Bedrock Geology of Franklin Grove Quadrangle
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STATEMAP Franklin Grove-BG Bedrock Geology of Franklin Grove Quadrangle Lee County, Illinois Franck Delpomdor and Joseph Devera 2020 615 East Peabody Drive Champaign, Illinois 61820-6918 (217) 244-2414 http://www.isgs.illinois.edu © 2020 University of Illinois Board of Trustees. All rights reserved. For permission information, contact the Illinois State Geological Survey. Introduction Previous work The first geological features of Lee County were illustrated Geographic location and geomorphological framework very generally on early statewide geologic maps at scale The Franklin Grove 7.5-minute Quadrangle is located in 1/500,000 (Worthen 1875; Weller 1906). Stratigraphy and north-central Illinois in the north-central part of Lee County, structural geology investigations in the Franklin Grove area Illinois, about 32 miles southwest of Rockford (Winnebago include those by Cady (1920), Leighton (1922), Templeton County), 45 miles east of Illinois-Iowa border, 50 miles and Saxby (1947), Templeton and Willman (1952, 1963), south of the Illinois-Wisconsin border, and 90 miles west Kolata and Buschbach (1976), Willman and Kolata (1978), of Chicago (Cook and DuPage Counties). Map coverage and Kolata et al. (1978). In addition, a map showing the bed- extends to the east from the Dixon East Quadrangle and rock geology of Lee County, including the Franklin Grove south of the Daysville Quadrangle. The quadrangle cov- Quadrangle, was published by McGarry (1999). Geologic ers approximately a 55 square mile area that is bounded by features were generalized in the Geologic Map of Illinois 41°45’00” and 41°52’30” North latitude and 89°15’00” and at scale 1/500,000 (Kolata 2005). Since 1993, the USGS- 89°22’30” West longitude. The residential and commercial STATEMAP geological mapping program has mapped eight developments are mainly centered in the village of Franklin quadrangles in the Dixon-Oregon-Rochelle area (Oregon and Grove, located north central of the quadrangle, and Center Mount Morris by Seid in 2010 and 2011 respectively; Grand Lee Township, an unincorporated community that is located Detour, Dixon East and West and Daysville by Kolata in extreme south-central of the quadrangle. As of the 2018 U.S. 2012, 2013a,b and 2014 respectively; Chana by Delpomdor Decennial census, the population covered by the quadrangle and Wirth in 2018; Rochelle by Delpomdor in 2019), which was estimated less than 1,500. greatly increased the regional knowledge of lithostratigraph- ic successions and structural geology. The quadrangle is divided into two distinct topographic surface morphologies: (1) a flat upland up to 800 feet in The early statewide stratigraphy of the Ordovician rocks, de- elevation that covers most of the area, and (2) an alluvial fined by Templeton and Willman (1952, 1963), then revised plain along Franklin Creek and Chamberlain Creek with by Willman and Kolata (1978), was divided, from the oldest an elevation lower than 750 feet to the western edge of the to the youngest: the Knox Dolomite Megagroup including quadrangle. Hills up to 850 feet in elevation are located in the Prairie du Chien Group -itself subdivided into Gunter the south part and the north-east edge of the Franklin Grove Sandstone, Oneota Dolomite, New Richmond Sandstone Quadrangle. The surface units are part of the Illinois Prairie and Shakopee Dolomite- and the Everton Dolomite, and ecoregion that consists of Till Plains section of the Rock Riv- the Ottawa Megagroup including the Ancell Group -itself er Hill Country of the Central Lowlands Province that was subdivided into St. Peter Sandstone and Glenwood Forma- formed during the Wisconsinan Glacial Episode. Sediments, tion-, the Platteville Group -itself subdivided into Pecatonica that were formed during the Illinoian Glacial Episode, are Formation and Plattin Subgroup, and Galena Group, itself located in the south-eastern edge of the quadrangle. Most of subdivided into Decorah and Kimmswick Subgroups, and the quadrangle is covered by Quaternary (i.e. Pleistocene- locally Dubuque Formation. Holocene) surficial sediments, which are mainly used for agriculture. Bedrock surface is largely concealed beneath The stratigraphic nomenclature system of the North-Central till plains that range in thickness from less than 10 to 170 Illinois was revised by Kolata (2013a,b) following the “test of feet following the degree of the differential erosion. Bedrock mappability” of map at the current scale of 1:24,000 recom- exposures are mainly located along the Franklin Creek River mended in the North American Stratigraphic Code (2005). It and its tributaries that are located in the Franklin Creek State proposed to change the stratigraphic rank as follows: Natural Area in the northwestern part of the quadrangle. 