PALEOGEOGRAPHY OF THE MIDDLE EOCENE TYEE GROUP, COAST RANGE OF OREGON, USA

The newly defined Tyee group is a 2-3 km thick clastic succession in western Oregon that includes the Tyee Formation (Tyee Mountain, Hubbard Creek, and Baughman members), Elkton Fm, Bateman Fm, and other related units. The Tyee Fm unconformably overlies the deformed Umpqua Group (~ 54–48 Ma) which filled a syn-orogenic foredeep basin during collision and accretion of the oceanic Siletz terrane. The Tyee group accumulated during a transitional period (approx. 47–41 Ma) between the end of terrane accretion and ~40-Ma onset of magmatism in the ancestral Cascades arc. Previous workers suggested that the Tyee group is overlain by the Coaledo Formation in the Coos Bay area, but new detrital zircon (DZ) data suggest that the lower member of the Coaledo Fm is equivalent to the Bateman fm. The Tyee group represents a series of basin-margin clinoforms that prograded to the NW or NNW (in modern coordinates) and were fed by a regional braided river network that flowed NW away from the Klamath Mountains. Clinoform topset deposits (fluvial, deltaic/shoreline, shelf) are represented by the Baughman mbr and Bateman Fm. The slope component is represented by the Hubbard Creek mbr and Elkton Fm, and the entire basin floor component is included in the Tyee Mountain mbr. The NE-trending Tyee coastline prograded nearly 50 km to the NW or NNW (modern coordinates) over the Umpqua Arch (former flexural bulge of the pre-Tyee Umpqua basin); the base of slope presumably prograded in a similar fashion. Clinoform progradation was interrupted by at least one major marine flooding event represented by mudstones of the Elkton Fm. New DZ data yield indistinguishable maximum depositional ages (MDAs) of ~46 ± 1 Ma for the base and top of the Tyee Fm, and statistically similar MDAs for the Bateman (45.1 ± 1.4 Ma) and lower Coaledo fms (44.9 ± 1.1 Ma). Thus, it appears that the Tyee group accumulated rapidly in a short time, and that the lower Coaledo Fm is part of this regional succession. Future work is needed to understand: (1) how the regional crustal structure and basin-filling patterns were influenced by tectonic transition from terrane collision to arc magmatism; and (2) whether the lower Coaledo Fm represents a contiguous lateral component of the Tyee deposystem, a distinct sub-basin, or was fed by a separate sediment dispersal system.