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wt wt wt wt wt wt wt ws wt wt ws wt wt ws wl ws wl pe pe wl pe wl ws wl la ws wl ws wl la wl ws wl la wl ws wl la ws wl la ws wl wl pe ws pe sa wl wt ws ws wt wt ws sa wt ws la la la wt ws wt wt wl ws pe wl A A’ 1150 1100 0 1500 1450 1400 1350 1300 1250 1200 1550 1600 Aqp Avs Avs Acv Acv Avs Avs Acv Acv Acv Agm Aqm Amv Aif Avs wt wt wt wt wt ws wt wt pe pe wt ws wl wl ws ws wt wl wt wt wt ws ws wl la wl la la la la wl la wl wt ws wl ws2 wt2 wt2 wt2 ws2 wl2 wtx wtx B B’ 1150 1100 1050 1500 1450 1400 1350 1300 1250 1200 1550 Pas Mlt Agt Agm Pai wt3 wl2 ws wl js js ws3 wtx2 ws wt ws wt2 ws2 la ws3 wt wtx js ws2 wl ws pe wl wl la ws0 ml wtx wl wl2 ws js js sa wl ws wt sa la 1150 1100 1050 1500 1450 1400 1350 1300 1250 1200 0 C C’ D D’ Aqm Agt Agm Pas Aqm Pai Agm Agu Ams Aqm Agd Ams Ami Agt Ams Agt Mlt Mlg Mau wt ml wt wt wt wt wt ml wt ws wt wt ws la pe sa js sa js sa wl pe pe la js js pe wl wl pe la wl la sa sa pe wl pe js wl sa js wl wl wt pe ws js js wl wl pe pe sa pe ml pe pe pe pe pe pe pe sa pe wt ws wt wl wl ws ws ws wt wt sa wl wl sa ws wl ws wl wl sa ws ws la la la ups wl2 itx wt2 itt il2 itx wl2 wtx ws2 wtx wtx wtx wl2 wl2 wtx wtx wt2 ws2 ws3 wl3 wtx ws3 ws2 wl3 wtx wt3 ups wt2 wtx ws2 wt3 ws2 wl2 ws3 wt2 wtx wtx wl2 wt2 ws2 wl2 wtx wtx itt2 wtx2 wtx wt3 ml itx2 1850 1800 1750 1700 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 0 Agt Agm Aqm Agm Aqm Aqm Agt wt pe ws pe wt pe pe wt ws wt ws la ws ws wl wl sa ws wt ws ws ws sa sa ws wt wt ws ws wl sa sa wl ws ws wl wl ws3 wt2 wl2 wt2 wt2 wl2 ws3 wt2 wtx wtx wtx wtx ws2 wt2 wtx wt2 wtx wl3 wt3 wl2 wt3 wl3 wl3 ws2 ws2 1550 1500 1450 1400 1350 1300 1250 1200 1150 E E’ Mlt Mau Mau Mau Mlg pe pe pe pe vs wl pe pe wl wl pe pe wl pe wl pe vt wl pe sa sa sa wl wl pe itt itt itt itt itt iss iss iss iss iss wtx wtx wtx itt sa iss iss iss F F’ 1850 1800 1750 1700 1650 1600 1550 1500 Asg Pai Paq Pas Mlt ws wt wt wt wt pe ws ws pe pe pe wt wt ws ws ws ws ws la la ws sa ws ws ws ws la ml ml ml ml ws ups wt2 ws2 wt3 wt3 wt2 ws3 ws2 wtx wtx wtx atx aat ws0 aal aat aat aal atx aat aal2 aal3 aal3 aal2 aal2 aal2 aal2 wt aal pe wtx wt 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 450 Elevation in feet above mean sea level G’ G wtx ws vtx atx MINNESOTA GEOLOGICAL SURVEY Harvey Thorleifson, Director Every reasonable effort has been made to ensure the accuracy of the factual data on which this map interpretation is 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 of the Minnesota Geological Survey in St. Paul. In addition, effort has been made to ensure that the interpretation conforms to sound geologic and cartographic principles. No claim is made that the interpretation shown is rigorously correct, however, and it should not be used to guide engineering-scale decisions without site-specific verification. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government. This map is submitted for publication with the understanding that the U.S. Government is authorized to reproduce and distribute reprints for governmental use. Supported by the U.S. Geological Survey, National Cooperative Geologic Mapping Program, under assistance Award No. G14AC00210. PRELIMINARY QUATERNARY STRATIGRAPHY AND SAND DISTRIBUTION MODELS OF THE CENTRAL ARROWHEAD AREA, LAKE AND ST. LOUIS COUNTIES, NORTHEASTERN MINNESOTA By Elizabeth L. Dengler, Kaleb G. Wagner, and Jacqueline Hamilton Minnesota Geological Survey University of Minnesota 2017 OPEN FILE REPORT OFR-2016-04 Prepared and Published with the Support of THE ST. LOUIS AND LAKE COUNTY BOARDS OF COMMISSIONERS, AND THE MINNESOTA ENVIRONMENT AND NATURAL RESOURCES TRUST FUND AS RECOMMENDED BY THE LEGISLATIVE-CITIZEN COMMISSION ON MINNESOTA RESOURCES LOCATION DIAGRAM The University of Minnesota is an equal opportunity educator and employer ! ! ! ! ! ! ! ! Ely Tower Aurora Winton Biwabik Babbitt McKinley Hoyt Lakes Figure 5. Sand and gravel of the Boundary Waters Formation (units ws0, ws, ws2, ws3). Model-generated map of the extent, depth from surface, and thickness of sand and gravel bodies lying stratigraphically immediately above till units wt, wt2, and wt3, but in places may overlie older units. Primarily includes proglacial outwash and ice-contact meltwater deposits generated by the Rainy lobe during its last recession and several minor re-advances. 48 0 00’ 00” 92 0 30’ 00” 48 0 00’ 00” 91 0 30’ 00” 47 0 30’ 00” 91 0 30’ 00” 47 0 30’ 00” 92 0 30’ 00” Lake County St. Louis County Lake County St. Louis County ! ! ! ! ! ! ! ! Ely Tower Aurora Winton Biwabik Babbitt McKinley Hoyt Lakes Figure 4. Surficial sand and gravel. Model-generated map of the extent and thickness of sand and gravel bodies generally occurring at or near the land surface. Includes units sa, wsd, ws, wsk, csd, cs, isd, iss, isk, and vsk from Dengler and Wagner (2016). The surficial sand and gravel is overlain in places by water and peat and other fine-grained sediment (unit pe, Dengler and Wagner, 2016), not shown above, but visible on the Quaternary cross sections (this plate). Surficial sand and gravel bodies are largely channelized south of the Laurentian continental drainage divide, whereas they are less laterally continuous north of the divide, forming ice-marginal deltas and fans, and positive-relief ice-contact landforms (e.g., eskers and kames). 48 0 00’ 00” 92 0 30’ 00” 48 0 00’ 00” 91 0 30’ 00” 47 0 30’ 00” 91 0 30’ 00” 47 0 30’ 00” 92 0 30’ 00” Lake County St. Louis County Lake County St. Louis County CROSS SECTION SYMBOLS Geologic contact – Approximately located. Contacts are not shown where data were insufficient to reliably extend units (above ups). Dashed lines separate numbered units of the same formation where they are stratigraphically adjoining. Bedrock contact – Contact point shown at the base of the Quaternary deposits with the associated bedrock map unit label from Jirsa et al. (2016). Bedrock fault – Contact point shown at the base of the Quaternary deposits, derived from symbols in Jirsa et al. (2016). Drill hole – Includes well records from the Minnesota County Well Index (CWI) within a 0.3 mile (0.5 kilometer) threshold distance north and south of a given cross section line. Where the top of a drill hole does not coincide with the sur- face elevation depicted along a cross section line, the actual location of the drill hole (within the threshold distance) lies at an elevation above or below that of its east-west position on the cross section line. DESCRIPTION OF CROSS SECTION UNITS Surficial and modified surficial geology units derive from those introduced in previous mapping. Superscript denotes reference material: * = CA Surficial Geology (Dengler and Wagner, 2016); † = SE Surficial Geology (Dengler et al., 2017). Units lacking a superscript appear as map units on both plates. HOLOCENE sa – Sand and gravel, cobbles, boulders, and sandy loam to silt loam-textured bedded sediment (modified unit) – Includes surficial units sa and in places, pe. ml – Disturbed lands (Surficial Geology unit). pe – Decomposed organic debris with clay, silt and loamy sand (Surficial Geol- ogy unit). la – Silt, clay, and loamy sand with organic debris (modified unit) – Includes surficial units la and in places, pe. ! ! ! ! ! ! ! ! Ely Tower Aurora Winton Biwabik Babbitt McKinley Hoyt Lakes Figure 6. Sand and gravel of the Independence Formation (units iss, iss2) – Model-generated map of the extent, depth from surface, and thickness of sand and gravel bodies lying stratigraphically immediately above till units itt and itt2, but in places may overlie older units. Primarily includes proglacial outwash and ice-contact meltwater deposits generated by the Brainerd lobe during its last recession. 48 0 00’ 00” 92 0 30’ 00” 48 0 00’ 00” 91 0 30’ 00” 47 0 30’ 00” 91 0 30’ 00” 47 0 30’ 00” 92 0 30’ 00” Lake County St. Louis County Lake County St. Louis County ! ! ! ! ! ! ! ! Ely Tower Aurora Winton Biwabik Babbitt McKinley Hoyt Lakes Figure 7. Mixed sand and gravel of the Boundary Waters and Independence Formations (unit vs). Model-generated map of the extent, depth from surface, and thickness of sand and gravel bodies lying stratigraphically immediately above till unit vt, but in places may overlie older units. Primarily occurs near or at the surface, forming an extensive interlobate esker system which abuts the eastern margin of the Allen and Wampus Lake moraines (Dengler and Wagner, 2016). Deposited subglacially and supraglacially by meltwater sourced from both the Rainy lobe and the Brainerd lobe. 48 0 00’ 00” 92 0 30’ 00” 48 0 00’ 00” 91 0 30’ 00” 47 0 30’ 00” 91 0 30’ 00” 47 0 30’ 00” 92 0 30’ 00” Lake County St. Louis County Lake County St. Louis County ! ! ! ! ! ! ! ! Ely Tower Aurora Winton Biwabik Babbitt McKinley Hoyt Lakes Figure 8. Mixed sand and gravel of the Independence and Cromwell Formations (units js, js2). Model-generated map of the extent, depth from surface, and thickness of sand and gravel bodies attributed to mixed proglacial meltwater sources emanating fron the Brainerd lobe and the Superior lobe. These units overlie till and associated bedded sediments of both the Boundary Waters and Independence Formations. 48 0 00’ 00” 92 0 30’ 00” 48 0 00’ 00” 91 0 30’ 00” 47 0 30’ 00” 91 0 30’ 00” 47 0 30’ 00” 92 0 30’ 00” Lake County St. Louis County Lake County St. Louis County ! ! ! ! ! ! ! ! Ely Tower Aurora Winton Biwabik Babbitt McKinley Hoyt Lakes Number of sands 1 2 3 4 5 Figure 10. Stacked sand and gravel units. Model generated map of the extent and number of Pleistocene sand and gravel units encountered between the land surface and bedrock. Note that overlying sand and gravel units are not necessarily interconnected. White areas have no mapped sand and gravel units. 48 0 00’ 00” 92 0 30’ 00” 48 0 00’ 00” 91 0 30’ 00” 47 0 30’ 00” 91 0 30’ 00” 47 0 30’ 00” 92 0 30’ 00” Lake County St. Louis County Lake County St. Louis County ! ! ! ! ! ! ! ! Ely Tower Aurora Winton Biwabik Babbitt McKinley Hoyt Lakes Figure 9. Undifferentiated Pleistocene sediment (unit ups) – Model-generated map of the extent, depth from surface, and thickness of Pleistocene sediment for which no or minimal descriptive data are available. 