Prepared and Published with the Support of COUNTY ATLAS SERIES THE BLUE EARTH COUNTY BOARD OF COMMISSIONERS AND ATLAS C-26, PART A MINNESOTA GEOLOGICAL SURVEY the Minnesota Environment and Natural Resources Trust Fund Blue Earth County Harvey Thorleifson, Director as recommended by the Legislative-Citizen Commission on Minnesota Resources Plate 2—Bedrock Geology

NIC OLL ET COU NTY

7 Minnesota B BEDROCK GEOLOGY Ka 44°15' N. 285 300 R. 29 W. Kd m STRATIGRAPHIC COLUMN River 285 By 240 s

94° W. e

i Lithostratigraphic

r Composite natural gamma log 255 NICOLLET LE SUEUR COUNTY R. 26 W. COUNTY R. 25 W. e unit S  270 - e 315 Increasing count 330 Era m Lithology Kd  e T. 109 N. sl Os 315 Julia R. Steenberg t Group, 0 100 s

24 Map symbol 19 y 300 300 24 24 Formation 270 lr 19 S API-G units

Oo Thickness (in feet) 300 River 285 JAMESTOWN 345 94°15' W. 255 Kd Wita Kd Creek 315 2012  315 )68 j Kd Lake j Dakota Kd 5-90 m.y.)

Upper Formation

Os Y

300 (99.6-93.5

T Duck 300 lr Morgan Os Lake N T. 109 N. CAMBRIA U

315

O

300 Minnesota 240 Ballantyne

C

LOCATION DIAGRAM Oo 300 Lake R 270

Madison MESOZOIC R. 28 W. U 31 Kd 300 31 Oo Unnamed Ka Os E

Kd 30-90 36 31 Eagle Lake lr lr 36 315 36 U

36 S Cretaceous 300 ) 31 300 22 (112-93.5 m.y.) (112-93.5 Gilfillin 315 E Lower to Upper Lake L Judson Lake 315 300 lr 240

1 6 Y 6 240 Os Platteville Formation 255 315 T Opg 6 Madison 5-20 1 1 N 6 Ph 1 255 U

sl 255 1 CORRELATION OF MAP UNITS O )60 Upper 315 315

C Lake

A

j (450 m.y.) 315 C ? E Kd Upper Cretaceous

285 14 Kd S Lithology Key

315 A St. Peter Osp W unconformity MESOZOIC

Mankato Mud Os 90-100 Lake   Ka Lower to Upper Cretaceous? w j R. 27 W. Armstrong 300 )68 315 Lake Os 315 unconformity Lieberg Middle (458 m.y.) 315 Dolostone Lake Eagle Os T. 108 N. 300 300 Opg Lake Upper n a Oo 255 Kd i c Sandy dolostone  i sl  v

255 o j 315 T. 108 N. Osp 169  Middle Ordovician d 255 j )83 r Ops Sandstone A 285 A' O Oo unconformity Very fine- to fine-grained Minneopa 300 Oo Oo Shakopee 300 300 Oo j 300 Os 300 300 315 Os Formation Creek 80-140 Fine- to medium-grained Kd 300 300 300

Y 315 300 Kd Born T unconformity 240 Kd

N Lower Ordovician BUTTERNUT VALLEY MANKATO Lake U JUDSON SOUTH BEND Kd 300 LERAY Medium- to coarse-grained 300

315 O Oo j 300

285 C

Kd

N Oo 315 unconformity Kd 315  Lower (488-478 m.y.) Shaly Prairie du Chien Group W 31 300 j PALEOZOIC O 36 31 285 300 R Lily 270 Kd B Lake 300 36 31 31 Oneota 300 Oo36 31 300 300 36 36 sl sl Kd Oo Siltstone Crystal 300 300Oo 60-100 Le Sueur Upper Lake 300  270 315 lr Shale 300 1 6 6 1 6 Kd 1 Loon 1 6 unconformity 255 6 315  Lake Crystal Lake  300 1 w Chert 300 j Kd 315 Kd  300 e Oolites )60 lr Jordan River Oo Os  300 Kd 255 Middle Cambrian Sandstone j )66 St. Clair  G Glauconite m 60-100

WATONWAN COUNTY WATONWAN Mills 255 Oo Lake Oo Oo unconformity Ph Phosphate grains 300  300 lr j Kd Le Sueur Phs Algal domes; stromatolites Blue j 300 unconformity j T. 107 N. LINCOLN 285 G (not used in limestone or GARDEN CITY Oo 315 dolostone units) Earth RAPIDAN DECORIA St. Lawrence C River MCPHERSON sl G Burrowed T. 107 N. Formation sl G 45-80 River  PALEOZOIC Pebbles (gravel in unconsolidated units) j 315 G 300 G River Flat-pebble conglomerate

Cobb G Garden Kd Rice Cross-bedded (festoon) Lake G City 285 300 River 300 285 300 315 MAP SYMBOLS Cross-bedded (planar to tangential) Lone Rock G lr G

