q 2003 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.Geology; June 2003; v. 31; no. 6; p. 545–548; 4 figures. 545

Anatomy of an embayment in an Ordovician epeiric sea, UpperMississippi Valley, USAJ.A. (Toni) Simo Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison,

Wisconsin 53706, USANorlene R. Emerson Department of Geology and Geography, University of Wisconsin–Richland, 1200 Highway 14 West,

Richland Center, Wisconsin 53581, USACharles W. Byers Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison,

Wisconsin 53706, USAGregory A. Ludvigson Iowa Department of Natural Resources and Department of Geosciences, University of Iowa, North

Capitol Street, Iowa City, Iowa 52242, USA

ABSTRACTThe integration of stratigraphic, geochemical, and biostratigraphic data from Middle

Ordovician carbonates and shales indicates that the North American epeiric sea was par-titioned into shelf areas with distinct characteristics. The Upper Mississippi Valley partof the epeiric sea was appraised by using regionally traceable and geochemically ‘‘finger-printed’’ K-bentonites, as well as detailed lithologic correlation. In the Midcontinent, theDecorah Formation records a time of high clastic sediment influx and abundant freshwaterrunoff from the Transcontinental Arch that created a salinity-stratified water column andled to episodic dysoxia. Later, relative flooding of the clastic source areas greatly reducedboth the clastic sediment and freshwater runoff. As a result, the salinity stratificationbroke down, oxygenating the seafloor and permitting carbonates to form. Associated withthis change, clarity of the water improved and the photic zone expanded, allowing seasonalblooms of Gloeocapsomorpha prisca to occur, resulting in increased burial of organic mat-ter. The increase in G. prisca and total organic carbon coincided with, but lagged behind,a regional d13C excursion. In addition, the timing of the initiation of the isotopic anomalyis different across the studied area, suggesting that local environmental conditions influ-enced the isotopic record. Data presented in this study support the partitioning of distinctareas within epeiric seas and the importance of this setting in storing inorganic and or-ganic carbon and recording environmental and biological changes.

Keywords: epeiric sea, Midcontinent, Ordovician, Decorah Formation, depositionalenvironments.

INTRODUCTIONThe Ordovician of the North American

Midcontinent epeiric sea has been viewedtraditionally as layer-cake stratigraphy withremarkably homogeneous and continuousstratigraphic units (Templeton and Willman,1963; Willman and Kolata, 1978). Thesestratigraphic units are dominated by carbon-ate rocks with small amounts of shale. Re-cent work indicates that deposition on cra-tonic epeiric seas was complex; subtlechanges in topography, sea level, and hy-drology had profound impacts on the ocean-ography and resulting ocean-water chemis-try, lithology, and biota.

In this paper we reevaluate the MiddleOrdovician stratigraphy of the northernMidcontinent on the basis of a new K-bentonite correlation within the HollandaleEmbayment (Fig. 1). We also provide acomprehensive depositional model in whichinherited topography and runoff budget playimportant roles in the depositional history,and we review data supporting a complexoceanographic setting within the Midconti-nent epeiric sea.

GEOLOGIC AND STRATIGRAPHICSETTING

During the Middle and Late Ordovician, theNorth American craton was subjected to oneof the largest marine floodings in Phanerozoichistory (Witzke, 1980). The Middle Ordovi-cian stratigraphy (Fig. 1) starts with a karstunconformity overlain by the St. Peter Sand-stone. A shift to carbonates occurred with thedeposition of the Platteville Formation. Theoverlying Decorah Formation is a mixedshale-carbonate unit, whereas younger forma-tions (Dunleith, Wise Lake, and Dubuque) arecomposed of thin-bedded carbonates withhardgrounds and no shale. The youngest Ordo-vician is the Maquoketa Shale. Fossils and struc-tures within these packages indicate normal-marine environments below wave base.

