Ronald D. Stieglitz, M. Carol McCartney and Paula E. Allen
On this field trip we will examine outcrops of Upper Ordovician and Silurian rocks along the face of the prominent escarpment on the east shore of Green Bay, and exposures of glacial materials, including the Two Creeks Buried Forest Bed, in eastern Wisconsin (Figure 1). The major objectives of this trip are: (1) to examine some sections of glacial materials that are significant to working out the stratigraphy of the red tills in eastern Wisconsin, (2) to present some preliminary results of current research on the sedimentology of a portion of the Maquoketa Formation, and (3) to examine the glacial and post-glacial effects on the bedrock of the area as well as the reciprocal influence of the bedrock, particularly the Silurian escarpment, on the Late Wisconsinan ice.
The bluffs which fringe much of the east shore of Green Bay are usually referred, to as the Niagaran Escarpment, however, this designation is incorrect because they are formed of Lower Silurian, Alexandrian, strata in this area. The lowland to the west, containing the bay and the Fox River Valley, Is developed on the Maquoketa Formation and older Middle Ordovician dolomites west of the river. Tills, outwash and lacustrine clays overlie the bedrock and beach features are found at several elevations throughout the Valley. East of the bluffs, Alexandrian or Middle Silurian, Niagarian, strata form the bedrock beneath glacial material of variable thickness.
Outcrops of the Maquoketa Formation are found along the shore of Green Bay as far north as the Little Sturgeon Bay area in Door County, Wisconsin, and in stream cuts and in quarries scattered throughout the. area west of the escarpment. No complete surface section of the Maquoketa Formation is known in the state, however, a complete section taken from a drill core at Oostburg In Sheboygan County is described by Froming (1971). One of the better exposures in this area, at Wequiock Falls, will be the first stop. The sequence of shale, argillaceous dolomite and dolomite is abundantly fossiliferous and displays some interesting sediment-organism relationships.
The second stop will be at Bay Shore County Park in northern Brown County. A road cut to provide access to the bay shore exposes Alexandrian rocks and upper Maquoketa beds as well as the talus at the base of the escarpment. Features observable at this atop provide a transition from the Paleozoic part of the trip to Pleistocene part by providing an opportunity to observe the effects of glacial and post-glacial conditions on the bedrock. The next part of the trip will focus on the Late Wisconsinan stratigraphy and history of eastern Wisconsin. After leaving the second stop we will drive south and east to the lake shore near Two Creeks. In so doing, we
Figure 1. Glacial geology of the Door County peninsula, from Hadley and Pelham (1976).
We will first pass across drift deposited by ice from the Green Bay Lobe and then enter onto an area most recently covered by Lake Michigan Lobe ice. Four stops, two in Green Bay Lobe deposits and two in Lake Michigan Lobe deposits, are described here. We will not see all the red tills in each lobe but we will see some of the typical relationships of the tills. Stop 3 at the lake shore exposure of the Two Creeks Forest Bed and the succeeding Stop 4 at the Valders Quarry are both well known localities that figured prominently in the establishment of the stratigraphic and chronologic terminology applied to Lake Wisconsinan materials and events in the Great Lakes region. The stratigraphic relationships between the two sites have recently been reevaluated resulting in an as yet not totally accepted revision of the previous nomenclature (for a complete discussion see the paper by McCartney elsewhere in this guidebook).
Stop 5 at the Brillion Quarry and Stop 6 at a pit east of De Pere provide an introduction to the till stratigraphy of the eastern limb of the Green Bay Lobe and further evidence tearing on the revision of Late Wisconsinan nomenclature.
The trip begins on the flood plain near the mouth of the Fox River. Subsurface borings for construction projects in the city have revealed that the channel and mouth of the river have migrated widely over the valley in the past. The route, primarily County Highway A, initially crosses flat, low-lying land along the southern and southeastern margin of Green Bay. The entire area was submerged during glacial and post-glacial times. In the vicinity of the University campus prominent abandoned shoreline features can be seen. North of the campus, recent utility work exposed Maquoketa Formation, lake beds and till. Several sets of shorellnes have been mapped (Goldthwait, 1907), however, the exact timing of the events and water levels still require additional work for specific definition.
We will turn eastward at Van Lanen Road and cross an undulating area of drift and lake deposits which rise to the base of the Silurian Escarpment. Weather permitting, we will walk into the first stop and meet the buses at the wayside. In the event of bad weather we will enter from the wayside. When we leave the first stop we will follow State Highway 57, which closely follows the edge of the escarpment, northward to the next stop at Bay Shore County Park.
Fort Atkinson and Brainard members of the Maquoketa Formation; Alexandrian Strata. S 1/2, SW 1/4, Sec. 7, T24N, R22R, Brown County, Wisconsin. Green Bay East 75-mlnute Quadrangle.
The rocks of the Maquoketa Formation are generally easily eroded and less resistant than the overlying Silurian dolomites and as a result form covered slopes. This outcrop is one of the better and most accessible of those exposed as streams flowing over the escarpment cut away the most resistant Silurian beds. Weather permitting, we will traverse upstream and examine the upper part of the Maquoketa Formation and the Mayville Dolomite (Figure 2) The Neda Formation is not present.
