geologic cross sections across the mahomet bedrock …

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9

GEOLOGIC CROSS SECTIONS ACROSS THE MAHOMET BEDROCK VALLEY CHAMPAIGN, FORD, MCLEAN, PIATT, AND VERMILION COUNTIES, ILLINOIS

Andrew J. Stumpf and Lisa A. Atkinson 2015

Prairie Research InstituteIllinois State Geological Survey 615 East Peabody Drive Champaign, Illinois 61820-6918 (217) 244-2414http://www.isgs.illinois.edu

© 2015 University of Illinois Board of Trustees. All rights reserved. For permission information, contact the Illinois State Geological Survey.

Location map base data from Champaign County GIS, 2013.

Digital cartography by Jennifer E. Carrell, Andrew J. Stumpf, and Deette Lund, Illinois State Geological Survey.

This research was supported in part by funding provided by Illinois Ameri-can Water (award number IL Amer Water 2007-02899) and the Illinois Department of Natural Resources as part of the Water Supply Planning for Illinois program (award number IDNR OWRWS1).

The Illinois State Geological Survey and the University of Illinois make no guarantee, expressed or implied, regarding the correctness of the inter-pretations presented in this document and accept no liability for the con-sequences of decisions made by others on the basis of the information presented here. The geologic interpretations are based on data that may vary with respect to the accuracy of the geographic location, the type and quantity of data available at each location, and the scientific and technical qualifications of the data sources. The cross sections in this document are not meant to be enlarged.

Recommended citation: Stumpf, A.J., and L.A. Atkinson, 2015, Geologic cross sections across the

Mahomet Bedrock Valley, Champaign, Ford, McLean, Piatt, and Ver-milion Counties, Illinois: Illinois State Geological Survey, Illinois Map 19, 1:48,000.

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26423(CHAM-08-05)

26484(LW Well 73)

26413(CHAM-08-02A)

26428(CHAM-08-07A)

26429(CHAM-08-09A)

26536(CHAM-09-03A)

21340(FORD-08-01A)

21539(PIAT-07-02A)

21589(PIAT-09-01A)

24066(CG-3-92)

24814(CHM-95C)

26306(CHAM-07-01A)24522

00644

20580

24648

23342

25348

20585

23220

Cross section line

Boundary of theMahomet Bedrock Valley[from Stumpf and Dey (2012)]

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IntroductionThis map product was developed as part of a study by the Illinois State Geological Survey (ISGS) to improve our understanding of the hydro-geology and geology of the Mahomet aquifer in east-central Illinois (Stumpf and Dey 2012), with a specific focus on Champaign County and adjacent counties. The geologic cross sections portray the uncon-solidated sediments and bedrock in two dimensions, in such a way that they can be viewed as a vertical slice through the earth’s surface down to bedrock. To represent the stratigraphic and lithologic relationships in the subsurface at the scale shown, the vertical scale was exaggerated 20 times. Also, to make it easier to cross reference information shown on the profiles, the cross sections are arranged so that the deepest channel of the Mahomet Bedrock Valley (MBV) is aligned with the center of the page. The cross sections show the distribution and thickness of geologic materials and how they relate to one another. Bold vertical lines on the cross section locate boreholes where information about the surface and subsurface geology was collected. The boreholes were drilled for several different purposes, including geologic mapping, engineering testing, construction of water wells, and oil, gas, and coal exploration.

The cross sections were drawn to document the geologic diversity in the MBV and adjacent areas. Each cross section depicts sequences of depos-its that resulted from a complex and varied history of sediment erosion and deposition during the Quaternary Period. The age and location of such events significantly influenced the preservation and modification of the glacial and postglacial deposits and topography of the bedrock surface.

The geologic mapping units portrayed on the cross sections and de-scribed in the accompanying legend were defined in a revised geologi-cal framework for the region developed by Stumpf and Dey (2012) and Atkinson et al. (2014). Several new informal mapping units were introduced for deposits encountered in the region. The revisions were necessary to classify deposits of glacial and nonglacial sediments (1) not encountered or identified in the past; (2) not differentiated in the previ-ous classification systems of Willman and Frye (1970), Willman et al. (1975), and Kempton et al. (1991); or (3) reinterpreted and correlated with other lithostratigraphic units. The cross sections were primarily drawn along lines connecting a series of boreholes. However, in some places where subsurface information was sparse or not available, the geological and geophysical information from boreholes adjacent to the lines was used to construct the profiles. In addition, where data are scarce or not available, unit boundaries are primarily drawn with dashed lines.