Upper Cambrian (501-488 m.y.) Formation Geologic contact—Approximately located. G Ripple cross-laminations Oo 100-130 31 36 31 300 Bedrock outcrop. 36 31 315 G 36 31 31 City Group Tunnel Dolomitic 36 Osp Location of geologic cross section. G 36 A A' 300 315 G G Calcareous 300 G Os Contact marks a major erosional surface 6 1 285 285 300 sl 6 285 1 6 1 6 300 Cobb Wonewoc 300 Watonwan 1 6 )83 1 w Facies change Maple Sandstone

315 50-80

River Good River 285 Kd Thunder 315 93°45' N. This gamma log is a compilation of five borehole Little Os geophysical logs collected from water wells in Blue 315 Pemberton G G 93°45' N. 800 Earth and Nicollet Counties; unique numbers 270341,  sl 300 770449, 213694, 213696, and 209876. G G M Eau Claire 300 315 e G M 60-80

 Formation G

sl Severson315 300 Perch Lake F Kd Perch  Lake

T. 106 N. sl

300 Creek VERNON CENTER F sl LYRA BEAUFORD 285 MEDO 315 CERESCO  T. 106 N. j M

Oo Figure 1. Elevation of the stratigraphic

lr 315 Vernon Cobb F tops of the Jordan and Wonewoc  285

j Center Kd 315

315 300 315 in Blue Earth County, 315 River 800 F 800 315 315 showing the geologic structure of 750 Paleozoic bedrock. Contour interval is

M Mt. Simon  F 50 feet (15 meters), scale 1:450,000. In m 315 Sandstone 300 315 areas where some or all of the Jordan 31 Oo Cottonwood Middle Cambrian (504-501 m.y.) 36 31 36 31 Lake 700 and Wonewoc Sandstones are missing

650 approximately 150-350 315 36 31 36 36 31 650 because of , the contours are 700 300 315 300 inferred from projection vertically 315 315 315 from other formation contacts, using

6 Kd

6 315 1 1 F 6 169 1 6 the estimated full thickness of the 1 6 1 900 Willow eroded formations, and shown with Mapleton Os 600

315 800 315 850 dashed line. The inferred traces of Creek 315 315 750 Ida Maple folds are shown. MESOPROTEROZOIC Lake Phs Perch (1200-900 M.Y. AGO)

EXPLANATION thousands j 315 Knights Top of the Jordan Sandstone PLEASANT River sl Lake Osp 315 Os Top of the Wonewoc Sandstone Creek Blue

315

Osp Axial trace, syncline MOUND 315 SHELBY Oo STERLING MAPLETON DANVILLE MF 315 Axial trace, anticline T. 105 N. 315 )30 )30 Earth )30 T. 105 N. S N Amboy Rice 315 River Y 330 Lura T 315

N Lake

U

315 Maple 315 O Kd 315 C Slough Y Kd

Creek T

N

330 N A Opg

Big U

W 315 )22 O

N

Os Cobb River C

O 31 315 T 36 36 31 A

A 31 36 31 C Hagar City Member

31 River E W 315 36

36 S Figure 2. Map of the first vertical derivative

315 Opg A MARTIN COUNTY R. 29 W. W F Oneota FARIBAULT COUNTY R. 28 W. magnetic anomaly data of Blue Earth County 94°15' W. R. 27 W. R. 26 W. Dolomite 94° W. R. 25 W. FARIBAULT COUNTY (Chandler and Lively, 2007). Lighter colors Digital base modified from the Minnesota Department of B' 800 Transportation BaseMap data; digital base annotation by indicate more magnetic. The structure contours GIS compilation by Julia Steenberg 750 the Minnesota Geological Survey. SCALE 1:100 000 of the stratigraphic top of the Wonewoc Sandstone

Edited by Lori Robinson 775 M 1 0 1 2 3 4 5 MILES Elevation contours were derived from the U.S. Geological F 500 are overlain on this image showing a prominent

Survey 30-meter Digital Elevation Model (DEM) by the anticline feature in the western portion of the map Blue Earth 1 0 1 2 3 4 5 6 7 8 KILOMETERS F siltstone Minnesota Geological Survey. area, based on the current well coverage. There is a Universal Transverse Mercator Projection, grid zone 15 550 Coon Valley subtle contrast in trends of the magnetic anomalies Member 1983 North American Datum contour interval 15 METERS Kasota ? from southwest–northeast in the southwestern sandstone ? 725 portion of the map area to southeast–northwest G G ORDOVICIAN 700 in the west-central part of the map area, possibly ? ? CAMBRIAN 600 reflecting a deep basement fold or fault structure. Jordan 650 This trend appears to coincide with the structural Sandstone cross section B-B' A A' grain of the Wonewoc Sandstone, suggesting that 1 meter 1,000 1,000 Paleozoic bedrock was displaced by reactivation unconsolidated Oo Oo Kd Oo unconsolidated Quaternary sediment Kd Os of the deep basement structural feature. Black 800 sl j Oo Os 800 Figure 3. Schematic stratigraphic section through the Late Cambrian Jordan Sandstone and the  Oo line shows county boundary and pink lines show lr j Early Ordovician depicting the stratigraphic relationships within the Hager City  600 w sl 600 structure contours. Inferred folds are also shown e and Coon Valley Members of the Oneota Dolomite from the Mankato area (N) to southeast Blue lr in black; scale 1:500,000. 400 m  400 Earth County (S). The Kasota sandstone, though equivalent in age to the lower part of the Coon ? w e Valley Member (Early Ordovician), was mapped as Jordan Sandstone (Late Cambrian) because 200 ? Archean and rocks, undivided m 200 ? Phs the units cannot be distinguished from each other based on subsurface data. See stratigraphic 0 0 column for lithology key. Elevation (feet above sea level) Vertical exaggeration = 10x

