Permian-Triassic unconformity in the Terrace Mountains, northwestern Utah

Abstract

A paucity of information about the Permian-Triassic boundary in the northern Great Basin has resulted in considerable speculation about the nature of this surface. Two exposures of the unconformity in the Terrace Mountains, Box Elder County, Utah, provide significant information on the contact in this area. At one location, a thin, unnamed, Upper Permian phosphatic unit is disconformable with the underlying Upper Permian Gerster Formation, and the Lower Triassic Dinwoody Formation disconformably overlies the phosphatic unit. At another locality 0.6 km to the northwest, the Lower Triassic Dinwoody Formation rests disconformably on the Upper Permian Gerster Formation, and the younger Permian phosphatic unit is absent. This documents the reports of previous workers about the existence of a locally preserved, erosional remnant of Upper Permian phosphatic rock in this area. This relation and the presence of a basal Lower Triassic conglomerate containing reworked Permian fossils establish that the Permian in this area was eroded to a gently irregular surface with slopes that were probably less than 1 m/km prior to transgression of Early Triassic (Griesbachian) seas.