Karst features of a glaciated dolomite peninsula, Door County, Wisconsin

Geomorphology, 4 (1990) 37-54 Elsevier Science Publishers B.V., Amsterdam

37

Karst features of a glaciated dolomite peninsula, Door County, Wisconsin Scot B. Johnson a and Ronald D. Stieglitzb aMinnesota Department of Natural Resources, Mississippi Team, Route 2, Box 230, Frontenac, MN 55041 (U.S.A.) bNatural and Applied Sciences, ES 317, University of Wisconsin-Green Bay, Green Bay, W154311- 7001 (U.S.A.) (Received January 25, 1989; accepted after revision May 10, 1990 )

ABSTRACT Johnson, S.B. and Stieglitz, R.D., 1990. Karst features of a glaciated dolomite peninsula, Door County, Wisconsin. Geomorphology, 4: 37-54. A geologic investigation of the northern part of Door Peninsula, Wisconsin for a state funded water quality project revealed that karstification of the Silurian aquifer is more extensive than previously believed. Sinkholes and small insurgent features, solution modified crevices, pavements, caves and springs were inventories and mapped. These features are generally smaller and less densely developed than those in most limestone terranes; however, they are important to the geomorphology and water quality of the peninsula. Continental glaciation has strongly influenced both the distribution and the present surface morphology of the karst features. Ice scour has formed a stepped bedrock topography, contributed to pavement formation and may have removed some preglacial features. Deposition has plugged and masked features in places. In addition, subglacial water circulation, and ice loading and unloading may have influenced karst development.

Introduction Eastern Wisconsin is a karst area that has only recently been recognized and investigated (Fig. 1). The karst features are developed in the Silurian dolomites that form the bedrock of the area (Fig. 2 ). These formations are for the most part, dense crystalline dolomites with little primary porosity (Table 1 ). The vertical development of the karst is limited by the underlying Ordovician Maquoketa Formation, a shale-rich unit, that separates the dolomite aquifers from deeper, primarily sandstone aquifers. Surface water and groundwater flow is strongly influenced by the Niagara Escarpment which is the dominant topographic feature of the county. In 1984, the University of Wisconsin-Green Bay began a study of the northern portion of the Door Peninsula of eastern Wisconsin as

part of a priority watershed project funded by the State of Wisconsin. The Priority Watershed Program provides funds to inventory and assess factors that influence water quality and to share the costs of remedial actions with rural landowners (Bachhuber and Schuster, 1986). Before the Priority Watershed Project, very little had been published in the scientific literature concerning karst on the peninsula. Martin ( 1916) inferred that the lack of caves was a result of deep scouring of the dolomite by glaciers. Bretz (1942) noted that Horseshoe Bay or Tecumseh Cave located on the west side of the peninsula is an excellent example of a cave with joint controlled walls and ceiling cavities. A cave survey of the area lists the location of 30 littoral and solution generated caves along with 25 sinkholes (Henning et al., 1972 ). Stieglitz et al. ( 1980 ) reported periglacial expansion joints and related gliding in the

38

S.B. JOHNSON AND R.D. STIEGLITZ

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Fig. 1. Location map of the Door Peninsula,Wisconsin. southern portion of the peninsula where the Maquoketa Formation crops out below the Silurian dolomites. Glide block caves like those

reported by Hedges ( 1972 ) in Iowa have been noted by us in southern Door County along the Niagaran Escarpment. To date only one has

39

KARST FEATURES OF A GLACIATED DOLOMITE PENINSULA tq29E

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been reported in northern Door County (Johnson, 1986). Barden (1980)concluded that the majority of caves and sinkhole in the county are influenced by joints.

Stieglitz (1984) was the first to publish a descriptive report of the karst features observed in Door County. Wiersma et al. ( 1986 ) investigated the hydrologic function of karst fea-

4O

S.B. JOHNSON AND R.D. STIEGLITZ

TABLE 1 Geologic materials of Door County, Wisconsin.

