> What is *surprising* about the Surprise Valley slump is that it is huge and
> does not appear to be associated with any known faults (except it is of
> course itself fault bounded) and is a Toreva-type slump (base outward
> rotation). This type of slump occurs most readily in unconsolidated or
> partially consolidated materials, and the present condition of the canyon
> walls certainly does not encourage this kind of development.
"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 fault.
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).
It's hard to say much without seeing the block, or at least clear
photographs, but perhaps the brecciation is a fault breccia?
-- Steven H. Schimmrich KB9LCG email@example.com Department of Physical Sciences Kutztown University 217 Grim Science Building, Kutztown, PA 19530 (610) 683-4437 http://www.uiuc.edu/ph/www/s-schim Fides quaerens intellectum