University of Wisconsin–Madison

Keweenaw Peninsula Field Trip

sudents in mine
Undergrads go underground

Undergrad group explores UP Michigan

by Jason Huberty

Eighteen undergraduates traveled to the Keweenaw Peninsula in Michigan’s Upper Peninsula from October 12-14, 2007. We used the Self-Guided Geological Field Trip to the Keweenaw Peninsula, Michigan by Ted Bornhorst (1994) to choose stops that highlighted the rift geology and copper deposits. The weather was outstanding with sunny days in the 50s and nights in the 30s.

The Keweenaw Peninsula is part of the Lake Superior segment of the 1.1 Ga Mid-continent rift which extends from Kansas to Lake Superior and through lower Michigan. The rift formed due to crustal extension over an asthenospheric plume and was filled in by large volumes of magma. There are more than 25 vertical km of volcanic rocks under Lake Superior including 10 km of the Portage Lake Volcanics (PLV). Rift-filling clastic sedimentary rocks, including the Copper Harbor Conglomerate, overlie the volcanic rocks. The Keweenaw fault, which was originally a graben-bounding normal fault, is now a high-angle reverse fault. The Jacobsville sandstone was deposited during and after reverse movement on the fault covering the entire basin with 3 km of red sandstone. The Keweenaw region is famous for its world-class native copper deposits. Copper in the basalt was released during burial metamorphism of deep parts of the lava flows.

We began our trip at the South Range Quarry where we were observed amygdaloidal lava flows within the PLV, a basal conglomerate bed and glacially-polished surfaces above the old quarry. Although the Keweenaw fault is a major feature of the area, it is not easily observed; rather, it is inferred from the changing lithologies of the PLV to the Jacobsville sandstone. The Jacobsville was spectacularly seen at the Natural Wall Ravine, a must-see for anyone visiting the Keweenaw.  At this stop, we met up with a mineralogy class from Eastern Illinois on a field trip which was a pleasant surprise.

In the afternoon, we visited the Seaman Mineral Museum on the Michigan Tech campus. George Robinson, the museum director, gave us a private tour and shared information about the geologic history of the copper district, the varied habit of copper crystals, ore deposit types and other famous Michigan minerals. From there, we took in a panoramic view on top of Bumbletown Hill where we could see Isle Royale on the horizon and a cuesta to the north formed by the Greenstone flow. After digging at the Phoenix mine tailings, we drove north to Copper Harbor and camped at historic Fort Wilkins State Park.

Sunday, we viewed two exposures of the Copper Harbor Conglomerate on the north shore including Hebard Park. Next, we climbed 1000 feet of elevation to the top of a conglomerate ridge to the Brockaway Mountain viewpoint. It was a clear day and fall colors remained on some of the trees giving us a magnificent view. Then it was off to Horseshoe Harbor for a date with billion-year old stromatolites, Collenia Undosa. Finding the harbor was a real adventure as we went too far down a seasonal road and were turned around by some hunters before we reached a meteorological rocket launch site used in the 1960s. After lunch, we headed to the Delaware mine and took a self-guided underground tour. The group was weary after our two-day excursion but the rainy trip home did not diminish sunny memories of the Keweenaw Peninsula.

We thank alumni and friends of the Department of Geoscience  for their generous support of undergraduate field trips.

Originally published in The Outcrop for 2007, UW-Madison Department of Geology and Geophysics, p. 14.