Prepared and Published with the Support of COUNTY ATLAS SERIES THE U.S. GEOLOGICAL SURVEY AS PART OF THE 2008 STATE GEOLOGIC MAPPING PROGRAM ELEMENT (STATEMAP) ATLAS C-22, PART A MINNESOTA GEOLOGICAL SURVEY Plate 2—Bedrock Geology Harvey Thorleifson, Director OF THE NATIONAL GEOLOGIC MAPPING PROGRAM, THE CHISAGO COUNTY BOARD OF COMMISSIONERS, AND the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources
KANABEC 93°7'30'' 93°00' 92°52'30'' COUNTY R. 22 W. PINE COUNTY R. 21 W.
280 6 m cv 300 1 6 1 6 BEDROCK Geology
e CORRELATION OF MAP UNITS 2 3 m 3 361 e su 4 Early to Late By unconformity Ordovician 280 Osp
280 Opo Early Ordovician e m e Anthony C. Runkel Terrence J. Boerboom unconformity RushDOUGLAS FAULT Rush Lake 1 City RUSHSEBA j T. 37 N. NESSEL cv T. 37 N. 280 5 sl 2010 Rush PALEOZOIC e U D Late Cambrian 5 260 tc
River e D U 260 unconformity m Creek 280 w 300 300 Osp MAP SYMBOLS 7 e Middle Cambrian e Geologic contact—Approximately located. 280 280 St. Croix m 8 COUNTY LOCATION 31 300 280 Fault—Faults in Paleozoic rocks are interpreted to be dip-slip. Letters indicate what is inferred ISANTI COUNTY 36 31 9 30 36 31 w e unconformity to be the most recent relative vertical displacement: U, up; D, down. Faults are concealed Goose su by Quaternary sediments and recent alluvium and are inferred from subsurface geologic 7 61 data supplemented by aeromagnetic and gravity data. Contrasts in magnetic intensity on 280 1 cv 6 6 1 6 MESOPROTEROZOIC the aeromagnetic map are inferred to be largely the expression of faults in the underlying 280 35 m Lake Mesoproterozoic volcanic and sedimentary rocks. Offsets of Paleozoic strata indicate Neander Lake cf that some of these faults were rejuvenated during and after Early Paleozoic time. ° ° 45 37'30'' tc 45 37'30'' Bedrock outcrop 280 e w Mud Lake
300 _ Wonewoc Sandstone (Late Cambrian)—This sandstone unit, formerly referred to as cv Goose Introduction DESCRIPTION OF MAP UNITS w the Ironton-Galesville Sandstone, is composed mostly of fine- to coarse-grained, This geologic map, cross section, and stratigraphic column depict the bedrock formations exposed PALEOZOIC ROCKS FISH LAKE moderately to well sorted, light gray, cross-stratified, quartz sandstone (Mossler, T. 36 N. at the land surface or lying directly beneath unconsolidated Quaternary deposits of variable thickness 300 SUNRISEe T. 36 N. 4 Robour Creek MINNESOTA Nomenclature has been revised for some Paleozoic formations in Minnesota, and some formation 2008). White, brown, and black linguliform brachiopod shells are locally abundant. 9 WISCONSIN across Chisago County. The map shows how the bedrock would appear if it were viewed from an Lake Horseshoe names formerly in use at the Minnesota Geological Survey have been replaced by names widely The upper part is the coarsest-grained; the lower part is finer-grained, better sorted, Lake aerial perspective, as if all the overlying unconsolidated material was stripped away. Quaternary accepted elsewhere in the region (Mossler, 2008). Rocks formerly included in the Franconia and progressively finer-grained toward its base. The very fine-grained sandstone in glacial till, lake sediments, outwash, and river alluvium cover the bedrock across most areas (Meyer w su Formation are now assigned to the Tunnel City Group, which is subdivided into the Mazomanie the lower part is feldspathic. The