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

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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. 121032060000; 121030007100; 121030006500, 121030006600) (Fig. 2). The unit consists of undifferentiated Cambrian System gray, glauconitic, silty, argillaceous, fine-grained, dolomitic The Cambrian rocks are only identified in subsurface geol- sandstone with green shale partings. Tts maximum thickness ogy from seven oil and gas wells and one water well. The ranges between 100 feet and 150 feet. The Franconia Forma- Cambrian rocks include, from the oldest to the youngest, the tion is usually overlain conformably by the Potosi Dolomite Mt. Simon Sandstone, the Eau Claire Formation, the Ironton (Buschbach 1975). Sandstone, the Franconia Formation, and the Potosi Dolo- mite. These five formations within the Cambrian succession Potosi Dolomite The Potosi Dolomite is conformable with are mapped together, because surface exposure is missing, the overlying Oneota Dolomite of the Prairie du Chien and well records are widely scattered and not detailed. The Group. This unit is identified in cuttings of same previous Mt. Simon Sandstone is only identified in the Colwell’s water wells cited above for the Franconia Sandstone (Fig. 2). The well record (API121032429400) located half miles north formation consists of pure glauconitic dolomite, locally ar- Naylor Rd in NE ¼SW¼NE¼ sec. 25, T22N, R10E (Fig. gillaceous or sandy. Its maximum thickness ranges between 2). However, no samples allow a detailed description of the 100 feet and 150 feet. In northern Illinois, the Potosi Dolo- lithology. The unit consists of up to 200 feet of sandstone. mite is estimated at 100 feet thick (Buschbach 1975). In north-central Illinois, the thickness of the Mount Simon Sandstone is estimated up to 1,500 feet (Buschbach 1975). Eminence Formation The Eminence Formation, overlying the Potosi Dolomite, is absent in the Franklin Grove Quad- Eau Claire Formation The Eau Claire Formation is identi- rangle due to erosion at the sub-Tippecanoe unconformity fied in the Colwell’s water well record (API121032429400) over the Ashton Anticline. and in cuttings from Franklin Grove village’s water well (API121030000100, sec. 1, T21N, R10E) and Delhotal’s Ordovician System oil and gas well (API121030006400, sec. 33, T22N, R10E) The Ordovician System is identified in water, oil and gas well (Fig. 2). It consists, from the oldest to the youngest, of 47 records and cuttings, as well as bluffs and the Old Quarry, feet of gray, fine- to medium-grained, sandstone, that is that are all located in the Franklin Creek State Natural Area, attributed to the Elmhurst Sandstone Member, 154 feet of north-central part of the quadrangle. In the Franklin Grove grayish brown to buff, glauconitic, partly sandy dolomite area, the Ordovician System comprises, from the oldest to containing greenish shale beds, here attributed to the Lom- the youngest: (1) the Prairie du Chien Group that includes the bard Dolomite Member, and at least 12 feet of gray to buff, Oneota Dolomite, the New Richmond Sandstone, the Shako- fine-grained, dolomitic sandstone, locally sandy dolomite, pee Dolomite, (2) the Ancell Group that includes the St. Peter here attributed to the Proviso Siltstone Member, near the Sandstone and the Glenwood Formation, and (3) the Sinnipee contact with the overlying Ironton Sandstone. Its maximum Group, that only includes the Platteville Formation (Willman thickness is estimated around 215 feet, but this unit can and Buschbach 1975). The Gunter Sandstone, basal unit of reach 400 feet in north-central Illinois (Buschbach 1975). the Prairie du Chien Group is regionally missing in Lee and The contact with the Ironton Sandstone is gradational. Ogle Counties (Willman and Buschbach 1975).

Ironton Sandstone Sandstone strata of the Ironton Sand- Prairie du Chien Group The Prairie du Chien Group is well stone was only identified in cuttings from Franklin Grove exposed along the Franklin Creek River and its tributaries 3 -89.2500° MountSimonSandstone EauClaireDolomite IrontonSandstone Franconia Dolomite Potosi Dolomite Cambrian duChienGroup Prairie Ancell Group Platteville Group Sinnipee Group Quaternary

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123 04 000m 46 E 25 25 000m 03 -89.2500° 41.7500° 41.7500°N -89.3750° covering about 4 square miles in the north-central part of the light buff, medium-grained, moderately sorted, very fine- quadrangle. The geologic contour of the Prairie du Chien grained sandy dolomite at the top; Group is constrained by the Daysville 7.5-minute Quad- rangle published by Kolata (2014), and 99 water, oil and gas 3. 17 feet thick thinning-upward massive beds of light gray well records including 9 water well cuttings. to light buff, pure, crystalline dolomite at the base, then pink-gray burrowed and argillaceous dolomite with green- Oneota Dolomite Description of the Oneota Dolomite is gray shale partings, and cherty dolomite with abundant thin based on 8 well cuttings from Franklin Grove village’s green shale partings at the top (Plate 1g, h,i). Symmetrical water well (API121030000100), Schafer and Delhotal’s ripples, NNW oriented, and hummocky cross-beddings are and Northern Illinois Gas Co’s oil and gas wells (API present at the top; numbers: 121030006400, 121030006900; 121032060000; 121030007100; 121030006500; 121030006600; 4. 1 foot thick massive beds of green to dark gray dolomitic 121032059900), because no outcrops were found in the map shale; area (Fig. 2). The unit consists of light buff-gray, medium- to coarse-grained, pure dolomite with white to light gray 5. 11 feet thick thin, wavy, massive and lenticular beds of cherts. Its thickness ranges between 80 feet and 120 feet, but light gray to light buff, fine-grained, pure, crystalline do- the maximum thickness is regionally estimated at 100 feet lomite, locally stromatolitic, with interbeds of light gray to (Kolata 2014). The Oneota Dolomite is conformably overlaid light buff, medium-grained, moderately sorted, cross-bedded by the New Richmond Sandstone. sandy dolomite and dolomitic sandstone;

New Richmond Sandstone Sandstone strata of the New 6. 10 feet thick thin, wavy and massive beds of light gray to Richmond Sandstone outcrops in many places along Franklin light buff, very fine-grained, pure, crystalline dolomite with Creek and its tributaries in the Franklin Creek State Natural thin green shale partings; Area (Fig. 3). This unit consists of light gray to white, very fine- to medium-grained, quartz arenite, slightly dolomitic, 7. 12 feet thick thin wavy and massive beds of light gray to sandstone with low-angle cross-bedding oriented southwest light buff, pure, very fine-grained, often crystalline, cherty (Plate 1a,b,c). Its thickness ranges from 20 feet and 25 feet, dolomite with thin green shale partings. Cross-bedded sandy but the maximum thickness is regionally estimated at 20 feet dolomite is common (Plate 1j) (Kolata 2014). The New Richmond is conformably over- laid by light gray to light brown, pure, crystalline or sandy The Shakopee Dolomite ranges between 60 feet and 165 feet dolomite of the Shakopee Dolomite (Plate 1d). The New in the map area, although (Kolata 2014) cited a maximum Richmond Sandstone intergrades laterally with greenish gray thickness of 140 feet. The contact with the St. Peter Sandstone argillaceous dolomite of the Shakopee Dolomite (Plate 1e). is identified in cuts along tributaries of the Franklin Creek River, located 1/5 miles east side of Twist Rd (SW¼NE¼SE¼ Shakopee Dolomite Conformably overlying the New Rich- sec. 33, T22N, R11E, N41.849209° W89.343102°) and 1/5 mond Sandstone is the Shakopee Dolomite, which is best miles north side of Old Mild Rd (SW¼NE¼NE¼ sec. 33, recorded in the same water and oil and gas wells cited above T22N, R10E, N41.856549° W89.343463°). The contact is for the Oneota Dolomite (Fig. 2). Outcrops of the Shako- marked by an erosional unconformity that is attributed to pee Dolomite are mainly exposed in the Franklin Creek the Sauk Sequence (below) from the Tippecanoe Sequence State Natural Area along the Franklin Creek River and its (above) (Willman et al. 1975) (Plate 2a). tributaries. The best exposures are at Mill Spring, Beecher Brown’s Spring, Senger’s Pond, Whipple’s Cave and Lover’s Ancell Group Leap. Fifty-one outcrop section points from tributaries of The Ancell Group comprises, from the oldest to the young- the Franklin Creek, and the Old Quarry were used for the est, the St. Peter Sandstone and the Glenwood Formation. compilation of the detailed lithostratigraphic description of Both formations are mapped as a single stratigraphic unit the Shakopee Dolomite (Figs. 3 and 4). It can be described because the Glenwood Formation is relatively thin, poorly as follows, from the oldest to the youngest: exposed, or locally unexposed. The geologic contour of the Ancell Group is constrained by the Daysville 7.5-minute 1. 4 feet thick thin wavy beds of light gray to light buff, pure, Quadrangle (Kolata 2014) to the north, the East Dixon crystalline dolomite with interbeds of light gray to light 7.5-minute Quadrangle (Kolata 2013b) to the west, (3) buff, medium-grained, moderately sorted, very fine-grained 62 well records, and (4) 11 cuttings from wells bored for sandy dolomite or light gray to white, very fine- to medium- groundwater and petroleum resources (Fig. 2). grained, quartz arenite, slightly dolomitic, sandstone (Plate 1d,f). Layers of conglomerate are common; St. Peter Sandstone The St. Peter Sandstone largely covers the central and northwestern parts of the Franklin Grove 2. 6 feet thick of massive and thick beds of light gray to light Quadrangle. This unit is exposed at near surface in cuttings buff, pure, crystalline dolomite at the base, and light gray to from water, oil and gas wells. The St. Peter Sandstone is well

