Minnesota Geology Pictures - Gunflint Iron Formation & Magnetic Rock

In areas of Northeastern Minnesota, the bedrock is comprised of the 1,878 million year old Gunflint Iron Formation.  The iron formation consists of black or gray chert alternating with black iron-rich layers.  During the late Wisconsin glaciation, the southwest trending Rainy lobe left a large slab of iron formation laying vertical.  The bedding planes of the surrounding iron formation is nearly horizontal, contrasting with the vertical bedding planes of this slab.  The name given to this 25 foot slab is Magnetic Rock (though many areas of the iron formation are magnetic).

For a more picture heavy look at the hike into Magnetic Rock, click here.  The trail is over a mile in length to the rock.

Minnesota Geology Pictures - Sudbury Impact Breccia

At the beginning of 2012, I started what I thought would be a weekly series called Minnesota Monday Geology, with posts regarding different geological topics around the state.  For nine months the series went by well, until the beginning of the school year where my time was taken up with work associated with school, like planning lessons, grading jail, and taking care of my three children at home.  Although I have numerous posts to finish the series somewhere in my mind, as well as several including geology from neighboring states like South Dakota and Wisconsin (including a recent trip to Van Hise rock) and several with gigapans that I've begun taking, I'm starting another series today.

Nearly 1,850 million years ago, the region that is now Sudbury, Ontario was struck by an asteroid at least six miles in diameter.  A record of this catastrophic moment in time is preserved in the rock record along the Gunflint Trail, near the Canadian border in Minnesota.  The impact breccia at this location has been interpreted as a submarine debris flow 450 miles from the impact site.  At the time of the impact, this area of Minnesota was located near the shoreline of an ancient sea.  Found directly beneath the breccia layer is the 1,878 million year old Gunflint Iron Formation, directly above the breccia layer is the 1,850 million year old mudstones and shales of the iron-free Rove Formation.  The breccia layer is almost 7 meters thick, with the lower levels containing angular pieces of Iron Formation several meters across.  The iron formation found in the upper levels of breccia are much smaller.

A more complete post on the Sudbury Impact Breccia can by found here.

Both of my sons went on a hike to the breccia outcrops last June (2012).  It can be a challenging hike through some dense underbrush for 6 year old legs.

My 4 year old was more interested in the leaf that he 'captured' than what was around him, oh well.

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).  

Minnesota Geology Monday - Sudbury Impact Breccia

Approximately 1,850 million years ago, an asteroid between 6-12 miles in diameter struck Earth at what is now Sudbury, Ontario.  This impact is the second-largest known impact, as well as one of the oldest.  The effects of this impact is left behind in the rock record that have recently been discovered near Thunder Bay, the Gunflint Iron Range, Michigan's Upper Penisula, among others.

There are several locations near Gunflint Lake, along the Gunflint Trail (County Road 12), that bear witness to the impacting event even though the impact was nearly 450 miles to the east.  One location is found approximately one mile down National Forest Road 1347, off of County Road 12.

Approximately a quarter mile after leaving the Gunflint Trail (County Road 12), you drive past a gravel pit in late-Wisconsin glacial deposits of the Rainy lobe.

After a little over a mile drive on National Forest Road 1347, a hiking trail intersects the road.  One exposure of the breccia is several hundred feet to the north of this intersection.

After following the trail north of the road for a few hundred feet, you must leave the trail and hike through the brush for another 100-200 feet to reach the exposure.  The breccia is found on the far side of the middle ridge in the picture below.

At the time of the impact, this area of Minnesota was located along the shore of an ancient sea.  The impact breccia lies directly upon the Gunflint Iron Formation (1,878 million years old) that was being deposited prior to impact.  The breccia at this location, has been interpreted as a submarine debris flow caused by earthquakes resulting from the impact.  The breccia layer is almost 7 meters thick, with larger pieces of iron formation (some almost 3 meters long) found near the bottom and smaller pieces found near the top.

After depostion of the impact breccia, the ancient sea again began to deposit fine-grained material, though no more iron.  Mudstones and shales of the Rove Formation (1,850 million years old) were deposited directly above the impact breccia.  These mudstones and shales were later metamorphosed into slate by the 1,115 million year old diabase sills of the Logan Intrusion found above the Rove formation.

Beyond the intersection of Cut Across Trail and National Forest Road 1347 approximately a quarter mile is another interesting exposure of impact breccia that shows ejecta.  As part of the impact, most material would have fallen back into the newly formed crater, though some of it was thrown nearly 450 miles to Minnesota.  The ejecta is found at the uppermost portion of the impact breccia and includes accretionary lapilli.  These lapilli are comprised of multiple layers of ash and melted fragments that were removed from the impact site.

The Sudbury Impact Breccia located in Minnesota demonstrates a catastrophic moment in Earth's history.  A much more detailed read on the exposures, prepared by Mark Jirsa of the Minnesota Geological Survey, found along the Gunflint Trail can be found here,

Minnesota Geology Monday - Magnetic Rock

Near the end of the Gunflint Trail, in northeastern Minnesota is a hiking trail that leads to Magnetic Rock.  The trail is approximately one mile in length and mostly uphill (at least on the hike in).  In the picture below, you can make out a small portion of the Gunflint Trail near the trailhead, the picture was taken about 1/4 mile into the hike.

Due to a forest fire in 2007, there are relatively few mature trees or vegetation along the trail.  This fire also did an excellent job of exposing bedrock outcrops throughout the area.  When looking north at several points along the trail, you see numerous exposures of the white, granitic Saganaga Tonalite.

The majority of the trail is on top of the 1,878 million year old Gunflint Iron Formation, which would have been deposited near the shoreline of an ancient sea.  The Gunflint Iron Formation consists of black to gray chert alternating with black iron-rich layers.  A goal this summer is to also acquire better photographs of the Gunflint Iron Formation, in particular the bedding planes, which is largely horizontal in the area.

Along the trail to Magnetic Rock, you are also able to view stromatolites within the Gunflint Iron Formation.  Though the day I traveled to the area a storm was coming in and I needed to complete the hike and set up camp, so I didn't have time to look for the stromatolites.  Maybe that can happen this summer.

Magnetic Rock is a large piece of the iron formation that has been turned vertical, probably by the southwestern trending Rainy lobe during the late Wisconsin glaciation.  Magnetic Rock is almost 25 feet from ground level to the top.  Due to it's height, the rock is visible for some distance along the trail before you actually reach it.

The bedding planes of the Gunflint Iron Formation is largely horizontal in the near vicinity of Magnetic Rock.  The bedding plane of Magnetic Rock is vertical, indicating that it has been moved from it's initial location.