established. However, as the ISWPTF (1997) vector map were corrected by 1) increasing the density of the elevation data on the vec- E tor map (especially in topographically flat areas and near large changes in slope gradi- E Farmer City 0 states, “The desire to unduly or unfairly restrict or control ground-water resource devel- B 25 100 0 ents), 2) re-gridding the map, and 3) removing upland-area data from the raster map T 0 opment generally stems from a lack of information about the resources or from a fear 200 0 2 (because, as noted above, the middle Banner Formation does not occur on the 1 40 C of becoming economically disadvantaged due to unknown adverse impacts 1 2 V 5 5 H 75 * * * 2 Mahomet E 0 0 . A 0 T R uplands). This method is useful for units whose depositional pattern is predictable. For O Consequently, timely appraisal of these ground-water resources is important so that

M C M .

P 250 E I L the basal sands of the Glasford Formation (sheet 2, fig. 16), data are sparse and the B T O 5 A 5 50 4 12 their development and use will enhance the region while minimizing unnecessary con- 0 T C 0 I 7 I 00 0 O 0 3 5 I

3 0 G 3 T 3 M5 W N 0 0 0 unit’s distribution is not so predictable. There, we gridded the unit thickness data and E 5 250 T 10 N flicts and preventing degradation of the resource.” The maps provided on these sheets 20 0 3 E A C Lincoln 0 I Clinton 0 D O C 25 150 D 4 0 P O computed the elevation of the upper surface by adding unit thickness to the elevation 20 O are intended to help decisionmakers address this issue.

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H . 12 of the underlying unit. 5 R 10 A Champaign Comparison of maps for each layer revealed potential inconsistencies such as areas O M Urbana 1 00 C 0 QUATERNARY STRATIGRAPHY where an older, lower unit was mapped at a higher elevation than the unit above. For 300

L K example, the initial raster map of the upper Banner Formation was computed without

O 3 5 Figure 7 depicts the relations and current classification of sediments deposited by 0 G considering the topography of underlying units. Comparison of bedrock and upper- A DE WITT CO. glacial, periglacial, and fluvial activity in east-central Illinois. The figure is based on this C Original measurements of N V

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A Banner elevation maps revealed the control that bedrock topography imposes on the

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MACON CO. A

3 P 2 0 L stratigraphic-unit thicknesses study and on studies by Larson and others (1997), Hansel and Johnson (1996), Herzog O L 5

M 0 E I 0 Y A distribution of upper Banner deposits. Revision of contour lines and re-gridding pro- 2

. Monticello 1

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0 and others (1995), Wilson and others (1994), Kempton and Visocky (1992), and 5

1 were made in feet, so standard 2

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0 0

Mount Pulaski 5 .

2

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duced a map showing the correct spatial relation—progressive thinning and then 0

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O units are retained here. To Kempton and others (1991). Stratigraphically from top to bottom, these sediments are C

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Tolono absence of upper Banner Formation, from the valley to the bedrock uplands.

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T convert to meters, multiply by grouped into three major units whose distribution and thickness are portrayed in fig-

O O

T 150 25 C 200 . Refinement of the map of each stratigraphic unit proceeded in this fashion until an 7 A

5 I

A 0.3048. ures 8, 9, and 10, respectively. Figure 8 shows the interfingering Mason and Wedron P

M internally consistent stack of maps was created. 2 Groups and overlying Cahokia Formation alluvium (Wisconsin and Hudson Episodes); 50 00 Internal consistency between the elevation map of the Wedron and Mason Groups 2 250 00 89° 88° figure 9 shows the Glasford Formation (Illinois Episode); and figure 10 shows the LOGAN CO. 150 and the elevation map of the upper Glasford Formation (sheet 2, figs. 14E and F) was 150 Pesotum Banner Formation (pre-Illinois Episode). The lower two units are separated into two SANGAMON CO. difficult to achieve, especially in the southwestern part of the map area at the limit of FORD CO. and three subunits, respectively. The distribution and thickness of the lower and mid- WOOD 0 20 Wisconsin ice, where thick Wedron Group end-morainal deposits abut an area under- MC LEAN CO. 50 0 dle Banner Formation subunits are significantly influenced by the configuration of the Decatur 2 20 0 50 5 5 5 lain by outwash, alluvium, and loess of the Mason Group and Cahokia Formation (sheet 1 2 5 1 2 Mahomet Bedrock Valley system, which generally trends east-west across the map 1 0 7 1 125 10 2, fig. 15E). There, Mason Group deposits and Cahokia Formation are in places thick, Base from ISGS data that was digitized from USGS SCALE 1:500 000 100 area, and the Mackinaw Bedrock Valley in the northwestern part of the map area. 150 0 thin, or absent; all overlie upper Glasford Formation deposits, which are in places 1:24,000- and 1:62,500-scale topographic maps 200 5 10 0 10 20 MILES 5 0 These bedrock valleys and their tributaries are shown in figure 5. Ages of these sedi- Mackinaw 2 exposed. Considering the thin, discontinuous nature of the overlying deposits in that 0 0 Universal Transverse Mercator projection Normal 1 mentary bundles are not precisely known, but are constrained by the following gener- 10 10 5 area, the upper Glasford Formation reasonably could be mapped at the land surface. ° 1 7 al estimates: Hudson Episode—less than 12,000 years before present (yBP); 10 0 10 20 KILOMETERS 40 30' 50 IROQUOIS CO. Bloomington However, joining the complex land-surface topography with the far more generalized 12 FORD CO. Wisconsin Episode—75,000 to 12,000 yBP; Illinois Episode—180,000 to 125,000 5 5Gibson0 City contours of the buried upper Glasford Formation surface to the north and east, under Paxton yBP; and pre-Illinois Episode—more than 500,000 and mostly less than 730,000 yBP. 25 Soils referred to as the Sangamon Geosol and the Yarmouth Geosol formed during the Wedron Group end moraine, and then integrating that composite surface with the map of the overlying unit (Wedron and Mason Groups) did not give satisfactory results. Figure 4.—Thickness of glacial sediments, including minor overlying sediments of nonglacial origin. Sediments over- . interglacial episodes between the Wisconsin and Illinois glacial episodes and the Illinois O The fine topographic detail southwest of the end moraine could not readily be meshed

