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Home , Geology of Minnesota, Glenwood Formation

Prepared and Published with the Support of COUNTY ATLAS SERIES THE WASHINGTON COUNTY BOARD OF COMMISSIONERS, ATLAS C-39, PART A GEOLOGICAL SURVEY Washington County Harvey Thorleifson, Director THE MINNESOTA DEPARTMENT OF NATURAL RESOURCES, DIVISION OF ECOLOGICAL AND WATER RESOURCES, AND the Minnesota Legacy Amendment's Clean Water Fund Plate 2—Bedrock Geology

CHISAGO COUNTY Y CHISAGO COUNTY T 93° W. 92° 52' 30" W. 92° 45' W. N R. 21 W. R. 20 W. U R. 19 W. O    U C s Sea s t   Opo t BEDROCK Geology 6 j D POLK A 275 Bone   j t Lake COUNTY 1 6 s Lake  1 Ops6 O K j

N 275 Nielsen

275 A  ¤8 t U s Lake300 D Forest U 300 By 300 )95 C 35 C' ¦§  j 300 Opo Forest Lake Opo Goose Mud Lake 300 Julia R. Steenberg and Andrew J. Retzler Lake  300 Os w Lake GermanOps Os 275 300 Lake

Clear Lake Ops 275  s 300 Opo w T. 32 N. t 2016 T. 32 N. Lithostratigraphic Composite natural gamma log   unit 275 j 300 Ops s  97 225 45° 15' N. Era Lithology 45° 15' N.  s ) 250 97 j Os Os Group, Increasing count LOCATION DIAGRAM ) FOREST LAKE Sylvan 300 SCANDIA 300 300 Formation, Lake 0 100

M System-Series  Map symbol Hydrostratigraphic properties j Os Member API-G units Thickness (in feet) s j Fish Ops Lake Hay 300 Lake Opo Decorah Od <40 Galena Group Opo 300 j s t ¤61 300 300 Sand t White Ops Lake Platteville and 300 Opg Glenwood Formations 30-35 INTRODUCTION distinguish the Oneota from the Shakopee Formation and St. Peter Ph Lake Long Lake from the sandy upper Shakopee Formation. Therefore, the mapped distribution of these Opo CORRELATION OF MAP UNITS The , cross sections, and stratigraphic column on this plate depict the type, distribution, 31 Big Marine Lake units is more speculative in these areas. The thickness of the du Chien Group 31 36  36 250 31 Os s 300 Upper

 POLK and structure of the bedrock units in Washington County that are either exposed at the land surface or lie Old Mill Stream  w beneath the St. Peter Sandstone varies greatly across Washington County, from less than

j j F Os COUNTY directly beneath unconsolidated glacial sediments of variable thickness (see cross sections j 50 to nearly 300 feet (15 to 91 meters; Fig. 5). This is interpreted to be due to several and Plates 3, 4, and 5, Surficial Geology, Quaternary Stratigraphy, Depth to Bedrock, and Bedrock s ST. CROIX Od factors, including syndepositional faulting along the Hudson–Afton horst, and of Horseshoe 1 6 Lake Tonti Upper Ordovician Topography). The map shows how the bedrock surface would appear if it was viewed from an aerial Creek Creek 6 1 COUNTY the Shakopee Formation prior to deposition of the St. Peter Sandstone. This erosional 6 Ops St. Peter 300 Opg perspective and the overlying Quaternary sediments were stripped away. The bedrock units near the land 300 Sandstone Os surface is the unconformity that marks the contact between the Shakopee Formation and Os Marine275 On 140-160 surface in Washington County consist of sedimentary rocks of age that form distinguishable St Croix overlying St. Peter Sandstone. Opo Os 300 Middle Ordovician and mappable layers designated as formations. These units are commonly exposed along the Mississippi Ops Shakopee Formation (Lower Ordovician)—A heterolithic unit composed mainly of light 300  unconformity and St. Croix River bluffs, within rock quarries, and along roadcuts within the county. Several of the Hardwood Ops j brown, thin- to medium-bedded dolostone, sandy dolostone, sandstone, and shale. It Hardwood Terrapin Paleozoic bedrock formations are major reservoirs for water supply in Washington County and also 275 Ops 300 Mud Ops Lake 300 contains oolites, intraclasts, fossilized microbial mounds, chert nodules, quartz sandstone, Oneka Turtle Middle Ordovician Pigs LITHOLOGY KEY provide a source of crushed carbonate rock and silica sand. Lake Opo Lake Lake Eye unconformity and green-gray shale partings. Thickness of the Shakopee Formation beneath the St. Ops )95 Lower Ordovician Characteristics of each formation are given in the stratigraphic column (Fig. 1) and in the description Os Peter Sandstone is quite variable within the area of the Hudson–Afton horst, ranging Opo of map units. The accompanying bedrock geologic cross sections add the dimension of depth and illustrate HUGO 275MAY from almost absent to nearly 200 feet (61 meters) thick. It appears to be thickest in the Os 300  the stratigraphic, structural, and topographic relationships of the bedrock units, as well as the variable T. 31 N. Hugo j T. 31 N. unconformity PALEOZOIC most southeast part of the county, east of the Hastings , where it is nearly 200 feet 275 Os St. Croix Shakopee Dolostone thickness of the overlying Quaternary sediments. Surfaces representative of the elevation of the tops of   Ops (61 meters) thick. On the opposite side of the horst, on the west side of the Cottage

w e Formation j 250 Rice 90-120 the mapped formations are also available as Digital Elevation Models (DEMs) for use in GIS programs. Os Grove fault, it reaches thicknesses of 115 feet (35 meters) and appears to progressively Egg Lake s Clearwater Os 300 Square Sandy dolostone The geologic formations are thin in relation to their aerial extent, and would only be one-tenth as thick Lake Opo s thin towards the northwest. Based on a limited amount of drill cuttings and geophysical

Creek Lake  as shown on the cross sections if no vertical exaggeration were used. The exaggeration necessary to j Sandstone Upper data within the Hudson–Afton horst, it appears that the Shakopee Formation thins to 300Ops  show the thin rock formations gives the appearance of steeper slopes on bedrock unit contacts, the land

Lower Ordovician t 300 Hager Very fine- to fine-grained less than 50 feet (15 meters) and may even be absent beneath the St. Peter Sandstone at Y

Os Prairie du Chien Group Oneota surface, and bedrock topography. Most of the Paleozoic units shown on this plate, with the exception T  City Opo several locations (see cross section A–A').

