Keweenaw Peninsula Geology Notes

May 2024

Written in preparation for a visit…

Contents

The Keweenaw Fault

The Keweenaw Fault is a ~100 mile reverse fault that bisects the Keweenaw Peninsula of the Upper Peninsula of Michigan and marks the boundary between the Mid-continent rift and continental rocks. The fault is a reverse fault that thrusts lava flows of the Midcontinent Rift System onto sedimentary rocks of the Jacobsville Sandstone, and is part of the inversion of the Midcontinent Rift where a region that had previously undergone extension experienced significant contraction during the final stages of the Grenville orogeny. There are few (no) surface indications of the fault; but the Pilgrim river runs along the sandstone side of the fault trace.

The fault’s activity has been crucial in shaping the geological and topographical features of the peninsula, including the uplift of significant copper deposits. The copper deposits – which, unusually, are found in almost pure metallic form — result from the lava flows, later modified by hydrothermal solutions

The Keweenaw Peninsula

The Keweenaw peninsula is dominated by the Portage Lake Volcanics along its north and western sides – these consist of large volumes of basalt from the Greenstone flow, and are well-known for containing large volumes of copper. The Copper Harbor Conglomerate 100-800 meters in thickness, lays unconformably on top of the Portage Lake Volcanics, and is composed of calcite-cemented conglomerates and sandstone that eroded from the surrounding highlands and were deposited in fluvial and marine environments. Jacobsville sandstone consists of sandstone and small conglomerates – red marked with light-colored streaks and spots caused by leaching and bleaching – and is found along the south/east side of the Keweenaw peninsula. It was deposited around 992–980 Ma. One significant feature along the Keweenaw Fault is known as the Natural Wall. The Wall is a near-vertical slope of the Jacobsville Sandstone which dips as steeply 85°. The steep dip of the Jacobsville Sandstone is the result of it being folded due to motion on the fault.

The Midcontinent Rift Valley

The rift valley was filled with a combination of extrusive volcanic rocks, called the Greenstone flow, with minor amounts of intrusive igneous rocks and sediments derived from the igneous rocks. The Greenstone flow is one of the largest lava flows in the world. It can be mapped for 90 km on the peninsula and is also found on Isle Royale 90 km northwest on the other side of the rift valley. It has a thickness of up to 400 m with an volume estimated to be between 800 to 1,500 km³ of lava. Other flows on the peninsula can be mapped for lengths up to 160 km.

More generally, rifts are segmented linear depressions formed by crustal extension, and are filled with sedimentary and igneous rocks. In this case, unusually, the valley is filled with flood basalts, which is one type large igneous province (LIP). Typical rifts are not filled with flood basalts, and typical flood basalts are not associated with significant crustal extension and faulting. North America’s Midcontinent Rift is an unusual combination, because its 3000-km length formed during an aborted continental breakup event at 1.1 Ga. MCR volcanic rocks are significantly thicker than other flood basalts, due to their deposition in a narrow rift rather than across a broad region, giving the MCR a rift’s geometry but a LIP’s magma volume.

Geology-related Places in the Area

https://www.google.com/maps/d/u/0/edit?mid=1-smPNdeDGbZtJNy_EOa5AJEMLbQAM2E&usp=sharing

