Soils and Geology of Wisconsin Field Trip, Fall 2011:
Transect of the Penokean Orogen

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

The Penokean Orogeny I

About 2000 m.y. ago, the Superior Craton constituted the core of North America. It consists of most of Ontario and Quebec, northern Minnesota, the Upper Peninsula of Michigan, and part of northwestern Wisconsin. The craton consists of a basement of gneiss and granite-greenstone belt terranes and was covered with shallow water sedimentary rocks (purple, below) including sandstone, carbonate rocks and iron formations. Off the southern margin, the continental slope consisted of deeper water mudrocks (brown).

The Superior Craton converged southward toward a volcanic island arc, the Wausau-Penbine Terrane (also known as the Central Wisconsin Magmatic Belt). The subduction zone probably dipped southward since all the intrusive rocks related to subduction are in the island arc rocks, and virtually none in the Superior Craton.

In all diagrams, north is to the left.

Eventually, plate convergence brought the Superior Craton and the Wausau-Pembine Terrane into contact, deforming and metamorphosing the rocks on the Superior Craton enge, and folding and thrusting them.

Day 1: October 15, 2011

Iron Mountain Iron Mine

Big John, mascot of the mine.

Outside, the loudspeakers play mining songs. All two of them: "Big John" and "Sixteen Tons." Thank goodness they never heard of "Timothy" (Look it up).
Mine cars
A totem pole of sorts with axes and shovels, for some reason.
Suiting up. Hard hats by law, raincoats to catch the drips.
Geologic map of the mine. In the days before core drilling, they explored the hard way, by digging tunnels.
An ore car. Below: other equipment.
Waiting for the commuter train.
An early core drill. Core drilling made it possible to probe underground without tunneling, making mining faster and safer, as well as more economical.
The most interesting geology in the mine was the breccia zones.
Water cooled drills like this were used for emplacing explosives.
Fish were once used to monitor air quality. This pool has been stocked with trout that live here permanently.

Below: Other denzens hang around the mine.
The small stope, where ore was mined from below.
The Big Stope. Again, ore was mined from below. Part of the roof later collapsed, leaving a dome-shaped chamber.
Behind the rubble pile is an 1100-foot deep lake where the stope flooded.
"Saftey First." Spelling Later

Iron Formation


Superior Craton Archean Gneiss

About ten miles north of Iron Mountain on M-95 is the first exposure of Archean basement of the Superior Craton, which covers most of Ontario and Quebec as well as the U.P. and northern Minnesota.

Continental Slope Metasediments

About a quarter of a mile north of the Wisconsin line on M-95 is a nice exposure of slate. As the margin of the Superior Craton subsided, shallow water shelf rocks gave way to deeper water mudrocks of the continental slope.

Here and below are folds.
Slate with siltstone bed
Slickensides on fault surface

Dunbar Gneiss

A mile west of Dunbar at the intersection of County U and US 8 is an exposure of the Dunbar Gneiss. There are numerous outcrops in the area but this one is clean and conveniently located.
Gneiss domes form when continental crust is heated and becomes plastic, and rises by buoyancy. The mobilization of these gneiss domes probably happened around 1700 m.y. ago during the Yavapai Orogeny. The Yavapai Orogeny occurred further south and is not exposed in Wisconsin, but this area was probably buried tectonically by 15 kilometers or more of thrust sheets.

Twelevefoot Falls County Park

Actually it's only 11 feet, 10-3/8 inches. I want a refund.
The rock here is Penokean quartz diorite with numerous local shear zones, part of the Twelvefoot Falls Shear Zone.

Day 2: October 22, 2011

Thunder Mountain Hatchery

The Thunder Mountain Quartzite is the eastern end of the McCaslin Syncline, which may or may not be an actual syncline.

The syncline is defined by three metasedimentary units that have not really been shown to be stratigraphically related. It has an oddly steep plunge for such a large structure. A possible alternative interpretation is that it represents collapse of blocks stoped by the Wolf River Batholith.
The quartzite extends north along a ridge from the hatchery to Thunder Mountain.
In the grazing light, bedding planes stand out in relief.
The steep face is approximately parallel to bedding.
Down in the lowlands, looking at soil.
It's an entisol.
Heading back to the hatchery
The rearing ponds.
Near the parking area is a large outcrop.
Bedding in the quartzite.
Small garnets occur in the quartzite, especially in the mossy area at lower center.
Several garnets are visible in the closeup, the best one about even with the end of the moss on the left and above it.
Zooming in on the garnets, which are a couple of millimeters in diameter. One is at far right center, the other at upper left.