thickness of the formation is 75 to 110 feet (23 to and Lusardi, 2001). The unconsolidated deposits reach a maximum thickness of about 500 feet 280 280 300 Formation and the Lone Rock Formation. The interval formerly referred to as the Ironton-Galesville 34 meters), except where it pinches out in unconformable contact against knobs of 10 (152 meters). Bedrock exposures are limited to the eastern part of the county, mostly along the St. Fish Sandstone is now named the Wonewoc Sandstone. More detailed discussion of these revisions is Keweenawan basalt near Taylors Falls. The Wonewoc Sandstone is conformable with Lake Harris 9 Croix River and its tributaries near Taylors Falls. e given in Mossler (2008). overlying and underlying formations; however, there is a subtle unconformity marked ° 280 260 260 92 52'30'' There are two fundamental kinds of bedrock mapped across Chisago County. Mesoproterozoic by a pebbly sandstone layer within the formation (Runkel and others, 1998). 280 bedrock of the Keweenawan Supergroup is about 1.1 billion years old, and consists of reddish-brown St. Peter Sandstone and Prairie du Chien Group (Early to Middle Ordovician)—Limited tc R. 20 W. Osp _e Eau Claire Formation (Middle to Late Cambrian)—The formation is composed of 260 cv D U mudstone and siltstone, lithic and feldspathic sandstone, and volcanic rocks such as basalt. The information (one set of borehole cuttings and one natural gamma log) indicates that 36 D U yellowish-gray to pale olive-gray, fine- to very fine-grained, feldspathic sandstone, 31 31 36 cf U Keweenawan Supergroup accumulated as part of the development of the Midcontinent Rift System 31 D 36 31 the St. Peter Sandstone and Prairie du Chien group locally unconformably overlie siltstone, and shale. White and brown linguliform brachiopod shells are common. 8 PINE ° FAULT 92 45' (for example Cannon and others, 2001), a large tear in the crust of North America. A second type 280 cv 240 the Eau Claire Formation and Wonewoc Sandstone west of the Douglas fault at Rush The Eau Claire Formation gradually coarsens to the north across Chisago County, Chain U ° R. 22 W. 280 of bedrock consists of relatively thin layers of sandstone, siltstone, dolostone, and shale deposited 93 7'30'' Lake 280 City. The St. Peter Sandstone consists largely of fine- to medium-grained, quartzose and as a result is composed dominantly of very fine- to fine-grained sandstone in su D m 260 in shallow seas during the Paleozoic Era, about 500 million years ago (for example Mossler, sandstone, with interbeds of very fine-grained, feldspathic sandstone that may be 6 1 6 1 260 30 m 6 the northern one-half of the county. The formation ranges from 60 to 90 feet (18 Digital base modified from 1990 Census TIGER/Line Files D U R. 19 W. 2008). These bedrock layers eventually buried the Keweenawan Supergroup across nearly all the the Pigs Eye Member (Mossler, 2008). Strata beneath the St. Peter Sandstone in Sunrise to 27 meters) in thickness, except where it pinches out in unconformable contact of U.S. Bureau of the Census (source scale 1:100,000); e 300 county. An exception was at Taylors Falls, where resistant knobs of Keweenawan basalt stood as this area are largely fine- to coarse-grained quartzose sandstone, with interbeds of against knobs of Keweenawan basalt near Taylors Falls. The contact with the Mt.