48 0 00’ 00” 92 0 30’ 00” 48 0 00’ 00” 91 0 30’ 00” 47 0 30’ 00” 91 0 30’ 00” 47 0 30’ 00” 92 0 30’ 00” Lake County St. Louis County Lake County St. Louis County SAND-DISTRIBUTION MODEL INTRODUCTION The sand and gravel deposits of Minnesota are the products of a long gla- cial history that complicates the mapping of potential water-bearing units in the subsurface. Establishing the location and characteristics of these sand and gravel units is an essential step toward identifying potential aquifers, and ensur- ing their appropriate use and protection. In an effort to delineate the sand and gravel bodies in the Central Arrowhead, geologists familiar with the region and a Geographic Information System (GIS) specialist collaborated to create a three- dimensional sand-distribution model showing the depth, thickness, and extent of these potential aquifers. Although the sand-distribution model and the resulting interpretations are based on the best available data, they are unavoidably incom- plete due to an uneven distribution of data. Surficial sand and gravel deposits (Figure 4) were mapped using exposures, shallow drill holes and trenches, water-well logs, and interpretations of sediment-landform associations (Dengler and Wagner, 2016). In contrast, the depiction of sand-distribution in the subsur- face was primarily based on water-well records as recorded by drillers, scientific drill core, and drill cuttings as described by geologists (Quaternary Stratigraphy, this plate). Each well record or log is a description of the stratigraphy at that precise location. A geologist assessed the accuracy of these descriptions and used this information to predict the thickness and lateral extent of each subsur- face unit around existing data points. These predictions are based on an analysis of the available data, consideration of the processes that deposited the glacial sediment, and an understanding of the regional glacial history. The distribution of data greatly influences the resolution and accuracy of the sand-distribution model. Where data are lacking, both at the surface and at depth, limited interpre- tations can be made. In general, the availability of reliable data decreases with depth below the land surface, and hence the nature and spatial distribution of buried sand and gravel bodies are not as well characterized as surficial units. The unconsolidated Quaternary sediments that occur between the land surface and the top of bedrock in the Central Arrowhead vary in character and thickness, and were deposited during several glacial episodes throughout the Pleistocene Epoch (Quaternary Stratigraphy, this plate). These deposits include multiple sequences of sand and gravel, though in the Central Arrowhead, most are related only to the most recent glacial advance and retreat across the region. Unsorted sediment deposited directly by ice (till), and bedded sediments of clay, silt, and fine-grained sand which settled into supra- and proglacial basins, locally form confining layers that enclose the sand and gravel bodies. Where the stratigraphy is more complex and two till layers related to different depositional events are not separated by a sand and gravel layer, lacking scientific drill core or cuttings data, their contact can be recognized in some instances from a nearby water-well record by a change in a driller’s description of the sediment texture, density, or color. Till units are both more pervasive and laterally continuous in the Central Arrowhead than are sand and gravel units. Whereas tills were deposited by an ice sheet that covered the entirety of the study area, sand and gravel bodies were primarily deposited by meltwater within subglacial conduits, subaerial chan- nels, or as deltas into ponded meltwater fronting the ice margin, and hence these tend to be more localized and both laterally and vertically discontinuous. Sand and gravel between layers of till may represent postglacial outwash from one or more ice lobes, proglacial outwash associated with the ice that deposited the overlying till, or a combination of both. For simplicity, the sand body naming convention associates sand and gravel units on the cross sections with the under- lying till (Quaternary Stratigraphy, this plate). Fifty-five 1 kilometer (0.6 mile) spaced east–west cross-section lines were used (Quaternary Stratigraphy, Figure 1) to create a three-dimensional model of the Quaternary deposits in the Central Arrowhead mapping area. The avail- able data were used to manually construct contact lines along each cross-section representing continuous base elevations for individual units in two-dimensional space. Examples of interpretations along seven segments of these lines (A–A’ through G–G’) are shown on the Quaternary Stratigraphy part of this plate. GIS software was used to extract elevation values from vertices along each unit line, and convert those into raster data representing the elevation of the base and areal distribution of the unit. Base surfaces were iteratively modified until the geolo- gists were confident that they adequately represented a stratigraphic interpreta- tion consistent with the majority of the data. These base surface grids were then processed using GIS raster calculations to create a set of top and bottom surfaces and a thickness for each geologic unit. The result is a three-dimensional geologic model of unconsolidated geologic materials for the Central Arrowhead. Model outputs are available digitally as raster data sets for the top and bottom elevation surfaces and thickness of each interpreted unit. Sand-distribution model outputs (Figures 4-10) should be considered high- probability maps for the occurrence and approximate thickness of major sand and gravel units. These visualizations do not guarantee sand and gravel bodies will be found at all places shown, nor do they preclude such deposits from being found in areas where they are not shown. Sand and gravel bodies less than 10 feet (3 meters) thick occasionally do not survive the processing that creates the multiple surfaces. Water-wells typically do not penetrate the complete thickness of sand and gravel layers, resulting in drillers’ logs commonly underreporting sand and gravel-body thickness. As a result, some of the sand and gravel bodies shown on the cross sections may be thicker and more widespread than portrayed. At increasing depths in the stratigraphic section, data availability diminishes and delineated sand and gravel bodies may be more or less continuous than shown. Not all water-wells and drill cores in the Central Arrowhead extend through the full thickness of unconsolidated sediments and therefore deeper Pleistocene deposits cannot be differentiated in all places (Figure 9). Additional sand and gravel units, or extensions of those mapped, are undoubtedly present in these undifferentiated parts of the Pleistocene section. Despite these limitations, the sand-distribution model provides a realistic depiction of the characteristics and distribution of geologic units in the subsurface of the Central Arrowhead. The model should be used as a guide, however, and should not preclude further site- specific investigations or inspection of individual well logs. A number of factors determine whether or not the sand and gravel units de- picted here are usable aquifers. To be considered an aquifer, earth materials must be saturated and capable of transmitting water to a well. Their capacities for water storage and transmission depend on their extent and thickness, as well as factors such as sediment coarseness, degree of sorting, consolidation, and poten- tial for recharge. Connectivity between sand and gravel units is also important. In many places, two or more sand and gravel units may connect to form a single aquifer if there is no intervening non-sand deposit. Water quality also deter- mines whether an aquifer is suitable as a source of drinking water. The maps and reports produced by the Minnesota Department of Natural Resources, as Part B of the County Geologic Atlas mapping program, take these and other factors into consideration to characterize hydrogeologic conditions. PLATE REFERENCES Dengler, E.L., and Wagner, K., 2016, Preliminary surficial geologic map of the Central Arrowhead area, Lake and St. Louis counties, northeastern Minnesota, scale 1:100,000, in Jirsa, M.A, Boerboom, T.J., Radakovich, A.L., Chandler, V.W., Peterson, D.M., Schmitz, M.D., Dengler, E.L., Wagner, K.G., Meyer, G.N., Horton, J.M., Setterholm, D.R., and Lively, R.S., 2016, Preliminary geologic maps of Lake and St. Louis Counties, northeastern Minnesota: Min- nesota Geological Survey Open File Report #OFR2016-04. Dengler, E.L., Wagner, K.G., and Meyer, G.N., 2017, Preliminary surficial geo- logic map of the Southeastern Arrowhead area, Lake and St. Louis counties, northeastern Minnesota, scale 1:100,000, in Jirsa, M.A, Boerboom, T.J., Rada- kovich, A.L., Chandler, V.W., Peterson, D.M., Schmitz, M.D., Dengler, E.L., Wagner, K.G., Meyer, G.N., Horton, J.M., Setterholm, D.R., and Lively, R.S., 2016, Preliminary geologic maps of Lake and St. Louis Counties, northeastern Minnesota: Minnesota Geological Survey Open File Report #OFR2016-04. Jirsa, M.A., Boerboom, T.J., Radakovich, A.L., Chandler, V.W., and Peterson, D.M., 2016, Bedrock geologic map of the Central Arrowhead area, Lake and St. Louis Counties, northeastern Minnesota, scale 1:100,000, in Jirsa, M.A, Boerboom, T.J., Radakovich, A.L., Chandler, V.W., Peterson, D.M., Schmitz, M.D., Dengler, E.L., Wagner, K.G., Meyer, G.N., Horton, J.M., Setterholm, D.R., and Lively, R.S., 2016, Preliminary geologic maps of Lake and St. Louis Counties, northeastern Minnesota: Minnesota Geological Survey Open File Report #OFR2016-04. Jirsa, M.A., and Meyer, G.N., 2007, Bedrock and Quaternary geology of the Central Mesabi Iron Range, northeastern Minnesota: Minnesota Geological Survey Open File Report #OFR07-03. Hobbs, H.C., 1992, Description of Rotasonic core from the Toimi drumlin field, northeastern Minnesota, in Lehr, J.D., and Hobbs, H.C., leaders, 1992, Field trip guidebook for the glacial geology of the Laurentian divide area, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Guidebook Series 18, p. 65-73. ! ! ! ! ! ! ! ! ! !! !! ! !!! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! !! ! ! " " " " " " " " Figure 1. Diagram of the Central Arrowhead showing the locations of 55 east-west cross sections constructed at regular 0.6 mile (1 kilometer) north-south intervals. Cross section segments A-A’ through G-G’ and the locations of the data points used in compiling the stratigraphy for all cross sections are also shown here. Open circles represent water-well sites in the County Well Index (only shown where within 0.3 mile [0.5 kilometer] of cross section segments). Solid circles represent cuttings sets analyzed by the Minnesota Geological Survey (MGS), labeled with corresponding unique number. Squares depict the location of rotary-sonic core sites drilled for the MGS, labeled with corresponding hole number. Sites with the prefix “SLL” were drilled in support of the present mapping project; those with the prefix “CDC” were drilled during earlier mapping (Hobbs, 1992). Though CDC-23, CDC-33, CDC-40, SLL-1, SLL-2, and SLL-6 are not located within the boundaries of the Central Arrowhead mapping area, they were used in interpretation of the regional stratigraphy. (( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( (((( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (((((( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ((( ( ( ( (( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( (( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( (( ( (( ( ( ( ( ( (( ( ( ( ( (( (( (( ( ( ( ( ( ( ( ( ( (( ( ( (( (( (( ( ( ( (( (( (((( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( (( ( ( ( (( (( (((( (( (( ((( ( (( (( (( (( ( ( ( ((( ((( (( (( (( ((( ( ( ( ( ( ( ( ( (( ( ( ( ( ( (( ( (( ( ( ( ( ( ( ( ( ( ( (((( (( ((( ( ( ( ( ( ( ( ( ( (( ( ( (( (( ( ( ( ( ( ( (( ( ((( ( ( (( ( (( ( (( (( (( ( ( ( ( (( (( ( ( ( (( (( ( ( (( (( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( (((( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ((( ( ( ( ( ( ( ( ( ( (( (( ( ((( ( ( ( ( ((( ( (( ((( (( ( ( ( ((( ((( (( ( (( ((((((( ( ( ( ( ( ( ( ( ( ( SLL-1 SLL-6 SLL-4 SLL-3 SLL-5 SLL-8 SLL-2 SLL-7 CDC-23 CDC-33 CDC-40 1 135 169 1 00331895 00331892 00331880 00331812 00652097 00270141 00020224 00243139 00243130 00571280 00243036 00161401 00776949 00786199 00773847 00243025 00161424 00798914 00786198 A A’ B B’ C C’ D D’ E E’ F F’ G G’ St. Louis County Lake County Biwabik McKinley Aurora Ely Winton Hoyt Lakes Tower Babbitt Gilbert Virginia ws0 la pe ml sa atx aat aal wl ws aal2 wtx wl2 wt ws2 wtx2 wl3 wt2 aal3 ws3 wt3 ups js iss vtx itx vs vt itt il2 ws0 sa ws ws2 ws3 js iss vs itt2 itx2 il js2 iss2 wl4 js2 iss2 Figure 3. Stratigraphic column of sand and gravel units identified in the Central Arrowhead mapping area. Numbered units are digital constructs used in the sand modelling process, those outlined in red do not appear in the cross sections shown on this plate. The full stratigraphic column with associated till and lake sediments appears in Figure 2. Figure 2. Stratigraphic position of till, lacustrine sediments, and sand and gravel bodies depicted in the cross sections. Units outlined in red were identified within a sediment sequence, but not at a boundary. They were part of the sand modeling process and are included as separate units in the digital files, but may not appear as such on the adjacent cross sections. By convention, the name designations of depicted sand and gravel bodies are associated with their underlying till. However, sand units are likely an admixture of material from immedately above or below the actual named unit. Tills and associated sand bodies of several formations are subdi- vided into multiple layers, depicted by sequential numbering (from land surface down) following the unit codes. Formation Peat Disturbed lands Aitkin Formation Alborn member Boundary Waters Formation Central Arrowhead Mesabi Iron Range pe p ml ob, rd, t aat, atx rct rt sg wt, wt2, wt3 ws0, ws, ws2, ws3 wl, wl2, wl3, wl4 sic Table 1. Correlation of stratigraphic units in the Central Arrowhead with units in the nearby Central Mesabi Iron Range map area (Jirsa and Meyer, 2007). Geologic units shown on cross sections Total #of samples Sand Silt Clay Crystalline Carbonate Shale Light Dark Red Aitkin Formation Alborn member, Till (unit aat ) Boundary Waters Formation Till (units wt, wt2, wt3 ) Independence Formation Till (units itt, itt2 ) 35 26 32 42 99 1 0 74 24 2 258 66 26 8 100 0 0 79 20 1 42 62 30 8 100 0 0 54 37 9 Total #of samples 31 167 31 Table 2. Average values for the composition and matrix texture of till units identified in the Central Arrowhead study area. Matrix texture (the less than 2 mm grain size fraction) is expressed as relative proportions of sand, silt, and clay, in percent. Gravel (the greater than 2 mm grain size fraction) is reported as percentage by mass of each bulk 50 g sample. The lithologic composition of the very coarse-grained sand component (the 1-2 mm fraction of the till matrix) is expressed in percent as relative proportions of crystal- line (mostly Precambrian), carbonate (mostly Paleozoic), and shale (mostly Cretaceous) grain-types. The crystalline category is further subdivided into light (granite, gneiss, monomineralic quartz), dark (mafic-rich igneous and metamorphic), and red (reddish basalt, rhyolite, arkose, and iron formation). Gravel (5) (23) (16) PLEISTOCENE Aitkin Formation – Reddish-brown (5YR 4/4) to reddish-grey (5YR 4/1 to 5YR 4/2) to brown (7.5YR 4/4) to greyish-brown (2.5Y 4/3) clay to clay loam-textured till, and bedded glaciofluvial and glaciolacustrine sediment of mixed Rainy, Riding Mountain, Winnipeg, and Superior provenances. Includes sequenced units atx, aat, aal, aal2, and aal3. Alborn Member – Clay to clay loam-textured till, and bedded sediments of mixed Rainy, Riding Mountain, Winnipeg, and Superior provenances. atx – Sand and gravel/till/lacustrine complex (modified unit) – Includes surficial units atx, and in places, pe. aat – Clay to clay loam till (modified unit) – Includes surficial units aat, and in places, pe. Unnamed Member – Reddish-grey (5YR 4/1) to brown (7.5YR 4/4) to greyish- brown (2.5Y 4/3) massive to bedded sediment of mixed Rainy, Riding Moun- tain, Winnipeg, and Superior provenances. aal – Undifferentiated lacustrine sediment (modified unit) – Includes surficial units aal*, and in places, pe. Boundary Waters Formation – Yellowish-brown (10YR 5/6) and brown (10YR 4/3) to grey-brown (2.5Y 4/2) to dark grey (5Y 4/1) sandy loam-textured till, and bedded glaciofluvial and glaciolacustrine sediments of Rainy prove- nance. Includes sequenced units wl, wl2, wl3, wl4, ws0, ws, ws2, ws3, wtx, wtx2, wt, wt2, and wt3. wl – Undifferentiated lacustrine sediment (modified unit) – Includes surficial units wl, wlc*, wli†, wls*, and in places, pe. ws – Sand and gravel (modified unit) – Includes surficial units ws, wsi†, wsk, wsd, and in places, pe. wtx – Sand and gravel/till/lacustrine complex (modified unit) – Includes surfi- cial units wtx, wl, wlc*, wli†, wls*, and in places, pe. wt – Sandy loam till (modified unit) – Includes surficial units wt and pe. In places, incorporation of lacustrine sediment near the base of this unit contributes to a denser, loam matrix texture with a relative enrichment of fine grained silt and clay. Mixed Independence and Cromwell Formations – Includes sediments depos- ited by mixed meltwater sources issuing from the Superior lobe and Brainerd lobe. js – Sand and gravel (modified unit) – Includes surficial units cs*, csd*, js†, jsd†, and pe. Note that surficial units originally mapped as cs and csd in Dengler and Wagner (2016) are now considered equivalent to units js and jsd in Dengler et al. (2017). Independence Formation – Brown (10YR 4/3) to dark grayish-brown (2.5Y 4/2) sandy loam till and bedded glaciofluvial and glaciolacustrine sediments of mixed Superior and Rainy provenance; deposited by the Brainerd lobe. Includes sequenced units il, il2, iss, iss2, itt, itt2, itx, and itx2. il – Undifferentiated lacustrine sediment (modified unit) – Includes surficial units il, and in places, pe. iss – Sand and gravel (modified unit) – Includes surficial units iss, isi†, isk, isd, and in places, pe. itt – Sandy loam till (modified unit) – Includes surficial units itt, and in places, pe. itx – Sand and gravel/till/lacustrine complex (modified unit) – Includes surficial units itx, ili†, and in places, pe. Mixed Boundary Waters and Independence Formations – Dark greyish- brown (10YR 4/2) sandy loam till and bedded glaciofluvial sediments of mixed Rainy and Superior provenances. vs – Sand and gravel (modified unit) – Includes surficial units vsk, and in places, pe. vt – Sandy loam to loamy sand till (modified unit) – Includes surficial unit vt†. vtx – Sand and gravel/till/lacustrine complex (modified unit) – Includes surficial units vtx*, and in places, pe. Unknown Units ups – Undifferentiated Pleistocene sediment (new unit) – Includes till, lacus- trine sand/silt/clay, and sand and gravel of unknown provenance. Shown in areas where data are scarce to absent and no interpretations of the material could be made. QUATERNARY STRATIGRAPHY INTRODUCTION The Quaternary Stratigraphy plate for the Central Arrowhead region shows the unconsolidated material expected to be encountered between the land surface and bedrock. Fifty-five west–east oriented cross sections with 0.6 mile (1 kilom- eter) spacing (Figure 1) were constructed to create a three-dimensional model of the Quaternary deposits in the Central Arrowhead. All major sand and gravel bodies are presented under the Sand Distribution Model