Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
Upper Ordovician and lower Silurian strata of eastern Wisconsin, by Ronald D. Stieglitz and Paula E. Allen (below)
Ronald D. Stieglitz and Paula E. AllenUniversity of Wisconsin-Green Bay
In northeastern Wisconsin the Paleozoic is represented by strata of theCambrian, Ordovician and Silurian Systems. East of the Fox River and thebay of Green Bay, in the area of this field conference, the bedrock is theUpper Ordovician Maquoketa Formation and Silurian dolomite. The Neda Formation is present between the two locally.Surprisingly little is known of these rocks in this area. Most of the work in the Maquoketa Formation has been done in Iowa and in parts of Indiana,Kentucky and Minnesota. Sivon (1979) reported on work done in eastern Wisconsin but details of that study have not been published. Froming (1971)collected conodonts from Maquoketa sections throughout the state includingseveral from the area. The Neda Formation has received considerable attention but uncertainties still exist as to its system assignment. Paull(1977) provides an excellent summary of the formation. Almost all of thework done on the Silurian rocks in Wisconsin has focused on the reef complexes. Shrock (1939) set the stage for the subdivision of the section andMikulic (1977) discussed the rocks in southeastern Wisconsin. No recentcomprehensive stratigraphic or paleontologic work has been published onthe Paleozoic rocks of the area and Chamberlin (1877) remains the mostimportant reference.
The Maquoketa Formation was first described and named by White in 1870.The type section is an exposure of "bluish and brownish shales which weather into tenaceous clay," located on the Little Maquoketa River in NorthWestern Iowa (Agnew, 1955). The Iowa Section, described by Calvin (1906)consists of four distinct members: the Elgin (limestone & shale), theClermont (shale), the Fort Atkinson (limestone) and the Brainerd (shales). In Illinois, the Maquoketa Group is divided into 5 formations. Cape Limestone, Scales Shale, Fort Atkinson Dolomite, Brainerd Shale, and the NedaFormation in ascending order (Buschback, 1964). The Elgin and Clermontare recognized as members of the Scales Shale. In Wisconsin the ScalesShale, Fort Atkinson, Dolomite, and the Brainerd Shale are recognized,however, they are considered members of the Maquoketa Formation. In eastern Wisconsin the lithology of the formation is somewhat differentthan that at Oostburg on which the Wisconsin nomenclature is based. Thelower member of the formation, the Scales Shale, is usually covered byPleistocene deposits but occasionally construction work exposes the shalealong the east shore of Green Bay. It appears to consist mainly of shalewith argillaceous dolomite interbeds. Gypsum accumulations occur locally.
The overlying Fort Atkinson Dolomite Member consists of dolomite and dolomitic limestone interbedded with thin, poorly consolidated calcareous mudand clay layers. The member in places forms a bench below the escarpmentor a series of small cascades in stream beds. The upper member, theBrainerd Shale, consists predominantly of poorly consolidated calcareousmudstone with interbeds of dolomitic shale and argillaceous dolomite. Itusually weathers to form covered slopes. In Illinois, the Scales Formation is assigned an Edenian to Richmondian ageand the Fort Atkinson Dolomite and Brainerd Shale are considered Richmondian in age (Templeton and Willman, 1963). Specific age relationships ofthe formation in eastern Wisconsin are still open to some question, however,Fronting (1971) concluded from a study of conodonts that equivalent lithologic units are older in Wisconsin than in Iowa.The Maquoketa Formation overlies the Galena Dolomite and is overlain byMayville Dolomite or the Neda Formation.