MethodsA customized tool for the ESRI ArcMap software programmed by the ISGS (Carrell 2015) was used to generate georeferenced profiles for the mapping units. For mapping units h-a, g-v3, pl-g1, pl-g2, b-m2, and |, grids of the unit’s upper surfaces in a 3-D geologic model were imported

into ArcMap. For additional graphical editing, the profile lines were imported to Adobe Illustrator via the MAPublisher software of Avenza Systems Incorporated (version 8.4). The final cross sections were ex-ported from Adobe Illustrator as ESRI shapefiles so they could be further analyzed using ArcMap software.

AcknowledgmentsFieldwork necessary to collect additional subsurface data was undertak-en between 2007 and 2009 by A.J. Stumpf, W.S. Dey, T.C. Young, and T.O. Hodson. Some borehole logs of natural gamma radiation shown are from Burch (2008).

We appreciate the support provided by the many public organizations, private companies, municipalities, and land owners that was critical for developing this product. We also thank the staff at the ISGS for their able assistance. Staff provided cartographic and GIS support, data entry and database development, assistance drilling boreholes to collect continu-ous core or samples, and assistance conducting borehole geophysical surveys. The mapping was supported in part by funding from Illinois American Water, the Illinois Department of Natural Resources through the Water Supply Planning for Illinois program, and State of Illinois General Revenue Funds.

ReferencesAtkinson, L.A., M. Ross, and A.J. Stumpf, 2014, Three-dimensional

hydrofacies assemblages in ice-contact/proximal sediments forming a heterogeneous ‘hybrid’ hydrostratigraphic unit in central Illinois, USA: Hydrogeology Journal, v. 22, p. 1605–1624. http://dx.doi.org/10.1007/s10040-014-1156-7.

Bleuer, N.K., 1975, Remnant magnetism of Pleistocene sediments of In-diana: Indiana Academy of Science Proceedings, v. 85, p. 277–294. https://journals.iupui.edu/index.php/ias/article/view/8266/8417.

Burch, S.L., 2008, Development of an observation well network in the Mahomet aquifer of east-central Illinois: Illinois State Water Survey, Data/Case Study 2008-01, 111 p. http://hdl.handle.net/2142/8856.

Carrell, J.E., 2015, Create 2D and 3D geologic cross sections: Illinois State Geological Survey [includes user guide, tutorial data, and computer codes]. http://www.arcgis.com/home/item.html?id=54584a5e302e4014a495b8fc37fe0663.

Grimley, D.A and N.D. Webb, 2010, Surficial geology of Red Bud Quad-rangle, Randolph, Monroe, and St. Clair Counties, Illinois: Illinois State Geological Survey, Illinois Geologic Quadrangle Map, IGQ Red Bud-SG, 1:24,000. http://hdl.handle.net/2142/43399.

Hansel, A.K., and W.H. Johnson, 1996, Wedron and Mason Groups: Lithostratigraphic reclassification of deposits of the Wisconsin Episode, Lake Michigan Lobe area: Illinois State Geological Survey, Bulletin 104, 116 p. http://hdl.handle.net/2142/43938.

Hansel, A.K., and E.D. McKay, III, 2010, Quaternary Period, in D.R. Kolata and C.K. Nimz, eds., Geology of Illinois: Illinois State Geo-logical Survey, p. 216–247.

Herzog, B.L., B.J. Stiff, C.A. Chenoweth, K.L. Warner, J.B. Sieverling, and C. Avery, 1994, Buried bedrock surface of Illinois, 3rd ed.: Illi-nois State Geological Survey, Illinois Map 5, 1:500,000. http://www.isgs.uiuc.edu/nsdihome/browse/statewide/zips/IL_Bedrock_Topog-raphy_1994_Ln.zip.

Kempton, J.P., W.H. Johnson, P.C. Heigold, and K. Cartwright, 1991, Mahomet Bedrock Valley in east-central Illinois: Topography, gla-cial drift stratigraphy, and hydrogeology, in W.H. Melhorn and J.P. Kempton, eds., Geology and hydrogeology of the Teays-Mahomet Bedrock Valley system: Geological Society of America, Special Paper 258, p. 91–124. http://dx.doi.org/10.1130/SPE258-p91.

