Abstract: Paleoecology, Biostratigraphy, and Depositional History of Paleocene to Middle Eocene Rocks of the Tyee Basin, Western Oregon

Elisabeth Brouwers, David Bukry, Louie Marincovich Jr., In-Chang Ryu

Paleogene rocks that crop out in the Tyee Basin provide a record of the depositional history and paleogeography of a forearc basin sequence. Knowledge based on fossil faunal and floral assemblages yield inferred water depths for the basal oceanic basalts and for interbedded and overlying marine sediments provide constraints on the environment of deposition for these rocks. Ostracode and mollusk faunas contribute information on water depth, and calcareous nannofossils give precise time lines for the stratigraphic sequence.

The basement Siletz River Volcanics consist of subaqueous basalt flows and pillow basalts with interbedded basaltic siltstones. K-Ar dates indicate 59-62 Ma; nannofossils yield an age greater than 54 Ma. Sparse mollusks from the siltstones indicate shallow water depths for the pillow basalts near Roseburg, ranging in depth from shoreline to inner shelf (0-100 meters). The conglomeratic Bushnell Rock Formation is 53-54 Ma (nannofossil zone CP 9); mollusks indicate inner to middle shelf water depths (0-150 meters), but they may have been transported downslope. The Tenmile Formation is a turbidite sequence and the White Tail Ridge Formation consists of shelf sandstones and siltstones. Both formations were deposited rapidly between 52.5 and 53 Ma (CP 10); mollusks and ostracodes indicate middle shelf water depths (50-150 meters), shallowing to inner to middle shelf water depths toward the basin margin at Roseburg. The Camas Valley Formation is 50.5-51.5 Ma (CP 11); ostracodes and mollusks indicate middle shelf water depths within the basin (50-150 meters), shallowing to inner to middle shelf water depths (0-150 meters) toward the margin around the Umpqua River.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California