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  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 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 , 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 (), 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 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, , 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 (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 . 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  300 into a single map unit with a full thickness of 30 to 35 feet (9 to 11 meters). COUNTY D tl 300 s 300 (Olsen and Bloomgren, 1989), a larger-scale seven-county Twin Cities metropolitan area bedrock Os Orono ¦§394j 6 Os 6  1 6 tl 6 1 Shakopee Platteville Formation—The Platteville Formation is 25 to 30 feet (8 to 9 meters) thick. It j Ox Yoke LakeU F  1 300 275 Willow geology map (Mossler and Tipping, 2000), and several quadrangle bedrock geology maps covering e j  300 275 Formation 300 j Wayzata 275 River is composed of tan to gray limestone and dolostone. It is commonly burrow mottled  North 300 275 Ops

35-40 portions of Hennepin County (Mossler, 2007, 2009). Significant improvements and modifications j  New s Arm Ops Maxwell Os Cedar Richmond LITHOLOGY KEY were made to the interpretations of the previous maps based on additional water-well records, and fossiliferous. It contains fine- to coarse-grained quartz sand and phosphate  Bay Opo 275 Lake Opg 225  w 300 Os grains in the lowermost 2 feet (0.6 meter). In the Twin Cities metropolitan area, m Lake of Hager drill cuttings, and borehole geophysics, enabling better definition of hydrogeologically significant

300 the Isles the Platteville Formation is subdivided into four members: (from top to bottom)

e 300 U Grays Opo City units. These improvements include: the Prairie du Chien Group is separated into the Shakopee  M 36 Limestone tl 494 300 31 31 300 Bay ¦§ Od the Magnolia, Hidden Falls, Mifflin, and Pecatonica (Runkel and others, 2015b). D M Opg 275 G Formation and Oneota Dolomite, the St. Lawrence Formation is mapped separately from the Tunnel Ops St. Louis Park 275 Oneota Dutch Lake Lake Dolomite Opo These correspond to major depositional facies and are distinguished primarily by Lower Ordovician  West Arm Prairie du Chien Group 300 w U Os Opg Od G 85-90 Dolostone City Group (previously ), and the Tunnel City Group is separated into the D D s Crystal Bay 300 Coon lithology and bedding style (Runkel and others, 2015b). The Platteville Formation

275 (Absent to the west) 300  Minnetonka Bde Valley uppermost Mazomanie Formation and lowermost Lone Rock Formation. Further refinement was U w j 325 tl 6 Maka is the dominant uppermost bedrock unit across the eastern part of the county, in Whaletail tl Langdon Opo 1 6 G also made to the unconformable contact between the St. Peter Sandstone and Jordan Sandstone e 300 Ska Sandy dolostone 300 Lake Hopkins particular the Minneapolis metropolitan area. A 300 Mississippi (areas where the Prairie du Chien Group is absent) in western Hennepin County. Faults and folds w 300 A' T. 117 N. Lake Mound Sandstone Glenwood Formation—The principal lithology of the Glenwood Formation is a grayish- )55 in the county were mapped to reflect significant, apparent offset in the Paleozoic formations. This 300 Os Minnetonka Os Very fine- to fine-grained jCook's M 275 provides greatly increased detail in the depiction of the subsurface geologic characteristics, which green to brownish-gray, calcareous, sandy, and phosphatic shale. The Glenwood 300 Bay Opo Ops 275 300 )7 has implications for modeling groundwater flow and contaminant transport. Formation is 3 to 7 feet (1 to 2 meters) thick. Os Ops Lake Os Lake Fine- to medium-grained ¦§35W Jordan Hiawatha  St. Peter Sandstone (Middle to Late Ordovician)—The upper 110 to 140 feet (34 to 43 275 Harriet j The different data sources and their irregular distribution and density can be seen on Plate 1, Os

Sandstone 85-90 w Lake Minnehaha275 s 325 St. Bonifacius ¤169 300 Medium- to coarse-grained Data-Base Map. The spatial distribution and coverage of these data should be considered when meters) of the St. Peter Sandstone, referred to as the Tonti Member, are mostly a white Halsted 275 e  Minnetonka Os St. Albans to tan, fine- to medium-grained, friable quartzose sandstone. Bedding and structures m Bay Lake assessing the reliability of the map at any particular location. Areas with a high density of bedrock 300  Bay 275 tl s Os River Creek Nokomis Shaly control points are more likely to have an accurate interpretation of the bedrock geology, whereas are generally absent. It is exposed in outcrops along the Mississippi River bluffs. Os 300 300 Opg tl  Christmas Opg areas with widely spaced control points may be less reliable. The bedrock surface digital elevation The lowermost 20 to 40 feet (3 to 12 meters), referred to as the Pigs Eye Member,  j  275 w s )7 s 300 Lake T. 28 N. 275 include white, gray, red, and green feldspathic shale and siltstone interbedded with 36 300 Siltstone models supplementing this map were interpolated using the ESRI ArcMap Topo to Raster tool and 31 300 F Excelsior Ops 275 St. Lawrence 300 31 31   Os 36 31 Formation s coarser-grained sandstone similar to that of the overlying Tonti Member. The Pigs j  G 35-45 similarly inherit these biases in the data. Furthermore, the accuracy of the digital elevation models j Shorewood 300 Edina RAMSEY tl tl Opo 36 Opg Os Ops COUNTY G Shale is dependent upon the contour interval used when mapping the topographic surface of each unit and Eye Member is not exposed in Hennepin County. The thickness of the St. Peter 275 250 R. 24 W. R. 23 W. 300 Sandstone varies from about 150 to 175 feet (45 to 53 meters). The basal contact of 93°45' the errors inherit in the Topo to Raster interpolation method. For this map, the bedrock surface 93°37'30" 275 Mazomanie )62 Opg Formation digital elevation models on average are accurate within ± 12.5 feet (± 3.8 meters). Significant the formation with the underlying Shakopee Formation (unit Ops) is a major erosional 6 Bryant DAKOTA  Chert Digital base modified from the Minnesota Department of Lake 1 6 tm Os 250 COUNTY efforts were made to ensure the accuracy of the bedrock surfaces; however, users should always unconformity (Smith and others, 1993). In much of western Hennepin County, the Transportation BaseMap data; digital base annotation by Os Ops Richfield 250 250 Oolites St. Peter Sandstone lies unconformably atop the Jordan Sandstone, with the Prairie the Minnesota Geological Survey. PALEOZOIC refer to and consider the two-dimensional geologic map, as it represents the most accurate and Elevation contours were derived from the U.S. Geological Survey 44°52'30" Purgatory du Chien Group absent (Retzler and others, 2016; this map). In these places, the 275 Os 44°52'30" G G Glauconite reliable representation of uppermost bedrock across the county. 30-meter Digital Elevation Model (DEM) by the Minnesota 100 275 ) )77 lowermost 15 to 20 feet (4 to 6 meters) commonly include a dark red-brown, iron- Geological Survey. 275 36 To create this map, over 20,000 located water-well records and 6,000 located engineering test 31 31 G Ph Phosphate grains Universal Transverse Mercator Projection, grid zone 15 cemented sandstone with intraclasts of light to dark gray chert and ooidal chert that Os )5 494 and giddings soil probe borings were accessed in the County Well Index, nearly 9,400 of which Creek ¦§ 250 1983 North American Datum G 250 Stromatolites reached Paleozoic bedrock. Geologic interpretations of subsurface material were made by the author are inferred to be weathered remnants of the Prairie du Chien Group.