Q

Holocene Series

U A T E

Unconsolidated, deposits including alluvium, beach sand, peat, and modern soils

Pleistocene Series

Kewaunee Formation

R

Horicon

N A

Formation

O-lOm

Red clay till Liberty Grove Member

R

0-18 m O-lOm

Sandy, pebbly till and outwash

Y Engadine formation

Niagaran Series

Burnt Bluff Group

Alexandrian Series 0

0-15 m

Manistique

Cordel Member

Buff, thin wavy bedded cherty fossiliferous dolomite

23-27 m

Formation

Schoolcraft

Gray thin bedded cherty fossiliferous dolomite

10-15 m

Hendricks Formation

Gray thin-to-thick dolomite interbedded with bluish- gray dense dolomite

Byron Formation

Bluish-gray dense dolomite

35-40 m

Mayville Formation

Gray thick-bedded cherty rough weathering dolomite

65-70 m

Neda Formation

Lenticular red-brown clays and fossiliferous ironstones

0-2 m

Maquoketa Formation

Blue-gray shale and fossiliferous dolomite

R

D 0 V

Brown medium bedded white weathering dolomite

6-24 m

Cincinnatian Series

tures, groundwater flow rates, groundwater flow directions, and the pollution potential of an area in southern Door County. Rosen ( 1984 ) related geologic parameters to the formation and location of known karst features, and termed the region glaciokarstic in accordance with Sweeting's ( 1973 ) classification.

100-125 m

Data collection methods The comprehensive mapping of the karst in northern Door County began with a thorough search of published and unpublished literature (Johnson, 1987; Link et al., 1978; Henning et al., 1972; Rosen, 1984). A considerable

41

KARST FEATURES OF A GLACIATED DOLOMITE PENINSULA

amount of information concerning karst features came from personal interviews with landowners. Interviews consisted of a set of predetermined questions concerning karst features and the individual's familiarity with the area. Information was then field checked for accuracy, location and description. Fracture traces were also m a p p e d using techniques and definitions developed by Lattman ( 1958 ), and others (Lattman and Parizek, 1964; Trainer, 1967; Wobber, 1967). In addition to the field mapping of karst features and air-photo mapping of fracture traces, areas of exposed bedrock and historic karst feature development were also delineated. Historic karst areas were defined as areas of emphemeral karst features which have been obliterated naturally, or filled and graded by landowners to avoid accidents and to maintain or expand tillable acreage. Such areas are considered significant to the evaluation of groundwater systems and pollution potential. Types of karst features in northern Door County Classification schemes of features based on geomorphic parameters are often used in karst research, however, these systems are difficult to apply everywhere. This difficulty may be attributed to the transitional and interrelated nature of karst geomorphic forms, their complex origins, and interactive geologic influences. One of the most widely used classification schemes is that of Sweeting (1973) in which karst landforms are divided into five major groups based on size, process, form and position. Some of the features observed in northern Door County are difficult to place in Sweeting's scheme. Another classification system in which features are grouped by topographic position and hydrologic function was proposed by Mylroie ( 1981 ). White (1988) provides a thorough discussion of karst features and presents a geometrical classification based on width, length and depth relationship.

Ford and Williams (1989) have produced a comprehensive treatment of karst landforms and processes. Five specific medium-scale karst features were m a p p e d and inventoried for the watershed project: (1) sinkholes (dolines), (2) crevices (grikes), (3) dolomite pavements, (4) springs, and (5) caves. The first four are considered landforms of the epikarst zone (Williams, 1983 ). Small-scale karren features were noted but not described or mapped in detail.

Sinkholes Sinkholes are generally described as small to moderate size basins or funnel-shaped depressions formed by solution related processes. Four types of sinkholes were recognized in the study area ranging in size from less than a meter in diameter and depth to more than 20 m in diameter and over 2 m in depth. Suffosion sinkholes are formed where unconsolidated materials subside into a solutionwidened joint or crevice in the bedrock. The intersection of crevices is not a prerequisite for suffosion sinkhole formation. The process may occur gradually as fine materials are piped into the subsurface or it may be discrete episodic events. In the latter case, water flow and groundwater fluctuations in the subsurface opening undermine a plug of unconsolidated material. The void is propagated upward to the surface where it is expressed as a new sinkhole. The depression is almost entirely within the regolith and, therefore, the size of the features is related to the thickness of the overburden. Suffosion sinkholes are generally small and are the most abundant type found throughout the study area. Newly formed subsidence sinkholes of this type are often nearly circular and steep-sided. Mass wasting and sheet wash may modify the walls into a cone-shaped depression. Surface water insurgence to these sinkholes is initially of small volume; however, the drainage area gradually increases as material is lost into the bedrock opening. In many cases,

