Abstract Transport of sand across shelves to deeper water still poses questions in spite of the acknowledged importance of submarine canyons as accessways, especially because canyons are little recognized in the ancient record. Eocene strata in southwestern Oregon, U.S.A. contain many small-scale channels to 100 m wide and 25 m deep that acted as conduits of much sand from a sandy littoral and deltaic zone across a narrow shelf and slope to feed deeper marine turbidity currents and other gravity flows, which built deepsea fans. The middle Eocene Elkton Siltstone Member of the Tyee Formation (500-600 m thick) is transitional stratigraphically from thick-bedded, mid-fan sandy turbidites also of the Tyee Formation beneath to the coal-bearing deltaic Coaledo Formation above. Foraminifers suggest depths of upper bathyal at the base to inner neritic at the top of the generally fine-grained Elkton; megafauna is extremely sparse although trace fossils are common. Some channels are filled with laminated mudstone-siltstone identical with surrounding material. Many other channels, however, are filled with massive to faintly parallel-laminated and rarely graded light-colored sandstone lacking fauna; spectacular mudstone intraclast conglomerate lenses are associated. At least one small channel levee is identifiable. Sedimentary structures in the channel sands suggest gravity-flow transport and considerable post-depositional deformation. Rare thin Bouma T a and T ab graded beds in the slope mudstones attest to occasional overbanking or levee breaching by gravity flows. Symmetrical ripples and hummocky cross stratification at the top of the Elkton, together with changes in foraminifers, indicate a shoaling trend. In sharp contrast, overlying Coaledo sandstones are coarser, show large-scale cross bedding, much of it contorted, and contain abundant wood, coal, zones of shallow-marine megafossils, and trace fossils. These deposits occur in a series of coarsening-upward cycles that reflect episodic shoreline progradation. Formerly we interpreted the channels as short delta distributary extensions onto the shelf. Recent investigations suggest channels formed deeper and farther from delta fronts, apparently as an array of sea gullies crossing the shelf and slope toward deeper water where they fed sand to deepsea fans. Modern slopes probably have many such channels that are not resolvable by conventional profiling techniques.

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.