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Deposition of the Cambrian Eau Claire Formation, Wisconsin: Hydrostratigraphic Implications of Fine-Grained Cratonic Sandstones

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Deposition of the Cambrian Eau Claire Formation, Wisconsin: Hydrostratigraphic Implications of Fine-Grained Cratonic Sandstones DEPOSITION OF THE CAMBRIAN EAU CLAIRE FORMATION, WISCONSIN: HYDROSTRATIGRAPHIC IMPLICATIONS OF FINE-GRAINED CRATONIC SANDSTONES Wasinee Aswasereelert1, J.A. (Toni) Simo1, 2, and David L. LePain3 ABSTRACT Understanding the link between the sedimentology and the hydrogeology of the Eau Claire Formation within Dane and adjacent counties in western to south-central Wisconsin is critical to fl uid fl ow studies. The Eau Claire is a relatively fi ne-grained fossiliferous sandstone unit that lies between coarser-grained, highly porous, unfossiliferous sandstone of the underlying Mount Simon and the overlying Wonewoc Formations. It consists primarily of very fi ne- to medium-grained, variably feldspathic, glauconitic, and dolomitic sandstone locally interbedded with argillaceous siltstone and silty mudstone that has a coars- ening and thickening upward succession. On the basis of sedimentary structures, lithology, and bedding characteristics, the Eau Claire is divided into fi ve lithofacies representing different paleowater depths of an epeiric shelf environment. The Eau Claire shallowing-upward succession is subdivided into up to fi ve depositional cycles laid down by repetitive shoreface progradation ranging from offshore–shoreface– foreshore facies bounded at the base by a marine fl ooding surface. In places, sharp-based shoreface facies rest directly over offshore facies attesting to lowering of sea level. The depositional cycles and the structural contour and isopach maps suggest that the Eau Claire lithofacies deposition and distribution were controlled by the substrate, including the Wisconsin Arch and syndepositional faults, and sea level. The Eau Claire depositional facies model parallels a hydrostratigraphic model in which confi ning prop- erties of the Eau Claire Formation decrease from offshore to foreshore facies. The aquitard qualities in- crease from the Wisconsin Arch to the western outcrop belt because the formation is more heterogeneous to the west. INTRODUCTION Regional aquitards, extensive layers of relatively im- through rock, is a function of permeability, porosity, permeable rock units, can restrict the movement of grain size, sorting, cementation, composition, sedi- contaminants and thus protect aquifers used for mu- mentary structures, and stratifi cation (Freeze and nicipal water supplies. The Eau Claire Formation, Cherry, 1979). Factors such as geologic origin, depo- which consists of fi ne-grained sandstones, siltstones, sitional and post-depositional processes, and lithology and mudstones, is a regional aquitard that impedes the can result in signifi cant heterogeneity and preferential exchange of groundwater between the Mount Simon fl ow pathways within aquitards. Consequently, knowl- aquifer and other main overlying aquifers in Wiscon- edge of the depositional environment and lithofacies sin (Bradbury and others, 1999). distribution of the Eau Claire Formation should en- Hydrogeologic studies have been hampered by a hance understanding of its role as an aquitard region- lack of understanding of the variable thickness and li- ally and locally. thology of the Eau Claire Formation (Krohelski and Our study focused on the link between the sedi- others, 2000; Gotkowitz and others, 2005). Hydrau- mentology and hydrogeology of the Eau Claire For- lic conductivity, a parameter describing fl uid fl ow mation in southern and western Wisconsin (fi g. 1). 1 Department of Geology and Geophysics, University of Wisconsin–Madison, Madison, Wisconsin 53706 2 Now at Exxon Mobil, URC GW3 980B, P.O. Box 2189, Houston, Texas 77252-2189 3 Wisconsin Geological and Natural History Survey, Madison, Wisconsin 53705-5100; now at Alaska Division of Geological & Geophysical Surveys, 3354 College Road, Fairbanks, Alaska 99709 Wisconsin Geological and Natural History Survey GEOSCIENCE WISCONSIN • Volume 19, Part 1 • published online 2008 A 0 100 MI B LAVA FLOW 02100 00 KM RIDGES LAKE SUPERIOR MICHIGAN WISCONSIN DOME TILDEN WILLOW QUARRY U OUTCROP RIVER U WISCONSIN STUDY AREA HWY 12-124 WISCONSIN ARCH STRUM BRUCE VALLEY MINNESOTA IOWA MI KM NESHONOC BARABOO RIDGE 0 0 U LAKE MICHIGAN 20 30 CORE STUDY AREA 40 60 DANE 60 90 UU 131486 HOLLANDALE 131467 EMBAYMENT 80 120 ILLINOIS 100 150 570055 570003 C 570001 110047 110095 110045 110052 110677 570168 COLUMBIA 570008 130889 130123 570010 131264 DODGE SAUK 130888 131371 130027 131091 JEFFERSON 130893 130941 250003 131275 131462 131100 130957 130038 130960 131440 131127 130698 130050 130968130983 280308 130105 131069 131485 130715 131143 130052 280090 130146 131486 131488 130066 130958 130947 130961 130929 130988 280857 130954 131467 130991 