42

this type of sinkhole remains small because of the very shallow soils over much of the peninsula. In fact, they are commonly plugged or tilled over by farmers and destroyed. Solution sinkholes are characterized by a cone-shaped depression within the bedrock. They are apparently formed by the localized dissolution of the dolomite and are often obscured by a thin veneer of colluvium. Sinkholes of this type are not very c o m m o n in northern Door County, seldom display a smooth funnel-shaped, and are usually associated with a crevice or the intersection of joints. Small features formed by suffosional and dissolutional processes were distinguished during field investigations and m a p p e d separately. These isolated features include open joints less than 10 cm wide and 50 cm in extent, and nearly cylindrical pits a few centimeters in diameter that appear to be important inflow locations. Although they are smallscale forms of the two previously described types of sinkholes, they were differentiated because they occur in areas of disturbed overburden and where water is channeled, such as road ditches or grass water ways. Collapse sinkholes are formed by the collapse of bedrock into a subsurface void. These sinkholes begin as nearly vertically walled jointand bedding-plane-controlled features. The form is modified over time by surficial processes and further collapse. Eventually the sinkhole becomes conical in shape with bedrock breakdown partially filling the sinkhole floor. Only two collapse sinkholes were mapped, both of which form entrances to caves located near the Niagaran Escarpment. Alluvial sinkholes have a well-defined stream channel leading to the sink opening. A covering of alluvium usually masks the weathered bedrock surface. These sinkholes range up to 2.5 m deep, by 23 m across and are hydrologically mature suffosion or solution sinkholes that drain relatively large surface areas. Such features appear to be very significant to aquifer recharge, especially during spring snowmelt.

S.B. JOHNSON AND R.D. STIEGLITZ

Crevices Crevices or grikes develop as water penetrates fractures in the bedrock and flows vertically or laterally along the joint opening. The dissolution of dolomite from the two faces of the joint widens the fracture. The rims of crevices may be rounded or angular depending on local conditions, and small-scale karren are often developed on the walls. Widened joints are found throughout northern Door County, especially in woodlots and fields with shallow soils, along crests of outcropping ridges, near ancient shoreline bluffs and in areas of dolomite pavements. Crevices may be found as isolated features or in groups or clusters covering large areas. A few crevices appear to be located in a favorable hydrologic position to collect significant amounts of runoff. Most, however, are found in relatively gently sloping woodlots or on topographic highs where overland flow is restricted and surface drainage is at a minimum.

Dolomite pavements Dolomite pavements are generally level areas of exposed bedrock that exhibit a variety of karst features. These unmantled areas are found primarily in two topographic settings. One setting occurs on hilltops and on the edges of bedrock outcrops, the other is associated with highly creviced areas of very low relief. Karren features, such as runnels, pans, splits, pits and grooves similar to those reported by Pluhar and Ford (1970) from the Bruce Peninsula of Canada, are numerous on dolomite pavements. Wide and laterally extensive crevices may be so numerous in pavements that they divide the dolomite into isolated blocks termed clints. Several pavements display unusual tunnel-like features with circular crosssections of 15 to 20 cm in diameter, extending up to a half meter into the bedrock before terminating at a bedding plane. Not all are circular, however, and some appear similar to the

KARST FEATURES OF A GLACIATED DOLOMITE PENINSULA

kamenitzas reported from limestone pavements in Britain by Rose and Vincent (1986a). These features have not been studied in detail and their association with veins within the dolomite has not been documented as it has been in the British examples. Several models of pavement development are reviewed by Rose and Vincent (1986b). Their measurements of grikes indicates that the pavements of northwest England reflect two distinct phases of pavement development, one before and one following the last glaciation of the area. Other lines of evidence reported on in this paper suggest that pavements on the Door Peninsula may also be of different ages. Springs

Most springs in the area are diffuse and ephemeral. During snowmelt, small springs and seeps occur in fields and woodlots, on hill slopes, and along dolomite cliffs. Some are found in the headwater reaches of stream channels, bubbling up through the stream alluvium. Other springs are found along shorelines and offshore in Green Bay and Lake Michigan (Bradbury, 1982 ). These may be related to karst development when water levels were lower during postglacial or even interglacial periods. Ca ves

There are three types of caves known in northern Door County: littoral (sea) caves, groundwater dissolution caves, and one example of a glide block cave. Littoral caves are formed by wave action upon the side of a dolomite cliff. Most such caves are only small rock shelters; however, some extend to lengths of 12-15 m. The largest caves are formed in cliffs along one of the dominant joint trends and in some instances they intersect groundwater generated voids. Many of the littoral caves formed along shorelines of ancient post-glacial

