Monday, July 16, 2012

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.

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