county border files modified from Minnesota Department of 280 m topographic highs, emergent as islands in the Paleozoic seas (Fig. 1). This setting also led to local Transportation files; digital base annotation by Minnesota e sandy, oolitic dolostone. They most likely correlate to the Shakopee Formation, but e Simon Sandstone is conformable. Geological Survey. deposition of unusual Paleozoic bedrock that includes boulders and cobbles of basalt that were could instead be equivalent to the lowest part of the Oneota Dolomite (Coon Valley 9 _ Mt. Simon Sandstone (Middle Cambrian)—The Mt. Simon Sandstone is pale yellowish- shed from these islands into the shallow sea. These deposits have been informally called the "Mill Member) of the Prairie du Chien Group. The maximum thickness of this map unit m Universal Transverse Mercator Projection, grid zone 15 brown to grayish-orange-pink to light gray, medium- to coarse-grained, quartz 1983 North American Datum D U Street conglomerate" (Berkey, 1897). is about 70 feet (21 meters). North280 U D sandstone. Interbeds of shale, siltstone, and very fine-grained, feldspathic sandstone Mud cf The most prominent feature of the bedrock topography (Plate 6) is an anomalously deep and Lake Branch Opo Prairie du Chien Group–Oneota Dolomite (Early Ordovician)—Dominantly tan to gray, are common, particularly in its upper half (Mossler, 1992), and in the lower several 12 linearly extensive buried valley in the southern half of the county that approximates the position medium to thickly bedded, finely crystalline dolostone, present only in the extreme tens of feet of the Mt. Simon Sandstone in the North Branch area. Inarticulate of the modern Sunrise River system and chain of lakes around the cities of Lindstrom and Chisago southeast corner of the county. Microbial build-ups (thrombolites) are common in T. 35 N. NORTH BRANCH SUNRISE AMADOR T. 35 N. brachiopod shells are locally common in the upper one-third of the formation. Thin N. (see cross section). The buried valley deepens to the north, and crosses the St. Croix River into some beds. The lowermost few feet include interbeds of fine- to coarse-grained, Branch beds of quartz-pebble conglomerate occur at several stratigraphic positions, and 300 Wisconsin near the town of Sunrise, where the base of the valley reaches an elevation of less than intraclastic, quartzose sandstone, very fine-grained, feldspathic sandstone and 95 River e are especially abundant near the base of the formation. The Mt. Simon Sandstone Sunrise 400 feet (122 meters). Other highlights of the bedrock topography are the knobs of relatively hard, siltstone, shale, and sandy dolostone of the Coon Valley Member. The maximum A unconformably overlies Mesoproterozoic rocks, burying a surface of erosion that had River e 16 ° resistant Mesoproterozoic basalt (Clam Falls Volcanics, unit < ) that stand at high elevation near 45°30' 45 30' cf preserved thickness of the Oneota Dolomite is about 30 feet (9 meters). hundreds of feet of relief within the county. As a result, the thickness of the Mt. Simon Taylors Falls. The tops of some of the highest knobs exceed 1,050 feet (320 meters), about 300 w _j Jordan Sandstone (Late Cambrian)—Dominantly tan to light gray sandstone characterized Sandstone is markedly variable even within small areas such as near Taylors Falls. 14 15 feet (91 meters) above the bedrock surface across most of the rest of the county. Almelund by coarsening-upward sequences consisting of two interlayered facies (Runkel, Based on a limited number of full penetrations of the formation, it appears to have cv 280 280 320 Buried stream channels incised into bedrock, in combination with faults with up to about 200 1994), which are not separated on the map. They are medium- to coarse-grained, a maximum thickness of about 250 feet (76 meters) across most of the county. feet (61 meters) of displacement, and a gentle, overall southward dip of Paleozoic bedrock (Fig. su e cross-stratified, generally friable, quartz sandstone; and very fine-grained, commonly 15 m e w 2), are a primary influence on the map distribution of bedrock geologic units. Across much of the bioturbated, feldspathic sandstone and lenses of siltstone and shale. The major MESOPROTEROZOIC ROCKS Sunrise 31 36 31 300 county the uppermost bedrock consists