5 April 2012
Getting back to my West Texas field trip now… remember the Castner Marble that we looked at earlier this week? Well, that very same marble is intruded by the Red Bluff Granite a bit further to the east. Here’s a collection of shots along the contact zone. The vertical layered rocks are the Castner Marble’s distinctive layers, and some meta-mudstone as well.
Small fractures on the bedding plane of the greenish meta-mudstone. Looks like a decent conjugate pair of orientations:
Further downhill, the Red Bluff Granite intrudes into some other distinctive rocks.
Can you identify them?
Yes, indeed! Those are the tops of basaltic cooling columns!
And when you get close to the contact with the Red Bluff Granite, you’ll find little squirts of felsic magma appear to have worked their way in between these stiff fingers of basalt…
Columns viewed from the side, so you can see that they are, in fact, columnar:
A bit further along the outcrop, and there is a lovely dike of pegmatite, bearing big “teeth” of riebeckite.
Check out the edge of this fine injection:
Annotated, to show what a cool contact this is:
The Red Bluff Granite may have a partner in Antarctica. It’s been suggested that based on age and isotopic signatures, a block of similar granite in Coats Land may be the Red Bluff’s long lost twin — separated by plate tectonics. This is a very cool geologic unit, both in outcrop, and in application!