INITIATION, UPLIFT, AND ISOLATION OF THE COOS BAY BASIN, SOUTHWESTERN OREGON: NEW INSIGHTS FROM U-PB DETRITAL ZIRCON DATA

Despite more than a century of study along the Cascadia margin of western North America, significant uncertainties remain regarding fundamental aspects of the evolution of this classic arc-forearc system, including the: (1) paleogeography of the margin prior to arc development; (2) inception of Cascade arc magmatism; and (3) timing of forearc uplift and exhumation. We present a large-n (n=4610) U-Pb detrital zircon (DZ) age data set from an ~5 km-thick Cenozoic sequence near Coos Bay, OR. These data provide new constraints on absolute ages, stratigraphic correlations, and provenance that yield insight into the tectono-sedimentary evolution of the Oregon Coast Range and Cascade arc. Facies associations and DZ data indicate that the upper Sacchi Beach fm and Coaledo, Bastendorff and Tunnel Point Fms represent a conformable and overall regressive sedimentary sequence deposited between ~44.9 and ~24.4 Ma in a deep marine slope to shallow marine setting. These units are unconformably overlain by the Tarheel fm and Empire Fm, which are both Miocene-aged based on seven DZ samples and a new age of 8.1 ± 0.1 Ma for the Empire tuff. These data and sandstone petrography confirm a stratigraphic correlation between the coastal Sacchi Beach fm and the inboard Tyee Fm exposed throughout the Coast Range, suggesting that the maximum age of the Tyee should be reduced from ~48 (Wells et al., 2014) to ~45 Ma. We suggest that abundant early Eocene and Late Cretaceous zircons were probably derived from Clarno volcanism and recycled from the Hornbrook Fm in the Klamath Mountains, respectively, implying more proximal sediment sources in central Oregon and northern California rather than input from Idaho via a hypothetical “Tyee paleoriver” (Dumitru et al., 2013). We interpret the paucity of Cascade arc grains (<40 Ma) in the Tarheel and Empire Fms to reflect Oligocene or early Miocene uplift and exhumation of the southern Coast Range, which effectively isolated the Coos Bay Basin from the Cascade arc source. Initial and ongoing uplift in the Coast Range may be driven by sediment underplating and crustal thickening beneath the forearc wedge, perhaps related to enhanced plate coupling due to slab flattening that has been postulated to explain the eastward migration of magmatism from the western to the High Cascades since the Oligocene (Priest, 1990).