principally of moderately consolidated layers of sandstone, part of the very fine-grained facies forms a regionally continuous interval that 36 31 KEWEENAWAN SUPERGROUP siltstone, and shale of Cambrian age. The southward dip of the Paleozoic strata is reflected on the gradationally overlies the St. Lawrence Formation (unit _sl), although there are map by a pattern of progressively older formations northward across the county. Mesoproterozoic lithologically similar intervals intercalated with the medium- to coarse-grained (460 m.y.) pt. A, scale 1:100,000, 7 pls.. Middle Ordovician Cannon, W.F., Daniels, D.L., Nicholson, S.W., Phillips, J., Woodruff, L.G., Chandler, V.W., Morey, Figure 3. Faults mapped in Chisago A Area in Figure 1 A' 2 G.B., Boerboom, T.J., Wirth, K.R., and Mudrey, M.G., Jr., 2001, New map reveals origin and County superimposed on first vertical Osp 70 geology of North American Mid-continent Rift: Eos, v. 82, p. 97-101. Chisago County Wisconsin derivative aeromagnetic data (Chandler and ISANTI CHISAGO Chandler, V.W., Boerboom, T.J., and Lively, R.S., 2002, Investigation of stream-like magnetic others, 2004). All faults except those in COUNTY COUNTY Taylors Falls Shakopee anomalies in Pine County, Minnesota, in Boerboom, T.J., project manager, Contributions to the North Branch tc (Mill Street extreme southeastern Chisago County are Formation geology of Pine County, Minnesota: Minnesota Geological Survey Report of Investigations conglomerate) part of major fault zones along the western 1,000 Sunrise River 1,000 60, p. 43-53. DOUGLAS FAULT side of the Midcontinent Rift System, 900 900 Chandler, V.W., Lively, R.S., and Wahl, T.E., 2004, Gravity and aeromagnetic data grids of Minnesota: e recognizable by strong, abrupt, linearly Not present? On file at the Minnesota Geological Survey. 800 800 tc Unconsolidated Quaternary sediment tc tc extensive contrasts in magnetic intensity. Prairie du Chien Group Cordua, W.S., 1989, Bedrock geology of the Dresser–St. Croix Falls area, Polk County, Wisconsin, 700 w w 700 The faults in the southeastern part of the Oneota m e 2 e Dolomite Chisago County, Minnesota, pl. 1 of Williams, I.S., ed., Paleogeography and structure of the cf Lower Ordovician (488-478 m.y.) e 600 Opo 30 600 county do not appear to correspond to LITHOLOGY KEY m St. Croix River valley: Tri-State Geological Field Conference, 53rd Annual Meeting, 71 p., 500 m 500 distinct, linear anomalies. Faults that m cv Dolostone 1 pl. 400 FAULT DOUGLAS Pmsu su 400 displace Paleozoic bedrock, shown as solid U D cf FAULT su PINE Johnson, M.D., 2000, Pleistocene geology of Polk County, Wisconsin: Wisconsin Geological and 300 D U 300 pink lines, were reactivated during and after Jordan PINE FAULT Sandy D U D U Early Paleozoic time. Their position at Sandstone j Natural History Survey Bulletin 92, 70 p., 1 pl. Vertical exaggeration ~x16 60-80 Sandstone 1 mile the bedrock surface does not everywhere Very fine- to fine-grained Meyer, G.N., and Lusardi, B.A., comps., 2001, Aggregate resource potential of Chisago correspond to the magnetic anomaly that County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-116, 2 pls., scale St. Lawrence sl Fine- to medium-grained G marks the position of the major faults in Formation 25-40 1:100,000. underlying Mesoproterozoic bedrock, but Medium- to coarse-grained Mooney, H.M., Craddock, C., Farnham, P.R., Johnson, S.H., and Volz, G., 1970, Refraction seismic Mazomanie their similar trends and close proximity Formation investigation of the northern midcontinent gravity high: Journal of Geophysical Research, v. indicate they are related features. Dashed Shaly 75, no. 26, p. 5056-5086. Today's Mill Street conglomerate outcrop 850 800 Lone Rock G Figure 2. Elevation of the stratigraphic tops of the Wonewoc pink line shows another major Midcontinent G Formation G Siltstone Morey, G.B., 1977, Revised Keweenawan subsurface stratigraphy, southeastern Minnesota: Minnesota 800 and Mt. Simon Sandstones in Chisago County, showing geologic Rift System fault that does not appear to PALEOZOIC Mazomanie tc Geological Survey Report of Investigations 16, 67 p. structure of Paleozoic bedrock; scale 1:300,000. Contour Shale displace Paleozoic bedrock. Formation 150-180 750 Mossler, J.H., 1992, Sedimentary rocks of Dresbachian age (Late