Distal signatures of Late Ordovician oceanic anoxia—New data from a classic epeiric ramp transect
Published:January 01, 2011
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Patrick I. McLaughlin, Norlene Emerson, Brian Witzke, Bryan Sell, Poul Emsbo, 2011. "Distal signatures of Late Ordovician oceanic anoxia—New data from a classic epeiric ramp transect", Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America, James D. Miller, George J. Hudak, Chad Wittkop, Patrick I. McLaughlin
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New data and review of classic sections from the Middle and Upper Ordovician North American Midcontinent in the Upper Mississippi Valley provide a refined picture of the age, stable isotope geochemistry, faunal composition, and—ultimately—origin of this epeiric ramp succession. Sequence stratigraphic analysis reveals a series of unconformity-bounded, genetically related facies packages. Shallowing and deepening trends are sometimes difficult to resolve due to a paucity of hydrodynamic indicators, yet unconformity surfaces are well marked by hardgrounds and confirmed by negative C-isotope spikes. Recent conodont biostratigraphy, new U-Pb radioisotopic ages for K-bentonites, and correlation of C-isotope profiles to global trends suggest that the succession spans the Darriwilian to Hirnantian epochs. Focus on Platteville to lower Galena Group strata (Sandbian to early Katian) provides a temporally high-resolution look at the onset and evolution of a long-term (>2 m.y.) positive carbon-isotope excursion, short-term perturbations in that record, and relationship to the preservation and diversity of the enclosed fauna and strata. Major changes in authigenic mineral suites and organic carbon content throughout the Upper Ordovician Upper Mississippi Valley suggest at least three major redox cycles. The combined evidence for globally recognized, positive carbon-isotope excursions coincident with these redox cycles, as well as high-frequency, sea-level fluctuations and successive faunal turnover events, suggests far-field responses to multiple global oceanic anoxic events.
Upper Ordovician strata of the Upper Mississippi River Valley are some of the most rigorously studied rocks in the world. They are well exposed and contain a broad range of lithologies, fossils, and geochemistry. The views provided in this field trip primarily build from decades of classic lithologic study of these rocks by the Illinois and Iowa geological surveys, Iowa Geological Society, and geology departments at the universities of Iowa, Wisconsin, Illinois, and Minnesota. These lithologic studies have recognized a hierarchy of lithofacies packages, each of which has been assigned a name. While this practice adds a great deal of complexity to the literature, these units are well established and known by many specialists outside of the U.S. (e.g., "Guttenberg" is the namesake for the globally recognized "GICE" carbon-isotope excursion), thus we retain their use here.
Ongoing studies, presented here, integrating facies analysis and geochemistry have greatly expanded our understanding of the Upper Ordovician in the Upper Mississippi Valley. Witzke and McLaughlin have recently restudied the lithofacies characteristics and distributions in nearly all cores penetrating the St. Peter to lower Galena Group in Iowa, Wisconsin, and Illinois. Emerson has complimented this study by detailed examination of the brachiopod biofacies from the upper Platteville to lower Galena Group. McLaughlin has expanded the C-isotope sampling interval and area presented in Ludvigson et al. (2004) to test regional correlation hypotheses and—working with Emsbo—has tested for the temporal correspondence of shifts in redox facies. New biostratigraphic and Sr-isotope data from the St. Peter Sandstone in western Iowa by Witzke and Emsbo and geochemistry of Platteville and Galena Group bentonites by Emerson and Sell have revolutionized our understanding of the chronostratigraphy of this interval. Cumulatively, the addition of these new data challenges long-held depositional interpretations that typically invoke only local to regional phenomena. In contrast, we suggest a strong component of far-field response to global episodes of Late Ordovician oceanic anoxia.
Sequence Stratigraphy (Witzke)
The Upper Mississippi Valley area of Iowa, Wisconsin, and Minnesota exposes a classic Middle and Upper Ordovician cratonic succession of sandstone, shale, and carbonate strata (Fig. 1). The basal part of this succession overlies a deeply incised erosional surface that divides the Sauk and Tippecanoe Mega-sequences (Sloss, 1963). This basal sequence is marked by the St. Peter Sandstone over most of the area, a unit dominated by mature quartzarenites. Although previously considered a transgressive shoreface-sand facies by many workers, this is probably an oversimplification of a more complex regional depositional regime (Witzke and Metzger, 2005). Conodonts recovered from the lower St. Peter Sandstone of western Iowa support a late Middle Ordovician (late Whiterockian, late Darriwilian) age (Witzke and Metzger, 2005). A slightly older (mid-Whiterockian), sub–St. Peter marine transgression into the region is fortuitously preserved as a shale fill (the Winneshiek Shale) within an impact structure at Decorah, Iowa, where a Lagerstätte has yielded conodonts and a variety of soft-bodied fossils that are currently under study (Liu et al., 2006). The St. Peter is overlain by the thin (0.1–5 m) Glenwood Shale across most of the region (Fig. 1). The Glenwood Shale is dominantly a green shale with sandstone commonly in the lower and upper part. The Glenwood contains abundant conodonts that indicate an earliest Late Ordovician age (early Turinian; Fig. 1). The Glenwood rapidly grades into dramatically thicker sections to the south (northern Illinois and Missouri), where it includes quartzarenites of the Starved Rock Sandstone and peritidal carbonate facies of the Joachim Formation.
The overlying Platteville Formation (upper Turinian) is a carbonate-dominated interval that is subdivided into several members in the region: (1) the basal Pecatonica Member—a dolostone and sandy dolostone unit that is capped by a regionally persistent ferruginous hardground surface; (2) the overlying McGregor Member—characterized by wavy-bedded limestone (lower Mifflin and upper Grand Detour units of the Illinois classification); northward in Minnesota, the McGregor interval includes additional dolostone and shale facies, where additional member names are applied; and (3) the uppermost Platteville strata are assigned to the Quimbys Mill Member, which is characterized by lime mudstones with brown organic shale seams. This member is absent in Minnesota and much of northeastern Iowa. The Platteville succession in Iowa has been subdivided into four discrete stratigraphic sequences (P1–P4; Fig. 1). Sandy carbonate and sandstone facies mark the bases of the lower three sequences in central Iowa and northern Missouri.
The succeeding Decorah Formation, the basal unit of the thick Galena Group, is subdivided into four members in the region, where it is marked by complex stratal geometries of shale and limestone facies (uppermost Turinian–lower Chatfieldian). Each member may represent a discrete stratigraphic sequence (G1–G2; Fig. 1). The basal Carimona Member (limestone and shale), which includes the widespread Deicke K-bentonite, is present in northern sections but is absent to the south. The Spechts Ferry Shale is a widespread, green-gray calcareous shale with thin limestones and brachiopod-rich stringers with minor brown shale. Sandy facies are recognized in central Iowa. The member includes the widespread Millbrig K-bentonite. The southward convergence of the Deicke and Millbrig K-bentonites in Iowa, and the disappearance of the Deicke (and Carimona) in that direction, suggest southward condensation of lower Decorah strata. The overlying Guttenberg Member is marked by wavy-bedded limestone facies in the southern area, but the interval is replaced northward by green-gray shale facies with decreasing carbonate content. The basal Guttenberg (Garnavillo bed) is generally phosphatic across the region, and oolitic ironstone facies are seen in Minnesota. The wavy-bedded Guttenberg carbonate beds (Glenhaven interval) are separated by shaly partings and seams, and many of the shales in unoxidized sections are organic rich and chocolate brown in color. The bulk of the organic matter is represented by Gloeocapsomorpha, a palynomorph of uncertain affinity. The Guttenberg in many sections is capped by a widespread hardground surface. The upper Decorah Ion Member includes carbonate, shaly carbonate, and shale facies in the region, and disseminated quartz sand is widely recognized within portions of the member. Shaly Ion strata are replaced southward by carbonate facies that are included within the lower Dunleith Formation (Buckhorn and St. James members). By contrast, Ion strata are replaced northwestward in northern Iowa and Minnesota by typical Decorah green-gray shales. A widely recognized epibole of trepostomes bryozoans is generally present in the upper part ("Prasopora beds").
