Prepared and Published with the Support of COUNTY ATLAS SERIES THE DODGE COUNTY BOARD OF COMMISSIONERS, ATLAS C-50, PART A MINNESOTA GEOLOGICAL SURVEY Dodge County Harvey Thorleifson, Director THE MINNESOTA DEPARTMENT OF NATURAL RESOURCES, THE MINNESOTA ENVIRONMENT AND NATURAL RESOURCES TRUST FUND Plate 2—Bedrock Geology AS RECOMMENDED BY THE LEGISLATIVE-CITIZEN COMMISSION ON MINNESOTA RESOURCES, AND THE MINNESOTA LEGACY AMENDMENT'S CLEAN WATER FUND RICE GOODHUE 93° W. 92° 52' 30" W. 92° 45' W. GOODHUE COUNTY COUNTY R. 18 W. R. 17 W. R. 16 W. COUNTY CORRELATION OF MAP UNITS 345 Od 330 STEELE 375 Ogs 345 Opg 360 330 Ogp 345 OLMSTED COUNTY Ogc 1 Kw 6 345 360 COUNTY Upper Cretaceous MESOZOIC 6 360 1 6 1 Ogp 330 unconformity Mi 375 unconformity A ddle Fork A' 375 Os Dlp BEDROCK GEOLOGY Od Middle Devonian Zum Ogs 315 Ops )57 360 bro Riv Ds er 330 Opg 330 unconformity By )56 Od Ogc Om Ogc 360 375 Ogc 360 Ogp Ogp 360 Od Odu Od ELLINGTON 375 CONCORD Andrew J. Retzler T. 108 N. MILTON T. 108 N. LOCATION DIAGRAM West Concord 345 Opg 360 Ogs 345 M Ogc Ogp Od Upper Ordovician 2019 390 390 Ogc Ogc 390 360 m Creek Lithostratigraphic Composite natural gamma log Milliken Creek Od co unit Milken Creek Increasing count to the right (API) ark M 44° 7' 30" N. Era 44° 7' 30" N. H Group, 0 A 100 Opg Formation, 0 C 100 Map symbol Lithology Ogs System-Series Thickness (in feet) Member Hydrostratigraphic properties 0 B 100 it commonly is stained black or dark gray with manganese oxides on the 360 36 PALEOZOIC INTRODUCTION unconsolidated glacial sediment and water wells that penetrate bedrock tend to draw from 375 36 31 Os Middle Ordovician 31 36 31 Od Ostrander Fe higher carbonate aquifers in the Galena Group and above. outermost surface. Total thickness of the St. Peter Sandstone is about 110 Opg Windrow Kw Fe unconformity The geologic map, cross sections, and stratigraphic column on this plate depict the type, <110 Upper feet (34 meters). Formation Most layers designated as aquitards in the county have a much lower permeability in MESOZOIC Iron Hill Cretaceous Fe A distribution, and structure of the bedrock units in Dodge County that are either exposed at Ogp Ops Prairie du Chien Group (Lower Ordovician)—Dominated by dolostone interlayered 6 390 the vertical direction than do aquifers. Examples in Dodge County include the Decorah 1 390 Little the land surface or lie beneath unconsolidated Quaternary sediments of variable thickness 6 1 Cedar Fe C unconformity Shale and Glenwood Formation, whose rocks are composed mostly of shale with small, with lesser amounts of quartzose sandstone. The Prairie du Chien Group is 1390 6 Lower Ordovician (see cross sections and Plate 3, Surficial Geology, Plate 4, Quaternary Stratigraphy, and Ogs Ogc Formation poorly connected pore spaces. Carbonate rock containing a relatively sparse network formally divided into two formations: the Shakopee Formation and underlying 375 Opo Plate 6, Bedrock Topography and Depth to Bedrock). The map shows how the bedrock 375 Dlp <77 Oneota Dolomite. It is commonly difficult to discern the contact between Little Cedar of fractures and/or interbedded with shale, such as the Maquoketa Formation, Dubuque Valley Group Valley Bassett surface would appear if it were viewed from an aerial perspective and the overlying Ogs unconformity Formation, or the lower part of the Prairie du Chien Group (Oneota Dolomite), are also the Shakopee Formation and Oneota Dolomite without high-quality drill Quaternary sediments were stripped away. The bedrock units nearest the land surface Middle Fork Zumbro Pinicon Ridge River 375 Fe Cj aquitards. However, layers designated as aquitards with very low permeability in the cuttings and borehole geophysical logs. Total thickness of the Prairie du Odu Ogc Os Formation in Dodge County consist of sedimentary rocks of Paleozoic and Mesozoic age that form 56 Ogs Middle Devonian Fe South Branch ) vertical direction may locally contain horizontal fractures that are conductive enough to Chien Group is about 300 feet (91 meters). 360 distinguishable and mappable layers designated as formations. Some of these units are Group Spillville Cs yield large quantities of water. 360 Ds B 60 Ops Shakopee Formation (Lower Ordovician)—A heterolithic unit composed Formation commonly exposed along small creeks and branches of the Zumbro River that flow eastward Wapsipinicon Ogp EXPOSED IN DODGE COUNTY NOT Mantorville Ogp Ogp Upper Cambrian Horizontal and vertical fractures are more common where bedrock layers are at or near mostly of tan to gray, thin- to medium-bedded dolostone, sandy dolostone, 375 into neighboring Olmsted County, and within quarries generally concentrated within the 375 Clr 330 Od Maquoketa the bedrock surface. As a result, aquitards in such conditions are likely to have higher sandstone, and shale. It contains oolites, intraclasts, microbial mounds, CLAREMONT WASIOJA Om east-central portion of Dodge