5 Dolomite 1 Clay, dark gray, silty. GE OUP A 2 Sand, gray to yellow-gray, fine-grained, poorly sorted, argillaceous. Rare TION A S T Pure Soil Shaley FORM- Sandy Shale Sand Sandstone

G R gravels. Sharp erosional contact with the bedrock. SYSTEM 1 60

Y 3 Shakopee Dolomite: Dolomite, light gray to light buff, pure, cristalline, very

TER 58 fine-grained, with very thin green shale partings. Beds are thin, wavy and

A 2

U massive. The bed boundaries are sharp. Fossils are rare but Thalassinoides -NA R Q 56 burrows are observed.

3 54 4 Shakopee Dolomite: Dolomite, light gray to light buff, pure, cristalline, very fine-grained, with sandy dolomite, light gray to light buff, medium-grained, 52 moderately sorted. Cross-bedding is common within sandy dolomite beds. Beds are thin, wavy, massive and lenticular. Fossils are not abundant, but stromatolite domes are common. 50 5 Shakopee Dolomite: Green to dark gray dolomitic shale. The bed is 1/2 feet 48 thick in size and massive. 4 46 6 Shakopee Dolomite: Dolomite, light gray to light buff, pure, cristalline, very fine-grained, with very thin green shale partings. Shaley dolomite, dark gray to 44 dark brown, at the base. Alternating light gray to buff argillaceous dolomite with Thalassinoides-, Palaeophycus-, and Planolites-type burrows and green-gray 42 dolomitic shale. Thin layers of light gray to light buff sandy dolomite at the top. Light gray to light buff, pure, cristalline, fine-grained dolomite with symmetrical 40 ripples and hummocky cross-beddings. Chert is present in the upper part. 5 Beds are thick at the base, thin at the top, wavy and massive. The bed 38 boundaries are sharp. Fossils are rare.

36 7 Shakopee Dolomite: Dolomite, light gray to light buff, pure, cristalline, very fine-grained, at the base, sandy dolomite, light gray to light buff, 34 medium-grained, moderately sorted at the top. Beds are thick (up to 3 feet thick), wavy and massive. Unfossiliferous. 32 8 Shakopee Dolomite: Alternation of dolomite, light gray to light buff, pure,

Shakopee Dolomite 6 cristalline, very fine-grained and sandy dolomite, light gray to light buff, 30 medium-grained, moderately sorted. Beds are thin and wavy. Conglomeratic IBEXIAN layers, light gray to light buff, in a medium-grained matrix. Fragments are ORDOVICIAN 28 composed of subrounded to rounded, spherical to tabular, granule- to PRAIRIE DU CHIEN pebble-sized grains of dolomite. Grain surface texture is smooth. Unfossilifer- 26 ous.

24 9 New Richmond Sandstone: Quartz arenite, light gray to white, composed of very fine- to medium-grained, well sorted, subrounded to rounded quartz sand 22 grains. Beds are thick, massive, lenticular, poorly cemented and friable. Low-angle cross-bedding, oriented SW, is common. A few beds of sandy 20 dolomite are common. The contact with the Shakopee Dolomite is gradational. 7 18

16 Geological features 8 14 Nodules

12 Dolomite

10 Burrows

Stromatolites 8 9 6

4

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New Richmond Sandstone Figure 3 Composite lithologic log of the base of Shakopee Dolo- 0 feet mite in contact with the New Richmond Sandstone located in the Franklin Creek State Natural Area. Soil Pure Sand Shale Sandy Shaley

Dolomite Sandstone 6 1 Clay, dark gray, silty. Dolomite GE OUP A 2 Sand, gray to yellow-gray, fine-grained, poorly sorted, argillaceous. TION A S T FORM- Shale Shaley Sandstone Sandy

Pure Rare gravels. Sharp erosional contact with the bedrock. Soil Sand G R SYSTEM

1 3 Tonti Member: Quartz arenite, light yellow-brown, composed of Y 22

TER moderate-grained, well sorted, well rounded quartz sand grains. The A 2 bed is massive, poorly cemented and friable. Planar parallel bedding U

-NA R 20

Q is present. Unfossiliferous. 18 3 4 Kress Member: Conglomerate, yellow-brown to dark brown, in a coarse-grained St. Peter-like sandy and dolomitic matrix. Fragments

-KIAN 16 are dominantly rounded to well rounded, spherical, and vary from sand ANCELL MOHAW St. Peter 4

Sandstone to pebble in size. Grain surface texture is smooth and exhibits common 14 crescentic impact marks. Fragments contain dolomite and chert.

12 5 Dolomite, light gray to light buff, pure, very fine-grained, often cristalline, with beds of medium-grained cross-bedded sandy dolomite 10 and very thin green shale partings. Flaser-like and crossed laminations are common. Chert is present in the upper part. The beds are wavy and 8 massive. Lenticular bedding is also observed. The bed boundaries are ORDOVICIAN 5 sharp. The unit shows a relative thinning-upward bedding, from up to 3 feet to as much 1/2 feet thick at the top. The contact with the St. Peter IBEXIAN 6 Sandstone is sharp or erosional when the Kress Member is present. Shakopee Dolomite

