Prepared and Published with the Support of COUNTY ATLAS SERIES THE WASHINGTON COUNTY BOARD OF COMMISSIONERS, ATLAS C-39, PART A MINNESOTA 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 t K j Lake COUNTY 1 6 s Lake 1 Ops6 O 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 WISCONSIN 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 Shale <40 Galena Group Opo 300 j s t ¤61 300 300 Sand t White Ops Lake Platteville and 300 Opg Rock Glenwood Formations 30-35 INTRODUCTION distinguish the Oneota Dolomite from the Shakopee Formation and St. Peter Sandstone Ph Lake Long Lake from the sandy upper Shakopee Formation. Therefore, the mapped distribution of these Opo CORRELATION OF MAP UNITS The geologic map, 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 Prairie du Chien Group 31 36 36 250 31 Os s 300 Upper Ordovician 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 Quaternary 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 erosion 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 Paleozoic 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 Limestone 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 fault, 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 Cambrian 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 Sandstones and Eau Claire Formation, can be seen at the land surface U ¤61 School w Opo Oneota Dolomite (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 Os Lake medium- to thick-bedded dolostone that generally lacks sedimentary features such as A Bald Lake Long e 31 River erosion, as indicated by the weathering profile on the lithology column of Figure 1. The units that cover K Medium- to coarse-grained Lake Big oolites and quartz sand characteristic of the Shakopee Formation, except in its lowermost O 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 MESOPROTEROZOIC 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.