Depositional Relations of Umpqua and Tyee Formations (Eocene), Southwestern Oregon

Studies of the depositional relations of the Umpqua and Tyee Formations (Eocene) in southwestern Oregon indicate a need for reassessing the correlations and currently used terminology. The Umpqua Formation (as herein restricted) consists of as much as 10,000 ft (3,000 m) of mudstone, sandstone, and conglomerate of nonmarine to deep marine origin. A basaltic basement that underlies the sedimentary rocks in most of the area and was formerly included in the Umpqua is herein considered a separate unit and assigned to the Siletz River Volcanics. A proposal to subdivide the Umpqua into three unconformity-bounded formations (in ascending order: Roseburg, Lookingglass, and Flournoy Formations) in the area west of Roseburg, Oregon, is not recognized in this report because of quest onable correlations and limited extent of some units. Foremost of these questionable correlations is that of the Flournoy Formation with rocks formerly assigned to the Tyee Formation in areas as far north as the latitude of Salem, Oregon. My investigations of depositional facies and lithologic criteria do not substantiate this correlation. The Tyee Formation, which conformably overlies the Umpqua, is a predominantly sandstone unit about 6,000 ft (1,800 m) thick, deposited in environments ranging from shallow marine and nonmarine deltaic on the south, to slope and deep marine basinal to the north. Deposition across the Umpqua-Tyee boundary contact represents a change in tectonic setting from active plate margin of the Umpqua to more stable marginal basin deposition of the Tyee. This change is also reflected in a change in provenance as indicated by the characteristic arkosic and micaceous aspects of Tyee sandstones.