Minnesota Geology Monday - Sioux Quartzite

The most prominent bedrock in southwestern Minnesota is comprised of the 1,700 million year old Sioux Quartzite that extends into South Dakota.  The quartzite ranges in color from pink to purple depending on the amount of iron oxides present.  The quartzite is derived from quartz sand, the source of which was the erosion of the Penokean Mountains, which were located across central Minnesota and into Wisconsin during Proterozoic time.  As the mountain ranges were weathered and eroded, most minerals were broken down into clay minerals and transported away.  The resistant quartz grains were transported south and deposited in a braided stream environment.

Cross beds and ripples found in the quartzite indicate the direction of movement of the flowing river systems as the sandgrains were being deposited.  Most of the cross beds and ripples indicate a southward direction.  Across the border in South Dakota, there are a few locations that have southward dipping cross beds that are found directly beneath northward dipping cross beds.  This potentially suggests that the depositional environment found in this location would have been near an ocean shore under the influence of tides.

Being highly weathering and erosion resistant, the Sioux Quartzite tends to be cliff forming in several locations around the region.  At Blue Mounds State Park, near Luverne, pioneers moving westward used the cliffs as a navigational marker.  The cliffs appearred blue at a distance and were given the name 'blue mounds'.

At Pipestone National Monument, the quartzite also forms cliffs.  The quartzite slowly weathers, primarily via ice wedging along vertical fractures.

During late-Wisconsin glaciation, glaciers also left their mark on the Sioux Quartzite.  Rocks and pebbles embedded in the ice left many glacial striations on exposed quartzite throughout the region.

Early Native Americans also found uses for such a resistant bedrock.  At the Minnesota Historical Site called the Jeffers Petroglyphs, carvings were made into the quartzite beginning 7,000 years ago and continued until recent times.  Carvings at the site include birds, turtles, bison, thunderbirds, weapons and a great number of other objects.  The site most likely represents religious illustrations of impartant aspects of the lives for these earlier Native Americans.

The Sioux Quartzite has been (and still is) used widely as both dimension stone and aggregate.  Modern quarries of Sioux Quartzite can be found as far east as New Ulm and extend west into South Dakota.  Early quarries used for these purposes include this late 1880s quarry found near Luverne.

Many towns in the region have buildings made out of the resistant Sioux Quartzite, including the Rock County Courthouse.

Minnesota Geology Monday - Pipestone National Monument

Pipestone National Monument is located on US Highway 75, approximately 25 miles north of Interstate 90.  The monument focuses on how Native Americans have quarried catlinite out of the region to be carved into pipes or other structures.  Catlinite is a metamorphosed mudstone with a fine-grained texture and it can be easily carved.  Due to the presence of iron oxides, the catlinite is deep red color.

At the entrance to the park office is the first stop, a series of granite boulders called the Three Maidens where Native Americans would leave offerings before proceeding to the quarries.  Native Americans realized that these boulders did not match the local bedrock and thus were significant spiritually.  These glacial erratics were transported from the Ortonville, MN area and broken into many pieces by the process of ice wedging.

Prior to the late 1880s, there were 35 pieces of rock containing petroglyphs that had been placed at the Three Maidens location.  These were removed in 1888 or 1889 because some of the petroglyphs had been defaced, but were returned to the monument in the mid-1900s.  These petroglyphs are now on display at the visitor center.

Bedrock in southwestern Minnesota is the 1,700 million year old Sioux Quartzite.  The quartzite ranges in color from white to pink to purple and derived from quartz sand.  The source location of the sand grains is the erosion of the Penokean Mountains which were located across central Minnesota and into Wisconsin during Proterozoic times.  Cross-beds and ripples found in the quartzite indicate the direction of movement of the flowing water as the sand grains were being deposited.  The majority of the cross-beds and ripples suggest a southward direction of flow.  The Sioux Quartzite correlates well with other quartzites in the region, including Wisconsin's Baraboo Quartzite.

Because the quartzite fractures along vertical joints, it tends to form cliffs.  Though weathering of this nature has also formed feature known locally as the Old Stone Face.

Or this structure known as the Oracle.