N 275 Dolomite

t 50-80 Fine- to medium-grained unconformity of the Mt. Simon and Wonewoc and , can be seen at the land surface U ¤61 School w Opo (Lower Ordovician)—Predominantly a yellowish-gray to light brown, O Section Coon in places in Washington County. The sedimentary rocks differ in their resistance to weathering and C Sunset Valley G Lake Os  medium- to thick-bedded dolostone that generally lacks sedimentary features such as Bald Lake Long e 31 River Medium- to coarse-grained erosion, as indicated by the weathering profile on the lithology column of Figure 1. The units that cover Lake Big oolites and quartz sand characteristic of the Shakopee Formation, except in its lowermost O K A Eagle Os 31 Middle Cambrian the largest areas of the map are the most resistant to weathering and generally form plateaus composed N 36 Carnelian 36 Ops 31 275  A Lake Lake m part. The formation contains two members, the Hager City and the Coon Valley, but Round Opo s Shaly of carbonate rock (limestone and dolostone). The soft sandstone and shale formations are more easily 45° 7' 30" N. Os Os 275 they are not mapped separately. The basal Coon Valley Member is a heterolithic unit Fish Lake 45° 7' 30" N. unconformity eroded and commonly occur on bedrock plateau or valley walls. Lake 300 275  Jordan  RAMSEY COUNTY j  j composed of thinly bedded dolostone, sandy dolostone, and beds of fine- to coarse-

j Sandstone 85-100 Louise 225 Lake Little Siltstone Mss Production of the map and associated products relied on several data sources, including outcrops, grained, poorly sorted quartz sandstone. Thickness of the Coon Valley Member is quite 6 1 6 Opo Carnelian 275 1 water-well and scientific drilling records from the County Well Index, rock core, drill cutting samples, Mann Loon Lake variable, it is locally absent to 30 feet (9 meters) thick. There appears to be a slight Benz Lake Os s OpsLake Shale Mbv borehole geophysical logs, seismic soundings, geophysical images, and previously published geologic

Lake Brown's trend in its thickness along the Hudson–Afton horst whereby it is thickest on the down

Dellwood 300 w Ops maps of Washington and adjacent counties (Mossler and Bloomgren, 1990; Mossler and Tipping, 2000; Pine M 300 Silver 275Opo dropped sides of the faults (20 to 30 feet [6 to 9 meters]) and thinner between the faults Tree  Chert Mossler, 2005a, b, 2006a, b, c, d, 2013; Anderson, 2009). This map supersedes the previous bedrock w St. Lawrence Mpv (10 to 15 feet [3 to 5 meters]). It also appears to be thin to absent towards the western Os Lake )95 s Os Lake 275 275 92° 45' W. Formation G geology map of Washington County (Mossler and Bloomgren, 1990). Significant improvements and 35-45 edge of the county and within the central and northern parts of the county. The Hager B 300 Ops G Oolites

Os Silver modifications were made to the previous map based on additional water-well records, drill cuttings,

 City Member is primarily very finely crystalline dolostone, with microbial textures. Its 300 jSTILLWATER M G borehole geophysics, and refined geophysical images. The Prairie du Chien Group is separated into Creek Glauconite GRANT Opo thickness also shows a similar trend to the Coon Valley Member in being thicker along Creek the Oneota Dolomite and Shakopee Formation and the St. Lawrence Formation is mapped separately

Mazomanie Ph Phosphate grains the down-dropped sides of the Hastings and Cottage Grove faults (50 to 70 feet [15 to 244 275 ) F MAP SYMBOLS Formation from the Tunnel City Group (previously the ). Faults and folds in the southeastern 21 meters]) and thinner between them (40 to 50 feet [12 to 15 meters]). Os )96 Twin Long Lakes G Stromatolites Geologic contact, approximatly located part of the county were mapped to reflect significant offset in the Paleozoic formations. This provides T. 30 N. T. 30 N. _j (Upper Cambrian)—Dominantly white to yellow, very fine- to coarse- Lake Lake 300 Opo detail of the subsurface geologic conditions, which has implications for modeling groundwater flow.

PALEOZOIC Shells Geologic contact, inferred White Masterman Brown's Creek G grained, friable quartz sandstone characterized by coarsening-upward sequences consisting

Upper Cambrian The different data sources and their irregular distribution and density can be seen on Plate 1, Data-Base  Opo  Fault—Faults are inferred from abrupt changes in the elevation of of two interlayered facies (Runkel, 1994). They are medium- to coarse-grained, Lake s StillwaterOps t Bioturbation U Map, and these should be considered when assessing the reliability of the map at any particular location. Lake G 160-180 D stratigraphic units from subsurface and outcrop data. Letters cross-stratified, generally friable, quartz sandstone; and very fine-grained, commonly Os Lone Rock Areas with a high density of bedrock control points are more likely to have an accurate interpretation McKusick275 G Pebbles bioturbated, feldspathic sandstone with lenses of siltstone and shale. The major part of Tunnel City Group Tunnel indicate relative vertical displacement: U—up, D—down. Dashed Mahtomedi Ops Formation of the bedrock geology, whereas those areas with widely spaced control points may be less reliable