Phillips, A.C., 2004, Surficial geology of Collinsville Quadrangle, Madison and St. Clair Counties, Illinois: Illinois State Geological Survey, Illinois Preliminary Geologic Map, IPGM Collinsville-SG, 1:24,000. http://hdl.handle.net/2142/77818.

Reimer, P.J., E. Bard, A. Bayliss, J.W. Beck, P.G. Blackwell, R.C. Bronk, C.E. Buck, H. Cheng, R.L. Edwards, M. Friedrich, et al., 2013, IntCal13 and Marine13 radiocarbon age calibration curves 0–50,000 years cal BP: Radiocarbon, v. 55, no. 4, 1869–1887. http://dx.doi.org/10.2458/azu_js_rc.55.16947.

Soller, D.R., S.D. Price, J.P. Kempton, and R.C. Berg, 1999, Three-dimensional geologic maps of Quaternary sediments in east-central Illinois: U.S. Geological Survey, Geologic Investigations Series Map I-2669, 3 sheets. http://pubs.usgs.gov/imap/i-2669.

Stuiver, M., P.J. Reimer, and R.W. Reimer, 2015, CALIB radiocarbon calibration, version 7.1. http://calib.qub.ac.uk/calib/.

Stumpf, A.J., and W.S. Dey, eds., 2012, Understanding the Mahomet aquifer: Geological, geophysical, and hydrogeological studies in Champaign County and adjacent areas: Illinois State Geological Survey, Draft Contract Report No. IL Amer Water 2007-02899.

Willman, H.B., and J.C. Frye, 1970, Pleistocene stratigraphy of Illinois: Illinois State Geological Survey, Bulletin 94, 204 p. http://hdl.handle.net/2142/43629.

Willman, H.B., E. Atherton, T.C. Buschbach, C. Collinson, J.C. Frye, M.E. Hopkins, J.A. Lineback, and J.A. Simon, 1975, Handbook of Illinois stratigraphy: Illinois State Geological Survey, Bulletin 95, 261 p. http://hdl.handle.net/2142/35115.

Figure 1 Location of geologic cross sections in Champaign County and adjacent areas. The cross section lines lie over a shaded relief map of the bedrock surface. This map was compiled from bedrock topography maps published by Stumpf and Dey (2012), Soller et al. (1999), and Herzog et al. (1994).

* The procedure for applying the use of episodes is after Hansel and Johnson (1996) and Hansel andMcKay (2010). Time periods are not to scale. The till units were deposited during separate ice advances,and their duration is not represented by the size of the boxes.

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GLACIO-LACUSTRINE

DEPOSITS BEDROCKGLACIAL DEPOSITSICE-CONTACT

DEPOSITSGLACIOFLUVIAL

DEPOSITS

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CORRELATION OF MAPPING UNITS

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QUATERNARY DEPOSITS

Unit

HUDSON EPISODE [~14,600 years before present (B.P.) to today]2

Cahokia Formation

HUDSON AND WISCONSIN EPISODES (~23,000 years B.P. to today)

Equality Formation

WISCONSIN EPISODE (~23,000–14,600 years B.P.)

Henry Formation

Yorkville Member, Lemont Formation

Batestown Member, Lemont Formation

Tiskilwa Formation

Ashmore Tongue,Henry Formation

ILLINOIS EPISODE (~200,000–130,000 years B.P.)