lie a bedrock surface of moderate relief; thicker sediments occur in the Mahomet Bedrock Valley and beneath C and pre-Illinois glacial episodes, respectively. The units discussed in this paragraph are N FORD CO. with the coarser, less detailed contour data on the upper Glasford surface under the A also shown in maps on sheets 2 and 3. Wedron Group end moraines (fig. 3), and somewhat thinner sediments occur on the bedrock uplands. The E CHAMPAIGN CO. 200 L end moraine. Attempts to mesh the contours were unsuccessful, resulting in abrupt Mahomet Bedrock Valley is a preglacial and early glacial drainage system in the region (see Kempton and others, C In the map area, the Wedron Group is composed of two formations, the Lemont and

M topographic breaks across the toe of the end moraine. These breaks could be cor- 125 the Tiskilwa, each composed predominantly of diamicton; two members are recog- 1991). M 50

C Le Roy rected, mostly through editing of individual cells or pixels. It was decided not to do so,

nized in each formation. The Mason Group is composed of water-laid and windblown L 0 TAZEWELL CO. E 15 because of the time and effort needed and because the corrections would produce a A 75 sediments subdivided into formations, members, and tongues, which occur as deposits

N 0 5 0 Heyworth 10 2 5 map that could not readily be updated or revised during development of the integrat- LOGAN CO. 1 C 1 0 Rantoul both at the land surface and intertonguing with or underlying the Wedron Group units. O 150 0 2 ed geologic model and set of maps. Also, future revisions to the map by the process

. 125 100 Within the Mason Group, the Peoria Silt (loess) is the most widespread surface unit; it 75 MC LEAN CO. described above (that is, revisions to the vector map and re-gridding) could not be made 50 DE WITT CO. caps much of the map area but is thickest west of the Wedron boundary in the south- Atlanta25 if the raster map were extensively edited. Therefore, the following assumption was 10 western part of the map area. The Morton Tongue of the Peoria Silt locally underlies Farmer City 0 applied to the map information, prior to rasterization: the topographic break beneath 15 the Wedron Group diamicton. The Henry Formation occurs predominantly in the prin- 5 the end moraine was resolved by assuming a minimum 15-foot thickness for surface 2 1 cipal river valleys, commonly beneath modern alluvium, as ribbons of sand and gravel

C deposits of the Wedron and Mason Groups and Cahokia Formation across the study 00 V

1 H

Mahomet E outwash; the Ashmore Tongue of the Henry Formation locally occurs below the . 5 0

A ONARGA BEDROCK VALLEY 7 5 5 R O 2 area. Southwest of the end moraine, this effectively lowered the top of the Glasford M DANVERS BEDROCK VALLEY M C Wedron Group diamicton. The Roxana Silt (loess) is the predominant subsurface unit 12 . 10 P 10 5 I 0 0 T O L 0 0 A Formation by 15 ft, removing the topographic break and permitting a smooth inte- T C I

75 1 I O 5 I within the Mason Group. It is most easily recognized and identifiable in the subsurface 7 0 T G

W N 5 50 T N gration of the Wedron and Mason Group map with the upper Glasford Formation map E A

C Lincoln 25 I (by water-well drilling contractors as well as by geologists) by presence of the Robein BEDROCK MAP UNITS Clinton D C

P O

O across the area. This assumption also affects areas where modern streams have

. Silt Member, a dark-brown to black organic-rich silt (a part of the Farmdale Geosol,