B cross section A-A' B' 1,000 Opg 1,000 Oo j Ka unconsolidated Quaternary sediment unconsolidated Quaternary sediment Osp   Os 800 Kd sl j 800 lr Oo   w j Every reasonable effort has been made to ensure the accuracy of the factual data on which this map 600 e  600 sl interpretation is based; however, the Minnesota Geological Survey does not warrant or guarantee that lr m there are no errors. Users may wish to verify critical information; sources include both the references listed 400 w 400  here and information on file at the offices of the Minnesota Geological Survey in St. Paul. In addition, effort ? e 200 has been made to ensure that the interpretation conforms to sound geologic and cartographic principles. m 200 Archean and Paleoproterozoic rocks, undivided ? No claim is made that the interpretation shown is rigorously correct, however, and it should not be used to 0 Phs 0 guide engineering-scale decisions without site-specific verification. ? Phs Elevation (feet above sea level) Vertical exaggeration = 10x

INTRODUCTION DESCRIPTION OF MAP UNITS grains. The Platteville Formation is only present in the extreme southeastern part of contains Ordovician-age fauna (Powell, 1935; Stauffer and Thiel, 1941) equivalent in Glauconite is present in the upper part of the formation and absent in the remainder. Chandler, V.W., Lively, R.S., and Wahl, T.E., 2004, Gravity and aeromagnetic data grids of Minnesota: On the county overlying the Glenwood Formation. age to the lower part of the Coon Valley Member. The lithic similarity of the Kasota The formation is present throughout the subsurface of Blue Earth County and subcrops file at the Minnesota Geological Survey. This geologic map depicts the type, structure, and distribution of bedrock units in Blue Earth Mesozoic Rocks Glenwood Formation—Greenish-gray, calcareous shale containing fine- to medium-grained, sandstone to the Jordan Sandstone makes it impractical to map as a separate unit using only in the extreme northwestern portion of the map area. The Wonewoc Sandstone Chandler, V.W., McSwiggen, P.L., Morey, G.B., Hinze, W.J., and Anderson, R.R., 1989, Interpretation of County that are either exposed at the land surface or lie directly beneath unconsolidated Quaternary Dakota Formation (Upper Cretaceous)—Variegated clay, siltstone, sandstone, and well-rounded quartz sand and sand-sized phosphate grains. The is subsurface data, therefore it was mapped as the Jordan Sandstone (unit _j). The Blue ranges in thickness from 50 to 80 feet (15 to 24 meters). Contacts with formations seismic reflection, gravity, and magnetic data across Middle Mid-Continent Rift System, deposits of variable thickness (see Plate 3, Surficial Geology). It shows how the bedrock would appear Kd conglomerate that lies above Paleozoic bedrock and below Quaternary deposits. The estimated to be only 5 to 10 feet (1.5 to 3 meters) thick and at the scale of this map has Earth siltstone along the Minnesota River is generally not thicker than 3 feet (1 meter) above and below are conformable and transitional. A subtle unconformity is present northwestern Wisconsin, eastern Minnesota, and central Iowa: American Association of Petroleum if it were viewed from above and all the overlying Quaternary deposits were removed. The bedrock sandstone is mostly quartzose, light gray, white, and orange. Grains are fine- to nearly the same lateral distribution as the overlying Platteville Formation; therefore, and contains a high abundance of Ordovician-age conodonts equivalent in age to the in this formation (Runkel and others, 1998); however, it is not practical to subdivide Geologists Bulletin, v. 73, p. 261-275. units of Blue Earth County consist of sedimentary rocks of Paleozoic and Mesozoic age that form coarse-grained and angular to well-rounded. The sandstone is generally friable and they are mapped as one unit. upper Coon Valley Member. the formation into two separate units on the basis of subsurface data. Graham, W.A.P., 1933, Petrology of the Cambrian-Ordovician contact in Minnesota: Journal of Geology, distinguishable and mappable layers designated as formations, such as the St. Lawrence Formation. locally well-cemented with and iron-oxide cement. Thin conglomerate beds Large, meter-scale, solution-enlarged vugs and fractures are particularly common _ (Middle to Upper Cambrian)—Variegated light green, feldspathic v. 41, p. 468-486. These units are locally exposed in bluffs, riverbanks, rock quarries, and roadcuts, particularly along Osp St. Peter Sandstone (Middle to Upper Ordovician)—Fine- to medium-grained, well- e are interstratified with quartz sandstone and contain highly polished pebbles of vein in exposures of the lowermost Hager City Member along the Minnesota River from siltstone, shale, and fine-grained sandstone with subordinate light pink dolomite. Shale Hu, S., Jarzen, D.M., and Dilcher, D.L., 2008, New species of angiosperm pollen from the Dakota Formation the Minnesota River and its tributaries. rounded, white to tan, quartzose sandstone that