Pearl Formation

Vandalia Member,Glasford Formation

upper unit3

Vandalia Member, Glasford Formation

middle unit3


lower unit3

Bellflower tongue,Teneriffe Silt4

Pearl Formation4, 5

Grigg tongue 2

Grigg tongue 1

Petersburg Silt

PRE-ILLINOIS EPISODE (>1,000,000–430,000 years B.P)6

Tilton Member,Banner Formation

Hillery Member,Banner Formation

Pesotum silt,Banner Formation4

Harmattan Member, Banner Formation

Belgium Member,Banner Formation

Fisher member,Banner Formation4

Mahomet Sand Member,Banner Formation3

upper unit

West Lebanon Member7

Banner Formation


lower unit

PRE-ILLINOIS EPISODE AND OLDER

Canteen member,Banner Formation3, 8

Dewitt facies

Canteen member,Banner Formation8

Pennsylvanian, Mississippian, Devonian,

Silurian bedrock

1 The materials mapped at the land surface may be overlain by 1 to 5 feet of wind-deposited silt (loess).2 The time periods for the Wisconsin and Hudson Episodes are reported in calibrated radiocarbon years before present (where “present” is considered to be 1950). Radiocarbon ages were calibrated with the online program Calib 7.1 (Stuiver et al. 2015) using the IntCal13 correction curve (Reimer et al. 2013).3 Materials are assigned to subunits or facies to account for their stratigraphic position, changes in lithology, or mode of deposition.4 New (informal) lithostratigraphic unit introduced by Stumpf and Dey (2012).5 Correlative to glacial meltwater deposits (outwash) mapped in southwest Illinois by Grimley and Webb (2010).6 The lower subdivision of the pre-Illinois Episode is based on dates from terrestrial cosmogenic nuclide analyses (TCN) conducted on quartz sand grains sampled in the lower unit of the Mahomet Sand Member and Dewitt facies of the Canteen member in Piatt County (unpublished data).7 The West Lebanon Member till was previously mapped only in west-central Indiana by Bleuer (1975).8 Correlative to preglacial deposits mapped in southwest Illinois by Phillips (2004).

Interpretation1

Alluvium (stream deposits); mapped in floodplains along creeks and drainageways and in fan-shaped deposits where streams emerge from the moraines onto lower gradient slopes

Proglacial and postglacial lake deposits; fills depressions or low-lying areas

Glaciofluvial sediment (outwash); deposited by glacial meltwater in streams and rivers that flowed from glaciers that deposited the Yorkville, Batestown, and Tiskilwa tills

Till and associated sediment; encountered only northeast of a line running through Gibson City and Gifford

Till and associated sediment; in the subsur-face, directly underlies the Yorkville Member

Till and associated sediment; in the subsurface directly underlies the Batestown Member

Glaciofluvial sediment (outwash); deposited by glacial meltwater streams and rivers that flowed from Tiskilwa ice; not consistently differentiable from underlying deposits of sand and gravel correlated with the Pearl Formation

Glaciofluvial sediment (outwash); deposited by glacial meltwater in streams and rivers that flowed from Vandalia ice; inset into the Vandalia Member (upper unit) of the Glasford Formation; contains the Sangamon Geosol in the upper part, except where eroded

Proglacial or ice-contact sediment; deposited by glacial meltwater or sediment gravity flows (debris flows) on or in front of Vandalia ice; contains the Sangamon Geosol in the upper part, except where eroded

Subglacial or ice-contact sediments; derived directly from glacial ice or deposited by glacial meltwater; deposition is interpreted to have occurred within an area of fast-flowing ice, possibly an ice stream, and is associated with the deglacial phase of the Illinois Episode

Till and associated sediment; overlain by deposits that accumulated on, in front of, or beneath Vandalia ice; nearly continuous deposit

Glaciolacustrine sediment; deposited in front of Vandalia ice; fills depressions or channels

Fluvial and glaciofluvial sediment; deposited in front of Vandalia ice and may include sediments deposited by outflows from lakes ponded behind the glaciers; over the Mahomet Bedrock Valley (MBV) not consistently differentiable from the underlying Mahomet Sand Member when intervening tills or lake sediment of the Banner Formation are absent; includes deposits of sand and gravel (pl-g1) mapped over and outside the MBV, deposited before unit pl-g2

Proglacial sediment; deposited in slackwater lakes or ice-marginal lakes

Till and associated sediment; may contain Yarmouth Geosol weathering profile in upper 10 feet (typically truncated); unit not well preserved.

Till and associated sediment; may contain Yarmouth Geosol weathering profile in upper 10 feet (typically truncated)

Deposits of a large proglacial lake; this lake that inundated part of the MBV and tributary valleys; associated with glaciers that deposited the Hillery Member till

Till and associated sediment; encountered most often in tributary bedrock valleys of the MBV

Glaciolacustrine sediment; deposited primarily in the MBV and tributary valleys; associated with the Harmattan ice

Fluvial or lacustrine sediment; deposited on a former interglacial floodplain of a river flowing in the MBV; the land surface was poorly drained and occasionally covered by overbank deposits, slope wash, or debris flows

Glaciofluvial sediment (outwash); deposited in the MBV by glacial meltwater flowing from West Lebanon ice during the retreat phase; glaciers were located northeast of the area

Till and associated sediment; deposited by glaciers flowing into the area from a northeastern or eastern ice source

Glaciofluvial sediment (outwash); deposited in the MBV by glacial meltwater flowing from West Lebanon ice during the advance phase; glaciers were located northeast of the area

Fluvial sediment; deposited in the MBV prior to the earliest pre-Illinois Episode glaciation