43

lakes and are now above the present level of Green Bay. Dorchester Cave, the second longest cave in northern Door County, is the only known glide block cave. The northeast wall of the N20 °W trending cave appears to have moved vertically downward about 15 cm and horizontally about 1 m to the north-northeast. Neither fault breccia nor slickensides are found on the bedding plane roof. The cave, located in the fossiliferous Cordel Member of the Manistique Formation, is not near the escarpment nor any basal glide stratum such as the Maquoketa Formation, and, therefore, not easily explained by gravity sliding. The mechanism of movement has not been determined but it quite probably is an ice shove or drag p h e n o m e n o n (Schroeder et al., 1986). Little dissolution activity is apparent, the cave floor is littered with breakdown, and there are no cave decorations, although many fossils can be observed. Caves formed by groundwater dissolution are more numerous than once thought. They include: Tecumseh Cave, Bear Cave, Large Pit and Small Pit (when the depth of a cave entrance exceeds the longest horizontal dimension, the cave is called a pit). Small Pit is entered via a subsidence sinkhole, whereas the entrances to Bear Cave and Large Pit are collapse sinkholes. Cave passages are often filled with glacial materials, breakdown, or sealed by illuviated clays. Tecumseh Cave is the longest known cave in Wisconsin with approximately 740 m of explored passages and forty-nine rooms (Kox, 1986). The cave entrance is in the face of the Niagara Escarpment and the cave appears to have been glacially truncated. It has developed primarily along dissolution-widened bedding planes. D o m e d rooms are formed where vertical dissolution-widened joints intersect the passageway. Great varieties of flowstone, stalagmites, stalactites, columns, and rimstone dams occur in parts of the cave. The cave is considered to have formed by a combination of phreatic and water table influences accord-

44

ing to Ford and Ewers' (1978) classification scheme. Fracture traces

It has been shown that a parallelism exists between fracture traces and major joint trends, the former being the surface manifestation of major joints or groups of closely spaced joints (Lattman and Nickelson, 1958; Lattman and Matzke, 1961 ). This is important hydrologically because joints, or groups of joints, have greater recharge, transmissivity and storage capacity than the intervening dolomite (Lattman and Parizek, 1964). Our goals in mapping fracture traces were to identify areas with a high potential for groundwater contamination and karst feature development. Areas with dense fracture traces have been used in past studies to predict the formation of suffosion sinkholes and to discover existing but unrecognized karst features (Littlefield et al., 1984; Brook and Allison, 1983). The majority of fracture traces mapped in northern Door County were from vegetative and soil tonal differences that apparently reflect the presence of bedrock fractures. There is. however, little correlation between fracture traces and areas of known karst interface features. It is unclear why there is little correlation but it may be related to the drainage characteristics of fractures or to the fact that the fractures have been plugged with glacial and/ or lake sediment and sufficient time has not passed for sinkholes to have formed. Orientation of karst features

Sinkhole elongation and crevice orientations were measured in the field with a Brunton compass. The orientations of fracture traces and of Sherrill's ( 1978 ) joint directions were also plotted. The crevice, fracture trace, and joint rose diagrams clearly illustrate that two joint orientations, N70°-N80°E, and N 2 0 ° - N 3 0 ° W are the dominant trends (Fig.

S.a. JOHNSON AND R.D. STIEGLITZ

3). The fracture trace diagram denotes another concentration of measurements at N50 °60°E that was not observed in the field but which corresponds with joint orientations in the surrounding region (Holst, 1982). The sinkhole rose diagram is dominated by the N70°-80°E joint trend. This over-representation or skewness toward the N70 °-80°E trend in the sinkhole population may indicate that joints with this orientation are the dominant water transporting joints. The orientation of domed rooms in Horseshoe Bay Cave also follows the N70°-80°E trend. Further analysis may indicate that joints with this orientation are responsible for a large proportion of the vertical recharge, and may also help to identify master joints or other karst-feature populations. Master joints are defined by Wopat (1974) to be features which are vertically extensive and persist through time. Distribution of karst features

The karst features of northern Door County are not distributed uniformly throughout the study area. Field mapping has revealed that there is a greater density ofkarst features south of the village ofFish Creek (Fig. 2 ). Maps prepared at a scale of 1:155,000 and 1:24,000 clearly show the variable distribution (Stieglitz and Johnson, 1986). Small-scale summary maps at page-size are more difficult to interpret because of the elongated shape of the peninsula and the number and variety of features plotted; however, the variation in the density of features is distinctly visible (Fig. 4 ). The distribution of karst features in the area studied is primarily influenced by hydrologic setting, overburden characteristics, and glaciation, and to a lesser degree by bedrock lithology and land use. Unconsolidated material The type and thickness of the overburden have a profound effect on karst development

