Minnesota Geology > Soudan Banded Iron Formation detail by ahochstaedter

A closer look at the Archean Soudan Banded Iron Formation in northern Minnesota. The outcroop is located at Soudan State Park, site of the oldest Iron mine Minnesota.

More details and a broader view here:
gigapan.org/gigapans/94833Glacial striations on this outcrop trend from the top towards the bottom of the image.

In this outcrop the prominant folds are F2, and have near vertical axes.

The structure in the upper left of the image is puzzling. I orignally thought it was a doubly plunging fold, but I can't get my mind around that explanation if the isoclinal folds have near vertical axes. Any ideas?

Zooming in on this image reveals millimeter scale laminations, perhaps resulting from differing proportions of Silica and Iron precipitating out of the anciant ocean. Why these variations occur is open to debate. Seasons? El Nino or PDO-like oceanic variations? Decades? Centuries? Millenia?

An even closer view is here:
zoom.it/O0KE  I am indebted to Mark Jirsa and John Green for leading a pre-GSA field trip to these rocks in October, 2011.