DETAILED GEOLOGIC MAP OF THE SOUTHERN PORTION OF THE THREE PEAKS 7.5’ QUADRANGLE, IRON COUNTY, UTAH, USA: A RECORD OF SEVIER OROGENESIS, IRON-OXIDE-APATITE MINERALIZATION, AND BASIN AND RANGE EXTENSION

The Three Peaks range lies ~12 km NW of Cedar City, Utah in a fault block near the eastern edge of the Basin and Range Province. Here, field relationships are vital in understanding Sevier orogenesis, emplacement of iron oxide-apatite (IOA) deposits, and water resources management adjacent to an expanding Cedar City. While the area was previously mapped in the late 1940s, our work provides new insights through the recognition of previously unknown faults and map units.

Exposed in the map area are Jurassic to Cretaceous sedimentary rocks, including the synorogenic Iron Springs Formation (ISF), deformed in the hanging wall of the Iron Springs thrust, the Paleogene Claron Formation, deformed, along with the older rocks, by laccolith intrusion, the Miocene Three Peaks laccolith, and Paleocene to modern unconsolidated alluvial fan sediments. Regional extension contributed to recent deformation. Our mapping was motivated by the following goals: update the stratigraphic nomenclature of the ISF, define petrographically distinct units in the Three Peaks laccolith, and refine the mapping of unconsolidated deposits on the alluvial fan complex.

Based on our results, the ISF at Three Peaks is subdivided into five distinct informal members: The Marshall Creek breccia (which we consider to be the basal member of the Iron Springs Formation), the recently discovered Three Peaks Tuff Member, the lower ISF, consisting of interbedded limestone, sandstone, mudstone, and conglomerate, the middle ISF, consisting predominately of mudstone interbedded with thin intervals of sandstone with sparse pebble to cobble conglomerate lenses, and the upper ISF, marked by the appearance of a cobble to boulder conglomerate. We recognize three petrographically distinct units in the laccolith, separated by gradational contacts; unit 1 has a color index (CI) of ~7-10, unit 2 has a CI of ~ 4-8, and unit 3 has a CI of less than 4. The alluvial fan complex was remapped using updated terminology and units.

Our map provides crucial contextual field relationships necessary for understanding the origin of the IOA deposits, the stratigraphy of the Iron Springs Formation, and deformation related to the regional tectonism; it is also useful to land managers for evaluating seismic risk from Basin and Range faults, and aquifer properties of unconsolidated sediments.