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

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 properties of the Eau Claire Formation decrease from offshore to foreshore facies. The aquitard qualities increase 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 sections 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 scattered 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 southern 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 Claire liability of well information. The most reliable infor- was deposited closer to the shoreline within shallower mation came from those wells that had cuttings with water and higher energy conditions, so it is less feld- detailed descriptions supported by gamma-ray bore- spathic and has coarser sediments. hole logs; less reliable were wells that had detailed descriptions, but no gamma-ray measurements; next EAU CLAIRE LITHOFACIES in reliability were wells that had gamma-ray logs, but The Eau Claire Formation was fi rst divided into litho- poor descriptions; the least reliable were wells that facies on the basis of bedding type and mudstone con- had poor descriptions and no gamma-ray measure- tent by Morrison (1968) and Huber (1975) in geo- ments. graphically restricted investigations in west-central

VOLUME 19 2008 • 3 A B C GAMMA RAY (CPS) GAMMA RAY (CPS) GAMMA RAY (CPS) 3255 6270 9285 0100200300 020406080 55 D 250 320

B C 50 C 255 325 B C C B 45 260 330 C

B WONEWOC FORMATION 40 265 335 D D C LITHOFACIES D (column A) C B AND C (columns B & C) 35 270 A 340 B C D LITHOFACIES D C B 30 275 345 B A C B C LITHOFACIES C 25 280 350 B DEPTH (FT) B C THICKNESS (FT) B LITHOFACIES B 20 C 285 355 B

A C A LITHOFACIES A 15 290 360 EAU CLAIRE FORMATION B B B 10 295 365 D MOUNT SIMON FORMATION D D LITHOFACIES D A 5 B 300 370 D 0 305 375 Figure 2. Gamma-ray characteristic of the Eau Claire Formation, from outcrop: A. Tilden composite section; and from core: B. Well 131467 (Nine Springs) and C. Well 131486 (Cottage Grove MP6). See ﬁ gure 1 for locations of outcrop and wells.

675 650

600 Core samples 550 Outcrop samples 500 Quartz 450

400 Potassium feldspar

350

300

NUMBER OF GRAINS 250

200

150

100

0 5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 GRAIN SIZE (PHI UNIT)

4 • GEOSCIENCE WISCONSIN Table 1. Eau Claire lithofacies A–E. HCS = hummocky cross-stratifi cation; SCS = swaley cross- stratifi cation; TCS = trough cross-stratifi cation; FWWB = fair-weather wave base. Ichnofacies and ichnofabric index are described in Pemberton and others (1992) and Droser and Bottjer (1986), respectively.

Contact relation Bedding Sedimentary Depositional Lithofacies Body fossil with characteristics structure environment index underlying Ichnofacies Ichnofabric lithofacies A: Mudstone At least 1-ft interval Soft sediment Brachiopod 3–4 Gradational, Offshore and siltstone of very thin- to thin- deformation with fragments with litho- below with minor bedded mudstone loaded sandstone facies B, sharp storm wave sandstone and siltstone; with others base very thin-bedded sandstone also present B: Thin-bedded, Sandstone beds are Microhummocky Brachiopod 3 Gradational, Offshore

micro-HCS generally less than cross-stratiﬁ cation Cruziana fragments with change transition zone sandstone with 2 in. thick; very thin- and ripple bed form in bedding between storm siltstone and bedded siltstone and characteristics and FWWB mudstone mudstone less than 0.1 in. thick

C: Thick- to very Sandstone beds are Ripple bed forms, Brachiopod 2 Gradational Lower thick-bedded generally thicker plus Ca: hummocky fragments and sharp- shoreface HCS to SCS than 2 ft; siltstone cross-stratifi cation; throughout; based; sharp Ca: just below and low-angle and mudstone Cb: hummocky hyolithid jump in FWWB to planar- laminae less than cross-stratifi cation, and trilobite bedding style Cb: at FWWB laminated 0.04 in. thick gutter casts and casts in Cb Cc: just above sandstone planar lamination; FWWB with minor Cc: swaley cross- Cd: above siltstone and stratifi cation; Cd: FWWB mudstone planar to low-angle (divided into 4 lamination sub-lithofacies:

Ca, Cb, Cc, Cd) Skolithos D: Thick- to very Sandstones appear Trough cross-bedding Brachiopod 1–2 Gradational Upper thick-bedded to be massive fragments shoreface TCS sandstone without internal below low tide breaks E: Very thick- Massive sandstone Planar lamination Brachiopod 1 Gradational Foreshore bedded, without parting fragments within planar-laminated surfaces intertidal zone sandstone

Wisconsin. The most recent regional study related to bite zones, showing that the formation is a distal facies the Eau Claire Formation was completed by Runkel of the overlying Wonewoc Formation (Crepicepha- and others (1998). They divided the Eau Claire into lus and Aphelaspis trilobite zones) and the underlying three units on the basis of the Upper Cambrian trilo- Mount Simon Formation (Cedaria trilobite zone). We have divided the Eau Claire Formation into M Figure 3. Frequency diagram, showing ﬁ ve lithofacies on the basis of sedimentary structures, grain-size distribution of quartz and lithology, and bedding characteristics (table 1). Inter- potassium feldspar in core and outcrop bedding of the thin- and thick-bedded lithofacies with- samples. in the formation delineates sandier-upward succes-

VOLUME 19 2008 • 5 gray in outcrop; in core, the color ranges from green to gray, pink to maroon, and buff. The sandstone is very light gray to yellowish brown in outcrop; the sandstone in core is gray, pink, dark green, and buff. This lithofacies is locally glauconitic, dolomitic, and micaceous. Sedimentary structures are not well preserved, possibly because of the intense bioturbation. Lithofacies B consists of 0.5- to 11.6-ft intervals of thin-bedded, very fi ne- to fi ne-grained (to medium- grained in core), variably glauconitic sandstone intercalated with substantial argillaceous siltstone and mudstone (fi gs. 5 and 7). The sandstone is light Figure 4. Interbedding of thin- and thick-bedded lithofacies of the Eau Claire Formation at Tilden Quarry. gray, light tan, yellowish brown, and dark green; the siltstone and mudstone are green to dark gray. In core, this lithofacies is also pink to maroon and buff. Outcrop samples are feldspathic and micaceous; cores are dolomitic. Sand- stone beds pinch and swell laterally and are locally lenticular (fi g. 8). They commonly display sharp, scoured bases. Mudstone and argillaceous siltstone are found in discontinuous drapes and very thin, laterally continuous beds. The microhumocky cross-stratifi cation is characterized by glauconite concentration and very thin-bedded siltstone and mudstone. It contains thin hummocks and swales that have wave- lengths of 1.5 to 2.5 ft. Sets of laminae intersect at low angles. Low-angle to Figure 5. Very thin- to thin-bedded silty mudstone and argillaceous wavy lamination could be associated siltstone of lithofacies A overlain by lithofacies B at Neshonoc Lake. with the microhummocky cross-stratifi cation. The upper surface of the sandstone beds commonly has oscillation sions (fi g. 4). Because sedimentary structures are dif- ripples. Bioturbation obscures the original bedding in fi cult to defi ne in cores, our Eau Claire lithofacies de- places. scription is mostly based on outcrop observation. Lithofacies C consists of less than 1.0-ft to Lithofacies A is composed of 1.0- to 6.4-ft in- 15.0-ft intervals of thick to very thick-bedded, very tervals of very thin to thin-bedded, relatively well ce- fi ne- to fi ne-grained (medium-grained at the top of mented, silty mudstone, argillaceous siltstone, and the formation in outcrop and throughout the forma- sandy siltstone (fi g. 5). Very thin-bedded, very fi ne- tion in cores), variably glauconitic sandstone inter- grained (fi ne grained in core) sandstone is common calated with green siltstone and mudstone laminae (fi g. 6). The siltstone and mudstone are green to dark or drapes (fi g. 9). The laminae are rare compared to

6 • GEOSCIENCE WISCONSIN AB

Figure 6. A. Lithofacies A of Nine Springs core, showing presence of very thin-bedded, very ﬁ ne-grained sandstone within more prominent siltstone and mudstone. B. Photomicrograph taken from sample 131467-289.8 with plane-polarized light.

A B

Figure 7. Thin-bedded sandstone of lithofacies B intercalated with argillaceous siltstone and mudstone. A. Outcrop at Willow River. B. Core from well 131486 (Cottage Grove MP6). C. Photomicrograph taken from sample 131486-346.3 with plane-polarized light. Q = quartz; G = glauconite; D = dolomite; and P = pore space.

VOLUME 19 2008 • 7 lamination and less-prominent hummocky cross-stratifi cation. Sublithofa- cies Cc contains swaley cross-stratifi - cation with broad, very low-angle concave-upward laminae that have some planar to low-angle internal truncation surfaces. Although the swales rarely pass laterally into hummocks, the presence of some convex-upward laminae in a swaley sandbody indicates a genet- ic similarity to hummocky cross-stratifi cation. Sublithofacies Cd is dominated by planar to low-angle lamination that has a trace of swaley cross-stratifi cation. All four sublithofacies locally display preserved asymmetric and sym- metric ripple forms on top of sandstone Figure 8. Bedding style of lithofacies B at Tilden Quarry. beds. Sandstone beds pinch and swell laterally and locally lenticular. In core, we separated lithofacies C into two sublithofacies: C1 and C2. A B Sublithofacies C1 is similar to sublithofacies Ca, Cb, and Cc; sublithofacies C2 is comparable to sublithofacies Cd. Sublithofacies C1 can be recognized by high- to low-angle cross-lamination and wavy lamination, which are likely to be associated with hummocky to swaley cross-stratifi cation. Sublitho- facies C2 can be defi ned by low-angle to planar lamination and lack of clear sedimentary structures as is sublithofacies Cd. Figure 9. Thick- to very thick-bedded sandstone of lithofacies C Lithofacies D is entirely sand- intercalated with siltstone and mudstone laminae. A. Outcrop at stone, typifi ed by 1.6- to 8.0-ft inter- Tilden Quarry. Core from well 131486. The outcrop also shows B. vals of thick- to very thick-bedded suc- amalgamated thin bedded, planar-laminated sandstone. cessions of very fi ne- to fi ne-grained (up to lower medium in cores), trough those in lithofacies B. Thick sandstone beds locally cross-stratifi ed, variably glauconitic sandstone (fi g. compose amalgamated thin-bedded, plane parallel- 10). Color ranges from light tan to rusty tan, plus laminated sandstone. Color ranges from very light white to light pink in cores. Rip-up clasts of sand- gray to light tan to pink. This lithofacies is feldspathic stone and dolomite are present. Well organized cross- and micaceous in outcrop and dolomitic in core. sets are typically 0.3 to 1.0 ft thick. They are com- The variation of sedimentary structures in litho- monly superimposed on one another to form thick, facies C in outcrop can be used to separate it into four amalgamated successions. sublithofacies. Sublithofacies Ca consists of hum- Lithofacies E is entirely sandstone, character- mocky cross-stratifi cation; its low-angle, gently un- ized by a 5.0-ft interval of very thick-bedded, very dulating laminae are formed by broad convex-upward fi ne- to fi ne-grained, light tan to rusty tan, planar-lam- hummocks and concave-upward swales with wave- inated sandstone (fi g. 11). Laminations dip very gen- lengths of 3.0 to 5.0 ft. Sublithofacies Cb has planar tly from each other.

8 • GEOSCIENCE WISCONSIN LITHOFACIES INTERPRETATION AND DEPOSITIONAL ENVIRONMENT Our study indicated that the lithofacies and accompanying sedimentary structures of the Eau Claire were deposited in an epeiric shelf environment ranging from offshore to foreshore facies (fi g. 12). We interpreted lithofacies A to have been deposited in a quiet-water environment below the storm-wave base representing an offshore facies (Howell and Flint, 2003). Lithofacies B represents an offshore transition facies deposited by storm-generated os- cillatory waves below the fair-weather- wave base, but above the storm-wave Figure 10. Thick- to very thick-bedded, trough cross-stratifi ed base (Dott and Bourgeois, 1982; Leckie sandstone of lithofacies D at Willow River. The broomstick is and Walker, 1982; Duke, 1985). Dur- marked every 6 in. ing storms, sands were eroded from the shoreward zone and redeposited in a microhummocky, cross-stratifi ed sheet-like blanket across the shelf. Dur- ing fair-weather periods, clays and silts were deposited from suspension and covered the sheet of sand laid down during the storms. We interpreted lithofacies C to represent lower shoreface facies deposited by storms in water depths close to the fair-weather-wave base, where the seabed was constantly agitated and clays and silts were rarely deposited (Howell and Flint, 2003). The sublithofacies Ca–Cd indicate slight- ly different settings: from a deeper and lower energy environment of sublithofacies Ca to a shallower and higher energy environment of sublithofacies Cd (Leckie and Walker, 1982; Walker, Figure 11. Very thick-bedded, planar-laminated sandstone of 1985; Duke, 1990). We interpreted the lithofacies E at Willow River. The broomstick is marked every 6 in. trough cross-bedded sandstone, lithofacies D, and planar-laminated sandstone, lithofacies E, to be upper shoreface and foreshore fa- mon Formations. This is consistent with fl ooding of cies, respectively. The sedimentary structures and the the shoreline (Mount Simon–Eau Claire contact), fol- lack of mica suggested conditions of high fl ow veloc- lowed by shallowing upward and progradation of the ity and shallow water depth (Collinson and Thomp- Wonewoc over the deeper-water Eau Claire. son, 1989). Lithofacies D and E are rare in the Eau Lithofacies D and E were deposited within an en- Claire, but common in the Wonewoc and Mount Si- ergetic nearshore environment, close to the arch, in a

VOLUME 19 2008 • 9 high tide level low tide level BACKSHORE FORESHORE ZONE ZONE UPPER supratidal fair-weather-wave base LOWER intertidal area area storm wave base SHOREFACE OFFSHORE TRANSITION Lithofacies E OFFSHORE ZONE ZONE ZONE

Lithofacies A Lithofacies B Lithofacies C D

LITHOFACIES A LITHOFACIES E

CLAY MATRIX DETRITAL GRAIN

LOWER POROSITY HIGHER POROSITY

HIGH GAMMA LOW GAMMA

NOT TO SCALE

Figure 12. Shallow marine profi le, showing the typical succession of the Eau Claire Formation. (Modifi ed from Coe and Church, 2003.) storm-dominated shoreface and in an intertidal zone, er corresponded to the peaks of any cycles, except the respectively. The lower shoreface sediment (lithofa- uppermost cycle of the Cottage Grove MP6 core (well cies C) was transported by storm-enhanced currents— 131486). along the shore by longshore currents and away from the shore by rip currents (Runkel, 1994; Byers and STRATIGRAPHIC SUCCESSIONS Dott, 1995; Runkel and others, 1998) following topo- AND CYCLICITY OF THE EAU CLAIRE graphic differences on the seafl oor. Immediately sea- Correlation of a composite section based on outcrops ward, lithofacies B sediment was buried during storms in Chippewa County and the descriptions of two cores and graded basinward to the mudstone of lithofacies A. in Dane County demonstrates cyclicity and lithofacies The lithofacies of the Eau Claire Formation are repetitions within the Eau Claire stratigraphic suc- also distinctly expressed in the relationship between cessions (fi g. 13). Note that the two cores show more the Eau Claire sediments and the total gamma of these proximal facies than the Tilden section, which repre- sediments as measured in outcrop and in the subsur- sents more distal deposits. face. The total gamma-ray measurements of the Wone- The overall vertical stacking of the Eau Claire woc and Mount Simon Formations (table 2) are for lithofacies represents shallowing-upward successions the base and the top, respectively. Figure 2 and table 2 and internally can be subdivided into depositional cy- show that the gamma decreases from lithofacies A to cles characterized by repetitions of lithofacies. The lithofacies D. On the basis of the subsurface data, the cycles are defi ned by deeper-water facies overlying Eau Claire lithofacies of some intervals can be iden- shallower-water lithofacies. In outcrop it is possible to tifi ed with the total gamma. The highest total gamma recognize up to eight lithofacies repetitions; the cores mostly corresponds to the lithofacies A (>100 cps) or show no more than six lithofacies repetitions. With re- B (<100 cps) of the second depositional cycle from spect to subsurface correlations based on total gamma the base of the formation. If the highest gamma repre- ray measurement, each Eau Claire depositional cy- sents lithofacies A, the peak of the bottom deposition- cle is bounded at its base by a relatively high gamma al cycle can be interpreted as representing shallower- count; we interpreted this to be a marine fl ooding sur- water facies because the highest gamma-ray values of face, which is marked by an abrupt change from shal- each depositional cycle decrease in magnitude up-sec- low- to deeper-water facies. We interpreted the inter- tion. If the highest total gamma of the bottom cycle is vals that have lower gamma counts to be grading into more than 200 cps, this interval consists of lithofacies a shallowing-upward succession. The outcrop shows B. If it is less than 200 cps, then lithofacies C com- at least fi ve cycles, but the cores show only four cy- poses this interval. Notably, lithofacies D and E nev- cles. It is important to note that the cores and outcrop

10 • GEOSCIENCE WISCONSIN Table 2. Total gamma-ray measurements showing geophysical trends within the Eau Claire Formation. The difference between core and outcrop measurements is due to the different tools used in the study. W = Wonewoc Formation; MS = Mount Simon Formation.