250 Upper Cambrian Mitchell Os 275 250 G based on material descriptions by drillers or gathered from other data sources and are recorded in Prairie du Chien Group (Early Ordovician)—Dominated by dolostone interlayered with lesser

140-145 Shells Lake 275 Os Ops Lone Rock Eden Prairie 6 1 6 the County Well Index. amounts of quartzose sandstone. The Prairie du Chien Group is formally divided Formation G 275 Anderson Tunnel City Group Tunnel Bioturbation into two formations: the Shakopee Formation and underlying Oneota Dolomite. It Lakes j 250 tl In Hennepin County, Paleozoic bedrock lies on top of a thick sequence of Mesoproterozoic T. 116 N. 250 Long Red Rock is commonly difficult to discern the contact between the Shakopee Formation and ¤212 Ninemile G Pebbles rocks of the Keweenawan Supergroup, associated with the Midcontinent Rift (see cross sections and Lake 275 Opo 275 Oneota Dolomite using geophysical logs and drill cuttings penetrating the Prairie 250 G Fig. 2; Jirsa and others, 2012). These rocks include sandstone, siltstone, and shale of the Hinckley Opo  Ops s Intraclasts Sandstone and Solor Church and Fond du Lac Formations (unit ), as well as volcanic rocks du Chien Group in Hennepin County. The boundary of these two formations as MAP SYMBOLS Lake Staring B' Mss 275 Lake R. 23 W. depicted on this map is therefore more speculative than for other unit contacts. 275 275 250 225 Meadow composed mostly of basalt, including the Chengwatana volcanic rocks (unit ) and the North Riley Cross-bedded (planar) Mcv 250 EXPOSED IN HENNEPIN COUNTY NOT Geologic contact—Approximately located. Dashed where the line marks the erosional 250 The thickness of the Prairie du Chien Group beneath the St. Peter Sandstone varies Branch mafic volcanic sequence (unit Mbv; Jirsa and others, 2012). Due to their deep burial and 275  greatly across Hennepin County, from absent to 130 feet (40 meters). contact between the St. Peter and Jordan Sandstones. 275 j Cross-bedded (trough) limited subsurface data, the distribution of these individual units is less certain than those of the Hyland 169 s 250 Fault—Faults are inferred from abrupt changes in the elevation of stratigraphic ¤ Lake Lake Paleozoic formations. Therefore, the top of these units and the overlying base of the Mt. Simon Ops Shakopee Formation (Early Ordovician)—A heterolithic unit composed mostly of light 275 275 tl Cross-bedded (hummocky) units from subsurface and outcrop data. Letters indicate relative vertical 250 275Bloomington 250 Opo Sandstone is dashed on the cross sections. No new mapping of these Precambrian units or their brown to red-brown, thin- to medium-bedded dolostone, sandy dolostone, sandstone, 225  T. 27 N. j 35W 250 Wonewoc Dolomitic

displacement: U—up, D—down.  Creek § associated faults was done for this project. Instead, a recent compilation map of the Precambrian and shale. It contains oolites, intraclasts, microbial mounds, chert nodules, quartz  ¦