subcrops in the southeastern portion of quartz, Paleozoic chert, and rare claystone clasts. Thin to thick beds of clay and Blue Earth County. The unit is 90 to 100 feet (27 to 30 meters) thick where overlain Mankato to St. Peter. These features are entirely filled with a gray to white shale, is dominantly green, white, and red. Sandstone is white to tan to pink and well-cemented (Cenomanian, Upper Cretaceous) of Minnesota, U.S.A.: Palynology, v. 32, p. 17-26. Characteristics of each formation are given in the stratigraphic column and in the description siltstone of variable colors including light to dark gray, white, blue, green, tan, and red by the Platteville and Glenwood Formations, but may be much thinner where partially siltstone, and sandstone (Fig. 3). The similarity of this material to the underlying Blue with calcite. Glauconite and white to brown linguliform brachiopod shells are present Morey, G.B., 1977, Revised Keweenawan subsurface stratigraphy, southeastern Minnesota: Minnesota of map units. The accompanying bedrock geologic cross sections add the dimension of depth and are interbedded within the sandstone. Black lignite, leaf impressions, and phosphatic eroded. The lower 30 feet (9 meters) includes variegated beds of white to gray shale, Earth siltstone, Kasota sandstone, and Jordan Sandstone, as well as the deformational throughout the formation and are most commonly associated with the dolostone beds. Geological Survey Report of Investigations 16, 67 p. illustrate stratigraphic relationships of the bedrock units, their structure, topography, as well as the fish material have also been found in outcrops and drill cuttings in the clay, siltstone, siltstone, and poorly sorted quartz sandstone. The basal contact of the formation with fabrics in these units in some outcrops, suggests much of the fill was injected upward The Eau Claire Formation is 60 to 80 feet (18 to 24 meters) thick. This formation Mossler, J.H., 2003a, Bedrock geology of the Mankato East quadrangle, Blue Earth and Le Sueur Counties, variable thickness of the overlying Quaternary deposits. The geologic formations are thin in relation and sandstone. These rocks are present throughout Blue Earth County as scattered the underlying Shakopee Formation of the Prairie du Chien Group is an erosional into the lower Hager City Member voids (Graham, 1933; Powell, 1935). Other outcrops subcrops at the base of a buried bedrock valley in the extreme northwestern portion Minnesota: Minnesota Geological Survey Miscellaneous Map M-136, scale 1:24,000. to their areal extent and would only be one-tenth as thick as shown on the cross sections if no vertical erosional remnants of variable thicknesses ranging from 5 to 90 feet (1.5 to 27 meters). unconformity representing a significant hiatus (Smith and others, 1993). There appears show a clear gradational contact between the cavity fill and weathered carbonate of the county. The contact with the Mt. Simon Sandstone is gradational and contains ———2003b, Bedrock geology of the Mankato West quadrangle, Blue Earth, Le Sueur, and Nicollet Counties, exaggeration were used. The exaggeration needed to show the thin rock formations gives the appearance They have been found to unconformably overlie the Wonewoc Sandstone, Lone Rock to be up to 30 feet (9 meters) of erosional relief on the top of the Shakopee Formation rock of the Hager City Member, representing a residuum of dissolved carbonate. A beds of medium- to coarse-grained Mt. Simon Sandstone interbedded within the very Minnesota: Minnesota Geological Survey Miscellaneous Map M-135, scale 1:24,000. of steeper slopes on the land surface and bedrock topography. In areas where bedrock is concealed Formation, St. Lawrence Formation, Jordan Sandstone, Oneota Dolomite, Shakopee based on the limited number of well logs that penetrate this contact in southeastern high abundance of conodonts in some samples of fill might reflect a concentration of fine- to fine-grained sandstone of the Eau Claire Formation. ———2008, Paleozoic stratigraphic nomenclature for Minnesota: Minnesota Geological Survey Report by a thick package of Quaternary , the geology is strongly dependent on borehole records Formation, and the undifferentiated unit (Ka) over a wide range of elevations between Blue Earth County. phosphatic material as part of the residuum. The origin of the fill material remains _ Mt. Simon Sandstone (Middle Cambrian)—Fine- to coarse-grained, white to light gray of Investigations 65, 76 p. and is generalized where borehole data are sparse. m 750 and 900 feet (229 to 274 meters) above sea level. They are interpreted to represent Prairie du Chien Group (Lower Ordovician)—Dominantly dolostone interbedded with lesser problematic and most likely has no single origin. to pale yellowish-brown, quartzose sandstone with thin interbeds of white and green Powell, L.H., 1935, A study of the Ozarkian faunas of southeastern Minnesota: St. Paul, Minn., St. Paul The production of this map, cross sections, and stratigraphic column