Outcrop Core

Average total Total gamma Average total Total gamma (cps) gamma (cps) (cps) gamma (cps) formation Lithofacies/

W 1,431–2,587 2,182 1.7–14.0 8.4 E 2,121–2,991 2,724 — — D 3,187–3,617 3,407 4.2–14.1 8.7 C 3,259–6,833 4,972 1.7–196.3 26.5 B 3,909–10,101 6,565 1.6–273.2 57.3 A 4,125–9,738 7,259 12.1–289.4 105.7 MS 2,973–5,784 4,638 7.2–37.4 20.4 are separated by approximately 150 miles and that tacts and sudden changes in lithology between the part of this variability can be explained by changes Eau Claire and the Wonewoc Formations may be as- in lithology and stacking along depositional strike. sociated with a signifi cant unconformity. However, On the other hand, the cores are separated by only 12 Havholm (1998) and Runkel and others (1998) de- miles; the variability between them may be a result of scribed the Eau Claire–Wonewoc contact as gradation- deposition at different locations in relation to the Wis- al and becoming younger to the west, which would consin Arch. likely be the result of the nearshore marine Wonewoc Generally, the contacts between lithofacies are prograding across the Eau Claire sediment. Our out- gradational, with two exceptions. The basal contact crop investigation showed that local wave-base ero- of each cycle refl ects an abrupt change from shallow- sion resulted in sharp contacts between offshore tran- to deeper-water facies that we interpreted as marine sition strata and shoreface sandstones and in turn fl ooding. Cementation and faunal lags are common at could be responsible for the thickness variability of these surfaces. In some places, sharp-based shoreface the Eau Claire Formation. Therefore, we concluded sandstone (lithofacies C and D) rests directly over that the unconformity-like feature between the Eau lithofacies B (Willow River section), interpreted by Claire and the Wonewoc Formations was due to local Walker and Plint (1992) to be the result of prograda- wave-base erosion and facies changes, not to any sig- tion due to a relative sea level fall. In other places, the nifi cant period of nondeposition or extensive erosion microhummocky cross-stratifi ed sandstone of lithofa- of previously deposited sediment. cies B is overlain by hummocky cross-stratifi ed sandstone of sublithofacies Cb; the contact is marked by REGIONAL CONTROLS gutter casts (Bruce Valley Quarry), which Aswaser- ON EAU CLAIRE DISTRIBUTION eelert (2005) interpreted as being formed by wave The amount of data from outcrops and cores (high- scour on the bed. Lithofacies Cc and Cd are present quality data) was insuffi cient to allow us to adequately only in the sharp-based successions; we interpreted lo- defi ne the regional distribution of the Eau Claire For- cal wave-base erosion during regression to have led to mation; therefore, we used subsurface logs derived sharp and, in some locations, erosive contacts between from cuttings (generally lower quality or more inter- the offshore transition microhummocky strata and the pretive data) from the Wisconsin Geological and Nat- shoreface sandstones. ural History Survey to interpolate between high-quali- Morrison (1968), Ostrom (1970), DiStefano ty data points. (1973), and Huber (1975) suggested that erosive con-

VOLUME 19 2008 • 11

40 20 Gamma (cps) 2 4 2 5 4 1 3 mU Fe Not to horizontal scale Fe PP G Fe PP G G G Fe G G G G Fe vfU fU G G G G G Fe Fe G G G st G G cl B B B D B Wavy lamination (outcrop) Wavy lamination (core) Wavy Stylolite Firmground Uncertain structure Missing interval W C/ C2 C1 C2 C2 D/ C1 MS Feet ?

330 335 340 345 350 355 360 365 370

240

180

120 60 Gamma (cps) Planar tangential cross-stratification 3 2 2 5 4 3 1 mL ? P G G P Mn Cross-stratification cross-stratification Trough Plane parallel lamination Low-angle lamination Hummocky cross-stratification Swaley cross-stratification G M M Mn G G P Mn Mn M M vfL fL G P G Mn G G G G G G G Mn cl B A B A B C/ W D/ B? A? MS Feet 275 280 285 290 295 300 265 270

Sandstone rip-up clast Siltstone rip-up clast Mudstone rip-up clast Dolomite rip-up clast Disseminated clay Liesegang band

Base of 4th Cycle 4th of Base

Base of 5th Cycle 5th of Base

Base of 2nd Cycle 2nd of Base Base of 3rd Cycle 3rd of Base Glauconite Mica Iron oxide Manganese oxide Pyrite Dolomitic cement thin bed/lamina of finer- or coarser-grained material Very Base of Wonewoc Formation Base of Wonewoc

P G M

Fe Mn

Base of Eau Claire Formation (1st Cycle) (1st Formation Claire Eau of Base

9285

7275

5265 3255 Gamma (cps) Outcrop–core correlation and comparison. Notice that the Eau Claire Formation in Formation and comparison. Notice that the Eau Claire correlation Outcrop–core 4 3 2 3 5 1 2 2 5 5 1 1/1 cL Fe mL ? ? ? ? Figure 13. Figure successions shallowing-upward more and represents composite section (left) is thicker outcrop Tilden and wells. 1 for locations of outcrop gure than in well 131467 (middle) and 131486 (right); see ﬁ G G G M G G G Fe G M M Fe M G G M Fe M G vfL fL G G M G G M G Fe G G G G G M G G Fe M cl B B B A B B B W D/ D/ Ca Cd Cb MS Cb Feet 5 0 20 15 35 30 25 10 45 40

12 • GEOSCIENCE WISCONSIN We used the relative amounts of sandstone and The structural contour and isopach maps show mudstone to defi ne the Eau Claire lithofacies in the a north–south trending structural high (fi g. 14). This cuttings. Cuttings composed of shale and siltstone high is evidence that the Eau Claire deposition was were interpreted to represent lithofacies A. When also infl uenced by the north–south trending positive sandstone dominated, depending on the amount of feature of the Precambrian substrate associated with shale and siltstone present, that interval could be lith- the Wisconsin Arch, which modifi ed the paleocurrents ofacies B (large amounts of clay- to silt-sized sedi- and sediment distribution. These maps indicate the ments), C (trace amount of clay- to silt-sized sedi- shape of the north–south trending high was modifi ed ments), or D and E (no sediments fi ner than sand were by syndepositional faults. In addition to the infl uence present). of the bedrock substrate varying over time, the cyclic- The structural contour map of the Precambrian ity within the Eau Claire Formation can be reasonably basement in Dane County shows a north–south trend- attributed to relative sea-level fl uctuations that forced ing high and three east–west trending faults (fi g. 14A). shorelines to move seaward and landward. The high and the faults are also refl ected in the structural contour maps of the base of the Eau Claire For- HYDROSTRATIGRAPHIC IMPLICATIONS mation and the Tunnel City Group (fi g. 14B and 14C). The permeability contrast between lithofacies and The isopach maps of the Eau Claire and the combined between depositional cycles, the vertical and lateral Eau Claire–Wonewoc (fi g. 14D and 14E) refl ect the trends in porosity and permeability, and the amount, fact that the Eau Claire Formation progressively thins thickness, and continuity of mudstones are particular- from 110 ft in southwestern Dane County to 40 ft in ly important to fl uid fl ow through the Eau Claire For- central Dane County; it is absent in northeastern Dane mation. The Eau Claire is a heterogeneous and aniso- County. This is consistent with the interpretations of tropic aquitard due to the discontinuity and vertical Twenhofel and others (1935), Odom (1975), and Run- stacking pattern of the lithofacies within the forma- kel and others (1998), who suggested that the Eau tion. The aquitard nature of the Eau Claire Formation Claire is thinned by virtue of its upper and lower parts is defi ned primarily by two components: low perme- grading laterally into the progressively thicker Mount ability mudstone and siltstone and higher permeability Simon and Wonewoc Formations toward the Wiscon- sandstone. sin Arch. Some of the best evidence for the Eau Claire be- The Eau Claire and the combined Eau Claire– ing an aquitard comes from test well 131467 at the Wonewoc isopach maps (fi g. 14D and 14E) demon- Nine Springs site, where a packer slug test was used strate that faulting is a major control on the thickness to measure horizontal hydraulic conductivity (Kh) and variation and the stratigraphic boundaries of the Eau static head of the Eau Claire (Bradbury and others, Claire Formation (fi g. 14D and 14E). The faults were 2006; fi g. 16). A signifi cant drop in hydraulic head— identifi ed in areas where the contour lines were close approximately 30 ft across the Eau Claire–Mount Si- to each other or where the elevations of the Eau Claire mon contact—shows that the Eau Claire Formation structural contour map and the thickness of the Eau acts as a major aquitard. The Kh values for the Eau Claire isopach map changed so abruptly that it could Claire fall into the clay–silt category, even though be evidence of displacement. The thickness contours the sandstone dominates lithofacies B; this is con- of the Eau Claire Formation are substantially infl u- sistent with our outcrop investigation, which showed enced by the faults; the faults have relatively less ef- that the mudstone and siltstone strata of this lithofa- fect on the combined Eau Claire–Wonewoc thickness. cies are generally more laterally continuous than the Cross sections on the regional surface at the base of sandstone beds (Aswasereelert, 2005). This continu- the Tunnel City Group show that the Wonewoc For- ity along with the thickness of the mudstone and silt- mation (fi g. 15) thins across faults corresponding to stone is likely to be signifi cant to fl uid fl ow through abrupt changes in the Eau Claire–Wonewoc boundary the aquitard. The vertical hydraulic conductivity is elevations, and the elevations change abruptly at the very diffi cult to measure directly; we inferred it to be base of the Eau Claire across the faults. Consequently, low because the aquitard holds up about 30 ft of hy- we interpreted the faults to be active during the depo- draulic head. sition of the Eau Claire and the Wonewoc. The mudstone and siltstone of the Eau Claire For-

VOLUME 19 2008 • 13 A 598 465 451

COLUMBIA 230 187 213 160 160 120 120

SAUK 80 DODGE 40 91 JEFFERSON 0 -40 78 -80 160 40 90 80 126 21 120 -120 174 160 -2 39 139 0 150 -221 -40 100 64 -80 -94 -120 -160 -154 83 -200 -88 -276 -80 -82 -160 -120 -185

DANE GREEN -200 ROCK IOWA -240 LAFAYETTE

-280 Kilometers 051015202530354045 Miles 0 5 10 15 20 25 30

B 600 576 674 620 736 COLUMBIA 640 478 600 533 759 478 732 600 560

560 664

480 DODGE SAUK 590 648 535 520 JEFFERSON 424 461 636 532 605 586 528 485 672 675 440 632 479 568 596 520 633 609 540 506 579 614 538 493 609 585 546 535 615 620 249 480 574 474 434 543 525 423 447 529 474 476 503 590 435 498 419 486 530 550 559 451 610 400

675 440 440 480 679 524 329 586 400 520 523 307 548 520 360 478 DANE 520 411 GREEN 399 IOWA 360 ROCK 384 320 LAFAYETTE 320 280 276 246 275 283 280 Kilometers 240 051015202530354045 216 240 Miles 0 5 10 15 20 25 30

Figure 14. Structural contour maps: A. Precambrian basement; B. Base of the Eau Claire Formation. Faults (dashed line) based on Eau Claire structural contour map. Elevation (in feet) based on sea level. Solid stars, circles, triangles, and squares represent more reliable to less reliable control points, respectively. Cores represented by open stars.

14 • GEOSCIENCE WISCONSIN C Kilometers 051015202530354045 Miles 666 784 685 0 5 10 15 20 25 30 846 755 COLUMBIA

839 792 760 720 719 DODGE 750 SAUK 773 685 800 815 JEFFERSON 702 756 760 750 706 680 678 720 847 790 702 640 709 748 790 766 660 616 720 666 774 754 600 588 769 675 663 765 750 354 680 713 604 574 682 579 673 687 668 735 600 720 680 631 560 616 649 681 723 675 630 698 681 790 824 834 629 502 746 680 497 640 665 598 708 648 539 520 DANE 631 680 644 591 GREEN 650 ROCK 574 499 554 IOWA 568 600 504 LAFAYETTE 560 456 520 516 455 480 473 436 440

D Kilometers 051015202530354045 0 Miles 0 0 5 10 15 20 25 30 0 15 0 COLUMBIA 0

0 20

10 0 0 DODGE 0 SAUK 29 40 0 0 JEFFERSON

85 0 30 25 0

160 40

30 35 0 10

65 30 0

80 40

37 40 20 20 20 40 29 70 35 35 25

90 60 31 28

40 60 60 70 30

50 40 50 25 60 60 40 45 80 70 50 45 70 50 32 80 75 40 45 15 80 44 60 55 40 60 18 15 110 50 80 45 25 120 30 15 50 80

70 70 30 60 DANE 60 30 GREEN 50 ROCK 30 IOWA 35 50 40 LAFAYETTE

40 25 139 45 60

Figure 14 (continued). C. Base of the Tunnel City Group. Isopach maps: D. Eau Claire Formation. Faults (dashed line) based on Eau Claire structural contour map. Elevation (in feet) based on sea level. Solid stars, circles, triangles, and squares represent more reliable to less reliable control points, respectively. Cores represented by open stars.

