AR-AR DATING OF DETRITAL MICAS FROM THE EARLY EOCENE UMPQUA GROUP, SW OREGON: IMPLICATIONS FOR SEDIMENT PROVENANCE AND PALEORIVER MODELS

New detrital mica single-grain Ar-Ar ages from strata of the Umpqua Group in SW Oregon provide a new constraint on currently debated models for Early Eocene sediment sources, tectonic evolution and regional paleo-rivers. Previous studies asserted that relatively young (Late Cretaceous, ca. 60-90 Ma) K-Ar and Ar-Ar ages measured on detrital mica grains from the overlying Tyee Formation require that the Tyee sediments were derived from western Idaho, not the Klamath Mountains. While this assertion is broadly in accord with reported bedrock mica ages from the Klamaths, published bedrock ages by no means represent all potential source regions in the Klamath Mountains. Furthermore, to the best of our knowledge no detrital ages from micas of the Umpqua Group have been published to date. To test a new hypothesis that Late Cretaceous muscovites of the Tyee formation may be derived from an as-yet undiscovered Klamath Mountains source, we have started measuring detrital Ar-Ar ages of detrital micas from the Umpqua Group. These strata are immediately subjacent to the Tyee Formation, have a well-established and undisputed source in the Klamath Mountains, and locally contain abundant detrital muscovite grains. Three samples from the Umpqua Group were irradiated for analysis at the AGES lab at LDEO. The first set of data record muscovite Ar-Ar ages ranging from 80-150 Ma and biotite Ar-Ar ages ranging from 50-150 Ma. The presence of a significant number of 60-90 Ma detrital mica grains requires that micas of this age were exposed in the Klamath Mountains during deposition of the Umpqua Group. These new data refute a long-standing argument that micas of this age in the Tyee Formation must have been derived from western Idaho (Renne et al., 1990; Heller et al., 1992), and raise the possibility that both the Umpqua Group and Tyee Formation were derived from an eroding source in the Klamath Mountains, as was concluded in most pre-1980 studies. This interpretation is supported by previous studies that document a single phase of rapid subsidence, basin filling, and progressive unroofing of a single sediment source from the lower Umpqua Group through the Tyee Formation (Ryu, 1995; Ryu and Niem, 1999) during Early Eocene growth of the northern Klamaths orogen due to collision of Siletzia with North America (Wells et al., 2014; Dorsey et al., 2019).