10 April 2013

Primary structures in the Lake Vermilion Formation

Posted by Callan Bentley

This is the final outcrop that we visited on the fall 2011 “Structural Geology of the subprovince boundaries in the Archean Superior Province” field trip that I took just before the fall 2011 annual meeting of the Geological Society of America in Minneapolis. It was a great trip, full of great outcrops, but this one was one of the best. I’m going to split my discussion of the place into two posts – one for the primary structures (today, here), and another later in the week for the secondary (tectonically-induced; deformational) structures.

The Lake Vermillion Formation is well exposed at Pike River Dam in northern Minnesota. It’s a gorgeous succession of turbidite strata. Graded bedding is very common there, and exquisitely photogenic.

According to the field trip guide, 64% of the 201 graywacke beds in this outcrop exhibit grading, as do 9% of the inter-lying silt beds.

There are also lovely examples of soft-sediment load-induced deformation:

This means that a fresh load of turbidity-current-deposited sand was dumped atop some squishy mud, and the sand sagged downward in broad lobe-like forms, while the less viscous mud squished upward into pointy cusps called “flame structures.”