lines represent areas where it is inferred. the very fine-grained facies forms a regionally continuous interval that gradationally F Intraclasts and inappropriate for site-specific needs. During production of this map, the records of over 14,000 overlies the St. Lawrence Formation (unit _s), although there are lithologically similar 300 F M —Axial trace of anticline, syncline. Fold limbs typically have located water wells existed within Washington County in the County Well Index and nearly 10,000 of 300 Cross-bedded (planar) shallow dips and are inferred from subsurface data. intervals intercalated with the medium- to coarse-grained facies at higher stratigraphic 300 Opo those reached Paleozoic bedrock. Geologic interpretations of subsurface material described by drillers 275 intervals. An unconformity, locally marked by thin beds of quartz pebble conglomerate Lily Active quarry Pine Springs Os Long Cross-bedded (trough) or gathered from other data sources were made by the authors and are represented in the County Well Opg 31 Lake and silcrete-cemented sandstone clasts (Runkel and others, 1999, 2007), separates the 300 36 Lake 250 Index records. Every reasonable effort has been made for the geologic interpretations in the County 31 325 Cross-bedded (hummocky) Bedrock outcrop Jordan Sandstone from the overlying Oneota Dolomite. The Jordan Sandstone is exposed Long Ops Wonewoc Od )36 Ops  Well Index to correspond to the map. Lake w along the Mississippi and St. Croix River bluffs and ranges in thickness from 80 to 100 Sandstone 50-60 Location of geologic cross section Dolomitic A' Paleozoic bedrock lies on top of a thick sequence of Mesoproterozoic (approximately 1,100 million 300 A feet (24 to 29 meters). In the northernmost part of the county near Scandia, it appears j years, or Ma, ago) rocks of the Keweenawan Supergroup, associated with the Midcontinent (see Os 1 6 Oak Park Heights 2 Vugs to thin to 65 to 70 feet (20 to 21 meters) thick. 6 G cross sections and Fig. 2). These rocks include sandstone, siltstone, and shale of the Hinckley Sandstone, Clear300 Opo _ 300 Cloverdale s St. Lawrence Formation (Upper Cambrian)—The St. Lawrence Formation is principally Lake Lake G Contact marks a major erosional surface Solor Church and Fond du Lac Formations (unit Mss), and volcanic rocks composed mostly of

Od )36 Demontreville 300 Lake light gray to yellowish-gray and pale yellowish-green, dolomitic, feldspathic siltstone

275 including the Powder Mill (unit Mpv) and North Branch (unit Mbv) volcanic sequences. Due to their

Olson 275 275 McDonald M with interbedded, very fine-grained sandstone and shale. Lenses and layers of light Lake Lake Os 275 Jane Lake Eau Claire e deep burial and limited subsurface data, the distribution of these individual units is less certain than the Os Os Bayport gray, finely crystalline, sandy dolostone occur locally, especially in the lowermost few Os Formation 80-100 Paleozoic units. Therefore, the contact between the top of these units and the overlying base of Mt. Od 300 feet of the formation (Runkel and others, 2006a). The formation is 35 to 45 feet (8

G Simon Sandstone is dashed in cross sections. No new mapping of these units was done for this project;

300 HYDROSTRATIGRAPHIC PROPERTIES KEY to 12 meters) thick. The upper contact with the Jordan Sandstone is conformable and 694 SunfishM ¦§ Ops F instead a recent compilation map of the bedrock geology of Minnesota is depicted (Fig. G 275 Lake BAYTOWN Relatively high permeability (aquifer) gradational. The gradational nature of the contact in well cuttings and on natural gamma 45° N. 5 45° N. 2; Jirsa and others, 2012). ) 275 logs can make selecting a precise contact between these formations difficult. T. 29 N. The Paleozoic rocks of Washington County are characterized by relatively thin, widespread layers of

)5 275 Relatively low permeability (except for fractures, aquitard) _t Tunnel City Group (Upper Cambrian)—The Tunnel City Group, formerly named the Franconia

T. 29 N. sandstone, shale, and carbonate deposited in shallow seas during the Cambrian and Ordovician Periods Os Os F Os Formation (Berg, 1954), varies from about 160 to 180 feet (49 to 55 meters) in thickness Opg of the Paleozoic Era, from about 500 to 450 million years ago. The older Cambrian-age formations are LAKE ELMO 275 )95 High permeability bedding fracture known to be common across Washington County. It is formally divided into two formations: the Mazomanie dominated by siliciclastic including sandstone and siltstone with minor shale, such 250 Every reasonable effort has been made to ensure the accuracy of the factual data on 325 Os and the Lone Rock Formations (Mossler, 2008). The Mazomanie Formation is dominantly Os  which this map interpretation is based; however, the Minnesota Geological Survey as the Jordan Sandstone and Mazomanie Formation (Tunnel City Group). Carbonate rock occurs only Eagle 275 Lakes white to yellowish-gray, fine- to medium-grained, cross-stratified, generally friable, quartz

Elmo 300 does not warrant or guarantee that there are no errors. Users may wish to verify critical as relatively thin layers within these units. Ordovician-age formations, in contrast, are dominated by Oakdale 325 Point Opo WEST information; sources include both the references listed here and information on file at the sandstone. Glauconitic grains typically are rare to absent and never exceed 5 percent 300Lake Os thicker units of carbonate rock with less sandstone and shale, such as the Prairie du Chien Group and offices of the Minnesota Geological Survey in St. Paul. In addition, effort has been made (Berg, 1954). Some beds contain brown, intergranular dolomite cement. Skolithos D the Platteville Formation. Where carbonate rock and silica-rich (quartzose) sandstone exist near the land LAKELAND U to ensure that the interpretation conforms to sound geologic and cartographic principles. burrows and sandstone intraclasts are common along discrete horizons. The Lone Rock Od surface (or within 50 feet [15 meters]), they are considered a valuable geologic resource. Carbonate 325 No claim is made that the interpretation shown is rigorously correct, however, and it should

Middle Cambrian Formation underlies the Mazomanie Formation and intertongues with it. It consists of Os 94 Mt. Simon m COUNTY EXPOSED IN WASHINGTON NOT Figure 1. Generalized stratigraphic column depicting 275 ¦§ not be used to guide engineering-scale decisions without site-specific verification. rock products are crushed, sorted, and used as construction material for roads and buildings, and in Tanners Goose Lake Sandstone 275 pale yellowish-green, very fine- to fine-grained glauconitic, feldspathic sandstone and