County. Several of the Paleozoic bedrock formations are MANTORVILLE Formation 30-55 chert nodules, quartzose sandstone, and green-gray shale partings. The T. 107 N. 390 T. 107 N. unconformity major reservoirs for water in the county, while others closer to the surface provide aggregate permeability compared with more deeply buried portions of the same formation, and may Om Ogs ek F Fe Cw Shakopee Formation is the oldest unit to subcrop in Dodge County, in the 390 re Dubuque and crushed stone materials. have a diminished ability to retard water flow to underlying aquifers. There is no precise Odu C 345 Odu 25 Formation Fe boundary between shallow and deep conditions of burial, but in most areas of southeastern northeastern corner of the county within the Middle Fork Zumbro River 375 Ce Characteristics of each formation are given in the stratigraphic column (Fig. 1) and in r e t n e 57 Minnesota, about 50 feet (15 meters) of depth below the bedrock surface is considered a valley. Total thickness of the Shakopee Formation is about 180 feet (55 Ogc)345 C Creek Middle Cambrian the description of map units. The accompanying bedrock geologic cross sections add the 360 Cm best approximation (Runkel and others, 2006a, 2018). meters). Om 375 dimension of depth and illustrate the stratigraphic, structural, and topographic relationships Claremont Om e ek Stewartville ¤14 Ogs e r Ogs Oneota Dolomite (Lower Ordovician; shown only on cross sections)—The odg 375 ek 85 In addition to this hydrostratigraphic classification, the Minnesota Department of Natural Opo Odu D C re Thompkins Formation of the bedrock units, as well as the variable thickness of the overlying Quaternary sediments. C 390 Resources, as Part B of the Dodge County Atlas, will conduct a thorough hydrogeologic Oneota Dolomite is separated into two members, the Hager City and the 390 Odu Spring Ogp Projected onto these cross sections are the locations and approximate depths drilled Kw on Kasson Coon Valley. The Hager City Member is predominantly a yellowish-gray Odu 390 375 Uni k (shown as black lines) from available water-well data within a 1,640-foot (0.5 kilometer) study of the groundwater flow system, aquifer capacity, and aquifer sensitivity that may or B de Cree B' Casca may not result in modifications or additions to this classification. Furthermore, designations to light brown, medium- to thick-bedded dolostone that generally lacks Dodge Center 375 buffered area surrounding each cross-section line. Surfaces representing the elevation of Masten Prosser 36 Ogc Ogp of aquifers versus aquitards made here may not correspond precisely with those made for sedimentary features, such as oolites and quartz sand, characteristic of 31 36 390 Formation 40-45 the tops of the mapped formations are also available as Digital Elevation Models (DEMs) Ogs Om 31 36 31 for use in GIS programs. The geologic formations are thin in relation to their aerial regulatory purposes by the Minnesota Department of Health. the overlying Shakopee Formation. Thickness of the Hager City Member Kw Galena Group extent, and would only be one-tenth as thick as shown on the cross sections if no vertical varies from about 90 to 95 feet (27 to 29 meters). The basal Coon Valley Henslin Creek Upper Ordovician Cummingsville Member is a heterolithic unit composed of thinly bedded dolostone, sandy 14 Ogc 65 exaggeration were used. The exaggeration necessary to show the thin rock formations gives Om 6 Om ¤ Formation 1 1 dolostone, and beds of fine- to coarse-grained, poorly sorted quartzose 6 14 6 1 the appearance of steeper slopes on bedrock unit contacts, the land surface, and bedrock DESCRIPTION OF MAP UNITS Om Odu ¤ Ogs topography. About half of the units shown as first bedrock on this plate are exposed at sandstone. Thickness of the Coon Valley Member varies from about 25 to Odu MESOZOIC the land surface, with the exception of the Windrow Formation, Little Cedar Formation, 30 feet (8 to 9 meters). Total thickness of the Oneota Dolomite is about Decorah Windrow Formation (Upper Cretaceous)—The Windrow Formation 120 feet (37 meters). Od Pinicon Ridge Formation, Spillville Formation, Maquoketa Formation, Dubuque Formation, Kw Shale 44° N. 44° N. 45-60 and Prairie du Chien Group, which are buried by Quaternary sediments. Where exposed, unconformably overlies the Devonian and Ordovician carbonate units from Cj Jordan Sandstone (Upper Cambrian; shown only on cross sections)—Dominantly 390 the Bassett Member of the Little Cedar Formation down to the Stewartville white to yellow, medium- to coarse-grained, friable quartzose sandstone 360 the sedimentary rocks differ in their resistance to weathering and erosion, as depicted Platteville and Formation of the Galena Group and is divided into two members: the characterized by coarsening-upward sequences consisting of two interlayered Opg 30 graphically by the weathering profile on the lithology column of Figure 1.