Geologic History of New Hope Cave, Manitowoc County, Wisconsin
John A. Luczaj ([email protected]) and Ronald D. Stieglitz ([email protected])
Department of Natural & Applied Sciences
University of Wisconsin - Green Bay
2420 Nicolet Drive
Green Bay, WI 54311
INTRODUCTION
This article presents the preliminary results of an ongoing study of the geologic
history of New Hope Cave in Cherney Maribel Caves County Park, Manitowoc County,
Wisconsin. The sediments in the cave contain an important record of the events
responsible for filling the cave, and may shed light on the geological processes
operating on the surface. A brief introduction to the regional geology of northeastern
Wisconsin is given here to help the reader place this research into a broader context.
GEOLOGIC SETTING
The bedrock of Northeastern Wisconsin is primarily composed of the
sedimentary rocks dolostone, shale, and sandstone that were deposited during the first
half of the Paleozoic Era between
540 and 350 million years ago
(Figure 1). Most of these were
deposited in warm shallow oceans
when this part of North America
was situated very near the equator,
and at times when global sea level
was episodically higher. The
limestone deposited in those
oceans was altered into a new type
of magnesium-rich carbonate rock,
known as dolostone (made from
the mineral dolomite), by waterrock interaction processes early in
their history (Luczaj, 2006). As a
result, northeastern Wisconsin’s
caves are hosted exclusively by
dolostone.
In total, up to 600 meters (~2,000
feet) of Paleozoic sedimentary
rocks are preserved in the area
Figure 1. Bedrock Geology of Wisconsin
above the Precambrian basement
showing the distribution of Paleozoic rocks.
rocks (Figure 2). These Paleozoic
Figure courtesy of WGNHS.
rocks thickens to as much as 4
kilometers under the central part of
the lower peninsula of Michigan. These rock units have been tilted slightly to the east
and thicken to as much as 4 kilometers in the ancestral Michigan basin, a folded
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structure underlying lower Michigan. This is
the reason that the well-known Niagara
escarpment trends northeastward from the
region south of Fond du Lac to the tip of Door
County and beyond. The Niagara escarpment
is essentially a prominent cliff or ridge of
Silurian dolostone that stands high because it
is more resistant to erosion than the softer
underlying Maquoketa Shale.
The youngest rocks preserved in
eastern Wisconsin are the Devonian (359-416
million years ago) sediments near Milwaukee.
Rocks younger than this that were deposited
in the region have since been removed after
long periods of river erosion and, more
recently, glacial erosion. As a result, there is a
large gap in the geologic record of Wisconsin
that stretches from about 360 million years
ago until around 20,000 years ago when the
glaciers began their most recent retreat from
the area. Glacial sediments from at least
three major advances of the Late Wisconsin
lobes of the Laurentide Ice sheet are
Figure 2. Stratigraphic column
preserved in northeastern Wisconsin. They
for northeastern Wisconsin.
are interspersed with thick sequences of fineModified from Mugel and Pratt,
grained sediments from Glacial Lake Oshkosh
1991; Harris and others, 1998.
(Hooyer, 2007). These glacial deposits from the
Pleistocene Epoch rest directly upon freshly scoured
bedrock and conceal much of the underlying geology of the region, including much of
the karst landscape.
The glacial till that covers the Cherney Maribel Caves County Park is mapped as
the Valders Member of the Kewaunee Formation. This unit was deposited by the Lake
Michigan Lobe of the Laurentide ice sheet between about 12,000 and 13,000 14C years
before present (Clayton and others, 2006). Little is known about the age or character of
glacial sediments that might have been present over the park before this event.