. 2 incised Glasford Formation and older deposits; there, the top of the older deposits in 0 50 0 Champaign whose radiocarbon ages range between 20,000 and 25,000 yBP). The Roxana Silt Mattoon and Bond Formations (Pennsylvanian) Urbana represents the youngest part of the ice-free interval after deposition of the Glasford these valleys has been effectively lowered by 15 ft, producing a minimum 15-foot thick- EY 25 MACKINAW BEDROCK VALLEY LL ness of Cahokia Formation alluvium in the riverbeds. The presence of Cahokia VA Formation. The youngest of the three sedimentary bundles includes the Cahokia

L Formation alluvium is reasonable and is generally supported by field observations. K 1 0 C O

0 Formation, composed of alluvial deposits which fill the river and creek bottoms.

O G R Modesto Formation (Pennsylvanian) 1 D A DE WITT CO. 50 However, as a consequence of the assumed minimum 15-foot thickness, thin surficial E C B N The Glasford Formation as defined for this study includes four recognized members:

H

T

C deposits also are shown along valley margins where they may not actually occur (see

MACON CO. A E P 0

O the Berry Clay Member (an accretion-gley included as part of the Sangamon Geosol),

M 1 M 125 I

A

O . Monticello

P fig. 17 on sheet 3); there, modern erosion has in places exposed upper Glasford H Carbondale Formation (Pennsylvanian) T M A A the Radnor Till Member, the Vandalia Till Member, and the Smithboro Till Member.

Mount Pulaski 10 . T . M 0 I A

G O Formation and older deposits. C

H O C N Locally significant sand and gravel commonly occur between the Radnor and Vandalia

° O

KENNEY BEDROCK VALLEY 40 C

7 O Tolono

5 5 . 25 N 0 C Both data quality and certainty of interpretation varied significantly for each strati- M T

O

O diamictons and at the base of the Vandalia diamicton. For purposes of this report, the

2 T 50 C (Pennsylvanian) . E 1 5 A graphic unit. We used the most certain of the units as the starting point to develop the 0 I 75 A 2 thin, discontinuous Berry Clay Member and overlying Robein Silt Member and Roxana T P 5 M set of maps, relying on them to constrain the mapping of less well-understood units. 1 0 100 2 1 Silt have been grouped with the Radnor Till Member as upper Glasford Formation. 5 25 0 The top of the Mason and Wedron Groups, which corresponds to the land surface, 10 rocks 15 Although included in the upper Glasford Formation, the basal sand and gravel also is LOGAN CO. 0 0 was an obvious starting point. Among the buried units, we had the most confidence B 5 Pesotum mapped separately. The lower Glasford Formation includes the Vandalia Till Member, SANGAMON CO. 2 E 5 in maps of the bedrock surface and the top of the middle Banner Formation, for two 5 1 5 7 5 present in most of the map area, the Smithboro Till Member, recognized only locally, D 0 7 2 0