VOLUME 19 2008 • 15 E Kilometers 051015202530354045 85 Miles 110 90 0 5 10 15 20 25 30 110 135 COLUMBIA 80 60 130 110 120 55 DODGE 160 SAUK 179 125 150 170 295 JEFFERSON 145 180 120 150 140

150 290 175 115 70

160

200 210

280 230 180 110 157 170 120 110

190 145 270 165 120 129 131 130 95 125 130 150 165 105 140 140 170 105

250 240 153 165 150

260 145 165 250 200 155 230 164 145 155

195 195 170 180

240 230 149 160

220 230 155 170 105 173 160 180 75 190 160 160 145 170 DANE

180

210 200 190

180 GREEN 170 175 IOWA ROCK 170 LAFAYETTE 270 180 180 190 160 220

Figure 14 (continued). E. Combined Eau Claire and Wonewoc Formations. Faults (dashed line) present in all maps based on Eau Claire structural contour map. Elevation (in feet) based on sea level. Faults (dashed line) based on Eau Claire structural contour map. Elevation (in feet) based on sea level. Solid stars, circles, triangles, and squares represent more reliable to less reliable control points, respectively. Cores represented by open stars. mation, which are interpreted to act as a major barrier cies A) or offshore transition (lithofacies B) facies and to fl uid fl ow, dominate lithofacies A and lithofacies B. the various shoreface facies (lithofacies C, D, and E) On the basis of Kh values (Bradbury and others, 2006) of the underlying strata. The difference in facies above and lithofacies distribution, the amount, thickness, and and below the cycle boundaries can result in fl ow-unit continuity of mudstone and siltstone are particularly boundaries that have signifi cant permeability differ- important to fl uid fl ow through the Eau Claire aqui- ences. For instance, the permeability and porosity of tard. The thickness of the least permeable strata, lith- the Tilden Quarry sandstones studied in outcrop in ofacies A, can vary from approximately 2 ft up to 25 Chippewa County increase from 328.3 millidarcies ft. The distribution of lithofacies A shows the most ir- (md) and 28.1 percent (lithofacies B) to 811.1 md and regularity: It can be present in one well, but absent in 29.4 percent (lithofacies C); the samples are only 3 ft another less than 10 miles away. Lithofacies A might apart (fi g. 17). have been preserved as either extensive strata or dis- The most porous and permeable strata are the continuous patches; lithofacies B, which is present relatively high depositional energy lower and upper in all the wells (fi g. 15), comprises much of the Eau shoreface and foreshore sandstones (lithofacies C, D, Claire Formation. and E; fi g. 12). The increase in porosity and permea- The stacking pattern of the Eau Claire succession bility can be tied to an increase in depositional energy, is also signifi cant to fl uid fl ow. The Eau Claire depo- which resulted in 1) better grain sorting and an associ- sitional cycles, bounded above and below by fl ood- ated decrease in the amount of fi ne-grained sediment ing surfaces, represent the building block of the Eau within pore spaces; 2) fewer and thinner mudstone Claire aquitard being separated by a surface of high layers; and 3) a decrease in cementing materials. As a permeability contrast between the offshore (lithofa- result, porosity and permeability are lower from upper

16 • GEOSCIENCE WISCONSIN A Base of Tunnel City Group A 0 A' 131485 DE Base of Wonewoc Formation 100 DE B C DE 130105 C 110 B C C C B B B 280308 C C C 120 B B B 130052 B C C B C Base of Eau Claire Formation 130947 B 130 B

Feet 131486 131488 B A 140 A Kilometers 03691215 150 Miles

A 0246810 160 B

170 Base of Tunnel City Group B 0 131486 131485 B' 100 131060 DE B DE C Base of Wonewoc Formation 110 131467 B C C C B B B B B 130981 A C C C 120 B C B C B B A C B B C 130 C B B A 131488 130030 A Base of Eau Claire Formation 140 B B B B Feet 150 C A D E Lithofacies D and E A B C Lithofacies C 160 C Kilometers B Lithofacies B 170 036912 15 Miles A Lithofacies A 180 C 0246810 Interpreted fault 190

200

Base of Tunnel City Group C 0 131486 131485 C' 100 DE B DE C 131138 B 110 C C B C B B B A C C C 120 B B B B C C B C 130 Base of Wonewoc Formation C B 131488

Feet 131273 140 C C

B 150 B B 160

170 C Base of Eau Claire Formation JEFFERSON Kilometers 180 131485 280308 0246810 130105 131486 130052 Miles 130947 A'

012345 A 131467 B' 131488 131060 C' 130030 130981 131138

B C 131273

Figure 15. A. Cross sections of the Eau Claire DANE Kilometers

Formation datum at the base of the Tunnel City Group, 0 20 40 Miles showing lithofacies variations and syndepositional 0 10 20 faults. B. Lines of cross sections.

VOLUME 19 2008 • 17 Lithology and structure Gamma (cps) Kh (ft/day) Total head (ft below datum) -3 -2 60 240 120 180 Facies cl st vfLvfU fL fU mL 10 10 -80 -70 -60 B G M M M M G

290 A

? 295 B G G G G G G G G G G G

P P D MS 300

Figure 16. Stratigraphic section of an Eau Claire interval, showing results from a packer-slug test at the Nine Springs site (well 131467). Bars show

horizontal hydraulic conductivity (Kh) derived from slug test using inﬂ atable straddle packers to isolate various sections of the borehole. Width of bars represents width of packed intervals. Hydraulic heads were measured using

pressure transducers; Kh and hydraulic head data from Bradbury and others (2006). MS = Mount Simon Formation. shoreface sandstones to offshore mudstones. This cor- gy and bedding characteristics representing a shallow responds to the previously mentioned increase in gam- marine depositional shelf deposit ranging from off- ma-ray values from lithofacies D to lithofacies A. We shore-shoreface-foreshore facies. In general, the for- concluded that the greater the permeability, the lower mation thickens to the south and the west. The number the gamma-ray values and the higher the depositional of vertical stackings, potassium feldspar grains and energy. sediments fi ner than fi ne sand increase from eastern The relationships among the various lithofacies Dane County to the western outcrop belt due to the imply that the hydraulic properties of the aquitard are proximity to the shoreline or the Wisconsin Arch. The directly related to the geometry and distribution of structural contour and isopach maps and the cyclicity lithofacies, which are a function of the depositional of the Eau Claire Formation suggest that the Precam- environment. Therefore, it should be possible to relate brian substrate (Wisconsin Dome and Arch), synde- trends in the distribution of hydraulic properties to the positional faulting, and sea level fl uctuation are major distribution of lithofacies in the aquitard system. Re- controls on the Eau Claire lithofacies deposition and gionally, more depositional cycles and more shallow- distribution. The Eau Claire aquitard is likely to retard ing-upward successions are present along the western fl uid fl ow to the west from the arch to the western out- outcrop belt than in the area near the Wisconsin Arch. crop belt. As a consequence, the Eau Claire Formation becomes more of a barrier to fl uid fl ow from the Wisconsin ACKNOWLEDGMENTS Arch to the west. We thank the Wisconsin Geological and Natural His- tory Survey for providing the subsurface data and pre- SUMMARY paring the thin sections. Funding for this project was We have divided the Eau Claire Formation into fi ve provided by a grant from the American Petroleum In- lithofacies based on sedimentary structures, litholo- stitute and National Ground Water Association. The

18 • GEOSCIENCE WISCONSIN Gamma (cps) Lithology and structure Permeability (md) 100 200 300 400 500 600 700 800 2435 3440 4445 5450 6455 7460 Facies cl st vfLvfU fL fU mL mU cL

G M 40 Cb G M

G G B G 35 G G Ca M M G G

G M G M 30 B G G G G Cb G G G 25 G G

G B G G Fe G 20 Fe Fe G

G 15 Cb G

M G

G G M

G M 10 G

B G G

Fe G G 5 Fe G Fe Fe ?

D ? MS ?

? 0 MS Figure 17. Stratigraphic section at the Tilden Quarry (location 90516), showing the relationship between the Eau Claire lithofacies, total gamma, and permeability. Permeability was measured by conventional core analysis techniques, using Core Laboratories CMS-300 instrument. MS = Mount Simon Formation.

VOLUME 19 2008 • 19 authors thank Anthony C. Runkel for his support, and Droser, M.L., and Bottjer, D.J., 1986, A semiquantitative Charles W. Byers, for his valuable discussion. Susan fi eld classifi cation of ichnofabric: Journal of Sedi- K. Swanson, Madeline B. Gotkowitz, and Thomas J. mentary Petrology, v. 56; no. 4, p. 558–559. Evans also provided helpful reviews. Grateful appre- Duke, W.L., 1985, Hummocky cross stratifi cation, tropi- ciation is due to the Wisconsin Geological and Natural cal hurricanes, and intense winter storms: Sedimen- History Survey staff and colleagues at the University tology, v. 32, p. 167–194. of Wisconsin–Madison Department of Geology and Duke, W.L., 1990, Geostrophic circulation or shallow Geophysics for their assistance, encouragement, and marine turbidity currents? The dilemma of paleofl ow suggestions. patterns in storm-infl uenced prograding shoreline systems: Journal of Sedimentary Petrology, v. 60, p. REFERENCES 870–883. Aswasereelert, Wasinee, 2005, Facies distribution and Freeze, R.A., and Cherry, J.A., 1979, Groundwater: New stacking of the Eau Claire Formation, Wisconsin: Jersey: Prentice-Hall, 604 p. Implications of thin shale-rich strata in fl uid fl ow: Madison, University of Wisconsin, M.Sc. thesis, Gotkowitz, M.B., Zeiler, K.K., Dunning, C.P., Thomas, 200 p. J.C., and Lin, Yu-Feng, 2005, Hydrogeology and simulation of groundwater fl ow in Sauk County, Bradbury, K.R., Eaton, T.T., and Evans, T.J., 1999, Wisconsin: Wisconsin Geological and Natural His- Characterization of the hydrostratigraphy of the tory Survey Bulletin 102, 43 p. deep sandstone aquifer in southeastern Wisconsin: Wisconsin Geological and Natural History Survey Havholm, K.G., 1998, Pre-Quaternary geologic his- Open-File Report 1999-02 (1 CD-ROM). tory of western Wisconsin, with an emphasis on the Cambrian sandstones, in Syverson, K.M., and Bradbury, K.R., Gotkowitz, M.B., Hart, D.J., Eaton, T.T., Havholm, K.G., eds., Geology of western Wiscon- Cherry, J.A., Parker, B.L., and Borchardt, M.A., sin: Guidebook for 61st annual Tri-State Geological 2006, Contaminant transport through aquitards: Field Conference: Tri-State/University of Wisconsin Technical guidance for aquitard assessment: AWWA System, p. 1–14. Research Foundation, American Water Works As- sociation, and IWA Publishing, 143 p. Howell, J.A., and Flint, S.S., 2003, Siliciclastic case study: The Book Cliffs, in Coe, A.J., ed., The Sedi- Byers, C.W., and Dott, R.H., Jr., 1995, Sedimentology mentary Record of Sea-Level Change: Cambridge, and depositional sequences of the Jordan Formation Cambridge University Press, p. 135–208. (Upper Cambrian), Northern Mississippi Valley: Journal of Sedimentary Research, v. 65, no. 3b, p. Huber, M.E., 1975, A paleoenvironmental interpreta- 289–305. tion of the Upper Cambrian Eau Claire Formation of west-central Wisconsin: Madison, University of Coe, A.L., and Church, K.D., 2003, Sequence stratigra- Wisconsin, M.Sc. thesis, 110 p. phy, in Coe, A.J., ed., The Sedimentary Record of Sea-Level Change: Cambridge, Cambridge Univer- Krohelski, J.T., Bradbury, K.R., Hunt, R.J., and Swanson, sity Press, The Open University, p. 57–98. S.K., 2000, Numerical simulation of groundwater fl ow in Dane County, Wisconsin: Wisconsin Geolog- Collinson, J.D., and Thompson, D.B., 1989, Sedimentary ical and Natural History Survey Bulletin 98, 44 p. Structures: London, Unwin Hyman Ltd, 207 p. Leckie, D.A., and Walker, R.G., 1982, Storm- and tide- DiStefano, M., 1973, The mineralogy and petrology of dominated shorelines in Cretaceous Moosebar-Low- the Eau Claire Formation, west-central Wisconsin: er Gates interval-outcrop equivalents of Deep Basin De Kalb, Northern Illinois University, M.Sc. thesis, Gas Trap in Western Canada: American Association 132 p. of Petroleum Geologists Bulletin, v. 66, p. 138–157. Dott, R.H., Jr., and Bourgeois, Joanne, 1982, Hummocky Morrison, B.C., 1968, Stratigraphy of the Eau Claire stratifi cation: Signifi cance of its variable bedding Formation of west-central Wisconsin: Madison, sequences: Geological Society of America Bulletin, University of Wisconsin, M.A. thesis, 41 p. v. 93, p. 663–680.

20 • GEOSCIENCE WISCONSIN Odom, I.E., 1975, Feldspar—Grain size relations in Minnesota Geological Survey Report of Investiga- Cambrian arenites, Upper Mississippi Valley: tions, v. 43, p. 60–71. Journal of Sedimentary Petrology, v. 45, no. 3, p. Runkel, A.C., McKay, R.M., and Palmer, A.R., 1998, 636–650. Origin of a clastic cratonic sheet sandstone: Stra- Ostrom, M.E., 1970, Field trip guidebook for Cambrian– tigraphy across the Sauk II–Sauk III boundary in Ordovician geology of western Wisconsin: Wiscon- the Upper Mississippi Valley: Geological Society of sin Geological and Natural History Survey Informa- America Bulletin, v. 110, p. 188–210. tion Circular 11, 131 p. Twenhofel, W.H., Raasch, G.O., and Thwaites, F.T., Ostrom, M.E., 1978, Stratigraphic relationships of lower 1935, Cambrian strata of Wisconsin: Geological Paleozoic rocks of Wisconsin: Wisconsin Geological Society of America Bulletin, v. 46, p.1687–1744. and Natural History Survey Field Trip Guide Book Walker, R.G., 1985, Geological evidence for storm 3, p. 3–22. transportation and deposition on ancient shelves, in Pemberton, S.G., MacEachern, J.A., and Frey, R.W., Tillman, R.W., Swift, D.J.P., and Walker, R.G., eds., 1992, Trace fossil facies models: environmental and Shelf Sands and Sandstone Reservoirs: Society of allostratigraphic signiﬁ cance, in Walker, R.G., and Economic Paleontologists and Mineralogists Short James, N.P., eds., Facies Models–Response to Sea Course Notes 13, p. 243–302. Level Change, Ottawa: Geological Association of Walker, R.G., and Plint, G.A., 1992, Wave- and storm- Canada, p. 47–72. dominated shallow marine systems, in Walker, R.G., Runkel, A.C., 1994, Revised stratigraphic nomenclature and James, N.P., eds., Facies Models: Response for the Upper Cambrian (St. Croixan) Jordan Sand- to Sea Level Change: Geological Association of stone, southeastern Minnesota, in Southwick, D.L., Canada, p. 219–238. ed., Short contributions to the geology of Minnesota:

VOLUME 19 2008 • 21 Published by and available from Wisconsin Geological and Natural History Survey 3817 Mineral Point Road • Madison, Wisconsin 53705-5100 ¤ 608/263.7389 FAX 608/262.8086 www.uwex.edu/wgnhs/ James M. Robertson, Director and State Geologist

ISSN: 0164-2049 This report is an interpretation of the data available at the time of preparation. Every rea- sonable effort has been made to ensure that this interpretation conforms to sound scientifi c principles; however, the report should not be used to guide site-specifi c decisions without verifi cation. Proper use of this publication is the sole responsibility of the user. The use of company names in this document does not imply endorsement by the Wisconsin Geological and Natural History Survey. Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, University of Wisconsin–Extension, Cooperative Extension. University of Wisconsin–Extension provides equal opportunities in employment and programming, including Title IX and ADA requirements. If you need this information in an alternative format, contact the Offi ce of Equal Opportunity and Diversity Programs or the Wisconsin Geological and Natural History Survey (¤ 608/262.1705).