The Dunleith Formation, the second formation of the Galena Group, is dominated by carbonate and cherty carbonate strata across most of the area, and represents part of an extensive carbonate shelf (or ramp) developed across much of interior North America. The formation is marked by limestone facies in northeast Iowa and Minnesota, but it becomes progressively dolomitized southward. It is dominated by skeletal mudstones to wackestones with thin packstone units. The Dunleith has been subdivided into a series of widely traceable members in the region, and a number of K-bentonites are recognized in the succession, especially in the upper half. The lower half of the Dunleith includes increasing shale content within some members to the north, and the entire interval becomes shale dominated in areas bordering the Galena erosional edge at St. Paul, Minnesota, and northwestern Iowa, where these strata have been assigned to an expanded "Decorah Shale." This northern shale facies includes intervals of phosphatic enrichment, and oolitic ironstones occur at two or more positions. Rhythmic interbedding of shale and limestone is evident in the carbonate-to-shale transition belt in southeastern Minnesota (Cummingsville Member). Numerous hardground surfaces, many blackened by impregnated pyrite and apatite, punctuate the Dunleith carbonate succession. The Dunleith includes two broadly traceable intervals with common receptaculitid and/or ischaditid algae ("Receptaculites Zones"), and dasyclad algal grains are seen in thin section in many, but not all, units of the Dunleith (Bakush, 1985). Witzke and Ludvigson (2005a) proposed two stratigraphic sequences (G3 and G4; Fig. 1) within the Dunleith Formation: a lower unit that becomes increasingly shaly northward (Beecher through Mortimer members), and an upper regional carbonate unit (Rivoli through Wyota Members). The decrease in argillaceous content in the upper Dunleith and overlying Wise Lake Formation reflects drowning of siliciclastic source areas on the Transcontinental Arch (Witzke, 1980) during the Edenian and Maysvillian.
The Wise Lake Formation is the least argillaceous unit of the Galena Group, and it has been interpreted as a discrete strati-graphic sequence by Witzke and Ludvigson (2005a). It is characterized by limestone to the north and dolostone to the south; mixed skeletal wackestone to packstone fabrics predominate. Much of the formation includes prominent networks of thalassi-noid burrows, and receptaculitids and dasyclads span much of the formation. The Dygerts K-bentonite occurs near the base. Benthic faunas are abundant and diverse in the Dunleith–Wise Lake, but much of the fauna remains poorly studied. The Wise Lake notably includes intervals with common gastropods, including large macluritids.
The Dubuque Formation caps the Galena Group, and it is distinguished by the reappearance of shales within the succession. The Dubuque is marked by wavy to planar beds of limestone (or dolostone to the south) separated by thin shales. These shales are likely sourced from the distant Taconic mountains, and presage the incursion of eastern-sourced siliciclastics for the succeeding Maquoketa Formation. The Dubuque limestone beds are dominated by crinoidal wackestones, and organic specks (chitinozoans) are common. Receptaculitids and algal grains are entirely absent above the basal beds. The Dubuque is considered to be a discrete depositional sequence (M1; Fig. 1) allied genetically with the succeeding Maquoketa sequences (Witzke and Ludvigson, 2005a). The base of the Richmondian Stage (Ash-gill, upper Katian) is identified in the upper part of the Dubuque Formation. The overlying Maquoketa Formation is entirely of Richmondian age.
The Maquoketa Formation includes complex and varied lithofacies of shale and carbonate. Limited page space does not permit an in-depth discussion of regional Maquoketa lithofacies and deposition, but a few generalizations are appropriate to this guidebook. A complex phosphatic unit and phosphorite with multiple stacked hardground surfaces marks the basal condensed unit of the Maquoketa across a broad area, but the Dubuque-Maquoketa contact is apparently conformable and nonphosphatic to the north and west in Minnesota and western Iowa. The Maquoketa succession is shale dominated in the southern area, but the formation becomes carbonate dominated to the north and west (with increasing shelly faunas). The Maquoketa succession is subdivided into four shallowing-upwards depositional sequences (Witzke, 2010). Each sequence is marked by a recurring stack of depositional-facies groups over most of the area, in ascending order: (1) a basal hardground and/or condensed phosphatic facies with diminutive molluscan faunas; (2) a lower unburrowed shale, often graptolitic and organic rich; (3) a middle nonskeletal dysoxic shale to carbonate interval, partly burrowed; (4) a middle trilobite-rich (primarily asaphids) shale to carbonate facies; and (5) a capping interval of shale and carbonate strata with shelly faunas, in part with skeletal packstones. The first two facies groups are generally absent to the north and west. The uppermost sequence of the Maquoketa, the Neda Member, includes widespread oolitic ironstone facies, and late Richmondian graptolites are noted in eastern Iowa. Hirnantian strata may be present in eastern Iowa, northwestern Illinois, and eastern Wisconsin (Mosalem Formation; see Bellevue stop discussion).
While there have been many attempts at obtaining radio-isotopic ages for Upper Ordovician K-bentonites (primarily the Millbrig and Deicke beds) in various locations in the eastern United States (e.g., Huff, 2008), there have been relatively few age determinations for ash-fall–derived beds in the Upper Mississippi Valley. Chetel et al. (2004) attempted to obtain Ar-Ar sanidine ages for the Millbrig K-bentonite, as well as the Dygerts K-bentonites from the Wise Lake Formation and the Rifle Hill K-bentonite in the Elgin Member of the Maquoketa Group (see Fig. 1 for stratigraphic position of K-bentonites); however, these ages are likely too young and imprecise because of decay constant errors, inter-laboratory biases, and other geologic complexities associated with this dating method. Other available radioisotopic ages, including older U-Pb zircon data, are similarly affected with imprecision and bias.
New U-Pb zircon data have been acquired for the Dickeyville, Elkport, Millbrig, and Deicke K-bentonites from the new Highway 151 road cut near Dickeyville, Wisconsin. These new data suggest that the lower three bentonites fall within the range of ages for two beds in the Womble Shale in Atoka, Oklahoma—the global stratotype section and point (GSSP) for the base of the Katian Stage. Two beds in the upper portion of the latest Sandbian Womble Shale yield highly precise ages between 452.76 ± 0.19 and 453.53 ± 0.28 Ma (Sell et al., 2011) using current EARTHTIME standardized methods (see http://www.earth-time.org/).
These new ages suggest a need to revisit regional biostratigraphic, sequence stratigraphic, and chemostratigraphic correlation schemes (Fig. 2). First, the new dates for the Elkport, Millbrig, and Deicke, compared to those in the Katian GSSP section, suggest that the Plectodina tenuis–Phragmodus undatus Conodont Zone boundary and the Sandbian-Katian Stage are somewhere in the base of the Dunleith Formation in the Upper Mississippi Valley. While the index species P. tenuis has not been found in the Highway 151 Dickeyville section (Leslie and Bergström, 2005), the faunal assemblage suggests that the Elkport K-bentonite may be in the upper portion of the P. tenuis Conodont Zone (Stephen Leslie, 2011, personal commun.). Additionally, these precise dates permit a correlation between three different biostratigraphic schemes: North American Mid-continent conodont zonation, North Atlantic conodont zonation, and North American graptolite zonation. Second, in terms of sequence stratigraphy, comparing dates and correlated tephra to the GSSP and elsewhere highlights the varied interpretations of sequence boundaries among different researchers (Fig. 2). Third, the variable expression, and in some places apparent lack of, the GICE positive carbon-isotope excursion in eastern North America suggests potential correlation errors. These miscorrelations have been verified through apatite trace-element fingerprinting of beds from the Upper Mississippi Valley (Emerson et al., 2004) and elsewhere in North America (Carey et al., 2009), which suggests that what is often referred to as the "GICE" may be in fact various parts of a single long-term positive carbon-isotope excursion (see sections on carbon isotopes below).
The tephra in the Upper Mississippi Valley suggest unusually good preservation of the host rocks and, now that the beds have been precisely dated, indicate a relatively intense period of large-magnitude volcanism. Extensive examinations of Sand-bian-Katian tephra across eastern North America (Carey et al., 2009; Adhya, 2009; Sell, 2010) have yet to find the Dickeyville, Elkport, Millbrig, and Deicke K-bentonites all preserved in the same outcrop or core outside of the Upper Mississippi Valley. The Dickeyville bed, as well as the Dygerts bed of the Wise Lake Formation and the Rifle Hill bed of the Maquoketa Group, have not been chemically identified elsewhere. The Calmar and Haldane K-bentonites of the Dunleith Formation, which may be from the same eruption, both appear to correlate on the basis of apatite–trace element chemistry with a bed approximately six meters above the base of the Antes Shale near Reedsville, Pennsylvania and the Manheim K-bentonite in the Dolgeville Formation of New York. All of the Upper Mississippi Valley K-bentonites, like the Millbrig and Deicke, have potential sources that are nearly 1500 km away (Huff et al., 1996). Given that the Millbrig and Deicke appear to be the result of large-magnitude eruptions (M8–M9; see Mason et al., 2004), it seems likely that the other beds of the Upper Mississippi Valley derive from eruptions of similar magnitude. Our new U-Pb zircon dates for the Elkport, Millbrig, and Deicke beds indicate a large-magnitude eruption frequency of 9.1 eruptions per million years during the height of the Sandbian-Katian volcanic episode. The rate is higher yet when including new data from the Kinnekulle K-bentonite and may increase with the addition of data from the Dickeyville K-bentonite. These rates are noteworthy because large-magnitude eruption rates during the height of similar volcanic episodes of the Cenozoic are much less at 1.4 eruptions per million years (Mason et al., 2004). The physical stratigraphic evidence for these large-magnitude eruption rates can only be viewed in outcrop exposures near Dickeyville, Wisconsin.