Rice tl Grass j Sandstone w 55-65 F Lake Opo Fold—Axial trace of anticline, syncline. Fold limbs typically have shallow dips and s Lake 36 bedrock geology of Minnesota is depicted (see Fig. 2; Jirsa and others, 2012). sandstone, and green-gray shale partings. Based on a limited amount of high quality M CARVER Minnesota 31 Vugs 31  250 drill cuttings and geophysical data, the Shakopee Formation is on average 40 feet are inferred from subsurface data. COUNTY j 93°15' The Paleozoic rocks of Hennepin County are characterized by relatively thin, widespread layers R. 24 W. Contact marks a major erosional surface (12 meters) thick. Prairie du Chien Group erosion limit—Approximately located. Line marks the SCOTT of sandstone, shale, and carbonate deposited in shallow seas during the Cambrian and Ordovician 93°30' R. 22 W. Ops G COUNTY 6 Periods of the Paleozoic Era, from about 500 to 450 million years ago. The older Cambrian formations Oneota Dolomite (Early Ordovician)—Predominantly a yellowish-gray to light brown, erosional extent of the Prairie du Chien Group beneath the St. Peter GIS compilation by A.J. Retzler Opo Sandstone. 225 250 G are dominated by siliciclastic sedimentary rock, including sandstone and siltstone with minor shale, medium- to thick-bedded dolostone that, except in its lowermost part, generally SCALE 1:100 000 Edited by Lori Robinson T. 115 N.  such as the Jordan Sandstone and Tunnel City Group. Carbonate rock occurs only as relatively lacks sedimentary features such as oolites and quartz sand characteristic of the Bedock outcrop 1 0 1 2 3 4 5 MILES  s j DAKOTA Shakopee Formation. The formation contains two members, the Hager City and the Eau Claire HYDROSTRATIGRAPHIC PROPERTIES KEY thin layers within these units. Ordovician formations, in contrast, are dominated by thicker units Line of geologic cross section COUNTY e A 1 0 1 2 3 4 5 6 7 8 KILOMETERS Formation 85-90 of carbonate rock (for example the Prairie du Chien Group and the Platteville Formation), with the Coon Valley, but they are not mapped separately. The basal Coon Valley Member A' R. 21 W. River SCOTT Relatively high permeability (aquifer) 93°22'30" COUNTY exception of the St. Peter Sandstone. Use of these Paleozoic bedrock resources as aggregate or is a heterolithic unit composed of thinly bedded dolostone, sandy dolostone, and CONTOUR INTERVAL 25 METERS G dimension stone is no longer practical within Hennepin County, because the areas in which bedrock beds of fine- to coarse-grained, poorly sorted quartzose sandstone. The thickness Relatively low permeability (except for fractures, aquitard) exists within 50 feet (15 meters) or less of the land surface coincide with dense urban areas. of the Coon Valley Member is variable—it is locally absent to 55 to 60 feet (16 G High permeability bedding fracture known to be common Across most of Hennepin County, bedrock units are slightly tilted (less than 1°) southeast towards to 18 meters) thick. The Hager City Member is primarily very finely crystalline the central Twin Cities metropolitan area. As a result, progressively older bedrock formations subcrop dolostone, with microbial textures. Its thickness ranges between 30 and 35 feet (9 from eastern to western Hennepin County. This general trend is locally interrupted by deep valleys and 10 meters). A A' that incise older formations, and by faults and folds. A fault is recognized where changes in the _j Jordan Sandstone (Late Cambrian)—Dominantly white to yellow, medium- to coarse- Mound Spring Park Lake Minnetonka Interstate 494 St. Louis Park Cross section Minnesota Bde Maka Ska Minnesota Highway 55 Mississippi 1,200 Whale Tail Lake Interstate 35W elevation of a bedrock contact occur within a very short distance, generally elevation changes of 50 grained, friable quartzose sandstone characterized by coarsening-upward sequences B–B' Highway 100 River Figure 1. Generalized stratigraphic column depicting Middle Cambrian feet (15 meters) or more within a distance of 1,000 feet (305 meters). A fold is inferred where these consisting of two interlayered facies (Runkel, 1994). They are medium- to coarse- the lithology, most common range in thickness, vertical 1,000 elevation changes indicate more gradual slopes. The stratigraphic top of the Jordan Sandstone and grained, cross-stratified, generally friable, quartzose sandstone; and very fine-grained, Mt. Simon m succession, age, and hydrostratigraphic properties for all Sandstone Wonewoc Sandstone were contoured at 25-foot (15-meter) intervals to show the inferred location of commonly bioturbated, feldspathic sandstone and lenses of siltstone and shale. The Opo Unconsolidated Quaternary sediments ~125-300 units shown on the map. The gamma log is a compilation 800 tl major part of the very fine-grained facies forms a regionally continuous interval  faults and folds in Hennepin County. Figure 3 shows an example of the contoured elevations for the w j Os Os Opg Opg of the following borehole geophysical logs on file at the  Os top of the Wonewoc Sandstone and the inferred faults and folds. These two units were selected to that gradationally overlies the St. Lawrence Formation (unit _s), although there 600 e Opo Minnesota Geological Survey: County Well Index unique  s j Opo Ops portray these structures because both are well-recognized and distinct contacts to well drillers, and are lithologically similar intervals intercalated with the medium- to coarse-grained U m numbers 200393, 223723, and 782102. D U j s they provide the best spatial coverage from the numerous wells that penetrate them. Displacement facies at higher stratigraphic intervals. An unconformity, locally marked by thin 400 D   tm tl of Paleozoic bedrock along faults in Hennepin County can be as much as 250 feet (76 meters) beds of quartz pebble conglomerate and silcrete-cemented sandstone clasts (Runkel w tl tm U and others, 1999, 2007), separates the Jordan Sandstone from the overlying Oneota 200 D e w depending on location; sufficient in places to juxtapose several different formations along the fault  Precambrian rocks, undifferentiated e (not shown) contact (see cross sections and Fig. 3). Bedrock that has dropped alongside a fault (labeled with a Dolomite of the Prairie du Chien Group. In western Hennepin County, the Prairie m 0 Mss Mcv Mss "D" on the map) preserves relatively younger formations at the surface and bedrock that has been du Chien Group is absent and the Jordan Sandstone unconformably underlies the  Elevation (feet above sea level) m Mbv uplifted (labeled with a "U" on the map) brings older formations closer to the surface. Most faults St. Peter Sandstone. In these areas, the Jordan Sandstone is typically thinner than -200 PINE FAULT Mss where it is overlain by the Prairie du Chien Group. It ranges in total thickness from ST. CROIX HORST trend northeastward, with a few trending approximately orthogonally to the northwest. DOUGLAS FAULT Mss Current and previous structural mapping throughout Hennepin County and adjacent counties 80 to 90 feet (24 to 27 meters). -400 has identified a regional Paleozoic structural low named the Twin Cities basin (Fig. 4; Austin, _s St. Lawrence Formation (Late Cambrian)—The St. Lawrence Formation is principally 1972; Mossler, 1972, 2013; Olsen and Bloomgren, 1989; Retzler and others, 2016; this map). light gray to yellowish-gray and pale yellowish-green, dolomitic, feldspathic siltstone B Cross section A–A' B' with interbedded, very fine-grained sandstone and shale. Lenses and layers of light 1,200 Crow County Highway 10 County Highway 50 Minnesota Highway 55 Plymouth Interstate 394 St. Louis Park State Highway 100 Interstate 35W Interstate 494 Long Meadow Minnesota The formation of the Twin Cities basin is not well understood, but has long been attributed to River Lake River isostatic and/or thermal adjustments accommodated by Midcontinent Rift System structures during gray, finely crystalline, sandy dolostone occur locally, especially in the lowermost REFERENCES 1,000 Middle to at least Late Ordovician time (Austin, 1972; Mossler, 1972). Several of the faults in the few feet of the formation (Runkel and others, 2006a). The formation is 35 to 45 feet Os Os  Unconsolidated Quaternary sediments Anderson, J.R., Runkel, A.C., Tipping, R.G., Barr, K., and Geological Society of America Abstracts with Programs, v. Paleozoic strata of Hennepin County are directly above or subparallel to faults in the underlying (8 to 12 meters) thick. The upper contact with the Jordan Sandstone is conformable s  800 j Os Alexander, E.C., Jr., 2011, Hydrostratigraphy of a fractured 48, no. 5, 1 pl. Mesoproterozoic Midcontinent Rift rocks, as inferred from geophysical imagery, and are therefore and gradational. The gradational nature of the contact in well cuttings and on Opg Os tm tl urban aquitard: Geological Society of America Field Guide Opo Runkel, A.C., 1994, Deposition of the uppermost Cambrian (St. interpreted to represent reactivation of these older structures during development of the Twin Cities natural gamma logs can make selecting a precise contact between these formations 600 w Ops j Ops Opo 24, p. 457-475. Croixan) Jordan Sandstone, and the nature of the Cambrian- challenging. e s j basin (see Fig. 2 and cross sections). Furthermore, the Prairie du Chien Group thins abruptly and is 400 m U   Austin, G.S., 1972, Paleozoic lithostratigraphy of southeastern Ordovician boundary in the upper Mississippi valley: absent toward these faults, unconformably placing the St. Peter Sandstone directly atop the Jordan Tunnel City Group (Late Cambrian)—The Tunnel City Group, formerly named the D U tm tl  U D  tl s Minnesota, in Sims, P.K., and Morey, G.B., eds., Geology Geological Society of America Bulletin, v. 106, p. 492-506. D w  Sandstone (Fig. 5). The erosional limit of the Prairie du Chien Group is depicted by a thick dashed Franconia Formation (Berg, 1954), varies from about 135 to 145 feet (41 to 44 meters) 200 D/U e w of Minnesota: A centennial volume: Minnesota Geological e ———1996, Bedrock geology of Houston County, Minnesota: line on the map. This phenomenon has not been documented in other parts of the state, and is in thickness across Hennepin County. It is formally divided into two formations: the m Survey, p. 459-473. Minnesota Geological Survey Open-File Report 96-4, 11 p., 0 Mss  interpreted to be the result of subaerial exposure and erosion of the Prairie du Chien Group during Mazomanie Formation and the Lone Rock Formation (Mossler, 2008). The upper ZONE Mss m Berg, R.R., 1954, Franconia Formation of Minnesota and 3 pls., scale 1:100,000. post-depositional structural movement related to the formation of the Twin Cities basin. contact of the Tunnel City Group with the St. Lawrence Formation is conformable. Elevation (feet above sea level) Mbv -200 Mss Wisconsin: Geological Society of America Bulletin, v. 66, Runkel, A.C., McKay, R.M., Miller, J.F., Palmer, A.R., and Taylor, PINE FAULT The contact is fairly sharp, and the contrast between the siltstone and shale of the