Lacustrine sediment, slope wash, or weath-ered bedrock; formed or accumulated in the MBV and tributary valleys during preglacial times

Bedrock; includes strata of both marine and terrestrial origin; the upper surface of bedrock is undulating or irregular (including paleovalleys and paleouplands) shaped by multiple cycles of erosion; rocks Silurian in age and younger are warped into structural highs, expressed on the bedrock surface along the La Salle Anticlinorium, a significant structural feature of Illinois

Description

Sand, silt, clay, and gravel; massive to strati-fied; locally oxidized; poorly sorted; contains beds of organic material; up to 15 feet thick

Silt and clay; stratified to massive; grayish brown; calcareous; may contain beds of diamic-ton, sand, or gravel; 5 to 20 feet thick

Sand and gravel; contains some beds of silt and pebbles; brown to yellowish brown; calcareous; well to poorly sorted; up to 25 feet thick

Diamicton; silt loam to silty clay; gray to brown; calcareous; contains beds of sand and gravel; 10 to 60 feet thick

Diamicton; sandy loam to silt loam; gray to grayish brown; calcareous; contains beds of sand, silt, and gravel; 25 to 75 feet thick

Diamicton; loam; grayish brown to reddish gray; calcareous; very stiff; 10 to 130 feet thick

Sand and gravel with silt; contains pebbles and cobbles; brown to grayish brown; calcareous; well to poorly sorted; 5 to 75 feet thick

Fine to coarse sand with gravel; yellowish brown to grayish brown; calcite-cemented in places; upper part may contain weathered sand and gravel with silt and clay that is part of the Sangamon Geosol; 10 to 150 feet thick

Diamicton, sand and gravel, and silt and clay; interstratified; includes sediments assigned to the Berry Clay, Hagarstown, Radnor, and Toulon Members, or Roby Silt; upper part contains weathered silty to clayey materials that are part of the Sangamon Geosol; 15 to 200 feet thick

Diamicton, with layers of sand and gravel and silt and clay; diamicton is less compacted than unit g-v1; includes sediments assigned to the Berry Clay, Radnor Member, Toulon Member, or Roby Silt; 25 to 175 feet thick

Diamicton; silt loam to loam; grayish brown; calcareous; contains beds of sand, silt, and gravel; hard; 5 to 100 feet thick

Silt, sand, and clay; stratified; gray to brown; calcareous; may contain beds of diamicton; in some places organic rich; 5 to 20 feet thick

Sand and gravel; pebbly; grayish brown; contains some beds of silt or diamicton; calcare-ous; well to moderately well sorted; 25 to 125 feet thick

Silt, sand, clay, and diamicton; stratified to massive; gray to brown; calcareous; 20 to 60 feet thick

Diamicton; silt loam to loam; grayish brown; calcareous; hard; upper part may contain weathered material that is part of the Yarmouth Geosol; 5 to 15 feet thick

Diamicton; loam; reddish brown to grayish brown; calcareous; contains beds of sand, silt, or gravel; hard; upper part contains weatheredmaterial that are part of the Yarmouth Geosol; 5 to 35 feet thick

Silt, clay, and fine sand; bedded or massive; reddish brown; calcareous; clayey at the bottom of the unit; may contain beds of diamicton; 5 to 80 feet thick

Diamicton; loam to silt loam; grayish brown to light olive brown; calcareous; contains beds of sand, silt, or gravel; contains numerous clasts of the local bedrock; hard; 5 to 100 feet thick

Silt, sand, and clay; bedded to massive; olive brown to gray; calcareous; 10 to 35 feet thick

Sand, diamicton, and silt; sandy loam to silty clay loam; very dark gray to greenish gray; weakly calcareous to leached; may contain humus, peat, wood, and/or fossil snails; hard; these materials were previously assigned to Lierle Clay unit (e.g., Soller et al. 1999); 5 to 40 feet thick

Sand and gravel; brown to grayish brown; contains some beds of silt; calcareous; well to moderately well sorted; 20 to 150 feet thick

Diamicton; sandy loam to clay loam; brown to pinkish gray; calcareous; includes basal fine and coarse facies of sand and gravel or silt and clay; hard; 5 to 90 feet thick

Sand and gravel; pebbly to cobbly; brown; locally contains beds of silt or diamicton; calcare-ous; well to moderately well sorted; 30 to 150 feet thick