The traverse will begin in the uppermost part of the Fort Atkinson Member of the Maquoketa Formation which represents the last carbonate deposition before the Silurian. Hardgrounds which contain Spongll lomorpha type burrows and a coralline zone, that has yielded large stromatolltes, tabulate corals, coralline algae, horn corals, and numerous brachipods are present. The contact between the Fort Atkinson and the overlying Brainard Shale Member is also exposed.
The Brainard Member is largely soft green mudstone but thin interbeds of dolomitic shale and argillaceous dolomite containing brachiopods, arborescent bryozoans, cornulltes worm tubes, and occasionally stromatolltes are found.80-
Figure 2. Generalized stratigraphic section at Wequiock Falls
About 2 meters of greenish-gray thin bedded dolomite lies above the mudstone. This unit forms the lower part of the steep valley walls just west of the bridge and the middle part of the face of the falls. The contact between the Maquoketa Formation and the Mayville Dolomite is tentatively placed below this unit. The top of the section is 3 to 4 meters of gray medium-to-coarse-grained dolomite characteristic of the Mayville Dolomite.
Alexandrian strata; Geomorphology of the Silurian Escarpment. NW 1/4, SW 1/4, Sec. 14, T25N, R22E, Brown County, Wisconsin, Dyckesville 75-Minute Quadrangle.
Alexandrian strata are exposed along a road cut through the escarpment to provide access to boat launching facilities constructed on the bay shore. In this area the escarpment front is mantled by coarse talus and loose material which have also been truncated by the road. The parking lot and breakwater at the base of the bluff are built on artificially filled material. The grade of the road, although still steep, was lowered after Its initial construction because of the difficulty of towing boat trailers back up the slope.
The free face of the bluff consists of about 105 meters of Mayville Dolomite which can be subdivided into three informal units (Figure 3). Theuppermost unit C capping the lip of the escarpment is, for the most part, light gray, fine-grained, thin-bedded dolomite. Some beds are very dense whereas others contain open vugs. The middle unit B is composed of dolomite Interbedded with nodular layers of gray to black chert. The entire unit breaks into small pieces and often has a fractured and rubbly appearance.
Figure 3. Generalized stratigraphic section at Bay Shore County Park.
It forms a reentrant in the cliff face. The lowermost unit A consists of gray mostly fine-grained, thick-bedded dolomite. Some vugs and nodular layers of chert are present.
Immediately below unit A a soft red clay several centimeters thick forms a small reentrant. This material appears to be a thin representation of the Neda Formation.
The base of the bluff is mantled by talus, but in a few places less than a meter of greenish-gray, fine-grained, laminated dolomite outcrops. These beds are dolomitic layers in the Brainard Member of the Maquoketa Formation The layers show anomalous dips and appear to be slightly deformed perhaps by the movement of blocks of the overlying dolomite.
The effects of solution on the dolomite is evident both at the surface beneath the thin soil and along prominant joints. These features of the dolomite, well displayed at this location, graphically illustrate why the area east of the escarpment is concerned about ground water pollution.
Figure 4. Relationships of the large blocks of Silurian dolomite to the escarpment, underlying rocks and talus.
Large blocks of dolomite, that have separated and moved away from the face of the escarpment are also visible (Figure 4). The blocks appear to have slid along the surface of the underlying shale or argillaceous beds and may have been forced by ice wedging (Stieglitz, Moran and Harris, in press). Elsewhere in the park. In the picnic and camping areas, a series of open joints are present which separate similar size block that remain in contact with the escarpment face. The outermost of these blocks has moved down about a meter and a similar distance toward the bay shore. Back from the escarpment edge several other open but narrower joints are present that separate blocks of similar width. Downslope adjustments of the blocks and coarse talus has produced what appear to be pressure ridges along the talus slope. The slope is nearly stable and talus production and block movement may have occurred shortly after the last recession of the ice from the bay.
From Bay Shore Park we will turn right on Highway 57 and go about one mile to County Highway T where we will turn left (south). We will follow Highway T for about 20 miles to the city of Denmark. One mile south of Highway 57 the road passes over a hill of Silurian dolomite. Glacial deposits are primarily ground moraine from Green Bay Lobe ice.
Just south of the small community of Poland the topography becomes much more rugged and we will cross the Neshota River which is thought to be a glacial drainageway which carried water to the southeast. We are traveling near the maximum southward and eastward extent of the Greatlakean ice of the Green Bay Lobe in the area as marked by the Denmark Moraine (Evenson and Mickelson, 1974). The moraine complex near Denmark is the northern extension of the interlobate moraine and is an older feature deposited between Green Bay Lobe and Lake Michigan Lobe ice during an earlier advance.
At Denmark we will turn onto Highway 141, cross the moraine and then turn east (left) onto County Highway BB. We will follow BB across Port Huron age Lake Michigan Lobe deposits onto younger Two Rivers Till near Stop 3 on the lake shore.
Figure 5. Generalized stratigraphic section of the lake bluff at Stop 3 north of Two Creeks, Wisconsin.