DOUGLAS FAULT Drill cutting samples and bedrock core in areas where the Prairie du Chien Group is missing Mss p. 857-882. ST. CROIX HORST J.F., 2007, High resolution sequence stratigraphy of lower show a deposit of variably thick iron-cemented sandstone and ooidal chert intraclasts incorporated St. Lawrence Formation and the underlying fine- to medium-grained, quartzose -400 Chandler, V.W., 1991, Aeromagnetic anomaly map of Minnesota: Paleozoic sheet sandstones in central North America: The role Vertical exaggeration = 10x within the basal Pigs Eye Member of the St. Peter Sandstone (Fig. 1) at or near the unconformable sandstone in the Mazomanie Formation and the very fine- to fine-grained glauconitic, Minnesota Geological Survey State Map S-17, scale of special conditions of cratonic interiors in development of Elevation of the top of the contact with the Jordan Sandstone (Retzler and others, 2016; this map). The ooidal chert intraclasts feldspathic sandstone and siltstone in the Lone Rock Formation of the Tunnel City 1:500,000. stratal architecture: Geological Society of America Bulletin, F Anticline Wonewoc Sandstone (in feet) are likely derived from the Prairie du Chien Group because it is the only Paleozoic unit in the Twin Group is distinct and usually marked by an intraclastic conglomerate. D/U Green, J.A., Runkel, A.C., and Alexander, E.C., Jr., 2012, Karst v. 119, no. 7/8, p. 860-881. U\D U 751-800 Cities metropolitan area known to contain ooids. Additionally, the presence of an unusual iron- _tm Mazomanie Formation (Late Cambrian)—The Mazomanie Formation is dominantly D M Syncline conduit flow in the Cambrian St. Lawrence confining unit, Runkel, A.C., McKay, R.M., and Palmer, A.R., 1998, Origin of