Paleoecology And Taphonomy (Emerson)
Middle to Late Ordovician sediments of the Upper Mississippi Valley region preserve an abundance of fauna that occupied marine environments of mid–North America during deposition of the Tippecanoe Megasequence. The basal strata of the St. Peter Sandstone and Glenwood Shale siliciclastics are not very macrofauna rich; while the carbonates and interbedded shales of the overlying Platteville Formation, Galena Group, and Maquo-keta Shale yield tremendous quantities of fossil specimens from this region.
Articulate brachiopods are the most common skeletal macro-fossil in the Late Ordovician fauna of the Upper Mississippi Valley. Dominant groups include orthids (Doleroides, Pionodema, Dalmanella, and Paucicrura) and strophomenids (especially Strophomena and Sowerbyella), with rhynchonellids (e.g., Rhyn-chotrema and Rostricellula) and atrypids (e.g., Zygospira) present in some beds. Associated taxa include a variety of bryozoans in various forms (e.g., ramose, domal, and encrusting), along with lesser amounts of crinoids, trilobites, rugose corals, gastropods, inarticulate brachiopods, cephalopods, bivalves, ostracods, and receptaculitids, as well as microfauna and trace fossils.
Since brachiopods were abundant benthic members of the Ordovician marine communities across this region, they are useful paleoenvironmental indicators. Brachiopod specimens were collected from the Decorah Formation to examine how the benthic faunal communities responded to paleoceanographic changes observed in the Late Ordovician record of the Upper Mississippi Valley. Focus was given to the Decorah Formation, a mixed shale-carbonate unit, because it records several of the regionally traceable events of interest such as: deposition of Taconic orogeny volcanic ash beds (e.g., the Deicke, Millbrig, Elkport, and Dickeyville K-bentonites); the Spechts Ferry and Guttenberg (GICE) positive carbon-isotope excursions (Lud-vigson et al., 2004); and the M4-M5 sequence boundary (Patz-kowsky and Holland, 1996).
Species-level analysis reveals major faunal turnover at multiple levels within the Decorah–lower Dunleith interval. Over 7000 brachiopod specimens were collected and assigned to 38 species belonging to 23 genera (see Emerson, 2002, for complete list). Figure 3 illustrates that most species had their first appearance datum (FAD) and last appearance datum (LAD) within the Decorah Formation with major faunal turnover at the M4-M5 boundary and very few ranges extending into the overlying Dunleith Formation. Of the 23 genera collected, only 39% had ranges that spanned across the M4-M5 sequence boundary and the onset of the GICE. As illustrated in Figure 3, most brachiopod species that cross the M4-M5 boundary are considered to be "rare" in abundance. These may have been "generalist" species that could adapt to a wide variety of environmental conditions and could withstand greater ecological change than the dominant "specialist" species. Doleroides and Pionodema dominated the communities during the M4 sequence. Similarly Dalmanella, Paucicrura, and Sowerbyella dominated the Guttenberg and Ion of the M5 sequence and are absent from the M4 sequence (Fig. 3).
Biodiversity analyses using multivariate Q-mode cluster analyses, along with SHE indexing (Figs. 4 and 5) indicate that a rather stable brachiopod diversity structure existed throughout deposition of the Decorah. Multivariate Q-mode cluster analyses (Fig. 4) produced two major brachiopod clusters reflecting the change from the M4 to M5 sequences (Emerson, 2002). As shown in Figure 5, the overall number of species increased from 23 within the Spechts Ferry Member (M4 sequence) to 33 within the Guttenberg and Ion Members (M5 sequence), but the diversity and evenness indices shown in the bottom of Figure 5, virtually remained the same when comparing the two sequences. The results of this paleoecological analysis suggest that broad ecological tolerances of rare species allowed them to persist in the face of fluctuating oceanographic conditions, while species that dominated the benthic communities were decimated.
Taphonomic analysis of brachiopod shell beds in the Decorah–lower Dunleith interval reveals evidence for quiescent hydrodynamic conditions and unusual seawater chemistry that altered shell composition prior to final burial. In outcrop, the brachiopod specimens occur typically in brachiopod-rich shell beds that are interbedded with generally fossil-poor shales but that at times may contain thin (<1 cm) brachiopod "pavements." Associated bryozoans most often are well-preserved broken segments of ramose forms with lengths <3 cm and may themselves form shell beds. Crinoids, cephalopods, and trilobites are less abundant as separated ossicles and fragments, respectively, while the remaining fauna (i.e., rugose corals, gastropods, bivalves, and ostracods) occur as fractured, but mostly whole, individuals. The majority of brachiopod fossils, both in shell beds and shale pavements, are found as disarticulated valves with the majority in a hydrodynamically stable convex-up orientation.
Within the Spechts Ferry Member, shell beds range in thickness from pavements to 10-cm-thick, mud-rich shell beds and have very low diversity (almost monospecific). Most beds are composed of valves of the orthids—Pionodema or Doleroides (or both) with very few other brachiopod genera present (see Emerson and Simo, 2006 for a more thorough description). As an example, a 30 × 20 cm surface count of a shell bed from the lower Spechts Ferry Member resulted in 321 shells of Pionodema with only two shells of Strophomena and one Rhynchotrema. Pionodema and Doleroides are both subequally biconvex with valves that are relatively thin, fragile, and have multicostellate (fine-ribbing) ornamentation. From a sample of 2047 identified Pionodema and Doleroides fossils, 54% of the specimens are brachial valves, 41% are pedicle valves, and only 5% consist of whole individuals. The valves often show very little to moderate signs of abrasion or bioerosion. Most valves lack borings or encrustation; but if present, most are bryozoan epibionts.
The brachiopod shells are preserved with a variety of coloration. Within the lowermost beds of the Spechts Ferry, many shells (but not all) are blackened and may be coated or stained. A few brachiopod pavements are composed of shells that are entirely white or very light gray. This assortment of white, shades of gray, and blackened shells is not specific to any particular brachiopod genus nor to just the brachiopod fossils. Associated bryozoans, crinoid ossicles, trilobite fragments, and ostracods as well have similar color alterations with particular note to blackened bryozoans. The general absence of bioerosion and encrustation and well-preserved condition of the shells suggests that they may have been buried rapidly, spending only a short period on the seafloor. If this is not the case, the environment must have been very low energy most of the time with seawater chemistry that was inhospitable to burrowers who might have dislodged the shell, exposing them at the sediment-water interface. Sedimentation rates within the Decorah Formation were very slow. Utilizing new radioisotopic ages for bentonites from the Highway 151 Dickeyville road cut, the estimated duration for deposition between the Deicke and Elkport K-bentonites was likely on the order of 0.5 m.y.; therefore the intervening ∼1.5 m of strata may have been deposited at a rate of ∼3 mm/ka.
Fossils in the lower Guttenberg are often found as well-preserved pavements on upper bedding surfaces of thin carbonate mudstones to wackestones (lower-mid to outer ramp environment). These beds also show well-preserved fossils internally that lack evidence of compaction. In the uppermost Guttenberg and the Ion Member, brachiopods are mostly broken, disarticulated fragments deposited within cross-stratified bioturbated packstones with intercalating grainstone stringers (shallower mid-ramp environment). While these upper members contain richer diversity than in the Spechts Ferry Member (discussed above), they are dominated by three genera—the strophomenid Sowerbyella and the orthids Dalmanella and Paucicrura. As an example, out of 2155 specimens of Sowerbyella, Dalmanella, and Paucicrura, 52% of the specimens are brachial valves, 35% are pedicle valves, and 14% consist of whole individuals. Many of the brachiopod genera retrieved from these carbonate-rich beds are slightly smaller (e.g., Dalmanella and Paucicrura) and have slightly more robust valves (e.g., Dalmanella, Paucicrura, Platystrophia, and Oepikina) than the dominant genera in the shale-rich facies below (e.g., Pionodema and Doleroides). Within the uppermost Guttenberg and Ion Members, fossils show greater abrasion and fragmentation and are deposited within a matrix of bioturbated coarse sand-sized skeletal grains, all suggesting an oxygenated, more energetic environment. Within the Gutten-berg and Ion Members, the fossils also occur in various colors as described above for the Spechts Ferry Member, but significantly fewer are blackened, and a majority of the fossils are red or reddish brown.
Many questions remain with the interpretation of fauna within this succession. Some of these questions center on the discoloration of the fossils. For example, what is, and how does the chemical composition vary among shells that are black, white, gray, or red? What is the cause of the discoloration? Is there a link between fossil color and sedimentation rate? Does the fossil color relate to the amount of time it was exposed on the seafloor? Do the colors relate to multiple cycles of burial and exhumation? These as well as other questions are being addressed with ongoing research.