45

KARST FEATURES OF A GLACIATED DOLOMITE PENINSULA

Joint Orientation

Fracture Trace Orientation

from Sherril11978 117 measurements

864 measurements

t

.t

Sinkhole Orientation

Crevice Orientation

24 measurements

139 measurements

Fig. 3. Rose diagramsof the orientationsof sinkholes,crevices,joints and fracture traces mapped in the study area. and expression. Trudgill ( 1985 ) discusses the relationship between soil and karst development in detail. He reiterates Ford's (1983) finding that a calcareous till can shield the bedrock from aggressive water by neutralizing the water before it reaches the soil/bedrock interface. Shielding of the bedrock in the county has been substantiated by the preservation of fresh glacial striations and polish under till sections less than half a meter thick. The distribution of karst features in relation to soil types in Jacksonport and Egg Harbor Townships is shown in Table 2. In that area of northern Door County, 88% of all features are expressed in three soil units; the Longrie, Summerville and Namur Formations. All three soils

formations have developed within glacial till and are in the same soil association. Areas of the thin Summerville and Namur soils contain the vast majority of crevices and creviced areas. Areas of Longrie soil contain fewer features but exhibit a proportionally higher number of sinkholes which may be a function of the thicker profile of this soil. Table 3 illustrates the relationship of features and soil thickness. Specific comparisons cannot be made because areas represented by each thickness interval were not calculated; however, the proportionally larger number of sinkholes in the deeper soils is apparent. Comparatively minor amounts of erosion will reveal crevices in shallow soils whereas in deep soils loss of material

46

S.B. JOHNSON AND R.D, STIEGLITZ

NORTHERN DOOR COUNTY, WISCONSIN KARST

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Fig. 4. Karst feature distribution map of the northern Door Peninsula, Wisconsin.

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47

KARST FEATURESOF A GLACIATEDDOLOMITE PENINSULA TABLE 2 Distribution of karst features in relation to soil types, Jacksonport and Egg Harbor Townships. Soil

Depth to bedrock (cm)

Reactivity

Drainage

Percent oftotal karstfeatures

Bonduel

50-100

No C horizon Solum mildly alkaline to medium acid

Somewhat poorly drained Permeability 1.5-2.0 c m / h

Bonduel Shallow Varient

25-50

No C horizon Solum mildly alkaline

Somewhat poorly drained Permeability 1.5-2.0 c m / h

2

Mildly to moderately alkaline

Well drained to moderately well drained Permeability 1.5-15 c m / h

3

Emmet

> 150

Longrie

50-100

Mildly to moderately alkaline

Well drained Permeability 1.5-15.0 c m / h

21

Namur

12- 30

No C horizon Solum neutral to mildly alkaline

Well drained Permeability 1.5-5.0 c m / h

25

Mildly to moderately alkaline

Well drained Permeability 1.5-5.0 c m / h

5

No C horizon Solum slightly acid to neutral

Well drained Permeability 1.5-5.0 c m / h

42

Omena Summerville

> 150 25-50

TABLE 3 Karst features and soil thickness. Soil Namur Bonduel Shallow/ Summerville Bonduel/Longrie Emmet/Omena

Depth (cm)

Sinkholes

Crevices

Creviced areas

Total

< 25 25- 50

16 36

33 52

37 53

25 44

50-150 > 150

32 16

13 2

5 5

23 8

Values in percentage of individual types or total number of features in each category.

into the fracture will more often be reflected at the surface as isolated suffosion sinkholes. More features were found on gently sloping surfaces than on steep slopes (Table 4). Isolated crevices far exceed all other features in the 6-12% slope category, perhaps reflecting karst development along a buried bedrock step. Trudgill (1985) found an overall decrease in solution activity beneath soils on steeper slopes in comparison to more gently sloping areas. This relationship may be attributed to less in-

TABLE 4 Karst features and slope. Slope (%)

Sinkholes

Crevices

Creviced areas

Total

<2 2- 6 6-12 > 12

37 60 2 1

39 56 14 1

18 78 2 2

32 62 5 1

Values in percentage of individual types or total number of features in each category.

48

filtration of potentially aggressive water on steep slopes. In addition, increased mass wasting keeps the dolomite covered and inhibits the formation of an acidic A horizon.