KARST DEVELOPMENT IN NORTHEAST WISCONSIN
Caves have been found throughout much of Wisconsin where carbonate bedrock
is present. They are best accessible in regions where no glacial materials are present
or in areas with thin glacial drift. In northeastern Wisconsin, there are two principal
bands of carbonate rocks (in this case dolostone) that are susceptible to the
development of karst features such as caves, sinkholes, and solution-enlarged
fractures. To the east of Lake Winnebago and the Fox River Valley, the Silurian
dolostones extend all the way to the Lake Michigan shoreline. To the west of the Fox
River Valley, two layers of Ordovician age dolostone bedrock of the Sinnipee and Prairie
du Chien Groups are quite extensive and cover most of the region as far west as
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Shawano and New London. While they have essentially the same chemical
composition, the difference in development of karst features in each group of rocks (or
lack thereof) is striking. A wide range of karst landforms has been recognized and
mapped during studies focusing on groundwater resources in the Silurian bedrock (e.g.,
Wiersma and others, 1984; Johnson and Stieglitz, 1990). These include small karren,
dolines, solution-enlarged fractures, and dry valleys. In contrast, very few karst features
have developed in the Ordovician dolostones of northeastern Wisconsin. In fact,
extensive research conducted by Luczaj (e.g., Luczaj, 2006, ongoing) has shown that
fractures within Ordovician dolostone still preserve relatively unweathered sulfide
minerals along vertical fractures in the region. There may be several reasons for this
systematic difference,
but perhaps the
existence of the
Maquoketa Shale has
had the most profound
effect. Prior to
Pleistocene glaciation, it
is likely that the
Maquoketa Shale
extended considerably
farther west than its
present location. This
would have, in effect,
isolated the underlying
Ordovician dolostone
from aggressive waters
at the surface or in
Figure 3. Solution-enlarged fracture in Silurian dolostone
shallow groundwater. In
from southern Brown County. Photo by John Luczaj.
addition, the topographic
elevation of the Silurian
might have also amplified the karst development to the east because of high
groundwater flow rates (Bloom, 1998). In any event, it seems clear that the Silurian
rocks east of the Fox River Valley are host to the majority of the karst features of
northeastern Wisconsin.
One of the most difficult questions to address while unraveling the karst history of
northeastern Wisconsin is the question of its timing. Because the process of karst
development is primarily one of bedrock dissolution, geologists are left with little
evidence that records precisely when this takes place. To establish an age range for a
particular event, geologists use cross-cutting relationships between rocks of known age
as well as numerical age dates determined for certain materials such as isotopicallydateable materials.
Most of the karst developed in northeastern Wisconsin appears to predate the
Pleistocene glaciation, which likely began about 2.4 million years ago in this part of
North America (Clayton and others, 2006). A wide variety of debris has been observed
in sinkholes, caves, and solution enlarged joints of northeastern Wisconsin. It appears
likely that the upper portion of the pre-glacial karst landscape has been removed,
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leaving open or sediment filled features (Figure 3). Unfortunately, no isotopic age dates
have been obtained from speleothems or cave decorations, and organic materials have
yielded age dates that are too young to indicate much about the timing of cave
development.
WHAT HAS BEEN FOUND IN EASTERN WISCONSIN CAVES?
A wide variety of materials have actually been discovered in caves or other karst
features from Door County in the north to places as far south as Waukesha County,
Wisconsin. This paper does not attempt to provide a comprehensive review of all
Wisconsin cave discoveries. Instead, it is an attempt to illustrate the variety of materials
that can be found and to provide examples of how this information can help geologists
reconstruct the history of our region.
Probably the oldest karst feature preserved in the Silurian bedrock of eastern
Wisconsin was found in the Halquist Quarry in Waukesha County, Wisconsin. Early
Devonian non-marine plant fossils were found atop an estimated 16 meter section of
mudstone in a large, solution-enlarged fissure in the Silurian Waukesha and Racine
formations. These non-marine plant fossils record the earliest plant fossils preserved in
Wisconsin karst (Kuglitsch and others, 1998; Jahren and others, 2003).
When one thinks of caves, stalactites, stalagmites, and other cave decorations
often come to mind. These precipitates of calcium carbonate (CaCO3) usually form
when the CO2 is released from solution when water enters a cave (Bloom, 1998). In
some cases, these “speleothems” can be age dated to reveal the timing of precipitation
and have major implications for our understanding of past climate changes (e.g.,
Drysdale and others, 2007).
Bones, teeth, wood, pollen, and other organic materials have revealed much
about the history of caves throughout the world. In Door County, Wisconsin, the
Brussels Hill Pit Cave preserves a sequence of cave sediments and flowstone that are
interpreted to be approximately 10,000 years old. This cave, which is the deepest in
Wisconsin at 28 meters, preserves numerous remains of wood, leaves, shrews, bats,
deer, bear, beaver, muskrat, and otter. Two 14C dates on organic sediments at the 28
meter depth were dated at 671 and 1,820 14C years before present. While the record of
organisms preserved at this location appears much younger than any glacial activity in
the region, important information about Wisconsin’s pre-European settlement faunal
assemblages can be learned (Brozowski and Day, 1994).