Carbon-Isotope Stratigraphy And Redox Facies (Mclaughlin And Emsbo)
Carbon-isotope analysis in the Upper Ordovician rocks of the Upper Mississippi Valley has been ongoing intermittently for nearly a quarter of a century (i.e., Hatch et al., 1987). While δ13Ccarb values remain virtually unknown for the St. Peter–Glenwood and age-equivalent units, the published carbonisotope stratigraphy for the Sandbian age Platteville-Decorah interval in the Upper Mississippi Valley is arguably one of the most comprehensive in the world. This interval features several closely spaced sections sampled at high resolution across a major facies change from carbonate to shale dominated. Less well sampled are the overlying Katian age strata of the Dunleith and Wise Lake formations. Recent work by the Wisconsin Geological and Natural History Survey (WGNHS) and Illinois State Geological Survey (ISGS) generated a large (N>2000) unpublished δ13Ccarb data set for the Dubuque Formation and Maquo-keta Group in eastern Wisconsin and northern Illinois (Fig. 6). This new data set also includes substantial documentation of late Hirnantian–age paleovalley fill deposits.
Carbon-isotope analyses in the Upper Mississippi Valley are currently focused primarily on the St. Peter, Glenwood, and lower Platteville formations. Regionally the temporal relationships of this Darriwilian to Sandbian age interval are poorly resolved. Two hurdles impede precise chronostratigraphy in this interval: (1) the largely unfossiliferous and carbonate-poor nature of the quartz arenites and (2) the complex lateral facies relations. For instance, across northern Illinois and into southeastern Iowa the ∼30+-m-thick Starved Rock Sandstone separates the western St. Peter–Glenwood succession from southern carbonate-dominated Everton-St. Peter-Dutchtown-Joachim succession (Templeton and Willman, 1963; Witzke and Metzger, 2005). The general lack of biostratigraphic index fossils and primary calcite cements (prohibiting δ13Ccarb analysis) makes it nearly impossible to directly date the Starved Rock Sandstone. The same is true of the basal St. Peter, which appears to be the only unit that can be traced continuously through the region. Thus, our δ13Ccarb sampling focused on the more carbonate-rich enclosing strata. For instance, we extended the sampled interval downward into the upper calcareous St. Peter Sandstone in the previously analyzed Big Spring #4, Elkader A1, and SS-9 cores held by the Iowa Geological Survey (Fig. 6). At the base of the interval, we sampled the calcareous shales and interbedded carbonates of the Readstown Member of the St. Peter at 30 cm intervals from the ∼150-m-thick U.S. Geological Survey "James" core drilled in Lafayette County, Wisconsin. Additionally, we collected highresolution samples through the Dutchtown-Joachim succession from multiple cores in southern Illinois, south of the Starved Rock Sandstone tongue.
Ongoing efforts to bolster published data sets from the Platteville and Decorah formations through the GICE include regional analysis of additional drill cores. Ludvigson et al. (2004) published a comprehensive series of δ13Ccarb profiles through the upper Platteville to lower Dunleith formations for eastern Iowa and one for central Illinois. Additional data sets were published by Ludvigson et al. (2000, 2004). Our ongoing work to further resolve the δ13Ccarb profile of this interval includes: (1) sampling the complete carbonate-dominated Platteville-Decorah interval at 5 cm intervals through a core taken at Governor Dodge State Park in southwestern Wisconsin, (2) sampling of an age-equivalent shale-dominated facies succession at 15 cm intervals from a core drilled in Pierce County, Wisconsin, and (3) sampling at 3060 cm intervals the age-equivalent carbonates in cores drilled in Pike and Clark counties in southern Illinois. The results of this expanded analysis suggest a slightly modified view of the δ13Ccarb profile than that presented previously (Fig. 6). The primary features of this new data set include: (1) a long-term positive shift in carbon isotopes beginning with the basal Platteville Formation and peaking in the Decorah Formation, (2) a series of superimposed, partially preserved, negative excursions, and (3) negative spikes and flat-line offsets coincident with major facies changes at hardground surfaces suggestive of subaerial exposure and modification by meteoric diagenesis. In detail, C-isotope values rise abruptly by ∼1‰ through the phosphatic Chana Member, plateau at that level through the Dane, and begin to rise slightly through the New Glarus members (Fig. 6). A sharp negative spike marks the unconformable hardground contact between the Pecatonica and McGregor formations. The δ13C b values rise in the lower to middle Mifflin Member (i.e., Mifflin Excursion of Ludvigson et al., 2004). This small positive excursion is immediately followed by a negative excursion in the upper Mifflin that reaches its lowest point near the Mifflin–Grand Detour contact. Values rise rapidly through the lower Grand Detour to plateau in the middle before starting a steady climb in the upper. The contact with the Quimbys Mill is marked by a negative spike of nearly 1‰. Values rise rapidly through the lower Quimbys Mill and then establish a gentle positive slope through most of the member. The thin Carimona bed of the Spechts Ferry has similarly positive values. The Glencoe bed of the Spechts Ferry and overlying phosphatic Garnavillo bed of the Guttenberg contain a negative excursion. Carbon-isotope values show a flat-line offset of up to 2‰ across the base of the Glenhaven, which slowly decrease upward. In most sections, a positive flat-line offset proceeds plateau values near the base of the overlying Ion Formation. The δ13Ccarb values presented by Fanton and Holmden (2007) show that this plateau gives way upward to decreasing values through the upper Dunleith and lower Wise Lake formations to a low of ∼2‰, before climbing again through the upper Wise Lake to achieve a new plateau at –0.5‰ in the Dubuque. A pilot study by the WGNHS to resolve the position of the Ordovician–Silurian boundary in eastern Wisconsin has resulted in the genesis of a δ13C b data carb set from the upper Dubuque Formation, Maquoketa Group, and basal "dark Mayville" Dolostone. This ∼70-m-thick interval was typically sampled at 30 cm resolution through four cores in eastern Wisconsin and one core from northern Illinois. This interval was also previously sampled in other parts of the Upper Mississippi Valley by Raatz (1992), Guthrie and Pratt (1994), and Fanton and Holmden (2007).
The Upper Ordovician succession in the Upper Mississippi Valley records at least three large-scale redox cycles that may represent the regional expression of Late Ordovician oceanic anoxic events. Preservation of organic matter, precipitation of authigenic minerals, bioturbation, and faunal compositions provide valuable information about redox conditions. The δ13C values provide a proxy for organic-carbon burial as do shale colors (darker colors = more organic carbon). Many positive carbon-isotope excursions throughout the Phanerozoic show a positive correlation with carbon burial during oceanic anoxic events. Red and green marine shales and carbonates of the Reedstown Member of the St. Peter Formation indicate a well-oxygenated interval. Organic-rich subtidal carbonates and shales of the Dutchtown Formation and age-equivalent brown shales and organic-rich sandstones of the upper St. Peter indicate increased organic-carbon burial that may equate with the Middle Darriwilian positive carbon-isotope excursion (MDICE). This succession is capped by regionally extensive hematite deposits that locally thicken into a 4+ m oolitic ironstone. The return to overlying green-shale deposition in the north and peritidal carbonates with green shales and evaporites in the south suggests a transition to warm oligotrophic conditions. The sharp contact with sandy phosphorite at the base of the Platteville Formation is coincident with a positive shift in C-isotopes and appearance of brown and gray shales and blue phosphatic carbonates. This redox phase reaches a peak in the Guttenberg Member with abundant brown shales and highly positive C-isotopes. Occurrence of greenish- and bluish-gray shales of the Spechts Ferry Member containing iron ooids to the north suggests an interface between highly reducing and more oxidizing seawater chemistry. Overall the reducing conditions continue for an extended period, ending only with the return of green and red marine shales in the Brainard Member of the Maquoketa Formation. The final reducing phase of the Upper Ordovician occurs with the appearance of dark-gray Hirnantian strata.
Stop 1. Madison East Quarry, Sauk-Tippecanoe Unconformity, Prairie du Chien– St. Peter Sandstone Contact
Location: UTM: 16T 314281E 4774261N. Quarry operated by Homburg Construction Co., Madison, Wisconsin. Request permission for access.
The field trip begins on the east side of Madison, Wisconsin, at the base of the Tippecanoe Megasequence (see field trip route map; Fig. 7). This locality represents one of the best exposures of the karstic unconformity between the Early Ordovician Prairie du Chien Group and the Middle Ordovician St. Peter Sandstone. The quarry is located on a topographic high of Oneota and Sha-kopee Formations of the Prairie du Chien Group. Logs of nearby wells show that the high is between two valleys incised into the Lower Ordovician dolostones of the Prairie du Chien Group that are filled by the St. Peter Sandstone, thus the modern expression reflects an Ordovician landscape (Fig. 8). A small exposure of the overlying Platteville Formation (once visible here) has been removed via quarrying.