275 ~200-280 St. Croix the lithology, thickness, vertical succession, age, and concrete operations. The silica-rich sandstone is texturally mature and well-rounded, which makes it a Lake Ops 31  35 36 t 250 siltstone, with thin, greenish-gray shale partings. Thin beds with dolomitic intraclasts 31 Lakeland D hydrostratigraphic properties for all units shown on sought after resource in the oil industry. It also has a wide variety of other uses including glassmaking, 94 275 225 N are common. In northeastern Washington County, individual tongues of Mazomanie § 225 ¦§94 ¦ Shores the map, as well as the schematic depiction of relative foundry operations, ceramics, filtration, and agriculture. However, viability of extraction is dependent

300 A Formation are as thick as 50 feet (15 meters), and the Mazomanie Formation as a

 L competence in outcrop where exposed. The gamma log is on many other factors, including detailed geologic conditions at individual sites, proximity to bulk Os e whole can reach thicknesses of 100 feet (30 meters). The Mazomanie Formation thins 225 Woodbury E Battle Margrafs 1 6 2 a compilation of the following borehole geophysical logs transportation, current land ownership and use, market prices, regulatory requirements, and many others. 3006 Creek Opg Lake K m  to the south, where it is progressively replaced laterally by the Lone Rock Formation. Lake Lake e on file at the Minnesota Geological Survey: County Well Figure 3 depicts where carbonate or quartzose sandrock is within 50 feet (15 meters) of the land surface 275 j A As a result, in southern Washington County the Mazomanie Formation is less than 25 275 275  Edith t L Index unique numbers 783609, 777305, and 256005. and highlights the active quarry operations at the time this map was created. 300  F 494 250 t B' feet (8 meters) thick to absent. The upper contact of the Tunnel City Group with the Od ¦§ Across most of Washington County, bedrock units are slightly tilted (less than 1°) southwest towards

275 Wilmes River Powers  St. Lawrence Formation is conformable. The contact is fairly sharp and the contrast Valley w the central Twin Cities metropolitan area, as part of the eastern margin of a shallow structural depression Lake Lake Opg Lake between the siltstone and shale of the St. Lawrence Formation, and underlying fine- to 325 St Croix known as the Twin Cities basin. As a result, progressively younger bedrock formations subcrop from 325 D U medium-grained, quartzose sandstone in the Mazomanie Formation of the Tunnel City D Beach w F U eastern to western Washington County. This general trend is locally interrupted by deep valleys that Group, is distinct and typically marked by an intraclastic conglomerate. Os Branch incise older formations, and by faults and folds. A fault is recognized where changes in the elevation of Valley Mesoproterozoic and older _w Wonewoc Sandstone (Upper Cambrian)—This sandstone unit, formerly referred to as the Os Creek St Mary's rocks, undifferentiated

Not Shown a bedrock contact occur within a very short distance, generally elevation changes of 50 feet (15 meters) Od w Colby D Point Ironton-Galesville Sandstone, is composed mostly of fine- to coarse-grained, moderately Carver Lake Ops U s or more within a distance of 1,000 feet (305 meters). A fold is inferred where these elevation changes

Lake M to well sorted, light gray, cross-stratified, quartz sandstone (Mossler, 2008). White,

T. 28 N. 275 275 T. 28 N. indicate more gradual slopes. The stratigraphic top of the Jordan Sandstone was contoured at 25-foot  300 F brown, and black linguliform brachiopod shells are locally abundant. The upper part RAMSEY Opg j AFTON  (15-meter) intervals to show the inferred location of faults and folds in Washington County (Fig. 4). COUNTY t is the coarsest-grained; the lower part is finer-grained, better sorted, and progressively 300 The top of the Jordan Sandstone was selected to portray these structures because it is a well-recognized 93° W. A WOODBURY )95 finer-grained toward its base. The very fine-grained sandstone in the lower part is R. 22 W. Opo DAKOTA w Military Road Jamaica Avenue Manning Avenue St. Croix River and distinct contact, has numerous control points including outcrops that expose the contact, and has Cross sections—Every attempt has been feldspathic. The thickness of the formation is 45 to 75 feet (14 to 23 meters). The COUNTY La 300 Os A Unconsolidated Os A' water wells that penetrate it. Displacement along faults in Washington County is on the order of 25 to 494 Lake 1,000 Quaternary sediments Wonewoc Sandstone is conformable with overlying and underlying formations; however, ¦§ Os 300 U made for the cross sections to match all Od 275 300 feet (8 to 91 meters), which is sufficient in places to juxtapose several different formations along 25 300 Ops D  300 300 j Os 800 Os Opg geologic interpretations made from the there is a subtle unconformity marked by a pebbly sandstone layer within the formation

Opg Opo Ops Ops the fault contact (see cross sections and Fig. 4). Bedrock that has dropped alongside a fault preserves

300 j s t County Well Index data. Symbology is (Runkel and others, 1998). F D 600 Opo Opo relatively younger formations (shown with a D on the map) at the surface and bedrock that has been 44° 52' 30" N. D U 44° 52' 30" N. j the same as on the bedrock map. Only _e Eau Claire Formation (Middle to Upper Cambrian)—The formation is composed of U 300 w  uplifted brings older formations closer to the surface (shown with a U on the map). Most faults trend 275 300 COTTAGE GROVE e Newport Ops 400    a small number of drill holes intersected yellowish-gray to pale olive-gray, fine- to very fine-grained, feldspathic sandstone, 36 31 36 Os31 35 t s FAULT ZONE m in the northeast direction but several trend northwest.

