The eight field trips in this volume, associated with GSA Connects 2021 held in Portland, Oregon, USA, reflect the rich and varied geological legacy of the Pacific Northwest. The western margin of North America has had a complex subduction and transform history throughout the Phanerozoic, building a collage of terranes. The terrain has been modified by Cenozoic sedimentation, magmatism, and faulting related to Cascadia subduction, passage of the Yellowstone hot spot, and north and westward propagation of the Basin and Range province. The youngest flood basalt province on Earth also inundated the landscape, while the mighty Columbia watershed kept pace with arc construction and funneled epic ice-age floods from the craton to the coast. Additional erosive processes such as landslides continue to shape this dynamic geological wonderland.

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.

Microfossils recovered from the Roseburg, Lookingglass, and Flournoy Formations (Umpqua Group) of southwest Oregon include 33 species and subspecies of planktonic foraminifers. Some of the faunas from these units include as many as 14 species per sample, and specimen preservation is moderate to good. The assemblages are similar to those from central and southern California but differ in that they are dominated by long-ranging, high-latitude species. The Roseburg and Lookingglass Formations were deposited during early Eocene time and are assigned on the basis of planktonic foraminifers to Zone P7-8 of the standard tropical zonation. The planktonic foraminiferal faunas of these units are indistinguishable. The Flournoy Formation is assigned to lower middle Eocene Zone P10. Planktonic foraminifers and calcareous nannofossils indicate that the unconformity between the Lookingglass and Flournoy Formations represents an interval of approximately 1 to 2 m.y. Despite reports of Paleocene microfossils occurring in the Roseburg and Lookingglass Formations, no microfossils definitely of that age were found during this study. Cretaceous microfossils were recovered from a small outlier currently included in the Roseburg Formation. Although the rocks containing these fossils closely resemble those of the Roseburg Formation, they probably belong to a separate unit.