The Neda Formation is a red shaly and oolitic iron rich deposit locally overlying the Brainerd Shale and underlying Alexandrian dolomite. Thename Neda was applied to the unit by Savage and Ross (1916). The thickest known Neda location is at Manitowoc, Wisconsin where 55 feet of the unit were found in a well (Rosenzweig, 1951). It is exposed at the surface ina few locations along the Silurian escarpment, the most prominent of whichare in the old mining district in Dodge County, Wisconsin. The formation is also exposed at Kittell Falls in southern Brown County. It is absent in Wequiock Falls but may be represented by a thin, red clay at Bay Shore Park in the northern part of the county. It is not recorded in any of thedeep water wells in the county.Most work suggests that the Neda is conformable with the subjacent MaquoketaFormation and unconformable with the superjacent Silurian dolomites. In Illinois, where the Maquoketa is considered to be a group, the Neda isincluded and assigned a Late Ordovician age (Templeton and William, 1963). In Wisconsin the formation is as yet not placed in either the Ordovicianor Silurian Systems (Ostrom, 1967).
The Alexandrian Series was named by Savage (1908) for exposures in AlexanderCounty, Illinois. Savage (1916) later revised the series and applied thename to rocks in Wisconsin. Currently, the Lower Silurian rocks of easternWisconsin are included in one formation, the Mayville Dolomite.
The Mayville Dolomite was described by Chamberlin (1877). In eastern Wisconsin the formation forms all or part of the Silurian Escarpment in BrownCounty. The rocks are, for the most part, buff weathering gray medium- tocoarse-grained dolomites with somewhat undulating bedding planes. Vugs,some of which are partially filled with calcite crystals are present in somebeds, and layers of modular chert are often present. The maximum reportedthickness of the unit in the area is approximately 230 feet in Door County(Sherrill, 1978).Willman (1973) correlates the Mayville Dolomite with the Wilhelmi Formation, Elwood Formation and the lower part of the Kankakee Formation in Illinois. In Iowa, the Mayville Dolomite correlates with the Edgewood, Kankakee and lower part of the Hopkinton formations (Willman, 1973). TheMayville Dolomite is undivided in Wisconsin, however, the same authorreports a few feet of Wilhelmi-like argillaceous Dolomite overlying theNeda Formation at Kittell Falls (spelled various other ways in the literature) in southern Brown Count~, Wisconsin.Middle Silurian, Niagaran, rocks overlie the Mayville throughout theregion.
Paula E. AllenUniversity of Wisconsin-Green Bay
The Maquoketa shale is a widespread and distinctive unit, recognized as aformation or a group over much of the Midwest (Templeton and Willman, 1963). In eastern Wisconsin, it is considered a formation consisting mainly of mudstone with variable amounts of carbonate rocks and divisible into three members. The complete section of the formation is not exposed, however, more than 200 feet have been penetrated by water wells east of the escarpment. A predominately carbonate member, the Fort Atkinson Dolomite separatestwo predominately fine clastic members; the Scales Shale below from theBrainerd Shale above.
In 1978 a large stromatolite was discovered in the Fort Atkinson in the bedof Wequiock Creek downstream from Wequiock Falls Wayside Park about 10 kmnortheast of Green Bay. A systematic collection was initiated of the associated lithologles and fossils in order to determine the depositional environments and their modern analogs. Repetitive sample collection was difficultbecause different portions of the outcrop were exposed from season to season.The stream's regime is such that spring or sometimes late summer floodingerodes exposed units and/or covers them with debris from up stream.
This study concentrates on approximately the uppermost two meters of theFort Atkinson Dolomite immediately below the Brainerd Shale. The contactbetween the two members is placed at the top of a series of more resistantcarbonate layers that form small cascades in the stream bed (Figure 1). Twosuch cascades are present in the study area below Wequiock Falls, both ofwhich are interpreted as being hardgrounds on the basis of sedimentary structures and associated depth related organic remains. Similar appearing layersare present at other localities, for example. In Baird Creek and most notablybelow Kittell Falls, but they have not been investigated in detail. Lateralchanges in lithology and community elements along strike are not well established because of the narrowness of the outcrop along the stream. The contact between the Fort Atkinson Dolomite and the underlying Scales Shale isnot exposed apparently occurring in a covered interval downstream.
Channel sampling of each unit was undertaken. Samples were cut with a diamond saw and either thin sectioned or cut into slabs for classification withthe aid of a binocular microscope. Unconsolidated material was wet sievedand the allochems of each size fraction determined. Conodonts were extracted by the method of Collison (1963). The ostracods are under investigation.