PRAIRIE DU CHIEN 4

2

0 feet Soil Pure Sand Shale Sandy Shaley

Dolomite Sandstone

Figure 4 Composite lithologic log of the top of Shakopee Dolomite in contact with the St. Peter Sandstone located in the Franklin Creek State Natural Area. exposed in several bluffs located near the Mondays Bridge located along northern side of the Franklin Creek River near and crops out along northern side of the Franklin Creek the Mondays Bridge in sec. 28, T22N, R10E (N41.859774° River in the northwestern part of the Franklin Grove Quad- W89.354248°) (Fig. 5; Plate 2c). The member displays a rangle, which allows a detailed lithological description of thickening-upward succession that is composed of poorly this unit. The St. Peter Sandstone is divided into two mem- cemented and moderately sorted, light gray to white, yellow- bers. These are the Kress (oldest, 0-5 feet thick) and Tonti brown when weathered, fine- to moderate-grained quartz Sandstone (20-200 feet thick). arenite including planar parallel and NNW-oriented low-angle crossbedding at the base (Plate 2d). The uppermost part of this In the Franklin Grove area, the Kress Member is only member is only exposed in cores of the Gittleson’s stratigraph- identified in a stream, tributary of the Franklin Creek River, ic test boring in sec. 13, T21N, R10E (API121032436800) located 300 feet south Old Mill Rd. in sec. 34, T22N, R10E (Fig. 2). It consists of 65 feet thick light gray well sorted, (N41.85328° W89.339863°). This member consists of a medium-grained, quartz arenite, locally finely planar laminat- coarse rubble or conglomerate of chert in clayey and sandy ed, with yellow or gray-greenish shale partings. On the base matrices with tabular to spherical, subrounded to rounded, of well records, the thickness of the St. Peter Sandstone is fragments from the Prairie du Chien Group (Plate 2b). estimated between 20 feet and 220 feet. The contact with the Glenwood Formation is unexposed in the Franklin Grove area, When the Kress Member is not exposed, the base of the Tonti but a stratigraphic test boring (API121412617800) drilled in Sandstone displays very thin bedded, poorly cemented and fri- the Chana 7.5-minute Quadrangle adjacent northeast of the able light gray, very fine- to medium-grained quartz arenite, as Franklin Grove area shows that it is sharp and conformable only observed the two previously cited streams, tributaries of (Delpomdor and Wirth 2018). the Franklin Creek River, located 1/5 miles east side of Twist Rd (SW¼NE¼SE¼ sec. 33, T22N, R11E, N41.849209° Glenwood Formation Southeast of the quadrangle, this W89.343102°) and 1/5 miles north side of Old Mild Rd formation is only identified from cuttings of the Illinois (SW¼NE¼NE¼ sec. 33, T22N, R10E, N41.856549° State’s and Shriver’s water wells API121032057300 in W89.343463°). The lowermost part of the Tonti Sandstone sec. 34, T21N, R10E and API121030005400 in sec. 6, is identified in approximately 18 feet of a well exposed bluff T20N, R11E, N41.752107° W89.282951°, 807 feet el- 7 1 Quaternary: Clay, dark gray, silty. Dolomite GE OUP A 2 St. Peter Sandstone: Quartz arenite, light yellow-brown, fine- to TION A S T FORM- Sand Shaley Shale Soil Sandstone Sandy Pure moderate-grained, moderately sorted. Beds are slightly wavy and they G R SYSTEM 18 range from 1 feet to as much 3 feet in thickness. The base exhibits QUATER 1 -NARY planar parallel and low-angle cross-bedding, dominantly oriented N89° 16 to N110°, while the top displays a 1 feet thick cross-bedding that is 2 overlain by a 3/4 feet-thick massive bed. Unfossiliferous. 14 3 not recovered 3 12 4 St. Peter Sandstone: Quartz arenite, light yellow-brown, fine- to moderate-grained. Grains are composed of moderately sorted, well 10 rounded quartz sand grains. Beds are relatively thin, ranging between 1/2 feet to 3 feet in thickness, massive, poorly cemented and friable. 8 Planar parallel and low-angle cross-bedding are common at the base, ANCELL while massive beds are present at the top. The unit is unfossiliferous. MOHAWKIAN ORDOVICIAN 4 6 St. Peter Sandstone 4

