Paleogeography and Structural Development of the Late Pennsylvanian to Early Permian Oquirrh Basin, Northwestern Utah

Abstract

Abstract Deposition in the Oquirrh Basin of northwestern Utah produced up to 7.5 km thickness of late Paleozoic limestone and sandstone. Study of Upper Pennsylvanian and Lower Permian lithofacies, trace fossils, and body fossils relative to a time framework determined by fusulinid biostratigraphy reveals a spectrum of depositional environments, which shifted through time. Shallow-water, organically productive marine environments were recorded in accumulation of a great thickness of Middle Pennsylvanian limestone and sandstone. In the Late Pennsylvanian, vast quantities of subarkosic sand and increased water depth reflect a significant change in basin morphology. A northwest-trending trough developed from the northwest part of the basin during the Late Pennsylvanian, extending the length of the basin by early to middle Wolfcampian. Water depths may have reached 300-400 m (1000-1300 ft), and stagnant water in the northwest part of the basin ultimately became poorly oxygenated. Shallower water siltstones capped the sequence in the late Wolfcampian to Leonardian. Coarse conglomerates, commonly composed of older clasts from the Oquirrh Group, are common in the uppermost Pennsylvanian and lower Wolfcampian deep-water deposits. They record rapid lithification, uplift, and erosion of the margins of the trough, and imply that the Oquirrh Basin was bounded by active northwest-trending high-angle faults at this time. These faults were apparently initiated in the Late Pennsylvanian and became inactive by the late Wolfcampian. Sedimentary rocks near the upper limits of the Oquirrh Group record passive filling of the remnant graben-like trough. The evolution of the Oquirrh Basin is similar to that of basement uplifts and yoked basins throughout the region to the south and east. The basin’s northwest trend, high-angle fault margins, and rapid structural development in the latest Pennsylvanian and early Wolfcampian demonstrate a tectonic association with the regional deformation which formed the Ancestral Rocky Mountains.