-
2 Geologic Overview
- Indiana University Press
- Chapter
- Additional Information
13 2 Geologic Overview Alan L. Titus, Eric M. Roberts, and L. Barry Albright III Cretaceous strata in southern Utah were deposited in the proximal portion of the Sevier Foreland Basin. Total thickness of Cretaceous sediments probably exceeded 3000 m in the region before mid-Laramide uplift and erosion . Exposures are primarily found at the Kaiparowits Plateau and around the margins of the Markagunt and Paunsaugunt plateaus and the Pine Valley Mountain region. The Cretaceous section is divided up into the Cedar Mountain, Dakota, Tropic, Straight Cliffs, Wahweap, and Kaiparowits formations east of Parowan Canyon and is contained almost entirely within the Iron Springs Formation west. The sections are highly fossiliferous and yield one of the best records of Late Cretaceous terrestrial ecosystem evolution known in North America. Introduction The state of Utah lies within both the Cordilleran Thrust Belt and Cordilleran Foreland Basin System (Fig. 2.1). The boundary between these two provinces, called the Cordilleran or Wasatch Hingeline (DeCelles, 2004), roughly parallels the east margin of the Sevier Fold and Thrust Belt. West of the Wasatch Hingeline are the extended and dissected remnants of thrust sheet stacks of Precambrian through early Mesozoic sedimentary rocks. East of the Wasatch Hingeline are thick sections of largely flat-lying Paleozoic, Mesozoic, and Paleogene sedimentary rocks (Hintze, 1988). Cretaceous strata crop out widely east of the Wasatch Hingeline, especially in the eastern Wasatch Plateau, Book Cliffs, Henry Basin, La Sal-Abajo Mountains, and the southern portion of the state (Fig. 2.2). In southern Utah, Cretaceous strata crop out almost continuously along a 190-km-long belt from St. George at the west end to the Fifty Mile Cliffs at the east end (Fig. 2.3). This outcrop belt is contained almost entirely within four prominent physiographic features: from east to west, the Markagunt Plateau, the Paunsaugunt Plateau, the Tropic Amphitheater, and the Kaiparowits Plateau (Doelling, 1975; Doelling and Davis, 1989). Of these four physiographic features , the Kaiparowits Plateau (Fig. 2.3), inside Grand Staircase –Escalante National Monument, contains the largest continuous exposures of Cretaceous strata (~850,000 acres) in the southern portion of the state (Sargent and Hansen, 1982). All Cretaceous rock on the Kaiparowits Plateau is within the western portion of the Kaiparowits Basin, a structural low between the East Kaibab Monocline and the Circle Cliffs uplift (Titus et al., 2005). The remaining three areas to the west are collectively part of the Grand Staircase physiographic region. Cretaceous strata also crop out around the 2.1. Reference map showing Utah’s location within the Cordilleran Thrust Belt and Cordilleran Foreland Basin System. Modified from DeCelles (2004). Titus, Roberts, and Albright 14 flanks of the Pine Valley Mountains near St. George, near Gunnison Reservoir, and at Parowan Gap, west of the town of Parowan (Fig. 2.3). Forty kilometers northeast of the east edge of the Kaiparowits Plateau, vast exposures of Cretaceous strata also occur within the Henry Basin, around the flanks of the Henry Mountains (Doelling, 1975). With the exception of those in the Pine Valley Mountains and Parowan Gap areas, most of the Cretaceous outcrops are structurally uncomplicated and are horizontal to gently dipping, except where they intersect the Hurricane Cliffs, Sevier, and Paunsaugunt fault systems, and the East Kaibab , Echo, and Waterpocket monoclines (Fig. 2.3). Both the Pausaugunt and Markagunt plateaus are extensively capped with Paleogene strata (Bowers, 1972, 1990; Biek et al., 2010), whereas the Kaiparowits Plateau had almost all of its Paleogene units removed by erosion during the Neogene. In addition to its substantial middle and upper Campanian terrestrial stratigraphic and fossil record, the Kaiparowits Basin is regionally important for one other reason: its paleogeographic position along the western margin of the Cretaceous Western Interior Seaway. For an extended 20-million-year period, a complex interplay between eustasy, tectonics, and sedimentation rates caused alternating marine and terrestrial deposition that has provided critical biostratigraphic constraints on the largely terrestrial Cretaceous record located to the west. In contrast, in the nearby Henry Basin, marine conditions existed almost continuously from late Cenomanian to early Campanian time (Peterson and Kirk, 1977; Eaton, 1990). Institutional Abbreviation UMNH, Utah Museum of Natural History, Salt Lake City, Utah. Tectonic and Paleogeogr aphic Setting Cretaceous sedimentation in southern Utah initiated in the late Early Cretaceous in response to lithospheric flexure caused by thrust loading within the Sevier Fold and Thrust Belt coupled with eastward progradation of a thick clastic wedge into the Cordilleran Foreland Basin System (DeCelles and Giles, 1996). Because pre-Barremian...