2

0 feet Soil Pure Sand Shale Sandy Shaley

Dolomite Sandstone

Figure 5 Composite lithologic log of the lower part of the St. Peter Sandstone of the Ancell Group located in the Franklin Creek State Natural Area. evation) respectively, and the Schafer’s oil and gas well of the water well, the stratigraphy of the Platteville Forma- API121030007100 (Fig. 2). The formation comprises green- tion comprises from the oldest to the youngest: ish gray argillaceous and silty sandstone (oldest, 10 feet thick) of the Kingdom Sandstone Member, greenish gray Pecatonica Member 25 feet of light gray, light buff or argillaceous and silty dolomite (5 feet thick) of the Daysville yellow-gray when weathered, slightly argillaceous to pure Dolomite Member, greenish gray argillaceous and silty dolomite, sandy and phosphatic at the base. sandstone (20 feet thick) of the undifferentiated Loughridge Sandstone and Nokomis Sandstone members, and maroon Mifflin Member18 feet of light gray, light yellow-buff to greenish gray sandy shale and silty sandstone (youngest, 5 white when weathered, very fossiliferous, nodular and argil- feet thick) of the Harmony Hill Shale Member. Its maximum laceous dolomite. thickness ranges between 15 feet and 40 feet. The Glenwood Formation is locally eroded or unconformable or marked by a Grand Detour Member 40 feet of light gray, light buff-yel- hardground surface with the overlying Platteville Formation. low gray when weathered, locally cherty, pure to argillaceous dolomite. Sinnipee Group Platteville Formation Previously regarded as a group in Quaternary System Illinois (e.g. Templeton and Willman 1963), the Platteville Pleistocene and Holocene (Quaternary) sediments mantle Formation is herein classified as a formation because its bedrock throughout the Franklin Grove Quadrangle. Four subdivisions are too thin and similar in lithology to be mapped formations can be recognized here, from the oldest to the separately at 1:24,000 and smaller scales. In the Franklin youngest: Grove area, three out of five members of this formation can be identified. There are the Pecatonica (oldest), Mifflin and 1. the Glasford Formation that includes gray compact silty Grand Detour Members. No outcrops of the Platteville Forma- sand with gravel, sand and silt interlays of the Sterling Till tion are found within the Franklin Grove Quadrangle, but its Member; greatest thickness is 80 feet as exposed in cuttings of 4 well records, all located in the southeastern edge of the quadrangle. 2. the Winnebago Formation that includes pinkish-tan sandy to gravelly till of the Argyle Till Member; The best subsurface record of the Platteville Formation is in cuttings from the Hutten’s water well just southeast of the in- 3. the Equality Formation that includes dark gray well-bed- tersection between the Hillison Rd. and Pine Hill Rd. in sec. ded silt and clay of the Carmi Member; 29, T21N, R11E (API121030010600) (Fig. 2). On the base 8 4. the Cahokia Formation that includes sandy gravel, sand, contains gastropods and algal masses, while the Shakopee silt or clay observed as alluvial deposits in Franklin Creek Dolomite displays gastropods and cephalopods (Buschbach and Chamberlain Creek rivers. 1975). In the Franklin Grove Quadrangle, algal mats form- ing, ½-1 inch of amplitude and 2-3 inches of diameter, close- The Illinoian Glacial Episode comprises the Glasford For- linked lateral linked hemispheroids (mode C as proposed mation, while the Wisconsinan Glacial Episode comprises by Logan et al. 1964) and, a few inches to more than 10 feet the Winnebago and Equality Formations. The thickness of thick, unbranched and finely laminated domes are observed Quaternary sediments ranges from 10 feet to 190 feet, being in the Shakopee Dolomite located 500 feet north-northwest greatest on the southwest. The best subsurface record of the of the Old Quarry in sec. 34, T22N, R10E (N41.850915° Quaternary System is in cores from the Gittleson’s strati- W89.328919°) (Plate 3a). Trace fossils are common in the graphic test boring API121412617800 in sec. 13, T21N, Shakopee Dolomite as observed in a tributary of the Franklin R10E, which about 89.9 feet thick of sediments are well ex- Creek River located NW of the Old Quarry. They include posed. The sediments display, from the oldest to the youngest: Cruziana-type ichnofacies (subtidal soft-grounded marine fossils): Thalassinoides (T- and Y-shaped branching vertical 1. approximately 45 feet of dark gray to dark green silty clay and horizontal burrows; Plate 3b), Palaeophycus (cylindri- till with intercalated sand and gravel of the Glasford cal, straight or slightly curved, unbranched subhorizontal Formation; burrows; Plate 3b), and Planolites (cylindrical, straight and tortuous horizontal burrows; Plate 3b). 2. approximately 44 feet of gray to yellow-gray moderately sorted fine- to medium-grained sand, locally silty or clayey, The Ancell Group is sparsely fossiliferous. However, Skolith- and dark gray silty to gravelly till of the Winnebago os-type worm borings (vertical, cylindrical, unbranched bur- Formation; rows), and a few scolecodonts (polychaete worm jaws) were identified in the St. Peter Sandstone. In the Chana area, dark 3. 1 feet dark organic-rich silty clay materials, regionally red algae and spores were observed in the Glenwood Forma- identified as the “black dirt” Drummer soil that consist of tion (Kolata 2014). a poorly drained soil formed by loess or other silty materi- als and in the underlying loamy stratified clays and silts on The Platteville Formation is known only from well records in nearly level or depressional parts of outwash plains, stream the Franklin Grove Quadrangle; thus, its paleontology here terraces, and till plains. This soil is used for agriculture, is unknown. Regionally, the Platteville Formation is very mainly corn, soybeans, and wheat. fossiliferous. The Pecatonica Member dominantly contains a brachiopod-molluscan fauna, but is generally less fossilifer- The gravels comprise tabular to spherical, subrounded to ous than the overlying Mifflin and Grand Detour Members. well rounded, rock fragments of quartz, sandstone, lime- Brachiopods, bryozoans, ostracods, corals, trilobites, and stone, dolostone, magmatic and metamorphic rocks. echinoderms, and bryozoans Nicholsonella and Mono- trypella that are associated with Trypanites (cylindrical and Paleontology unbranched vertical burrows), Balanoglossites (U-shaped cylindrical burrows) and Thalassinoides borings are common A stratigraphic evaluation and assessment of the micropalae- within the middle and upper parts of the Pecatonica Member ontological groups with the highest biostratigraphical resolu- (Kolata 2013a,b). The paleontological content of the Mifflin tion, i.e. Bolboforma, dinoflagellate cysts, calcareous nanno- and Pecatonica Members is similar, but fossils of echino- plankton, planktonic foraminifera and pteropods, follows. derms including crinoids, cystoids, and a paracrinoid differ (Kolata 1975). Fossils include brachiopods (Oepikina, Stroph- Cambrian System omena, Campylorthis, Hesperorthis, Doleroides), bryozoans Paleontology of the Cambrian System is scarce in north- (Stictopora, Diplotrypa, Eridotrypa, Hemiphragma, Ceramo- central Illinois. Trilobites (Dickelocephalus, Illaenurus and pora), gastropods (Lophospira, Trochonema, Phragmolites, Saukiella), oboloid brachiopods and worm borings within Ectomaria, Hormotoma), bivalves (Vanuxemia, Cryptodonta, the Franconia strata have been previously identified at Or- Ctenodonta), cephalopods (Whitfieldoceras, Proteoceras, egon, Illinois (Willman and Templeton, 1951). Algal domes Beloitoceras, Actinoceras), trilobites (Ceraurus, Thaleops, (Cryptozoon), rare trilobites (Saukiella), Hypseloconus Isotelus, Vogdesia, Walencrinuroides), ostracods (Leperditia, (high-conical monoplacophoran molluscs) and gastropods Leperditella, Schmidtella), echinoderms (Cupulocrinus, Ab- remains have been described within the Potosi Dolomite ludoglyptocrinus, Cremacrinus, Porocrinus) and trace fossils (Buschbach 1975). ichnogenus such as Chondrites (root-like branching burrows) and Balanoglossites (Kolata 2014). A few beds at the base of Ordovician System the Grand Detour Member contains abundant lithistid fossil In north-central Illinois, fossils of the Ordovician System are sponge and trilobite fauna including Anthaspidella, Zittelella, rare in the Prairie du Chien and Ancell Groups, and abundant Dystactospongia and Anapliomera (Kolata 2014). in the Platteville Formation. The Oneota Dolomite mostly 9 Biostratigraphy and geochronology the overlying Carimona Member of the Decorah Formation (Kolata et al. 1998). It gave a U-Pb zircon age of 454.5 ± The biostratigraphy of the bedrock exposed within the 0.5 Ma (Tucker 1992; Tucker and McKerrow 1995). These Franklin Grove Quadrangle ranges in age from Lower Ordo- stratigraphic relations support assignment of the Platteville vician (Ibexian Series) to the north and Middle Ordovician Formation to the Turinian Stage of the Mohawkian Series (Mohawkian Series) to the south. Bedrock of the Cambrian (Sandbian-Katian Stages of the Upper Ordovician Series). System was assigned based on the occurrence of Elvinia and Saukia (including Rosettia and Saukiella-Calvinella) trilo- Paleogeography bites to Upper Cambrian (Franconian-Trempealeauan Stages of the Croixan Series) in age (Howe et al. 1972; Kurtz 1975; The sediments that constitute the Wisconsin Arch were Palmer 1982). deposited during the formation of Reelfoot aulacogen in southern Illinois that led to late Proterozoic-Early Cambrian Cordylodus intermedius and Loxodus bransoni conodont zo- continental breakup along the Appalachian and Cordilleran nations within the Oneota Dolomite of the Prairie du Chien margins of the Laurentian plate (Sloss 1988; Hoffman 1989; Group have been correlated to the Upper Cambrian-Lower Kolata and Nelson 1991). The rifting occurred a major inun- Ordovician (Stairsian Stage of the Ibexian Series) Rossodus dation of the sea from the south over the proto-Illinois basin manitouensis conodont zonation (Ethington and Clark 1971, that widened significantly during the Sauk megasequence 1981; Repetski and Ethington 1983; Repetski et al. 1993; from Cambrian through earliest Middle Ordovician time Ross et al. 1997). Oepikodus communis conondont zona- (Dalziel et al. 1994). During this time, the Illinois Basin be- tion within the New Richmond Sandstone and the Shakopee longed to the Interior Cratonic Basin, part of the Laurentian Dolomite of the Prairie du Chien Group has been assigned plate situated within 30° latitude of the equator and in warm to the Black Hillsian Stage of the Ibexian Series in North humid climate (Glonka 2002; Scotese 2002). America (Floian Stage of the Lower Ordovician Series) (Ethington and Clark 1971, 1981; Repetski 1982). Prior to a Late Cambrian-Lower Ordovician sea-level rise, land-derived sands of the Mount Simon Sandstone from Pre- Faunas of the Glenwood Formation of the Ancell Group cambrian rocks eroded to low relief, due to a long interval include conodont Belodina compressa Zone that has been at- of erosion, came mainly from the north, northeast and east tributed to the base of the Sandbian-Katian Stages (Turinian (Willman 1975; James et al. 1989) (Fig. 6). The sea-level Stage of the Mohawkian Series in North America) (Mos- transgression marked favorable conditions for extensive kalenko 1983; Sweet 1984). In the central and northern carbonate epeiric shelf deposition on and around the Lau- Midcontinent, the conodonts Phragmodus cognitus-Eris- rentian plate (Morgan 2012). Mixed carbonate-siliciclastic modus sp.-Chirognathus duodactylus within the Glenwood cycles of the Eau Claire Formation were deposited in a distal Shale suggest an early Mohawkian association (Witzke and carbonate shelf, while carbonate muds were formed in a Metzger 2005). The Ancell Group sequence spans parts of proximal carbonate shelf that was favored with the reduction the upper Chazyan and Turinian Stages (Sandbian-Katian in terrigenous mud input. Minor uplift of the Wisconsin Arch Stages of the Upper Ordovician Series). located north of the basin occurred a new cycle of marine transgression, initiated by the deposition of the Galesville- The conodont Phragmodus undatus Zone and the graptolite Ironton Sandstone, that pushed northward the land-bordering Amplexograptus maxwelli Zone within the Mifflin Mem- sandstone belt along the shoreline of the proto-Illinois basin ber of the Platteville Formation have been assigned to the (Watso and Klein 1989). Stabilization of the Wisconsin Arch Turinian Stage of the Mohawkian Series (Sandbian-Katian permitted local transgression and deposition of carbonate Stages of the Upper Ordovician Series) (Elles and Wood sands and muds of the Franconia Formation and the Potosi 1907; Bergström and Sweet 1966; Sweet 1984; Williams Dolomite respectively. 1994; Leslie 2000; Leslie and Bergström 1995, 2005; Gold- man et al. 2002). A local minor uplift developed erosional unconformities in some place through the basin (Watso and Klein 1989). In the The Platteville Formation lies below the Millbrig K-benton- Franklin Grove area, local tectonics eroded sandstones of the ite Bed (Kolata et al. 1986; Kolata et al. 1996; Kolata et al. Jordan Formation. The subsidence in the basin became low 1998). This bed yielded a U-Pb age for single-grain zircons due the stability of the Wisconsin Arch that created favored of 453.1 ± 1.3 Ma (Tucker 1992; Tucker and McKerrow conditions for carbonate production of the Oneota Dolo- 1995) and Ar-Ar age for biotite of 454.1 ± 2.1 Ma (Kunk et mite (Fig. 7a). Local deformation provided small amounts al. 1985). The Millbrig K-bentonite is assigned to the Global of land-derived sand materials, here attributed to the New Boundary Stratotype Section and Point marking the bound- Richmond Sandstone, that only accumulated in littoral ary between the Turinian and Chatfieldian Stages (Sandbian- zone of the basin (Fig. 7b). The limit of the New Richmond Katian Stages of the Upper Ordovician Series) (Bergström Sandstone is along a line trending southwestward from about et al. 2009). The Deicke K-bentonite Bed occurs beneath the Danville, in east-central Illinois, toward Cape Girardeau, Millbrig K-bentonite at the top of the Platteville or within Missouri (Workman and Bell 1948). The New Richmond 10 Paleoequator Land Shallow-water sea Epeiric sea Deep ocean

Figure 6 Paleogeographic reconstruction of the lower Ordovician North America continent, and location of the Franklin Grove area (modified from Scotese 2002).