relied on several different sources the easternmost extension of a large, nearly continuous subcrop area of strata generally Institute of Science Museum Bulletin 1, 80 p., 17 pls. amounts of quartz sandstone. The group is divided into two formations, the Shakopee _j Jordan Sandstone (Upper Cambrian)—Dominantly white, light gray and tan, fine- to feldspathic shale and siltstone. The sandstone is generally very friable to poorly of information including outcrop mapping, water-well and scientific drilling records (including holes correlative with the Upper Cretaceous Dakota Formation (Setterholm, 1990). Formation and the underlying Oneota Dolomite. Together they range in thickness coarse-grained, quartz sandstone, siltstone, and shale. The Jordan Sandstone is cemented. Pebbles of vein quartz are present near its base and scattered white Rongstad, J.M., and Wohlfeil, J.D., 1991, Bedrock geology, in Geologic atlas Blue Earth County, Minnesota: drilled for this project), drill cuttings, drill core, borehole geophysical logs (see Plate 1 for distribution The mapped distribution of the Upper Cretaceous rocks on this map is more from 230 to 250 feet (70 to 76 meters), but commonly are much thinner where they characterized by two interlayered facies (Runkel, 1994). They are a medium- to coarse- linguliform brachiopod shells are common in the upper one-third of the formation. The Mankato, Minn., Mankato State University Water Resources Center, 23 p. of these data), and published geologic maps of parts of Blue Earth and adjacent counties (Sloan, 1964; speculative than for other map units because prediction of the subsurface distribution are present as uppermost bedrock and have been partially eroded. These formations grained, cross-stratified, friable, quartzose sandstone more common near the top; and Mt. Simon Sandstone is 150 to 350 feet (46 to 107 meters) thick. It unconformably Runkel, A.C., 1994, Deposition of the uppermost Cambrian (St. Croixan) Jordan Sandstone, and the nature Rongstad and Wohlfeil, 1991; Mossler, 2003a, b; Runkel and Mossler, 2004a, b; Runkel, 2005a, b). of these strata is rendered difficult by the presence of profound unconformities were subdivided in the map area based mainly on well cuttings and geophysical logs a very fine-grained, bioturbated, feldspathic sandstone with lenses of siltstone and overlies the Mesoproterozoic Hinckley Sandstone throughout most of Blue Earth of the Cambrian-Ordovician boundary in the upper Mississippi valley: Geological Society of America The Paleozoic rocks of Blue Earth County are characterized by thin, widespread layers of sandstone, bounding these strata, and because information from water-well records alone is and to a lesser degree on drilling logs. shale most common near the base. The Jordan Sandstone is 60 to 100 feet (18 to 30 County and undivided Archean and Paleoproterozoic rocks near the western border Bulletin, v. 106, p. 492-506. shale, and deposited in shallow seas during the Paleozoic era from about 500 to 450 ———2005a, Bedrock geology of the Cambria quadrangle, Blue Earth, Brown, and Nicollet Counties, typically inadequate to consistently recognize these strata as distinct from quartz-rich Shakopee Formation—Yellow-gray to grayish-orange, thin to thickly bedded, sandy, meters) thick where it is undissected by erosion. Abundant exposures of the Jordan of Blue Earth County. million years ago. The Cambrian age formations are dominated by siliciclastic os Minnesota: Minnesota Geological Survey Miscellaneous Map M-150, scale 1:24,000. Quaternary sand, organic-rich Quaternary lake sediments, or from clay-rich, in situ oolitic dolostone divided into two members. The Willow River Member is composed Sandstone can be seen along the Minnesota, Blue Earth, and Le Sueur Rivers. The including sandstone, siltstone, and subordinate shale. Carbonate rock occurs only as relatively thin residuum developed on Paleozoic bedrock. Compared to the patchy strata classified as of a light brown to grayish-orange, thin to medium bedded, sandy, oolitic, intraclastic Jordan Sandstone subcrops in the central portion of the map area and within deeply Mesoproterozoic Rocks ———2005b, Bedrock geology of the Courtland quadrangle, Blue Earth, Brown, and Nicollet Counties, layers in these units. Ordovician age formations, in contrast, are dominated by carbonate rock, such Minnesota: Minnesota Geological Survey Miscellaneous Map M-149, scale 1:24,000. age in eastern Blue Earth County, strata in western Blue Earth County dolostone. Meter-scale microbial mounds, chert nodules, fine- to coarse-grained incised and buried bedrock valleys. An unconformity exists between the Jordan as the Prairie du Chien Group and the Platteville Formation. The Paleozoic rocks are unconformably Phs Hinckley Sandstone, Fond du Lac Formation, and Solor Church Formation (shown can be assigned to the Late Cretaceous period with some confidence. Interbedded quartz sandstone beds, and gray-green shale partings are common. The underlying Sandstone and the overlying