Honors College Thesis
 

⁴⁰Ar/³⁹Ar Dating of Umpqua Group Sediments within the Oregon Coast Range: Implications for Provenance

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https://ir.library.oregonstate.edu/concern/honors_college_theses/dn39x862k

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  • New detrital mica ages from the Umpqua Group of the Oregon Coast Range provide constraints on models of Eocene sedimentation. Based on sandstone petrography, previous studies assert that the Klamath Mountains to the south are the most likely source region for Umpqua Group sedimentary provenance. However, the presence of Late Cretaceous to Paleocene (60-90 Ma) muscovites and Eocene zircon grains within the Tyee Formation have led to the assertion that it was derived from the Idaho Batholith and Challis Volcanics (Dumitru et al., 2013; Heller et al., 1985). To test the hypothesis that the Tyee may share a source area with the Umpqua Group, we have dated detrital micas from Umpqua sandstones using the ⁴⁰Ar/³⁹Ar method. Three samples from Umpqua Group strata were irradiated for analysis at the AGES lab at LDEO, and yielded muscovite ages ranging from 80-150 Ma and biotite ages ranging from 50-150 Ma. The presence of a significant number of 60-90 Ma grains suggests that either the Umpqua Group may not entirely be derived from the Klamath Mountains, or that detrital micas of this age within the Umpqua and Tyee sediments were derived from the Klamath Mountains. The ages of Tyee micas could be explained by a source area of the Idaho Batholith and Challis volcanics, but could also be explained by a progressive unroofing sequence originating from the Klamath Mountains (Gaschnig et al., 2010; Heller et al., 1985). If the micas in the Umpqua Group are derived from an eastern source, there are implications for paleotopography during and before deposition. If the young micas are derived from the Klamath Mountains, that would indicate the presence of Cretaceous-Paleocene aged plutonic activity and/or exhumation that has yet to be discovered.
  • Keywords: Oregon Coast Range, Idaho Batholith, Tyee Formation, Radiogenic Isotopes, Thermochronology
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