oxide coating and cement, as well as intraclasts, lends itself to an interpretation of a lag deposit of white to yellowish-gray, fine- to medium-grained, cross-stratified, generally friable, M M 701-750 southeast Minnesota, USA: Carbonates Evaporites, v. 27, a classic cratonic sheet sandstone: Stratigraphy across the Paleozoic fault weathered Prairie du Chien Group material that was later reworked and deposited as the lowermost quartzose sandstone with interbedded, brown, dolomitic sandstone. Glauconitic grains F no. 2, p. 167-172. Sauk II–Sauk III boundary in the upper Mississippi valley: U U/D Fault movement 651-700 part of the St. Peter Sandstone (Retzler and others, 2016; this map). typically are absent and never exceed 5 percent (Berg, 1954). Skolithos burrows and D D Howell, P.D., and van der Pluijm, B.A., 1990, Early history of Geological Society of America Bulletin, v. 110, p. 188-210. D U Syndepositional structural movements and thickness changes in the Prairie du Chien Group sandstone intraclasts are common along discrete horizons. In northern Hennepin F F 601-650 the Michigan basin: Subsidence and Appalachian tectonics: Runkel, A.C., Miller, J.F., McKay, R.M., Shaw, T.H., and Bassett, U U have also been noted on the eastern rim of the Twin Cities basin atop a preexisting Midcontinent County, the Mazomanie Formation can reach as much as 50 feet (15 meters) thick. D Geology, v. 18, p. 1195-1198. D.J., 1999, Cambrian-Ordovician boundary strata in the 551-600 Rift horst in Washington County to the east and into western Wisconsin (Steenberg and others, 2015; The Mazomanie Formation thins to absent towards the south–southwest, where it F Jirsa, M.A., Boerboom, T.J., and Chandler, V.W., 2012, Geologic central mid-continent of North America: Acta Universitatis M F Steenberg and Retzler, 2016). Folding within the Prairie du Chien Group, believed to be tectonic is progressively replaced laterally by the Lone Rock Formation.

( ( Mesoproterozoic thrust fault 501-550 map of Minnesota, Precambrian bedrock geology: Minnesota Carolinae Geologica, v. 43, p. 17-20.

( in origin, has also been documented in Wisconsin and Iowa by Ludvigson and McAdams (1980). _tl Lone Rock Formation (Late Cambrian)—The Lone Rock Formation underlies the

Paleozoic fault 451-500 Geological Survey State Map S-22, scale 1:500,000. Runkel, A.C., Mossler, J.H., Tipping, R.G., and Bauer, E.G.,

M Several other midcontinent structural features (such as the Michigan Basin, Wisconsin Arch, and Mazomanie Formation and intertongues with it. It consists of pale yellowish-green, ( ANOKA

( D WASHINGTON Jirsa, M.A., Boerboom, T.J., Chandler, V.W., Mossler, J.H., 2006a, A hydrogeologic and mapping investigation of the

Line of cross section U Hollandale Embayment) were active roughly contemporaneously and possibly in response to far- very fine- to fine-grained glauconitic, feldspathic sandstone and siltstone, with thin,

( 401-450 St. Lawrence Formation in the Twin Cities metropolitan ( Runkel, A.C., and Setterholm, D.R., 2011, Geologic map of

field stresses generated during orogenic activity along present-day eastern North America (Howell greenish-gray shale partings. Thin beds with dolomitic intraclasts are common.

B F WRIGHT Minnesota, bedrock geology: Minnesota Geological Survey area: Minnesota Geological Survey Open-File Report (

( U 351-400 and van der Pluijm, 1990; Smith and others, 1993). The development of the Twin Cities basin The Lone Rock Formation ranges in thickness from 85 feet to as much as 145 feet

F D F State Map S-21, scale 1:500,000. 06-4, 20 p. ( DOUGLAS FAULT (

M 301-350 and its effect on Prairie du Chien Group deposition may also be an early manifestation of these (26 to 44 meters) in southern Hennepin County where the Mazomanie Formation

ZONE Ludvigson, G.A., and McAdams, M.P., 1980, New evidence of Runkel, A.C., Steenberg, J.R., Retzler, A.J., and Tipping, R.G., ( ( F orogenic far-field stresses (Steenberg and others, 2015; Retzler and others, 2016; Steenberg and is absent.

251-300 early Ordovician tectonism in the Upper Mississippi Valley: 2015a, Fracture stratigraphy in lower Paleozoic bedrock of

( Retzler, 2016; this map).

( F

PINE FAULT Iowa Geological Survey Technical Information Series no. the central midcontinent, North America: Geological Society _w Wonewoc Sandstone (Late Cambrian)—This sandstone unit, formerly referred to as

( 201-250 the Ironton-, is composed mostly of fine- to coarse-grained, ( F of America Abstracts with Programs, v. 47, no. 5, p. 12.

9, 29 p. HYDROSTRATIGRAPHY

RAMSEY Runkel, A.C., Steenberg, J.R., Tipping, R.G., Jansen, S., moderately to well sorted, light gray, cross-stratified, quartzose sandstone (Mossler, ( ( Less than 200 HENNEPIN Luhmann, A.J., Covington, M.D., Peters, A.J., Alexander, S.C.,

Anger, C.T., Green, J.A., Runkel, A.C., and Alexander, E.C., and Retzler, A.J., 2015b, Hydraulic conductivity and The Paleozoic bedrock formations contain significant sources of groundwater that provides the 2008). White, brown, and black linguliform brachiopod shells are locally abundant.

