Abstract

Upper Cretaceous to Paleogene nonmarine sedimentary rocks of southwest Utah record Sevier foreland basin sedimentation, Laramide-style folding and intermontane sedimentation, and cessation of Laramide deformation. The formations that record this tectonic evolution are, from oldest to youngest, the Iron Springs, Kaiparowits, Canaan Peak, Grand Castle (informal name), Pine Hollow, and basal part of the Claron.

The upper part of the Santonian to lower Campanian(?) Iron Springs Formation represents synorogenic, fluvial deposits derived from the Wah Wah and Blue Mountain thrust sheets of southwestern Utah. The middle to upper Campanian Kaiparowits and upper Campanian(?) to lower Paleocene Canaan Peak Formations are an upward-coarsening sequence derived from southeastern California and southern Nevada. Initial Laramide-style deformation occurred during latest Cretaceous or early Paleocene time, influencing the depositional pattern of the Canaan Peak fluvial system. The lower Paleocene Grand Castle formation represents an east- to southeast-flowing, braided-river system with the same source as the Iron Springs Formation (the Wah Wah and Blue Mountain thrust sheets). Conglomerate of Grand Castle onlaps the easternmost Sevier thrusts and is folded by Laramide structures. Although strata of the Grand Castle formation represent post-thrust and, in part, pre-Laramide deposition, initial development of a south-southwest-trending, Laramide-style upwarp controlled the geometry of the Grand Castle basin.

The lower Paleocene to middle Eocene Pine Hollow Formation unconformably overlies post-Sevier conglomerates and records Laramide partitioning of the foreland basin. The Pine Hollow basin received sediment from both the west and northeast, and it is associated with development of the Johns Valley anticline and possibly the Circle Cliffs uplift. Small alluvial fans developed on the limbs of these structuresand graded laterally into playa mudflats. Fluvial, deltaic, and lacustrine deposits of the basal part of the Claron Formation are coeval with the Pine Hollow Formation, but they developed in a separate basin to the southwest. Lacustrine deposits of the Claron Formation overlap paleotopographic highs of the Pine Hollow basin and indicate cessation of Laramide deformation by middle Eocene time.

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