1. GREGORICH, Holly G., MALONE, David H., and SHIELDS, William E.
Surficial Geologic Map of the Pontiac Northwest 7.5 Minute Quadrangle in Livingston County, Illinois
GREGORICH, Holly G., MALONE, David H., and SHIELDS, William E.
Department of Geography-Geology, Illinois State University, Campus Box 4400, Normal, IL 61761
Results and Discussion
Surficial geology of the Livingston quadrangle consists mainly of Cahokia Formation, Equality Formation, Henry Formation, and Lemont Formation; all of which are Quaternary in age. Older Paleozoic deposits Pennsylvanian in age are exposed just north of Pontiac. The Cahokia Formation is Holocene in age and consists of bedded alluvial deposits ranging in grain size from clay to gravel in floodplains and river channels. The Equality Formation consists of yellowish brown to light brownish silt and clay with lenses of gravel, sand, diamicton, organic debris and wood. Equality Formation is interpreted as slack water or lacustrine environments of ponded meltwater between the Marseilles and Minonk Moraines (i.e. “Glacial Lake Pontiac”). The Henry Formation is represented by the Mackinaw facies in the mapping area and consists of stratified sand and gravel with lenses of silt, clay, organic debris, wood and shells. Mackinaw deposits are interpreted as valley train outwash forming terraces. The Batavia facies also occurs as alluvial fans long the margin of the moraines and is deposited proglacially by meltwater from distant glaciers. The Lemont Formation (Wedron Group) consists of calcareous gray diamiction with lenses of gravel, sand, silt, and clay and is interpreted as till deposits from the retreating glaciers during the Wisconsinan glacial episode. The Lemont Formation exists on and to the north of the Normal moraine.
The Pennsylvanian Undivided consists of sediment deposited in fluvial, deltaic, and shallow marine depositional systems. Limestones formed as shoals in shallow marine environments and mudstones are floodplain, delta, and prodelta deposits. Sandstones formed in fluvial or delta distributary channels.
Abstract
The Pontiac Northwest 7.5 Minute Quadrangle (Scale 1:24,000) is located in Livingston County, Illinois. This area reveals an interesting assemblage of Quaternary glacial deposits and Pennsylvanian strata that are exposed along the Vermilion River and its tributaries. Methods used in the preparation of this surficial geologic map were the analysis and conversion of soil survey data, the analysis of LiDAR data and the interpretation of well water logs, and traditional field observations. The surficial geology of the Quadrangle is comprised of Quaternary glacial sediment units that were deposited during the Wisconsin episode. Quaternary units include the Equality Formation, the Henry Formation, the Lemont Formation, and the Cahokia Alluvium. The Henry Formation and the Lemont Formation make up the majority of the surficial geology of the Quadrangle. The Lemont Formation (Wedron Group), is a gray, silty clay diamicton, which was deposited as a ground moraine locally. The Equality formation is a yellow-brown sandy silt that was deposited as loess. The Henry Formation, a sand and gravel unit of the Mason Group, is either an outwash plain or valley train deposit. The Cahokia Alluvium contains poorly sorted sand, silt and clay sized grains and can be found in abundance along the Panther Creek, a main tributary to the Vermilion River. The Pennsylvanian strata of the Bond Formation, which occur in quarries and along some of the lower order streams, consists of gray shale, siltstone, coal, limestone and sandstone.
Figure 1: Quaternary geologic map of Illinois are. Extent of the Wisconsin Episode shown in green. Livingston County is outlined in red.
Figure 2: Unit groupings created from soils data before edits (colors different than final map).
Figure 4: Quaternary Geologic Map of Livingston County. From Stiff (2000).
Methodology
Data was collected from the Illinois State Geological Survey (ISGS) upon which various data analysis techniques were used. Livingston County soil polygon data was used to separate and group geologic unit boundaries based on parent material identified in the Livingston County Soil Survey. Water well log data from the ISGS was used both in conjunction with soil polygon to obtain surficial geologic unit boundaries, and to develop subsurface interpretations. After the parent material was determined Canvas 15 was used to assign formations to soils polygons. Finally, the geology of the area was field checked and the soil survey data was modified as needed to construct a viable and admissible surficial geologic map.
The final surficial geologic map is portrayed in the adjacent plate. Figure 1 shows the location of Livingston County and the Pontiac NW Quadrangle with respect to the Quaternary Geology of Illinois. Figure 2 is provides the soil parent material data. Figures 3-4 portray the regional bedrock and surficial geology of Livingston County, respectively.
Figure 3. Bedrock map and formation explanation for Livingston County. From Kolata (2005)
References
Curry, B.B., Grimley D.A., and McKay E.D., 2011, Quaternary glaciations in Illinois (Chapter 37), in J. Ehlers, Gibbard, P.L. and Hughes, P.D., eds., Developments in Quaternary Science, v. 15, Amsterdam, The Netherlands, Elsevier, p. 467–487.
Hansel, A.K. and McKay, E.D., 2010, Quaternary Period, in Kolata, D.R. and Nimz, C., eds., Geology of Illinois: Champaign, IL, University of Illinois at Urbana-Champaign, Illinois State Geological Survey, p
Hansel, A.K, and Johnson, H.W., 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.
Kolata, D.R., 2005, Bedrock geology of Illinois: Illinois State Geological Survey, Illinois Map 14, scale 1:500,000.
Natural Resources Conservation Service, 2004, Soil Survey of Livingston County, Illinois: United States Department of Agriculture, Soil repot no. 159, 421 p.
Soller, D.R., Price, S.D., Kempton, J.P., and R. C. Berg, 1999, Three-dimensional geologic maps of Quaternary sediments in east-central Illinois: United States Geological Survey Geologic Investigations Series Map I-2669, 3 sheets.
Stiff, B.J., 2000, Surficial Deposits of Illinois, scale 1:500,000: Illinois State Geological Survey Statewide Map.
Descriptions
Quaternary Deposits
Cahokia Formation: Silt loam to silty clay loam to sandy loam; may contain thin sandy or loamy zones; dark brown to grayish brown; massive to weakly stratified; noncalcareous; soft consistency; weak and thin modern soil profile is typical (<1 foot); localized beds with organic matter; up to 30 feet thick
Equality Formation: Silt loam to sandy loam; dark yellowish brown to light brownish gray to grayish brown; massive to finely laminated; weak soil structure; gray mottles and bright iron stains; noncalcareous; soft to medium stiff consistency.
Henry Formation: Sand and gravel with cobbles and boulders; stratified; yellowish brown to grayish brown; calcareous; usually clean and moderately well sorted; unconformably overlies older sand and gravel deposits, glacial diamictons 3, or bedrock; variable in thickness
Lemont Formation (Wedron Group): Pebbly silty clay loam diamicton; unstratified; olive (oxidized) to grayish brown (unoxidized); firm to hard; compact; calcareous; massive to jointed; some cobbles, few
boulders; discontinuous beds of sand, gravel, silt, or clay; overlain in places by wind-blown silt (loess) of the Peoria Silt.
Paleozoic Strata
Pennsylvanian Undivided: Limestone and calcareous claystone and shales; the upper limestone member of the Bond Formation is as much as 50 feet thick and the lower is in places as much as 30 feet thick and overlies the Shelburn and Patoka Formations which is mostly shale and sandstone; also includes thin beds of limestone and coal. Gray shales, siltstones, and sandstones are similar to those in the underlying Modesto Formation. Gray shales constitute the greatest portion of the formation, but thick channel sandstones are developed locally. Qc Qe Qh Ql Pu