Mss Mss (

( D Figure 3. Map depicting the elevation of the stratigraphic top Jr., 2011, Classification of thermal patterns at karst springs hydrostratigraphy of the Platteville Formation, Twin Cities water supply for much of Hennepin County. This map and associated products, such as the bedrock The upper part is the coarsest-grained; the lower part is finer-grained, better sorted, ( F

( U of the Wonewoc Sandstone and corresponding mapped fold axes and progressively finer-grained toward its base. The very fine-grained sandstone in

M and cave streams: Ground Water, v. 49, no. 3, p. 324-335. Metropolitan Area, Minnesota: Minnesota Geological Survey unit digital elevation models, provide a three dimensional depiction of the rock properties that control

( D

( M and faults in Hennepin County. Shaded intervals represent 25- Open-File Report 15-01, 34 p. flow in these water-bearing layers. Such rock properties are called hydrostratigraphic properties. the lower part is feldspathic. The thickness of the formation is 45 to 75 feet (14 to ( U Morey, G.B., 2001, Compositions of rift-related volcanic rocks

Mbv foot (7.6-meter) elevation intervals, with blue colors indicative Runkel, A.C., Steenberg, J.R., Tipping, R.G., and Retzler, A.J., 23 meters). The Wonewoc Sandstone is conformable with overlying and underlying ( U of the Keweenawan Supergroup atop the St. Croix horst, The hydrostratigraphic classification, shown by brown and blue colors on the hydrostratigraphic

(with thin U D M of lower elevations and red colors of higher. In areas where the formations; however, there is a subtle unconformity marked by a pebbly sandstone ( 2014a, Geologic controls on groundwater and surface water ( southeastern Minnesota: Minnesota Geological Survey properties column of Figure 1, distinguishes layers that are dominated by relatively high permeability DOUGLAS FAULT Mss cover) D

Wonewoc Sandstone is not penetrated by water-well data or is Information Circular IC-47, 36 p. flow in southeastern Minnesota and its impact on nitrate (easily transmitting water) material, versus layers dominated by lower permeability (relatively harder layer within the formation (Runkel and others, 1998).

( F ( Mss absent because of erosion, its top elevations are inferred from concentrations in streams: Minnesota Geological Survey

Mcv CARVER Mossler, J.H., 1972, Paleozoic structure and stratigraphy of the to transmit water) material. This generalized characterization for Hennepin County is based on _e (Middle to Late Cambrian)—The formation is composed of

( Open-File Report 14-02, 70 p.

( vertical projections of formation contacts stratigraphically higher Twin City region, in Sims, P.K., and Morey, G.B., eds., hydrogeologic reports by Runkel (1996), Paillet and others (2000), Runkel and others (2003, 2006a, b, yellowish-gray to pale olive-gray, fine- to very fine-grained, feldspathic sandstone,

( or lower in the section, respectively. Furthermore, fold axes and Geology of Minnesota: A centennial volume: Minnesota Runkel, A.C., Tipping, R.G., Alexander, E.C., Jr., and Alexander, 2014a, b), Tipping and others (2006), Anderson and others (2011), Luhmann and others (2011), Green siltstone, and shale. White and brown linguliform brachiopod shells are common. A ( A' 0 2 4 8 Miles

faults were interpreted using a culmination of stratigraphic top S.C., 2006b, Hydrostratigraphic characterization of ( Geological Survey, p. 485-497. and others (2012), and unpublished borehole and core data collected by the Minnesota Geological The formation ranges from 80 to 90 feet (24 to 27 meters) in thickness. The contact SCOTT