Michigan Technical University: Boulder Garden – Boulders, curated by Bill Rose
Michigan Technical University: Seaman Museum — Minerals. This stunning museum features a jaw-dropping showcase of minerals from the Great Lakes Region, as well as minerals from around the world
Quincy Mine — In Hancock, across the canal from Houghton.
Delaware mine, just east of Eagle Harbor. Self-guided tour of ruins on the ground level, and part of the first level of the mine
Cliff mine ruin. The old Cliff Mine site is located just south of Phoenix, off US-41 on the Cliff Drive
- “Put “parking for Cliff Mine” into google maps. There is a nice parking area and and large tailing piles. The forest around the tailings made for a beautiful hike too.”
- “Currently, the only mine tailing piles that are accessible and legal to pick are Cliff Mine and Central Mine. Please note that visitors are only welcome to pick at Central Mine when the gates are closed and no work is being done on the site. “
Esrey is a small Roadside park between Eagle Harbor and Copper Harbor that offers incredible Lake Superior views from atop giant ancient boulders with natural stairs carved to make the climb to the top easier. The park offers picnic tables and pit toilets, as well as a small pebble beach for rock picking. This is a great spot to find small whole agate nodules, as well as beautiful pieces of prehnite and thompsonite. Views of the rugged shoreline at Esrey are hard to beat.
Great Sand Bay is a beautiful, protected beach between Eagle River and Eagle Harbor. Despite what the name implies, the beach at Great Sand Bay actually offers both sandy and rocky sections of shoreline. This makes it a great spot for swimming, as well as rock hunting. A variety of agates, quartz, and other treasures can be found along the shore.
Calumet Waterworks Park is one of our favorite rock picking destinations because of its close proximity to town. Agates, prehnite, beach glass, and Yooperlites are commonly found along this stretch of shoreline. Also recommended is the beautiful Shute’s bar in Calumet, an old haunt for the mining community.

ILSG Field Trip #1

Rift-filling Mesoproterozoic Strata and Native Copper Deposits of the Keweenaw Peninsula, MI, Wed, May 15, 2024. 7:45am @MTU Memorial Union Bldg, US 41 side (get parking pass from leader) Leader: Ted Bornhorst (MTU)

Bedrock strata and native copper deposits of the Keweenaw Peninsula are part of the Mesoproterozoic Midcontinent Rift System. The elevated, central, bedrock “spine” of the Keweenaw Peninsula mostly consists of rift-filling, subaerial, basaltic lava flows with minor interflow, clastic, sedimentary rocks that make up the Portage Lake Volcanics (PLV). The PLV of the Keweenaw Peninsula hosts the world’s largest native copper mining district from which approximately 5 billion kg of refined copper was recovered. Rift-filling clastic sedimentary rocks overlie the PLV. The oldest of these is the Copper Harbor Formation that is mostly composed of volcanic-derived conglomerate but includes an informal member termed the Lake Shore Traps, which is composed of basaltic to andesitic lava flows. Younger rift-filling clastic sedimentary strata consist of the Nonesuch and Freda Formations. This one-day field excursion will traverse the Keweenaw Peninsula from South Range to Houghton/Hancock to Copper Harbor and will focus on lithologies of the PLV and Copper Harbor Formation and on the native copper hosted by the PLV.

ILSG Field Trip #5

7:45am @MTU Memorial Union Bldg, US 41 side (no pass; but cold in mine)

The Adventure Mine: Geology and History of a Native Copper Mine, Ontonagon County, MI, Sat, May 18, 2024 Leaders: Matt Portfleet (Adventure Mining Company), Ted Bornhorst (MTU)

The historic Adventure Mine in the Greenland-Mass subdistrict is located about 40 km southwest of the major copper deposits of the Keweenaw Peninsula native copper district. From 1851–191 about 5 million kg of native copper were produced from several amygdaloidal tops of the Portage Lake Volcanics at the Adventure Mine. The character of mineralization at the Adventure Mine is similar to the major deposits of the Keweenaw Peninsula native copper mining district. The field excursion will include a walking tour of the 1st level and will focus on the geology and history of the Butler lode. The tour will provide an opportunity to collect native copper and associated mineral specimens in an underground stope that will be blasted and safety inspected prior to the tour. After the collecting opportunity, another blast will be conducted just before participants leave the mine. After the underground tour, participants will visit the Adventure Mining Company’s facility in Painesdale, about 40 km northeast, where specimens are prepared for sale to tourists and collectors. A short tour of the Champion No. 4 shaft-rockhouse adjacent to the workshop will be conducted. Dress appropriately for the underground tour walk-in, which is about 760 m on a wet surface. Underground temperatures are about 7o C (45 o F). Hard hats and lights will be provided.