Oneota Dolomite that represents a major hiatus (Runkel Runkel, A.C., McKay, R.M., Miller, J.F., Palmer, A.R., and Taylor, J.F., 2007, High resolution sequence overlain by scattered erosional remnants of unconsolidated clay and sandstone deposits of Mesozoic age. only on cross sections) , siltstone, and medium-grained sandstone from drill core near the town of New Richmond Member is a yellow-gray, fine- to coarse-grained quartz sandstone and others, 1999, 2007). However, due to the complex stratigraphic relationship at stratigraphy of lower Paleozoic sheet sandstones in central North America: The role of special conditions These rocks were probably deposited continuously across the Paleozoic bedrock surface as sediment Hinckley Sandstone and Fond du Lac Formation—The Hinckley Sandstone is salmon- of cratonic interiors in development of strata architecture: Geological Society of America Bulletin, Cambria is lithologically similar to well-dated , siltstones, and sandstones and sandy dolostone. As much as 16 feet (5 meters) of the New Richmond Member this contact (Fig. 3), the medium- to coarse-grained Ordovician-age Kasota sandstone in a fluvially-dominated delta plain on the shallow, eastern margin of the Western Interior Seaway colored, very fine- to medium-grained, moderately to well sorted, quartzose sandstone, v. 199, p. 860-881. from three nearby localities in Nicollet and Brown Counties. Sedimentary rocks from was once exposed in quarries east of the Minnesota River from Mankato to Le Sueur is mapped as Jordan Sandstone in Blue Earth County because the two units cannot be that covered much of the western interior of North America during the Late Cretaceous period. Due siltstone, and shale. Sandstone is variably cemented with quartz. Red-brown shale and these localities provide a rich and diverse assemblage of palynomorphs containing County (Stauffer and Thiel, 1933; Austin, 1971). In eastern Blue Earth County the distinguished on subsurface data alone. Runkel, A.C., McKay, R.M., and Palmer, A.R., 1998, Origin of a classic cratonic sheet sandstone: Stratigraphy to subsequent erosion, these deposits have been separated into scattered, discontinuous outliers. pollen taxa that are considered to be no older than Late Cretaceous (late Cenomanian) siltstone are present in the lower part of the formation. The Fond du Lac Formation is across the Sauk II-Sauk III boundary in the upper Mississippi valley: Geological Society of America Shakopee Formation ranges from 80 to 140 feet thick (24 to 43 meters) and appears _sl St. Lawrence Formation (Upper Cambrian)—Light gray, pale yellowish-green to light reddish-orange to yellow-brown, feldspathic sandstone, siltstone, and shale. Sandstone Bulletin, v. 110, p. 188-210. Paleozoic bedrock units lie disconformably on an eastward-thickening sequence of Mesoproterozoic in age (Hu and others, 2008). to thicken eastward toward the center of the Hollandale Embayment. pink, dolomitic, feldspathic siltstone with interbedded, very fine-grained sandstone rocks of the Keweenawan Supergroup that accumulated as part of the development of the Midcontinent is poorly sorted with grains that tend to be subrounded to angular (Morey, 1977; Runkel, A.C., Miller, J.F., McKay, R.M., Shaw, T.H., and Bassett, D.J., 1999, Cambrian-Ordovician boundary Ka Undifferentiated (Lower to Upper Cretaceous?)—Interbedded pale red-brown to pale Oneota Dolomite—Mostly very thick to thickly bedded, light brown to grayish-orange, and shale. Dolostone and dolomitic sandstone beds typically have shale and sandstone Rift System. The rift formed during a thermo-tectonic event 900 to 1,200 million years ago that oo Boerboom, 2001). The combined thicknesses of the Fond du Lac Formation and strata in the central mid-continent of North America: Acta Universitatis Carolinae Geologica, v. 43, olive siltstone, clay, and very fine- to medium-grained sandstone with calcareous and finely crystalline, microbial dolostone that except in its lowermost part generally lacks intraclasts at the base. Laminated green shale and pink to red dolomite interbeds involved extension of the earth's crust with concurrent faulting, mafic volcanism, plutonism, and Hinckley Sandstone are interpreted to be up to thousands of feet in the easternmost p. 17-20. iron-oxide cement occur in the northwesternmost part of Blue Earth County. Similar sedimentary features such as oolites and quartz sand characteristic of the Shakopee are particularly abundant in the lower half of the formation. The glauconite content later deposition of a thick sequence of sedimentary rocks (Chandler and others, 1989). These rocks part of Blue Earth County based on the sparse number of wells that penetrate these Runkel, A.C., and Mossler, J.H., 2004a, Bedrock geology of the Good Thunder quadrangle, Blue Earth outcrops across the Minnesota River in Nicollet County also contain concretionary Formation. Exposures of the Oneota Dolomite are common in Blue Earth County in the dolostone decreases toward the top of the formation