Mississippi ST. CROIX HUDSON–AFTON HORST F 275 Mesoproterozoic bedrock, thus the relief is siltstone, and shale. White and brown linguliform brachiopod shells are common. The

10 F 200 e  Several of the faults observed in Paleozoic strata are subparallel to faults in the underlying

¤ COUNTY w HASTINGS FAULT

Opg M U Opo generalized and inferred from the thickness formation ranges from 80 to 100 feet (24 to 30 meters) in thickness. The contact with

 D U Mesoproterozoic Midcontinent Rift rocks, as inferred from geophysical imagery, and are therefore D D t t m F D U 0 the Mt. Simon Sandstone is conformable. ¤61 Opg U Trout U PIERCE U D U U D of the Mt. Simon Sandstone. Dashed 275 Brook D D interpreted to represent reactivation of these older structures (Fig. 2). The aeromagnetic and gravity

1 Os 1  COUNTY vertical lines represent Precambrian faults _

6 6 U s U Opo 2 -200 m Mt. Simon Sandstone (Middle Cambrian)—The Mt. Simon Sandstone is pale yellowish- M data show where a great thickness of dense and magnetic have been brought near the surface in 275 D D 275 j Mpv and the long dashed lines in C–C' represent 300 Os U Elevation (feet above sea level) Mss Mss brown to grayish-orange-pink to light gray, medium- to coarse-grained, quartz sandstone. F M D -400 an inverted graben known as the Hudson–Afton horst (Sims and Zeitz, 1967; Cannon and others, 2001). F 300 volcanic flows inferred from geophysical Interbeds of shale, siltstone, and very fine-grained feldspathic sandstone are common, 275 Ops D Paleozoic rocks overlying the Hudson–Afton horst in Washington County are also uplifted relative to D St Paul Os Ops -600 imagery. particularly in its upper half (Mossler, 1992). Inarticulate brachiopod shells are locally U D U U the rocks on either side, although the displacement is of a lesser magnitude than that in the underlying Park 275 Cottage Grove D Opo common in the upper one-third of the formation. Thin beds of quartz-pebble conglomerate j U Mesoproterozoic bedrock (Fig. 4). Lower Ordovician Prairie du Chien Group rocks thin (less than 50 D 275 D D t feet [15 meters]) within the uplifted horst and thicken along either side (less than 300 feet [91 meters]), occur at several stratigraphic positions, and are especially abundant near the base of the U U formation. The Mt. Simon Sandstone unconformably overlies Mesoproterozoic rocks. River  300 250 suggesting displacement was occurring during Early to Middle Ordovician time (see cross section A–A' j 275 B B' 250 275 1,200 U D Dellwood Minnesota Highway 36 Lake Elmo Airport Interstate 94 Lakeland Based on a limited number of full penetrations of the formation, it appears to have a Opg and Fig. 5). COTTAGE GROVE St. Croix River maximum thickness of about 280 feet (85 meters). 300 1,000 Unconsolidated Os Os U 225 Quaternary Os HYDROSTRATIGRAPHY T. 27 N. U Os D Os T. 27 N. 800 sediments Ops MESOPROTEROZOIC Ops D F Ops  275 t The Paleozoic bedrock formations contain significant sources of groundwater, which provide the DENMARK Os j  D  600 Opo Opo j e majority of the water supply for Washington County. This map and associated products, such as the Keweenawan Supergroup U Os )95 FAULT j Ops t s  m COTTAGE GROVE FAULT s t bedrock unit DEMs, provide a three-dimensional depiction of the rock properties that control flow Mss Sandstone, siltstone, and local conglomerate (shown on cross sections and Fig. 2)—Includes GREY D U Opo 400 HUDSON–AFTON HORST    in these water-bearing layers. Such rock properties are called hydrostratigraphic properties. The 275 e w w e the Hinckley Sandstone and Fond du Lac (youngest detrital zircons ~1,000 Ma) and CLOUD Ops 200

