Prepared and Published with the Support of COUNTY ATLAS SERIES THE MOWER COUNTY BOARD OF COMMISSIONERS AND ATLAS C-11, PART A MINNESOTA GEOLOGICAL SURVEY Plate 2—Bedrock Geology D.L. Southwick, Director THE MINNESOTA DEPARTMENT OF NATURAL RESOURCES, DIVISION OF WATERS ° R 18 W 93 00' B' R 17 W R 16 W 92°45' Omaq 56 1 6 6 1 6 1 Ogal R 15 W R 14 W 92°30' Odub er v Odub 16 i Dspl 00 R 1100 reek 11 UDOLPHO 1150 C 7 oot Omaq k BEDROCK GEOLOGY R ree 8 Ogal 1 Omaq PLEASANT C AA y AA' Waltham re Odub Dspl VALLEY a C Dclp By 1 Sargeant 63 T 104 N A' n so 90 in Omaq John H. Mossler ch b n o a R Br Dspl 1250 SARGEANT T 104 N 1100 1998 C Dspl e Omaq d a WALTHAM 12 218 r 20 RACINE Dclp 00 1 STRATIGRAPHIC COLUMN 1050 12 50 North Dclp U 31 R 36 31 31 iv 56 36 31 36 31 36 D 36 e Racine r Group, Natural Gamma Log 1250 Era Formation, Lithology Increasing count BB Unit N BB' o Series Member r 0 100 th R Omaq Unit Symbol o 6 API-G units 6 b 1 6 1 Hydrostratigraphic LANSING 1 erts 1 6 Thickness (in feet) Lansing Creek 6 1 43°45' Dclp 7 Bear 43°45' 2 Ostrander Fork Member Kwos Dspl Creek Upper Omaq 250 Iron Hill Fe Fe 20–35 DEXTER Windrow 2 1 (<100 m.y.) Kwih Brownsdale Formation Cretaceous Member Omaq MESOZOIC D 1050 Dclp Lithograph Dclp k U ee City Dcuu Cr Dspl Formation k r Up to 50 1000 ree GRAND ea Odub Wolf C B Dclc MEADOW th Fork Ramsey ou 1100 S Coralville Mill Pond Creek T 103 N T 103 N Deer Formation Dspl D RED ROCK Dexter Grand 57–61 Ogal Dcum 16 16 Dcum U U Meadow D k 1050 1100 16 e 20 e Hinkle & Mur r Dcum phy 1150 C Dclc Eagle CC C CC' re Center Mbrs 25–35 e 1050 k U D U Cedar Valley Group ins FRANKFORD Chickasaw 1100 D b Dclc Dclp b Member o Dclp Maple View D Dspl 15–43 1150 8 Dspl Bassett 36 Little Cedar Formation 31 31 1200 14 Member 36 31 36 31 36 31 40–70 1100 1100 Dclc Dclp Pinicon 16 90 1250 Dclc Dcum Elkton Ridge 6 1 6 1 6 1 6 1 6 1 Formation 20–47 90 Middle Devonian (387–374 m.y.) ek re r C ee er 3 Dclc Dclp D Riv Dcum Spillville Dspl 1050 Formation Dclc 21–84 3 Dcum Root Wapsipinicon Group DD 1000 DD' Austin 56 Dclc Dcum Branch 1050 O 1000 7 r c h 105 South a Maquoketa r 3 Omaq T 102 N d CLAYTON 1100 T 102 N C Formation AUSTIN 36–85 r C Dclp e Dcum e MARSHALL k BENNINGTON Dubuque Dcuu 3 Odub Formation 23–40 Dclp 1150 D U Dcuu 1050 Dclc 4 Dcuu 1050 1200 Varco er Rose v er WINDOM i U er iv Creek 4 4 R D iv R R a R o w s e Creek o a Stewartville Dcum I w r Io Formation e 218 p 75–85 31 ar 31 36 31 36 31 p 36 31 36 Dspl d 36 1100 e Dclc U C EE EE' Group 56 7 Dclp Ogal 6 1 6 Dclc 1 6 nch 1 a 6 1 6 r 1 e tl Galena Prosser B it L h Limestone Adams t 1000 r 40–50 o 1000 Dcum N Dclc 5 Dclc Taopi 5 1000 LE ROY 11 Cummings- ville 7 Dspl U Formation 70–75 p p 950 er 950 Dcum Little U LYLE 5 5 Fe Fe T 101 N NEVADA D T 101 N Upper Ordovician (458–444 m.y.) Decorah 1000 Dcum Dcuu Dcuu LODI Shale 950 Cedar Dclp Dcum Dcum 56 45–50 105 Dcuu Odpg Dcum Platteville Iowa 14 Dcum Formation 20–30 FF W R FF' Glenwood Ph o iver Ph o 900 Dclc 5–6 d 900 Formation Ph b ur 1100 Fe Fe y C R O i reek v Dcuu er t 0 ter 105 6 Creek 6 6 St. Peter Ostp Dcuu Sandstone Le Roy 70–80 Lyle 31 31 36 7 36 31 Dcum 31 36 31 36 C' 36 Dclc 43°30' 43°30' 93°00' R 18 W B R 17 W A R 16 W 92°45' R 15 W R 14 W 92°30' Digital base modified from 1990 Census TIGER/Line Files SCALE 1:100 000 Cartography by Joyce Meints and Philip Heywood of U.S. Bureau of the Census (source scale 1:100,000); Graphic design by Philip Heywood 1 0 12345 MILES county border files modified from Minnesota Department of Transportation files; digital base annotation by Minnesota 1 0 12345 6 7 8 KILOMETERS Shakopee Geological Survey Formation Universal Transverse Mercator Projection, grid zone 15 1927 North American Datum INTRODUCTION The Bassett Member, though very thick bedded to massive, is jointed and pump sand). Also, a low-permeability regional confining layer at the bottom Many zones of well-cemented siltstone and shale may reduce yields of The map on this plate shows the bedrock formations that crop out or lie immediately contains a few large, open vugs. This relatively permeable unit forms a minor of the formation—found farther north and west in the state—is missing in wells developed in the Mt. Simon. The effect of Precambrian bedrock highs beneath