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 water- rock 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 Page 1 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 preserved in northeastern Wisconsin. They Figure 2. Stratigraphic column are interspersed with thick sequences of fine- for northeastern Wisconsin. grained sediments from Glacial Lake Oshkosh Modified from Mugel and Pratt, (Hooyer, 2007). These glacial deposits from the 1991; Harris and others, 1998. 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 Page 2 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 isotopically- dateable 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, Page 3 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.