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Prepared and Published with the Support of COUNTY ATLAS SERIES THE HENNEPIN COUNTY BOARD OF COMMISSIONERS, ATLAS C-45, PART A MINNESOTA GEOLOGICAL SURVEY Hennepin County Harvey Thorleifson, Director THE MINNESOTA ENVIRONMENT AND NATURAL RESOURCES TRUST FUND SHERBURNE AS RECOMMENDED BY THE LEGISLATIVE-CITIZEN COMMISSION ON MINNESOTA RESOURCES, Plate 2—Bedrock Geology COUNTY ANOKA AND THE MINNESOTA LEGACY AMENDMENT'S CLEAN WATER FUND COUNTY WRIGHT 93°30' COUNTY 275 U 31tm D j Mississippi 6 s 1 R. 22 W. j 275 s R. 23 W. s tm M 275 M )101 CORRELATION OF MAP UNITS 93°37'30" 275 D275 D Lake tm Laura River U U s tl w 275F BEDROCK GEOLOGY North Twin Diamond j Od 275 w tl Lake Cowley Lake tl Upper Ordovician 275 Lake w U T. 120 N. j T. 120 N. Opg s D LOCATION DIAGRAM 275 Sylvan 275 tm By Prairie Lake tm 275 Os Middle Ordovician Lake tl D tl ¤52 j U 93°22'30" unconformity 300 tm Rogers French s Ops F s D Hayden Andrew J. Retzler Lake U Lake Lemans tm 94 F tm unconformity s ¦§ Lake s Lower Ordovician tl j Henry s Champlin U Lake s Opo 2018 D s Powers unconformity Lake R. 21 W. 31 s PALEOZOIC 36 31 36 31 j j 275 w 275 27536 j Dayton Goose B Lake s U tm s tm tl F j ¤169 D RiverOs tm 1 6 j tm 1 6 Mud 1 6 Upper Cambrian M j Lake R. 24 W. F j tl j tm w e s s unconformity tl 275 w s 300 j tm 45°7'30" Crow s j e Osseo 250 45°7'30" 275 tl Middle Cambrian j 275 s Rice Lake m Schwappauff s tm LakeOs j s unconformity T. 119 N. Os tm Weaver T. 119 N. Greenfield w Lake j Mss INTRODUCTION Most layers designated as aquitards in the county have a much lower permeability in the vertical Schendel M MESOPROTEROZOIC Os Brooklyn Park direction than do aquifers. Examples in Hennepin County include the lower Jordan Sandstone, and s Lake Corcoran tl tm j tm The geologic map, cross sections, and stratigraphic column on this plate depict the type, D Mbv St. Lawrence and Eau Claire Formations, whose rocks are composed mostly of very fine-grained U s Mcv distribution, and structure of the bedrock units in Hennepin County that are either exposed at the Hafften 275 s sandstone and shale with small, poorly connected pore spaces. Carbonate rock with relatively sparse Lake 300 s land surface or lie beneath unconsolidated Quaternary glacial sediments of variable thickness Os fractures, such as the lower part of the Prairie du Chien Group (Oneota Dolomite), are also aquitards. 325 300 Fish j U Lake Opo (see cross sections and Plate 3, Surficial Geology; Plate 4, Quaternary Stratigraphy; and Plate 6, Os However, layers designated as aquitards with very low permeability in the vertical direction may Os 300 300 D300 )252 93°45' Jubert 300 Scott Os Os 275 Bedrock Topography and Depth to Bedrock). The map shows how the bedrock surface would appear Lake F locally contain horizontal fractures that are conductive enough to yield large quantities of water, Lake j tm 275 ¦§94 Ops if it was viewed from an aerial perspective and the overlying Quaternary sediments were stripped 275 Os Cedar Island including the St. Lawrence Formation. Lake ¤52 Ops Palmer Lake away. The bedrock units near the land surface in Hennepin County consist of sedimentary rocks 300 sRattail j Eagle Brooklyn Center Horizontal and vertical fractures are more common where bedrock layers are at or near the tl Lake 325 31 j Lake Opo of Paleozoic age that form distinguishable and mappable layers designated as formations. Some of 31 36 36 31 36 Lake 300 s 36 31 bedrock surface. As a result, aquitards in such conditions are likely to have higher permeability Os Morin F these units are commonly exposed along the Mississippi River bluffs within the county. Several of Lake Lake Sarah F Maple Grove Opo compared with more deeply buried portions of the same formation, and may have a diminished 300 s s Os Pike the Paleozoic bedrock formations are major reservoirs for water supply in the county, and provide 275 Lake ability to retard water flow to underlying aquifers. There is no precise boundary between shallow Rebecca s 300 a stable platform that supports major metropolitan buildings and bridges. 