St. Croix Croix St. hydrostratigraphic classification, shown by brown and blue colors on the hydrostratigraphic column of m  Solor Church Formations; deposited in eolian, fluvial, and lacustrine environments. ISLA225ND ¤61 HASTINGS m Mooers D U 250 Figure 1, distinguishes layers that are dominated by relatively high permeability (easily transmitting Lake U Opo F 0 Mbv North Branch volcanic sequence (shown on cross section C–C' and Fig. 2)—Part of the 225 U WISCONSIN 225 ¤10 U U Ops water) material, versus layers dominated by lower permeability (relatively more difficult to transmit s  D Mpv St. Croix horst. s U D -200 275  j j D Elevation (feet above sea level) water) material. This generalized characterization for Washington County is based on hydrogeologic j U Mss Mss GROVE FAULT COTTAGE D Mpv Powder Mill volcanic sequence (~1,099 Ma; shown on cross sections and Fig. 2)—Part of Os -400 reports by Runkel (1996), Paillet and others (2000), Runkel and others (2003, 2006a, b, 2014a, b), 36 U 36D 31300 D U the St. Croix horst. Baldwin 31 s Tipping and others (2006), Anderson and others (2011), Luhmann and others (2011), Green and others 225  U Lake t t -600 D (2012), and unpublished borehole and core data collected by the Minnesota Geological Survey. The 6 R. 21 W. Ops River high permeability layers are potential aquifers, able to yield economic quantities of water in most places. REFERENCES Mississippi U St. Croix River  Anderson, J.R., 2009, Bedrock geology of the Lake Elmo quadrangle, Ramsey and Washington Counties, R. 22 W. 1 j Forest Lake Manning Trail St. Croix Trail The low permeability layers are potential aquitards that retard vertical flow, hydraulically separating the U 6 4 A' 93° W. D U 275 C C' aquifer layers from one another in many places, and protecting water resources in the underlying layers Minnesota: Minnesota Geological Survey Miscellaneous Map M-185, scale 1:24,000. 92° 52' 30" W. D 1,000 Opo Anderson, J.R., Runkel, A.C., Tipping, R.G., Barr, K., and Alexander, E.C., Jr., 2011, Hydrostratigraphy of a  Opo j Od from surface contamination. Fractures parallel to bedding that have been demonstrated to have high Digital base modified from the Minnesota Department of D j Unconsolidated Quaternary sediments fractured urban aquitard: Geological Society of America Field Guide 24, p. 457-475. ¤61 ConleyU T. 26 N. 800 Transportation BaseMap data; digital base annotation by permeability are likely to be present, at least locally, in all formations in Washington County, but are Lake   s  Berg, R.R., 1954, Franconia Formation of Minnesota and Wisconsin: Geological Society of America Bulletin, the Minnesota Geological Survey. U  ¤10 s t t placed on the column where the hydrogeologic reports cited above indicate they are most common. The D s 600 v. 66, p. 857-882.     e Elevation contours were derived from the U.S. Geological 44° 45' N. t e w w locations of springs in Washington County most commonly occur within the lower Jordan Sandstone, s 44° 45' N. Cannon, W.F., Daniels, D.L., Nicholson, S.W., Phillips, J., Woodruff, L.G., Chandler, V.W., Morey, G.B., Survey 30-meter Digital Elevation Model (DEM) by the 400 m m St. Lawrence Formation, and upper Tunnel City Group, where high permeability bedding fractures PIERCE Boerboom, T.J., Wirth, K.R., and Mudrey, M.G., Jr., 2001, New map reveals origin and geology of North Minnesota Geological Survey. River R. 20 W. are known to be common. The hydrogeologic properties of the Mesoproterozoic bedrock are poorly COUNTY 200 American Mid- Rift: Eos, v. 82, p. 97-101. Universal Transverse Mercator Projection, grid zone 15 DAKOTA COUNTY understood. It does not supply water to Washington County at least in part because sufficient water 1983 North American Datum 0 Mbv Chandler, V.W., 1991, Aeromagnetic anomaly map of Minnesota: Minnesota Geological Survey State Map SCALE 1:100 000 U CREEK GRABEN FALLS U resources are available at shallower depths in the Paleozoic rocks. GIS compilation by R.S. Lively D S-17, scale 1:500,000. 1 0 1 2 3 4 5 MILES Elevation (feet above sea level) -200 In Washington County, most aquifers are layers dominated by relatively coarse-grained sandstone, Edited by Lori Robinson Vertical exaggeration = 10x Green, J.A., Runkel, A.C., and Alexander, E.C., Jr., 2012, conduit flow in the Cambrian St. Lawrence 1 0 1 2 3 4 5 6 7 8 KILOMETERS such as within the upper part of the Jordan Sandstone, in which water can be fairly easily transmitted in confining unit, , USA: Carbonates Evaporites, v. 27, no. 2, p. 167-172. both horizontal and vertical directions through the pore spaces between sand grains, as well as through Jirsa, M.A., Boerboom, T.J., and Chandler, V.W., 2012, Geologic map of Minnesota Precambrian bedrock contour interval 25 METERS fractures. Other aquifers, such as the upper part of the Prairie du Chien Group, are composed mostly geology: Minnesota Geological Survey State Map S-22, scale 1:500,000. of carbonate rock in which water is transmitted through a relatively dense network of fractures and Luhmann, A.J., Covington, M.D., Peters, A.J., Alexander, S.C., Anger, C.T., Green, J.A., Runkel, A.C., and solution cavities. However, layers designated as aquifers can locally contain low permeability strata Alexander, E.C., Jr., 2011, Classification of thermal patterns at karst springs and cave streams: Ground that serve as small, internal aquitards including parts of the upper Mt. Simon Sandstone and the lower Water, v. 49, no. 3, p. 324-335. Jordan Sandstone. Mossler, J.H., 1992, Sedimentary rocks of Dresbachian age (Late Cambrian), Hollandale embayment, Most layers designated as aquitards in the county have a much lower permeability in the vertical southeastern Minnesota: Minnesota Geological Survey Report of Investigations 40, 71 p. direction than do aquifers. Examples in Washington County include the lower Jordan Sandstone, St. ———2005a, Bedrock geology of the Hudson quadrangle, Washington County, Minnesota: Minnesota FCG Lawrence, and Eau Claire Formations, whose rocks are composed mostly of very fine-grained sandstone Geological Survey Miscellaneous Map M-154, scale 1:24,000. M and shale with small, poorly connected pore spaces. Carbonate rock with relatively sparse fractures, ———2005b, Bedrock geology of the Stillwater quadrangle, Washington County, Minnesota: Minnesota Mbv such as the lower part of the Prairie du Chien Group (Oneota Dolomite), are also aquitards. However, Geological Survey Miscellaneous Map M-153, scale 1:24,000. layers designated as aquitards with very low permeability in the vertical direction may locally contain ———2006a, Bedrock geology of the Hastings quadrangle, Washington and Dakota Counties, Minnesota:

Minnesota Geological Survey Miscellaneous Map M-169, scale 1:24,000. F horizontal fractures that are conductive enough to yield large quantities of water, including the St. Lawrence Formation. ———2006b, Bedrock geology of the Prescott quadrangle, Washington and Dakota Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-167, scale 1:24,000. Horizontal and vertical fractures are more common where bedrock layers are at or near the bedrock ———2006c, Bedrock geology of the St. Paul Park quadrangle, Washington and Dakota Counties, Minnesota: surface. As a result, aquitards in such conditions are likely to have higher permeability compared with Elevation of the top of the Minnesota Geological Survey Miscellaneous Map M-166, scale 1:24,000. more deeply buried portions of the same formation, and may have a diminished ability to retard water Jordan Sandstone in feet ———2006d, Bedrock geology of the Vermillion quadrangle, Dakota County, Minnesota: Minnesota Geological flow to underlying aquifers. There is no precise boundary between shallow and deep conditions of Survey Miscellaneous Map M-168, scale 1:24,000. Less than 550 burial, but in most areas of southeastern Minnesota about 50 feet (15 meters) of depth below the bedrock ———2008, Paleozoic stratigraphic nomenclature for Minnesota: Minnesota Geological Survey Report of surface is considered a best approximation (Runkel and others, 2006a). Investigations 65, 76 p., 1 pl. 551-600 In addition to this hydrostratigraphic classification, the Minnesota Department of Natural Resources, ———2013, Bedrock geology of the Twin Cities ten-county metropolitan area, Minnesota: Minnesota 601-650 as Part B of the Washington County atlas, will conduct a thorough hydrogeologic study of the groundwater Geological Survey Miscellaneous Map M-194, scale 1:125,000.

