1 March 2010

Crucifix site 2: Horsts & grabens

Posted by Callan Bentley

So, those sediments we saw yesterday? They’re faulted in the area around the Crucifix Site.

As this image shows, the style of faulting is normal faulting:chalk_bluff_stuff_05

Annotated with some color to jazz things up a bit:chalk_bluff_stuff_05anno

In normal faults, the upper block moves downward with respect to the lower block. They are typical of extensional tectonic settings.


Numerous small faults here — a little bit of displacement on each:chalk_bluff_stuff_01

Ditto for this exposure (same place, just zoomed out a bit):chalk_bluff_stuff_18

Nice! Two horsts and a central graben:chalk_bluff_stuff_02

The normal faults exposed at the Crucifix Site are a lot like those we observe a kilometer to the north, up on the Volcanic Tableland. The difference is that the Volcanic Tableland faults disrupt the stiff upper layer of the Bishop Tuff (“Ig2”), while these Crucifix faults are smaller and more subtle — cutting through weaker stuff.