Sand with gravel; brown to gray; calcareous or leached; contains a higher proportion of fragments of the local bedrock; encountered in the deepest channel of the MBV; often contains mussel and gastropod shells; 5 to 20 feet thick

Silt, clay, and diamicton; crudely stratified; olive brown to brownish gray; calcareous or leached; contains a higher proportion of fragments of the local bedrock; 10 to 60 feet thick

Shale, siltstone, limestone, sandstone, and dolomite; Pennsylvanian rocks include relatively thin, but continuous units of of black shale, limestone, shale, siltstone, sandstone, coal, and claystone that may be soft, fissile, and fractured; rocks locally contain siderite nodules, plant and marine fossils, and paleosol features

Cross Sections

Sand and gravel

Mainly sand; may contain some gravel or silt

Laminated silt and clay

Mixture of loam, sand and gravel, diamicton, and/orsilty clay

Diamicton, massive silt, or other fine-grained sediment

Contact

Inferred contact

CPS = counts per second

Horizontal scale: 1:48,000 (1 inch = 4,000 feet)Vertical scale: 1 inch = 200 feetVertical exaggeration: 20�

Note: Boring labels indicate the county identification number and field identification number (in parenthe-ses). The county number is a portion of the 12-digit API number on file at the ISGS Geological Records Unit that references records in the ISGS Institutional Database. Most reports for the boreholes are available from the ISGS website (https://www.isgs.illinois.edu).

(CHAM-08-09A)

26429

county identificationnumber

project site identificationnumber

0 150CPS

entire depth shown only upper part shown

Log of natural gamma radiationin borehole

Borehole Information

Sand, diamicton, and silt; organic-rich

c

h

t

l-b

l-y

h-a

e

tr-b

pb

g-v1

g-v3

pl

g-v2

pl-g1

pl-g2

b-f

b-c(d)

b-c

b-tl

b-hl

b-hm

b-m2

b-m1

b-wl

b-ps

b-b

|

QUATERNARY DEPOSITS

Unit

HUDSON EPISODE [~14,600 years before present (B.P.) to today]2

Cahokia Formation

HUDSON AND WISCONSIN EPISODES (~23,000 years B.P. to today)

Equality Formation

WISCONSIN EPISODE (~23,000–14,600 years B.P.)

Henry Formation

Yorkville Member, Lemont Formation

Batestown Member, Lemont Formation

Tiskilwa Formation

Ashmore Tongue,Henry Formation

ILLINOIS EPISODE (~200,000–130,000 years B.P.)

Pearl Formation


upper unit3

Vandalia Member, Glasford Formation

middle unit3


lower unit3

Bellflower tongue,Teneriffe Silt4

Pearl Formation4, 5

Grigg tongue 2

Grigg tongue 1

Petersburg Silt

PRE-ILLINOIS EPISODE (>1,000,000–430,000 years B.P)6

Tilton Member,Banner Formation

Hillery Member,Banner Formation

Pesotum silt,Banner Formation4

Harmattan Member, Banner Formation

Belgium Member,Banner Formation

Fisher member,Banner Formation4


upper unit

West Lebanon Member7

Banner Formation


lower unit

PRE-ILLINOIS EPISODE AND OLDER

Canteen member,Banner Formation3, 8

Dewitt facies

Canteen member,Banner Formation8

Pennsylvanian, Mississippian, Devonian,

Silurian bedrock

1 The materials mapped at the land surface may be overlain by 1 to 5 feet of wind-deposited silt (loess).2 The time periods for the Wisconsin and Hudson Episodes are reported in calibrated radiocarbon years before present (where “present” is considered to be 1950). Radiocarbon ages were calibrated with the online program Calib 7.1 (Stuiver et al. 2015) using the IntCal13 correction curve (Reimer et al. 2013).3 Materials are assigned to subunits or facies to account for their stratigraphic position, changes in lithology, or mode of deposition.4 New (informal) lithostratigraphic unit introduced by Stumpf and Dey (2012).5 Correlative to glacial meltwater deposits (outwash) mapped in southwest Illinois by Grimley and Webb (2010).6 The lower subdivision of the pre-Illinois Episode is based on dates from terrestrial cosmogenic nuclide analyses (TCN) conducted on quartz sand grains sampled in the lower unit of the Mahomet Sand Member and Dewitt facies of the Canteen member in Piatt County (unpublished data).7 The West Lebanon Member till was previously mapped only in west-central Indiana by Bleuer (1975).8 Correlative to preglacial deposits mapped in southwest Illinois by Phillips (2004).