Mission of the Wisconsin Geological and Natural History Survey The Survey conducts earth-science surveys, ﬁ eld studies, and research. We provide objective scientiﬁ c information about the geology, mineral resources, water resources, soil, and biology of Wisconsin. We collect, interpret, disseminate, and archive natural resource information. We communicate the results of our activities through publications, technical talks, and responses to inquiries from the public. These activities support informed decision-making by government, industry, business, and individual citizens of Wisconsin. LITHOFACIES, K-BENTONITE GEOCHEMISTRY, AND SEQUENCE STRATIGRAPHY OF THE ORDOVICIAN (MOHAWKIAN–CINCINNATIAN) GALENA GROUP, NORTHEASTERN IOWA

Steve R. Beyer1, 2, J.A. (Toni) Simo1, 3, and Charles W. Byers1

ABSTRACT The Ordovician (Mohawkian–Cincinnatian Series) Galena Group, a mixed carbonate–siliciclastic unit, was deposited in a near-equatorial epeiric sea that covered most of the North American craton. The Galena Group is widely distributed in the Upper Mississippi Valley and is non-dolomitized and well preserved in northeastern Iowa. Generally, the Galena Group is composed of bioturbated wackestones and packstones that preserve abundant and diverse, predominantly benthic fauna. We recognized four lithofacies groups on the basis of subtle lithologic and ichnofaunal variations in the northeastern Iowa outcrop belt. Lithofacies group stacking patterns are broadly cyclic and defi ne depositional cycles on the scale of 1 to 15 m. We correlated broadly cyclic depositional cycles of the Galena Group along an approximately 100 km northwest–southeast transect in northeastern Iowa in a sequence stratigraphic framework. Our correlations were aided by the chemical fi ngerprinting of single primary apatite phenocrysts separated from the prominent and widespread Calmar K-bentonite bed. The Dunleith, Wise Lake, and Dubuque Formations of the Galena Group form one composite sequence that contains fi ve high frequency sequences, which are composed of subordinate cycles. Vertical and lateral facies variations are a result of changes in eustatic sea level and terrigenous sediment supply. We interpreted the Galena Group to have been deposited on a subtidal, low energy, storm-dominated epeiric ramp.

INTRODUCTION In the Upper Mississippi Valley (UMV), USA, an 1B). In the northeastern Iowa outcrop belt, the Gale- epeiric sea fl ooded Laurentia during a period of high na Group is 75 to 85 m thick and consists of shaly to global sea level in the Middle to Late Ordovician. This shale-free, bioturbated, fossiliferous limestone. Galena sea was bounded by the active Taconic Island Arc to Group strata in northeastern Iowa are well preserved: the south and east (modern azimuth directions), Lau- they escaped the pervasive dolomitization that masks rentian continental highlands to the north, and the fauna and depositional fabrics in most of the lower Pa- Trans-Continental Arch to the west (fi g. 1A) (Witzke, leozoic carbonates of the UMV. The Galena Group 1980). The Hollandale Embayment, an early Paleozoic contains thin, laterally extensive, altered volcanic ash tectonic depression, was an active depocenter within beds termed K-bentonite beds. Each K-bentonite bed the UMV epeiric sea (fi g. 1). represents a geologically instantaneous ashfall event Our study focused on the predominantly carbon- and has value as a timeline for stratigraphic correla- ate, Middle to Late Ordovician (Mohawkian–Cincin- tion. Stratigraphic correlation is especially desirable in natian Series) Galena Group (Templeton and Will- the northeastern Iowa Galena Group belt, where shale man, 1963) that is widely distributed in the UMV (fi g. and shaly limestone facies to the northwest and rela-

1 Department of Geology and Geophysics, University of Wisconsin–Madison, 1215 West Dayton Street, Madison, Wisconsin 53706 2 Now at Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6 Canada 3 Now at Exxon Mobil, URC GW3 980B, P.O. Box 2189, Houston, Texas 77252-2189 Wisconsin Geological and Natural History Survey GEOSCIENCE WISCONSIN • Volume 19, Part 2 • published online 2008 A equator 10°S

h A r c a l t n t i n e n t R i n - C o f t e i d i n M n t C o s - 20°S a n T r Wisconsin Arch Hollandale Embayment

Cincinnati Arch

? S.E. Nebraska carbonate Arch sediments Taco nic Orogenic clastic Ozark Uplift Belt sediments

exposed land

B C WA M ississip

SERIES i SYSTEM River WISCONSIN

TIAN Maquoketa

STUDIED

CINCINNA

N Galena INTERVAL MN

Platteville HE Decorah

O I C I A V

Volney St. Peter BS-4 core O R D 50

MOHAWKIAN

Prairie Guttenberg du Chien 0 IOWA m

trough cross-strata hardgrounds N large-scale cross-bedded 0 100 km IL limestone

Ordovician outcrop area, shale HE Hollandale Embayment including Galena Group dolostone WA Wisconsin Arch

Figure 1. A. Paleogeographic reconstruction of the north-central United States during the Middle to Late Ordovician. The Galena Group, the focus of this study, was deposited in the Hollandale Embayment, an early Paleozoic depocenter. The Hollandale Embayment was part of a larger epeiric sea that covered much of the present-day eastern United States. Modifi ed from Witzke (1980). B. Idealized stratigraphic column for Ordovician sedimentary rocks of the UMV. Modifi ed from Byers and Dott (1995). C. Location of study area. Stratigraphic sections were measured at Decorah, Volney, and Guttenberg, Iowa. One section was measured using core BS-4. Ordovician outcrop area includes the Galena Group. Trends of two structural features, the Wisconsin Arch and the Hollandale Embayment, are shown. Modifi ed from Kolata and others (1996).

24 • GEOSCIENCE WISCONSIN tively shale-free limestone and dolomite facies to the a fossil-cataloging project and were subsequently southeast are transitional and complicate stratigraphic awarded the Strimple Medal for amateur paleontology relationships. by the Paleontological Society in 1987. Their works Our research built upon the studies of Emer- are a voluminous paleontological, stratigraphic, and son (2002) and Simo and others (2003), which fo- sedimentological resource for those studying the Gale- cused on the basal Decorah Formation of the Galena na Group in the UMV. Group, and complements previous studies of the Ga- Witzke and Bunker (1996) proposed a sequence lena Group in the Wisconsin outcrop belt (Choi, 1995, stratigraphic interpretation for lower Paleozoic stra- 1998; Choi and others, 1999). Our research contrib- ta in Iowa, and identifi ed two higher-ordered and fi ve utes higher stratigraphic resolution on a local scale to subordinate lower-ordered transgressive-regressive cy- previous regional-scale stratigraphic interpretations of cles for the Galena Group in Iowa. Their cycles were the Galena Group (Witzke and Ludvigson, 2005; Wit- represented by shallowing-upward patterns bounded zke and Bunker, 1996; Witzke and Kolata, 1988) and by hardground surfaces and/or phosphatic lags. provides a more comprehensive geologic interpreta- Emerson (2002) and Simo and others (2003) pro- tion to previous lithostratigraphic studies of the Ga- posed a K-bentonite-aided sequence stratigraphic cor- lena Group in northeastern Iowa (Levorson and Gerk, relation of the basal Decorah Subgroup of the Galena 1972a, 1972b, 1975, 1983; unpublished data, 1972). Group. Some important results of this work are that We had three objectives for this paper: 1) docu- 1) geochemical “fi ngerprinting” of K-bentonite beds ment the lithostratigraphy and lithofacies of the Ga- via single apatite phenocryst analysis is successful and lena Group in the northeast Iowa outcrop belt, where valuable for intraregional K-bentonite correlation, and comparatively fewer studies have been performed; 2) sediment supply and oceanic mixing were the pre- 2) identify a prominent K-bentonite bed, present at dominant controls on sedimentation. three localities across the study area, by primary apatite phenocryst chemistry; 3) elucidate sedimentary METHODS processes in epeiric seas. In our study we interpreted We measured detailed stratigraphic sections at 14 lo- observed facies stacking patterns and lateral facies re- cations near Decorah, Volney, and Guttenberg in Win- lationships in sequence stratigraphic terms, based on neshiek, Allamakee, and Clayton Counties, respective- K-bentonite correlation of Galena Group strata within ly, northeastern Iowa (fi g. 1C; see appendix: Locality a complex facies transition. We have placed the facies register). Exposures included bluff faces, quarries, and of the northeastern Iowa Galena Group outcrop belt in roadcuts; we studied one previously drilled core. Six- the context of a regional-scale depositional model. For ty-six rock samples were cut into standard 30 µm thin our proposed model, we drew upon research in more sections and/or hand-polished slabs and studied using geologically recent epeiric sea settings to help inter- standard petrographic methods. A composite strati- pret facies distribution in Ordovician UMV epeiric graphic column of the Galena Group near Decorah is seas. shown in fi gure 2. Additional stratigraphic control was established Previous work using gamma ray spectrometry. Measured sections Hall (1851) defi ned the Galena lithostratigraphic unit, were analyzed with an Exploranium GR-320 portable and Calvin (1906), Ulrich (1911), and Kay (1935) spectrometer. Measurements were obtained every 0.5 provided additional lithostratigraphic framework. Fur- m vertically, using 100-second count times. Variations ther lithostratigraphic splitting and current nomencla- in gamma response versus stratigraphic thickness were ture for the Galena Group was provided by Templeton found to be consistent among sections and provided a and Willman (1963) and Willman and Kolata (1978). reliable correlation tool. The potassium and to a less- Levorson and Gerk (1972a, 1972b, 1975, 1983; er extent thorium spectrums displayed the most clear unpublished data, 1972) synthesized the results of signal and were used instead of the uranium spectrum. an exhaustive lithostratigraphic study of the Gale- The uranium spectrum was highly variable and pro- na Group in northeastern Iowa and adjacent parts of duced a noisy signal for unknown reasons. A compos- Wisconsin and Minnesota. They measured hundreds ite potassium gamma curve for the Galena Group near of stratigraphic sections to provide a framework for Decorah is shown in fi gure 2.

VOLUME 19 2008 • 25 L I T H O F A C I E S ISC MCC-H BC Dubuque Fmn.

R unnamed R * R R Wise Lake Formation

* Haldane (?) G a l e n r o u p

I I I

R limestone Calmar R * shaly limestone R dolostone Dunleith Formation

shale R 10 major bedding plane R R K-bentonite bed R * hardgrounds 5 chert calcite or pyrite vugs R Receptaculites oweni I Ischadites iowensis

Beecher Point Eagle Fairplay Mortimer Sherwood / Rivoli Wyota / Wall Sinsinawa Stewartville 0 (m) Thalassinoides burrows

0 1.0 2.0 3.0 potassium (%)

Figure 2. Composite stratigraphic column for the Galena Group studied near Decorah, Iowa, with potassium gamma ray curve and distribution of facies groups within the Galena Group. Potassium gamma ray curve is a proxy for shale content. ISC = interbedded carbonate-shale; MC = massive carbonate; C-H = Chondrites-hardground carbonate; BC = bedded carbonate.

26 • GEOSCIENCE WISCONSIN

Maclurites above Calmar Calmar above Paleosynapta ﬂ accida, ﬂ Paleosynapta in cherty unit half lower upper 2 m basal 4 m; basal 4 m; upper 1 m sp. at base at sp. sp. at top sp. Prasopora FAUNA Ischadites iowensis Ischadites K-bentonite Receptaculites, Receptaculites, Receptaculites, (approximately) Isotelus Receptaculites, middle 1/3; sp., upper 2 m Receptaculites, (approximately) none epibole Prasopora Top: Widespread shaly bed plane Widespread Top: of top Base: Top: prominent hardground hardground prominent Top: surface prominent and widespread Top: shaly bed plane widespread and prominent Top: surfacehardground shaly bed plane Widespread Top: thicker SE to becomes that Top: Depauperate zone Depauperate Top: prominent, thick, cm 6 Top: bed and recessive widespread, of Levorson bed” “marker plane; and Gerk (1975) prominent and widespread Top: shaly bed plane hardground, prominent Top: prominent a by replaced locally bedding plane

Thalassinoides burrows, and burrows, Chondrites Upper: decimeter-scale bedded, grain-rich wackestones. grain-rich bedded, decimeter-scale Upper: grainstones wackestones to thin-bedded Middle: with shale wackestone interbedded grain-rich mud-rich to Base: and packstones and shaly nodular grainstones common nodular chert bands in upper half; Calmar K-bentonite K-bentonite Calmar nodular chertcommon bands in upper half; (NW) m 2 approximately present base the above (SE) m 4 to wacke- shaly bedded thinly and shale to interbedded Basal to upper mudstone wackestone; and shaly grain-rich packstone chert conspicuous bands several wackestone with cases in many bedded, meter-scale Decimeter- to several punctuated by packstones wackestones and burrowed nodular hardgrounds; with common thin (3–7 cm) grainstones chert near top common mud-rich to nearly massive-bedded, bioturbated, Pervasively wackestones with nodular chertgrain-rich to bands increasing bedded near base and top conspicuously more SE; wackestone to packstone, regularly interbedded with 1 to 5 cm cm 5 to 1 with interbedded regularly packstone, wackestone to shale calcareous shale and thin-bedded dark brown thick gray top the toward nodular hardgrounds, rich wackestones and minor packstones; at contains half upper chert studied; exposures in absent skeletal cm) (5–15 thick relatively and prominent three least decimeter-scale to bedded at base, meter-scale beds; grainstone top at bedded discontinuous wackestone punctuated by grain-rich to mud-rich chert scattered base at hardgrounds; common beds; grainstone abundant to rare grainstones; and packstones, wackestones, above m 0.5–1.0 present K-bentonite (?) Haldane chert; nodular the base grading base, at wackestones and packstones shaly Interbedded top; at packstones and wackestones shale-free relatively to common hardgrounds, prominent m (10.1 ft) SE THICKNESS LITHOLOGY CONTACTS KEY m (13.5 ft) SE m (25.9 ft) SE m (16.9 ft) SE 6.3 m (20.7 ft) NW crinoidal bioturbated mud-rich, gray, Decimeter-scale bedded, ft) 40 NW (approximately Summary of the lithostratigraphic characteristics of the Galena Group (Dunleith, Wise Lake, and Dubuque Formations). and Dubuque Lake, Wise (Dunleith, of the Galena Group characteristics Summary of the lithostratigraphic Beecher Member 4.4 m (14.4 ft) NW – 3.1 LITHOSTRATIGRAPHIC LITHOSTRATIGRAPHIC UNIT Mortimer Member Member Fairplay 3.3 m (10.8 ft) NW – 4.1 Member Point Eagle – 7.9 4.4 m (14.4 ft) NW ft) (18.7 m 5.7 5.2 – NW Dubuque Formation Formation Lake Wise Stewartville Member ft) (36 m 11.0 NW Member Sinsinawa grain- to mud-rich dolomite-mottled and shale-free Relatively Dunleith Formation ft) (23 m 7.0 NW MembersWall–Wyota 7.5 m (24.6 ft) NW shale-free to base) (at shaly bedded, meter-scale to Decimeter- Rivoli–Sherwood Members m 12 approximately dolomitic grain-rich bedded, meter-scale Decimeter- to Table 1. Table

VOLUME 19 2008 • 27 Decorah, IA

limestone

shaly limestone

dolostone

shale R R unnamed K-bentonite major bedding plane R R K-bentonite bed hardgrounds chert calcite or pyrite vugs R Receptaculites oweni I Ischadites iowensis Thalassinoides burrows

Haldane (?) K-bentonite Guttenberg, IA Wall

Volney, IA I Calmar I I I I K-bentonite I Calmar Sherwood R I Calmar K-bentonite R R R K-bentonite R R Rivoli R

R R Mortimer R R R R R R R R R R R R R R R 5 R R R R Eagle PointEagle Fairplay Mortimer Sherwood / Rivoli Eagle PointEagle Fairplay R

0 A = Delgado, 1983 Beecher Point Eagle Fairplay Mortimer Sherwood / Rivoli Wyota / Wall Sinsinawa Stewartville Dubuque Beecher Point Eagle Fairplay Mortimer Rivoli Beecher (m) Beecher B = this study A B Figure 3. Stratigraphic columns of sampled beds at Decorah, Volney, and Guttenberg, Iowa (indicated by arrows). Arrows point to K-bentonite beds (Calmar, Haldane (?), and unnamed); samples yielded abundant euhedral apatite and zircon phenocrysts. Unlabeled arrows indicate a detrital mineral assemblage and are not considered to be K-bentonite beds.