Thickness of Prairie du Chien flow system, aquifer capacity, and aquifer sensitivity, which may result in modifications to this

Figure 2. Faults that displace Paleozoic Mossler, J.H., and Bloomgren, B.A., 1990, Bedrock geology, pl. 2 of Swanson, L., and Meyer, G.N., eds.,

M 651-700 Group rocks in feet classification. Furthermore, designations of aquifers versus aquitards made here may not correspond

bedrock in Washington County superimposed Geologic atlas of Washington County, Minnesota: Minnesota Geological Survey County Atlas C-5, M precisely with those made for regulatory purposes by the Minnesota Department of Health. scale 1:100,000, 7 pls. on a map of the first vertical derivative 701-750 300

Mossler, J.H., and Tipping, R.G., 2000, Bedrock geology and structure of the seven-county Twin Cities aeromagnetic data (Chandler, 1991) and F 250 751-800 metropolitan area, Minnesota: Minnesota Geological Survey Miscellaneous Map M-104, scale the underlying Mesoproterozoic rock units DESCRIPTION OF MAP UNITS

200 1:250,000. (Jirsa and others, 2012). Paleozoic faults Active quarry F 801-850 150 Od (Upper Ordovician)—Dominantly grayish-green shale interbedded with thin Paillet, F.L., Lundy, J., Tipping, R., Runkel, A.C., Reeves, L., and Green, J., 2000, Hydrogeologic characterization and fault zones where they cluster in the Mss 851-900 beds of fossiliferous limestone. Fossiliferous, yellowish-brown limestone beds are most of six sites in southeastern Minnesota using borehole flowmeters and other geophysical tools: U.S. Quartzose sandrock within 100 southern part of the map (CGF—Cottage common at the base of the Decorah Shale, and are recognized as the basal Carimona Geological Survey Water-Resources Investigations Report 00-4142, 33 p. 50 feet of the land surface 50 Grove fault, CGFZ—Cottage Grove fault 901-950 Member (Mossler, 2008). The Decorah Shale is present as erosional remnants capping Runkel, A.C., 1994, Deposition of the uppermost Cambrian (St. Croixan) Jordan Sandstone, and the nature

zone, HF—Hastings fault, HFZ—Hastings

Carbonate rock within 50 951-1,000 0 Platteville mesas in the southwestern part of the county. Though no exposures exist of the Cambrian–Ordovician boundary in the upper Mississippi valley: Geological Society of America

fault zone) are subparallel to strong, abrupt, feet of the land surface M in the county, water well logs indicate the Decorah Shale has a maximum preserved Bulletin, v. 106, p. 492-506.

M linearly extensive contrasts in magnetic F 1,001-1,050 thickness of 40 feet (12 meters). ———1996, Bedrock geology of Houston County, Minnesota: Minnesota Geological Survey Open-File

( intensity in the underlying Mesoproterozoic Report 96-4, 11 p., 3 pls., scale 1:100,000. F Opg Platteville and Glenwood Formations (Upper Ordovician)—The Platteville Formation is ( rocks associated with the Midcontinent Rift. generally tan to gray, fossiliferous limestone and dolostone. The underlying Glenwood Runkel, A.C., McKay, R.M., Miller, J.F., Palmer, A.R., and Taylor, J.F., 2007, High resolution sequence ( The aeromagnetic data display where a great Formation is dominantly a green-gray, sandy shale. The Platteville Formation is the stratigraphy of lower Paleozoic sheet sandstones in central North America: The role of special conditions thickness of magnetic Mesoproterozoic rift of cratonic interiors in development of stratal architecture: Geological Society of America Bulletin, v. ( dominant uppermost bedrock unit across a large expanse of the southwestern part of the basalts (unit Mpv) have been brought near Figure 3. Map showing where carbonate Figure 4. Map of Washington County Figure 5. Color shaded map indicating county. The combined thickness of the formations is 30 to 35 feet (9 to 11 meters). 119, nos. 7/8, p. 860-881. ( Mpv the surface in an inverted graben known as Runkel, A.C., McKay, R.M., and Palmer, A.R., 1998, Origin of a classic cratonic sheet sandstone: Stratigraphy and quartzose sandrock are present within depicting the elevation of the stratigraphic the thickness of the Prairie du Chien Platteville Formation—The Platteville Formation is 25 to 30 feet (8 to 9 meters) thick. It is ( the Hudson–Afton horst (HAH), bounded by 50 feet (15 meters) of the land surface in top of the Jordan Sandstone showing the Group where it exists below the St. Peter across the Sauk II–Sauk III boundary in the upper Mississippi valley: Geological Society of America composed of tan to gray limestone and dolostone. It is commonly burrowed, mottled, Bulletin, v. 110, p. 188-210. ( thrust faults, shown by black lines with teeth. Washington County. Brown represents F mapped fold axes (thin black lines) and Sandstone. Blue colors represent areas where and fossiliferous. It contains fine- to coarse-grained quartz sand and phosphate grains Runkel, A.C., Miller, J.F., McKay, R.M., Shaw, T.H., and Bassett, D.J., 1999, Cambrian-Ordovician boundary ( Faults in the northeastern corner of the county quartzose sandrock (including the Jordan, faults (thick black lines). Colored intervals the Prairie du Chien Group is thinner and

F in the lowermost 2 feet (0.6 meter). strata in the central mid-continent of North America: Acta Universitatis Carolinae Geologica, v. 43, p.