Sandstone is overlain by deposits of the carbonate zone, the Limestone and Joachim Dolomite that were deposited to the Shakopee Dolomite, developed further seaward in a very south (Kolata 2010). The marine transgression flooded over shallow environment situated shoreward from an area of the bar complex, which, subsequently, open marine waters algal headlands (Ostrom 1970) (Fig. 7c). spread northward and deposition of the Platteville Formation carbonates commenced in the upper Midwest. The Shakopee Dolomite and older strata were eroded lasting approximately 10 Ma in northern Illinois and Wisconsin that Deposition of the Franklin Grove area recorded a known global lowering of sea level at the end of Early Ordovician time (Vail et al. 1977). The marine regres- In the Franklin Grove area, bedrock only outcrops in the sion coincided to the development of many new intracratonic Franklin Creek State Natural Area. Although the Oneota Do- structural features such as the Kankakee Arch (Ekblaw 1938) lomite is missing in near surface, the Prairie du Chien Group and Ozark Dome (Lee 1943; Dapples 1955). Karst sinkholes is well preserved and made this area one of the most impor- and valleys up to 650 feet deep are present along the Sauk/ tant hot spot for the observation of this group. To reconstruct Tippecanoe unconformity in Kansas (Meriam and Atkinson the deposition of the Prairie du Chien Group, the Machlin 1956) and northern Illinois (Buschbach 1964). Quarry in the southeastern part of the Daysville Quadrangle (N½NW¼ sec. 20, T22N, R11E), adjacent to the Franklin The initial deposition of the Tippecanoe megasequence was Grove Quadrangle to the south, is here selected as a local restricted to the continental margin of Laurentia plate. With reference for its stratigraphy and lithology. rising sea-level, a diachronous sheet of pure quartz sands of the St. Peter Sandstone was deposited in an advancing After a minor marine regression, the Oneota Dolomite re- shoreline in the Interior Cratonic Basin. A marine sand bar- corded a sea-level rise that drowned by shallow marine water rier complex, attributed to the Starved Rock Bar Complex, through the northern margin of the basin (Fig. 7a). Lime was developed in southwestern Michigan prograding through muds, that were formed by accumulation of death and decay northern Illinois, southeastern Iowa, northern Missouri of benthonic organisms (mainly calcareous algae), detritus into northeastern Kansas (Nunn 1986). This bar complex abraded from larger carbonate particles (microboring from isolated lagoon-type sediments of the Glenwood Formation fungi, algae and sponges), or accumulation of planktonic to the north to marine carbonate muds of the Dutchtown biota and possibly from direct precipitation from sea water

11 current transport Land Shallow-water sea Epeiric sea Sand dune Microbial domain Direction of longshore Direction of sediment lime mud BASIN MICHIGAN

KANKAKEEARCH longshore current longshore BASIN ARCH ILLINOIS

WISCONSIN

sand dune ARCH

Shakopee Dolomite

MISSISSIPPI RIVER MISSISSIPPI Paleoequator c colloform stromatolite colloform BASIN MICHIGAN

KANKAKEEARCH stromatolitic dome stromatolitic BASIN ILLINOIS ARCH

WISCONSIN

ARCH sand dunes MISSISSIPPI RIVER MISSISSIPPI

New Richmond Sandstone Paleoequator b lime mud BASIN MICHIGAN

KANKAKEEARCH sand dunes Simplified paleoenvironmental evolution of the Mid-Continent with deposition of the Prairie du Chien Group. Idealized 3-D Idealized of the Mid-Continent with deposition Prairie du Chien Group. evolution Simplified paleoenvironmental BASIN Stromatolite Cephalopod Brachiopod Echinoderm Gastropod Trilobite Burrow ILLINOIS ARCH WISCONSIN Figure 7 Dolomite Richmond Sandstone and the Shakopee the New system of the Oneota Dolomite, depositional model of the carbonate ramp area in red rectangle. Grove Location of the Franklin Quadrangle. Grove within the Franklin

Oneota Dolomite

ARCH MISSISSIPPI RIVER MISSISSIPPI

marine channel Paleoequator a Prairie du ien Group

12 (perhaps biochemically induced by phytoplankton blooms) subtidal zone of the epeiric shelf. After the maximum flood- (Wilson 1975), occupied a vast surface area of the shallow ing, a large surface area available and the high rate of accom- epeiric shelf. Coarser grains were formed from breakdown of modation conditioned favorable conditions for the production low Mg calcite shells and tests from brachiopods, bryozoans, and the preservation of carbonate systems (Catuneanu 2008). ostracods, and additionally by trilobites and rugose corals in The base-level rises generated enough space accommodation the Paleozoic (Cambrian to Devonian times). However, the for the accumulation of tidal flat deposits. Waves, and rarely post-mortem disintegration of algae was considered as the storms, transported quartz sand onto the sublittoral zone of possible biogenic aragonite precursor in the lime muds for offshore areas, where sand pinches out in accretionary sand the Platteville Formation (Lasemi 1983). The sandy materials sheet-like bodies. Cruziana-type worm- (Palaeophycus and were probably derived from sublittoral and near-littoral zones Planolites) and shrimp-produced (Thalassinoides) burrows by coastal erosion of strong currents such as wave-currents, developed in relatively cohesive substrates of shallow-marine longshore currents or heavy storms (Reineck and Singh subtidal zone (Knaust 2017). The high relief formed by sand 1980). The lenticular geometry of the bedding is therefore dunes in the subtidal zone became shallower, amplifying the interpreted as a series of coalescing deposits, which were potential retention of sand and the development of microbial spread out as sheet-like bodies of sand dunes shoreward over activity by trapping and binding sediments that formed algal the shelf. The flat lamination within sand dunes was gener- headlands (Ostrom 1970). ated by wave action when shoaling waves take much sand into suspension (Reineck 1967). The low-angle crossbedding Dolomitization was formed under high-energy conditions by the migration of sand bars (Reineck and Singh 1980). The Cambrian-Ordovician bedrock of the Franklin Grove area has been pervasively dolomitized from low magnesium At the onset of the deposition of the New Richmond Sand- limestone that outcrops eastward in the Michigan Basin, stone, a short-term sea-level fall occurred throughout the southward in the Illinois Basin and west-southwest in the region, causing a seaward shift in the depositional system Forest City Basin (Pratt and Wandrey 1996). In northern Illi- (Fig. 7b). Land-derived sands from northeast of the Illinois nois and southeastern Wisconsin, the dolomite was precipi- Basin developed a sand ridge belt on the continental margin. tated on the crest and flanks of the Wisconsin dome (Kolata, This belt was rimmed to the north by extensive tidal flats D., Illinois State Geological Survey, personal communica- that were deposited under semi-restricted water circula- tion, 2020). In Wisconsin, the emplacement of dolomite was tion. In the Franklin Grove area, the sand ridge accumulated formed by the mixing of marine water with fresh meteoric bidirectional cross-bedded sandstone that was deposited by water in a phreatic zone, a mechanism referred to as the ephemeral storms and longshore currents in a shallow-water “Dorag” model (Badiozamani 1973). However, the original environment. Dorag model, has been discredited by Luczaj (2006), who made a convincing case that the origin of the pervasive do- After the sea-level fall episode, the sea-level rise progressed lomitization from Wisconsin occurred by late hydrothermal landward by the deposition of the Shakopee Dolomite (Fig. pore-fluid circulations rather than early mixing zone. The 7c). The barrier ridge system, previously formed, displayed Cambrian Ordovician rocks from the Chana area recently a tidal flat system that was limited by the water influx. These showed a preferential dolomitization by deep penetration of conditions enhanced the precipitation of fine-grained carbon- extensive and pervasive dolomitization from density-driven ate sediments under low-energy conditions. The tidal flats flow of evaporitic brines (Jones et al. 2002). Evaporite reflux were reached by tidal currents, producing thinly interlayered dolomitization systems probably migrated westward from bedding of sand and lime mud layers. The sands were ac- the Michigan Basin during Devonian and Mississippian time cumulated during periods of current activity and rare storms, (Luczaj 2006). while lime muds were developed during slack water periods (Reineck and Singh 1980). Rare wave-worked gravels and lime mud pebbles formed thin layers due the strong erosion Electric and induction profiles on tidal flat surfaces. The flooding of the basin during the Electric logging is a passive wireline logging method used to marine transgression caused a landward shift in the depo- obtain a continuous record of a formation’s rock properties. sitional system. The sediments of the Shakopee Dolomite This technique measures the electrical resistivity of rocks, recorded the transition between tidal flat to marine shelf envi- which are the ability to impede the flow of electric current. ronments by the accumulated of sand dunes. Lime muds were The Spontaneous Potential (SP) log is commonly used to deposited respectively in a subtidal zone of the epeiric shelf. detect permeable beds and to estimate clay content and for- Siliceous sponges, forming nodules of chert that is the result mation water salinity. The SP log can be used to distinguish of the dissolution and the replacement of siliceous spicules between impermeable shale and permeable shale and porous of sponges during diagenesis (Trewin and Fayers 2005), were sands. The SP log is expressed in millivolt. The resistivity developed in subtidal shallow water and bathyal environ- (R) logging is used to differentiate between formations filled ments (Flügel 2010). The maximum flooding was marked with salty waters (good conductors of electricity) and those by deposition of greenish to dark grayish muds in the deep filled with hydrocarbons (poor conductors of electricity). The 13