Seventeen clay-rich and recessive beds, indicative abundant in euhedral apatite and zircon phenocrysts of K-bentonite, were sampled at seven localities (see were considered K-bentonite beds (ﬁ g. 3). Sampled locality register) and processed to separate primary beds not considered to be K-bentonites yielded organ- apatite phenocrysts from the bulk sample using proce- ic apatite (conodonts), quartz, calcite, dolomite, abrad- dures detailed in Beyer (2003), Emerson (2002), and ed zircons, and rare potassium feldspar. Potassium Mange and Maurer (1992). Beds that yielded samples feldspar in these beds yield Proterozoic 40Ar/39Ar ages

28 • GEOSCIENCE WISCONSIN (Chetel, 2004; Chetel and others, 2005) and suggested Interbedded shale-carbonate facies group a detrital, nonvolcanic origin. From each of the K-ben- Description tonite samples, 15 to 20 grains were analyzed by elec- The interbedded shale-carbonate facies group includes tron microprobe using fi ve points per grain. Elements all the interbedded and shaly carbonate lithologies of analyzed were P, Ca, F, Cl, Y, La, Ce, Fe, Mg, and Mn. the Galena Group that we studied. This group can be Electron microprobe analysis was performed with a recognized in outcrop by its resistant shaly carbonate Cameca SX51 microprobe in the Eugene Cameron beds intercalated with recessive shale-rich beds. Electron Microprobe Lab, Department of Geology and The recessive shale-rich beds are 1 to 20 cm thick Geophysics, University of Wisconsin–Madison. and generally consist of greenish-gray, poorly indu- LITHOSTRATIGRAPHY rated, non-fi ssile calcareous shales intercalated with grain-rich carbonate. One to three centimeter thick Table 1 provides a summary of the lithostratigraph- beds of carbonate-free greenish-gray or brown shales ic characteristics of the Dunleith, Wise Lake, and are rare. Gray-weathered, Chondrites-burrowed, dark Dubuque Formations of the Galena Group in north- brown fi ssile calcareous shale is a unique lithology eastern Iowa. We adopted the lithostratigraphic clas- and is restricted to the upper Dubuque Formation. In sifi cation of Levorson and Gerk (1972a, 1972b, 1975, general, recessive shale-rich strata of the interbedded 1983; unpublished data, 1972), which directly applied shale-carbonate facies group decrease in abundance to the classifi cation of Templeton and Willman (1963) the southeast. and Willman and Kolata (1978) to Galena Group stra- Carbonate lithologies belonging to the interbed- ta in northeastern Iowa. However, differentiation be- ded shale-carbonate facies group are predominantly tween the Rivoli and Sherwood Members and be- tan to light gray, centimeter- to decimeter-scale bed- tween the Wall and Wyota Members was not possible ded, grain-rich, shaly wackestones and packstones. at the measured sections due to similarities in lithol- Shale is present as 0.5 to 3.0 cm thick zones of anas- ogy and bedding style and lack of clear contact crite- tomosing hairline shale drapes. Beds display perva- ria. Therefore, the Rivoli and Sherwood Members and sive Planolites burrowing to the extent that all primary the Wall and Wyota Members are herein referred to depositional features have been destroyed. Two less as the Rivoli–Sherwood Members and the Wall–Wyo- predominant carbonate lithologies are 1) nodular car- ta Members, respectively. In addition, the Frankville, bonate consisting of medium to light gray (fresh and Luana, and Littleport Beds of the Dubuque Formation weathered) mudstone to grain-rich wackestone nod- (Levorson and Gerk, 1983) were not differentiated for ules separated by 1 to 30 mm thick undulatory to ir- this study. regular gray shale beds, and 2) centimeter-scale bedded bioturbated grainstones that may be graded and FACIES DESCRIPTION separated by shale drapes. AND INTERPRETATION Chert nodules are absent and hardground surfac- Most of the Galena Group studied in northeastern es are rare but prominent where observed. This group Iowa is non-dolomitized and very well preserved. displays the most diverse fauna of the four Galena Strata preserve fauna, ichnofauna, and primary depo- Group facies groups and includes brachiopods, gastro- sitional fabrics that suggest sedimentation took place pods, bryozoans (including Prasopora sp.), crinoids, during periods of cyclic sea-level change in storm- and solitary corals. dominated epeiric seas. The group composes the entire Beecher Member, The Galena Group comprises four lithofacies the lower half of the Mortimer Member, two parts of groups: 1) interbedded shale-carbonate, 2) mas- the lower Rivoli–Sherwood Members, the middle part sive carbonate, 3) bedded carbonate, and 4) Chon- of the Wall–Wyota Members, and the entire Dubuque drites-hardground carbonate. The distribution of these Formation in the northwest study area. Shale content groups within the Galena Group is shown in fi gure 2. decreases to the southeast, and this facies group is restricted to the basal Mortimer Member at Guttenberg, Iowa.

VOLUME 19 2008 • 29 Interpretation The massive carbonate facies group compris- Sediment of the interbedded shale-carbonate facies es the entire Eagle Point Member, the upper half of group does not display intraclasts, wave or current the Mortimer Member, and the upper 2 to 3 m of the cross stratifi cation, or any other fair-weather wave in- Wall–Wyota Members. dicators. Strata display variable degrees of storm-wave reworking, and we interpreted them to have been de- Interpretation posited above the storm-wave base (SWB). Shale-rich Sediments of the massive carbonate facies group dis- strata are intimately associated with packstones and play bioturbated mud-rich to grain-rich wackestone grainstones, suggesting deposition in a more land- textures and lack wave- or current-generated sedi- ward position closer to a siliciclastic source and in a mentary structures. We interpreted laminated very more frequently wave-reworked environment. Car- fi ne-grained grainstones to be the distal parts of tem- bonate-rich strata have decreased siliciclastic content pestites that may have been deposited at or just be- and wackestone to packstone textures, which suggests low the SWB. The massive carbonate facies group deposition farther offshore in a less frequently wave- and Chondrites-hardground carbonate facies group reworked environment. The diverse faunal assem- display the lowest siliciclastic content of the Galena blage and pervasive bioturbation indicate deposition Group that we studied; they represent deposition far- in normal marine conditions. However, Chondrites- thest from a terrigenous sediment source and prob- burrowed dark brown fi ssile calcareous shale beds of ably at the greatest relative depth. The origin of the the Dubuque Formation may indicate deposition in an chert within this facies group is unclear. Previous oxygen-defi cient environment. studies have invoked siliceous sponge spicules (Kor- pel, 1983; Delgado, 1983) and K-bentonites (Mossler Massive carbonate facies group and Hayes, 1966; Levorson and Gerk, 1972a) as the Description source of silica for nodular chert. Samples and exposures studied did not corroborate these explanations. The massive carbonate facies group includes all rela- The abundance of skeletal grains and pervasive tively shale-free, nearly massive-bedded carbonates bioturbation suggest deposition under normal marine with rare to absent hardground surfaces. This facies conditions. group is recognized in outcrop by its lack of prominent shale beds, homogeneous texture, and common Bedded carbonate facies group nodular chert bands. The most predominant lithology is light tan to Description light gray, decimeter- to meter-scale bedded, perva- The bedded carbonate facies group includes all con- sively bioturbated, relatively shale-free, mud-rich to spicuously bedded, commonly dolomitized carbonate grain-rich wackestones. The predominant ichnogenus lithologies containing common hardground surfaces is Planolites with subordinate Thalassinoides. Intense and Thalassinoides burrowing. It is recognized in out- Planolites burrowing homogenized the sediments to crop by decimeter- to meter-scale, hardground-bear- the extent that no primary depositional features are ing carbonate beds separated by prominent and lat- present. A less common but signifi cant lithology is erally continuous shaly bed planes. The presence of tan or gray, weakly to nonbioturbated, laminated, very abundant hardgrounds, Thalassinoides burrows, and fi ne-grained skeletal grainstone. common dolostone distinguishes this facies group Nodular chert bands are common within the mas- from the interbedded shale-carbonate facies group, sive carbonate facies group, and they may be associ- which also appears well bedded in outcrop. ated with laminated, very fi ne-grained grainstones. The bedded carbonate facies group consists of Chert content increases signifi cantly to the southeast. gray to tan, decimeter- to meter-scale bedded, perva- Hardgrounds are rare in the northwestern study area sively bioturbated mud-rich to grain-rich wackestones and increase in abundance to the southeast. Predomi- and packstones. One to three centimeter thick, later- nant fauna in the massive carbonate facies group are ally discontinuous grainstone beds are rare to com- brachiopods, gastropods, Receptaculites, and solitary mon. Several medium to coarse-grained, prominent, corals. and laterally continuous, 8 to 15 cm thick grainstone

30 • GEOSCIENCE WISCONSIN beds that display imbrication of clasts and burrowed conspicuous shaly bed planes, represent deposition in tops are present in the upper half of the Stewartville a more landward position when compared to the mas- Member. sive carbonate facies group. We interpreted Thalassi- Strata display weak to moderate dolomitization, noides-related cycles to represent an upward decrease with dolomite content increasing to the southeast. Do- in net sediment accumulation. Shaly cycle bases rep- lostone is unique to this facies group. resent high siliciclastic input and relatively high net Carbonate beds are bounded by shaly bed planes, sediment accumulation. Siliciclastic input decreases rare shale beds, and/or prominent hardground sur- into the middle part of the cycles as indicated by rel- faces. Shale is also present within the beds as rare to atively shale-free and Thalassinoides-burrowed car- abundant 2 to 20 mm thick zones of anastomosing bonates: Burrowing organisms were probably able to hairline drapes. colonize seafl oor sediments during periods of low net Thalassinoides is the predominant ichnogenus sediment accumulation. Cycle-terminating hardground of this facies group and imparts a distinct appearance surfaces represent a halt in sedimentation and seafl oor to exposures: Fresh exposures are mottled and dis- cementation. play dark-colored burrows that contrast with light-col- The presence of diverse and plentiful fauna and ored host sediments; weathered exposures display a abundant bioturbation suggest deposition under nor- pock-marked to vuggy appearance due to preferential mal marine conditions. The presence of pyrite-ce- weathering of dolomite burrow fi ll; highly weathered mented hardground surfaces may indicate periods of exposures can display complete removal of the dolo- nondeposition under oxygen-poor conditions. mite burrow fi ll, displaying casts of the three-dimen- sional geometry of Thalassinoides burrow networks Chondrites-hardground carbonate and imparting a Swiss cheese appearance to the rock. facies group Thalassinoides burrowing is either pervasive or cyclic. Description Cycles are decimeters to meters thick and consist of a shaly limestone base, overlain by Thalassinoides bur- The Chondrites-hardground carbonate facies group in- rowed wackestones and a hardground surface. cludes all relatively shale-free carbonates that display Hardground surfaces are abundant, weakly to abundant Chondrites burrows and prominent, strongly moderately mineralized, closely spaced (3–30 cm); mineralized hardground surfaces. These features, plus they have been scoured to notably planar surfaces. common nodular chert bands, characterize this facies Nodular chert bands are rare to common. Common group in outcrop. fauna include Receptaculites, gastropods, solitary cor- The most predominant lithology of this facies als, and bryozoans (including Prasopora sp.). group is dark to light gray to tan, centimeter- to deci- The bedded carbonate facies group comprises the meter-scale bedded, thoroughly bioturbated mud-rich entire Fairplay Member, the basal Rivoli–Sherwood to grain-rich wackestones and packstones. Zones of 1 Members, most of the Wall–Wyota Members, the en- to 40 mm thick, anastomosing dark brown, dark gray, tire Sinsinawa Member, and the upper seven-eighths or black very fi ne-grained drapes that contain Chon- of the Stewartville Member. drites burrows are common. The drapes preserve grap- tolites at some locations. Beds are punctuated by sev- Interpretation eral conspicuous, 3 to 7 cm thick laterally continuous grainstone beds. Grainstone beds are very fi ne to me- Sediments of the bedded carbonate facies group do dium-grained and weakly to nonbioturbated. Rarely, not display fair-weather wave sedimentary structures. the grainstone beds display the “sand wave” geome- The presence of discontinuous to continuous centime- try described by Delgado (1983), in which grainstone ter-scale grainstone beds, scoured hardgrounds, and thickness varies due to the presence of an undulatory grain-rich wackestone to packstone textures indicate top. Very fi ne-grained grainstones are in many instanc- reworking and erosion by storm currents above the es laminated. SWB. Scouring of hardgrounds may have been aid- Hardground surfaces are prominent and abundant ed by more frequent impingement of storm-generated in the Chondrites-hardground carbonate facies group. currents and storm wave action on the seafl oor at shal- Pairs or trios of hardgrounds may be vertically even- lower depths. Increased siliciclastic content, including ly spaced at intervals of 0.5 to 1.5 m. Hardgrounds

VOLUME 19 2008 • 31 are strongly cemented by micro- to macro-crystalline eastern North America, including northeastern Iowa pyrite, which imparts a stark black color to the sur- (Sardeson, 1924; Allen, 1929; Allen, 1932; Templeton face at relatively fresh exposures and a rusty brown and Willman, 1963; Mossler and Hayes, 1966; Will- or red-brown color at weathered exposures. Relief of man and Kolata, 1978; Kolata and others, 1996). The hardground surfaces ranges from 0.0 to 5.0 cm. Those K-bentonite beds serve as marker horizons and pro- hardgrounds that have no relief are exceptionally pla- vide correlation among sections in the facies transi- nar surfaces that may display Trypanites borings, trun- tion zone. The beds have been positively identifi ed via cation of burrows, and micro- and macro-scale trun- chemical fi ngerprinting. The fi ngerprint was provid- cation of fossils. Hardground “clasts,” or centimeter- ed by electron microprobe analysis (EMPA) of single scale, Trypanites-bored hardground rip-ups are present primary apatite phenocrysts, which quantifi es trace within 1 dm above or below prominent hardgrounds. elemental abundances of the phenocryst population. Blackened, sulfi de-mineralized skeletal grains are also Apatite phenocrysts are abundant and are most like- abundant above and below hardgrounds. ly unaffected by alteration of the parent ash, and thus Nodular chert bands are common within this fa- preserve primary magmatic chemistry (Samson and cies group, but are relatively less abundant than in others, 1988). Bi-elemental scatter plots of relatively parts of the massive carbonate facies group. Common abundant trace elements such as Mg and Mn provide fauna include brachiopods, crinoids, and Ischadites io- separate “clusters” for individual K-bentonites (Emer- wensis. son, 2002; Simo and others, 2003). Clusters show lit- The Chondrites-hardground carbonate facies tle to no variability among localities, allowing individ- group is restricted to the upper Rivoli–Sherwood ual K-bentonites to be identifi ed and used for regional Members and approximately the basal meter of the stratigraphic correlation (Samson, 1986; Samson and Wall–Wyota Members in the northwest study area. others, 1988; Emerson, 2002; Simo and others, 2003; This facies group is not present at measured sections Shaw, 2003). to the southeast. Seventeen clay-rich and recessive beds were sampled within the Dunleith and Wise Lake Formations Interpretation in northeastern Iowa (see appendix) and processed to Sediments of the Chondrites-hardground carbonate separate primary phenocrysts (see Methods section). facies group do not display fair-weather wave sedi- On the basis of phenocryst mineralogy, we recognize mentary structures. The presence of centimeter-scale the Calmar and Haldane K-bentonites of Willman and grainstone beds, scoured hardgrounds, and hard- Kolata (1978) and an unnamed K-bentonite within the ground “clasts” indicate reworking by storm currents Stewartville Member (fi g. 3). above the SWB. However, the presence of nonbioturbated, laminated, very fi ne-grained grainstones and Calmar K-bentonite bed non-scoured hardgrounds may suggest deposition just (Willman and Kolata, 1978) below the SWB. Relatively shale-free, predominantly The Calmar K-bentonite bed is widespread in north- wackestone textures indicate deposition far removed eastern Iowa and is stratigraphically located 2.0 to 3.5 from a siliciclastic sediment source. m above the base of the Rivoli–Sherwood Members The bioclastic-rich and bioturbated wackestones of the Dunleith Formation. The bed is 2 to 6 cm thick and packstones that compose much of this facies and consists of orange-gray to olive green plastic clay. group indicate deposition under normal marine condi- Prominent K-bentonite beds at Volney, Iowa (lo- tions. However, the presence of strongly mineralized cality 12), and Guttenberg, Iowa (locality 13), have hardground surfaces and Chondrites-burrowed dark- been identifi ed as the Conover K-bentonite (Levor- colored shale drapes throughout the lithofacies sug- son and Gerk, unpublished data, 1972) and the Nas- gests periods of deposition under oxygen-poor condi- set K-bentonite (Delgado, 1983; Samson, 1986; Sloan tions. and others, 1987; Kolata and others, 1996), respectively. Our EMPA of apatite phenocrysts from these K-BENTONITE GEOCHEMISTRY beds plus the Calmar K-bentonite (Levorson and Potassic-altered volcanic ash beds—K-bentonite Gerk, 1983) at Decorah, Iowa (locality 5), yielded a beds—are present in Ordovician strata throughout single cluster on a Mg vs. Mn bi-elemental plot (fi g.

