Upper Ordovician and lower Silurian strata of eastern Wisconsin, by Ronald D. Stieglitz and Paula E. Allen (below)
Ronald D. Stieglitz and Paula E. Allen University of Wisconsin-Green Bay
In northeastern Wisconsin the Paleozoic is represented by strata of the Cambrian, Ordovician and Silurian Systems. East of the Fox River and the bay of Green Bay, in the area of this field conference, the bedrock is the Upper 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 including several 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 the work done on the Silurian rocks in Wisconsin has focused on the reef complexes. Shrock (1939) set the stage for the subdivision of the section and Mikulic (1977) discussed the rocks in southeastern Wisconsin. No recent comprehensive stratigraphic or paleontologic work has been published on the Paleozoic rocks of the area and Chamberlin (1877) remains the most important 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 North Western Iowa (Agnew, 1955). The Iowa Section, described by Calvin (1906) consists of four distinct members: the Elgin (limestone & shale), the Clermont (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 Neda Formation in ascending order (Buschback, 1964). The Elgin and Clermont are recognized as members of the Scales Shale. In Wisconsin the Scales Shale, 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 different than that at Oostburg on which the Wisconsin nomenclature is based. The lower member of the formation, the Scales Shale, is usually covered by Pleistocene deposits but occasionally construction work exposes the shale along the east shore of Green Bay. It appears to consist mainly of shale with 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 mud and clay layers. The member in places forms a bench below the escarpment or a series of small cascades in stream beds. The upper member, the Brainerd Shale, consists predominantly of poorly consolidated calcareous mudstone with interbeds of dolomitic shale and argillaceous dolomite. It usually weathers to form covered slopes. In Illinois, the Scales Formation is assigned an Edenian to Richmondian age and the Fort Atkinson Dolomite and Brainerd Shale are considered Richmondian in age (Templeton and Willman, 1963). Specific age relationships of the 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 by Mayville 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. The name 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 in a few locations along the Silurian escarpment, the most prominent of which are 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 the deep water wells in the county. Most work suggests that the Neda is conformable with the subjacent Maquoketa Formation and unconformable with the superjacent Silurian dolomites. In Illinois, where the Maquoketa is considered to be a group, the Neda is included and assigned a Late Ordovician age (Templeton and William, 1963). In Wisconsin the formation is as yet not placed in either the Ordovician or Silurian Systems (Ostrom, 1967).
The Alexandrian Series was named by Savage (1908) for exposures in Alexander County, Illinois. Savage (1916) later revised the series and applied the name to rocks in Wisconsin. Currently, the Lower Silurian rocks of eastern Wisconsin 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 Brown County. The rocks are, for the most part, buff weathering gray medium- to coarse-grained dolomites with somewhat undulating bedding planes. Vugs, some of which are partially filled with calcite crystals are present in some beds, and layers of modular chert are often present. The maximum reported thickness 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). The Mayville Dolomite is undivided in Wisconsin, however, the same author reports a few feet of Wilhelmi-like argillaceous Dolomite overlying the Neda Formation at Kittell Falls (spelled various other ways in the literature) in southern Brown Count~, Wisconsin. Middle Silurian, Niagaran, rocks overlie the Mayville throughout the region.
Paula E. Allen University of Wisconsin-Green Bay
The Maquoketa shale is a widespread and distinctive unit, recognized as a formation 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 separates two predominately fine clastic members; the Scales Shale below from the Brainerd Shale above.
In 1978 a large stromatolite was discovered in the Fort Atkinson in the bed of Wequiock Creek downstream from Wequiock Falls Wayside Park about 10 km northeast 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 difficult because different portions of the outcrop were exposed from season to season. The stream's regime is such that spring or sometimes late summer flooding erodes exposed units and/or covers them with debris from up stream.
This study concentrates on approximately the uppermost two meters of the Fort Atkinson Dolomite immediately below the Brainerd Shale. The contact between the two members is placed at the top of a series of more resistant carbonate layers that form small cascades in the stream bed (Figure 1). Two such cascades are present in the study area below Wequiock Falls, both of which are interpreted as being hardgrounds on the basis of sedimentary structures and associated depth related organic remains. Similar appearing layers are present at other localities, for example. In Baird Creek and most notably below Kittell Falls, but they have not been investigated in detail. Lateral changes 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 is not 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 with the aid of a binocular microscope. Unconsolidated material was wet sieved and 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 lithologic types (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. In ascending 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 features that can be used to determine the general conditions and position of the depositional environment in which it formed (Figure 3).
Relatively thin-bedded intergrading shelly and muddy lithologles represent deposition 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 of the 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 smaller heads of Favestella are overturned. Some of the larger colonies are elongate, presumably the result of currents and their growth in close proximity to tidal channels. The thriving community was eventually buried, at least in part by the channel sands.
Coralline algae are important constituents of patch communities. Both calcareous algae and algal stromatolites are commonly found in subtidal channels in recent environments (Frost, 1974; Gebelein, 1969; Ginsburg, 1964; Walker, 1972; Ziegler and others, 1974). They have been described from ancient deposits (Chafetz, 1973; Hatfield, 1968; Walker,.1972; Ziegler and others, 1974) and they can influence sediment deposition considerably. The algae and stromatoiltes present in the section perhaps served as baffles or binders or 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 similar origins. No attached epifauna and only a few small borings exist on the hardground surface which suggest that cementation and subsequent burial were both rapid (Bromley, 1975). Calcarenites with low clay content appear to be the most common type of hardground (Gokring and Kaznlorzak, 1974). A low clay content enhances lithification, where as an abundance of clay prevents early diagenetic recrystallization (lbid, p. 958). The thick mud layer on top of the hardground was deposited rapidly enough to preserve the preburial topography as evidenced by the hummocky nature of the hardground surface (Bromley, 1975). Recent cementation of lime sands is occurring in the Persian Gulf in water between 3-5 meters deep (Bathurst, 1975).
Deposition of sediments in a subtidal channel is indicated by an abundance of 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 constant and nutrients are plentiful (Brondos and Kaesler, 1976; Scott and West, 1976). Channel environments with sand/mud bottoms are ideal for ostracod development because the currents are a natural energy subsidy continually supplying nutrients and removing wastes.
After the channel sands of the upper portion of the section were deposited and burrowed, another hummocky hardground was produced. The predominantly carbonate depositing environmental conditions were altered and the sequence terminated by the influx of large amounts of fine clastic sediment.
The lithologles and associated fossil communities of the upper Fort Atkinson Dolomite at the Wequiock Falls section (Table 1) represent deposition in shallow subtidal environments similar to those found in South Florida. Changes in the facies mosaic through time were produced by the alternation of shoaling water, subtidal channels, and banks. It appears that the area was variable over small lateral distances and temporally dynamic.
Grants in Aid of Research were provided by Sigma XI and the University of Wisconsin-Green Bay Research Council. Thin section work by the Milwaukee Public Museum is greatly appreciated.
Created 25 July 2001, Last Update 17 November 2011
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