Hydrogeologic influences The type and density of karst features are influenced by hydrologic factors, the most notable of which is the groundwater gradient. The steepest gradients are associated with the Niagara Escarpment where the water table may be more than 45 m below the surface. Relatively steep gradients also occur on the divides near to the bedrock valleys, particularly those containing the larger inland lakes. More and larger features occur in such areas because infiltrating water can circulate rapidly through the dolomite. This results in the exposure or unplugging of buried or infilled features and facilitates postglacial dissolution. With few exceptions, the large alluvial sinkholes are located in the central to western reaches of those valleys where gradients to the bay or Lake Michigan are also relatively steep. In the eastern part of the peninsula, the hydraulic gradients are generally lower and much of the lowlands that were once covered by the waters of postglacial lakes are now wetlands. The concentrations of creviced areas mapped on that side of the peninsula may have been exposed by reworking of the glacial deposits by the lake waters. Significant hydraulic gradients are also developed in the vicinity of smaller scale scarps resulting from glacial erosion and surrounding some high-capacity water wells. Each of these situations will be described in the following sections.

Glaciation Continental glaciation has had a major effect on karst of the area. Rosen (1984) characterized the area as glaciokarstic based on Sweeting's (1973) definition and classifica-

S.B. JOHNSON AND R.D. STIEGLITZ

tion scheme. Ford (1977) is more explicit in defining the relationship between karst formation and glaciation. Ford includes the readaptation of an area to a karstic function postglacially as glaciokarstic. In later papers, Ford (1983, 1987) lists the effects that glaciers may have on a karst region. Northern Door County is a true glaciokarstic region with features that have resulted from some of the effects recognized by Ford. Deeper and more extensive glacial scouring in the northern part of the study area is suggested by the fact that stratigraphically higher units, such as the Engadine Formation, are not widely present. Scouring is also suggested by the greater number of re-entrant points or low spots in the Niagara Escarpment in the north, which would have facilitated access of the ice to the interior of the peninsula. Glacial scouring may be responsible for the erasure of some karst features that predate the middle Woodfordian glacial advance identified by Schneider ( 1981 ) and could be at least partially responsible for the lower density of features in the northern portion of the area. Glacial scouring of pre-existing karst features may influence the extent to which the features are expressed postglacially. Figure 5 suggests two examples of the impact of glacial scouring. In the first case (A), a solution sinkhole developed in dolomite, before glaciation, is scoured away and later expressed as a subsidence sinkhole. In the second instance (B), a well-developed subcutaneous zone, the layer of weathered bedrock above the saturated zone, is reduced by glaciation to a zone of poorly developed widened joints that may form a dolomite pavement if the drift cover is thin. Glacial scouring above prominent bedding planes has formed a series of bedrock benches or steps along the sides of preglacial stream valleys and uplands. Preferential scouring of stepped areas may be related to the competence of preglacial dissolution weathered bedrock. On the top of the interfluves and the edges of the dolomite steps, the local hydraulic gra-

KARSTFEATURESOF AGLACIATEDDOLOMITEPENINSULA

A

Before

49

B

Fig. 5. Diagrammatic representation of the effects of glacial scour on pre-existing karst features.

dients are steep, postglacial dissolution is enhanced, and karst features are very dense. These locations often exhibit pits, runnels, tunnels, and other small-scale karren that have not been mapped or studied in detail. These stepped karst assemblages, or schichttreppenkarst areas, were first recognized in southern Door County by Rosen (1984) and are very similar to Sweeting's (1973) bench and scar areas in England (Rosen et al., 1987 ). Figure 6 is a hypothetical cross-section of schichttreppenkarst in northern Door County that illustrates some possible relationships

among karst features, bedrock, topographic position, overburden thickness, and major joints. The cross-section illustrates the difficulties associated with describing the karst hydrogeology and geology based on the surface expression of karst features. At location A, the step is not expressed topographically because of the thick overburden. At B, a master joint is expressed as a subsidence sinkhole. At C, dolomite pavement forms where soil was eroded and/or transported through crevices, or where drift was not deposited. At D, crevices have formed on the edge of a step where local hydraulic gradients are high. At E, dolomite pavement occurs on the edge of a step where soil is thin or absent. Here a master joint cannot be differentiated from other widened joints at the surface. At F, a crevice near the edge of a step is expressed as a subsidence sinkhole because of the thick soil layer. At G, dolomite pavement forms without the presence of a major joint. At H, a preglacial master joint is not expressed because of glacial filling. Northern Door County schichttreppenkarst is in an immature but dynamic state and, in all probability, is moving toward the mature development found by Sweeting in Yorkshire, England.