32 • GEOSCIENCE WISCONSIN A 0.09

0.08 M.D.L. = 0.01 wt. % = 0.01 wt. M.D.L.

0.07

Typical 1σ precision 0.06

0.05 Mn (wt. %) Mn (wt.

0.04

0.03 "Nasset" at Guttenberg, IA (n=17) "Conover" at Volney, IA (n=9)

0.02 Calmar at Decorah, IA (n=15)

M.D.L. = 0.01 wt. % 0.01

0 00.005 0.010.015 0.020.025 0.030.035 0.04 Mg (wt. %)

B 0.16

0.14 M.D.L. = 0.01 wt. % = 0.01 wt. M.D.L. 0.12

Typical 1σ precision 0.1

0.08 Mn (wt. %) Mn (wt.

0.06

unnamed K-bentonite at Decorah, IA (n=30) Dygerts K-bentonite at Galena, Il (n=20) 0.04 Haldane K-bentonite at Decorah, IA (n=17) Calmar K-bentonite - NE Iowa (n=39)

0.02 M.D.L. = 0.01 wt. %

0 0 0.02 0.04 0.06 0.08 0.1 0.12 Mg (wt. %)

Figure 4. A. Mg vs. Mn plot for apatite phenocrysts from the Calmar K-bentonite (Levorson and Gerk, 1983) at Decorah, Iowa (locality 5), the “Conover” K-bentonite (Levorson and Gerk, unpublished data, 1972) at Volney, Iowa (locality 12), and the “Nasset” K-bentonite (Delgado, 1983; Samson, 1986; Sloan and others 1987; Kolata and others 1996) at Guttenberg, Iowa (locality 13). Apatites from all three beds yielded a single cluster and are therefore a single K-bentonite rather than three separate K-bentonites. B. Mg vs. Mn plots for all K-bentonites studied. The Dygerts K-bentonite was collected at the type section of Willman and Kolata (1978) and was not present at studied exposures in northeast Iowa. Data for the Haldane K-bentonite are from Leslie (2002).

4A). These data suggested that the sampled beds are strained by an overlying and unique zone of Ischadites the same K-bentonite, not three separate K-bentonites. iowensis, we determined that the three sampled beds On the basis of stratigraphic position, speciﬁ cally con- are the Calmar K-bentonite. The Conover and Nas-

VOLUME 19 2008 • 33 set K-bentonite beds were not present at sections mea- minor euhedral zircon. The phenocryst mineralogy sured during this study. and plastic nature of the clay suggest that the bed is a The presence of the Calmar K-bentonite at three K-bentonite. locations across the study area makes the bed a valu- The EMPA of apatite for this bed yielded Mg vs. able stratigraphic marker and a candidate for further Mn ratios distinct from those of Calmar and Haldane K-bentonite correlation study. (?) K-bentonite in the study area (fi g. 4B). Ratios were also distinct from Dygerts K-bentonite (Willman and Haldane (?) K-bentonite bed Kolata, 1978) Mg vs. Mn ratios from samples at the (Willman and Kolata, 1978) type section in northwest Illinois (fi g. 4B). The Dy- The Haldane K-bentonite bed is present at two loca- gerts K-bentonite is not present at studied exposures tions in Decorah, Iowa, and is not widespread in the in northeastern Iowa. study area. Where observed, the bed is stratigraphically located 0.6 to 1.0 m above the base of the Wall– SEQUENCE STRATIGRAPHIC Wyota Members and consists of orange-gray to olive ARCHITECTURE green plastic clay. The bed is in stratigraphic proxim- Figure 5 displays a correlation of Galena Group stra- ity to the Haldane K-bentonite of Willman and Kola- ta across the study area in a sequence stratigraphic ta (1978). However, the Haldane K-bentonite was not framework. The Calmar K-bentonite was correlated sampled at the type section for comparison. Due to along the cross section via apatite fi ngerprinting and the uncertainty in correlation, the Haldane K-benton- serves as the datum. Additional correlation was pro- ite studied in northeastern Iowa is herein referred to as vided by potassium gamma ray curves and recognition the Haldane (?) K-bentonite. of key faunal zones, including the Prasopora epibole Apatite phenocrysts separated from the Haldane and Receptaculites–Ischadites zones. Potassium gam- (?) K-bentonite were analyzed by Leslie (2002) at ma ray values were assumed to be a proxy for clay locality 2, Decorah, Iowa. A bed at the same strati- content; highest potassium gamma ray values repre- graphic position was sampled at locality 1, Decorah, sent greatest infl ux of siliciclastic sediment and mark Iowa, during this study, but yielded an authigenic as- sequence, high frequency sequence, and cycle bound- semblage of minerals, including abundant barite and aries. ubiquitous, unweathered euhedral pyrite. The presence On the basis of interpretation and correlation of apatite phenocrysts at only one location precluded of measured sections and described samples, we in- use of this horizon as a stratigraphic marker. terpreted the Galena Group strata that we studied to The Mg vs. Mn ratios for Haldane (?) K-bentonite form one composite, “third order” (Kerans and Tinker, apatite phenocrysts by Leslie (2002) vary only slight- 1997) sequence (fi g. 6). This composite sequence is ly from Mg vs. Mn ratios for Calmar K-bentonite apa- named G2 and is consecutive with the Galena Group tites analyzed during this study (fi g. 4B). Clear strati- composite sequence G1 (defi ned by Emerson, 2002) graphic relationships allow differentiation of the Hal- that represents the underlying Decorah Formation. dane (?) and Calmar K-bentonites at Decorah, but it is Composite sequence G2 contains fi ve nested high-fre- unclear whether apatite analysis could distinguish the quency sequences and numerous nested complete and two beds if stratigraphic relations were unknown. partial cycles.

Unnamed K-bentonite bed Sequence boundaries (new) Composite sequence G2 is bounded by a lower se- A 1 cm thick bed plane consisting of dense, dark red- quence boundary (SB 2) and an upper sequence dish-brown plastic clay is present throughout the Dec- boundary (SB 3). SB 2 (deﬁ ned by Emerson, 2002) is orah, Iowa, area. It is stratigraphically located 2.0 m marked by the widely recognized Prasopora zonule. above the base of the Stewartville Member and ap- This key bed has been previously interpreted as a se- proximately marks the top of the upper Receptaculites quence boundary; it is laterally continuous, marks a zone. The bed was sampled during this study at locali- sharp lithologic boundary, and separates two distinct ties 7 and 8 and yielded abundant euhedral apatite and brachiopod communities (Emerson, 2002; Simo and

34 • GEOSCIENCE WISCONSIN

3 2 HFS G2-3 HFS G2-2 HFS G2-1 HFS

HFS G2-4

1 % potassium % I I I I

R R R R R R 0 C C C C C C C NW C Decorah, IA C C C C C C C C C C C C C C C % potassium 0123 I R R R R R R R R R burrows Haldane (?) K-bentonite Volney, IA Volney,

high frequencyhigh boundarysequence hardgrounds chertnodular Thalassinoides zone zone shale

major bedding plane Prasopora Prasopora K-bentonite bed K-bentonite Prasopora Receptaculites Ischadites I R sequence boundary2 limestone shaly limestone Calmar K-bentonite Cross section for the Galena Group studied. (See ﬁ g. 1 for study locations.) Datum is the Calmar K-bentonite. 1 for study locations.) Datum is the Calmar K-bentonite. g. studied. (See ﬁ section for the Galena Group Cross hardground carbonate facies % potassium I 012 R I R R R R R R R R R R R R R R Chondrites- bedded carbonate facies massive carbonate facies interbedded carbonate/shale facies Figure 5. Figure hemicycles. Shaded triangles = regressive SE C C

cycles

Guttenberg, IA Guttenberg, km

Mortime ood Fairplay Eagle Point Eagle Rivoli / Sherw / Rivoli Beecher 010 r 5 meters 0 high frequency sequences

VOLUME 19 2008 • 35 high cycles frequency sequences sequence boundary 3 Dubuque Fmn.

HFS G2-5

R R R R Prasopora zone Wise Lake Formation R Receptaculites I Ischadites hardgrounds HFS G2-4 nodular chert Thalassinoides burrows

calcite or pyrite vugs

limestone

G a l e n r o u p shaly limestone HFS G2-3 dolostone I I I shale major bedding plane R R K-bentonite bed HFS G2-2 R Dunleith Formation

R R R R HFS G2-1 5

0 Beecher Point Eagle Fairplay Mortimer Sherwood / Rivoli Wyota / Wall Sinsinawa Stewartville sequence (m) 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 boundary 2 potassium (%)

Figure 6. Composite stratigraphic column with high frequency sequences. We interpreted the Galena Group to form one composite sequence that is bounded by a lower sequence boundary and an upper sequence boundary. The composite sequence contains a nested hierarchy of higher-ordered high frequency sequences and higher-ordered cycles. HFS = high frequency sequence; shaded triangles = regressive hemicycles. Refer to the text for composite sequence and sequence nomenclature. others, 2003). the Dubuque Formation. This boundary is marked by Sequence boundary 3 is at the base of a locally high gamma ray (all spectra) values and a conspicu- widespread 2 dm thick bed of thin-bedded calcareous lithologic change. Above SB 3, green-gray calcar- ous shale that lies approximately 3 m below the top of eous shales typical of the underlying Galena Group

36 • GEOSCIENCE WISCONSIN are absent and replaced by brown, thin-bedded, cal- sequence G2-3 displays strongly mineralized hard- careous organic-rich shales that preserve common grounds and common Chondrites burrowing at the Chondrites burrows and specimens of the trilobite Iso- turnaround point. The regressive hemicycle of the telus sp. Witzke and Bunker (1996) and Witzke and HFS defi ned is represented by an upward increase in Kolata (1988) recognized the 2 dm thick bed of thin- siliciclastic content and gamma ray values, an upward bedded calcareous shale, but interpreted it as a mark- decrease in mud–grain ratio, numerous weak to prom- er for maximum fl ooding. Sequence boundary 3 may inent hardgrounds, and a Thalassinoides-dominated therefore be interpreted as a Type 3 sequence bound- ichnofossils assemblage. ary, or drowning unconformity, described by Schlager (1999). Cycles High frequency sequences are further divided into High-frequency sequences higher-ordered packages that have been termed cy- Composite sequence G2 is divided into fi ve higher-or- cles (fi g. 6), as defi ned by Kerans and Tinker (1997). dered packages that have been termed high-frequen- High frequency sequences contain 2 to 5 complete cy- cy sequences (HFS) (fi g. 6), as defi ned by Kerans and cles; partial cycles are observed in the transgressive Tinker (1997). The fi ve HFS are named G2-1 to G2-5 hemicycles of HFS G2-1 and G2-2. Cycles are deci- and are consecutive with HFS described by Emerson meters to meters thick and are defi ned by the same fa- (2002) for the underlying Decorah Formation. High- cies stacking patterns as HFS (fi g. 7). Cycle bases are frequency sequences G2-1 to G2-5 are 8 to 15 m thick marked by higher gamma ray values and are usually and are defi ned by vertical repetition of lithofacies shale-rich bed planes. Siliciclastic content decreases groups. upward in the transgressive hemicycle and is refl ected The ideal HFS consists of an interbedded shale- by lower gamma ray values. Cycle turnaround points carbonate base, an overlying massive carbonate mid- are marked by relatively shale-free carbonate litholo- dle, and a bedded carbonate top. This facies stacking gies that commonly display nodular chert and mud- pattern is recognized in HFS G2-1, G2-2, G2-4, and dominated textures. The regressive hemicycles of cy- G2-5. The pattern is modifi ed by the replacement of cles defi ned display an upward increase in siliciclas- the massive carbonate facies group by the Chondrites- tic content and gamma ray values, hardgrounds, and hardground carbonate facies group. This facies rela- Thalassinoides burrows. tionship is recognized in HFS G2-3. The upper 3 m Correlation of individual cycles across the study of the Dubuque Formation above SB 3 belongs to the area is poor to non-correlative, and cycles were gener- Dubuque–Elgin T–R subcycle of Raatz and Ludvigson ally more diffi cult to defi ne in the relatively shale-free (1996). We confi dently correlated HFS G2-1 and G2-2 southeast study area because of thin to absent shaly across the study area (fi g. 5). cycle bases. The decrease in siliciclastic sediment con- Figure 7 displays vertical trends for HFS de- tent to the southeast and the disruption of cycle fea- fi ned in this study. The bases of the sequences are tures by storm processes and bioturbation are probable marked by high gamma ray values and are surfaces causes for poor decimeter- to meter-scale correlation. across which siliciclastic content and distribution of grain-rich lithologies abruptly increase. The transgres- DISCUSSION sive hemicycle of these sequences is represented by Depositional model an upward decrease in siliciclastic content and gam- Distribution of Galena Group strata indicate deposi- ma ray values, an upward increase in mud–grain ra- tion over hundreds of square kilometers of the present- tio, hardgrounds that are rare but prominent where day upper and mid-Mississippi Valley. Strata studied observed, and a Planolites-dominated ichnofossil as- are devoid of bioclastic sand shoals, barriers, reefs, or semblage. The transgressive-regressive turnaround buildups. Sedimentary structures that indicate rework- point is marked by low gamma ray values and silici- ing by fair-weather waves or currents (that is, ripple clastic content and nonbioturbated to weakly Plano- marks, intraclasts, cross-bedding) were not observed, lites- or Chondrites-burrowed sediments. The turn- and no evidence for subaerial exposure was present around is commonly marked also by laminated fi ne- nor has any been reported by previous workers. These grained grainstones and nodular chert. High frequency

VOLUME 19 2008 • 37 high frequency sequence/cycle carbonate K-gamma Ichnogenera Variation 1 texture response distribution m wpg low high

non-bioturbated

Chondrites Thalassinoides Planolites ~1 – 10 m IS-C MC BC

high frequency sequence/cycle carbonate K-gamma Ichnogenera Variation 2 texture response distribution m wpg low high

non-bioturbated

Chondrites Thalassinoides

/HC BC Planolites Ch ~6 m IS-C

limestone laterally discontinuous grainstone chert laterally continuous grainstone Chondrites burrows shale laminated fine-grained grainstone Thalassinoides burrows dolostone BC bedded carbonate MC massive carbonate weak to prominent hardgrounds IS-C interbedded carbonate - shale prominent, strongly mineralized hardgrounds Ch/HC Chondrites hardground carbonate

