Deep-water deposits of the Eocene Tyee Formation, Oregon

Abstract

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.