NORTHERN DOOR COUNTY SCHICHTTREPPENKARST

sinkhole /B

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dolomite ~ pavement J1G

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Fig. 6. Diagrammatic cross-section of schichttreppenkarst in northern Door County illustrating the location and morphological expression of karst features. See text for explanation.

50

S.B. JOHNSON AND R.D. STIEGLITZ

Lithology A comparison ofkarst features with bedrock formations did not reveal a distinct association with any particular lithologic unit. Barden (1980), however, noted that caves and larger karst features are found in the Byron and Hendricks Formations probably because those formations comprise most of the escarpment in northern Door County and are associated with its high hydraulic gradients. Compositional variations may be more important than now perceived, particularly in the formation ofkarren. Chemical analyses of the dolomite units are limited. Those available suggest that the Manistique Formation is somewhat more dolomitic than the underlying units. Composition of the Byron Formation (Cal.o3Mgo.97) and the Hendricks Formation (Cal.olMgo.99) approximate a 1 : 1 ratio. Manistique values available range from (Cal.olMgo.99) to (Cao.94Mgl.o6). An additional consideration is that the facies within the units have not been well delineated and they may exert considerable control on dissolution activity.

Landuse practices Table 5 compares sinkholes and crevices with general landuse practices in northern Door County and shows that sinkholes predominate in fields, and crevices predominate in woodlots. This relationship reflects the fact that level areas of thick soil have been tilled, whereas shallow creviced areas have been left TABLE 5

in wooded lots or forests. Subsidence sinkholes are favored as the soil is disturbed and drainage altered by cultivation. Irrigation can also influence the distribution of karst features. The steep hydraulic gradients induced by the high-capacity wells, coupled with the increased artificial recharge and surface disturbance, appear to have stimulated the formation of suffosion sinkholes on the property of a large nursery in Sturgeon Bay Township. Rock dissolution may be enhanced; however, the dominant effect is unplugging of infilled or sealed joints. Time of karst formation

Bedrock dissolution and the modification of a karstic area due to geologic influences are ongoing processes. It is, therefore, neither an easy nor straightforward task to determine the time of karst formation. It was once believed that caves were lacking in eastern Wisconsin because of intensive glacial erosion; however, several large caves do exist in northern Door County and seem to have survived the effects of glaciation. Caves, master joints, and large isolated features on topographic highs probably predate, at least, the last ice advance. Insurgent points, most karren features, and small karst depressions (Rosen et al., 1987) appear to post-date the last ice advance. Collapse sinkholes, swallets in stream channels, and solution sinkholes may be either pre- or postglacial in age. Alluvial sinkholes and suffosion sinkholes have formed postglacially, however, they may be surface expressions of preglacial karst features which have become functional and unclogged of drift.

Karst features and landuse. Landuse

Sinkholes

Crevices

Field Woodlot Dilch Dolomite Pavement Other

52 7 30 1 10

14 48 18 10 10

Values in percentage of features in each category.

Karst drainage zones

The Bruce Peninsula in Ontario, Canada is very similar to the Door Peninsula in karst development, bedrock geology, and geologic history. The Bruce Peninsula is also dominated topographically by the Niagara Escarpment,

KARSTFEATURESOF A GLACIATEDDOLOMITEPENINSULA

51

NORTHERN DOOR COUNTY, WISCONSIN K A R S T D R A I N A G E ZONES

•

KEY Holokarst A Holokarst B Fluvio-Karst Fluvial Drainage Primary Spring MILES 5

0 0

;

KM

10

10 15

Fig. 7. Karst drainage zones and primary springs of the northern Door Peninsula, Wisconsin.

has been glaciated numerous times, and is mantled by very thin till. Cowell and Ford (1983) were able to characterize karst drainage zones on the Bruce Peninsula based on field observations and geochemical analyses of spring water. Characterization and delineation of karst drainage zones in northern Door County have been modeled after their work. Northern Door County has been divided into four drainage zones; holokarst A, holokarst B, fluviokarst, and fluvial drainage (Fig. 7 ). The

holokarst drainage zone parallels the Niagara Escarpment where hydraulic gradients are steep and karst integration is best developed. In this zone, there is no overload flow or surface drainage. Precipitation and meltwater enter the many vertical widened joints and sinkhole at or just below the surface. In this area the runin component of recharge is dominant. In the holokarst B zone, surface runoff is contained within closed drainage basins and is channeled primarily by intermittent streams