300 275 Os j Os and deep conditions of burial, but in most areas of southeastern Minnesota, about 50 feet (15 meters) 1 6 tl Peter Lake 6 Lithostratigraphic 6 Lake 1 6 Mud Lake 1 Meadow Lake 1 Characteristics of each formation are given in the stratigraphic column (Fig. 1) and in the Os Loretto Medina Bass unit Composite natural gamma log of depth below the bedrock surface is a best approximation (Runkel and others, 2006a, 2014a). tm j 300 300 Era )55 Os Pomerleau Lake description of map units. The accompanying bedrock geologic cross sections add the dimension of s Lake Twin Os Group, Formation, In addition to this hydrostratigraphic classification, the Minnesota Department of Natural j 300 Lithology Map symbol API M Lakes depth and illustrate the stratigraphic, structural, and topographic relationships of the bedrock units; System-Series Member Haughey s 300 T. 118 N. Resources, as Part B of the Hennepin County atlas, will conduct a thorough hydrogeologic study Hydrostratigraphic properties F Thomas Lake 494 0 100 Thickness (in feet) Lake Os Os 300¦§ as well as the variable thickness of the overlying Quaternary sediments. Surfaces representing the ¤169 Opo of the groundwater flow system, aquifer capacity, and aquifer sensitivity that may or may not result Robina Winterhalter Lake 300 elevation of the tops of the mapped formations are also available as digital elevation models for 275 RAMSEY Decorah 300 Od 12 Edward <40 in modifications or additions to this classification. Furthermore, designations of aquifers versus Lake 275 ANOKA Shale ¤ COUNTY Group 300 Spurzem 93°15' Galena use in GIS programs. The geologic formations are thin in relation to their aerial extent, and would 300 Lake Hamel Lake New Hope Ardmore300 tm Lake Os Crystal R. 24 W. COUNTY R. 23 W. aquitards made here may not correspond precisely with those made for regulatory purposes by the Opo only be one-tenth as thick as shown on the cross sections if no vertical exaggeration were used. The Os F Opo Platteville and Minnesota Department of Health. j Ops Opg 300 Lake Halfmoon j exaggeration necessary to show the thin rock formations gives the appearance of steeper slopes on Os 300 Os Glenwood Formations 30-35 Lake s Lost Lake275 Ph bedrock unit contacts, the land surface, and bedrock topography. Most of the Paleozoic units shown 300 Independence Medina Opo 1 St. 6 300 DESCRIPTION OF MAP UNITS Independence School Lake s Anthony T. 118 N. tl Plymouth on this plate, with the exception of the Platteville Formation, Glenwood Formation, and St. Peter Os j Robbinsdale Crystal Lake 250 Opg Os Holy Name Os Upper Ordovician Sandstone, are buried by Quaternary sediments and cannot be seen at the land surface in Hennepin PALEOZOIC j Maple Lake Os 52 County. The sedimentary rocks differ in their resistance to weathering and erosion, as depicted tl 275 ¤ Plain300 F Medicine )100 Od Decorah Shale (Late Ordovician)—Dominantly grayish-green shale interbedded with Opo Ops graphically by the weathering profile on the lithology column of Figure 1. The units that cover the 300 Katrina Thies Lake Ops Lake Opg 275 thin beds of fossiliferous limestone. Fossiliferous, yellowish-brown limestone beds Irene s Os largest areas of the map are the most resistant to weathering and commonly form plateaus composed Lake Lake Wolsfeld Medicine Tonti are most common at the base of the Decorah Shale, and are recognized as the basal 300 Lake Os Os 300 of carbonate rock (limestone and dolostone). The soft sandstone and shale units erode more easily Os Mooney 300Snyder Lake Carimona Member (Mossler, 2008). The Decorah Shale is present as erosional j Lake 94 St. Peter 45° ¦§ and typically occur as thin veneers atop the carbonate rock plateaus or lie along valley walls. Lake Sandstone Os s 275 275 Od 45° remnants capping buried mesas of Platteville Limestone in the eastern part of the s Parkers 150-175 Production of the map and associated products relied on several data sources, including outcrops, Lake 250 county. Although not fully preserved in the county, water-well logs in adjacent 300 275 35W 300 394 300 300 ¦§ ¦§ Opo water-well and scientific drilling records from the County Well Index data base, rock core, drill j Ops 275 T. 29 N. counties indicate the Decorah Shale has a maximum preserved thickness of 90 feet Long Lake 300 250 cutting samples, borehole geophysical logs, seismic soundings, geophysical images, and previously Os Middle Odrovician 300 Ops Opg (27 meters). 31 w 36 31 j Os 31 Plymouth )55 Od 300 300 36 36 31 published geologic maps of Hennepin and adjacent counties (Olsen and Bloomgren, 1989; Mossler and WRIGHT 300 300 275 Pigs Platteville Formation and Glenwood Formation (Late Ordovician)—The carbonate Os 300 Gleason Opg COUNTY 300 Eye Tipping, 2000; Runkel and others, 2006a; Mossler, 2013; Steenberg and Chandler, 2013; Steenberg 300 Lake Golden Valley Platteville Formation and underlying shale of the Glenwood Formation are combined CARVER 300 Opg 250 and Retzler, 2016). This map supersedes the previous bedrock geology map of Hennepin County tl 300 Minneapolis 300 into a single map unit with a full thickness of 30 to 35 feet (9 to 11 meters).