Seventy-six samples from the type Bastendorff Formation exposed near Coos Bay, Oregon, were examined to determine if significant calcareous plankton were present. Planktonic foraminifera and/or calcareous nannofossils are recorded from 24 of those samples. Planktonic foraminiferal zones believed present are the Globigerinatheka semiinvoluta and Globorotalia cerroazulensis s.l. Zones of Bolli and Zones P15, P16, and P17 of Blow. Nannoplankton zones recognized are the Chiasmolithus oamaruensis and Isthmolithus recurvus Subzones of Bukry; these are considered equivalent to Zone NP18 and Zones NP19 and NP20 of Martini, respectively. The middle and upper parts of the Bastendorff—assigned by Tipton to the provincial uppermost part of the Narizian Stage and the Refugian Stage, respectively, on the evidence of benthic foraminiferal distribution—are of late Eocene age based on calcareous plankton.

ABSTRACT This field guide reviews 19 sites providing insight to four Cenozoic deformational phases of the Cascadia forearc basin that onlaps Siletzia, an oceanic basaltic terrane accreted onto the North American plate at 51–49 Ma. The field stops visit disrupted slope facies, prodelta-slope channel complexes, shoreface successions, and highly fossiliferous estuarine sandstones. New detrital zircon U-Pb age calibration of the Cenozoic formations in the Coos Bay area and the Tyee basin at-large, affirm most previous biostratigraphic correlations and support that some of the upper-middle Eocene to Oligocene strata of the Coos Bay stratigraphic record represents what was differentially eroded off the Coast Range crest during ca. 30–25 Ma and younger deformations. This suggests that the strata along Cape Arago are a western “remnant” of the Paleogene Tyee basin. Zircon ages and biostratigraphic data encourages the extension of the Paleogene Coos Bay and Tyee forearc basin westward beyond the Fulmar fault and offshore Pan American and Fulmar wells. Integration of outcrop paleocurrents with anisotropy of magnetic susceptibility data from the middle Eocene Coaledo Formation affirms south-southeast to north-northwest sediment transport in current geographic orientation. Preliminary detrital remanent magnetism data show antipodal directions that are rotated clockwise with respect to the expected Eocene field direction. The data suggest the Eocene paleo-shoreline was relatively north-south similar to the modern shoreline, and that middle Eocene sediment transport was to the west in the area of present-day Coos Bay. A new hypothesis is reviewed that links the geographic isolation of the Coos Bay area from rivers draining the ancestral Cascades arc to the onset of uplift of the southern Oregon Coast Range during the late Oligocene to early Miocene.