Figure 7. Vertical trends in lithology, carbonate texture, K-gamma ray response, and ichnogenera distribution for the Galena Group studied; m = mudstone/micrite; w = wackestone; p = packstone; g = grainstone; shaded triangles = regressive hemicycles. observations support the interpretation that the Ga- similar distribution of facies and hydrodynamic pro- lena Group was deposited in a subtidal, low gradient, cesses to the Galena Group studied. storm-dominated epeiric sea. Specifi cally, the “epeiric We interpreted the Galena Group in our study ramp” model named by Wright and Burchette (1998) area to represent deposition in the proximal zone of and defi ned by Lukasik and others (2000) displays the epeiric ramp model (fi g. 8A). Deposition in this

38 • GEOSCIENCE WISCONSIN A A A’

attenuation of fair-weather waves

fair-weather wave base intertidal mudflats? landward transport of carbonate mud by waves ~100 m (erosionally truncated?) proximal distal ramp ramp

low energy, storm-dominated mixed low energy, storm-dominated winnowing of CO 3 mud, terrigenous shale- shale/carbonate zone of carbonate production scouring of hardgrounds dominated deposition deposition and deposition by wave action

STUDY AREA

Figure 8. A. Cross section showing epeiric ramp B depositional model proposed for the Galena Group, A with study area indicated. Modifi ed from Lukasik MINNESOTA and others (2000). B. Distribution of broadly defi ned WISCONSIN facies belts of the Galena Group and equivalent strata ed at in the Upper and Middle Mississippi Valley, with in om d line of cross section A'. Modifi ed from Witzke and shale Ludvigson (2005).

d te a IOWA in m o d ud m e at on zone is characterized by bioturbated, fossiliferous, rb ca ILLINOIS hardground-bearing carbonate of variable texture and lacking grainy shoal or barrier facies. Bathymetry of the proximal zone is above the fair-weather-wave base, but wave energy was minimal, reduced by fric- tion during travel over the broad and uniform expans- es of the ramp. Reworking and seafl oor scouring by A’ / n ed storm-generated waves were instead the dominant hy- ai t MISSOURI gr na al i drodynamic processes. let om ske d d N un The epeiric ramp model predicts a gradual de- gro ard crease in carbonate mud content and an increase in 0 100 mi h hardground frequency in a seaward direction as fair- 0 100 km weather-wave energy increases. This trend is present distribution of Galena Group in the Illinois and Missouri Galena Group-equivalent and equivalent strata outcrop belt, where strata become considerably thin- lithofacies boundary ner, grainier, and hardground-rich to the south (fi g. 8B) (Templeton and Willman, 1963; Witzke and Ko- lata, 1988; Thompson, 1991; Witzke and Ludvigson, terrigenous shale deposition is present in the southern 2005). Carbonate mud winnowed from this “middle Minnesota and northern Iowa Galena Group outcrop shelf” zone of Witzke and Ludvigson (2005) may have belt and is represented by the shale subfacies of the been transported by wave energy landward into the Galena Group studied in northeastern Iowa (fi g. 8A study area, a process similar to the epeiric ramp and and B). This distribution is similar to that of the Mur- epeiric platform (Irwin, 1965) models. ray Basin, south Australia, which is also interpreted A landward zone of low energy, storm-infl uenced to have been deposited on an epeiric ramp (Lukasik

VOLUME 19 2008 • 39 NW ~200 km SE

highsiliciclastic-sediment supply low hardgrounds chert Thalassinoides burrows BC BC BC bedded carbonate MC

MC MC massive carbonate

IS-C IS-C interbedded carbonate-shale IS-C BC BC MC MC HFS/cycle base IS-C younger falling sea level sea level decreasing oxygenation/mixing in epeiric sea that is becoming decoupled from Taconic water mass increasing siliciclastic input sea floor periods of sediment starvation and sea floor colonialization bedded carbonate facies

high sea level sea level

oxygenated, well mixed epeiric sea abundant, diverse benthic fauna that is coupled with Taconic water mass active carbonate deposition siliciclastic source area drowned sea floor chert content increases to southeast massive carbonate facies

rising sea level sea level increasing epeiric sea oxygenation/mixing siliciclastic input periods of sediment starvation sea floor

interbedded shale-carbonate facies massive carbonate facies

STUDY AREA older

Figure 9. Interpretation of sedimentological controls during deposition of high frequency sequences defi ned in this study. Columns represent the effect of siliciclastic sediment supply on Galena Group lithology, facies stacking, and outcrop expression from northwest to southeast. and others, 2000; Lukasik and James, 2003). Mud- We considered siliciclastic sediment supply to be fl at facies of the epeiric ramp model are not present in a signifi cant depositional control because it is intrinsi- the Galena Group that we studied, but may have been cally related to sea level in our depositional model for present in a more landward position to the northwest the Galena Group. The proposed low-gradient subtidal and are erosionally truncated. epeiric ramp is characterized by a lack of signifi cant inherited topography, by a lack of high relief features Controls on epeiric sea sedimentation such as reefs, buildups, or prominent shelf margins, We interpreted the observed vertical and lateral facies and by diminished wave competency in proximal set- variations to be primarily a product of relative sea-lev- tings. In low relief settings, large absolute values of el change. In addition, we interpreted siliciclastic sedi- sea level rise or fall need not be invoked to produce ment supply and regional-scale oceanic circulation to substantial retreats or advances of the shoreline, be related, signifi cant depositional controls. resulting in exposure or drowning of widespread ter-

40 • GEOSCIENCE WISCONSIN rigenous source areas, respectively. Diminished wave ifi cation was not determined. Local inherited topogra- competency in proximal settings may impede the bas- phy or increased epeiric sea stratifi cation may be pos- inward transfer of fi ne terrigenous sediment, resulting sible causes. in association of shaly facies with shallow, normally 3) Upward increases in siliciclastic and skeletal high energy environments. The constraints imposed grain content indicate falling sea level and gradual re- by the proposed depositional model suggest that sili- exposure of the Transcontinental Arch to the north- ciclastic sediment supply, rather than changes in west. Episodic inputs of siliciclastic-rich sediments absolute sea level, is a primary control of facies vari- are represented by conspicuous shaly bed planes in ation. the northwest study area. Common centimeter-scale, We determined that changes in sea level produced laterally discontinuous grainstone beds were depos- the following sedimentologic responses (fi g. 9): ited during storm events and later dissected by en- 1) Increased siliciclastic content and skeletal suing storm currents and/or bioturbation. Pervasive grain-rich facies indicate lower sea levels and in- Thalassinoides burrow networks represent colonial- creased exposure and subsequent erosion of the Trans- ization of seafl oor sediments during periods of slow Continental Arch to the northwest. This resulted in carbonate production and accumulation. Hardground high runoff and basinward transport of siliciclastic surfaces may represent periods of sediment starvation sediments. Under these conditions, shaly carbonate in- and diminished oceanic circulation in the Hollandale terbedded with calcareous grain-rich shale accumulat- Embayment during lowering of sea level. Scouring ed in the northwest part of the study area proximal to of hardground surfaces and deposition and dissection the siliciclastic source area; relatively shale-free car- of centimeter-scale grainstone beds were achieved by bonates with zones of hardground surfaces accumu- more frequent impingement of storm wave base on the lated more distally to the source area in the southeast seafl oor. part of the study area. Upward decreases in siliciclastic content and skeletal grain content indicate rising CONCLUSIONS sea level and subsequent reduction of exposed source • Much of the Galena Group is composed of per- areas. vasively bioturbated wackestones and packstones 2) Carbonate mud-rich and siliciclastic-free facies that are punctuated by thin (1–10 cm), laterally suggest higher sea level and signifi cant drowning of discontinuous grainstone beds. The rocks pre- siliciclastic source areas. Under these conditions, the serve diverse and abundant, predominantly ben- Hollandale Embayment experienced increased circula- thic invertebrate fauna. Four lithofacies groups tion and enhanced mixing of the local water mass with are recognized in the subtle lithologic and ichno- the more extensive “Taconic Sea” to the southeast. Di- faunal variations in the Galena Group of north- verse and abundant benthic communities thrived in eastern Iowa. Repetition of lithofacies groups is the well mixed and siliciclastic-free water mass and cyclic throughout the study area. Depositional cy- resulted in accumulation of hardground-free, rela- cles generally consist of a shale-rich base, a car- tively shale-free, and nodular chert-prone carbonate bonate-rich middle, and a hardground-bearing, across the study area. Deposition of laterally continu- Thalassinoides-burrowed carbonate top. ous, laminated fi ne-grained grainstones and the most shale-free carbonates accompanied maximum sea-lev- • Altered volcanic ash beds, or K-bentonites, are el rise. The presence of prominent and strongly miner- interstratifi ed within the Galena Group. Specifi - alized hardground surfaces and abundant Chondrites cally, the Calmar and Haldane (?) K-bentonites burrows in HFS G2-3, however, indicated dissimilar (Willman and Kolata, 1978) and an unnamed K- maximum fl ooding conditions. Apparently, the Hol- bentonite were identifi ed during this study. Elec- landale Embayment did not become well mixed dur- tron microprobe analysis of single apatite phe- ing sea-level rise during deposition of HFS G2-3, re- nocrysts provided a chemical fi ngerprint for indi- sulting in periodic dysoxia–anoxia and reducing con- vidual K-bentonites. The technique allowed cor- ditions. The geographic restriction of these conditions relation of Galena strata within a complex facies to the northwest study area suggests a modifi cation of transition. Positive identifi cation of the Calmar local conditions, although the direct cause of the mod- K-bentonite bed at three locations in northeast-

VOLUME 19 2008 • 41 ern Iowa corrected misidentifi cations by previous Beyer, S.R., 2003, Stratigraphy, sedimentology, and K- workers at two of these locations. bentonite geochemistry of the Galena Group, northeastern Iowa, Upper Mississippi Valley: Madison, • We interpreted the Galena Group in our study University of Wisconsin, M.S. thesis, 304 p. area to be a third-order composite sequence (Ker- ans and Tinker, 1997) named G2. Composite se- Byers, C.W., and Dott, Jr., R.H., 1995, Sedimentology and depositional sequences of the Jordan Forma- quence G2 is bounded by surfaces across which tion (Upper Cambrian), northern Mississippi Val- major sedimentological changes are evident; it ley: Journal of Sedimentary Research, v. B65, p. contains a nested hierarchy of fi ve lower-ordered 289–305. high-frequency sequences and numerous higher- ordered cycles. High-frequency sequences and Calvin, Samuel, 1906, Geology of Winneshiek County: cycles display repeating facies stacking patterns; Iowa Geological Survey, v. 16, p. 37–146. the ideal stacking pattern consists of a transgres- Chetel, L.M., 2004, 40Ar/39Ar geochronology of detrital sive, interbedded carbonate-shale base overlain and volcanic K-feldspar of the Upper Mississippi by maximum fl ooding, shale-free carbonate, and Valley: Implications for terrigenous provenance and a regressive, bedded carbonate top (where bed- the depositional and biostratigraphic history of an ding is provided by thin shale beds and hard- upper Ordovician epeiric sea: Madison, University ground surfaces). The cyclic manner in which the of Wisconsin, M.S. thesis, 490 p. facies groups are stacked is a result of changes in Chetel, L.M., Simo, J.A., and Singer, B.S., 2005, eustatic sea level and sediment supply during de- 40Ar/39Ar geochronology and provenance of detrital position. K-feldspars, Ordovician, Upper Mississippi Valley: Sedimentary Geology, v. 182, p. 163–181. • The Galena Group of northeastern Iowa was deposited in the proximal zone of an epeiric ramp, Choi, Y.S., 1995, Stratigraphy and sedimentology of the Middle Ordovician Sinnipee Group, eastern Wiscon- spatially transitional between a terrigenous-sup- sin: Madison, University of Wisconsin, M.S. thesis, plied siliciclastic rich zone to the northwest and a 299 p. siliciclastic-free zone of active carbonate production to the southeast. Deposition occurred under Choi, Y.S., 1998, Sequence stratigraphy and sedimentol- conditions of low fair-weather-wave energy and ogy of the Middle to Upper Ordovician Ancell and strong episodic storm energy. Sinnipee Groups, Wisconsin: Madison, University of Wisconsin, Ph.D. thesis, 284 p. Choi, Y.S., Simo, J.A., and Saylor, B.Z., 1999, Sedimen- ACKNOWLEDGMENTS tologic and sequence stratigraphic interpretation of We acknowledge John Fournelle for microprobe anal- a mixed carbonate-siliciclastic ramp, mid-continent ysis and thank Norlene Emerson, Lauren Chetel, and epeiric sea, Middle to Upper Ordovician Decorah Jarad Schmitt for insightful discussion and fi eld assis- and Galena Formations, Wisconsin, in Harris, P.M., tance. Financial support was provided by the Depart- Saller, A.H., and Simo, J.A. eds., Advances in ment of Geology and Geophysics, University of Wis- Carbonate Sequence Stratigraphy: Application to consin–Madison. We thank Dennis R. Kolata, John Reservoirs, Outcrops, and Models: SEPM Special Publication No. 63, p. 275–289. Mossler, and Bruce A. Brown for their helpful reviews of the manuscript. S.R. Beyer is grateful to C.W. By- Delgado, D.J., 1983, Deposition and diagenesis of the ers and J.A. Simo for supervision during his M.Sc. re- Galena Group in the Upper Mississippi Valley, in search of this project. Delgado, D.J., ed., Ordovician Galena Group of the Upper Mississippi Valley—Deposition, diagenesis, REFERENCES and paleoecology: Guidebook for the 13th Annual Field Conference, Great Lakes Section SEPM, p. Allen, V.T., 1929, Altered tuffs in the Ordovician of Min- A1–A17. nesota: Journal of Geology, v. 37, p. 239–248. Emerson, N.R., 2002, Sedimentology, sequence stratigra- Allen, V.T., 1932, Ordovician altered volcanic material in phy, and brachiopod biostratigraphy of the Ordovi- Iowa, Wisconsin, and Missouri: Journal of Geology, cian (Mohawkian) Decorah Formation, Midcon- v. 40, p. 259–269.