section of this plate. The units shown in the cross sections were defined using various data (Dengler and Wagner [2016], Figure 1) sourced primarily from the Minnesota County Well Index (CWI), including water well drillers’ logs and cuttings sets, records from exploratory boreholes, and new data in the form of rotary-sonic cores that were collected for this study. Almost all units presented here are amalgamations of those defined in Dengler and Wagner (2016) and Dengler et al. (2017); only unit ups is unique to this plate. Peat (unit pe) is shown on the cross sections where it is significantly thick (about 8 feet), or it is laterally extensive. Where thin or localized, unit pe is incorporated into the underlying unit. Although they do not intersect the Central Arrowhead study area, there are several shared units between this plate and the surficial geology and cross sec- tion plates (Plates 4A and 6) of Bedrock and Quaternary geology of the Central Mesabi Iron Range, northeastern Minnesota (Jirsa and Meyer, 2007). Though Quaternary deposits were not formally assigned to lithostratigraphic formations in Jirsa and Meyer (2007), based on description and stratigraphic order, they could be tentatively correlated to several units on this plate (Table 1). Depth to bedrock (i.e., the thickness of unconsolidated Quaternary deposits) in the Central Arrowhead is extremely variable, though across much of the area it is less than 10 feet (3 meters). Where there are thicker deposits within basins or channels in the bedrock, the stratigraphy varies depending on depositional process, paleo- meltwater flow direction, and the position of the basin with respect to continen- tal drainage divides. The seven cross section segments (A–A’ to G–G’; Figure 1) on this plate were selected to illustrate this variability. All 55 cross sections can be viewed in the supplemental digital files. Rotary-sonic drill holes (Figure 1) are the key stratigraphic control points in the study area. Core associated with these drill holes was collected, logged, sampled, and interpreted by the Minneso- ta Geological Survey (MGS). These data were used to interpret and correlate be- tween drillers’ logs housed in CWI. Because not all CWI data points fall directly on the cross-section line, drill hole surfaces may project above or below the land-surface elevation. This is a result of the difference in elevation between the data point’s actual location within 0.3 mile (0.5 kilometer) of the cross section line, and its east-west projection along that line. This difference was accounted for in the process of cross section creation. All cross sections are displayed at 50x vertical exaggeration in order to show thin and/or complex units. The term “till” refers to diamicton (unsorted to poorly-sorted sediment) that was deposited directly by glacial ice. Laboratory analyses of the texture (grain- size) and lithologic analyses of the 1-2 mm (very coarse-grained sand) fraction (Table 2) under a low magnification microscope indicate that the majority of sediment in the Central Arrowhead derives from a northeastern source and is of Rainy provenance. These indicators include a coarse-grained matrix texture and a preponderance of locally-derived rock-types constituting the counted 1-2 mm grain-size fraction. Several mixed-provenance units also occur in the Central Arrowhead, including those of mixed Rainy and Superior (il, iss, itt, and itx); mixed Riding Mountain, Winnipeg, Superior (units aat, atx, and aal), and Rainy (unit aal); and mixed Superior and Rainy (unit js) provenances. By convention, the name designations of sand and gravel bodies are associated with the underly- ing till, however, the upper- and lowermost portions of individual sand and till bodies are likely an admixture of material from immediately above or below the depicted contacts. Across the mapping area, the apparent localized complexity of subsurface units may reflect variability in data density. In regions of denser data, complex stratigraphic relationships can be deciphered and shown on the cross sections. This complex stratigraphy is probably representative of much of the Central Arrowhead, particularly in areas with a substantial thickness of Quaternary sedi- ments. In certain areas, cross sections may appear especially complex in order to properly reflect the geometry of sand bodies in the subsurface while still adher- ing to the stratigraphic constraints of the sand model. These constraints require that two sand bodies separated by a non-sand unit are independently defined, even when in reality they may belong to the same unit, or be associated with the same depositional event. In contrast, sparse data in other areas are inadequate to delineate actual subsurface complexity, and hence units may appear relatively continuous and uniform. Units are extended across areas with fewer data where it seems reasonable that they may be laterally continuous. In most places, strati- graphic complexity in the subsurface arises in response to minor readvances or changes in the geometry of an ice margin as it undergoes retreat. In these areas, the stratigraphy will be more complex than in areas where the ice margin retreated more rapidly or uniformly. It is important to consider the interpretive nature of the cross sections, especially for deeper units which are less frequently encountered during drill- ing, and are thus defined on the basis of fewer data. Commonly, in areas with little or no data at depth, units are continued from adjacent cross sections. Since data are sparse across much of the study area, the surficial mapping (Dengler and Wagner, 2016) was instrumental in developing a conceptual stratigraphic model. In many places, because the Quaternary package is less than 50 feet (15 meters) thick, it was possible to extend surficial units to the base of the section. This was done only where it was reasonable to do so and in conjunction with the limited data available in these regions. In areas of scarce data and a considerable thickness of Quaternary deposits (as determined remotely by passive seismic and gravitational methods), most notably in the southeast portion of the mapping area, inferences based on the surficial mapping were extended into the subsur- face beyond 50 feet (15 meters) where they were deemed consistent with more regional data. For example, scientific rotary-sonic cores SLL-1, CDC-23, CDC- 33 and CDC-40 (Lehr and Hobbs, 1992) all depict a relatively continuous pack- age of till extending from the surface to bedrock. Though these coring locations all exist outside of the Central Arrowhead (Figure 1), their findings informed the depiction of the subsurface in adjacent and geologically comparable regions within the Central Arrowhead. All units named here are correlated with formations designated in Quater- nary Lithostratigraphic Units of Minnesota (Johnson et al., 2016) on the basis of color, lithologic composition of the 1-2 mm (very coarse-grained sand) fraction, texture, and stratigraphic position. Correlations were made using information from Johnson et al. (2016), Dengler and Wagner (2016), Dengler et al. (2017), and from the Quaternary Database Index (QDI), an internal working database of the MGS. Though defined using the same nomenclature, certain properties of units presented here may differ from those of their type sections described in Johnson et al. (2016). These differences were deemed acceptable, given the textural and compositional heterogeneity that is characteristic of individual, laterally extensive till sheets. Figures 2 and 3 show the stratigraphic order of cross section units defined for the stratigraphic constraints of the sand model process. These figures do not strictly adhere to the correlation diagrams in Dengler and Wagner (2016) and Dengler et al. (2017); a discrepancy which arises in part due to the discontinuity of units throughout the study area. For example, though deposits of the Bound- ary Waters Formation occur across most of the region, Independence Formation deposits are confined only to the eastern half of the study area. Despite sedi- ments belonging to each of these formations being deposited near-contempora- neously, their vertical order on Figure 2 reflects stratigraphic position relative to the entire study area, rather than the exclusive relationship between formations. This is done to ensure the input requirements of the sand model process are met. Figure 2 indicates that several till units (i.e., units wt or itt) occur as mul- tiple layers separated by sand and gravel or lacustrine sediment, and are made discrete by appending a numerical suffix to the unit code; this is done solely to facilitate sand model processing and does not indicate that sequentially num- bered units are significantly older or compositionally distinct. The unnumbered version of the unit code (such as wt or itt) is the primary unit, which is refer- enced in the unit description. This naming scheme is also applied to sand bodies and lacustrine sediments where they occur within a single till body, or are inter- preted as the products of minor ice marginal readvances and stagnations. In each instance, sequentially numbered units retain the same fundamental properties used to define their primary unit. Sediments deposited into relatively deep basins may also have been dissected on occasion by outwash in response to shifting meltwater drainage pathways (i.e., glacial Lake Upham II sediment in the St. Louis sublobe basin dissected by Rainy lobe outwash near the Embarrass Gap). This may also introduce complexity in the stratigraphic order, however it does not alter the underlying properties of the deposits that are used to assign them to a primary unit. 1:200,000 0 5 10 2.5 Miles 0 7.5 15 3.75 Kilometers 1:400,000 0 5 10 2.5 Miles 0 7.5 15 3.75 Kilometers Thickness Contour Interval (ft.) 20 40 60 80 100 120 140 Depth below land surface (ft.) 0 - 50 51 - 100 101 - 150 151 - 200 201 - 250 251 - 300 301 - 350 351 - 400 Avs