and the contact with the are poorly known in Blue Earth County, represented by only a few drill cuttings and gamma logs. units. However, the Hinckley Sandstone is discontinuous and represents only a small County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-146, scale 1:24,000. carbonate within dark red and green clay, siltstone, and sandstone beds. This unit ranges where bedrock is at or near the surface. The Oneota Dolomite ranges from 60 to 100 Jordan Sandstone is conformable and gradational. The contact with the underlying Therefore, they cannot be assigned to a particular formation but likely correlate to parts of the Hinckley fraction of that thickness. ———2004b, Bedrock geology of the Judson quadrangle, Blue Earth and Nicollet Counties, Minnesota: from 30 to 90 feet (9 to 27 meters) thick in Blue Earth County and unconformably feet (18 to 30 meters) thick, thickening to the east toward the center of the Hollandale Lone Rock Formation is also gradational. The St. Lawrence Formation is typically Sandstone, Fond du Lac Formation, and Solar Church Formation. Counties that border Blue Earth Solar Church Formation—Interbedded sandstone, siltstone, and shale. Sandstone and Minnesota Geological Survey Miscellaneous Map M-145, scale 1:24,000. overlies both the Eau Claire and the Mt. Simon Formations between 700 and 730 feet embayment in a manner similar to the Shakopee Formation. 45 to 80 feet (14 to 24 meters) thick where it is undissected by erosion. It subcrops County to the west and northwest have a small number of drill cuttings and gamma logs that show (213 and 223 meters) above sea level. This unit has not yielded fossils in Blue Earth west of the erosional edge of the Jordan Sandstone and occurs at the base of several siltstone are generally red to pale reddish-brown and the shale is dark reddish-brown Setterholm, D.R., 1990, Geologic maps of the Late Cretaceous rocks, southwestern Minnesota: Minnesota The Oneota Dolomite in this area has been divided into a number of formal Paleozoic bedrock directly overlies several older Proterozoic and Archean rock units, such as the County, although gymnospermous pollen and dinoflagellate cysts extracted from deeply incised bedrock valleys throughout Blue Earth County. Exposures can be seen in color although dark to light greenish-gray shale beds occur at depth (Morey, 1977). Geological Survey Miscellaneous Map M-69, 2 pls., scale 1:750,000. members and informal beds (Fig. 3). Its carbonate dominated upper part, the Hager and the Morton Gneiss, marking the western boundary of preserved sediments related lithologically similar strata from drill core in McLeod County near Plato, Minnesota along the Minnesota River near Judson. Sandstone is poorly to moderately sorted with angular to subrounded grains. The rock Sims, P.K., and Zeitz, I., 1967, Aeromagnetic and inferred paleogeographic map of east-central City Member, consists of very thick to thickly bedded, finely crystalline dolostone to the Midcontinent Rift System. are indicative of a latest Albian to Cenomanian age (Early to Late Cretaceous; Ravn, is mineralogically immature and contains appreciable amounts of feldspar grains and Minnesota and part of Wisconsin: U.S. Geological Survey Geophysical Investigations Map GP-563, that has been extensively quarried for building stone near Mankato (Stubblefield, _lr Lone Rock Formation (Upper Cambrian)—Variegated sandstone, siltstone, and shale The distribution of uppermost bedrock units on the map is primarily influenced by the location of 2009, unpub. data). volcanic rock fragments (Morey, 1977). It may be carbonate rich, especially in its 6 p., scale 1:250,000. 1971). It ranges from 55 to 80 feet (17 to 24 meters) thick. The underlying Coon with subordinate beds of dolostone ranging from 100 to 130 feet (30 to 40 meters) in deep, buried river channels that are incised into the bedrock (see Plate 6, Bedrock Topography), and a upper part. The Solar Church Formation is thousands of feet thick like the Fond du Sloan, R.E., 1964, The Cretaceous system in Minnesota: Minnesota Geological Survey Report of Valley Member is a heterolithic unit containing dolostone, siltstone, shale, and thickness. Sandstone is yellowish-gray to green, very fine- to fine-grained, quartzose, gentle, overall eastward dip of the Paleozoic rock (less than 2°). This dip is part of the western margin Paleozoic Rocks Lac Formation. However, its thickness is extremely variable because of faulting of Investigations 5, 64 p., 2 pls. sandstone reaching thicknesses of 20 feet (6 meters). Throughout most of Blue Earth feldspathic, and glauconite-rich. It is commonly interbedded with greenish-gray shale of the broad structural depression known as the Hollandale embayment. As a result, progressively the Mesoproterozoic rocks. Smith, G.L., Byers, C.W., and Dott, R.H., Jr., 1993, Sequence stratigraphy of the lower Ordovician Prairie Nomenclature has been revised for some of the Paleozoic rock formations in Minnesota, and County the Coon Valley Member is