52

into alluvial sinkholes. This zone corresponds closely to a large portion of Sherrill's map where the water table is more than 30 m below the surface. The fluviokarst zone supports overland flow and intermittent surface channel flow but a significant proportion is directed into the bedrock through karst interface features. In the fluvial drainage zone, surface channel flow and surface water are perennial, indicating that there is little loss to interface features a n d / o r that the area is one of groundwater discharge. The karst drainage zones may be considered as environmental units. The holokarst regions have a high potential for infiltration and movement of contaminants throughout the aquifer. The fluviokarst region is one of lesser, yet significant potential for aquifer contamination. The fluvial drainage zone has the lowest potential for groundwater contamination. Few interface recharge features have been found in the lacustrine and alluvial deposits of the latter zone. It is possible to reduce the risk of aquifer degradation through planned land-use decisions that shift the location of potentially contaminating practices to less vulnerable areas. The use of the fluvial drainage zone as a relatively lower pollution potential environmental unit may be a practical solution for the siting of landfills and sewage treatment plants, and the spreading of holding and septic tank wastes. The problems associated with the spreading of manure and pesticide use can not be shifted. These potential contaminating practices must be addressed through site-specific handling techniques and landuse controls. Karst drainage zones, used as environmental units to assess contamination potential may also help to prioritize remedial actions.

Summary Karst features are smaller and less developed in northern Door County than those in limestone regions, such as Florida or other re-

S.B. JOHNSON AND R.D. STIEGLITZ

gions of Paleozoic rocks such as Iowa, Indiana and Kentucky. They are, however, more widespread and densely spaced than was appreciailed before our study. The somewhat restricted nature of karstification can be explained by the lower solubility of the dolomite in comparison to limestone (Thrailkill, 1977 ), and by severe and frequent Quaternary glaciation of the area. The expression of karst landforms is affected by glaciation in several ways. First, erosion by ice of the Green Bay Lobe may have removed many pre-existing karst features. Glacial scouring seems to have been more intense in the northern portion of the area which may explain the lower density of karst features there. This observation suggests that the greater karstification and the larger features found in the southern portion of the study area may predate the Woodfordian ice advance onto the peninsula from the Green Bay lobe (Schneider, 1981). Secondly, glacial erosion has produced a series of bedrock steps throughout the peninsula. Relatively steep hydraulic gradients have developed along the bedrock steps which result in an increased rate of dissolution and concentration of karst features. The effect is most pronounced on steps that lack of protective mantle of calcareous till either because of nondeposition or subsequent erosion. The resulting schichttreppenkarst is characteristic of the alrea. Thirdly, glacial deposition has buried and/ or sealed many karst interface features. This is evidenced by fracture traces unrelated to open interface features, dense fracture sets revealed in fields by vegetation patterns during dry periods, and by the large number of small, often ephemeral, subsidence sinkholes that form as tile buried features become unplugged and begin to function as part of the karst drainage system. Although more tentative, two other effects of glaciation listed by Ford ( 1983, 1987) may also be present. The water-bearing zones de-

53

KARST FEATURES OF A GLACIATED DOLOMITE PENINSULA

scribed by Sherrill ( 1978 ) m a y be partially attributable to the flexing caused by glacial isostatic adjustments. Additionally, large individual features, such as widened joints on topographically high areas may have been formed by a preglacial hydrologic regime. Isolation of the feature may be a result of glacial erosion or may reflect a subglacial meltwater system karst recharge point. Collapse sinkholes and solution sinkholes are not a b u n d a n t in the study area. Alluvial sinkholes are relatively large and hydrologically important because they collect water from significantly large areas. Suffosion sinkholes are generally shallow and small in diameter because the thickness of the soil is not great enough for a large size cone to form. Sinkholes are preferentially elongated in a N 7 0 ° - 8 0 ° E direction suggesting that joints of that trend are important conduits for vertical and perhaps horizontal water movement. Cultivation, ditching, establishment of grass waterways, and heavy pumping o f groundwater can stimulate the development of sinkholes and insurgent points. Geomorphologic defined karst drainage zones recognized as environmental units with different potentials for pollution will greatly aid the efforts to protect the quality of groundwater in this and other karst areas.

Acknowledgements

This study was funded in part by the Wisconsin Priority Watershed Program through a contract with Door County, Wisconsin. The cost of the preparation of some of the figures was provided by the Natural and Applied Sciences Concentration of the University o f Wisonsin-Green Bay. We are also grateful to William Schuster, Door C o u n t y Soil and Water Conservation Department, for his help in all phases of the study and to Bryn Fosburg for assistance with field work.

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