5 0 1 R rocks 0 10 reasons. First, the Mahomet Sand Aquifer and the bedrock surface were easy for O Y Decatur and the “lower” Vandalia Member, which currently is assigned to the Glasford C L E drillers and geologists to identify, relative to the gray-brown diamicton-dominated K V A L Formation but which may be a separate unit or correlated with the Tilton Till Member Base from ISGS data that was digitized from USGS SCALE 1:500 000 of the Banner Formation. Beyond the Radnor diamicton boundary in the southeast stratigraphy in the remainder of the section. Second, the fluvial processes that con- Silurian rocks trolled bedrock erosion and deposition of the middle Banner Formation are relatively 1:24,000- and 1:62,500-scale topographic maps 10 0 10 20 MILES corner of the map area, the Berry Clay Member, Robein Silt Member, and Roxana Silt Universal Transverse Mercator projection are included with the lower Glasford Formation map unit. The basal sand and gravel well understood; fluvial processes leave a relatively predictable pattern of deposits con- strained within a network of valleys. Approximate location of LaSalle Anticlinorium 10 0 10 20 KILOMETERS of the Vandalia Till Member is included in the map unit but also is shown separately. axis The Banner Formation contains three distinct subunits: an upper unit consisting We therefore began our modeling from the top (land surface) and the bottom principally of diamictons, a middle unit composed principally of coarse- to fine-textured (bedrock surface and top of middle Banner Formation) of the depositional sequence, and worked toward the middle, where interpretations of spatial patterns of buried MIDDLETOWN BEDROCK VALLEY PESOTUM BEDROCK VALLEY water-laid sediments, and a lower unit composed of interbedded diamictons and Figure 10.—Thickness of the Banner Formation (pre-Illinois Episode). The pattern of sediment deposition was coarse- to fine-textured water-laid sediments. The upper Banner Formation contains diamictons and associated sand and gravel are most difficult. For example, the bound- greatly influenced by the Mahomet Bedrock Valley. The fluvial phase of these sediments constitutes the thickest three named diamicton members, the Tilton, Hillery, and Harmattan, and a locally ary between the upper Banner Formation and lower Glasford Formation was particu- part of the section and delineates the valley and its tributaries. These mostly sand and gravel sediments are over- larly problematic because multiple diamictons commonly occur with scant evidence of Figure 5.—Perspective view, looking north, of the geology and topography of the bedrock surface in the map area. occurring uppermost member, the Lierle Clay, which is an accretion-gley facies of the lain by diamictons, which occur in the valley and on the surrounding bedrock uplands. In many areas, especially paleosols separating them or without the presence of a complete section. Our map of Bedrock geology is from Pius Weibel (Illinois State Geological Survey (ISGS), written commun., 1997). Bedrock Yarmouth Geosol developed in the uppermost Banner Formation unit. All of these on the bedrock uplands, the Banner Formation was eroded by later glaciations or fluvial action. units may be present over the bedrock valley or uplands. Of the three diamicton mem- a stratigraphic unit was vastly improved by comparing it to vertically adjacent, well- topography is from this study. The axis of the LaSalle Anticlinorium, of late Paleozoic age, also is shown. To defined units. enhance topographic detail, the image is vertically exaggerated approximately 30x. bers, the Hillery Till Member is the most widespread and easily recognized unit in sam- ples and in well-drillers’ logs (because of its distinctive reddish color) throughout the With a complete set of elevation maps generated, maps of unit thickness were then map area. The Harmattan Till Member may intertongue with the upper part of the computed, by calculating the difference in elevation between the top of the unit and middle Banner Formation sand and gravel just to the east of the map area. On the the top of the underlying unit. Because these thickness maps are derived from two uplands, lower Banner Formation deposits are described in well logs in various places, raster elevation maps, they reflect characteristics of each parent map. Consequently, but their distribution is apparently patchy and they are not easily separated; therefore, when displayed as interval rather than continuous data, they tend to show more of the for this report, lower Banner deposits on the uplands are included in the upper Banner characteristic jagged appearance of raster maps than do the elevation maps. As dis- Formation. cussed above, these are not mapping errors. The middle and lower Banner Formation units are restricted mainly to the bedrock valleys and compose most of the fill in these valleys. The middle Banner Formation VISUALIZING THE DEPOSITS IN THREE DIMENSIONS consists of the Mahomet Sand Member in the Mahomet Bedrock Valley and its equiv- 89° 88° alent, the Sankoty Sand Member in the Mackinaw Bedrock Valley. It is composed Geologists have traditionally used cross sections and fence diagrams to visualize geo- mainly of outwash sand and gravel within the main bedrock valleys, but intertongues logic units in three dimensions. Portraying the three-dimensional nature of geologic WOODFORD CO. with a silt facies in the tributary valleys, and grades upward into fine-textured lacustrine materials in as much detail as possible is essential to an improved understanding of MC LEAN CO. silts, and locally organic-rich alluvial silt, in some areas of the main bedrock valleys. geologic maps and geologic processes responsible for the distribution of deposits. This These silts were deposited in temporary lakes created by ice or sediment dams. The information is needed both during the iterative process of mapping and for the pub- approximate delineation between the fluvial and lacustrine facies of the Mahomet Sand lic’s comprehension of map information. The three-dimensional visualization products we provide here were developed in EarthVision software, from two-dimensional grids Mackinaw Member is shown in figures 14B and 15A (sheet 2). In most of the Mahomet Bedrock Normal Valley, the middle Banner Formation composes the entire fill of the valley and rests on of each stratigraphic horizon created in Arc/Info. They include block diagram per- ° 40 30' O. spective views of each stratigraphic horizon (sheet 2, figs. 13 and 16C and F) and the IROQUOIS C R 2 W R 1 W R 1 E89° R 2 E R 3 E R 4 E R 5 E R 6 E R 7 E R 8 E R 9 E R 10 E R 11 E88° R 14 W Pennsylvanian or older bedrock units; in some of the tributaries, its silt facies overlies Bloomington FORD CO. fence diagram and horizontal “slices” on sheet 3 (figs. 18 and 19). 340–516 Gibson City 711 older deposits of the lower Banner Formation. Lower Banner Formation deposits are Paxton 643 M 1.0 r RD CO. ac 745 753 Traditional methods of showing changes in subsurface-unit geometry along a verti- e WOODFO 748 kin k described in well logs and samples in various places in the Mahomet Bedrock Valley, T 25 N v a e i w e AN CO. R r cal plane (cross sections; see fig. 17 on sheet 3) and providing a three-dimensional per- R MC LE 645 i but their distribution is patchy and they are not easily separated; therefore, for this 330–385 w ver 631 657 C

. a kin 743 spective of these planar views (fence diagram; see fig. 18 on sheet 3) are useful for O 0.5 c 629 ng 752 report, lower Banner deposits in the Mahomet Bedrock Valley are included in the mid- a 744 pri C M 742 S illustrating the configuration of the Mahomet Bedrock Valley and progressive valley

N FORD CO. 695 626 747 750 658 dle Banner Formation. A

E infilling. Cross sections may be selected to emphasize a particular feature, such as unit CHAMPAIGN CO. The most extensive deposits of the lower Banner Formation are found in the west- L T 24 N Mackinaw 718 C Normal 751 ern confluence area of the Mahomet and Mackinaw Bedrock Valleys. In this region, thickness along the length of the Mahomet Bedrock Valley at its thalweg, or deepest M EXPLANATION

