1 – Notes

May 2024

Written in preparation for a visit…

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.

Summary of Periods and Mass Extinctions

Edicarian: 635-538. First appearance of wide-spread multi-cellular organisms in ocean: Soft-bodied microbial organisms forming mats and other structures, and free-floating filter feeders.

End-Edicarian extinction: ~448. 86% species went extinct.* Possibly due to advent of burrowing organisms that disrupted largely sessile ecosystem. Not an official mass extinction because of a very incomplete fossil record.

Cambrian: 538-485. Warm shallow seas flank margins of several continental remnants of the breakup of the supercontinent Pannotia. In ocean there is the advent of hard-bodied complex organisms, and subsequent explosion of diversity into all phyla known today. The land bare except for microbial crust; arthropods and mollusks begin to adapt to life on land towards the end of this period.

Ordovician: 485 – 433. High CO2 levels and continents inundated with vast shallow seas jammed with life: brachiopods; trilobites; cephalopods; eurypterids; grapholites; and jawless fish. Many isolated continents and islands, with continents at south pole and a global sea occupying most of the northern hemisphere. First spores of land plants (fungi and simple plants) at 467Ma, with their spread possibly releasing phosphorous into the ocean stimulating algal blooms and CO2 sequestration.

End-Ordovician extinction:~345 Ma. 75% species went extinct.* Major ice age, likely precipitated by biogenic CO2 depletion, followed by a whip-lash of warming.

Silurian: 443-419. Gondwanaland and island chains provide diversity of environments; in the ocean early fish diversify into jawed and bony fish. Terrestrial life expands in the Silurian-Devonian Terrestrial Revolution: vascular plants emerge from more primitive land plants, and three groups of arthropods (myriapods, arachnids and hexapods) became fully terrestrialized.

Devonian: 419-359. Gondwana supercontinent in the south, Siberia to the north, and Laurussia to the east. Free-sporing vascular plants form extensive forests (Archaeopteris); by the middle of the Devonian several groups have evolved leaves and true roots; by the end the first seed-bearing plants appear.

Late-Devonian extinction event: ~250 Ma. 96% species went extinct.* Two major extinction pulses, and many smaller pulses. One theory is that it is due to the release of nutrients by the punctuated spread of land plants as they developed vascular systems with leaves and roots, and seeds.

Carboniferous: 359-299. Age of amphibians — also first appearance of amniotes from which both reptiles and mammals came. Vast rainforests covered the land, and insects diversified. The latter part of the Carboniferous experienced glaciations, low sea level, and mountain building as the continents collided to form Pangaea. A minor marine and terrestrial extinction event, the Carboniferous rainforest collapse, occurred at the end of the period, caused by climate change

Permian: 299-251. On land: The Carboniferous rainforest collapse left behind vast regions of desert in the continental interior. Amniotes, which could better cope with the conditions, diversified into the synapsids (the ancestors of mammals which came to dominate the Permian) and the sauropsids (reptiles). . In the ocean fish diversify with placoderms dominating almost every known aquatic environment, alongside coeleocanths, with sharks and bony fishes on the sidelines.

End-Permian extinction: 251.9 Ma. 80% of species went extinct.* The Siberian Traps were created at 252 Ma and also interacted with the Tunguska sedimentary basin filled with carbonates, shale, coal and salt in layers up to 12 Km thick; it is the worlds largest coal basin. When the magma intersected the basin, it caught fire, detonated in multiple places, and released vast about of CO2 and methane, on top of the CO2 produced by the eruption contributing to global warming and ocean acidification and anoxia. Other chemicals produced by the incineration of the Tunguska basin contents may have destroyed the ozone layer.

Triassic: 252-201. Brannen argues for a long 5 – 10 million year recovery, but that is disputed. The ancestors of crodcodiles dominated the Triassic; ancestors of dinosaurs and first true mammals appear, but were not dominant. The global climate during the Triassic was mostly hot and dry. Pangea had deserts spanning much of its interior until ita began to gradually rift into Laurasia and Gondwana to the south. In line with this the climate shifted from hot and dry to more humid, with a massive rainfall event called the Carnian Pluvial Event that lasted a million years.

End Triassic Extinction: 200 Ma. 80% of species went extinct.* Volcanism from the rifting of Pangea produced flood basalt that covered more than 4 million square miles. The CO2 concentration doubled or tripled, raising the already warm temperatures by at least 3 ° C. The final extinction pulse was fast: on the order of 20,000 years.

Jurassic: 201.4 – 145. Gondwana begins to rift. Climate warm and humid.

Cretaceous: 145 – 66. Gondwana completes rifting and by the end of the period today’s continents are recognizable, but with shallow inland seas in North America and Africa and between Greenland and Norway.

End Cretaceous Extinction: xxx. 76% of species went extinct.* Most likely some combination of the eruption of the Siberian Traps and the Chixtulub impact lead to global warming and an extended period of darkness. Almost all large animals eliminated, including all dinosaurs excerpt ancestors of birds.

Percent of species that went extinct, for any one event, vary considerably among sources. These numbers are better read as an indicator of relative severity.

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Keweenaw Peninsula Geology Notes