ABSTRACT

Upper Cretaceous to Eocene clastic rocks of southwest Utah record active Sevier-style thrust deformation, cessation of thrusting, and sedimentation related to Laramide-style folding. The formations which record these tectonic events include, from oldest to youngest: the Iron Springs, Kaiparowits, Canaan Peak, Grapevine Wash, Grand Castle (informal name), Pine Hollow, and Claron formations.

Two petrographically distinct upward-coarsening petrofacies occur in the foreland basin and are herein named for the formations they comprise: the Kaiparowits-Canaan Peak and Iron Springs-Grand Castle petrofacies. The Kaiparowits-Canaan Peak petrofacies consists of volcanic and radiolarian chert lithic fragments derived from sources in southeastern California and southern Nevada. The finer-grained Kaiparowits Formation was deposited during active thrusting in the middle-to-upper Campanian while the conglomeratic Canaan Peak Formation was deposited during cessation of thrust activity between the upper Campanian(?) and lower Paleocene. The Iron Springs-Grand Castle petrofacies contains quartzite and carbonate clasts and sandstone compositions that suggest derivation from the Wah Wah and Blue Mountain thrust sheets in southwestern Utah. The Iron Springs Formation represents Upper Cretaceous, syn-thrusting deposition, whereas the Grand Castle conglomerates were deposited during lower Paleocene rebound of these thrust sheets.

The Upper Cretaceous to Paleocene Grapevine Wash Formation is an areally restricted conglomeratic alluvial fan sequence composed of carbonate and sandstone clasts derived from Paleozoic rocks in the upper plate of the Square Top Mountain thrust in southwestern Utah.

The Pine Hollow Formation received sediment during development of Laramide-style folding of the Sevier foreland basin during early Paleocene to middle Eocene time. Jurassic and Cretaceous sedimentary units tilted during deformation shed detritus to the east and southwest into an intermontane basin.

The clastic basal part of the Claron Formation (Paleocene to Eocene) also received sediment from exposed highlands related to Laramide structures. The abundance of silty carbonate lithic grains suggests that much of the sedimentation in the Claron basin was intraformational.