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Page 1: A A’ qUATERNARY STRATIGRAPHY INTRODUCTION Elizabeth …

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MINNESOTA GEOLOGICAL SURVEYHarvey Thorleifson, Director

Every reasonable effort has been made to ensure the accuracy of the factual data on which this map interpretation is 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 of the Minnesota Geological Survey in St. Paul. In addition, effort has been made to ensure that the interpretation conforms to sound geologic and cartographic principles. No claim is made that the interpretation shown is rigorously correct, however, and it should not be used to guide engineering-scale decisions without site-specific verification.The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government. This map is submitted for publication with the understanding that the U.S. Government is authorized to reproduce and distribute reprints for governmental use. Supported by the U.S. Geological Survey, National Cooperative Geologic Mapping Program, under assistance Award No. G14AC00210.

PRELIMINARY qUATERNARY STRATIGRAPHY AND SAND DISTRIBUTION MODELS OF THE CENTRAL ARROWHEAD AREA, LAKE AND ST. LOUIS COUNTIES, NORTHEASTERN MINNESOTA

By

Elizabeth L. Dengler, Kaleb G. Wagner, and Jacqueline Hamilton Minnesota Geological Survey

University of Minnesota2017

OPEN FILE REPORTOFR-2016-04

Prepared and Published with the Support ofTHE ST. LOUIS AND LAKE COUNTY BOARDS OF COMMISSIONERS, AND THE MINNESOTA ENVIRONMENT AND NATURAL RESOURCES TRUST FUND

AS RECOMMENDED BY THE LEGISLATIVE-CITIzEN COMMISSION ON MINNESOTA RESOURCES

LOCATION DIAGRAMThe University of Minnesota is an equal opportunity educator and employer

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Figure 5. Sand and gravel of the Boundary Waters Formation (units ws0, ws, ws2, ws3). Model-generated map of the extent, depth from surface, and thickness of sand and gravel bodies lying stratigraphically immediately above till units wt, wt2, and wt3, but in places may overlie older units. Primarily includes proglacial outwash and ice-contact meltwater deposits generated by the Rainy lobe during its last recession and several minor re-advances.

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Lake CountySt. Louis County

Lake CountySt. Louis County

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Figure 4. Surficial sand and gravel. Model-generated map of the extent and thickness of sand and gravel bodies generally occurring at or near the land surface. Includes units sa, wsd, ws, wsk, csd, cs, isd, iss, isk, and vsk from Dengler and Wagner (2016). The surficial sand and gravel is overlain in places by water and peat and other fine-grained sediment (unit pe, Dengler and Wagner, 2016), not shown above, but visible on the Quaternary cross sections (this plate). Surficial sand and gravel bodies are largely channelized south of the Laurentian continental drainage divide, whereas they are less laterally continuous north of the divide, forming ice-marginal deltas and fans, and positive-relief ice-contact landforms (e.g., eskers and kames).

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Lake CountySt. Louis County

Lake CountySt. Louis County

CROSS SECTION SYMBOLSGeologic contact – Approximately located. Contacts are not shown where data were insufficient to reliably extend units (above ups). Dashed lines separate numbered units of the same formation where they are stratigraphically adjoining.Bedrock contact – Contact point shown at the base of the Quaternary deposits with the associated bedrock map unit label from Jirsa et al. (2016).Bedrock fault – Contact point shown at the base of the Quaternary deposits, derived from symbols in Jirsa et al. (2016).Drill hole – Includes well records from the Minnesota County Well Index (CWI) within a 0.3 mile (0.5 kilometer) threshold distance north and south of a given cross section line. Where the top of a drill hole does not coincide with the sur-face elevation depicted along a cross section line, the actual location of the drill hole (within the threshold distance) lies at an elevation above or below that of its east-west position on the cross section line.

DESCRIPTION OF CROSS SECTION UNITSSurficial and modified surficial geology units derive from those introduced in previous mapping. Superscript denotes reference material: * = CA Surficial Geology (Dengler and Wagner, 2016); † = SE Surficial Geology (Dengler et al., 2017). Units lacking a superscript appear as map units on both plates.

HOLOCENEsa – Sand and gravel, cobbles, boulders, and sandy loam to silt loam-textured bedded sediment (modified unit) – Includes surficial units sa and in places, pe.ml – Disturbed lands (Surficial Geology unit). pe – Decomposed organic debris with clay, silt and loamy sand (Surficial Geol-ogy unit).la – Silt, clay, and loamy sand with organic debris (modified unit) – Includes surficial units la and in places, pe.

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Figure 6. Sand and gravel of the Independence Formation (units iss, iss2) – Model-generated map of the extent, depth from surface, and thickness of sand and gravel bodies lying stratigraphically immediately above till units itt and itt2, but in places may overlie older units. Primarily includes proglacial outwash and ice-contact meltwater deposits generated by the Brainerd lobe during its last recession.