The Prairie du Chien Group has been divided into two formations, the Oneota below and the Shakopee above. The Oneota consists of medium- to thick-bedded dolomitic quartz sandstone and quartzitic to pure dolostone with local zones of algal stromatolites, silicified oolites, intraclasts, thin greenish-gray to maroon shales, and chert (both as nodules and thin beds). The overlying Shakopee Formation consists of interbedded quartzitic dolostone, quartz sandstone, and shale with chert, oolites, and algal stromatolites locally common, and with the upper interval being less sandy and more variably cherty.
The main feature of interest at this stop is the erosional unconformity separating the Lower Ordovician Prairie du Chien dolostone from the overlying Middle Ordovician St. Peter Sandstone (rusty color, medium- to fine-grained, cross-bedded sandstone; Skolithos burrows occur only on the upper part of the section) (Fig. 9). The unconformity reflects a time of subaerial dissolution and fluvial incision of a shallow-marine carbonate platform (Great American Carbonate Bank). This erosion resulted in the development of irregular karst topography on the Prairie du Chien strata consisting of multiple valleys, caves, and sinkholes which were subsequently infilled by sediments of the St. Peter Sandstone during the Middle Ordovician transgression.
The St. Peter Sandstone is an economically significant blanket or sheet sandstone, covering most of six states. It is famous for extreme compositional and textural maturity as well as for being the basal unit of the Tippecanoe A Supersequence set. The quartz sandstone contains grains that are well rounded suggesting that much of the sand was derived by reworking of older sandstones. It has long been quarried for glass and foundry sand, and in more recent times for bedding sand used in this region's abundant dairy barns. The St. Peter is also an important aquifer, and is a natural gas reservoir in Michigan. The St. Peter Sandstone thickness averages 30–40 m but varies widely because of the relief of the unconformity. The St. Peter shows significant facies changes. In Wisconsin it is dominantly fine- to medium-grained eolian sandstones, whereas Skolithos-bearing marine sandstones increase in proportion westward, southward, and eastward.
Stop 2. Fitchburg West Road Cut, St. Peter Sandstone– Platteville Formation Contact
Location: UTM: 16T 298013E 4765092N. Road cut near Fitchburg, Wisconsin.
Recent development of a shopping center on the west side of Fitchburg, southwest of Madison, has resulted in the creation of an easily accessible exposure of the St. Peter–Platteville contact. This section was created in 2007, and the WGNHS has archived a short core from the locality that was drilled during the initial construction planning stage. Currently the section exposes ∼6 m of the upper marine beds of the St. Peter Formation. The lower 3 m of the St. Peter at this location is cross bedded with undulating beds and abundant liesegang banding. The upper 3 m of St. Peter is more tabular and includes successive horizons of limonitic Skolithos burrows. The contact with the Platteville locally forms an angular unconformity. The tabular, calcareous beds of the Platteville are distinctive from the more massive weathering St. Peter below. Note that this section contains a highly truncated St. Peter Sandstone. At Stop 4 we will see a marine St. Peter succession that is nearly 10 m thicker. During construction of this section, eolian strata of the Tonti Member and red and green paleosols of the Reedstown Member were exposed in a small ditch across the street. The Tonti was at least 4 m thick and showed the classic succession of highangle dune cross beds with a variety of bright-orange, yellow, and white alternating colors, cut by many small synsedimentary faults. A silcrete was developed near the top of the Reedstown. The exposed top of the Reedstown was just over 2 m thick and showed red shale over green shale. The shales displayed abundant soil slickensides and were cut by several thin lenticular sandstones, some displaying halite pseudomorphs.
Stop 3. Wisconsin Geological and Natural History Survey (WGNHS) Research and Education Center
Location: UTM: 16T 276308E 4765173N. 211 S. Blue Mounds Street, Mount Horeb, Wisconsin.
At the Research and Education Center, we will view several drill cores archived by the WGNHS. The Research and Education Center is a 25,000 ft2 facility that houses ∼600,000 linear feet of bedrock drill core, cuttings samples from ∼2.7 million linear feet of drilling, and an extensive hand-sample collection from across eastern North America. The facility also features well-equipped labs for rock processing, optical geology, photography, and geochemistry. Workshops and short courses are regularly held in the Education Center on the upper level.
The "James core," drilled in southern Lafayette County, Wisconsin, is 472 ft long and spans the mid-Dunleith through basal St. Peter formations. Of particular interest in this core is the St. Peter Sandstone, which is ∼300 ft thick with well-developed Tonti and Reedstown Members. The Reedstown Formation is ∼150+ ft thick in the James core and is composed of an alternating succession of red paleosols and marine carbonates and fine-grained sandstones. Intraformational conglomerates are present at several levels. The overlying Tonti is ∼150 ft thick and shows a basal unit made up of heavily cross-bedded fine sand with alternating pale-yellow and bright-orange sections. The upper Tonti is composed of poorly cemented and cryptically cross-bedded quartz arenite. Poor recovery at the Tonti-Platteville contact obscures relations slightly; however, the phosphatic sandstones of the Chana Member of the Pecatonica Formation are identifiable.
The "Dodgeville core" is ∼130 ft long and spans the uppermost St. Peter to lower-Dunleith interval, containing a facies succession typical of that found in southwestern Wisconsin. This core, drilled less than 2 miles from the Dodgeville Rule quarry (Stop 4) in central Iowa County, was sampled at very high resolution for C-isotope analysis. Micrite samples were drilled every 2.5 in, while vugs, stylolites, and other late diagenetic features were avoided. The resulting C-isotope profile is the first in the Upper Mississippi Valley to capture the full variability of the isotope record. Its resemblance to an age-equivalent profile sampled at a similar temporal spacing is striking.
The "Langer core," nearly 100 ft in length, spans the lower Dunleith to upper St. Peter interval. Drilled in Pierce County, western Wisconsin, it displays a shale-dominated facies succession that is typical of the area from Decorah, Iowa to Minneapolis, Minnesota. Goethite ooids are present at several horizons: lower Platteville, middle Spechts Ferry, and upper Spechts Ferry. These horizons of iron ooids are persistent through outcrops around Decorah and cores to the north and west. K-bentonite fingerprinting (Emerson et al., 2004) provided the first rigorous tie points for uniting the northwestern shale-facies belt and the carbonate-facies belt to the southeast. Carbon isotopes sampled from this core provide another critical tool for correlating between the shale and carbonate-facies belts.
Stop 4. Dodgeville North Quarry, St. Peter Sandstone–Glenwood Shale-Platteville Formation
Location: UTM: 15T 732441E 4765508N. Quarry operated by Rule Construction, Dodgeville, Wisconsin (private property; request permission for access).
The quarry north of Dodgeville, Wisconsin is operated by Rule Construction as a source of carbonate aggregate and bedding sand for dairy industry. The quarry exposure is in two parts, upper and lower. The lower exposure shows nearly 30 m of St. Peter Sandstone. This is one of the largest and cleanest St. Peter exposures in southwestern Wisconsin. At the base of the exposure, oblique sections through large eolian dunes show white sands alternating with brightly colored yellow and orange intervals. Cross laminations and color bands are cut by many small synsedimentary faults that offset strata by as much as 2 m. About halfway up the exposure, the eolian dunes are cut by tabular marine sandstones with trough cross bedding. Multiple Skolithos burrow horizons as seen at Stop 2 are present ∼15 m above the floor of the quarry. A prominent 5 cm phosphate band forms a distinctive marker at 20 m and is overlain by a thick interval of burrowed white sandstone. The upper meter and a half of the St. Peter shows a complex interbedding of hematitic and glauconitic sandstones with diverse marine ich-nogenera including large pyrite-cemented Thalassinoides. The uppermost surface of the St. Peter is a hematite and pyrite mineralized crust. A thin ∼30 cm succession of the greenish-gray to bluish-gray Glenwood Shale is present between the St. Peter and basal Platteville and is well exposed where the quarry is benched. Abundant Thalassinoides burrows filled with phos-phatic sand descend from the base of the Platteville to penetrate the Glenwood. The sharp contact with the base of the Platteville is present on the far southwestern end of the upper exposure. The basal Chana Member of the Pecatonica is a very phos-phatic coarse-grained quartz sandstone, containing irregular phosphate-stained surfaces, grain coatings, and large nodules. The overlying ∼13 m of the Platteville, including the Pecatonica and lower McGregor members, is bluish-gray, partially dolo-mitized and moderately fossiliferous phosphatic packstone and wackestone in thick tabular beds.
Stop 5. New Dickeyville Road Cut, Platteville–Lower Galena Group
Location: UTM: 15T 698605E 4723784N. U.S. Highway 151 road cut near Dickeyville, Wisconsin.