contour data for bedrock geologic units other than the Wonewoc intergranular and secondary porosity in part of the Cambrian Survey. The high permeability layers are potential aquifers, able to yield economic quantities of with the Mt. Simon Sandstone is conformable and transitional, which is commonly ( ———1992, Sedimentary rocks of Dresbachian age (Late Sandstone, most notably the Jordan Sandstone. Note the radial Cambrian), Hollandale embayment, southeastern Minnesota: sandstone aquifer system of the cratonic interior of North water in most places. The low permeability layers are potential aquitards that retard vertical flow, apparent on natural gamma-ray logs. pattern of the stratigraphic top elevation map in eastern Hennepin America: Improving predictability of hydrogeologic DAKOTA Minnesota Geological Survey Report of Investigations RI- hydraulically separating the aquifer layers from one another in many places, and protecting water ­_m Mt. Simon Sandstone (Middle Cambrian)—The Mt. Simon Sandstone is pale yellowish- properties: Sedimentary Geology, v. 184, p. 281-304. County, an effect of the Twin Cities basin (see Fig. 4). Elevation of the top of the 40, 71 p. resources in the underlying layers from surface contamination. Fractures parallel to bedding that brown to grayish-orange-pink to light gray, medium- to coarse-grained, quartzose 0 2 4 8 Miles Jordan Sandstone (in feet) ———2007, Bedrock geology of the Coon Rapids quadrangle, Runkel, A.C., Tipping, R.G., Alexander, E.C., Jr., and Green, J.A., have been demonstrated to have high permeability are likely to be present, as least locally, in all sandstone. Interbeds of shale, siltstone, and very fine-grained, feldspathic sandstone High : 1,057 Anoka and Hennepin Counties, Minnesota: Minnesota 2003, Hydrogeology of the Paleozoic bedrock in southeastern formations in Hennepin County, but are placed on the column where the hydrogeologic reports are common, particularly in its upper half (Mossler, 1992). Linguliform brachiopod B' Low : 443 Geological Survey Miscellaneous Map M-175, scale Minnesota: Minnesota Geological Survey Report of cited above indicate they are most common. Most bedrock springs in Hennepin County emanate shells are locally common in the upper one-third of the formation. Thin beds Investigations RI-61, 105 p., 2 pls. Figure 5. Color-shaded map indicating the Paleozoic fault 1:24,000. from the Platteville Formation in the metropolitan area and along the Mississippi River, where of quartz-pebble conglomerate occur at several stratigraphic positions, and are Runkel, A.C., Tipping, R.G., Green, J.A., Jones, P.M., Meyer, thickness of the Prairie du Chien Group in Hennepin Twin Cities basin extent ———2008, Paleozoic stratigraphic nomenclature for Minnesota: high permeability vertical and bed-parallel fractures are known to be common. The vast majority especially abundant near the base of the formation. The Mt. Simon Sandstone County. Red colors correspond to areas where the Minnesota Geological Survey Report of Investigations RI- J.R., Parker, B.L., Steenberg, J.A., and Retzler, A.J., 2014b, of Platteville Formation springs in the Twin Cities Metropolitan Area emanate from the contact unconformably overlies Mesoproterozoic rocks. Based on a limited number of drill Hydrogeologic properties of the St. Lawrence aquitard, Figure 2. Faults that displace Paleozoic bedrock in Hennepin County superimposed on the underlying Twin Cities basin extent Prairie du Chien Group is thicker and blue colors 65, 76 p., 1 pl. between the uppermost Magnolia Member and underlying Hidden Falls Member due to a contrast holes, the formation has a maximum thickness of about 280 feet (85 meters) in the (in Hennepin County) southeastern Minnesota: Minnesota Geological Survey Mesoproterozoic rock units and interpreted thrust faults modified from Jirsa and others (2012), and the to areas where it is thinner. The black outlined 0 4.5 9 18 Miles ———2009, Bedrock geology of the Anoka quadrangle, Anoka in mechanical properties leading to the preferential termination of vertical fractures at or near this southeastern portion of the county and thins to about 140 feet (42 meters) towards Eroded edge of the Open-File Report 14-04, 56 p. first vertical derivative magnetic data reduced to pole from Chandler (1991). The two Mesoproterozoic polygons represent the mapped extent of the St. and Hennepin Counties, Minnesota: Minnesota Geological contact (Anderson and others, 2011; Runkel and others, 2015a, b). Deeper into the subsurface, the the west–northwest. Prairie du Chien Group Smith, G.L., Byers, C.W., and Dott, R.H., Jr., 1993, Sequence faults, the Douglas fault and Pine fault, are the western-bounding, high-angle faults constraining the Peter Sandstone. Color-shaded areas within those Figure 4. Regional extent of the Twin Cities basin (blue outline), as illustrated by the elevation of the Survey Miscellaneous Map M-184, scale 1:24,000. hydrogeologic properties of the Mesoproterozoic bedrock are poorly understood, and it does not Mapped extent of stratigraphy of the lower Ordovician Prairie du Chien central horst structure of the Midcontinent Rift System, known as the St. Croix horst (Morey, 2001; polygons correspond to where a variable thickness stratigraphic top of the Jordan Sandstone, and its relation to the geographic location of Hennepin County ———2013, Bedrock geology of the Twin Cities ten-county supply water to Hennepin County, at least in part, because sufficient water resources are available MESOPROTEROZOIC the St. Peter Sandstone Group on the Wisconsin arch and in the Michigan basin: Jirsa and others, 2012). The locations of the Douglas and Pine faults were reinterpreted from Jirsa of the Prairie du Chien Group is present between (magenta outline) and mapped Paleozoic faults. The depicted extent of the basin was first interpreted metropolitan area, Minnesota: Minnesota Geological Survey at shallower depths within the Paleozoic rocks. Paleozoic fault American Association of Petroleum Geologists Bulletin, Keweenawan Supergroup and Midcontinent Rift Intrusive Supersuite the St. Peter Sandstone and Jordan Sandstone. by Mossler (2008) and reinterpreted herein using a digital elevation model of the stratigraphic top of the Miscellaneous Map M-194, scale 1:125,000. In Hennepin County, most aquifers are layers dominated by relatively coarse-grained sandstone, and others (2012) from new insights on the bedrock geology during this mapping effort. Due to their v. 77, p. 49-67. similar trends and close proximity to the Mesoproterozoic thrust faults, many of the Paleozoic faults Thickness of the Color-shaded areas outside of these polygons Jordan Sandstone as proxy. The digital elevation model was interpolated using the ESRI ArcMap Topo Mossler, J.H., and Tipping, R.G., 2000, Bedrock geology and such as the upper part of the Jordan Sandstone, in which water can be more easily transmitted in both Mss Sandstone, siltstone, and local conglomerate (shown only on