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Lake CountySt. Louis County

Lake CountySt. Louis County

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Figure 7. Mixed sand and gravel of the Boundary Waters and Independence Formations (unit vs). Model-generated map of the extent, depth from surface, and thickness of sand and gravel bodies lying stratigraphically immediately above till unit vt, but in places may overlie older units. Primarily occurs near or at the surface, forming an extensive interlobate esker system which abuts the eastern margin of the Allen and Wampus Lake moraines (Dengler and Wagner, 2016). Deposited subglacially and supraglacially by meltwater sourced from both the Rainy lobe and the Brainerd lobe.

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Figure 8. Mixed sand and gravel of the Independence and Cromwell Formations (units js, js2). Model-generated map of the extent, depth from surface, and thickness of sand and gravel bodies attributed to mixed proglacial meltwater sources emanating fron the Brainerd lobe and the Superior lobe. These units overlie till and associated bedded sediments of both the Boundary Waters and Independence Formations.

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Number of sands1

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Figure 10. Stacked sand and gravel units. Model generated map of the extent and number of Pleistocene sand and gravel units encountered between the land surface and bedrock. Note that overlying sand and gravel units are not necessarily interconnected. White areas have no mapped sand and gravel units.

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Figure 9. Undifferentiated Pleistocene sediment (unit ups) – Model-generated map of the extent, depth from surface, and thickness of Pleistocene sediment for which no or minimal descriptive data are available.

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Lake CountySt. Louis County

Lake CountySt. Louis County

SAND-DISTRIBUTION MODEL

INTRODUCTION The sand and gravel deposits of Minnesota are the products of a long gla-cial history that complicates the mapping of potential water-bearing units in the subsurface. Establishing the location and characteristics of these sand and gravel units is an essential step toward identifying potential aquifers, and ensur-ing their appropriate use and protection. In an effort to delineate the sand and gravel bodies in the Central Arrowhead, geologists familiar with the region and a Geographic Information System (GIS) specialist collaborated to create a three-dimensional sand-distribution model showing the depth, thickness, and extent of these potential aquifers. Although the sand-distribution model and the resulting interpretations are based on the best available data, they are unavoidably incom-plete due to an uneven distribution of data. Surficial sand and gravel deposits (Figure 4) were mapped using exposures, shallow drill holes and trenches, water-well logs, and interpretations of sediment-landform associations (Dengler and Wagner, 2016). In contrast, the depiction of sand-distribution in the subsur-face was primarily based on water-well records as recorded by drillers, scientific drill core, and drill cuttings as described by geologists (Quaternary Stratigraphy, this plate). Each well record or log is a description of the stratigraphy at that precise location. A geologist assessed the accuracy of these descriptions and used this information to predict the thickness and lateral extent of each subsur-face unit around existing data points. These predictions are based on an analysis of the available data, consideration of the processes that deposited the glacial sediment, and an understanding of the regional glacial history. The distribution of data greatly influences the resolution and accuracy of the sand-distribution model. Where data are lacking, both at the surface and at depth, limited interpre-tations can be made. In general, the availability of reliable data decreases with depth below the land surface, and hence the nature and spatial distribution of buried sand and gravel bodies are not as well characterized as surficial units. The unconsolidated Quaternary sediments that occur between the land surface and the top of bedrock in the Central Arrowhead vary in character and thickness, and were deposited during several glacial episodes throughout the Pleistocene Epoch (Quaternary Stratigraphy, this plate). These deposits include multiple sequences of sand and gravel, though in the Central Arrowhead, most are related only to the most recent glacial advance and retreat across the region. Unsorted sediment deposited directly by ice (till), and bedded sediments of clay, silt, and fine-grained sand which settled into supra- and proglacial basins, locally form confining layers that enclose the sand and gravel bodies. Where the stratigraphy is more complex and two till layers related to different depositional events are not separated by a sand and gravel layer, lacking scientific drill core or cuttings data, their contact can be recognized in some instances from a nearby water-well record by a change in a driller’s description of the sediment texture, density, or color. Till units are both more pervasive and laterally continuous in the Central Arrowhead than are sand and gravel units. Whereas tills were deposited by an ice sheet that covered the entirety of the study area, sand and gravel bodies were primarily deposited by meltwater within subglacial conduits, subaerial chan-

nels, or as deltas into ponded meltwater fronting the ice margin, and hence these tend to be more localized and both laterally and vertically discontinuous. Sand and gravel between layers of till may represent postglacial outwash from one or more ice lobes, proglacial outwash associated with the ice that deposited the overlying till, or a combination of both. For simplicity, the sand body naming convention associates sand and gravel units on the cross sections with the under-lying till (Quaternary Stratigraphy, this plate). Fifty-five 1 kilometer (0.6 mile) spaced east–west cross-section lines were used (Quaternary Stratigraphy, Figure 1) to create a three-dimensional model of the Quaternary deposits in the Central Arrowhead mapping area. The avail-able data were used to manually construct contact lines along each cross-section representing continuous base elevations for individual units in two-dimensional space. Examples of interpretations along seven segments of these lines (A–A’ through G–G’) are shown on the Quaternary Stratigraphy part of this plate. GIS software was used to extract elevation values from vertices along each unit line, and convert those into raster data representing the elevation of the base and areal distribution of the unit. Base surfaces were iteratively modified until the geolo-gists were confident that they adequately represented a stratigraphic interpreta-tion consistent with the majority of the data. These base surface grids were then processed using GIS raster calculations to create a set of top and bottom surfaces and a thickness for each geologic unit. The result is a three-dimensional geologic model of unconsolidated geologic materials for the Central Arrowhead. Model outputs are available digitally as raster data sets for the top and bottom elevation surfaces and thickness of each interpreted unit. Sand-distribution model outputs (Figures 4-10) should be considered high-probability maps for the occurrence and approximate thickness of major sand and gravel units. These visualizations do not guarantee sand and gravel bodies will be found at all places shown, nor do they preclude such deposits from being found in areas where they are not shown. Sand and gravel bodies less than 10 feet (3 meters) thick occasionally do not survive the processing that creates the multiple surfaces. Water-wells typically do not penetrate the complete thickness of sand and gravel layers, resulting in drillers’ logs commonly underreporting sand and gravel-body thickness. As a result, some of the sand and gravel bodies shown on the cross sections may be thicker and more widespread than portrayed. At increasing depths in the stratigraphic section, data availability diminishes and delineated sand and gravel bodies may be more or less continuous than shown. Not all water-wells and drill cores in the Central Arrowhead extend through the full thickness of unconsolidated sediments and therefore deeper Pleistocene deposits cannot be differentiated in all places (Figure 9). Additional sand and gravel units, or extensions of those mapped, are undoubtedly present in these undifferentiated parts of the Pleistocene section. Despite these limitations, the sand-distribution model provides a realistic depiction of the characteristics and distribution of geologic units in the subsurface of the Central Arrowhead. The model should be used as a guide, however, and should not preclude further site-specific investigations or inspection of individual well logs. A number of factors determine whether or not the sand and gravel units de-picted here are usable aquifers. To be considered an aquifer, earth materials must be saturated and capable of transmitting water to a well. Their capacities for

water storage and transmission depend on their extent and thickness, as well as factors such as sediment coarseness, degree of sorting, consolidation, and poten-tial for recharge. Connectivity between sand and gravel units is also important. In many places, two or more sand and gravel units may connect to form a single aquifer if there is no intervening non-sand deposit. Water quality also deter-mines whether an aquifer is suitable as a source of drinking water. The maps and reports produced by the Minnesota Department of Natural Resources, as Part B of the County Geologic Atlas mapping program, take these and other factors into consideration to characterize hydrogeologic conditions.

PLATE REFERENCESDengler, E.L., and Wagner, K., 2016, Preliminary surficial geologic map of the

Central Arrowhead area, Lake and St. Louis counties, northeastern Minnesota, scale 1:100,000, in Jirsa, M.A, Boerboom, T.J., Radakovich, A.L., Chandler, V.W., Peterson, D.M., Schmitz, M.D., Dengler, E.L., Wagner, K.G., Meyer, G.N., Horton, J.M., Setterholm, D.R., and Lively, R.S., 2016, Preliminary geologic maps of Lake and St. Louis Counties, northeastern Minnesota: Min-nesota Geological Survey Open File Report #OFR2016-04.

Dengler, E.L., Wagner, K.G., and Meyer, G.N., 2017, Preliminary surficial geo-logic map of the Southeastern Arrowhead area, Lake and St. Louis counties, northeastern Minnesota, scale 1:100,000, in Jirsa, M.A, Boerboom, T.J., Rada-kovich, A.L., Chandler, V.W., Peterson, D.M., Schmitz, M.D., Dengler, E.L., Wagner, K.G., Meyer, G.N., Horton, J.M., Setterholm, D.R., and Lively, R.S., 2016, Preliminary geologic maps of Lake and St. Louis Counties, northeastern Minnesota: Minnesota Geological Survey Open File Report #OFR2016-04.