Recent reconstruction activity (2001) of U.S. Highway 151 between Dodgeville and Dickeyville, Wisconsin, has created a tremendous new exposure of late Sandbian to Katian strata at this location. The inclined road cut provides good access to rock faces on both sides of the divided highway with access at road level to the Platteville, Decorah, and lower Dunleith Formations without break (Fig. 10). It is at this locality that all four of the K-bentonites (Dickeyville, Elkport, Millbrig, and Deicke), discussed elsewhere in the guidebook, have been identified and newly U-Pb zircon dated (Fig. 11).
At the north end of the road cut, rocks exposed at the base belong to the Pecatonica Member of the Platteville Formation. The Pecatonica here is characterized by fossiliferous dolostone. Much of the invertebrate fossil material has been lost during the dolomitization process and now appears as small open molds. The top of the Pecatonica is marked by a widespread hardground surface that can be noted at 1.5 m above the base. The overlying McGregor Member is an upward succession of fossiliferous, wavy-bedded to nodular limestone. Much of the McGregor here is pale yellowish-brown to light-gray micritic to argillaceous limestone with thin shale partings and fossil-rich grainstone stringers. Examples of the grainstone stringers can be seen at 6 m and 7.5 m above the prominent Pecatonica hardground. Macrofossils within the McGregor include numerous brachiopods and bryozoans with lesser amounts of crinoids, trilobites, rugose corals, ostrocods, nautiloids, and gastropods.
The bedding style of the uppermost 2 m of the McGregor becomes less wavy bedded and more horizontally bedded with more frequent hardground surfaces. This portion of the Platte-ville is classified as the Grand Detour Formation of the "Platte-ville Group" by the Illinois Geological Survey (Templeton and Willman, 1963). The top of the Grand Detour at this location is marked by a 3–5 cm recessive interval, which is overlain by an 80-cm-thick unit composed of chocolate-brown dense micrite interbedded with grainstone stringers. This unit is identified as the Quimbys Mill Member, which often breaks with conchoidal fracture, giving it the nickname "glass rock" by early lead miners of this region. Overlying the Quimbys Mill is a 20 cm carbonate unit identified as the Carimona Member of the basal Decorah Formation in Iowa and Wisconsin. The Carimona is a fossilifer-ous wackestone that thickens northwestward into Minnesota and Iowa and is absent farther eastward in Wisconsin. The reverse is true for the Quimbys Mill, which is generally present in Wisconsin but absent in Iowa and Minnesota. This is one of the few locations where the Quimbys Mill and Carimona are present together. Here the basal Carimona contains large rip-up clasts of the Qui-mbys Mill limestone. The Deicke K-bentonite can be found near the top of the Carimona.
The overlying Decorah Formation consists of a lower shale member (Spechts Ferry Member), a middle limestone unit (Guttenberg Member), and an upper shaly interval (Ion Member). The Spechts Ferry here is ∼1.6 m thick and consists of dark greenish-gray shale with thin interbedded limestone (packstones) and brachiopod shell beds with increased shale thickness upward. The Millbrig K-bentonite is located near the base of the Spechts Ferry as a 2–3 cm rusty-orange clay layer. The lowermost Guttenberg Member (Garnavillo of Templeton and Willman, 1963) is recognized by thin green shales and interbedded phosphatic carbonate mudstones. The Elkport K-bentonite can be located within the lower Guttenberg (∼35 cm from base) as a thin (∼2 cm) orange clay above the first two limestone beds. Above the Elkport, the Guttenberg consists of a wavy-bedded, white-weathering, gray packstone to grainstone interval with increasingly amalgamated grainstones upward (Glenhaven beds of Templeton and Willman, 1963). The Dickeyville K-bentonite (∼5 cm thick) can be found as a recessive sticky white to orange clay located ∼3.3 m from the base of the Guttenberg. The remaining upper Guttenberg and Ion interval of the Decorah is composed of calcareous shale and interbedded argillaceous, irregular bedded packstones and grainstones. A prominent mineralized hardground located at 4.8 m above the base of the Guttenberg may mark the contact between the Guttenberg and Ion, as it does at most localities in this area (although field confirmation was not possible before the publishing of this guidebook).
A change in weathering color of the rock face can be seen at ∼6.6 m above the base of the Guttenberg—from the gray Decorah to orange colors of the overlying Dunleith Formation. The Dunleith forms the remaining high cliffs of the road cut and consists of recrystallized dolostone and argillaceous dolomitic limestones with a considerable quantity of nodular white chert (the Decorah Formation lacks any chert).
Stop 6. Bellevue State Park, Ordovician–Silurian Boundary
Location: UTM: 15T 712102E 4680507N. Bellevue State Park, Iowa.
The imposing bluffs along the Mississippi River at Bellevue, Iowa, expose a succession of Ordovician and Silurian strata; however, the Maquoketa Formation is represented by mostly covered shale slopes in the area, with minor shale exposure in places. Lower Maquoketa exposures directly overlying the Galena Group can be seen in the northeastern part of the state park along Mill Creek, but these will not be examined for this trip. The prominent wavy-bedded argillaceous to cherty dolostone strata near the park entrance belong to the Mosalem Formation (Fig. 12). This formation regionally infills large east-trending erosional valleys incised into the Maquoketa Shale, varying between 0 and 30 m in thickness. Most previous workers assigned this formation to the Lower Silurian, largely by stratigraphic position, but Kleffner et al. (2005) raised the possibility that part or all of the Mosalem may be of Hirnantian age (latest Ordovician). Although graptolite biostratigraphy is equivocal (latest Ordovician or earliest Silurian), a positive carbon-isotope excursion within the Mosalem and the equivalent Wilhelmi Formation of Illinois (Kleffner et al., 2005) is consistent with Hirnantian time. Hirnantian glaciation in the Southern Hemisphere caused a major eustatic drawdown of sea level that likely produced the large regional sub-Mosalem erosional unconformity. The upper Mosalem contains a brachio-pod and coral fauna that previously has been interpreted to be of Lower Silurian (Rhuddanian) age (see summary in Witzke, 2008). If true, the Ordovician–Silurian boundary may occupy a position within the Mosalem Formation. The overlying Silurian dolostone strata of the Tete des Morts and Blanding formations (Fig. 12) contain Lower Silurian faunas.
Stop 7. Graf Road Cut, Lower Maquoketa Formation
Location: UTM: 15T 674584E 4706267N. Road cut near Graf, Iowa.
The roadside exposure at Graf exposes the lower dolostone and shale strata of the Maquoketa Formation (Fig. 13), an interval generally included in the Elgin Member (equivalent to the lower Scales Shale of Illinois). The mostly covered slope above this exposure and below the capping Silurian dolostone ledges spans a thick interval of middle and upper Maquoketa shale-dominated facies. The total formation here is ∼75 m thick, and the Graf section (road cut and slope) comprises the type locality of the Maquoketa (Witzke and Heathcote, 1997). The lower strata exposed here include phosphatic dolostone and dark-brown, organic-rich shale facies. The shales are graptolitic in part, and in some beds linguloids (Leptobolus) are abundant. Dense accumulations of orthoconic nautiloids are present in the middle part. Phosphatic enrichment of dolomitic beds includes apatite pellets and tiny phosphatized fossils. All dolostone beds are phosphatic to varying degrees, and unit 4 (see Fig. 13) is 60% apatite by weight. The phosphatized fossils comprise rich and diverse assemblages of molluscan-dominated faunas, most less than a few millimeters in size. These diminutive faunas vary in composition through the Graf succession. They are generally dominated by gastropods, nuculoid bivalves, scaphopods (Plagioglypta), and hyoliths, but other taxa are also present (Witzke and Heathcote, 1997). Phosphatic facies of the lower Maquoketa are best developed in the Dubuque area, although the basal phosphorite (not exposed here) is identified across a vast area from Iowa and Missouri eastward to Indiana.
The phosphatic and organic-rich sediments of the lower Maquoketa succession are interpreted to have been deposited within a stratified epicontinental seaway with anoxic to dysoxic benthic waters. Recurring oxygen stresses and episodic oxygen depletion have been proposed to explain the evolution of the diminutive benthic faunas through paedomorphic adaptation for reproduction (Witzke and Heathcote, 1997). The deposition of abundant phosphate and organic matter likely reflects both high primary surface productivity and oxygen-deficient bottom waters. The abundance of nautiloids and graptolites may be related to high surface productivity associated with impinging or upwelling nutrient-rich waters along the margin of a carbonate shelf (shallower Elgin Member carbonate-shelf facies are seen to the north). The nautiloid beds may represent episodic mass mortality events in the area, and such beds are largely restricted to the area around Graf and Dubuque. Nautiloid septa are commonly broken and imploded, and nautiloid shells commonly became telescoped during septal implosion. Raatz and Ludvigson (1996) calculated water depths of 180–270 m for deposition of the nautiloid beds at Graf.
Stop 8. Guttenberg North Road Cut, Ordovician Strata
Location: UTM: 15T 654588E 4743426N. Road cut along Highway X56 near Guttenberg, Iowa.