We focus our work on the Decorah For-mation, a mixed carbonate-shale depositionalsequence, which reflects the interplay betweencontinental flooding and weathering of ex-posed land. The Decorah Formation is bound-ed below and above by sequence boundaries(Witzke and Bunker, 1996). The lower surfaceis an angular discordance, and the upper is

marked by a key bed (Prasopora epibole) thatseparates two different lithologies and twobrachiopod communities. Brachiopods (32species) are very common, and 6000 speci-mens were collected and identified. A com-posite species range chart (FADs, first appear-ance datums; LADs, last appearance datums)shows that most species had their FADs andLADs confined within the upper and lowerboundaries of the Decorah and did not haveranges that crossed formation boundaries(Emerson, 2002). FADs and LADs and mul-tivariate Q-mode cluster analyses show twomajor brachiopod clusters reflecting shale-richversus carbonate-rich lithologies and indicatea close link between environment and fauna.

K-BENTONITE CORRELATIONFour K-bentonite layers (Willman and Ko-

lata, 1978) within the Decorah Formation(Fig. 1) were identified and correlated on thebasis of primary apatite phenocryst chemistrycontained within the altered ash. Figure 2shows Mg versus Mn (wt%) values that sep-arate the different K-bentonite apatite pheno-crysts into four clusters. Comparison of grainanalyses from various localities indicates thatapatite Mg and Mn values for a single K-bentonite remain within the same cluster anddo not show geographic variations, althoughvalues from a single crystal may overlap withother fields.

SEDIMENTOLOGY AND SEQUENCESTRATIGRAPHY

The integration of the K-bentonite and lith-ologic correlations allows for the subdivisionof the Decorah sequence into two wedges, alower shale and an upper carbonate. This cor-relation (Fig. 3) is different from previous in-terpretations in which shales and carbonatesinterfinger (Ludvigson et al., 1996). Our re-construction implies that the majority of theDecorah shale in Minnesota is older than themajority of the Decorah carbonates in Iowaand Wisconsin (Fig. 3). The lower shale faciesof the Decorah occurs primarily within theHollandale Embayment and thins against theWisconsin Dome. The overlying Decorah car-

546 GEOLOGY, June 2003

Figure 1. A: Geologic set-ting for Midcontinent andeastern United States. Po-sition of lat 158S duringOrdovician is shown.Note position of Hollan-dale Embayment, tecton-ic depression, betweenTranscontinental Archand Wisconsin Dome(Bunker et al., 1988). Gen-eralized isopachs of Ga-lena Group follow Witzkeand Kolata (1988) and Ko-lata et al. (2001). Thick-nesses suggest two dif-ferent basins and pos-sibly water masses(Holmden et al., 1998).B: Chronostratigraphyfollowing Templetonand Willman (1963).Kirk—Kirkfieldian, Rock—Rocklandian.

Figure 2. Plot of Mg vs. Mn (elemental wt%) of all apatite grains analyzed with electronmicroprobe. MDL—minimum detection limits. Each data point represents microprobe valuesaveraged from 3 to 5 points taken from single grain. N—number of grains.

bonates extend farther upramp onto the Wis-consin Dome and become thinner, less grainy,and more shale rich to the NW.

The lower Decorah shale wedge consists ofshale-rich cycles defined by fossil-poor, lam-inated, shale bases that grade upward into in-terbedded shales with carbonate mudstonesand shell pavements containing a nearlymonospecific assemblage of disarticulated,nonabraded brachiopod valves. Chondrite isthe dominant trace fossil. The shales have ahigh gammacerane ratio and homohopane in-dex (Pancost et al., 1998). These cycles areinterpreted to represent deposition in an outerramp below storm wave base. The bases ofthe shale cycles were deposited in dysoxic

conditions; oxygenation appears to have in-creased upward, owing to water mixing andwave reworking.

The upper Decorah carbonate wedge con-sists of carbonate-rich cycles with fossil-rich,bioturbated, calcareous shale bases that gradeupward into bioturbated packstones. These cy-cles are interpreted to have developed withinthe mid-ramp, where frequent storm waves re-worked bottom sediment.

Cycles at the transition from the shale tothe carbonate wedges have unique character-istics, suggesting that the lithologic changewas accompanied by modification in the oceanchemistry, productivity, and biota. The upper-most shale-rich cycle contains an interval rich

in phosphate (Fig. 3), suggesting a period ofslow sedimentation. The lowermost cycle ofthe carbonate wedge has very high values oftotal organic carbon (TOC) and Gloeocapso-morpha prisca, and also records a d13C ex-cursion (Fig. 4).