Jirsa, M.A., Boerboom, T.J., Radakovich, A.L., Chandler, V.W., and Peterson, D.M., 2016, Bedrock geologic map of the Central Arrowhead area, Lake and St. Louis Counties, northeastern Minnesota, scale 1:100,000, in Jirsa, M.A, Boerboom, T.J., Radakovich, A.L., Chandler, V.W., Peterson, D.M., Schmitz, M.D., Dengler, E.L., Wagner, K.G., Meyer, G.N., Horton, J.M., Setterholm, D.R., and Lively, R.S., 2016, Preliminary geologic maps of Lake and St. Louis Counties, northeastern Minnesota: Minnesota Geological Survey Open File Report #OFR2016-04.

Jirsa, M.A., and Meyer, G.N., 2007, Bedrock and Quaternary geology of the Central Mesabi Iron Range, northeastern Minnesota: Minnesota Geological Survey Open File Report #OFR07-03.

Hobbs, H.C., 1992, Description of Rotasonic core from the Toimi drumlin field, northeastern Minnesota, in Lehr, J.D., and Hobbs, H.C., leaders, 1992, Field trip guidebook for the glacial geology of the Laurentian divide area, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Guidebook Series 18, p. 65-73.

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Figure 1. Diagram of the Central Arrowhead showing the locations of 55 east-west cross sections constructed at regular 0.6 mile (1 kilometer) north-south intervals. Cross section segments A-A’ through G-G’ and the locations of the data points used in compiling the stratigraphy for all cross sections are also shown here. Open circles represent water-well sites in the County Well Index (only shown where within 0.3 mile [0.5 kilometer] of cross section segments). Solid circles represent cuttings sets analyzed by the Minnesota Geological Survey (MGS), labeled with corresponding unique number. Squares depict the location of rotary-sonic core sites drilled for the MGS, labeled with corresponding hole number. Sites with the prefix “SLL” were drilled in support of the present mapping project; those with the prefix “CDC” were drilled during earlier mapping (Hobbs, 1992). Though CDC-23, CDC-33, CDC-40, SLL-1, SLL-2, and SLL-6 are not located within the boundaries of the Central Arrowhead mapping area, they were used in interpretation of the regional stratigraphy.

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1

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169

1

00331895003318920033188000331812

00652097

00270141

00020224

00243139

00243130

0057128000243036

00161401

00776949

00786199

00773847

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00786198A A’

B B’

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G G’

St. Louis County Lake County

BiwabikMcKinley Aurora

Ely

Winton

Hoyt Lakes

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Figure 3. Stratigraphic column of sand and gravel units identified in the Central Arrowhead mapping area. Numbered units are digital constructs used in the sand modelling process, those outlined in red do not appear in the cross sections shown on this plate. The full stratigraphic column with associated till and lake sediments appears in Figure 2.

Figure 2. Stratigraphic position of till, lacustrine sediments, and sand and gravel bodies depicted in the cross sections. Units outlined in red were identified within a sediment sequence, but not at a boundary. They were part of the sand modeling process and are included as separate units in the digital files, but may not appear as such on the adjacent cross sections. By convention, the name designations of depicted sand and gravel bodies are associated with their underlying till. However, sand units are likely an admixture of material from immedately above or below the actual named unit. Tills and associated sand bodies of several formations are subdi-vided into multiple layers, depicted by sequential numbering (from land surface down) following the unit codes.

FormationPeatDisturbed landsAitkin Formation

Alborn member

Boundary Waters Formation

Central Arrowhead Mesabi Iron Rangepe pml ob, rd, t

aat, atx rctrtsg

wt, wt2, wt3ws0, ws, ws2, ws3wl, wl2, wl3, wl4 sic

Table 1. Correlation of stratigraphic units in the Central Arrowhead with units in the nearby Central Mesabi Iron Range map area (Jirsa and Meyer, 2007).

% of the less than2 mm fraction

% of total grains countedof the 1-2 mm fraction

Geologic units shownon cross sections Total #of

samplesSand

Silt ClayCrystalline

Carbonate

ShaleLight

Dark Red

Aitkin FormationAlborn member, Till (unit aat)

Boundary Waters FormationTill (units wt, wt2, wt3)

Independence FormationTill (units itt, itt2)

35 26 32 42 99 1 0 74 24 2

258 66 26 8 100 0 0 79 20 1

42 62 30 8 100 0 0 54 37 9

Total #of

samples

31

167

31

Table 2. Average values for the composition and matrix texture of till units identified in the Central Arrowhead study area. Matrix texture (the less than 2 mm grain size fraction) is expressed as relative proportions of sand, silt, and clay, in percent. Gravel (the greater than 2 mm grain size fraction) is reported as percentage by mass of each bulk 50 g sample. The lithologic composition of the very coarse-grained sand component (the 1-2 mm fraction of the till matrix) is expressed in percent as relative proportions of crystal-line (mostly Precambrian), carbonate (mostly Paleozoic), and shale (mostly Cretaceous) grain-types. The crystalline category is further subdivided into light (granite, gneiss, monomineralic quartz), dark (mafic-rich igneous and metamorphic), and red (reddish basalt, rhyolite, arkose, and iron formation).

Gravel

(5)

(23)

(16)

PLEISTOCENEAitkin Formation – Reddish-brown (5YR 4/4) to reddish-grey (5YR 4/1 to 5YR 4/2) to brown (7.5YR 4/4) to greyish-brown (2.5Y 4/3) clay to clay loam-textured till, and bedded glaciofluvial and glaciolacustrine sediment of mixed Rainy, Riding Mountain, Winnipeg, and Superior provenances. Includes sequenced units atx, aat, aal, aal2, and aal3.Alborn Member – Clay to clay loam-textured till, and bedded sediments of mixed Rainy, Riding Mountain, Winnipeg, and Superior provenances.atx – Sand and gravel/till/lacustrine complex (modified unit) – Includes surficial units atx, and in places, pe.aat – Clay to clay loam till (modified unit) – Includes surficial units aat, and in places, pe.Unnamed Member – Reddish-grey (5YR 4/1) to brown (7.5YR 4/4) to greyish-brown (2.5Y 4/3) massive to bedded sediment of mixed Rainy, Riding Moun-tain, Winnipeg, and Superior provenances.aal – Undifferentiated lacustrine sediment (modified unit) – Includes surficial units aal*, and in places, pe.Boundary Waters Formation – Yellowish-brown (10YR 5/6) and brown (10YR 4/3) to grey-brown (2.5Y 4/2) to dark grey (5Y 4/1) sandy loam-textured till, and bedded glaciofluvial and glaciolacustrine sediments of Rainy prove-nance. Includes sequenced units wl, wl2, wl3, wl4, ws0, ws, ws2, ws3, wtx, wtx2, wt, wt2, and wt3. wl – Undifferentiated lacustrine sediment (modified unit) – Includes surficial units wl, wlc*, wli†, wls*, and in places, pe.ws – Sand and gravel (modified unit) – Includes surficial units ws, wsi†, wsk, wsd, and in places, pe. wtx – Sand and gravel/till/lacustrine complex (modified unit) – Includes surfi-cial units wtx, wl, wlc*, wli†, wls*, and in places, pe. wt – Sandy loam till (modified unit) – Includes surficial units wt and pe. In places, incorporation of lacustrine sediment near the base of this unit contributes to a denser, loam matrix texture with a relative enrichment of fine grained silt and clay.

Mixed Independence and Cromwell Formations – Includes sediments depos-ited by mixed meltwater sources issuing from the Superior lobe and Brainerd lobe.js – Sand and gravel (modified unit) – Includes surficial units cs*, csd*, js†, jsd†, and pe. Note that surficial units originally mapped as cs and csd in Dengler and Wagner (2016) are now considered equivalent to units js and jsd in Dengler et al. (2017).Independence Formation – Brown (10YR 4/3) to dark grayish-brown (2.5Y 4/2) sandy loam till and bedded glaciofluvial and glaciolacustrine sediments of mixed Superior and Rainy provenance; deposited by the Brainerd lobe. Includes sequenced units il, il2, iss, iss2, itt, itt2, itx, and itx2.il – Undifferentiated lacustrine sediment (modified unit) – Includes surficial units il, and in places, pe.iss – Sand and gravel (modified unit) – Includes surficial units iss, isi†, isk, isd, and in places, pe. itt – Sandy loam till (modified unit) – Includes surficial units itt, and in places, pe.itx – Sand and gravel/till/lacustrine complex (modified unit) – Includes surficial units itx, ili†, and in places, pe. Mixed Boundary Waters and Independence Formations – Dark greyish-brown (10YR 4/2) sandy loam till and bedded glaciofluvial sediments of mixed Rainy and Superior provenances.vs – Sand and gravel (modified unit) – Includes surficial units vsk, and in places, pe. vt – Sandy loam to loamy sand till (modified unit) – Includes surficial unit vt†.vtx – Sand and gravel/till/lacustrine complex (modified unit) – Includes surficial units vtx*, and in places, pe. Unknown Unitsups – Undifferentiated Pleistocene sediment (new unit) – Includes till, lacus-trine sand/silt/clay, and sand and gravel of unknown provenance. Shown in areas where data are scarce to absent and no interpretations of the material could be made.