A more striking example of how a surface faunal assemblage can be preserved in
a karst feature is from the Pipe Creek Junior Sinkhole in Grant County, Indiana (Farlow
and others, 2001). There, an assemblage of numerous plant and animal species from
part of the Pliocene Epoch (~4 to 5 million years ago) were found in a sinkhole
uncovered by quarrying operations. The animals, some of which are now extinct,
included frogs, turtles, fishes, birds, snakes, as well as small and large mammals. The
deposit is a unique window into the continental biota and the climate present in the
continental interior of eastern North America during the Tertiary Period.
The contents of most northeastern Wisconsin caves usually consist of sediments
washed in from above, in the case of sinkholes and fissures, or less commonly
sediments deposited by water flowing through cave passages. Sediments deposited by
flowing water can preserve spectacular information about the direction, speed, and
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hydraulic characteristics of the system. It is this type of system that was responsible for
sediment deposition in the New Hope Cave at Cherney Maribel Caves County Park.
NEW HOPE CAVE
Cherney Maribel Caves County Park is located along the western side of the
West Twin River in northwestern Manitowoc County, Wisconsin. The New Hope Cave,
located near the northern border of the park, was discovered by Norb Kox and Paul
Pryes on February 5, 1984 (Figure 4). The cave contains over 400 feet of passageway,
Figure 4. Map of New Hope Cave courtesy of Letha Welch, slightly modified.
which was filled or nearly filled with sediment. Although excavation began in 1984,
more systematic excavation began in the past few years with floor to ceiling excavation
beginning in 2006 (Henry Welch, written communication). Systematic excavation of the
unconsolidated sediments in the cave is being performed by members of the Wisconsin
Speleological Society and the Friends of Maribel Caves and is facilitated by the
Manitowoc County Parks System. Once opened, passageways vary in height between
about 5 feet and 13 feet. The sediments in the cave have a variety of grain sizes,
indicating a series of events, some of which were high velocity flow events.
Sedimentary strata in the cave are somewhat variable in extent and thickness, but a
general picture of the stratigraphy shows three main packages of sediment that have
been documented in the cave.
Stratigraphy of Cave Sediments
The lowermost package is 0 to ~4 feet in thickness and is made mostly of fine to
medium grained sand, which overlies laminated greenish to red clay and sand
intercalations near its base. The lowermost package of sediments is the most visually
distinctive of the three because of its bright orange appearance (Figure 5). It exhibits
Page 5
S
C
W
S
Middle Package
Lower Package
Dolostone Wall
Rock
C
Figure 5. Exposed cut showing the lower and middle sediment packages at profile 7 in
the Southeast Passage. “C” indicates conglomerate layers, “S” indicates sandy layers
of the middle package, and “W” indicates a water escape structure. Photo taken on
11/12/2006, courtesy of Letha Welch.
what is known as liesegang banding, a chemically precipitated banding of iron oxides
that occurs in fluid saturated sediments or rock. The top of this orange unit shows a
distinct erosion surface and pieces of dark orange flowstone, indicating that the erosion
occurred after development of the iron oxide staining. Numerous pieces of collapsed
ceiling rocks up to a few feet in length (~1 meter) are preserved at the interface between
the lower and middle packages of sediment.
The middle package of sediments is present throughout most of the cave, and
has a light buff to dark brown color. Overall the package was best preserved in the
Southeast passage, where it was at least 6 feet thick. Two very coarse grained
conglomerate layers were preserved in this package of rocks, each of which is about 1
foot thick. The first gravel layer occurs at the base, directly above the orange colored
sediments of the lower Package (Figure 5). Overlying each of the two gravel layers is a
thick sequence of well sorted, buff to brown colored sand that illustrates tabular crossbedding and ripple cross-laminations.
Page 6
The conglomerate layers in the middle package of sediments consist of coarse
gravels up to 5 inches (13 cm) in diameter, with a sandy matrix between the grains.
These gravels are apparently the first record of glacially-derived sediment in the cave.
Clasts of gabbro, granite,
and other non-local rocks
are found in these two
layers, likely indicating
glacial transport to the
area from locations
hundreds of kilometers to
the north. The sand layers
above each gravel layer
show spectacular channel
scours, fluid-escape
structures, and angular
blocks of dark brown
coarse sand that were
apparently transported
while frozen (Figure 6).
