About This Item
Share This Item
A significant component of the Eocene Tyee Formation of Eocene age in the Oregon Coast Range was not derived from local source areas but appears to have come from terranes that presently lie far to the east. Traditional basin analyses, including studies of paleocurrents, lithofacies, and sandstone compositions, have generally been interpreted as indicating local derivation from the Klamath Mountains which lie to the south. However, isotopic analyses of whole-rock sandstone samples and mineral separates preclude derivation solely from the Klamath region.
Sandstone samples collected throughout the Tyee basin were analyzed for Sm-Nd, Rb-Sr, K-Ar, and 18O. Whole-rock negative ^egrNd values (-7.1 to -7.3) coupled with positive ^egrSr values imply a source area underlain by Precambrian continental crust. Whole-rock Rb-Sr data fall along a trend dissimilar to data from Klamath-derived sandstones. Detrital K-feldspar has 87Sr/86Sr ratios that are too high to have been derived from plutonic rocks in the Klamaths. Consistent Rb-Sr ages for white mica from samples throughout the basin indicate a Late Jurassic age, and K-Ar ages have been uniformly reset to about 68 Ma, an overprint not recognized in the Klamath terranes. ^dgr18O values of white micas cluster around 9.5 per mil, a d are similar to values for S-type granites like those in the Idaho batholith but higher than those of normal I-type granites. Also, these values are much lower than those of white micas from metamorphic rocks in the Klamath Mountains.
These data preclude the Klamath Mountains as the principal source for detritus feeding the Tyee basin. The most likely source region is the Idaho batholith. Derivation of abundant sediment from this eastern source area suggests that during deposition the Oregon Coast Range basin lay much farther east and has subsequently moved westward to its present position. Such a major displacement is consistent with the inferred tectonic rotational history that has been suggested for the Oregon Coast Range since the time of deposition of the Tyee Formation.
End_of_Article - Last_Page 485------------