16 April 2013
Posted by Callan Bentley
Today, let’s take a look at another site from the pre-GSA-Minneapolis field trip to examine the structural geology of the sub-province boundaries in the Superior Craton:
Do you recognize what you’re looking at there? Here’s another example, this time with scale:
Those are pillow basalts, metamorphosed into greenstone! Annotated:
Take a fresh look, and hopefully their round, pillowy shapes will pop out at you:
These pillow basalts crop out near Ely, Minnesota, and are part of the same broad package of rocks that includes the banded iron formation near Soudan, and the turbidites exposed at Pike River Dam.
The circled “4-X” numbers refer to field trip stops on the fourth day of the trip. These Archean pillow basalts were seen at stops 4-2 and 4-3.
Where primary structures survived, the pillows were defined by a differently-colored rind and a train of vesicles (amygdules) interior of that.
Here’s another vesicle train…
But these pillows have been squashed – and a penetrative cleavage has overprinted all the rock in the outcrop. Can you spot it?
Let’s zoom in.
Thank goodness for the occasional 3D exposure! Here’s that same scene, garnished with annotations:
Cleavage strikes northeast and dips steeply to the south. It’s a nice example of flattening strain (X=Y>Z, in terms of the axes of the strain ellipsoid).
Here’s an animated GIF to give you a sense of this “biscuit-like” geometry:
Plenty to talk about here:
Time to head on out to another outcrop…