SALINA CANYON
One of the world's most
impressive angular unconformities is located just alongside Interstate 70 in
Salina Canyon, Sevier County, central Utah, USA (see
map).
Angular unconformities are
formed by structural tilting of flat-lying sedimentary rocks, followed by
erosion, followed by deposition of more flat-lying sedimentary rocks. The
Salina Canyon angular unconformity can't get more angular than it is
already. You've got flat-lying beds atop vertical beds, separated by an
erosion surface having small-scale irregular topography.
This angular unconformity is
exposed throughout the lower parts of Salina Canyon (Salina Creek valley), east
of the town of Salina, on both sides of Interstate 70 (but better seen on the
northern side of the highway). Eastward of the 1st photo shown below, the
underlying rocks become less vertical, and eventually have a disconformable
relationship with the overlying rocks.
Salina Canyon angular
unconformity - reddish vertical beds below the contact are part of the Twist Gulch
Formation (Middle Jurassic). The light-colored horizontal beds above the
contact include a thin interval of Flagstaff Formation (mid-Paleocene), the
Colton Formation, and the Green River Formation (upper Paleocene to Eocene).
Salina Canyon angular
unconformity - reddish vertical beds below are the Twist Gulch Formation (Middle
Jurassic) - mainly siltstones, sandstones, and shales. The whitish
horizontal beds above are sandstones of the Flagstaff Formation
(mid-Paleocene). Halite salt casts, a rare sedimentary structure, occur
on some of the bedding planes in the Twist Gulch Fm. at this locality.
The structural tilting of
the Twist Gulch in Salina Canyon has been attributed to local salt dome
tectonics. Evaporites occurs in the underlying Arapien Formation (Middle
Jurassic). Some of the Arapien's rock salt intervals have domed upward
(salt diapirs), resulting in tilting of overlying beds. The Salina Canyon
angular unconformity provides information about the timing of Arapien rock salt
diapirism (post-Jurassic & pre-Paleocene - in other words, during the
Cretaceous).
For more info. on the
geology of this area, see:
Lawton & Willis (1987) -
The geology of Salina Canyon, Utah. in Rocky Mountain
Section of the Geological Society of America. Geological Society of
America Centennial Field Guide 2: 265-268.