About 120 m of Ordovician strata are accessible in a series of road cuts along the Mississippi Valley in the Guttenberg area, including the St. Peter Sandstone, Glenwood Shale, Platteville Formation, and most of the Galena Group (Decorah, Dunleith, Wise Lake, and Dubuque formations). Because of limited time and the considerable stratigraphic thickness, we will focus on the Platteville-Decorah succession at the large road cut north of Guttenberg (Great River Road, Highway X56). Above the St. Peter Sandstone and thin condensed Glenwood Shale at the base of the road cut, the Platteville Formation includes a lower dolostone and sandy dolostone interval (Pecatonica Member) and an upper interval of wavy-bedded limestone with shaly partings (McGregor Member). The closely similar type section of the McGregor Member is located 20 km north at the edge of Pikes Peak State Park (see Fig. 14). A regionally correlated hardground surface near the middle of the McGregor member marks the contact of the equivalent Mifflin and Grand Detour formations of the Illinois classification. The Pecatonica, Mifflin, and Grand Detour intervals are each interpreted as a stratigraphic sequence, but the uppermost Platteville, Quimbys Mill, sequence is absent in this part of the Upper Mississippi Valley (Ludvigson et al., 2004).
The overlying Decorah Formation includes three members at Guttenberg, the Spechts Ferry, Guttenberg, and Ion. The basal Decorah Carimona Member (and the contained Deicke K-bentonite) is absent at Guttenberg, but a thin Carimona appears to the north at McGregor (see Fig. 14). The Millbrig K-bentonite occurs near the base of the Spechts Ferry Shale at Guttenberg. The Guttenberg Member superficially resembles the Platteville with its wavy-bedded limestone strata. The limestones are inter-bedded with organic brown shale seams, but these are mostly oxidized on outcrop. The Guttenberg positive carbon-isotope excursion (GICE), originally identified in Iowa, was one of the first major carbon-isotope excursions recognized in the Paleozoic, and it is now identified at many localities worldwide.
The overlying Ion Member includes shale and limestone strata, and a widespread epibole of trepostome bryozoans (especially the hemispherical Prasopora) occurs in the upper part. The shales of the Ion are replaced to the south in Illinois by carbonate-dominated facies generally included within the Dun-leith Formation. Above the Ion, the road-cut section exposes a thick succession of mixed limestone and dolostone strata, partly cherty, of the middle and upper Galena Group (Dunleith and Wise Lake formations). This succession includes numerous submarine hardground surfaces, many irregularly sculpted by burrows and borings and darkened with pyrite and apatite. Delgado (1983) identified 74 hardgrounds within the Dunleith Formation at Guttenberg. These hardgrounds mark small-scale hiatuses within the Galena succession, and many are broadly correlatable. Thalassinoid burrow networks are common in the Dunleith and Wise Lake formations, and these are commonly preferentially dolomitized. The Galena Group at Guttenberg is partially dolomitized, especially upwards in the succession, and the section here forms part of a broad transition zone that separates the pervasively dolomitized sections to the south (as seen at Dubuque) and the nondolomitized limestone section to the north (as seen at Decorah).
Stop 9. Pike's Peak State Park, Ordovician Strata
Location: UTM: 15T 649703E 4762120N. Pikes Peak State Park, Iowa.
Pikes Peak State Park provides a spectacular view of the Upper Mississippi and Wisconsin River valleys. The stop will also provide a backdrop for continuing discussion of the regional Ordovician geology. The park is named after Zebulon Pike, who explored the region shortly after the area became part of the United States following the Louisiana Purchase in 1803. The viewing platform in the park is positioned above natural cliff exposures of dolostone strata belonging to the Dunleith Formation (Galena Group). Out of sight along the cliffs below and in the deeply incised stream drainages are exposures of the Decorah, Platteville, and Glenwood formations, and the St. Peter Sandstone (Anderson, 2000). If time permits, a short walk past the large bear effigy mound to Bridal Veil Falls will reveal natural exposures of the Decorah and Platteville formations. The steep drainages in the park also expose thick sections of St. Peter Sandstone (27–68 m) that infill deep erosional paleovalleys incised into dolostone and sandstone strata of the Lower Ordovician Prairie du Chien Group. The southernmost exposures of Cambrian strata (Jordan Sandstone) in the Mississippi Valley are seen in the lowest portions of the park.
A number of animal effigy mounds are known from this stretch of the Mississippi Valley, and these earthen features were created by native peoples of the "Effigy Mound culture" between about A.D. 600 and 1100. The large bear-shaped mound in the park is one of the most easily accessible effigy mounds in the area. The Wisconsin River enters the Mississippi on the opposite side of the valley, and an interconnected series of channels, sloughs, and backwaters are evident. The Mississippi Valley in the park displays over 150 m of vertical relief. The river valley was incised an additional 100 m into Cambrian bedrock during the Pleistocene, but this deeper incision is now filled with Quaternary alluvial deposits (see Fig. 15).
Stop 10. Bloody Run Road Cut, Decorah Formation
Location: UTM: 15T 646017E 4765018N. Bloody Run road cut along U.S. Highway 18/52 3.7 miles SW of Marquette, Iowa.
This stop provides a view of the basal shaly portion of the Decorah Formation in the Upper Mississippi Valley. The inclined road cut provides good access to rock faces on both sides of Highway 18/52 with close inspection of higher portions of the outcrop made possible by the creation of a cut bench along the outcrop (best accessed on the southeast side of the highway). Although this road cut is old and getting overgrown with vegetation, it provides a good stop for fossil collecting because of the rock weathering. Portions of the St. Peter and Platteville formations are accessible in road cuts along the highway downslope from this stop.
The Decorah Formation is ∼12 m thick at this location (Fig. 16) and consists of a succession of a lower shale-rich member (Spechts Ferry) and upper limestone members (Guttenberg and Ion). Exposure of the basal shaly Spechts Ferry Member is intermittently visible along the base of the section due to coverage by talus and vegetation. Access to the Deicke and Millbrig K-bentonites is possible with a little excavation. The Elkport K-bentonite is more easily accessible and found at the base of the lower limestone ledge on the southeast side of the highway (Gut-tenberg Member). The Dickeyville K-bentonite is not present at this location (Fig. 16).
Above the shaly Spechts Ferry, the lower cliff face is composed mostly of the Guttenberg Member. The lower limestone beds above the Elkport K-bentonite consist of thin, wavy-bedded, light-gray to brown (weathers whitish) mudstones with intercalating grainstones and interbedded shale partings. There are a few discontinuous thin grainstones with abundant whole or nearly whole macrofossils. Overlying these beds are irregular, medium-to thick-bedded, gray-green skeletal packstones and grainstones, which become grainier and amalgamated upward. This succession ends with a prominent hardground identified as the top of the Guttenberg Member (see Fig. 16) located near the top of the lower cliff face (southeast side of highway). The Ion Member here consists of greenish-gray to gray skeletal packstones to grainstones and calcareous shales located on the lower portion of the upper cliff face on the south side of Highway 18/52. The overlying Dunleith Formation is characterized by recrystallized tan to orangish vuggy dolostone making it difficult to distinguish the original sedimentary fabric and fossils.
Articulate brachiopods are the most common macrofossils of the Decorah fauna here as elsewhere. Orthids (Doleroides, Pionodema, Dalmanella, and Paucicrura) and strophomenids (especially Strophomena and Sowerbyella) are dominant, with rhynchonellids (e.g., Rhynchotrema and Rostricellula) and atrypids (e.g., Zygospira) present in some beds. Associated taxa include a variety of trepostome bryozoans in various forms (e.g., ramose, domal, and encrusting), which become more abundant in the upper Decorah, along with lesser amounts of crinoids, trilo-bites, rugose corals, gastropods, inarticulate brachiopods, cepha-lopods, and bivalves.
The above-mentioned fauna occurs mostly in brachiopodrich shell beds interbedded with shale lithofacies that are in general fossil poor but may contain thin (<1 cm) brachiopod "pavements." The majority of brachiopods are found as disarticulated valves with the majority of the valves in the convex-up, hydrodynamically stable orientation. Associated bryozoans are most often found as broken segments of ramose forms with lengths less than three centimeters. Crinoids, cephalopods, and trilobites usually occur as separated stem pieces and broken segments respectively, while the remaining fauna (i.e., rugose corals, gastropods, and bivalves) occurred as fractured, but mostly whole, individuals.
Stop 11. Decorah Bruening Quarry, Decorah and Dunleith Formations
Location: UTM: 15T 600134E 4793800N. Quarry 0.8 km east of Decorah, Iowa.
This stop provides an excellent view of a complete section of the Decorah Formation (Fig. 17) in its type area. The underlying uppermost portion of the Platteville Formation (including the Deicke K-bentonite) can be reached by a short walk along Highway 9 to the west, and the overlying lower Dunleith Formation can be accessed, with care, by climbing along the upper walls of this quarry.