McAuley Gneiss

McAuley Gneiss outcrop. What was once an open hilltop with a bewildering patchwork of gneiss, metavolcanics and aplite is now disappointingly grown over.
Large outcrop in a power line cut.
PLEASE turn into soil! Pleasepleaseplease!

Below: Microcosm on a tree stump. Cup and British Soldier lichens.
Outcrop of the McAuley Gneiss

Waupee Volcanics, Baldwin Conglomerate and Hagar "Rhyolite"

Widening of County W has created a nice large new road cut of Waupee Volcanics.

Below: Features in the metavolcanics, including possible sedimentary interbeds.
Headed to the Baldwin Conglomerate exposure.
The contact between Baldwin Metaconglomerate and Hagar "Rhyolite" is on a steep knob that's a dicey climb on a dry day and all but unclimbable if it's wet.
Baldwin metaconglomerate
Hagar "Rhyolite," so called because of its porphyritice texture and aphanitic groundmass, although there is no evidence of it being volcanic and it is generally considered a shallow intrusive within the Wolf River batholith.

Tornado Swath

On June 7, 2007, a tornado swept 40 miles across Menominee, Langlade and Oconto Counties. Even more than four years later the swath across Highway 64 is dramatically obvious.

Note the lack of clutter, probably the result of salvage logging.

Soils Stop

Looking for a wet soil
Still an alfisol but the iron oxide mottling confirms wet conditions, making it an aqualf.

Brokaw Metasediments

These rocks near the Wisonsin River on County W just east of Highway 51 look metavolcanic due to the green color, but close inspection shows they are mostly metasedimentary rocks.
Faint bedding is visible in this outcrop, with apparent dip about 20 degrees east (left).
And this is definitely metaconglomerate.

Wausau Syenite

The focus of the trip was not the Wolf River Batholith, but if you're in Wausau you have to stop to see the Wausau Syenite.
Once upon a time this was the easiest stop in the world to reach, but now thanks to highway construction and creation of a commercial park, it's at the end of a confusing maze, even though you can easily see it from far away.
Sunset view from Rib Mountain.
Alas, our plan to camp here collapsed because the campground was closed. So Plan B was Marathon Park in Wausau.
While looking at Jupiter, I asked if anyone had binoculars. Nobody did, but someone joked that Professor Fermanich or Luczaj might have an app on their phones.

I propped my camera on the van roof, set the camera for a one second exposure and racked the digital zoom out to about 60x. After numerous attempts I got this, clearly showing two of the moons, although a little jiggly.

So yeah, I got an app for that.

Day 3: October 23, 2011

Rib Mountain Quartzite

Above: Breaking camp at Marathon Park in Wausau.

Left: Geology briefing before setting off for the quarry on the west end of Rib Mountain.

Hi, ho, hi ho....
  Below: what looks like a bedding plane turns out to be a serpentinized dike.
Below: The quartzite is white and massive, and bedding planes maddeningly hard to see. Bedding results from changes in deposition conditions, and when the sediment and deposition conditions are extremely uniform, bedding can be very subtle.

Rib Falls Metavolcanics

Rib Falls is an attractive stop with several metavolcanic lithologies exposed.
Left and below: Foliated metatuffs and volcaniclastics
Waiting for the group photo.
Soon to be an Olympic event: the Luczaj Leap.

Professor Luczaj races his camera timer to get into the group picture on time.
Rib Falls also has wonderful potholes.
This circular mark is a puzzle. Is it the last remnant of a pothole that was eroded away?
  Subtle but clear bedding is visible here as alternating light and darker vertical green bands.
Porphyritic metavolcanic rocks.

Halsey Metaperidotite

This otherwise undistinguished road cut has outcrops of metaperidotite. Ultramafic rocks are extremely unusual in Wisconsin. If they're of the same origin as those in other mountain belts, they may represent fragments of mantle.
  The outcrops are just south of this bend in County H. 