DECORAH ISOTOPIC RECORDThe d13C profiles of the Decorah Formation

have been developed from numerous sites inIowa and show a positive carbon isotope ex-cursion that appears to be correlative to an ex-cursion found elsewhere (Ainsaar et al., 1999;Ludvigson et al., 1996; Patzkowsky et al.,1997).

The Iowa bulk stable carbon isotope valuesshow a positive d13C excursion of ;3‰.Within our high-resolution stratigraphicframework, it is possible to show that the tim-ing and magnitude of the isotopic excursionchanges across the studied area (Fig. 4). Thesesections also show an interruption in the ex-cursion with more negative d13C values withinthe interval containing high concentrations ofphosphate granules (Fig. 4) and interpreted toreflect an influx of phosphorus and 12C-richdeep basinal waters (Brasier et al., 1990;Kump and Arthur, 1999).

In sections dominated by thick carbonates(Wisconsin Dome, Fig. 3), the positive excur-sion postdates the Millbrig K-bentonite andthe phosphate-rich interval. The profile (Fig.4) shows that low d13C values coincide withthe phosphate-rich interval and that high d13Cvalues peak near the transition between theshale and carbonate wedges, which is geo-graphically and stratigraphically well con-strained by the Elkport K-bentonite (Fig. 4).These sections also show high TOC (to 50%),organic carbon (;8‰), and concentrations of

GEOLOGY, June 2003 547

Figure 3. Cross sectionshowing lithologic andK-bentonite correlationacross research area. SeeFigure 1 for location. Dia-gram illustrates compen-sating relationship be-tween lower shale-richfacies thinning to south-east, and upper carbonate-rich facies thinning tonorthwest. Elkport K-bentonite is datum. C—Carimona Member.

G. prisca (Pancost et al., 1998, 1999). How-ever, these high values occur above the Elk-port K-bentonite after the increase in, and inpart overlapping with, the d13C excursion.

In contrast, the shale-rich sections depositedin the deeper-water Hollandale Embayment(Fig. 4) show an earlier d13C increase, be-tween the Deicke and Millbrig K-bentonites,and the highest d13C values, near the ElkportK-bentonite. There is little or no presence ofthe brown shales that host the high G. priscaconcentrations in these sections.

DECORAH DEPOSITIONAL MODELDeposition of the Decorah occurred in two

stages. During the early stage, the Transcon-tinental Arch was exposed. Runoff from theland supplied clastic sediment that accumulat-ed within the Hollandale Embayment. Our in-terpretation is that the freshwater runoff cre-ated a salinity-stratified water column. Thisstratification led to times of dysoxic condi-tions in the Hollandale Embayment. Organicproductivity was restricted by the photiczone’s shallow depth, which was due to thesuspended clastic content.

At a later stage (soon after deposition of theElkport K-bentonite), relative flooding of theTranscontinental Arch greatly diminished thesupply of both clastic sediments and fresh-water. The salinity stratification broke down,oxygenating the seafloor, permitting carbon-ates to form on the Wisconsin Dome and pro-

grade into the Hollandale Embayment. Simi-larly, 143Nd/144Nd isotope ratios (Fanton et al.,2002) suggest high sea level and expansion ofthe Taconian water mass northward during de-position of the base of the Decorah’s upperwedge.

We hypothesize that the improved waterclarity expanded the photic zone and increasedprimary productivity, allowing for the anom-alous organic content and increased G. priscadeposition. These oxic conditions on the sea-floor coincided with the high organic carboncontent and the positive carbon isotope spikerecorded within the basal Guttenberg Member.The anomalous organic content cannot simplybe the result of enhanced preservation; it mustrecord an actual increase in phytoplanktonproductivity.