NIPEE

ANCELL CHIEN DU PRAIRIE CAMBRIAN

SIN-

ville

kopee wood

Oneota Franconia Ironton Potosi St. Peter St.

Platte- Sha- Glen- New New Quaternary Richmond 750 /m 2 + - Resistivity 20 millivolts ohms. m ohms. Electric and electromagnetic TD: 724 TD: Spontaneous Potential Spontaneous 121030007100 0 0 feet Figure 8 records induction logs of oil and gas well Quadrangle. Grove within the Franklin 100 200 400 300 500 700 Depth 600

750 N /m 2 Grove + Franklin Franklin 38 - Resistivity 20 millivolts ohms. m ohms. 121030006900 Spontaneous Potential Spontaneous TD: 545 TD: 121030006900 0 121030007100 0 feet 100 200 400 300 500 600 Depth 121032060000 121030006400 TD: 666 TD: 88 Induction 121032060100 121032059900 0 121032060000 feet 100 200 400 300 500 Depth 600 TD: 761 TD: Induction 0 121032060100 feet 100 200 400 300 500 700 Depth 600 Quadrangle Franklin Grove Grove Franklin TD: 592 TD: Induction 0 121032059900 feet 100 200 400 300 500 Depth 600 750 /m 2 + - Resistivity 20 millivolts ohms. m ohms. TD: 694 TD: Spontaneous Potential Spontaneous 121030006400 0 0 feet Quaternary Sinnipee Group Formation Platteville Ancell Group Prairie Du Chien Group Cambrian Dolomite Potosi Formation Franconia Ironton Sandstone Eau Claire Formation

100 200 400 300 500 700 600

Depth

kopee

Claire

Potosi Franconia Ironton Oneota St. Peter St.

Eau Sha- New New

Richmond

CAMBRIAN PRAIRIE DU CHIEN DU PRAIRIE ANCELL Quaternary

14 R log is expressed in ohms meter2/meter. The electromag- side of the SFZ, north of the Franklin Grove Quadrangle. netic induction (EI) log measures formation resistivities and The Ashton Anticline was originally defined by Willman and electrical conductivity in boreholes. This technique is useful Templeton (1951) and Nelson (1995). It is a rather subtle in distinguishing formations that contain water sands and oil- fold that parallels the south side of the Sandwich Fault Zone. bearing muds. The unit of EI log is expressed in ohm. Six oil and gas well records are used for stratigraphic correlations in Sandwich Fault Zone the Franklin Grove area (Fig. 8). The SFZ extends northwesterly from near Manhattan in Will County to west of Oregon in Ogle County, a distance of The Eau Claire Formation is characterized by a trend from about 85 miles. The fault zone separates downthrown Ancell low and moderate SP baselines that are inversed in the R log. Group (Ordovician) on the northeast from upthrown Cambri- This trend is interpreted as an alternation of non-porous and an strata on the southwest. The stress regime is dominantly impermeable dolomitic argillite and dolomite. The Ironton tensional. A minimum throw of 300 feet is estimated in the Sandstone shows a low and flat SP baseline typical of sandy adjacent Daysville and Chana quadrangles located north and rocks. The Resistivity log is principally flat. However, high northwest respectively. Age of the SFZ cannot be specified baselines, respectively at the top and the base in the Schafer’s more precisely than post-Silurian, pre-Quaternary because oil and gas wells API121030006900 and API121030007100, rocks of intervening ages are absent from the region (Kolata indicate a change of lithology that is interpreted as porous et al. 1978; Nelson 1995). sandy dolomite, possibly flooded by freshwater. The Fran- conia Formation shows a moderate and flat SP baseline with Ashton Anticline higher SP baselines at the middle part of the unit. The R The Ashton Anticline is an elongate, linear fold that roughly baselines are respectively low and high from north to south parallels the southwest side of the SFZ from southern of the Franklin Grove area. These baselines are interpreted as DeKalb to south-central Ogle County (Willman and non-porous and impermeable dolomitic sandstone that flood- Templeton 1951; Nelson 1995). The Cambrian Franconia ed by freshwater to the south. The overlying Potosi Dolomite Formation, the oldest bedrock unit at the surface in Illinois, shows low and flat SP and R baselines that are interpreted outcrops or subcrops along the axis of the anticline. In the as an alternation of non-porous and impermeable dolomite, Franklin Grove area, the occurrence of Prairie du Chien stra- locally argillaceous or sandy. The Prairie du Chien Group is ta, overlapping Cambrian rocks in the northeastern part of characterized by an increase- to decrease-upward sequence the adjacent Daysville and Chana quadrangles, which attests in SP and R baselines. These baselines are interpreted as to the southwest flank of the Ashton fold (see cross-section porous and permeable sandy dolomite flooded by freshwater A-A’). The stereographic projection shows preferential direc- that become less porous and permeable sandy dolomite and tions oriented northwest and southeast (Fig. 9). dolomite at the top. The SP and R baselines of the St. Peter Sandstone and Glenwood Formation in the Schafer’s oil and Economic Resources gas well API121030007100 are low and flat, which indicate an abnormal non-porous and impermeable sandstone. The EI Land use baselines show an increase-upward sequence within the St. Most of the Franklin Grove Quadrangle is devoted to Peter Sandstone that can be interpreted as a water-bearing agriculture commodities, primarily row crop farming with sandstone at the top. corn, soybeans, alfalfa hay, and wheat, the prevalent crops (Gradle 2005). Capability of pastures is minor in the area. Structural Geology Since 1970, the Franklin Creek State Natural Area contains a 238-acre natural park that preserves rocky bluffs shelter In north-central Illinois, the structural geology has been a perennial creek and create an environment in which a investigated using aeromagnetic and reflection seismic rich flora and fauna thrive (Illinois Department of Natural surveys by the U.S. Geological Survey and Northeastern Resources 2000). The residential and commercial commodi- Illinois Metropolitan Area Planning Commission during the ties are mainly centered in the Franklin Grove village and 1960s. Drilling samples from exploratory borings and water Lee Center unincorporated community, respectively located wells, logs, geophysical logs, refraction seismography, earth northcentral and southeast of the quadrangle. resistivity profiles, and outcrop studies have identified the Sandwich Fault Zone (SFZ) in northern Illinois (Kolata et al. Groundwater 1978). The Franklin Grove Quadrangle is located along the The stratigraphy of the Franklin Grove is controlled by the western part of the Wisconsin Arch, which is defined as the Sandwich Fault Zone, located north of the Franklin Grove structurally high portion of the area situated north of the SFZ area, dictates flow in northeastern and northcentral Illinois. and separating the Illinois Basin to the south from the Michi- The hydrostratigraphic units in the vicinity south of this fault gan Basin to the northeast and connecting the Wisconsin zone are vertically displaced by more than 100 feet, which and Cincinnati arches (McGinnis and Heigold 1961; Beck the permeability decreases and the cementation increases 1965; McGinnis 1966). A smaller structural feature related (Kolata et al. 1978; Abrams et al. 2015). The Franklin Grove to the Ashton Anticline is present on the south (upthrown) area indicates three types of aquifers: sand and gravel de- 15 Equal Area N Lower Hemisphere 350 10 340 20 330 30