1. The elimination of megagroups and subgroups; The geologic map lies on the west flank of the Wisconsin Arch and is located in a structurally complex area near the 2. the reassignment of the Ottawa Megagroup to the rank of western termination of the Sandwich Fault Zone (SFZ) Group; and the La Salle Anticlinorium (Kolata et al. 1978; Kolata 2013a,b). Locally, bedrock is controlled by the Ashton 3. the reassignment of Platteville and Galena Groups to the Anticline that is a structure parallel to and south of the SFZ rank of formations; in Lee and Ogle Counties. In the quadrangle, the anticline exposes Middle Ordovician Prairie du Chien Group to the 4. the reassignment of Pecatonica, Mifflin, Grand Detour, bedrock surface along the south edge of the SFZ. The rocks Nachusa and Quimbys Mill Formations to the rank of mem- at the bedrock surface in the quadrangle are, from the oldest bers of the Platteville Formation; to the youngest, the Middle Ordovician Prairie du Chien Group, Ancell Group and Sinnipee Group. 5. the reassignment of all corresponding members to the rank of beds. 1 The reassignment of the Decorah and Kimmswick Sub- bers of the Platteville Formation. The Quaternary surficial groups to the rank of formations is similarly made on the sediments are mapped together in a single unit due to the statewide stratigraphy. Recently, the Sinnipee Group, defined difficulty of differentiation within till plain sediments. by Mudrey et al. (1982) in Wisconsin, was proposed for the reassignment of the Ottawa Group (Nelson, J., Illinois State How this map was made Geological Survey, personal communication, 2020). The geologic map is based on the compilation of data from bedrock exposures in an abandoned quarry, natural bluffs Due to limited outcrop and subsurface control and widely along the Franklin Creek River and its tributaries, and scattered and not detailed water wells, the formations are from examination of 215 water well and 6 oil and gas well mappable in single stratigraphic units at the scale of the records, and 17 drill cuttings stored at the ISGS core library Oneota Dolomite, New Richmond and Shakopee Formation (Fig. 1). Cuttings from sample sets were examined by F. -Gunter Sandstone Member being missing in this area- of the Delpomdor. One hundred fifty-five feet of stratigraphic test Prairie du Chien Group, the St. Peter Sandstone and Glen- boring was drilled through the Quaternary and St. Peter wood Formation of the Ancell Group, and the Pecatonica, Sandstone for stratigraphic investigations in the Gittleson’s Mifflin, Grand Detour and Nachusa and Quimbys Mill Mem- property located east side of Whitney Rd half miles north 89°22’30” 89°15’ 20’ 17’30” 41°52’30” 41°52’30” OGLE Grand Chana Detour Daysville Dixon ranklin F Ashton East Grove 50’ 50’ Rockford LEE Walton Amboy Sublette Rock Island Peoria Bloomington Springfield 47’30” 47’30” St. Louis Carbondale 41’45” 41’45” 20’ 17’30” 89°22’30” 89°15’ Drill Holes 0 1 2 mi Oil and Gas Well Water-well Boring 0 2 km Stratigraphic Well Figure 1 Index map of Illinois showing the location of the Franklin Grove Quadrangle (yellow) and names of surrounding quadrangles in Lee and Ogle Counties. Index map of Franklin Grove Quadrangle and location of data points. Data density does not allow all individual points to be shown at this scale (1:120,000). 2 of the intersection between Herman Rd and Whitney Rd village’s water well (API121030000100) and Schafer and (API121032436800) (Fig. 1). The core and stratigraphic Delhotal’s gas and oil wells (API numbers: 121030007100, descriptions are archived at the ISGS core library and at the 121030006900, 121030006400, 121032060000), all located ISGS Geologic Records Unit, respectively. Cited boreholes in the northern half of the quadrangle (Fig. 2). Its maximum are described in “List of cited well records” at the end of this thickness ranges between 120 feet and 150 feet, but this unit report. Field work for this study was completed in 2019 and reaches a maximum thickness of 150 feet in the area (Bus- 2020 by the authors. chbach, 1975). It consists of white to buff, poorly sorted, medium-grained, locally dolomitic and/or glauconitic, Stratigraphy sandstone. The formation is not subdivided into members due to the poorly detailed lithological descriptions of wells. The stratigraphy of the Franklin Grove Quadrangle is based The Ironton Sandstone is conformable with the overlying on bedrock exposures, water well, oil and gas well records, Franconia Formation (Buschbach 1975). and a stratigraphic test boring. A map locates all drill holes used in this investigation, and a diagram shows via graphic Franconia Formation Sandy dolomite, locally dolomitic columns the distribution of borings that have sample studies sandstone, glauconitic, of the Franconia Formation is only (Fig. 2). Among surface rock exposures, bluffs and the Old recognized in cuttings from Franklin Grove village’s water Quarry, formerly named Brown’s Quarry, from the Franklin well (API121030000100), Schafer and Delhotal’s oil and Creek State Natural Area are by far the most informative, as gas wells (API numbers: 121030006400, 121030006900; they present many details of lithology and paleontology.