M point (see cross-section J–J’ on sheet 3). 40° 30' . recent studies and drilling (for example, Herzog and others, 1995) have shown exten- C Le Roy 673 638 738 IROQUOIS CO

L Public or industrial water-supply well sive areas containing interbedded diamictons, coarse-textured sand and gravel, and Computer software provides the opportunity to easily manipulate data in order to E O. TAZEWELL CO. 674 FORD C A 8.5 Bloomington Gibson City B 655 quickly view and evaluate subsurface deposits as maps are iteratively being developed. N 670 Heyworth 15–115 696 740 ig 739 Paxton fine-textured lacustrine sand, silt and clay beds. Although these deposits appear to be LOGAN CO.

C Rantoul rk 615 608 656 F Software also allows the geologist to view and evaluate the deposits from more per- O 1.3 Transmissivity in thousands of gallons per day o k 604 o F r 717 683 u highly variable in distribution, thickness and composition, it is possible with more data . o r T 23 N F 591 D spectives than would be efficient using traditional cartographic methods. For example, MC LEAN CO. per foot 609 . they will become more predictable, and mappable. To the east of the map area on the 1.7 t e S it M 2.6 152 (G) s l 619 O 741 c e d an h DE WITT CO. d g 737 id figure 19 on sheet 3 shows the pattern of units that would be seen along a horizontal 610 C d bedrock uplands, local remnants of older sediments are present and correlated mainly Atlanta Glasford Formation only W i 704 280 am l N e 1.0 M on FORD CO. F

A o rk with the lower Banner Formation. These deposits include the Belgium Silt Member, plane at a given elevation if overlying deposits were removed. Starting with the low- Farmer City E 474 Pumpage (1982) in millions of gallons per day 703 L CHAMPAIGN CO. 667

736 766 284 V est plane, this series of images shows the gradual infilling of the Mahomet Bedrock C Ri 730 which records a remanent magnetism with reversed polarity (Kempton and others, ver e

583 M r 698 m Valley, and may suggest the persistence of this feature throughout the section. When

M 1991). This information, along with amino-acid racemization determinations (Miller

C

V Thickness of Mahomet Sand 285 473 735 669 i

H C 716 T 22 N 697 Le Roy l Mahomet E . i displayed on the computer as an animation, this series of maps provides a new,

A o R and others, 1992) made on shells collected from a gravelly clay resting on bedrock and O L 584

M n M

E C . 0–50 ft TAZEWELL CO. 281 k A dynamic visualization of the distribution of deposits through time. Computer-based, P 82 275 I e directly below the Mahomet Sand Member close to the deepest part of the Mahomet T O 152 (G) L 472 R

N A e T C I 755 283 r i 689

I O

I LOGAN CO. 675 774 v

C 0.3 T 0.5 G C er Bedrock Valley, suggests that the age of these oldest deposits is more than 730,000 three-dimensional visualization offers a significant new opportunity for a more com- W N 50–100 ft Rantoul O T N k E A k Heyworth 282 t

C r Lincoln I . 702 734 l 724 D C e k 692 plete understanding of subsurface glaciogenic sediments, both to the geologist and to o yBP.

P O Clinton e e a O re MC LEAN CO. h r F 723 c S 300–510 . C 509 t . 100–200 ft i 20–200 (G) C o D other map users. po DE WITT CO. 681 15.9 T 21 N Atlanta a L 710 682 ig 340 0.7 r k 757 B S a ic on 505 648 MAPPING THE DEPOSITS Urbana g K g 756 679 p u 705 h o Champaign S 508 Po reek t int C r Farmer City 713 o 758 o 714 n N R L Stratigraphic Database O 205–490 i r v C h V

G e 687 e H 70 Mahomet v c E r

A .

i A DE WITT CO. 0.6 n 439 R C Over many years, an extensive ISGS collection of records from wells and borings has N O

M 712 R a M H Lake C 572 r 690 . 456

P

I C B

MACON CO. 3.5 (G) A

L P 706 T O A been used to interpret age relations and lithology for geologic mapping and ground- O 160–710

I M Clinton T I T 20 N C 759

I O

A n I e

. Monticello o G

P k 493 T

T W N k m n e T N 2.0 A water studies in cooperation with local, State, and Federal partners. A cornerstone of e a i

T

e Mount Pulaski e r E A g l

C Lincoln 485 I C I

r

n C G Clinton D a 437 C 680 C le P 571 a O REFERENCES CITED

i O N 440 our current effort was identifying a set of “key stratigraphic control points” (Kempton, ° O 487 S 688 S

40 M 488 .

Tolono .