42 • GEOSCIENCE WISCONSIN tinent, USA: Madison, University of Wisconsin, Lukasik, J.J., and James, N.P., 2003, Deepening-upward Ph.D. thesis, 490 p. subtidal cycles, Murray Basin, South Australia: Journal of Sedimentary Research, v. 73, no. 5, p. Hall, James, 1851, Lower Silurian System; Upper Silu- 653–671. rian and Devonian Series, in Foster, J.W., and Whit- ney, J.D., eds., Report on the geology of the Lake Lukasik, J.J., James, N.P., McGowran, B., and Bone, Y., Superior land district, pt. 2: U.S. 32nd Congress 2000, An epeiric ramp: Low-energy, cool-water car- Special Session, Senate Executive Document 4, p. bonate facies in a Tertiary inland sea, Murray Basin, 140–166; American Journal of Science, 2nd series, South Australia: Sedimentology, v. 47, p. 851–881. v. 17, p. 181–194. Mange, M.A., and Maurer, H.F.W., Heavy Minerals in Irwin, M.L., 1965, General theory of epeiric clear water Colour: New York: Chapman and Hall, 1992. sedimentation: AAPG Bulletin, v. 49, p. 445–459. Mossler, J.H., and Hayes, J.B., 1966, Ordovician benton- Kay, G.M., 1935, Ordovician System in the Upper Mis- ites of Iowa: Journal of Sedimentary Petrology, v. sissippi Valley, in Kansas Geological Society 9th 36, p. 414–427. Annual Field Conference Guidebook, p. 281–295. Raatz, W.D., and Ludvigson, G.A., 1996, Depositional Kerans, C., and Tinker, S.W., 1997, Sequence stratigra- environments and sequence stratigraphy of Up- phy and characterization of carbonate reservoirs: per Ordovician epicontinental deep water deposits, Tulsa, Society for Sedimentary Geology Short eastern Iowa and southern Minnesota, in Witzke, Course 40, 130 p. B.J., Ludvigson, G.A., and Day, J., eds., Paleo- zoic Sequence Stratigraphy: Views from the North Kolata, D.R., Huff, W.D., and Bergström, S.M., 1996, American Craton: Geological Society of America Ordovician K-bentonites of eastern North America: Special Paper 306, p. 307–330. Geological Society of America Special Paper 313, 84 p. Samson, S.D., 1986, Chemistry, mineralogy, and correlation of Ordovician bentonites: Minneapolis, Korpel, J.A., 1983, Depositional and diagenetic history University of Minnesota, M.S. thesis, 128 p. of the middle/upper Ordovician Dunleith Formation in northeastern Iowa: Iowa City, University of Iowa, Samson, S.D., Kyle, P.R., and Alexander Jr., E.C., 1988, M.S. thesis, 91 p. Correlation of North American Ordovician bentonites by using apatite chemistry: Geology, v.16, p. Leslie, E.R., 2002, Chemical ﬁ ngerprinting, 40Ar/39Ar 444–447. dating and diagenetic alteration of Ordovician K-bentonites, Upper Mississippi Valley: Madison, Sardeson, F.W., 1924, Volcanic ash in Ordovicic rocks of University of Wisconsin, M.S. thesis, 68 p. Minnesota: Pan-American Geologist, v. 42, no. 1, p. 45–52. Levorson, C.O., and Gerk, A.J., 1972a, A preliminary stratigraphic study of the Galena Group of Win- Schlager, W., 1999, Type 3 sequence boundaries, in Har- neshiek County, Iowa: Iowa Academy of Science ris, P.M., Saller, A.H., and Simo, J.A. eds., Advances Proceedings, v. 79, no. 3–4, p. 111–122. in carbonate sequence stratigraphy: Application to reservoirs, outcrops, and models: SEPM Special Levorson, C.O., and Gerk, A.J., 1972b, Revision of Ga- Publication No. 63, p. 35–45. lena stratigraphy: Geological Society of Iowa Field Trip Guidebook 25, 15p. Shaw, G.H., 2003, Trace element chemistry of individual apatite phenocrysts as a tool for ﬁ ngerprinting Levorson, C.O., and Gerk, A.J., 1975, Field recognition altered volcanic ash beds: Assessing interbed and of subdivision of the Galena Group within Win- intrabed variation at local and regional scales: Geo- neshiek County: Field Trip Guide for the Iowa, Min- logical Society of America Bulletin, v. 115, no. 8, nesota, and Wisconsin Academies of Science, 17 p. p. 933 – 942. Levorson, C.O., and Gerk, A.J., 1983, Field recognition Simo, J.A., Emerson, N.R., Byers, C.W., and Ludvigson, of stratigraphic position within the Galena Group G.A., 2003, Anatomy of an embayment in an Ordo- of northeastern Iowa (limestone facies), in Delgado, vician epeiric sea, Upper Mississippi Valley, USA: D.J., ed., Ordovician Galena Group of the Upper Geology, v. 31, no. 6, p. 545–548. Mississippi Valley—Deposition, diagenesis, and paleoecology, Guidebook for the 13th Annual Field Sloan, R.E., Kolata, D.R., Witzke, B.J., and Ludvigson, Conference, Great Lakes SEPM, p. C1–C11.

VOLUME 19 2008 • 43 G.A., 1987, Description of major outcrops in Minne- Witzke, B.J., and Bunker, B.J., 1996, Relative sea-level sota and Iowa, in Sloan, R.E., ed., Middle and Late changes during Middle Ordovician through Missis- Ordovician Lithostratigraphy and biostratigraphy of sippian deposition in the Iowa area, North American the Upper Mississippi Valley: Minnesota Geological craton, in Witzke, B.J., Ludvigson, G.A., and Day, Survey Report of Investigations 35, p. 197–223. J., eds., Paleozoic sequence stratigraphy: Views from the North American craton: Geological Society of Templeton, J.S., and Willman, H.B., 1963, Champlainian America Special Paper 306, p. 307–330. Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p. Witzke, B.J. and Ludvigson, G.A., 2005, The Ordovician Galena Group in Iowa and its regional stratigraphic Thompson, T.L., 1991, Paleozoic succession in Missouri, relationships, in Ludvigson, G.A., and Bunker, B.J., part 2: Ordovician system: Missouri Department of eds., Facets of the Ordovician geology of the Upper Natural Resources, Division of Geology and Land Mississippi Valley region: Iowa Geological Survey Survey Report of Investigations 70, 282 p. Guidebook Series 24, p. 3–21. Ulrich, E.O., 1911, Revision of the Paleozoic systems: Witzke, B.J., and Kolata, D.R., 1988, Changing structural Geological Society of America Bulletin, v. 22, p. and depositional patterns, Ordovician Champlainian 281–680. and Cincinnatian series of Iowa–Illinois, in Ludvig- Willman, H.B., and Kolata, D.R., 1978, The Platteville son, G.A., and Bunker, B.J., eds., New perspectives and Galena Groups in northern Illinois: Illinois State on the Paleozoic history of the Upper Mississippi Geological Survey Bulletin 502, 75 p. Valley: Iowa Department of Natural Resources, Geological Survey Bureau Guidebook 8, p. 55–77. Witzke, B.J., 1980, Middle and Upper Ordovician paleogeography of the region bordering the Trans- Wright, V.P., and Burchette, T.P., 1998, Carbonate ramps: continental Arch, in Fouch, T.D., and Magathan, An introduction, in Wright, V.P., and Burchette, T.P., E.R., eds., Paleozoic paleogeography of west-central eds., Carbonate Ramps: Geological Society, Lon- United States: West-Central United States Paleo- don, Special Publications 149, p. 437–456. geography Symposium 1: Society of Economic Paleontologists and Mineralogists, p. 1–18.

APPENDIX: LOCALITY REGISTER All thin sections and polished slabs are housed at the Department of Geology and Geophysics, Weeks Hall, University of Wisconsin–Madison and are available upon request to the senior author. Map on next page shows approximate locality locations. 1. Madison Road quarry • Fairplay Member: 4.1 m; Mortimer Member: 2.0 m; Rivoli–Sherwood Members: 4.8 m. • Northernmost of two quarries on north side of Madi- • UTM: NAD 83/15N/ 596649E/ 4794311N. son Road, approximately 0.8 km (0.5 mi) west of intersection with Highway 52, Decorah, Winneshiek County, Iowa. Owned by Wiltgen Construction Co. Roadcut in 2003. Named Pavlovec West in Levorson and • East side of Highway 52, 0.5 km (0.3 mi) north of Gerk (unpublished data, 1972). intersection with Highway 9, Decorah, Winneshiek • Mortimer Member: 1.9 m; Rivoli–Sherwood Mem- County, Iowa. bers: 12.1 m; Wall–Wyota Members: 1.0 m. • Rivoli–Sherwood Members: 7.8 m; Wall–Wyota • Sampled beds: Haldane (?) K-bentonite at 14.7 m. Members: 0.7 m. • UTM: NAD 83/15N/ 595708E/ 4795391N. • Sampled beds: Haldane (?) K-bentonite at 18.3 m. • UTM: NAD 83/15N/ 596649E/ 4794311N. 2. Stream diversion and roadcut Stream diversion 3. Highway 9 roadcut • West side of Highway 52, approximately 0.5 km • North side of Highway 9, approximately 1.2 km (0.3 mi) north of intersection with Highway 9, Dec- (0.6 mi) east of intersection with Highway 52, Dec- orah, Winneshiek County, Iowa. orah, Winneshiek County, Iowa. It is convenient to

44 • GEOSCIENCE WISCONSIN 52 W34

10 9

A34 8 Decorah 15 4 23 9 7 6 9 r e iv 51 R

52 i p p 76 i s 12 s i

11 i

X26 M Postville

Prairie du Chien Monona 14 18 Wisconsin River

Map of localities of studied Galena Group exposures

N 52 10 km 5 miles

Guttenberg park at the end of an alley that lies between Mill Street and Mechanic Street, just to the northwest. 13 Named “8h” in Levorson and Gerk (unpublished data, 1972). • Rivoli–Sherwood Members: 4.8 m; Wall–Wyota Members: 7.4 m; Sinsinawa Member: 4.2 m. 5. Highway 52 roadcut • Sampled beds: recessive, clay-rich bed (non-K-ben- • West side of Highway 52, approximately 0.7 km tonite) at approximately 15.2 m. (0.4 mi) south of intersection with Highway W20 • UTM: NAD 83/15N/ 598068E/ 4793828N. (also Pole Line Road), Decorah, Winneshiek Coun- ty, Iowa. 4. Ice cave • Rivoli–Sherwood Members: 12.6 m; Wall–Wyota Members: 8.2m; Sinsinawa Member: 1.0 m. • North side of Quarry Street–Ice Cave Road, approx- • Sampled beds: Calmar K-bentonite at 3.4 m; re- imately 1.3 km (0.8 mi) east of intersection with cessive, clay-rich bed (non-K-bentonite) at 7.1 m; College Drive, Decorah, Winneshiek County, Iowa. recessive, clay-rich bed (non-K-bentonite) at 11.2 • Beecher Member: 0.7 m; Eagle Point Member: m; recessive, clay-rich bed (non-K-bentonite) at 5.6 m; Fairplay Member: 4.6 m; Mortimer Member: 18.4 m. 2.8 m; Rivoli–Sherwood Members: 3.6 m. • UTM: NAD 83/15N/ 596114E/ 4796119N. • UTM: NAD 83/15N/ 598803E/ 4796119N.

VOLUME 19 2008 • 45 6. Holthaus quarry Quarry • Two quarries located on south side of Trout Run • East side of Highway W34, approximately 4.9 km Road, approximately 0.5 km (0.3 mi) east of inter- (3.0 mi) north of intersection with Highway 52, section with Highway W38 (also Division Street, north of Decorah, Winneshiek County, Iowa. Middle Calmar Road), south of Decorah, Win- • Mortimer Member: 1.8 m; Rivoli–Sherwood Mem- neshiek County, Iowa. Owned by Roverud Const. bers: 2.9 m. Co. in 2003. • UTM: NAD 83/15N/ 596362E/ 4807083N. • Rivoli–Sherwood Members: 3.8 m. • Sampled beds: recessive, clay-rich bed (non-K-ben- 10. Hitching Post Road roadcut tonite) at 1.5 m. • South side of Hitching Post Road, approximately • UTM: NAD 83/15N/ 598287E/ 4791908N. 2.4 km (1.5 mi) west of intersection with Highway 52, 1.4 km (0.9 mi) east–northeast of Bluffton, Win- 7. Hovey quarry neshiek County, Iowa. • West side of Highway W38 (also Division Street, • Ion Formation: 0.3 m; Beecher Member: 4.4m; Ea- Middle Calmar Road), approximately 2.0 km (1.2 gle Point Member: 6.5 m; Fairplay Member: 4.3 m; mi) south of underpass at Highway 9, south of Dec- Mortimer Member: 2.5 m. orah, Winneshiek County, Iowa. • Sampled beds: recessive, clay-rich bed (non-K-ben- • Sinsinawa Member: 6.3 m; Stewartville Member: tonite) at 4.7 m. 3.9 m. • UTM: NAD 83/15N/ 589966E/ 4806840N. • Sampled beds: unnamed K-bentonite at 8.6 m; recessive, clay-rich bed (non-K-bentonite) at 6.3 m. 11. Postville quarry • UTM: NAD 83/15N/ 597652E/ 4791936N. • Quarry located on north side of Quarry Road, off the east side of Highway 51, approximately 4.6 km (2.8 8. Pole Line Road roadcut mi) north of intersection of Highway 51 and High- • Long roadcut on north side of Pole Line Road, ap- way 52 in Postville, Allamakee County, Iowa. proximately 5.8 km (3.6 mi) west of the intersec- • Stewartville Member: 5.4 m; Dubuque Formation: tion with Highway 52, northwest of Decorah, Win- 6.3 m. neshiek County, Iowa. • UTM: NAD 83/15N/ 616739E/ 4775886N. • Wall–Wyota Members: 0.8 m; Sinsinawa Member: 6.1 m; Stewartville Member: 9.0 m. 12. Volney roadcut • Sampled beds: unnamed K-bentonite at 9.2 m. • West side of Highway X26 (also Volney Road), • UTM: NAD 83/15N/ 592744E/ 4798671N. approximately 6 km (3.7 mi) south of intersection with Highway 76, or approximately 11.8 km (ap- 9. Canoe Creek roadcuts and quarry proximately 7.3 mi) north of intersection with High- Roadcut way 18–52, north of Monona, Allamakee County, Iowa. • West side of Highway W34 (also N. Winn Road), • Ion Formation: 0.5 m; Beecher Member: 3.9 m; Ea- approximately 4.4 km (2.7 mi) north of intersection gle Point Member: 4.5 m; Fairplay Member: 7.7 m; with Highway 52, north of Decorah, Winneshiek don, Special Publications 149, p. 437–456. County, Iowa. • Mortimer Member: 4.1 m; Rivoli–Sherwood Mem- • Ion Formation: 1.3 m; Beecher Member: 2.9 m. bers: 4.0 m. • UTM: NAD 83/15N/ 596477E/ 4806578N. • Sampled beds: recessive, clay-rich bed (non-K-bentonite) at 4.4 m; Calmar K-bentonite at 23.2 m. Roadcut • UTM: NAD 83/15N/ 632081E/ 4777375N. • East side of Highway W34 (also N. Winn Road), approximately 4.7 km (2.9 mi) north of intersection 13. Guttenberg south roadcut with Highway 52, north of Decorah, Winneshiek • West side of Highway 52, approximately 1.9 km County, Iowa. (approximately 1.2 mi) south of intersection with • Eagle Point Member: 2.7 m; Fairplay Member: Highway C7X (also Garber Road), south of Gutten- 4.0 m; Mortimer Member: 1.0 m. berg, Clayton County, Iowa. • UTM: NAD 83/15N/ 596334E/ 4806870N.

46 • GEOSCIENCE WISCONSIN • Buckhorn Member: 2.8 m; St. James Member: 14. BS-4 core 2.8 m; Beecher Member: 3.1 m; Eagle Point Mem- • Drilled in Clayton County, Iowa, on east side of ber: 5.1 m; Fairplay Member: 8.0 m; Mortim- Highway W62, 6.4 km (approximately 4 mi) south er Member: 3.0 m; Rivoli–Sherwood Members: of Postville, Iowa. Housed at Iowa Geological Sur- 10.0 m. vey Bureau, Iowa City, Iowa. • Sampled beds: recessive, clay-rich bed (non-K-bentonite) at 21.8 m; recessive, clay-rich bed (non-K-bentonite) at 23.1 m; recessive, clay-rich bed (non-K-bentonite) at 24.9 m; Calmar K-bentonite at 29.2 m. • UTM: NAD 83/15N/ 656043E/ 4735239N.

VOLUME 19 2008 • 47 Published by and available from Wisconsin Geological and Natural History Survey 3817 Mineral Point Road • Madison, Wisconsin 53705-5100 ¤ 608/263.7389 FAX 608/262.8086 www.uwex.edu/wgnhs/ James M. Robertson, Director and State Geologist

48 • GEOSCIENCE WISCONSIN