Figure 6. Dark brown angular clasts of unconsolidated
These sand layers
sand. The angular shape and lack of a cohesive matrix
preserve sedimentary
suggests that these blocks were frozen when deposited.
structures such as cross
Block on left is approximately 4 inches long. Photo by
bedding and climbing
John Luczaj.
ripples that indicate flow
was apparently moving
eastward during deposition in the cave, toward the bluff along the West Twin River
valley. The sands of the middle package were likely deposited by water flowing at 10 to
20 cm/second, but the coarse gravels likely required flow rates upward of at least 1-2
meters/second (Bloom, 1998).
The upper package of sediment was best exposed in the West Passage at the
time of this article. This package also rests above a substantial erosion surface
developed on the top of the middle package of sediments. These sediments consist of
laminated brown clays with interbedded poorly sorted gravel containing clasts up to an
inch (~2 cm) in diameter. Overall, this package appears better preserved to the west,
but most attempts to carve vertical exposures of this unit have not been successful due
to collapse of the water-saturated sediment.
Above the three packages of sediment in the cave is a flowstone layer, usually
one-half to one inch (1-2 cm) thick, but rarely as thick as 4 inches (J.D. Skattebo,
personal communication). This layer was present in most parts of the cave where
excavation has been completed. In some places, collapsed ceiling rock (Silurian
dolostone) is preserved beneath the flowstone at or near the top of the upper
sedimentary package. A loamy layer occurs in the Southeast passage above the
collapsed ceiling rocks, but below the flowstone layer. Additional ceiling collapses have
been found on top of the flowstone layer.
Page 7
Water Escape Structures
At numerous locations along the
Southeast passage, vertical conical
and cylindrical-shaped water
escape structures have been
identified. They appear to emanate
from either the lowermost gravel
layer in the middle package of
sediments or from fractures at the
floor of the cave. They range in
size from a few inches wide and a
foot high to as much as three feet
in diameter and at least 6 feet high.
The largest water-escape structure
is over 3 feet wide and continues
Figure 7. A large gravel water escape structure in vertically from the bedrock floor
the Southeast passage. In this view, the crossthrough the middle package of
bedded sands have been excavated around the
sediments at the end of the
structure, which is about 1 foot in diameter. Photo southeast passage (J.D. Skattebo,
courtesy of Letha Welch and J.D. Skattebo.
personal communication). One of
these was a cylinder of gravel that
was cored by sand (Figure 7; J.D. Skattebo, personal communication). Others are
funnel shaped structures that widen upward, and some of these has graded bedding
that displays a fining-upward trend in grain size.
These structures are interpreted as water-escape structures for two main
reasons. First, they emanate from zones of high permeability that are capable of
carrying significant amounts of water. In addition, they cut across less permeable, finer
grained sediments toward the surface. One of these even shows evidence for at least
two episodes of water escaping.
Flowing water along the surface of the cave sediments is unlikely to have
generated these structures. Seasonal fluctuations of the water table are also not likely
to have generated the differential pressures needed to cut through the upper layers. A
more likely source for the high-pressure water is one of glacial origin. The
interconnection between karst systems and subglacial drainage systems was reported
by Sharp and others (1989, page 129) for a glacier in Switzerland. High-pressure water
at the base of the glacier during transport or melting might produce the hydraulic head
necessary for the formation of such structures.
Age and Origin of Sediments in the Cave
In an alcove north of West Dome #1 in the West passage, several disseminated
bones were found by J.D. Skattebo and Al Schema during January and February 2008
(Figure 4). The bones were found within sand and gravel layers of the upper package
of sediments, about 2 inches (5 cm) below the flowstone layer. While bones of several
animals have been found both above and below the flowstone throughout the cave,
including bats and possibly a woodchuck, these bones appeared to be contained within
the gravel. As a result, an age estimate for the upper sediment package can be made.
Page 8
The bones found have not yet been precisely identified, but they appear to
include thin bat bones, two bat mandibles, and
two larger vertebrae (Figure 8). The two
vertebrae were selected by Beta Analytic, Inc.
of Miami, Florida as ideal for radiocarbon
dating. According to the laboratory, the two
bones contained plenty of carbon for an
accurate measurement of carbon-14 (14C)
content. The bones were prepared for analysis
by standard methods, which involved bone
collagen extraction with alkali treatment. The
isotopic composition of the collagen was
analyzed to determine whether alteration had
likely occurred. The 13C/12C ratio for the
collagen was -20.2‰, which indicates that the
bone was likely not altered and that it is likely
to yield an accurate age date. The remaining
carbon was analyzed to determine the
radiocarbon age using standard accelerator
mass spectrometry (AMS) methods.