320 40

310 50

300 60

290 70

280 80 10 20 30 70 60 40 90 80 50 W E

260 100

250 110

240 120

230 130

220 140 210 150 200 160 190 170 S N = 13

Figure 9 Stereographic projection of dips and strikes measured within the Franklin Grove area showing preferential directions oriented northwest and southeast. posits of the glacial drift (minor units within 100-200 feet of Stone ground surface), shallow bedrock of Cambrian and Ordovi- The Old Quarry has been actively mined during the early cian age (major units within 500 feet of ground surface), twentieth century. The quarry outcrops approximately 20 and deep bedrock of Cambrian age (units greater than 500 feet thick of pure to crystalline dolomite, locally sandy or feet from ground surface) (Wehrmann and Hlinka 2003). argillaceous, with thin layers of greenish gray shale. This The deep sandstone aquifer of the Cambrian Mt. Simon rock has been commonly used in the construction and build- Sandstone and the lower Eau Claire Formation are the most ing product applications due to its hardness and density. productive groundwater resources, while the shallow sand- Dolomite is generally crushed and sized for use as a road stone aquifer of the St. Peter Sandstone is the most important base material, an aggregate in concrete and asphalt, railroad provider of water in the north-central Illinois. In the Franklin ballast, and rip-rap. Another uses for this rock are the acidity Grove area, where the Prairie du Chien dolomites immedi- control of soil by farmers, the production of magnesium salts ately underlies the glacial drift, aquifers yield low quantities including magnesia, which is used in pharmaceuticals, glass of groundwater. In the St. Peter-Glenwood sandstone aqui- and ceramics manufacture and an oil and gas reservoir rock. fers, sandstones have a moderate coefficient of transmissibil- ity and yield several hundred gallons per minute.

16 Acknowledgments Dapples, E.C., 1955, General Sandstone and Simpson Group: American Association of Petroleum Geologists The authors acknowledge with gratitude the assistance of the Bulletin, v. 39, p. 444-467. Franklin Creek Natural State Area, for permission to access Delpomdor, F.R.A., 2019, Bedrock geology of Rochelle outcrops in the Franklin Creek Natural State Area. We would Quadrangle, Lee and Ogle Counties, Illinois: Illinois like to thank Randy Gittleson at Franklin Grove, to drill on State Geological Survey, USGS-STATEMAP contract his property. Special thanks go to Emily Bunse for prepara- report, 2 sheets, 1:24,000. tion of the LiDAR basemap and the cartography of the map. The authors thank John Nelson for the review of the map and Delpomdor, F.R.A., and H. Wirth, 2018, Bedrock geology report. The new illustrations and tables for this report were of Chana Quadrangle, Lee and Ogle Counties, Illinois: crafted by Franck Delpomdor. This research was supported Illinois State Geological Survey, USGS-STATEMAP in part by the U.S. Geological Survey National Cooperative contract report, 2 sheets, 1:24,000. Geologic Mapping Program (STATEMAP) under USGS Ekblaw, G.E., 1938, Kankakee Arch in Illinois: Geological award number G19AC00310. The views and conclusions con- Society of America Bulletin, v. 49, p. 1425-1430. tained in this document are those of the authors and should Elles, G. L., and E.M.R. Wood, 1907. A monograph of Brit- not be interpreted as necessarily representing the official poli- ish graptolites. Monograph of the Palaeontographical cies, either expressed or implied, of the U.S. Government. Society, 539 p. Ethington, R.L., and D.L. Clark, 1971, Lower Ordovi- References cian conodonts in North America, in Sweet W.C., and Bergström, S.T., eds., Symposium on conodont biostra- Abrams, D.B. et al., 2015, Changing Groundwater Levels in tigraphy: Geological Society of America Memoir 127, the Sandstone Aquifers of Northern Illinois and Southern pp. 63-82. Wisconsin: Impacts on Available Water Supply: Illinois State Water Survey, Contract Report 2015-02, 114 p. Ethington, R.L., and D.L. Clark, 1981, Lower and Middle Ordovician conodonts from the Ibex area, Millard Badiozamani, K., 1973, The dorag dolomitization model- County, Utah: Brigham Young University Geology Stud- -application to the Middle Ordovician of Wisconsin: ies, v. 28, 155 p. Journal of Sedimentary Petrology, v. 43, p. 965-984. 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List of cited well records Vail, P.R., R.M.Jr., Mitchum, and S.Ill., Thompson, 1977, Global cycles of relative changes of sea level, in Payton, Stratigraphic test well C.E., ed., Seismic stratigraphy—Applications to hydro- API: 121032436800 carbon exploration: American Association of Petroleum Farm: Gittleson, Randy Geologists, Memoir 26, p. 83-97. Total depth: 155 feet Watso, D.C., and G. deV. Klein, 1989, Origin of the Cambri- Elevation: 820 feet an-Ordovician sedimentary cycles of Wisconsin using Location: 2570 feet from the west line, 2800 feet from the tectonic subsidence analysis: Geology, v. 17, p. 879-881. south line, sec. 13, T21N, R10E Latitude: N41.8082º; Longitude: W89.2929º Wehrmann, A.H, and H.J. Hlinka, 2003, Illinois Commu- nity Water Supply: Illinois State Water Survey, 1 sheet, Water well 1:750,000 API: 121032429400 Weller, S., 1906, The geologic map of Illinois: Illinois State Farm: Colwell, Robert Geological Survey Bulletin, v. 1, 24 p. Total depth: 1105 feet Williams, S. H., 1994, Revision and definition of the C. Elevation: / wilsoni graptolite Zones (Middle Ordovician) of southern Location: NE¼SW¼NE¼ sec. 25, T22N, R10E Scotland: Transactions of the Royal Society of Edinburg, Latitude: N41.870278 º; Longitude: W89.288611º Earth Sciences, v. 85, p. 143–157. Willman, H.B., 1975, Paleozoic Erathem, in Willman H.B., et API: 121030000100 al., eds., Handbook of Illinois Stratigraphy: Illinois State Farm: Village of Franklin Grove Geological Survey Bulletin, v. 95, p. 33-34. Total depth: 769 feet Elevation: 803 feet Willman, H.B., and T.C. Buschbach, 1975, Ordovician Location: 844 feet from the west line, 1038 feet from the System, in Willman et al., eds., Handbook of Illinois south line, sec. 1, T21N, R10E Stratigraphy: Illinois State Geological Survey Bulletin, Latitude: N41.842195º; Longitude: W89.299912º v. 95, p. 47-87. Willman, H.B., and D.R. Kolata, 1978, The Platteville and Ga- API: 121032057300 lena Groups in Northern Illinois: Illinois State Geological Farm: Illinois State Maintenance and Storage Survey, Circular 502, 75 p. Total depth: 200 feet Willman, H.B., and J.S. Templeton, 1951, Cambrian and lower Elevation: 807 feet Ordovician exposures in northern Illinois: Transactions of Location: SW¼SW¼SW¼ sec. 34, T21N, R10E the Illinois State Academy of Science, v. 44, p. 109-125; Latitude: N41.759083 º; Longitude: W89.341228º reprinted in 1952, Short Papers on Geologic Subjects: Il- linois State Geological Survey, Circular 179, p. 109–125. API: 121032057300 Farm: Shriver, A. Willman, H.R., E. Atherton, T.C. Buschbach, C. Collinson, Total depth: 125 feet J.C. Frye, M.E. Hopkins, J.A. Lineback, and J.A. Simon, Elevation: 807 feet 1975, Handbook of Illinois stratigraphy: Illinois State Location: 600 feet from the east line, 400 feet from the north Geological Survey Bulletin, v. 95, 261 p. line, sec. 6, T20N, R11E Wilson, J.L., 1975, Carbonate Facies in Geologic History: Latitude: N41.752107º; Longitude: W89.282951º Berlin, Heidelberg, New York, Springer-Verlag, 471 p. Witzke, B.J., and R.A. Metzger, 2005, Ordovician conodonts API: 121030010600 and stratigraphy of the St. Peter Sandstone and Glenwood Farm: Hutten, P. Shale, central United States: Bulletins of American Pale- Total depth: 140 feet ontology, v. 369, p. 53–90. Elevation: 830 feet Location: 400 feet from the west line, 400 feet from the Workman, L.E., and A.H. Bell, 1948, Deep drilling and north line, sec. 29, T21N, R11E deeper oil possibilities in Illinois: American Association Latitude: N41.778795 º; Longitude: W89.254900º of Petroleum Geologists Bulletin, v. 32, p. 2041-2062. Worthen, A.H., 1873, Geology and Paleontology: Geological Oil and gas well Survey of Illinois, v. 5, 619 p. API: 121030006400 Company: Natural Gas Pipeline Co. of America