250145 131092 131060 131138 130993 131442 131042 130981 131226 130013 131102 131064 250014 130030 131273 130316 130122 130925 130900 280043 130934 131088 280085 131096 130079 131126 131334 250016 DANE 130204 GREEN 230023 230022 540002 ROCK 540478 540564 540540 IOWA 230011 540548 LAFAYETTE 540037 540474 540514 Kilometers 230087 051015202530354045 Miles 330303 0 5 10 15 20 25 30 Figure 1. A. Regional map showing early Paleozoic tectonic features and major paleotopographic highs of Precambrian rocks in the central midcontinent region. Modifi ed from Runkel (1994). B. Core and outcrop location map. Diamonds represent the locations of the outcrop and well sec- tions shown in fi gure 2. C. Locations of water wells and coreholes used in this study. Solid stars, circles, triangles, and squares represent more reliable to less reliable control points, respectively. Cores represented by open stars. Well data provided by the Wisconsin Geological and Natural His- tory Survey. 2 • GEOSCIENCE WISCONSIN This work may assist in predicting locations of the EAU CLAIRE FORMATION Eau Claire lithofacies in areas for which rock data are The Eau Claire Formation overlies the Mount Simon limited and may help explain the regional distribution Formation and underlies the Wonewoc Formation. The of porosity and permeability, the major controls on Eau Claire Formation was deposited in a cratonic en- fl uid fl ow. vironment that included intracratonic basins and scat- tered arches and domes (Ostrom, 1978). During the METHODOLOGY Cambrian, the seafl oor in what is now northern Wis- We compiled outcrop and subsurface data within the consin was near the Wisconsin Dome. Our core data Upper Mississippi Valley for our study. The subsur- came from Dane County, which is on the southwest- face data come primarily from Dane County in south- ern side of the Wisconsin Arch, a southeast–trending central Wisconsin; the outcrop information, from extension of the Wisconsin Dome; our outcrop data western Wisconsin (fi g. 1). We used lithostratigraph- came from west of the dome (fi g. 1). ic and sedimentologic data from outcrops, cores, cut- The Eau Claire Formation is part of the Marju- tings, and borehole geophysical logs. Stratigraphic man (Cedaria and Crepicephalus trilobite zone) to sections were measured and logged using a handheld Steptoean (Aphelaspis trilobite zone) Stage of the Up- natural spectral gamma-ray detector (Exploranium per Cambrian Croixian Series and is predominantly spectrometer, GR-320 enviSPEC). Weathered parts of very fi ne- to medium-grained, moderately to well sort- outcrop faces were scraped before being measured. ed, variably feldspathic, glauconitic, and dolomitic Four cores with gamma-ray borehole logs (Mount So- sandstone with variable amounts of siltstone and mud- pris Instruments gamma-ray probe, 2PGA-1000) were stone. It has a distinctive gamma-ray signature that also described. With gamma ray tools, total gamma has a sharp base and up to fi ve peaks of high gamma- and the amount of potassium, uranium, and thorium ray values that decrease in magnitude up-section (fi g. were measured in counts per second (cps). We studied 2). Correlation of the gamma ray values and the lithol- 36 thin sections from cores and outcrops that are rep- ogy of the Eau Claire showed that the peaks repre- resentative of the upper parts of the Mount Simon and sent greater potassium feldspar content in the very fi ne Eau Claire Formations and the lower part of the Wone- sand and silt than in the coarser-grained sand. woc Formation to determine mineral composition and On the basis of petrographic analysis, we found grain size at 400 equally spaced points on each slide. two major differences between Eau Claire core and In addition to the sedimentologic study, porosity and outcrop samples (fi g. 3): 1) sand grains of core sam- permeability of 15 samples from Tilden Quarry, West ples are much coarser than those of outcrop samples, Tilden roadcut, and Willow River outcrop were mea- and 2) potassium feldspar is a major component in sured at 400 pounds per square inch confi ning stress outcrop samples, but almost absent in cores. These by conventional core-analysis techniques (using a differences support the feldspar and grain-size rela- Core Laboratories CMS-300 instrument) to investigate tionship proposed by Odom (1975), who suggested hydrostratigraphic trends. that, in the Upper Cambrian rocks, feldspar is concen- We used subsurface data to generate structural trated in sediments fi ner than 0.125 mm. We deter- contour and isopach maps. We reexamined 96 litho- mined that the differences between cores and outcrops logic logs derived from cuttings that had been previ- were infl uenced by the proximity to the shoreline, re- ously studied by Wisconsin Geological and Natural sulting in differences in the energy and the depth of History Survey staff. We developed a continuum of re- deposition. In Dane County (core area), the Eau
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