Because the quartzite is erosion resistant and cliff-forming, a creek has developed a waterfall in the park over the escarpment called Winniwissa Falls.  It must be one of the very few waterfalls present in southwestern Minnesota.

Further south of the waterfall, the escarpment continues, at the base of the escarpment are evidence of plunge pools and scour holes.  Looking west of the escarpment (the direction the creek flows) there appear to be dry channels.  As the glaciers where melting in the late Pleistocene, meltwater would have been pouring through the area creating a much larger waterfall.

Native Americans continue to quarry the catlinite in several quarries like shown below.  The park service needs to pump water out of the quarries before work proceeds.

Quarrying would have (and still does) as demonstrated by one of the park's informational signs.  The letter A represents the catlinite, B is the overlying Sioux Quartzite, C is the soil layer, while D is the rubble pile created by removing the Quartzite and soil.  Since the quartzite and catlinite layers dip be 5-10 degrees to the east, more and more quartzite must be removed in order to reach the catlinite layer.

The park service has cleaned up one of the older quarries for tourists to enter.  At the bottom of the quarry face is the catlinite layer, the layer is approximately 10-15 inches thick and much softer than the overlying Sioux Quartzite.

Because of the cultural and geological significance, Pipestone National Monument was recently included as one of the 101 American Geo-Sites You've Gotta See in the book by Albert Dickas.

Minnesota Geology Monday - Folded Banded Iron Formation

Minnesota's first iron ore mine opened in 1882 and is called the Soudan Mine, initially there were several open pit mines in the area.  In 1892, the work moved underground and continued until 1962.  During the time that the mine was in operation, over 14 million metric tons of iron ore were removed.  The state bought the mine and approximately 1,200 acres surrounding the mine for $1, on the condition that the area be converted to a park.  Today visitors to Soudan Underground Mine State Park can ride a metal skip to the 27th level (2,341 feet below the surface) for a tour of the mine or visit a physics laboratory that is also operated at this depth.

Near the mine is also a classic pavement outcrop of folded banded iron formation.  Some have called it the most photographed outcrop in Minnesota and it was recently added as one of the 101 American Geo-Sites You've Gotta See.  The day that I visited the site, it was unfortunately late in the day, with a slight drizzle and thunderstorm moving in, but here are some of the pictures taken that day.

The site consists of layers of metallic hematite, red jasper and white chert.  These layers were folded multiple times, though questions arise about whether the folding occurred while the sediments were still soft or after they had lithified.

The site is a short walk away from the mines headframe, the stands over the mine shaft supporting the cables and skips that brought people and ore from the lower levels to the surface.  Because of several small roads in the vicinity, you do not need to enter through the park's main entrance to get to the outcrop, Stunz Bay Road climbs a small hill and leads practically right past the outcrop.

In many places, the layers of red jasper and white chert have small fractures that are filled with milky quartz.  This suggests that the jasper and chert were brittle during the folding event.

Stromatolites in the Gunflint Iron Formation & the Sudbury Impact Layer

A few weeks ago, I took my two boys (Kieran, age 6 and Luke, 3) camping on the North Shore of Lake Superior and at the end of the Gunflint trail.  Day 4 of our camping trip found us almost sixty miles away from Lake Superior at the Trail's End campground at the end of the Gunflint Trail.  The plan for the morning was to accompany a group of Earth Science teachers and several geology professors on a couple of hikes to sites of geologic importance in the area.  The group of teachers were part of TIMES XIV, the TIMES Project is an intensive two-week long class teaching inquiry-based teaching methods and focusing them on Minnesota Earth Science.  As a past participant of TIMES (summer of 2009) and a member of the Minnesota Earth Science Teachers Association's Board of Directors, we were invited to go along on the days hikes.  My job would be to talk Earth Science with the teachers and get to know what they do in their classrooms, while giving them examples of what I do differently as a result of the TIMES Project.  Kieran and Luke's job for the day was to follow along and just be themselves, they actually do a great job with long hikes in uneven terrain for kids their age.