. C n 484 385 O 771 e 1990) from the ISGS collection of subsurface data. This collection was supplemented er T 193 203 693 429 Berg, R.C., and Kempton, J.P., 1988, Stack-unit mapping of geologic materials in . e 559 Champaign 382 D 482 G 568 Urbana by data from six test holes drilled for this project, including the Gifford site shown in 694 o Illinois to a depth of 15 meters: Illinois State Geological Survey Circular 542, 23 691 o 722 708 733 s the photographs. From these control points, we built a stratigraphic database. We e 328 275 T 19 N 770 476 k p., map scale 1:250,000.

. e L C . e identified 177 such borehole records, which, if they were evenly spaced, would aver-

O O k r r

O e 686 e Berg, R.C., and Abert, C.C., 1994, Large-scale aquifer sensitivity model: C LOGAN CO. e G e C

C r k

Pesotum A age about 1.5 per township. These data served as principal control for constructing

N C DE WITT CO. 668 719 372 T

N SANGAMON E

CO. C Environmental Geology, International Journal of Geosciences, v. 24, no. 1, p.

O T

t k m r

l H o

C C F maps of each stratigraphic unit. Figures 11 and 12 show the locations of the control A a 772 MACON CO. 548 r

I S

O A A

e P 726 b t 34–42. P s v l

M

i I 764 . a M d a S Decatur 546 R Monticello A points. Only 167 control points are shown in figures 11 and 12 because 10 points

n n P r

o T e . Bernknopf, R.L., Brookshire, D.S., Soller, D.R., McKee, M.J., Sutter, J.F., Matti, J.C., r

Mount Pulaski i m 729 A . 562 a T r a O g 721

T 18 N I are located outside the map area.

F O

n G s

C ° C a r

312

40 C and Campbell, R.H., 1993, Societal value of geologic maps: U.S. Geological

N

S O e Base from ISGS data that was digitized from USGS Tolono SCALE 1:500 000 N 765 727 v

T . i 731 C R

O 191 T Survey Circular 1111, 53 p.

1:24,000- and 1:62,500-scale topographic maps O R C 378 i 10 0 10 20 MILES A

v I

.

A 298 e

762 763 534 P Hansel, A.K., and Johnson, W.M., 1996, Wedron and Mason Groups: M Universal Transverse Mercator projection 555 a r 353 533 540 728 i L k Lithostratigraphic reclassification of deposits of the Wisconsin Episode, Lake 10 0 10 20 KILOMETERS ake Fork 725 s 528 553 a 761 k Michigan Lobe area: Illinois State Geological Survey Bulletin 104, 116 p. T 17 N LOGAN CO. s 294 a Pesotum 305 66 K Herzog, B.L., Wilson, S.D., Larson, D.R., Smith, E.C., Larson, T.H., and Greenslate, SANGAMON CO. 522 529 649 647 354 672 192 720 Lake 352 M.L., 1995, Hydrogeology and groundwater availability in southwest McLean and Decatur Decatur 297 Figure 6.—Representative hydrologic properties of municipal and industrial wells developed in the Mahomet Sand southeast Tazewell counties; Part 1: Aquifer characterization: Illinois State Geological Survey and Illinois State Water Survey Cooperative Groundwater and Glasford Formation aquifers (modified from Kempton and others, 1991, fig. 23). County boundaries from ISGS and land grid from SCALE 1:500 000 Report 17, 70 p. Illinois Department of Natural Resources (IDNR); 10 0 10 20 MILES both data sets digitized from USGS 1:24,000- and Horberg, Leland, 1945, A major buried valley in east-central Illinois and its regional 1:62,500-scale topographic maps relationships: Illinois State Geological Survey Report of Investigations 106, 11p. 10 0 10 20 KILOMETERS Universal Transverse Mercator projection ———1953, deposits below the Wisconsin drift in northeastern Illinois: Illinois State Geological Survey Report of Investigations 165, 61 p. Illinois State Water Plan Task Force, 1997, The Mahomet Bedrock Valley aquifer Figure 11.—Location of 167 of the 177 key stratigraphic control points used in this study (10 occur outside the system, knowledge needs for a vital resource: University of Illinois at Urbana- map area). They are high-quality data points at verified locations, derived from the ISGS collection of subsurface Champaign, Water Resources Center Special Report 21, 49 p. information. These data are in the form of continuous core samples from test borings, drill cuttings and washed Kempton, J.P., 1990, Key stratigraphic control (Quaternary): SSC area: Illinois State “grab” samples from water wells, drillers’ logs, geophysical logs, and, rarely, outcrops. In addition to the key strati- Geological Survey Open-File Series 1990–10, 42 p. graphic control points, many more well data were used in the preparation of the maps shown in this report. Kempton, J.P., Johnson, W.H., Cartwright, K., and Heigold, P.C., 1991, Mahomet Bedrock Valley in east-central Illinois: topography, glacial drift stratigraphy, and TIME UNITS LITHOSTRATIGRAPHY PEDOSTRATIGRAPHY hydrogeology, in Melhorn, W.N., and Kempton, J.P., eds., Geology and hydro- W E geology of the Teays-Mahomet Bedrock Valley system: Geological Society of DESCRIPTION OF UNITS USGS drill rig being set up in September 1994 at the Gifford site, Champaign County. Hudson Cahokia Modern soil America Special Paper 258, p. 91–124. Formation Cahokia Episode Peoria Silt Formation Henry Formation Equality Kempton, J.P., and Visocky, A.P., 1992, Regional groundwater resources in western Yorkville Member Formation upper Cahokia Formation—Coarse- to fine-grained alluvial These high-quality data points have been described in detail by ISGS geologists, and McLean and eastern Tazewell counties with emphasis on the Mahomet Bedrock Batestown Member deposits; is the predominant fill in most of the smaller creek Lemont Formation their locations have been field verified. The information is in the form of continuous Valley: Illinois State Geological Survey and Illinois State Water Survey Cooperative Piatt Member bottoms lower Oakland facies core samples from test borings, drill cuttings and washed “grab” samples from water Groundwater Report 13, 41 p. Tiskilwa Formation Farmdale Geosol Mason and Wedron Groups—Predominantly diamictons Group Mason Delavan Member wells, geophysical logs, and, rarely, outcrops. Some drillers’ logs having detailed Larson, D.R., Kempton, J.P., and Meyer, S., 1997, Geologic, geophysical, and hydro- Episode Wedron Group Wedron Peddicord Tongue on Roxana Silt (tills) of the Wedron Group overlain and underlain by for- Wisconsin Ashmore Tongue Sangamon Geosol descriptions were used in a few areas of sparse data. Samples provide an understand- logic investigations for a supplemental municipal groundwater supply, Danville, Robein Silt Member mations, members, and tongues of the Mason Group; only Roxana Silt ing of the geology at a given site, and are commonly compared to data in nearby well Illinois: Illinois State Geological Survey and Illinois State Water Survey Berry Clay the Mason occurs in surface valley fills and beyond the Member logs to develop interpretations of the distribution of any particular geologic surface, Pearl Formation Wedron margin. In the stratigraphic column, the Wedron Cooperative Groundwater Report 18, 62 p. both areally and at depth. When integrated with geologic surfaces above and below, Lineback, J.A., 1979, Quaternary deposits of Illinois: Champaign, Ill., Illinois State Radnor Till Member is shown in lighter green and the Mason in darker green these data served as principal control while we constructed a map of each stratigraph- Geological Survey, scale 1:500,000.

