A bit of a minor correction for the very first post of this thread. The Great Unconformity of the Grand Canyon is not always an angular unconformity. When the tilted layers run out, it is directly on the Vishnu Schist bedrock and thus becomes a nonconformity. In other words when horizontal Tapeats Sandstone is directly on top of the tilted strata of the Grand Canyon Supergroup, it is an angular unconformity and then it is directly on top of the Vishnu Schist which is not layered rock, it is a nonconformity. It is the same unconformity since it is one erosional surface, what changes is what is the rocks below that surface.
The Great Unconformity is not only exposed at the Grand Canyon, it is also exposed near Los Vegas at a place Frenchman Mountain. Thus one can visit the unconformity without going deep into the canyon. I recommend taking a look at the
Frenchman Mountain Great Unconformity site which is rather nice. The
last page of the site has an interesting image (which I shrink here):
I think we can add a few more steps in the formation of the geology in the area! I know this is pushing topic: someone might want to start a thread on faults. But this is where the subject of faults and unconformities overlap a bit since both are involved.
Until tomorrow when I plan to boldly take this thread quite a bit farther from home with an angular unconformity that is out of this world.
This message has been edited by Harlequin, 01-11-2005 15:38 AM