represented by a thinly bedded, sandy, oolitic, partings. Mica is abundant along laminations in the sandstone and siltstone. Dolostone older bedrock formations subcrop from eastern to western Blue Earth County. The Cretaceous age du Chien Group on the Wisconsin arch and in the Michigan basin: American Association of Petroleum several formation names formerly in use at the Minnesota Geological Survey have been replaced by and intraclastic dolostone with microbial laminations, mounds, and minor amounts of is light tan to pink, contains sandstone intraclasts, and becomes more abundant toward deposits do not appear to have a similar distribution that corresponds to the structural grain of the Geologists Bulletin, v. 77, p. 49-67. names that are widely accepted elsewhere in the region for the same units. Rocks formerly referred glauconite. Outcrops of this dolostone can be found along the Watonwan River near the base of the formation. Glauconite is present throughout but is most abundant in the REFERENCES Paleozoic units, which may indicate most of the folding predates their deposition. Structure contours Stauffer, C.R., and Thiel, G.A., 1933, and marls of Minnesota: Minnesota Geological Survey to as the are now named the Tunnel City Group and include the Lone Rock, Garden City. This unit is overlain by a 1 to 3 foot (1 meter) bed of white to green, basal part of the formation. Linguliform brachiopod fragments are rare but do occur drawn on the stratigraphic tops of the Jordan and Wonewoc Sandstones (Fig. 1) show several fold Austin, G.S., 1971, The stratigraphy and petrology of the Shakopee Formation, Minnesota: Iowa City, Bulletin 23, 193 p. Mazomanie, and Davis Formations, and the sandstone interval formerly referred to as the Ironton and thinly laminated, glauconitic, feldspathic, dolomitic siltstone informally referred to in the lower half of the formation. This unit subcrops across the westernmost part of structures in the county and the gentle eastward dip of the Paleozoic rock. University of Iowa, Ph.D. dissertation, 216 p. ———1941, The Paleozoic and related rocks of southeastern Minnesota: Minnesota Geological Survey Galesville Sandstones is now assigned to the Wonewoc Sandstone. More detailed discussion of these as the Blue Earth siltstone. Blue Earth County. The uppermost part of the formation is exposed in small outcrops The structural contours of the Wonewoc Sandstone show a broad anticline in western Blue near Judson on the north banks of the Minnesota River. The upper contact of the Lone Boerboom, T.J., 2001, Bedrock geologic map and sections, pl. 2 of Boerboom, T.J., project manager, Bulletin 29, 261 p. revisions is given in Mossler (2008). The dolomitic facies of the Coon Valley Member is absent along the Minnesota Earth County. This feature appears to coincide with subtle trends in the magnetic anomalies found Rock Formation with the St. Lawrence Formation is conformable and gradational, and Geologic atlas of Pine County, Minnesota: Minnesota Geological Survey County Atlas C-13, 7 pls., Stubblefield, W.L., 1971, Petrographic and geochemical examination of the Ordovician Oneota Dolomite River near Mankato in Blue Earth and Nicollet Counties and instead a light gray, on the aeromagnetic image of the area (Fig. 2; Chandler and Lively, 2007). The anticline possibly Opg Platteville and Glenwood Formations, undivided (Upper Ordovician) the lower contact is conformable with the underlying Wonewoc Sandstone. scale 1:100,000. in the building stone districts of southeastern Minnesota: Iowa City, University of Iowa, M.S. thesis, medium- to coarse-grained, quartz sandstone informally referred to as the Kasota developed in response to adjustments along an underlying deep basement fault structure not related Platteville Formation—Finely crystalline, yellowish-brown to light gray, fossiliferous Chandler, V.W., and Lively, R.S., 2007, Revised aeromagnetic data for Minnesota: Minnesota Geological 154 p. sandstone overlies the Jordan Sandstone (Fig. 3). The Kasota sandstone ranges _w Wonewoc Sandstone (Upper Cambrian)—Fine- to coarse-grained, moderately sorted, to the Midcontinent Rift System. The apparent absence of faults in Blue Earth County likely reflects limestone and dolostone with a maximum eroded thickness of 10 feet (3 meters). Survey Open-File Report 07-06, 32 p. from a few inches to 7 feet (2 meters) thick; it disconformably overlies the Jordan friable, white to tan, quartzose sandstone with white linguliform brachiopod shells. its position west of the major faults associated with the Midcontinent Rift System that are known to The basal dolostone bed contains fine- to coarse-grained quartz sand and phosphate Sandstone and conformably underlies the Blue Earth siltstone. The Kasota sandstone The lower portion of the formation is typically finer-grained and better sorted. have affected the deposition and distribution of Paleozoic bedrock units elsewhere (Sims and Zeitz, 1967; Chandler and others, 2004).

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