The upper Fort Atkinson at Wequiock Falls is dominated by four lithologictypes (Figure 2): biomicrite, biomicsparite, biosparite and micrite (Folk,1959). These lithologies reoccur and together with sedimentary structures,and fossil elements, can be used to divide the section into subunits. Inascending order seven subunits A to G are recognized.
The section under consideration at Wequiock Falls displays a set of intergrading llthologies, fossil elements, and sedimentary and biogenic featuresthat can be used to determine the general conditions and position of thedepositional environment in which it formed (Figure 3).
Relatively thin-bedded intergrading shelly and muddy lithologles representdeposition in barrier environments (Ziegler and others, 1974). The coarsening upward sequence and eventual establishment of a coral-algal assemblage is analogous to shoaling and coral patch community development on shoals ofthe South Florida shelf environment.
The discoidal shape of many of the coral colonies observed indicates significant water turbulence (Hatfield, 1968) and in addition, many of the smallerheads of Favestella are overturned. Some of the larger colonies are elongate,presumably the result of currents and their growth in close proximity totidal channels. The thriving community was eventually buried, at least inpart by the channel sands.
Coralline algae are important constituents of patch communities. Both calcareous algae and algal stromatolites are commonly found in subtidal channelsin recent environments (Frost, 1974; Gebelein, 1969; Ginsburg, 1964; Walker,1972; Ziegler and others, 1974). They have been described from ancientdeposits (Chafetz, 1973; Hatfield, 1968; Walker,.1972; Ziegler and others, 1974) and they can influence sediment deposition considerably. The algaeand stromatoiltes present in the section perhaps served as baffles or bindersor both and thereby affected the distribution of sediment.
Burrow structures in the crinoidal sands, which buried the coral community,most closely resemble Spongellomorpha paradoxica (Palmer, 1978). The similarities between Spongellomorpha and recent decapod crustacean burrows(Collionassa) have long been recognized and are believed to have similarorigins. No attached epifauna and only a few small borings exist on thehardground surface which suggest that cementation and subsequent burial wereboth rapid (Bromley, 1975). Calcarenites with low clay content appear tobe the most common type of hardground (Gokring and Kaznlorzak, 1974). Alow clay content enhances lithification, where as an abundance of clay prevents early diagenetic recrystallization (lbid, p. 958). The thick mudlayer on top of the hardground was deposited rapidly enough to preserve thepreburial topography as evidenced by the hummocky nature of the hardgroundsurface (Bromley, 1975). Recent cementation of lime sands is occurring inthe Persian Gulf in water between 3-5 meters deep (Bathurst, 1975).
Deposition of sediments in a subtidal channel is indicated by an abundanceof microscopic ostracods in the upper part of the section (Walker, 1972). Ostracods are generally more abundant in offshore shallow shelf environments'with a mixed sand and mud substrate where salinity remains relatively constantand nutrients are plentiful (Brondos and Kaesler, 1976; Scott and West, 1976).Channel environments with sand/mud bottoms are ideal for ostracod developmentbecause the currents are a natural energy subsidy continually supplyingnutrients and removing wastes.
After the channel sands of the upper portion of the section were depositedand burrowed, another hummocky hardground was produced. The predominantlycarbonate depositing environmental conditions were altered and the sequenceterminated by the influx of large amounts of fine clastic sediment.
The lithologles and associated fossil communities of the upper Fort AtkinsonDolomite at the Wequiock Falls section (Table 1) represent deposition inshallow subtidal environments similar to those found in South Florida.Changes in the facies mosaic through time were produced by the alternationof shoaling water, subtidal channels, and banks. It appears that the areawas variable over small lateral distances and temporally dynamic.
Grants in Aid of Research were provided by Sigma XI and the University ofWisconsin-Green Bay Research Council. Thin section work by the MilwaukeePublic Museum is greatly appreciated.
Created 25 July 2001, Last Update 13 January 2020
Not an Official UW-Green Bay site