that bound what is known as the Falls Creek Wonewoc, and St. Peter Sandstones) and represent 50-foot (15-meter) elevation orange represents areas where it is thicker. ( ( F Glenwood Formation—The principal rock type of the Glenwood Formation is a grayish- 17-20. HAH Graben (FCG) are subparallel to foliation beige represents carbonate rock (including M intervals; blue colors represent lower There is progressive thinning toward the ( ( green to brownish-gray, calcareous, sandy, and phosphatic shale. The Glenwood Formation Runkel, A.C., Mossler, J.H., Tipping, R.G., and Bauer, E.J., 2006a, A hydrogeologic and mapping investigation trends of underlying Mesoproterozoic the Platteville and Shakopee Formations elevations, and orange represents higher north, where it thins to 50 feet (15 meters) is 3 to 7 feet (1 to 2 meters) thick. of the St. Lawrence Formation in the Twin Cities metropolitan area: Minnesota Geological Survey

( ( flows (unit Mbv). Due to their similar and Oneota Dolomite). The active quarry

elevations. Dashed lines depict the 25-foot thick. Thicknesses reach nearly 300 feet (91 Open-File Report 06-4, 20 p.

F Os St. Peter Sandstone (Middle to Lower Ordovician)—The upper 100 to 140 feet (30 to 43

( ( trends and close proximity, all Paleozoic operations in Washington County are shown (8-meter) contour intervals. In areas where meters) in the southern part of the county

Runkel, A.C., Steenberg, J.R., Tipping, R.G., and Retzler, A.J., 2014a, Geologic controls on groundwater M F meters) of the St. Peter Sandstone is mostly a white to tan, fine- to medium-grained,

faults are interpreted to have originated F

( ( in black. This figure was generated using F the Jordan Sandstone is absent because of except within the Hudson–Afton horst and surface water flow in southeastern Minnesota and its impact on nitrate concentrations in streams: CGF M friable quartzose sandstone. Bedding and structures are generally absent. It is exposed ( HFZ from reactivation of deep Mesoproterozoic the bedrock topographic surface and bedrock M erosion, the map is not colored, and the (HAH), where it thins to less than 50 feet ( in patchy outcrops in the southern half of the county where glacial sediments are thin. Minnesota Geological Survey Open-File Report 14-2, 70 p. CGFZ structures. Abrupt thickness changes in F ( Mss geology polygons, which are standard F contours are inferred from vertical projection (15 meters) in places. Small areas where Runkel, A.C., Tipping, R.G., Alexander, E.C., Jr., and Alexander, S.C., 2006b, Hydrostratigraphic characterization ( The lowermost 10 to 40 feet (3 to 12 meters), referred to as the Pigs Eye Member, ( CGF the Prairie du Chien Group across the Geographic Information System (GIS) of the contacts of stratigraphically lower the Prairie du Chien Group appears to thin of intergranular and secondary porosity in part of the Cambrian sandstone aquifer system of the cratonic ( includes white to gray feldspathic shale and siltstone interbedded with coarser-grained Hudson–Afton horst indicate that faults products of a county geologic atlas. formations. abruptly are interpreted to represent areas of interior of North America: Improving predictability of hydrogeologic properties: Sedimentary Geology, HUDSON-AFTON HORST HF sandstone similar to that of the Tonti Member. The Pigs Eye Member is not exposed ( were reactivated during Early Paleozoic F significant erosion during a hiatus marked v. 184, p. 281-304. in Washington County. The thickness of the St. Peter Sandstone varies from about 130 (Early to Middle Ordovician) time. by an unconformity prior to the deposition Runkel, A.C., Tipping, R.G., Alexander, E.C., Jr., and Green, J.A., 2003, Hydrogeology of the Paleozoic bedrock to 160 feet (40 to 49 meters). The basal contact of the formation with the underlying F HF of the St. Peter Sandstone. CGF—Cottage in southeastern Minnesota: Minnesota Geological Survey Report of Investigations 61, 105 p., 2 pls. Shakopee Formation (unit Ops) is a major erosional unconformity (Smith and others, CGFZ Grove fault, CGFZ—Cottage Grove fault Runkel, A.C., Tipping, R.G., Green, J.A., Jones, P.M., Meyer, J.R., Parker, B.L., Steenberg, J.A., and Retzler, zone, HF—Hastings fault, HFZ—Hastings 1993). A.J., 2014b, Hydrogeologic properties of the St. Lawrence aquitard, southeastern Minnesota: Minnesota HFZ fault zone. Prairie du Chien Group (Lower Ordovician)—Dominated by dolostone interlayered with Geological Survey Open-File Report 14-4, 56 p. lesser amounts of quartz sandstone. Outcrops are exposed along the tops of bluffs of Sims, P.K., and Zeitz, I., 1967, Aeromagnetic and inferred Precambrian paleogeographic map of east-central the Mississippi and St. Croix River valleys; small patchy outcrops are also present in Minnesota and part of Wisconsin: U.S. Geological Survey Geophysical Investigations Map GP-563, 6 southeastern Washington County where the Quaternary sediments are thin. The Prairie p., scale 1:250,000. du Chien Group is a significant source of rock aggregate in this part of the county. The Smith, G.L., Byers, C.W., and Dott, R.H., Jr., 1993, Sequence stratigraphy of the lower Ordovician Prairie Prairie du Chien Group is formally divided into two formations: the Shakopee Formation du Chien Group on the Wisconsin arch and in the basin: American Association of Petroleum and underlying Oneota Dolomite. Geophysical logs and drill cuttings in the northern part Geologists Bulletin, v. 77, p. 49-67. Tipping, R.G., Runkel, A.C., Alexander, E.C., Jr., and Alexander, S.C., 2006, Evidence for hydraulic ©2016 by the Regents of the of the county and within parts of the Hudson–Afton horst that span the St. Peter through the Jordan Sandstones are lacking and water well records are typically inadequate to heterogeneity and anisotropy in the mostly carbonate Prairie du Chien Group, southeastern Minnesota, The University of Minnesota is an equal opportunity educator and employer GEOLOGIC ATLAS OF WASHINGTON COUNTY, MINNESOTA USA: Sedimentary Geology, v. 184, p. 305-330.