ILSG - Terrace Bay 2019

Pre-Meeting trips - Sunday April 23

Trip 1. Precambrian geology of the Chippewa River Valley.

Trip Leaders: Rob Lodge, Bob Hooper (both UW-Eau Claire).

This trip will explore some of the most southerly exposures of the Precambrian rocks of the Canadian Shield. Along the Chippewa River, there are numerous erosional outliers of the Paleoproterozoic Pembine-Wausau and Archean Marshfield terranes of the Penokean Orogen. Additionally, there are mafic intrusions associated with the Mid-Continent Rift. Since the last time that ILSG has visited the region in the 80’s and 90’s, our understanding of the geologic history and tectonic significance of rocks exposed along the Chippewa River and surrounding drainages has vastly improved. This trip will discuss some of the “recent” and “actually recent” data collected and how our understanding of the southernmost exposures of the Canadian Shield has changed. The trip will start by examining the nonconformity beneath the Cambrian Mt. Simon Formation and stop at several classic Precambrian outcrops that are exposed around several dams on the river while presenting our new interpretations of the Precambrian tectonic development of the southern Lake Superior region. Note that this trip will include moderate hiking on uneven, rocky terrain adjacent on the banks of rivers and streams. One location will require a 2 km return-trip hike and a small stream crossing.

REVISED* Trip 2. Paleozoic stratigraphy of NW Wisconsin.

Trip Leader: Carsyn Ames (Wisconsin Geological and Natural History Survey)

This trip will investigate exposures of Cambrian through Ordovician siliciclastic and carbonate rocks in western Wisconsin. These rocks were deposited during the major Cambrian – Ordovician Sauk and Tippecanoe marine transgressions onto the Laurentian interior. They compose the regional aquifer system, host disseminated sulfide mineralization related to the Mississippi Valley Type sulfide deposits of southwest Wisconsin, and their variable hardness in part controls the formation of ledges and hillslopes in the fluvially-dissected Driftless Area of southwestern Wisconsin. Cambrian sandstone of the Elk Mound Group overlies Precambrian basement rocks across the Great Unconformity and is locally mined as proppant for fracking in the oil and gas industry Above the Elk Mound Group, carbonate- and glauconite-rich siliciclastic sediments of the Tunnel City and Trempealeau Groups were deposited. Together, these rocks compose the lower sandstone aquifer of Wisconsin. The Ordovician Prairie du Chien Group dolomite, deposited at the end of the Sauk transgression, unconformably overlies the Cambrian sandstones, and is notable for karsting. Sandstone of the middle Ordovician St Peter Formation (Ancell Group) sits above the Prairie du Chien Group dolostone across a major unconformity, locally infilling paleovalleys with over 100 ft of relief. Middle to late Ordovician dolomite of the Sinnipee Group unconformably overlie the Ancell Group, and locally cap hilltops in the region. We begin our day with a tour of Crystal Cave approximately 1 hour west of Eau Claire, and spend the afternoon at outcrops of these units.

This field trip is limited to 20 participants.

*This description was updated on April 5, 2023

Post-meeting trips - Wednesday April 26

Trip 3. Precambrian Geology of the Eau Claire River Valley.

Trip leaders: Rob Lodge and Evan Weber (student) (UW-Eau Claire).

This trip will examine one of the Paleoproterozoic volcanic complexes within the Penokean orogen that is exposed in several erosional outliers in the Eau Claire River valley. The Eau Claire Volcanic Complex is the same age (~1.8 Ga) as the Ladysmith-Rhinelander Volcanic Complex to the north that hosts ~150 million tonnes of volcanogenic massive sulfide mineralization, but the tectonic setting and metallogeny of the Eau Claire Volcanic Complex is poorly constrained. Previous tectonic models interpret the Eau Claire Volcanic Complex as an arc system built upon the Archean Marshfield terrane, but new preliminary data questions that interpretation. This trip will discuss some of the new geochronological data and tectonic interpretations of the Eau Claire Volcanic Complex and its place in the geologic history of the southernmost Canadian Shield. The trip will start by examining the nonconformity beneath the Cambrian Mt. Simon Formation and stop at several classic Precambrian outcrops that are exposed around several dams and parks on the river. Note that this trip will include moderate hiking on uneven, rocky terrain adjacent on the banks of rivers and streams.

Trip 4. Quaternary Geology and Geomorphology of the Eau Claire Region.

Trip Leaders: Doug Faulkner (UW-Eau Claire) and Elmo Rawling (WI Geological & Natural History Survey)

We will begin the field trip at the David R. Obey Ice Age Interpretive Center located approximately 30 miles NNE of UW-Eau Claire. The Center is in the hummocky landscape of the Chippewa Moraine, dominated by stagnant ice features including ice-walled lake plains. Here we will discuss the late Wisconsinan history of the Chippewa Lobe of the Laurentide Ice Sheet. We will then proceed down the valley of the Chippewa River, the main meltwater stream that drained the Chippewa Lobe, to sites approximately 10 miles WSW of UWEC. We will make several stops along the way to discuss the river’s history of late Wisconsinan aggradation and post-glacial incision. The focus of these stops will be on landforms created when the river incised through glacigenic sediments and into a buried bedrock landscape. We will, however, also consider the many stream terraces found in this section of valley and the complex history of incision they represent. In addition, we will discuss Holocene aeolian activity in the valley, emphasizing its relationship to both regional climatic variations and erosion of the Chippewa River into its valley’s fluvioglacial fill.

Note that this trip will include moderate hiking on established trails, both paved and unpaved.

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