Quaternary sediments. The information used to create this map was compiled from aquifer (J.A. Green, pers. commun., 1997). The medial limestone member of Mower County (Woodward, 1986). Confining layers may be present locally. and structure on unit thickness is uncertain, but they may dramatically reduce Opdc Pinicon Ridge, which farther south in Iowa is broken, possibly brecciated, and Opdc Prairie du Chien Group (Lower Ordovician). the amount of sandstone available for well development: to the south in Cerro PALEOZOIC examination of exposures of bedrock and from subsurface data, including water well drillers' 320–340 cavernous (Libra and Hallberg, 1985), may form an aquifer. Shaly dolostone Shakopee Formation—Dense to finely crystalline, light-gray to light-olive-gray Gordo County, Iowa, the Mt. Simon is absent locally due to nondeposition logs, geologists' descriptions of water-well cuttings, wireline geophysical logs, engineering drawings, and geotechnical logs. Unpublished sources that were consulted include geologic and shale in the upper and lower parts of Pinicon Ridge have been interpreted dolostone. Locally sandy (medium to coarse quartz sand grains); siliceous (Hershey and others, 1970). Prairie du Chien Group field notes by R.E. Sloan (1954–1960), D.W. Kohls (1959–1960), Nikola Prokopovich to be confining units (Libra and Hallberg, 1985). oolites present locally. Oolitic dolostone possesses moldic porosity in places. Fond du Lac and Solor Church Formations (Middle Proterozoic)—Not penetrated (1955), and W.E. Parham (1966). J.A. Green, Minnesota Department of Natural Resources, Dspl Spillville Formation (Middle Devonian)—Finely crystalline, light-brown to grayish- Minor amount of white chert. Thickness about 150 ft. by any wells drilled in Mower County. Interpreted from adjacent areas to orange dolostone; abundant fossil-moldic porosity; dog-tooth calcite spar in Oneota Dolomite—Very light gray to yellowish-gray, finely crystalline, consist of red-brown siltstone, sandstone, and shale having low permeability. Oneota and G.N. Meyer provided the author with locations and descriptions of outcrops and sinkholes. Dolomite For further information on the bedrock geology of Mower County, see Mossler (1998). larger vugs. Very thick to thin bedded. Basal few feet are sandy to silty and nonfossiliferous. Thickness, about 170 ft. may contain minor amounts of shale or shaly dolostone. Middle to upper parts Coon Valley Member—Heterolithic basal unit; medium- to coarse-grained quartzose Lower Ordovician (505–478 m.y.) of unit have thin-bedded, light-olive-gray, dense dolomitic limestone. Unit sandstone, very fine grained, feldspathic sandstone and siltstone, and sandy to Contact—Approximately located; generally concealed. DESCRIPTION OF MAP UNITS thickness is as much as 78 ft in eastern Mower County; unit thins in western silty dolostone. Windrow Formation (Upper Cretaceous?)—The Windrow Formation unconformably Mower County, where it is 21–66 ft thick. Most permeability and porosity in the Prairie du Chien is related to fractures D Fault—Inferred; U, upthrown side, D, downthrown side. overlies Devonian carbonate rocks; the contact is generally marked by a zone Fracture porosity and permeability dominate upper part of unit; fossil- (joints, bedding planes), but intergranular porosity is present in sandstone U of weathered rubbly rock several feet thick at the top. The mapped extent of moldic, vuggy porosity is prominent in lower part. The basal shaly dolostone beds. Fractures near the sub–St. Peter unconformity are possibly solution Anticline—Showing trace of axial surface and plunge of fold. the two members as shown on the map and sections is approximate. of low permeability may function as confining unit locally, as studies suggest enhanced through weathering that occurred prior to deposition of the St. Peter. Coon Valley Member Kwos Ostrander Member—Conglomerate consisting of rounded pebble-sized clasts of it may in northern Iowa (Witzke and Bunker, 1985); however, gamma logs Strongly cemented, very fine grained sandstone and siltstone beds in the basal Syncline—Showing trace of axial surface and plunge of fold. quartz and chert set in matrix of fine to coarse quartz sand grains.