Isotopic provenance of sandstones from the Eocene Tyee Formation, Oregon Coast Range

Abstract

The Tyee Formation of Eocene age in the Oregon Coast Range has been studied by a variety of isotopic techniques in order to determine its provenance. Traditional basin analyses including paleocurrent measurements, lithofacies mapping, and study of sandstone compositions made previously suggest derivation from the Klamath Mountains, which lie to the south. In contrast, the isotopic compositions of whole-rock sandstone samples, white mica, and potassium feldspar separates preclude derivation solely from this local source area. Nd-Sm, Rb-Sr, K-Ar, 18 O/ 16 O, and D/H analyses of sandstones from the Tyee and related formations yield the following information about their source areas. (1) Whole-rock ϵ Nd values between −7.1 and −7.3 at the time of deposition indicate that an old crustal component (∼700 Ma) was incorporated in the source rocks. (2) Whole-rock Rb-Sr systematics implies an older age than those of sandstones clearly derived from the Klamath Mountains. These Rb-Sr values are similar to those of modern sands of the Columbia River that were derived from eastern source areas. (3) Either apparent Rb-Sr ages of potassium feldspars are too old or their initial 87 Sr/ 86 Sr ratios are too high to have been derived from plutonic rocks of the Klamath Mountains or Sierra Nevada. (4) White micas have a fairly consistent Late Jurassic Rb-Sr isochron but have K-Ar ages of 68 Ma, an overprint not recognized in the Klamath terranes. (5) White micas have δ 18 O values of ∼9.5, a value typical of S-type granges such as are found in the Idaho batholith, but too high for normal I-type granites such as in the northern Sierra Nevada and too low for metamorphic rocks in the Klamath Mountains. (6) White micas have δD values consistent with those observed for plutonic white mica in the Idaho batholith, but markedly lower than those of white mica from schist in the Klamath Mountains. (7) Potassium feldspars have δ 18 O values that vary widely and that mainly are not in oxygen-isotope equilibrium with coexisting white mica, suggesting that these minerals were not derived from the same source area. These results indicate that the provenance of these sandstones included S-type (two-mica) granites that formed in Late Jurassic time from sources that included an old crustal component. Minerals in the granites underwent subsequent thermotectonic age resetting in Late Cretaceous time. Rocks in the Klamath Mountains and northern Sierra Nevada do not possess these features and consequently are precluded from being major source areas for the Tyee Formation. The sandstones most likely were derived from the Idaho batholith. Abundant detritus from that source area is consistent with a model in which the Oregon Coast Range basin lay much farther east, closer to Idaho, during deposition and subsequently moved westward to its present position. Such major displacement is compatible with the tectonic-rotation history documented for the Oregon Coast Range that began during the time of deposition of the Tyee Formation.