upper Glasford Formation ic unit. These data also were used to develop the interpretive cross sections (sheet 3, McLean, L.R., Kelly, M.D., and Riggs, M.H., 1997, Thickness of Quaternary deposits Upper Glasford fig. 17), which were drawn by hand as the regional model of geologic history was devel- in McLean County, Illinois: Illinois State Geological Survey Open-File Series Radnor Till Member—Mainly diamicton (till) and some asso- oped. 1997–1E, map scale 1:100,000. Although only 177 boreholes constituted the key stratigraphic control, many more Pike Geosol ciated deposits; includes the Roxana Silt and Robein Silt Melhorn, W.N., and Kempton, J.P., 1991, The Teays System; A summary, in Vandalia Till Member Member (Mason Group), which, along with the Berry Clay well data were used in preparation of these maps. These secondary data were helpful Melhorn, W.N., and Kempton, J.P., eds., Geology and hydrogeology of the Teays- Smithboro Till Member Member (Glasford Formation), provide subsurface marker in refining the location of contour lines. For some stratigraphic surfaces, these sec-

lower Mahomet Bedrock Valley system: Geological Society of America Special Paper Glasford Formation Yarmouth Geosol horizon for the top of the Glasford ondary data were numerous; for example, 1355 boreholes were used in the construc- Illinois Episode 258, p. 125–128. Lierle Clay Member Basal Radnor sand and gravel tion of the bedrock-surface elevation map. Miller, B.B., Reed, Philip, Mirecki, J.E., and McCoy, W.D., 1992, molluscs from ? ? ? Tilton Till Member "lower" Vandalia pre-Illinoian sediments of the buried Mahomet Valley, central Illinois: Current ? Lower Glasford Member (Tilton?) Research in the Pleistocene, v. 9, p. 117–118. ? ? Vandalia and Smithboro Till Members— Mainly diamicton of Vandalia Till Member; the Smithboro occurs only locally Panno, S.V., Hackley, K.C., Cartwright, K., and Liu, C.L., 1994, Hydrochemistry of Hillery Till Member at the base. Includes “lower Vandalia” diamicton (till), the Mahomet Bedrock Valley Aquifer, east-central Illinois: indicators of recharge Pleistocene and ground-water flow: Ground Water, v. 32, no. 4, p. 591–604. upper

