Minnesota Geology Pictures - Mill City Conglomerate & 600 Million Years Missing

Within the Minnesota portion of Interstate State Park lies the informally named 504 million year old Mill City Conglomerate.  The picture above shows one of the outcrops of the conglomerate found in notches exposed through erosion.

The conglomerate consists of basalt boulders cemented together by a reddish matrix of sand and silt.  The size of the basalt boulders and rounded edges indicates a high energy environment during deposition, though some fossils of trilobites and brachiopods have been found in the conglomerate.  Quite likely, the conglomerate was deposited on the shoreline of the Cambrian seas near basaltic islands.  The basalt is part of the 1,100 million year old Mid-Continent Rift System. 

Not far from the campground, the contact between the 504 million year conglomerate and the 1,100 million year old basalt.  600 million years of time is missing between these two rock types.

Minnesota Geology Pictures - Stromatolites in the Biwabik Iron Formation

In northern Minnesota, the Biwabik Iron Formation was deposited 1,900 to 1,885 years old along the shores of the Animikie Sea.  At this time, surface water was busy eroding the continent located to the north bringing fine materials like silt and clay into this shallow sea that would later be incorporated into the iron formation.  Before the time of iron deposition though, reduced iron was easily dissolved and concentrated into the sea water because there was not free oxygen molecules in the atmosphere or seas at the time.

This was a unique time period in Earth's history, as photosynthetic organisms begin to appear in the rock record worldwide.  These organisms, called stromatolites, formed algal reefs along the shoreline of these ancient seas and as a waste product of photosynthesis, produced oxygen.  It is this oxygen that caused the iron to precipitate out of the seawater and be deposited as iron.  This time period is called the Great Oxidation Event (GOE) and is responsible for most of the iron deposition throughout the world.

Each domed structure above would have been an algal mound located at or near the surface of the water 1,900 years ago, busy precipitating iron to the seafloor as a consequence of photosynthesis.  This particular field of stromatolite domes is found within an inactive iron mine and is being preserved, there are no future plans to mine the iron in this location.

Minnesota Geology Pictures - Pillowed Greenstone

In Gilbert, Minnesota, is a site of 2,700 million year old pillowed greenstone.  Basaltic lava would have originally have erupted into an ocean or sea forming rounded structures called pillows.  Through weak metamorphosism, involving higher temperatures and pressures, original minerals found in the basalt like plagioclase and pyroxene changed to chlorite and epidote.  The edges of the pillows have crystals much smaller than the interior.  As the lava erupted into the colder water, the edges cooled faster resulting in small crystal growth, while the interior cooled relatively slower resulting in larger crystals.

Much later in time, during the late Wisconsin glaciation, glaciers from the Rainy Provenance scoured the area, largely cutting the pillows in half and leaving numerous striations across the pillows.