In the modern tropical ocean, the chloro-phyll maximum (the maximum concentrationof phytoplankton) is not at the surface whereavailable light is greatest, but is tens of metersdown in the water column. The plankton popu-lation is balanced between the zone of nutrient-poor water above and light-lacking water be-low. It is possible that G. prisca behaved likemodern phytoplankton and was most concen-trated at a deep chlorophyll maximum. Theoceanographic changes that led to the switchfrom shale to carbonate deposition increasedthe penetration of light at depth and acceler-ated photosynthesis, allowing for G. prisca to

thrive and increased burial of organic matter.Possibly a sea-level rise opened up new ave-nues for influx of nutrient-rich deep watersfrom the Taconian foreland basin into the cra-tonic interior (Fanton et al., 2002).

The environmental conditions in the Hol-landale Embayment during the change fromshale- to carbonate-dominated wedges appearto coincide with a craton-wide d13C excursion(Patzkowsky et al., 1997). However, the timelag between the TOC and G. prisca peaks andthe excursion indicates that these two pro-cesses were independent (Fig. 4). Further-more, the difference in initiation of the excur-sion across the study area (Fig. 4) suggeststhat local effects have influenced the timingand shape of the excursion. This offset hasbeen recorded in many sections across Iowaand Wisconsin (Ludvigson et al., 2000). It isunclear whether the apparent delay was a re-sult of sediment condensation, relatively low-resolution sampling, environmental condi-tions, or a combination thereof. We think thatthe consistency of the isotopic record rules outa sampling problem. Apparently, local envi-ronmental conditions prevailed over the re-gional carbon isotope signature. Furthermore,if the excursion was of global event, in thecratonic interior its signature was diminishedin magnitude and postponed in time.

CONCLUSIONS1. By using four regionally traceable and

geochemically ‘‘fingerprinted’’ K-bentonites,

548 GEOLOGY, June 2003

Figure 4. Lithostratigraphy of Decorah Formation including stratigraphic positions of key K-bentonites (horizontal dashed lines), concentrations of G. prisca, and d13C and total organiccarbon (TOC) values. Brachiopod species range lines A, B, and C refer to three major group-ings of species (see text). A—species generally confined to shale-rich facies, B—sharedspecies present in both facies, C—species generally confined to carbonate-rich facies. TOCand G. prisca values are adapted from Pancost et al. (1998), brachiopod data are from Em-erson (2002). C—Carimona Member, Sp. Ferry—Spechts Ferry Member.

the Midcontinent Decorah Formation was cor-related across a carbonate-shale facies change.

2. We envision two stages of deposition.Exposure of the Transcontinental Arch andfreshwater runoff created a salinity-stratifiedwater column accompanied by deposition of ashale-rich wedge of sediment during times ofintermittent dysoxic conditions. At a laterstage of deposition, following the submer-gence of the Transcontinental Arch, the clasticinput and freshwater runoff diminished. Thischange allowed for a clearer water column, anexpanded photic zone, and a switch to car-bonate deposition with times of high primaryproductivity and G. prisca burial, followed bydilution by prograding carbonates.

3. The lag time between the G. prisca peak(and subsequent TOC accumulation) and thed13C excursion suggests that these are not co-eval processes and probably do not representcause and effect.

4. Within the K-bentonite and sequencestratigraphic framework, across the study area(,100 km), the d13C excursion started at dif-ferent times, although it peaked at the sametime. This finding suggests that the initiationof a regional carbon isotope excursion was af-fected by local environmental conditions.

5. This scenario points to a North American

Ordovician epeiric sea wherein subtle topo-graphic changes divided the flooded continentinto different depositional areas with distinctphysical, biological, and chemical signatures(Holmden et al., 1998). Subtle topographymodified storm currents and restricted circu-lation within the sea in such a way that smallvariations in sea level, runoff, and evaporationmay have drastically changed the water chem-istry, leading to major facies and faunalchanges.

ACKNOWLEDGMENTSComments by Brian Pratt, Beverly Saylor, and an anon-

ymous reviewer greatly improved this work. Isotopic datagathering was supported by National Science Foundationgrants EAR-0000741 and EEC-9912191. Field work wassupported by grants from the Department of Geology andGeophysics, University of Wisconsin–Madison.

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Manuscript received 4 January 2003Revised manuscript received 28 February 2003Manuscript accepted 3 March 2003

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