Interpretation1

Alluvium (stream deposits); mapped in floodplains along creeks and drainageways and in fan-shaped deposits where streams emerge from the moraines onto lower gradient slopes

Proglacial and postglacial lake deposits; fills depressions or low-lying areas

Glaciofluvial sediment (outwash); deposited by glacial meltwater in streams and rivers that flowed from glaciers that deposited the Yorkville, Batestown, and Tiskilwa tills

Till and associated sediment; encountered only northeast of a line running through Gibson City and Gifford

Till and associated sediment; in the subsur-face, directly underlies the Yorkville Member

Till and associated sediment; in the subsurface directly underlies the Batestown Member

Glaciofluvial sediment (outwash); deposited by glacial meltwater streams and rivers that flowed from Tiskilwa ice; not consistently differentiable from underlying deposits of sand and gravel correlated with the Pearl Formation

Glaciofluvial sediment (outwash); deposited by glacial meltwater in streams and rivers that flowed from Vandalia ice; inset into the Vandalia Member (upper unit) of the Glasford Formation; contains the Sangamon Geosol in the upper part, except where eroded

Proglacial or ice-contact sediment; deposited by glacial meltwater or sediment gravity flows (debris flows) on or in front of Vandalia ice; contains the Sangamon Geosol in the upper part, except where eroded

Subglacial or ice-contact sediments; derived directly from glacial ice or deposited by glacial meltwater; deposition is interpreted to have occurred within an area of fast-flowing ice, possibly an ice stream, and is associated with the deglacial phase of the Illinois Episode

Till and associated sediment; overlain by deposits that accumulated on, in front of, or beneath Vandalia ice; nearly continuous deposit

Glaciolacustrine sediment; deposited in front of Vandalia ice; fills depressions or channels

Fluvial and glaciofluvial sediment; deposited in front of Vandalia ice and may include sediments deposited by outflows from lakes ponded behind the glaciers; over the Mahomet Bedrock Valley (MBV) not consistently differentiable from the underlying Mahomet Sand Member when intervening tills or lake sediment of the Banner Formation are absent; includes deposits of sand and gravel (pl-g1) mapped over and outside the MBV, deposited before unit pl-g2

Proglacial sediment; deposited in slackwater lakes or ice-marginal lakes

Till and associated sediment; may contain Yarmouth Geosol weathering profile in upper 10 feet (typically truncated); unit not well preserved.

Till and associated sediment; may contain Yarmouth Geosol weathering profile in upper 10 feet (typically truncated)

Deposits of a large proglacial lake; this lake that inundated part of the MBV and tributary valleys; associated with glaciers that deposited the Hillery Member till

Till and associated sediment; encountered most often in tributary bedrock valleys of the MBV

Glaciolacustrine sediment; deposited primarily in the MBV and tributary valleys; associated with the Harmattan ice

Fluvial or lacustrine sediment; deposited on a former interglacial floodplain of a river flowing in the MBV; the land surface was poorly drained and occasionally covered by overbank deposits, slope wash, or debris flows

Glaciofluvial sediment (outwash); deposited in the MBV by glacial meltwater flowing from West Lebanon ice during the retreat phase; glaciers were located northeast of the area

Till and associated sediment; deposited by glaciers flowing into the area from a northeastern or eastern ice source

Glaciofluvial sediment (outwash); deposited in the MBV by glacial meltwater flowing from West Lebanon ice during the advance phase; glaciers were located northeast of the area

Fluvial sediment; deposited in the MBV prior to the earliest pre-Illinois Episode glaciation

Lacustrine sediment, slope wash, or weath-ered bedrock; formed or accumulated in the MBV and tributary valleys during preglacial times

Bedrock; includes strata of both marine and terrestrial origin; the upper surface of bedrock is undulating or irregular (including paleovalleys and paleouplands) shaped by multiple cycles of erosion; rocks Silurian in age and younger are warped into structural highs, expressed on the bedrock surface along the La Salle Anticlinorium, a significant structural feature of Illinois

Description

Sand, silt, clay, and gravel; massive to strati-fied; locally oxidized; poorly sorted; contains beds of organic material; up to 15 feet thick

Silt and clay; stratified to massive; grayish brown; calcareous; may contain beds of diamic-ton, sand, or gravel; 5 to 20 feet thick

Sand and gravel; contains some beds of silt and pebbles; brown to yellowish brown; calcareous; well to poorly sorted; up to 25 feet thick