The day’s hikes were led by Jim Miller, geology professor of the University of Minnesota, Duluth and the Director of the Precambrian Research Center.  We met the group in the morning at the Magnetic Rock trailhead, the same place that we had hiked the day before, though we would not be repeating the entire hike this day.  Magnetic Rock is a large slab of the Gunflint Iron Formation that was tipped vertically by the late-Wisconsin glaciation.  The focus of the day were features found around the 1,878 million year old Gunflint Iron Formation.  An iron mine, the Paulson mine, opened near here in the late 1800s, but failed rather quickly because the Mesabi Iron Range (located to the south) was already in operation and a financial panic scared investors in 1893.  Before the discovery of iron, early explorers and/or voyageurs to the area, acquired black chert from the iron formation to be used as flint in their black powder muskets.

The group hiked approximately half way to Magnetic Rock to view stromatolites that are found along the trail.  Stromatolites are nearly circular algal mounds that can be several feet in diameter.  While alive, cyanobacteria (the living organism of the stromatolite) grow upward from the seafloor by trapping sediment on a sticky mat.  These organisms obtain energy by using the process of photosynthesis, essentially using carbon dioxide and releasing oxygen as a waste product.  Stromatolites of this time period are remarkable because of their role in the Great Oxidation Event (GOE).  They are the reason that we have the iron deposits around the world, before this period in Earth's history, the level of atmospheric oxygen was very low which allowed soluble iron to build in the world's oceans.  As the stromatolites began to release oxygen as a waste product, the iron in the world's oceans basically rusted (or precipitated) out of solution and was deposited on the ocean floor.  Stromatolites of this age are found around the world near most of the world's major iron formations.

Stromatolites are not unique to just this time period in Earth's history, they are found in many sedimentary rock sequences.  The much younger Ordovincian rocks of Southeastern Minnesota also have great examples of stromatolites.  They can even be found today in hypersaline waters, where predators are scarce, the best example being Shark Bay in Australia.

The next stop for the day was a short drive and short hike away that a catastrophic few hours in Earth's past are recorded in the rock record.  Approximately 1,850 million years ago, a meteorite struck near Sudbury, Ontario, almost 450 miles away from Minnesota.  It is the second largest impact structure found and very few are older than this impact.  The Minnesota outcrops showing evidence of the impact were found because a forest fire raged through the area in 2007 and removed most of the overlying vegetation, making it much easier to see the underlying rocks, especially the breccias of the Sudbury Impact Layer.

Essentially, the greater than 10 magnitude earthquakes caused by the impact fractured the iron formation and resulting tsunami re-worked or re-arranged the angular pieces of iron formation.  Near the base of the impact layer, these angular fragments are meters in length and cemented together in a green matrix.  Near the top of the formation, the size of the iron formation fragments are much smaller.  Curiously, Jim Miller explained that iron ceased to be deposited worldwide after the impact.  For a much more detailed read on the formation of the impact layer and a history of the impact layer itself, read the document published by the Minnesota Geological Survey.

The picture below shows Kieran sitting on the burnt remains of a tree, killed during the forest fire in 2007 that granted accessibility to the Sudbury Impact Layer.  This was just about one of the last outcrops that we would visit with the TIMES group, though they would continue on for at least one more stop before returning to the Twin Cities that evening.  It is located just a short hike down National Forest Service Road 1347 from the previous stop.

At this particular outcrop, you are able to stand near the uppermost portion of the Sudbury Impact Layer.  Deposited here are accretionary lapilli, essentially ash pellets that were thrown away from the impact site.  Using very high powered microscopes, shocked quartz (which is only found at impact sites) can be found in these centimeter-sized pellets, commonly called ejecta, and the surrounding matix.  Look for the small, concentric circle structures in the pictures below.

Many of the teachers in the group we were accompanying were amazed that two boys, like Kieran and Luke, could go through some of these hikes at their age.  A few were even surprised that they could camp in locations like these, this isn't a state park with a lot of people, it's fairly remote and there is no cell phone reception or any technology that we're used to using.  Both boys tend to do a great job doing their own thing when camping in locations like these and dad got a few hours of adult interaction (a very rare thing for a week-long father/son camping trip).