U-Pb Geochronology Of Igneous Rocks In Southeastern Arizona To Constrain A Minimum Age Of Deposition For The Fort Crittenden Formation With Implications For Laramide Tectonics
AuthorSmith, Joshua Martin
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PublisherThe University of Arizona.
AbstractThe Fort Crittenden Formation comprises a ~2.5 km thick sequence of fluvial-alluvial upper-Cretaceous sedimentary units of interest due to rich fossil record, and implications for Laramide tectonics in southeastern Arizona. Dating of the Fort Crittenden Formation is important for understanding geological evolution of Arizona and western North America. Previous studies (Dickinson and Hayes, 1986) provided an age of ca. 75 Ma based on K-Ar geochronology for the age of deposition for the Fort Crittenden Formation. The Salero Formation, which is stratigraphically on top of the Fort Crittenden Formation was dated to ca. 74 Ma (Inman, 1982) using K-Ar geochronology. Based on paleontological content, the Fort Crittenden Formation has been interpreted to be Campanian-Maastrichtian in age (84-67 Ma). This research aims at more precisely constraining the depositional age of this formation using zircon Uranium-Lead geochronology (LA-ICPMS) from four samples (Figure 1): a hypabyssal intrusion within the Fort Crittenden Formation, an andesite unit of the Salero Formation overlying the Fort Crittenden Formation near Mount Fagan, a dacite unit of the Salero Formation above the andesite unit, and a rhyolite unit of the Salero Formation. New zircon U-Pb data ages from the intrusion (75.8 Ma), dacite (75.4 Ma), and rhyolite (73.4 Ma) reveal an abundance of late-Cretaceous ages for these samples whereas U-Pb ages of the andesite sample reveal no late-Cretaceous ages. Our results indicate the top of the Fort Crittenden Formation is 75 Ma (older than Maastrichtian), and the base is older than 75 Ma. This has implications for the timing and mechanism of Laramide tectonics and magmatism in southeastern Arizona. Deposition of the upper Fort Crittenden Formation marks changes in volcanism from andesitic-dacitic (75.4 Ma) to rhyolitic (73.4 Ma) over 2 million years, which may reflect changes in subduction dynamics.