Fig. 2 and the cross Steenberg, J.R., and Chandler, V.W., 2013, Bedrock geology, are interpreted to have originated from reactivation of the deep-seated Mesoproterozoic structures— Prairie du Chien represent the Prairie du Chien Group as first to Raster tool from a culmination of 50-foot (15-meter) elevation contours interpreted by Mossler (2013), structure of the seven-county Twin Cities metropolitan area, horizontal and vertical directions through the pore spaces between sand grains, as well as through sections)—Includes the quartzose Hinckley Sandstone and the more feldspar- and Group (in feet) pl. 2 of Tipping, R.G., project manager, Geologic atlas of particularly those mapped just above and parallel to the deep-seated faults. Additionally, the mapped bedrock, and are thinner due to erosion during Steenberg and Retzler (2016), and this mapping effort. The interpreted edge of the Twin Cities basin Minnesota: Minnesota Geological Survey Miscellaneous fractures. Other aquifers, such as the upper part of the Prairie du Chien Group, are composed mostly lithic-rich sandstones and shale of the Fond du Lac and Solor Church Formations; Wright County, Minnesota: Minnesota Geological Survey Less than 25 deposition of the overlying glacial sediment. Note roughly corresponds to the 750-foot (228-meter) elevation contour of the stratigraphic top of the Jordan Map M-104, scale 1:250,000. of carbonate rock in which water is transmitted through a relatively dense network of fractures deposited in eolian, fluvial, and lacustrine environments (Jirsa and others, 2012). extent of the Twin Cities basin corresponds with the Paleozoic fault directly overlying the Pine fault County Atlas C-30, scale 1:100,000, 6 pls. (see Fig. 4), and the thickness trends and abrupt absence of the Prairie du Chien Group shows a similar the abrupt thinning and absence of the Prairie du Sandstone across much, but not all, of the interpolated extent shown. The Paleozoic faults represented Olsen, B.M., and Bloomgren, B.A., 1989, Bedrock geology, pl. and solution cavities. However, layers designated as aquifers can locally contain low permeability North Branch mafic volcanic sequence (shown only on Fig. 2 and the cross sections)— 26 - 50 Steenberg, J.R., and Retzler, A.J., 2016, Bedrock geology, pl. 2 of Mbv correspondence (see Fig. 5). The area in which the mapped Paleozoic fault diverges significantly from Chien Group along an erosional line (dashed line) are from Jirsa and others (2011), Steenberg and Retzler (2016), and this map. Note the close relationship 2 of Balaban, N.H., ed., Geologic atlas of Hennepin County, strata that serve as small, internal aquitards including parts of the upper Mt. Simon Sandstone and Part of the St. Croix horst volcanic sequence (Jirsa and others, 2012). Bauer, E.J., project manager, Geologic atlas of Washington the mapped trace of the Mesoproterozoic Douglas fault is herein termed the Douglas fault zone. Lines 51 - 75 in close proximity to the Paleozoic fault system and between the Twin Cities basin and the Paleozoic faults—in particular, faults in Washington and Hennepin Minnesota: Minnesota Geological Survey County Atlas C-4, the lower Jordan Sandstone. County, Minnesota: Minnesota Geological Survey County Mcv Chengwatana volcanic rock; primarily mafic flows (shown only on Fig. 2 and cross outermost extent of the Twin Cities basin (thick Counties that bound the eastern and western edges of the basin. The formation of the Twin Cities basin scale 1:100,000. of cross section are shown for reference. Please refer to the map unit descriptions for the labeled 76 - 100 Atlas C-39, scale 1:100,000, 6 pls. section A­–A')—Part of the St. Croix horst volcanic sequence (Jirsa and others, Mesoproterozoic rock units. Note that the Mesoproterozoic unit has a thin, approximately 30- black line) to the west–northwest. Areas in which is commonly attributed to isostatic and/or thermal adjustments accommodated by Midcontinent Rift Paillet, F.L., Lundy, J., Tipping, R.G., Runkel, A.C., Reeves, L., Mbv Steenberg, J.R., Retzler, A.J., and Runkel, A.C., 2015, Evidence 2012). foot-thick (9-meter-thick) cover of unit above it (as supported by water-well records), as well as 101 - 125 the mapped extent of the St. Peter Sandstone lies structures and reactivated Paleozoic structures during Middle to at least Late Ordovician time (Austin, and Green, J., 2000, Hydrogeologic characterization of six Mss for Early Ordovician reactivation of Mesoproterozoic rift foliation trends in the lava flows (as inferred from aeromagnetic data), both of which are not shown for beyond the erosional line (unshaded polygons) 1972; Mossler, 1972). sites in southeastern Minnesota using borehole flowmeters 126 - 150 structures within the Prairie du Chien Group: Washington indicate St. Peter Sandstone deposited directly and other geophysical logs: U.S. Geological Survey Water- the purposes of this figure. These are depicted in the corresponding cross sections, with the inferred County, Minnesota: Geological Society of America Abstracts 151 - 175 atop Jordan Sandstone. This phenomenon is Resources Investigation Report 00-4142, 33 p. foliation trends represented by long, dashed lines. with Programs, v. 47, no. 5, p. 81. thought to be due to movement along the Paleozoic Retzler, A.J., Steenberg, J.R., and Runkel, A.C., 2016, Depositional Tipping, R.G., Runkel, A.C., Alexander, E.C., Jr., and Alexander, fault system during the reactivation of underlying trends of the Prairie du Chien Group (Lower Ordovician) S.C., 2006, Evidence for hydraulic heterogeneity and Mesoproterozoic faults and the development of along the western edge of the Twin Cities basin, Minnesota: anisotropy in the mostly carbonate Prairie du Chien Group, the Twin Cities basin, and is unique to Hennepin Possible evidence of syn- to post-depositional basin formation: 0 2 4 8 Miles southeastern Minnesota, USA: Sedimentary Geology, v. County (Austin, 1972; Mossler, 1972, 2013; Olsen 184, p. 305-330. and Bloomgren, 1989; Mossler and Tipping, 2000; Every reasonable effort has been made to ensure the accuracy of the factual data on which this map interpretation is Retzler and others, 2016; this map). based; however, the Minnesota Geological Survey does not warrant or guarantee that there are no errors. Users may wish to verify critical information; sources include both the references listed here and information on file at the offices ©2018 by the Regents of the University of Minnesota ENVIRONMENT of the Minnesota Geological Survey in St. Paul. In addition, effort has been made to ensure that the interpretation AND NATURAL RESOURCES conforms to sound geologic and cartographic principles. No claim is made that the interpretation shown is rigorously The University of Minnesota is an equal opportunity educator and employer TRUST FUND GEOLOGIC ATLAS OF HENNEPIN COUNTY, MINNESOTA correct, however, and it should not be used to guide engineering-scale decisions without site-specific verification.