In Grand Canyon, there is a syncline in the eastern limb where the
Precambrian sediments are in apparent conformable contact with the Cambrian
Tapeats, from which they are indistinguishable lithologically. However, as
one moves laterally, the beds of the Precambrian rise up to cut the
Tapeats, and the unconformity becomes obvious. Also there are lots of
formations in the west (California, Nevada, Utah, etc,) which are
designated Precambrian/Cambrian or Precambrian? because until a trilobite
or some other metazoan fossil is located, the rocks are considered
Precambrian. This "logic" would be tautologic but for the occurrence
nearly everywhere of the boundary unconformity.
Art
http://chadwicka.swau.edu