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 The Eocene Tyee Formation of west central Oregon, USA, records deposition in a forearc basin. With outcrop exposures of fluvial/deltaic to shelf and submarine fan depositional environments and known sediment sourcing constrained by detrital zircon dating and mineralogy linked to the Idaho Batholith, it is possible to place deposits of the Tyee Formation in a source-to-sink context. A research program carried out by the Department of Geological Sciences at The University of Texas at Austin and ExxonMobil Research Company’s Clastic Stratigraphy Group has reconstructed the Eocene continental margin from shelf to slope to basin floor using outcrop and subsurface data. This work allows us to put observations of individual outcrops into a basin-scale context. This field trip will visit examples of depositional environments across the entire preserved source-to-sink system, but it will focus on the deep-water deposits of the Tyee Formation that range from slope channels to proximal and distal basin-floor fans. High-quality roadcuts reveal the geometry of slope channel-fills in both depositional strike and dip orientations. Thick, sand-rich medial fan deposits show vertical amalgamation and a high degree of lateral continuity of sandstones and mudstones. Distal fan facies with both classic Bouma-type turbidites and combined flow or slurry deposits are well exposed along a series of new roadcuts east of Newport, Oregon. The larger basin-scale context of the Tyee Formation is illustrated at a quarry in the northern end of the basin where the contact between the oceanic crust of the underlying Siletzia terrane and submarine fan deposits of the Tyee Formation is exposed. The Tyee Formation provides an excellent opportunity to see the facies and three-dimensional geometry of deep-water deposits, and to show how these deposits can be used to help reconstruct ancient continental margins.