The main floor of the quarry lies on the upper surface of the Carimona (Fig. 16), which here is marked by a mineralized, bored burrowed hardground, whereas north of Decorah, Iowa, the upper Carimona surface contains large 1 m wave ripples. The carbonates of the Carimona consist of wackestones and packstones, and interbedded thin (1–3 cm), fissile, calcareous shales. Fossils within these carbonates include brachiopods, cephalo-pods, trilobites, gastropods, ostracodes, and bryozoans.
The overlying Glencoe beds (Spechts Ferry Member) are composed of interbedded shales and thin carbonate beds. Near the base, the Spechts Ferry consists of fossil-poor, laminated, Chondrites burrowed, pyrite-rich dark-gray shales that grade upward into interbedded carbonate mudstones and thin mud-rich shell beds consisting of well-preserved nearly monospecific, disarticulated, blackened whole brachiopod valves. These shell beds are dominated by Doleroides and/or Pionodema with lesser abundance of Strophomena, Hesperothis, and rare Rostricellula and Diorthelasma. In the upper Glencoe, shales change to green (Fig. 18) and are interbedded with grain-rich shell beds consisting of broken disarticulated, mostly not blackened, brachiopod valves. Abundant coarse sand- to pebble-size phosphate-rich grains occur both in the green-shale facies and in the interbedded shell beds within a zone ∼60 cm thick (Fig. 18).
The overlying Guttenberg Member of the Decorah is more difficult to recognize at this locality and is much thinner (∼2 m thick) than at localities to the south and east. The lowermost Guttenberg (Garnavillo) is composed of green shales and interbedded mudstones and contains the Elkport K-bentonite (Fig. 17). Above the Elkport (lower Glenhaven beds) the lithofacies changes to interbedded wavy, brown carbonate mudstones to wackestones with well-preserved, diverse, open marine fauna (brachiopods, crinoids, trilobites, and gastropods), abundant carbonate mud, and organic-rich brown shale partings. The upper Guttenberg beds consist of cross-stratified packstones to grainstones inter-bedded with dark-brown to gray-green shales containing bra-chiopods, bryozoans, rugose corals, and trilobite segments overlain by skeletal packstones and intercalating grainstones with few whole fossils and abundant coarse-sand–sized bioclasts.
At this locality, strata of the overlying lower Ion Member consist of irregular wackestone to skeletal packstone and grain-stone nodules and lenses interbedded with gray to green calcareous shales containing abundant sand-size bioclasts (Fig. 17). Overall, brachiopods collected from the Ion are less abundant than in underlying Guttenberg and Spechts Ferry beds but include horizons rich in Dalmanella, Paucicrura, and Sowerbyella, with lesser amounts of Hesperorthis, Protozyga, Plaesiomys, Rhyn-chotrema, and Zygospira.
The upper boundary of the Decorah Formation is marked by a Prasopora epibole (Fig. 18) that consists of strongly biotur-bated (common Chondrites and Thalassinoides), medium to thin, wavy-bedded, bryozoan-rich, argillaceous packstones and/or wackestones. These beds contain intercalated brachiopod grain-stones dominated by Platystrophia, Rhynchotrema, Dalmanella, and Paucicrura; and bryozoans (including abundant Prasopora) with minor rugose corals and crinoid ossicles.
Stop 12. Decorah West Road Cut, Galena Group Strata
Location: UTM: 15T 594202E 4798928N. Road cut along Pole Line Road northwest of Decorah, Iowa.
The Decorah area is probably the best place to study the Galena Group succession in the Upper Mississippi Valley, not only because of the numerous exposures in the area that span its entirety, but especially because these exposures faithfully preserve original depositional fabrics (and fossils) in limestone facies. This contrasts with the partially to pervasively dolomitized sections to the south and east in Iowa, Wisconsin, and northern Illinois where primary fabrics and fossils are more diagenetically obscured. A composite stratigraphic section for Galena Group strata above the Decorah Formation is shown on Figure 19, and most of this section is accessible at the Pole Line Road road cut northwest of Decorah, the focus of this stop. The Dunleith, Wise Lake, and Dubuque formations are subdivided into numerous members that are widely correlated across the Midcontinent, and thin K-bentonite beds are noted at several positions within the succession (see Fig. 19).
Levorson and Gerk (1972) studied the Galena Group succession in the Decorah area over many years, and they successfully applied the stratigraphic nomenclature originally defined in northern Illinois. They made extensive fossil collections and identified several regionally persistent intervals with diverse articulated crinoid faunas. The bulk of the Dunleith Formation is characterized by mixed skeletal mudstone to wackestone fabrics, but thin packstone beds punctuate the succession, some seen as starved megaripples. The lower half of the formation includes shale and shaly limestone units that resemble facies seen in the Decorah Formation, but the upper half is notably less argillaceous. Two intervals contain common receptaculitid and ischaditid algae (the "Receptaculites Zones") as well as dasyclad grains. As seen at the Guttenberg stop, numerous hardground surfaces punctuate the succession.
The overlying Wise Lake Formation is the least argillaceous interval of the Galena Group, and it is prominently marked by extensive thalassinoid burrow networks through most of the succession. In general, the Wise Lake is more skeletal rich than the Dunleith and is dominated by amalgamated wackestones and packstones with several widely traceable packstone units (the "sparry calcarenite beds" of Levorson and Gerk, 1972). The "Upper Receptaculites Zone" encompasses the middle of the formation. The Dubuque Formation, which caps the Galena succession, includes numerous shale interbeds, and much of the formation is notably crinoidal.
Stop 13. Riffle Hill Quarry, Upper Galena Group and Lower Maquoketa Formation
Location: UTM: 15T 561393E 4827746N. Rifle Hill, northwest of Greenleafton, Fillmore County, Minnesota.
The Rifle Hill area northwest of Greenleafton, Minnesota, includes a deep quarry with adjoining road cuts that exposes a succession of the middle to upper Galena Group (upper Dunleith, Wise Lake, and Dubuque formations) and lower Maquoketa Formation (Fig. 20). We will focus on the upper Dubuque and lower Maquoketa succession seen above the quarry in the road cuts and ditches along the county road. The lower Maquoketa succession of southern Minnesota contrasts markedly with that seen at Stop 7 at Graf, Iowa—dark organic shales, phosphorites, and phosphatic dolostones are notably absent. The lower Maquoketa of southern Minnesota is a succession of interbedded skeletal mudstones to packstones with shelly benthic faunas (especially brachiopods) and argillaceous to shaly units with common graptolites and/or trilobites (especially Isotelus). Shelly faunas increase in abundance both northward (shoreward) and upsection (a shallowing-upward sequence). Three-dimensionally preserved graptolites are common in the basal beds of the Maquoketa at Rifle Hill, and these interstratify at a fine scale with shelly faunas (especially sowerbyellid brachiopods). These basal beds may be the northern facies equivalent of the basal Maquoketa phosphorite seen to the south. Unlike the prominent complex phosphatic hardground surface that marks the top of the underlying Galena Group to the south, the Dubuque-Maquoketa contact in southern Minnesota is apparently conformable, and the contact has proven difficult to place. In fact, the formational contact was drawn at different stratigraphic positions in the descriptive sections of Sloan and Kolata (1987) and Raatz (1992).
The underlying Galena Group succession at Rifle Hill was measured and described by Levorson and Gerk (see Sloan and Kolata, 1987, p. 88–91), and a thin K-bentonite was noted in the middle part of the Dubuque Formation (the "Rifle Hill K-bentonite"). The Dubuque Formation is marked by smooth to wavy-bedded crinoidal limestone beds separated by prominent shale partings. The underlying Dunleith and Wise Lake succession in the adjoining quarry closely resembles that seen at Decorah.
Thanks to Steve Maul of the Wisconsin Geological and Natural History Survey for creating the field trip route map. The quality of this article benefited from thoughtful reviews by Carlton Brett and Todd LaMaskin.
Figures & Tables
Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America
This volume of 25 field guides plus one paper on field instruction was prepared in conjunction with the 2011 GSA Annual Meeting in Minneapolis, Minnesota. The diverse slate of field trips spans a geologically broad range of topics, including the Precambrian geology of the southern Canadian Shield; the economic geology of the Lake Superior region; Phanerozoic strata in Minnesota, Wisconsin, Iowa, and North Dakota; glacial geology; hydrogeology and limnology; undergraduate and K12 geoscience field education; archaeological investigations in the upper Mississippi River valley; and geology by bicycle.
- absolute age
- anaerobic environment
- carbonate ramps
- epicontinental seas
- field trips
- Galena Dolomite
- isotope ratios
- marine environment
- Middle Ordovician
- mineral composition
- Mississippi Valley
- Platteville Formation
- road log
- sea-level changes
- sedimentary rocks
- sequence stratigraphy
- stable isotopes
- United States
- Upper Mississippi Valley
- Upper Ordovician