20 Farm: Delhotal, W. Location: 1335 feet from the east line, 108 feet from the Total depth: 694 feet south line, sec. 9, T21N, R10E Elevation: 710 feet Latitude: / ; Longitude: / Location: 50 feet from the east line, 50 feet from the south line, sec. 33, T22N, R10E API: 121030006500 Latitude: / ; Longitude: / Company: Natural Gas Pipeline Co. of America Farm: Delhotal, D. API: 121030007100 Total depth: 481 feet Company: Natural Gas Pipeline Co. of America Elevation: 793 feet Farm: Schafer, W.F. Location: 50 feet from the east line, 875 feet from the south Total depth: 724 feet line, sec. 34, T22N, R10E Elevation: 815 feet Latitude: / ; Longitude: / Location: 76 feet from the east line, 48 feet from the north line, sec. 22, T21N, R10E API: 121030006600 Latitude: / ; Longitude: / Company: Natural Gas Pipeline Co. of America Farm: Sanders API: 121030006400 Total depth: 475 feet Company: Natural Gas Pipeline Co. of America Elevation: 822 feet Farm: Schafer, A.M. Location: 75 feet from the west line, 75 feet from the south Total depth: 545 feet line, sec. 26, T22N, R10E Elevation: 770 feet Latitude: / ; Longitude: / Location: 96 feet from the west line, 30 feet from the south line, sec. 11, T21N, R10E API: 121032059900 Latitude: / ; Longitude: / Company: Northern Illinois Gas Co. Farm: / API: 121030006400 Total depth: 592 feet Company: Natural Gas Pipeline Co. of America Elevation: 773 feet Farm: Schafer, A.M. Location: 585 feet from the east line, 54 feet from the south Total depth: 545 feet line, sec. 5, T21N, R10E Elevation: 770 feet Latitude: / ; Longitude: / Location: 96 feet from the west line, 30 feet from the south line, sec. 11, T21N, R10E API: 121030006900 Latitude: / ; Longitude: / Company: Natural Gas Pipeline Co. of America Farm: Schafer, A.M. API: 121032060000 Total depth: 545 feet Company: Northern Illinois Gas Co. Elevation: 770 feet Farm: / Location: 96 feet from the west line, 36 feet from the south Total depth: 666 feet line, sec. 11, T21N, R10E Elevation: 813 feet Latitude: / ; Longitude: /

21 Plate 1 Outcrop photographs (caption continued on next page). 22 Plate 1 (continued from previous page) a-b) Stratigraphic contact between the New Richmond Sandstone (below) and the Shako- pee Dolomite (above). The New Richmond Sandstone (1) shows approximately 15 feet thick of quartz arenite with low-angle cross- bedding oriented southwest (to the left of the picture). The contact with the Shakopee Dolomite (2) is conformably. Location: Lover’s Leap, SW N1/2 sec. 34, T22N, R10E, N41.850551° W89.336928°. (c) Low-angle cross-bedding, preferentially oriented southwest (to the right of the picture), within the New Richmond Sandstone (1). The Shakopee Dolomite (2) shows at the base thin wavy beds of crystalline dolomite with interbeds of sandstone and sandy dolomite. Location: Whipple’s Cave, SW¼SW¼NE¼ sec. 34, T22N, R10E, N41.849587° W89.336188°. (d) Slightly undulated conformable contact between the New Richmond Sandstone (1) and the Shakopee Dolomite (2-3). The base of the Shakopee Dolomite shows at least 4 feet thick thin wavy beds of alternating dolomite and sandstone, and sandy dolomite (2), then 12 feet thick of massive and thick beds of crystalline dolomite and sandy dolomite that are overlain by thinning-upward massive beds of crystalline dolomite. Location: Senger’s Pond, SW¼NE¼SW¼ sec. 34, T22N, R10E, N41.848746° W89.335050°. (e) Lenticular bedding of the New Richmond Sandstone (1) over the Shakopee Dolomite (2). Location: SE¼NW¼NW¼ sec. 34, T22N, R10E, N41.848746° W89.335050°. (f) Thin beds of crystalline sandy dolomite with interbeds of sandstone. Location: ¼ miles SSE Beecher Brown’s Spring, NW¼NW¼NW¼ sec. 3, T21N, R10E, N41.842681° W89.323156°. (g) Nodules of chert within a bed of dolomite of the Shakopee Dolomite. Location: ¼ miles SSE Beecher Brown’s Spring, NE¼NW¼NW¼ sec. 2, T21N, R10E, N41.842791° W89.320595°. (h-i) Thin wavy beds of burrowed argillaceous dolomite capped by massive beds of dolomite and sandy dolomite. Joint is oriented south-southwest. Cruziana-type trace fossils of Palaeophycus worm forming cylindrical, straight or slightly curved and unbranched subhorizontal burrows are also observed. Location: 1/10 miles NNE Old Quarry, SE¼NW¼SE¼ sec. 34, T22N, R10E, N41.850952° W89.328889°. (j) Cross-bedded sandy dolomite within dolomite. Location: Mill Spring, NE¼NW¼SE¼ sec. 33, T22N, R10E, N41.850960° W89.346716°.

Plate 2 Outcrop photographs. (a) Unconformable contact between the Shakopee Dolomite (1) and the St. Peter Sandstone (2). Location: 1/5 miles SE Mill Spring, NE¼NW¼SE¼ sec. 33, T22N, R10E, N41.850960° W89.346716°. (b) Conglomerate of the Kress Member composed of chert and fragments of Prairie du Chien Group rocks in clayey and sandy matrices. Location: 300 feet south Old Mill Rd. in sec. 34, T22N., R10E, N41.85328° W89.339863°. (c) View of the bluff of St. Peter Sandstone at Mondays Bridge. Location: Mondays Bridge, SW¼SE¼SE¼ sec. 28, T22N, R10E, N41.859651° W89.354440°. (d) Planar parallel beds of quartz arenite (Tonti Member) including NNW-oriented low-angle cross-bedding. Location: Mondays Bridge, SW¼SE¼SE¼ sec. 28, T22N, R10E, N41.859774° W89.354248°.

23 Plate 3 Paleontology. (a) Collenia-type stromatolites forming ½-1 inch of amplitude and 2-3 inches of diameter of close-linked lateral linked hemispheroids. Location: 500 feet NNW Old Quarry in sec. 34, T22N, R10E, N41.850915° W89.328919°. (b) Thalassinoides-type trace fossils marked by T- and Y-shaped branching vertical and horizontal burrows. Location: Location: SW¼SW¼NE¼ sec. 34, T22N, R10E, N41.850871° W89.328928°. (c) Palaeophycus-type trace fossils marked cylindri- cal, straight or slightly curved and unbranched subhorizontal burrows. Location: SW¼SW¼NE¼ sec. 34, T22N, R10E, N41.850871° W89.328928°. (d) Planolites-type trace fossils marked by cylindrical, straight and tortuous horizontal burrows. Location: SW¼SW¼NE sec. 34, T22N, R10E, N41.850760° W89.328896°.

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