Significance: This locality is important because the dated forest bed (11,800 yrs BP, Broecker and Farrand, 1976) provides an absolute date for late-glacial events in the Lake Michigan Basin. Thwaites and Bertrand (1957) correlated the upper red till here with the red till at Valders. It is now believed that these tills do not correlate and that the Twocreekan was one of many retreat intervals which separated minor readvances (Evenson and others. 1976).
From Stop 3 we will follow Highway 42 southward over Two Rivers till. Near the city of Two Rivers we will cross the Two Rivers Moraine and enter an area of glacial lake sediments deposited in the lowlands of the Twin River Younger red till has not been found overlying the lacustrine sediments suggesting that post-Two Creekan ice did not cross the area. Just south of the city of Manitowoc we will turn west on State Highway 151 following it, first across Lake Michigan Lobe drift of Port Huron age and then over the Interlobate Moraine to Stop 4 at Valders.
Valders Till, unnamed till; Silurian dolomite. SW 1/4, NW 1/4, Sec. 32, T19N, R23E, Manitowoc County, Wisconsin. Valders 7 5-Minute Quadrangle.
This is also a well known and important site often mentioned in the literature (Figure 6). The bedrock here is Middle Silurian, Niagaran, in age. Directional indicators on the bedrock surface evidence two separate ice flow directions.
Figure 6. Generalized stratigraphic section of the till relationships in the Valders Ouarrv.
Description: In parts of the quarry, the unnamed Middle Woodfordian age (Gary) till overlies the Silurian dolomite; in places the Late Woodfordian Valders till directly overlies the bedrock. Beneath the buff, sandy Middle Woodfordian till are striations on the dolomite which trend nearly north-south. Two sets of striations (north-south and east-west) underlie the red, clayey Valders till. Cresentic gouges have also been observed (Stieglitz, Moran and Quigley, 1978).
Significance: The striations indicate the ice advanced from the north and deposited the sandy, unnamed till and that it later advanced from the east and deposited the red Valders Till. Thwaites (1943) named the Valders till for this locality and later (Thwaites and Bertrand, 1957) correlated it to the red till over the Two Creeks Forest Bed.
From Valders we will follow Highway 148 north to Highway 10. We turn west and follow Highway 10 to the city of Brilllon. During of the trip we will return to an area covered by deposits laid down by the east side of the Green Bay Lobe.
Chilton Till, Wayside Till; Silurian Dolomite. SW 1/4, SW 1/4, Sec. 24, T20N, R20E, Calumet County, Wisconsin, Brilliol 7 5-Minute Quadrangle.
At this stop we will examine the stratigraphic relationships and characteristics of several tills overlying Silurian bedrock (Figure 7). The effect of the escarpment on ice flow will also be considered.
Description: There are two tills here on the dolomite bedrock. The lower till is the Wayside till; it is overlain by the Chilton till. The Wayside is the Middle Woodfordian sandy till and the Chilton is a Late Woodfordian clayey till. The sandy clay Branch River till, which lies below the Chilton till north of Brillion, is missing at this locality.
Figure 7. Generalized stratigraphic section of the till relationships in the Brilllon Quarry.
Significance: The ice that deposited the Chllton till during the late Woodfordian (late Port Huron) behaved differently than earlier or later advances. The ice extended further south than the early Port Huron advance (which deposited the Branch River till) but did not extend as far east or west in the northern portion of the lobe. Tee flow was controlled by the Niagara escarpment and ice left the lowland only where the escarpment is very low. This suggests the ice had a lower profile than did ice of the preceding or succeeding advance. The lower profile may have been due to a higher water content at the bed. References: McCartney and Mickelson, in preparation.
From the Brillion Quarry we will follow Calumet and Brown County Highway PP north to State Highway 32. After traveling less than a mile on Highway 32 we will turn eastward (right) on Sportsman Drive and follow several town roads to County Highway X where we will turn east (right) to Stop 6. The route northward roughly parallels the Silurian Escarpment which lies to the west. The highway passes over the edge of the escarpment near Hilly Haven Golf Course and crosses a small sediment filled reentrant. From this area to the pit at Stop 6 we will be traveling on unconsolidated material deposited along the west face of the escarpment.
Glenmore Till over Chilton Till, separated by dated Two Creekan Forest Bed; outwash deposits. Southeast edge of French Grant 38, Northwest corner, T22N, R21E, Brown County, Wisconsin, De Pere 75-Mlnute Quadrangle.
At this site we will examine several tills and thick outwash deposited along the face of the Silurian Escarpment (Figure 8). The outwash is highly dissected by streams flowing off the escarpment toward the northwest. The pit exposes channels, cross-bedding and large fault blocks of sand and gravel. Part of the pit is being filled by paper mill sludge. Directly across High way X to the south is the Brown County East Landfill site.
Figure 8. Generalized stratigraphic section of the buried forest and the till relationships at the Scray Hill pit. References: Kessenich, pers. comm., 1976 McCartney and Mickelson, in preparation, Mickelson and Evenson, 1975
Created 25 July 2001, Last Update 17 November 2011
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