Cambrian), Hollandale embayment, 850 interval is 50 feet (15 meters). In areas where some or all of Ocean DOUGLAS FAULT Volcanics Tunnel City Group southeastern Minnesota: Minnesota Geological Survey Report of Investigations 40, 71 p. tc tc 750 the Wonewoc and Mt. Simon Sandstones are missing because G G Lone Rock G G 0 5 miles G G w w of erosion, the contours are inferred from projection vertically Formation Chert ———2008, Paleozoic stratigraphic nomenclature for Minnesota: Minnesota Geological Survey G G G G Upper Cambrian (501-488 m.y.) Report of Investigations 65, 76 p., 1 pl. e e from other formation contacts, using the estimated full thickness Oolites Basalt (cf) of the eroded formations. The inferred traces of faults and Mossler, J.H., and Bloomgren, B.A., 1990, Bedrock geology, pl. 2 of Swanson, L., and Meyer, G.N., G 700 Glauconite m m folds are shown. Faults in Mesoproterozoic bedrock that do eds., Geologic atlas of Washington County, Minnesota: Minnesota Geological Survey County Wonewoc Microbial domes; stromatolites/thrombolites 800 w 800 not displace Paleozoic strata are not shown. Sandstone Atlas C-5, scale 1:100,000, 7 pls. 700 75-110 Fossiliferous; fossils (symbols not used in 800 limestone or dolostone units) Mossler, J.H., and Tipping, R.G., 2000, Bedrock geology and structure of the seven county Twin 800 Cities metropolitan area, Minnesota: Minnesota Geological Survey Miscellaneous Map M-104, Burrows G G G scale 1:250,000. Minnesota Wisconsin G 100 feet Figure 4. Map of second vertical derivative Eau Claire G Pebbles e Palmer, A.R., and Geissman, J., 1999, 1999 geologic time scale: Geological Society of America, Formation 1 mile magnetic anomaly data in the northern part G 60-90 Flat-pebble conglomerate 700 1 p. Approximate scale of Chisago County showing "stream-like" G Cross-bedded (festoon) Runkel, A.C., 1994, Revised stratigraphic nomenclature for the Upper Cambrian (St. Croixan) Jordan Figure 1. Schematic illustration of the depositional conditions about 490 anomalies, traced with thin, magenta lines, Cross-bedded (planar to tangential) Sandstone, southeastern Minnesota, in Southwick, D.L., ed., Short contributions to the geology million years ago in the area known today as Taylors Falls, Minnesota, and that represent buried bedrock valleys. The Hummocky/swaley cross-bedded of Minnesota: Minnesota Geological Survey Report of Investigations 43, p. 60-71. St. Croix Falls, Wisconsin. The Cambrian sedimentary bedrock present in the EXPLANATION 750 650 buried valleys contain a relatively great area today was deposited in a shallow sea, with resistant knobs of Proterozoic thickness of moderately magnetic glacial Ripple cross-laminations Runkel, A.C., McKay, R.M., Miller, J.F., Palmer, A.R., and Taylor, J.F., 2007, High resolution Top of the Wonewoc Sandstone Dolomitic sequence stratigraphy of lower Paleozoic sheet sandstones in central North America: The role basalt ( Clam Falls cf 1000s Map GP-563, 6 p., scale 1:250,000. (1200-900 M.Y) (1200-900 PROTEROZOIC PROTEROZOIC unique numbers 612116, 475354, 706835, 737047 The views and conclusions contained in this document are those of the authors and should not be 0 5 miles Volcanics 500 Wirth, K.R., Cordua, W.S., Kean, W.F., Middleton, M., and Naiman, Z.J., 1998, Field guide to the interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. 1Numerical ages from Palmer (1999). Government. This map is submitted for publication with the understanding that the U.S. Government 2Maximum eroded thicknesses: units are eroded everywhere in Chisago County. geology of the southeastern portion of the Midcontinent Rift System, eastern Minnesota and is authorized to reproduce and distribute reprints for governmental use. Supported by the U.S. western Wisconsin: Institute on Lake Superior Geology, 44th Annual Meeting, Proceedings, Geological Survey, National Cooperative Geologic Mapping Program, under assistance Award No. v. 44, pt. 2, p. 33-78. 08HQAG0095. Yochelson, E.L., and Webers, G.F., 2006, A restudy of the Late Cambrian molluscan fauna of Berkey ©2010 by the Regents of the University of Minnesota (1898) from Taylors Falls, Minnesota: Minnesota Geological Survey Report of Investigations 64, 60 p. The University of Minnesota is an equal opportunity educator and employer GEOLOGIC ATLAS OF CHISAGO COUNTY, MINNESOTA