Figure 8. One of two vertebrae found
The Conventional Radiocarbon Age
near the top of the upper sediment
reported by Beta Analytic, Inc. is 4,930 ± 40
package, about 2 inches beneath the
years before present (BP). Because the
14
flowstone layer. Vertebra is
percentage of C relative to other isotopes of
approximately 1 cm long, and is of an
carbon in the atmosphere has not been
unknown animal, likely a mammal
constant through time, the radiocarbon age
larger than a bat, but smaller than a
needs to be calibrated. Using the INTCAL04
woodchuck. Photo by Dan Meinhardt.
Radiocarbon Age Calibration, Beta Analytic
determined the 2! calibrated result to be 5,590
to 5,740 Calendar years BP. As a result, the age of this material is one of the oldest, if
not the oldest, bone recovered from a Wisconsin cave.
Although the bones were transported and deposited along with the sediments,
they showed little or no abrasion. The gravel layer in the upper sediment package in
which the bones were discovered is less than or equal to 5,590 to 5,740 years old. The
nature of the non-abraded vertebrae and the unbroken thin bat bones suggests that
these bones were reworked only by a relatively mild event that modified the uppermost
portion of the upper sedimentary package. The middle and lower sediment packages in
the cave are likely considerably older than 5,590 to 5,740 years. While it is not
presently possible to interpret the exact origin of each of the layers, certain constraints
can be placed on their formation.
The middle sedimentary package contains sands and gravels deposited under
high enough flow rates to indicate very rapid flow of water in the cave. The gravel
layers contain small glacial erratics that indicate a glacial origin for the sedimentary
debris. After deposition of the middle sedimentary package, high-pressure water must
have escaped from high permeability fractures and gravel layers in communication with
those fractures. It seems likely that these sediments were deposited during or before
Page 9
the final melting of the last ice sheet that occupied the area. The presence of
apparently frozen blocks of sand in the cave indicates a significantly colder climate,
given that the temperature inside the cave has not been observed below 42°F (5.5°C),
despite exterior temperatures of -21°F (-29.5°C) (J.D. Skattebo, personal
communication).
The lowermost package is more difficult to interpret. The collapsed ceiling
interval and the liesegang banding preserved in the lowermost sedimentary package
occurred before the deposition of the middle package of sediments. While it is not easy
to determine when deposition of this package occurred, it is possible that the sediment
was deposited either during an interglacial period or perhaps even before the first
glaciers reached the region during the Late Pliocene or Early Pleistocene epochs. Nonlocal pebbles and grains have not been documented from this lower sedimentary
package, and a substantial amount of time likely elapsed to record the thick flowstones,
ceiling collapse blocks, and the liesegang banding of iron oxides. It is important to note
that the chemical and hydrologic conditions responsible for forming the liesegang
banding have not reoccurred for at least as long as the middle sedimentary package
has existed (> 5,590 years).
SUMMARY AND FUTURE WORK
The New Hope Cave in Cherney Maribel Caves County Park contains a
spectacular record of water-transported sediments that preserve a wide array of
sedimentary structures. Three main sedimentary packages are recognized in the cave
require that at least some of the sediment to be at least 5,590 to 5,740 years old.
Water-escape structures and high flow regime sedimentary structures are consistent
with glacial activity in the region. The lower unit in the cave displays an unusual orange
iron oxide banding, and is likely considerably older than the other two units, possibly
even pre-Pleistocene.
Work continues on the excavation along the West passage and a new branch
from the Southeast Passage, deemed “Hidden Passage” (Figure 4). Further grain size
and composition analyses will be performed, along with a search for more datable
materials such as bone, wood, or pollen to help constrain the age of sediments in the
cave.
ACKNOWLEDGEMENTS
The authors thank members of the Wisconsin Speleological Society and Friends
of Maribel Caves for access to the cave, collection of materials, and excavation work.
We especially thank J.D. Skattebo, Letha Welch, and Henry Welch for their reviews and
efforts, without which this article would not have been published.
The radiocarbon age dating was paid for by the UW-Green Bay Research
Council, Friends of Maribel Caves, and the Wisconsin Speleological Society.
Undergraduate student Jen Wessel also participated in sample collection, and
interpretation of sedimentary structures.
Page 10
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