Location: 45°03'10" N 89°57'54" W;
UTM: 16 T 266587 m E 4993123 m N
  The soil is loess-based but still an alfisol because of the weak organic content of the A horizon and the clay content of the B horison.
And on the way back to Highway 29...

The Penokean Orogeny II

South of the Penokean Orogenic Belt was a third terrane, another continental block called the Marshfield Terrane. Rocks of this terrane at Black River Falls are 2800 m.y. old, the oldest rocks in Wisconsin. In Minnesota, there are rocks in the Minnesota River Valley older than 3100 m.y., the oldest rocks in the U.S., and the Marshfield Terrane is often suspected to be a piece of the Minnesota River Valley Terrane.

No sedimentary rocks of the Marshfield Terrane are preserved. The subduction zone probably dipped north since all the Penokean intrusive rocks are in the Wausau-Pembine Terrane. The former descending slab attached to the Superior Craton probably detached and sank, but the interactions between the two descending plates are not known in detail.

The Marshfield Terrane collided with the southern margin of the Wausau-Pembine Terrane.

Eau Pleine Shear Zone

Considering all the hooh-hah in Wisconsin geology about how the Eau Pleine Shear Zone is a major suture, good outcrop descriptions are very rare. This railroad cut on State Highway 34 just south of the Wisconsin River bridge near Dancy is very good, though safe legal parking is hard to come by and the embankment is extremely steep.

Location: 44°42'36" N 89°41'55" W
UTM: 16 T 286242 m E 4954274 m N

Archean Marshfield Terrane at Conants Rapids

At Conants Rapids in Stevens Point are outcrops of Archean gneiss of the Marshfield Terrane.

Left is a gneiss outcrop. Below: Views of the dam.

Location: 44°29'17" N 89°34'42" W
UTM:16 T 294998 m E 4929309 m N
Left: The best outcrops are immediately south of the dam.

Below: Ripple marks.
Left: Shear zone with lineated gneiss.

Below: Views of the gneiss.
Below: Wisconsin River south of the dam.

Tigerton Anorthosite and Wolf River Batholith

No two geologic maps show the Tigerton Anorthosite the same, because it's not a single unit, but rather a swarm of xenoliths. Also, despite the name "Tigerton," almost all of it is well north of Tigerton. This is actually one of the southernmost exposures.
The anorthosite is gray, the granite is pink.
View of the anorthosite

Below: the granite here is banded and not at all uniform, as if the granite magma were reacting with the anorthosite.
Here we see concentric mafic bands around a possible small anorthosite xenolith.
Looking south to the intersection of Copunty J and US 45.
A short distance east on US 45 at the municipal waste plant is an outcrop of classic Wolf River Rapakivi granite.
Below: some phenocrysts are 5 cm in diameter. The rounding has been attributed to abrasion during flow of the magma. Note the obvious zoning and the rapakivi rims, especially in the last photo.

Possible Origin of the Tigerton Anorthosite

One of the things that has long intrigues Wisconsin geologists is that the Tigerton Anorthosite coincides with the deepest part of the gravity low over the batholith. The puzzle is that the anorthosite differs little in density from the granite. After seeing the Laramie Anorthosite, which underlies the Sherman Batholith in Wyoming (and both are almost identical to their Wisconsin counterparts), the explanation seems rather simple.

A rising mantle plume ascends under the crust of Wisconsin. Continental crust is pink, mantle is green, the plume is orange

The plume reaches the crust and heats and softens it (bright pink). Meanwhile the plume segregates, with mafic minerals sinking (brown) and feldspar rising to form anorthosite (light orange).

Partially molten crust begins to rise by buoyancy, entraining some of the anorthosite as it rises.

The batholith solidifies and is eroded to its present level. The mantle plume is long since solidified. The conduit of the Wolf River Batholith extends deep into the crust, accounting for the gravity minimum over the batholith (graph). Since the anorthosite ascended through the conduit, it coincides with the gravity low although neither its mass nor its density are sufficient to create the gravity low.

Reference: Wisconsin gravity minimum: Solution of a geologic and geophysical puzzle and implications for cratonic evolution; David J. Allen and William J. Hinze; Geology; June 1992; v. 20; no. 6; p. 515-518.

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Created 20 October 2011 , Last Update 15 January 2020