Quaternary Period which may be a separate unit or correlated with the Tilton Till Member (upper Banner). The Robein Silt Member and Soller D.R., 1993, Preliminary map showing the thickness and character of Sangamon Geosol, including the Berry Clay Member, local- Quaternary sediments in the glaciated United States east of the Rocky Mountains: ly cap the Vandalia beyond the Radnor boundary U.S. Geological Survey Open-File Report 93–543, map scale 1:3,500,000. Harmattan Till Member Basal Vandalia sand and gravel ———1998, Map showing the thickness and character of Quaternary sediments in the Unnamed Geosol glaciated United States east of the Rocky Mountains: Northern Great Lakes States Sankoty Sand Member N Banner Formation Belgium Silt Member and Central Mississippi Valley States, the Great Lakes, and southern Ontario (in Mackinaw Bedrock (lower Banner) o o middle Bedrock Valley) sand facies silt facies Upper Banner— Mainly diamictons and local sand and (80 31' to 93 West Longitude): U.S. Geological Survey Miscellaneous Hegeler Till gravel of the Tilton, Hillery, and HarmattanTill Members; Investigations Series Map I–1970–B, scale 1:1,000,000. R locally includes cap of the Lierle Clay Member and Mahomet Sand Member Member Soller, D.R., and Bernknopf, R.L., 1994, Geologic information and land use decisions: (in Mahomet Yarmouth Geosol. Thin, patchy lower Banner deposits Banner Formation Bedrock Valley) Geosol in silt facies may occur on bedrock uplands, and are included Geotimes, v. 39, no. 4, p. 12–14.

Pre–Illinois Episode Pre–Illinois Stiff, B.J., 1996, Quaternary deposits of Illinois, 1996: Champaign, Ill., Illinois State Middle and lower Banner—Mainly middle Banner sand Geological Survey GIS database “quat96”, version 1.0, scale 1:500,000. and gravel of the Mahomet and Sankoty Sand Members, Visocky, A.P., and Schicht, R.J., 1969, Ground-water resources of the buried with associated silt (lacustrine) facies. The lower Banner

lower red till occurs locally as diamictons, silt, and sand and gravel, main- Mahomet Bedrock Valley: Illinois State Water Survey Report of Investigation 62, Organic sediment ly in bedrock valleys in the northwestern part of the map 52 p. Willman, H.B., and Frye, J.C., 1970, Pleistocene stratigraphy of Illinois: Illinois State shells area Soil Sand and (or) gravel N–Normal polarity Geological Survey Bulletin 94, 204 p., map scale 1:500,000. Bedrock of Bedrock—Rocks of Pennsylvanian, Mississippian, Devonian, Silurian to Pennsylvanian age Silt and clay Mainly diamicton R–Reversed polarity and Silurian age Sediments from a geologic contact in the Gifford test hole. Diamicton (till) on left; Wilson, S.D., Kempton, J.P., and Lott, R.B., 1994, The Sankoty-Mahomet aquifer in sand and gravel (outwash) on right. the confluence area of the Mackinaw and Mahomet Bedrock Valleys, central Figure 12.—Perspective view of the map area showing distribution of 167 key stratigraphic control points relative INTERIOR—GEOLOGICAL SURVEY, RESTON, VA—1999 Illinois: A reassessment of aquifer characteristics: Illinois State Geological Survey to the bedrock topographic surface. Viewpoint is from the south. Each control point is represented by a linked, and Illinois State Water Survey Cooperative Groundwater Report 16, 64 p. vertical sequence of colored cubes. The top of each cube represents the top of a stratigraphic unit, as follows: Methodology Figure 7.—Diagrammatic stratigraphic column of glaciogenic sediments in east-central Illinois. The Cahokia green, land surface; magenta, upper Glasford Formation; pink, lower Glasford Formation; orange, Glasford Because of the thick sequence of geologic materials in the region, and the paucity of Formation was deposited mostly during the Hudson Episode. Geologic and stratigraphic names and intervals used Formation sands; brown, upper Banner Formation; yellow, middle Banner Formation; and black, bedrock. Cube exposures, subsurface information was a critical part of the geologic mapping project. 1Geologic and stratigraphic names and intervals used in this report are those accepted in this figure are those accepted by the ISGS. thickness does not imply unit thickness. To show topographic detail, the image is vertically exaggerated approxi- Subsurface information formed the basis for most geologic maps of the region and for by the Illinois State Geological Survey. mately 30x.

AUTHOR AFFILIATIONS THREE-DIMENSIONAL GEOLOGIC MAPS OF QUATERNARY SEDIMENTS IN EAST-CENTRAL ILLINOIS 1 U.S. Geological Survey, 908 National Center, Reston, VA 20192 Email: [email protected] By 2 Any use of trade, product, or firm names in this publication is for descriptive Illinois State Geological Survey (emeritus) purposes only and does not imply endorsement by the U.S. Government 3 1 1 2 3 Illinois State Geological Survey, Natural Resources Building, David R. Soller, Susan D. Price, John P. Kempton, and Richard C. Berg For sale by U.S. Geological Survey, Information Services, Box 25286, Federal 615 East Peabody Drive, Champaign, IL 61820 Center, Denver, CO 80225 Email: [email protected] 1999 Printed on recycled paper