Diamicton; silt loam to silty clay; gray to brown; calcareous; contains beds of sand and gravel; 10 to 60 feet thick

Diamicton; sandy loam to silt loam; gray to grayish brown; calcareous; contains beds of sand, silt, and gravel; 25 to 75 feet thick

Diamicton; loam; grayish brown to reddish gray; calcareous; very stiff; 10 to 130 feet thick

Sand and gravel with silt; contains pebbles and cobbles; brown to grayish brown; calcareous; well to poorly sorted; 5 to 75 feet thick

Fine to coarse sand with gravel; yellowish brown to grayish brown; calcite-cemented in places; upper part may contain weathered sand and gravel with silt and clay that is part of the Sangamon Geosol; 10 to 150 feet thick

Diamicton, sand and gravel, and silt and clay; interstratified; includes sediments assigned to the Berry Clay, Hagarstown, Radnor, and Toulon Members, or Roby Silt; upper part contains weathered silty to clayey materials that are part of the Sangamon Geosol; 15 to 200 feet thick

Diamicton, with layers of sand and gravel and silt and clay; diamicton is less compacted than unit g-v1; includes sediments assigned to the Berry Clay, Radnor Member, Toulon Member, or Roby Silt; 25 to 175 feet thick

Diamicton; silt loam to loam; grayish brown; calcareous; contains beds of sand, silt, and gravel; hard; 5 to 100 feet thick

Silt, sand, and clay; stratified; gray to brown; calcareous; may contain beds of diamicton; in some places organic rich; 5 to 20 feet thick

Sand and gravel; pebbly; grayish brown; contains some beds of silt or diamicton; calcare-ous; well to moderately well sorted; 25 to 125 feet thick

Silt, sand, clay, and diamicton; stratified to massive; gray to brown; calcareous; 20 to 60 feet thick

Diamicton; silt loam to loam; grayish brown; calcareous; hard; upper part may contain weathered material that is part of the Yarmouth Geosol; 5 to 15 feet thick

Diamicton; loam; reddish brown to grayish brown; calcareous; contains beds of sand, silt, or gravel; hard; upper part contains weatheredmaterial that are part of the Yarmouth Geosol; 5 to 35 feet thick

Silt, clay, and fine sand; bedded or massive; reddish brown; calcareous; clayey at the bottom of the unit; may contain beds of diamicton; 5 to 80 feet thick

Diamicton; loam to silt loam; grayish brown to light olive brown; calcareous; contains beds of sand, silt, or gravel; contains numerous clasts of the local bedrock; hard; 5 to 100 feet thick

Silt, sand, and clay; bedded to massive; olive brown to gray; calcareous; 10 to 35 feet thick

Sand, diamicton, and silt; sandy loam to silty clay loam; very dark gray to greenish gray; weakly calcareous to leached; may contain humus, peat, wood, and/or fossil snails; hard; these materials were previously assigned to Lierle Clay unit (e.g., Soller et al. 1999); 5 to 40 feet thick

Sand and gravel; brown to grayish brown; contains some beds of silt; calcareous; well to moderately well sorted; 20 to 150 feet thick

Diamicton; sandy loam to clay loam; brown to pinkish gray; calcareous; includes basal fine and coarse facies of sand and gravel or silt and clay; hard; 5 to 90 feet thick

Sand and gravel; pebbly to cobbly; brown; locally contains beds of silt or diamicton; calcare-ous; well to moderately well sorted; 30 to 150 feet thick

Sand with gravel; brown to gray; calcareous or leached; contains a higher proportion of fragments of the local bedrock; encountered in the deepest channel of the MBV; often contains mussel and gastropod shells; 5 to 20 feet thick

Silt, clay, and diamicton; crudely stratified; olive brown to brownish gray; calcareous or leached; contains a higher proportion of fragments of the local bedrock; 10 to 60 feet thick

Shale, siltstone, limestone, sandstone, and dolomite; Pennsylvanian rocks include relatively thin, but continuous units of of black shale, limestone, shale, siltstone, sandstone, coal, and claystone that may be soft, fissile, and fractured; rocks locally contain siderite nodules, plant and marine fossils, and paleosol features

ILLINOIS STATE GEOLOGICAL SURVEYPrairie Research InstituteUniversity of Illinois at Urbana-Champaign

Illinois Map 192015

geologic cross sections across the mahomet bedrock …

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