Re: Questions from a YEC convert

Arthur V. Chadwick (
Mon, 01 Dec 1997 19:09:33 -0800

At 07:52 PM 12/1/97 -0500, Steve wrote:

> "Toreva" blocks are slumps which occur along listric normal faults and
>are quite common in the Grand Canyon. They're briefly discussed in the
>great reference book "Grand Canyon Geology" edited by Beus and Morales
>(1990, Oxford University Press) and were named in a 1937 paper by Reiche
>("The Toreva Block, A Distinctive Landslide Type" Journal of Geology 45,
>538-548). They commonly occur when the Bright Angel shale at the base of a
>Redwall cliff is undercut by the river and the shale becomes too weak to
>buttress the lithostatic load imposed by the overlying Redwall, et al.
>strata and the whole package rotationally slides along a listric normal
> I fail to follow why you state that they're not associated with known
>faults (they are) and why they can't occur in consolidated sediments (as a
>structural geologist, I don't see any problem with this type of structure
>forming in fully lithified rock).

Whether it can form in fully consolidated rock is a subject for continued
discussion. The Surprise Valley slump is not associated with (I should
have added) known *regional* faults which could have explained its failure.
The extent of the slide in Surprise Valley is considerably larger than
anything that could have been due to undercutting alone (it is about 2
miles wide at the widest spot).
> It's hard to say much without seeing the block, or at least clear
>photographs, but perhaps the brecciation is a fault breccia?

No chance. It is a horizontal slab displaced laterally over a mile from
the nearest outcrop and about a thousand feet lower than Redwall outcrop.