qUATERNARY STRATIGRAPHYINTRODUCTION

The Quaternary Stratigraphy plate for the Central Arrowhead region shows the unconsolidated material expected to be encountered between the land surface and bedrock. Fifty-five west–east oriented cross sections with 0.6 mile (1 kilom-eter) spacing (Figure 1) were constructed to create a three-dimensional model of the Quaternary deposits in the Central Arrowhead. All major sand and gravel bodies are presented under the Sand Distribution Model section of this plate. The units shown in the cross sections were defined using various data (Dengler and Wagner [2016], Figure 1) sourced primarily from the Minnesota County Well Index (CWI), including water well drillers’ logs and cuttings sets, records from exploratory boreholes, and new data in the form of rotary-sonic cores that were collected for this study. Almost all units presented here are amalgamations of those defined in Dengler and Wagner (2016) and Dengler et al. (2017); only unit ups is unique to this plate. Peat (unit pe) is shown on the cross sections where it is significantly thick (about 8 feet), or it is laterally extensive. Where thin or localized, unit pe is incorporated into the underlying unit. Although they do not intersect the Central Arrowhead study area, there are several shared units between this plate and the surficial geology and cross sec-tion plates (Plates 4A and 6) of Bedrock and Quaternary geology of the Central Mesabi Iron Range, northeastern Minnesota (Jirsa and Meyer, 2007). Though Quaternary deposits were not formally assigned to lithostratigraphic formations in Jirsa and Meyer (2007), based on description and stratigraphic order, they could be tentatively correlated to several units on this plate (Table 1). Depth to bedrock (i.e., the thickness of unconsolidated Quaternary deposits) in the Central Arrowhead is extremely variable, though across much of the area it is less than 10 feet (3 meters). Where there are thicker deposits within basins or channels in the bedrock, the stratigraphy varies depending on depositional process, paleo-meltwater flow direction, and the position of the basin with respect to continen-tal drainage divides. The seven cross section segments (A–A’ to G–G’; Figure 1) on this plate were selected to illustrate this variability. All 55 cross sections can be viewed in the supplemental digital files. Rotary-sonic drill holes (Figure 1) are the key stratigraphic control points in the study area. Core associated with these drill holes was collected, logged, sampled, and interpreted by the Minneso-ta Geological Survey (MGS). These data were used to interpret and correlate be-tween drillers’ logs housed in CWI. Because not all CWI data points fall directly on the cross-section line, drill hole surfaces may project above or below the land-surface elevation. This is a result of the difference in elevation between the data point’s actual location within 0.3 mile (0.5 kilometer) of the cross section line, and its east-west projection along that line. This difference was accounted for in the process of cross section creation. All cross sections are displayed at 50x vertical exaggeration in order to show thin and/or complex units. The term “till” refers to diamicton (unsorted to poorly-sorted sediment) that was deposited directly by glacial ice. Laboratory analyses of the texture (grain-

size) and lithologic analyses of the 1-2 mm (very coarse-grained sand) fraction (Table 2) under a low magnification microscope indicate that the majority of sediment in the Central Arrowhead derives from a northeastern source and is of Rainy provenance. These indicators include a coarse-grained matrix texture and a preponderance of locally-derived rock-types constituting the counted 1-2 mm grain-size fraction. Several mixed-provenance units also occur in the Central Arrowhead, including those of mixed Rainy and Superior (il, iss, itt, and itx); mixed Riding Mountain, Winnipeg, Superior (units aat, atx, and aal), and Rainy (unit aal); and mixed Superior and Rainy (unit js) provenances. By convention, the name designations of sand and gravel bodies are associated with the underly-ing till, however, the upper- and lowermost portions of individual sand and till bodies are likely an admixture of material from immediately above or below the depicted contacts. Across the mapping area, the apparent localized complexity of subsurface units may reflect variability in data density. In regions of denser data, complex stratigraphic relationships can be deciphered and shown on the cross sections. This complex stratigraphy is probably representative of much of the Central Arrowhead, particularly in areas with a substantial thickness of Quaternary sedi-ments. In certain areas, cross sections may appear especially complex in order to properly reflect the geometry of sand bodies in the subsurface while still adher-ing to the stratigraphic constraints of the sand model. These constraints require that two sand bodies separated by a non-sand unit are independently defined, even when in reality they may belong to the same unit, or be associated with the same depositional event. In contrast, sparse data in other areas are inadequate to delineate actual subsurface complexity, and hence units may appear relatively continuous and uniform. Units are extended across areas with fewer data where it seems reasonable that they may be laterally continuous. In most places, strati-graphic complexity in the subsurface arises in response to minor readvances or changes in the geometry of an ice margin as it undergoes retreat. In these areas, the stratigraphy will be more complex than in areas where the ice margin retreated more rapidly or uniformly. It is important to consider the interpretive nature of the cross sections, especially for deeper units which are less frequently encountered during drill-ing, and are thus defined on the basis of fewer data. Commonly, in areas with little or no data at depth, units are continued from adjacent cross sections. Since data are sparse across much of the study area, the surficial mapping (Dengler and Wagner, 2016) was instrumental in developing a conceptual stratigraphic model. In many places, because the Quaternary package is less than 50 feet (15 meters) thick, it was possible to extend surficial units to the base of the section. This was done only where it was reasonable to do so and in conjunction with the limited data available in these regions. In areas of scarce data and a considerable thickness of Quaternary deposits (as determined remotely by passive seismic and gravitational methods), most notably in the southeast portion of the mapping area, inferences based on the surficial mapping were extended into the subsur-face beyond 50 feet (15 meters) where they were deemed consistent with more

regional data. For example, scientific rotary-sonic cores SLL-1, CDC-23, CDC-33 and CDC-40 (Lehr and Hobbs, 1992) all depict a relatively continuous pack-age of till extending from the surface to bedrock. Though these coring locations all exist outside of the Central Arrowhead (Figure 1), their findings informed the depiction of the subsurface in adjacent and geologically comparable regions within the Central Arrowhead. All units named here are correlated with formations designated in Quater-nary Lithostratigraphic Units of Minnesota (Johnson et al., 2016) on the basis of color, lithologic composition of the 1-2 mm (very coarse-grained sand) fraction, texture, and stratigraphic position. Correlations were made using information from Johnson et al. (2016), Dengler and Wagner (2016), Dengler et al. (2017), and from the Quaternary Database Index (QDI), an internal working database of the MGS. Though defined using the same nomenclature, certain properties of units presented here may differ from those of their type sections described in Johnson et al. (2016). These differences were deemed acceptable, given the textural and compositional heterogeneity that is characteristic of individual, laterally extensive till sheets. Figures 2 and 3 show the stratigraphic order of cross section units defined for the stratigraphic constraints of the sand model process. These figures do not strictly adhere to the correlation diagrams in Dengler and Wagner (2016) and Dengler et al. (2017); a discrepancy which arises in part due to the discontinuity of units throughout the study area. For example, though deposits of the Bound-ary Waters Formation occur across most of the region, Independence Formation deposits are confined only to the eastern half of the study area. Despite sedi-ments belonging to each of these formations being deposited near-contempora-neously, their vertical order on Figure 2 reflects stratigraphic position relative to the entire study area, rather than the exclusive relationship between formations. This is done to ensure the input requirements of the sand model process are met. Figure 2 indicates that several till units (i.e., units wt or itt) occur as mul-tiple layers separated by sand and gravel or lacustrine sediment, and are made discrete by appending a numerical suffix to the unit code; this is done solely to facilitate sand model processing and does not indicate that sequentially num-bered units are significantly older or compositionally distinct. The unnumbered version of the unit code (such as wt or itt) is the primary unit, which is refer-enced in the unit description. This naming scheme is also applied to sand bodies and lacustrine sediments where they occur within a single till body, or are inter-preted as the products of minor ice marginal readvances and stagnations. In each instance, sequentially numbered units retain the same fundamental properties used to define their primary unit. Sediments deposited into relatively deep basins may also have been dissected on occasion by outwash in response to shifting meltwater drainage pathways (i.e., glacial Lake Upham II sediment in the St. Louis sublobe basin dissected by Rainy lobe outwash near the Embarrass Gap). This may also introduce complexity in the stratigraphic order, however it does not alter the underlying properties of the deposits that are used to assign them to a primary unit.

1:200,000

0 5 102.5 Miles

0 7.5 153.75 Kilometers

1:400,000

0 5 102.5 Miles

0 7.5 153.75 Kilometers

Thickness Contour Interval (ft.)20

40

60

80

100

120

140

Depth below land surface (ft.)0 - 50

51 - 100

101 - 150

151 - 200

201 - 250

251 - 300

301 - 350

351 - 400

Avs

quaternary structure

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