Abstract

Conventional modal sandstone data from Paleogene units of the Pacific Northwest are not precise enough to pinpoint source areas and constrain displacement histories of accreted sedimentary terranes. Isotopic provenance study, used in conjunction with traditional basin-analysis techniques, provides a powerful means of identifying source areas. Analysis of Rb-Sr data in both wholerock and detrital white micas of Paleogene sandstones from allochthonous units in the eastern core of the Olympic Mountains and coeval autochthonous sandstones from coastal Pacific Northwest shows that the Needles-Gray Wolf and Grand Valley lithic assemblages of the core came from the same source as sandstones of the Chuckanut Formation and Puget Group in northern Washington. The source of these units is isotopically distinct from the source for units farther south, such as the Tyee Formation in Oregon. Chemical compositions, conventional K-Ar age determinations, and oxygen- and hydrogen-isotope compositions of white micas support this conclusion.

Similar analyses of sandstones from the Western Olympic lithic assemblage and the Yakutat terrane of southeastern Alaska suggest that these two units have a similar source, but that they differ slightly from sandstones of the eastern Olympic core and autochthonous Washington units.

The overall composition of sandstones (lithic arkosic), and the very high initial-Sr values (>0.710), moderately high δ18O values (∼+9) and Mesozoic age for detrital white mica of Olympic core rocks and sandstone of the Yakutat terrane suggest a source in the high-rank metamorphic and plutonic rocks from the eastern part of the Omineca Crystalline Belt in southeastern British Columbia. Furthermore, rapid uplift of this source area during Eocene time is consistent with the depositional age of the Olympic rocks. Sediment derived from this source area was transported westward by major river systems that crossed the low-lying North Cascade Range and supplied the deposits of the autochthonous units of the northern Washington coast and the offshore equivalents before the latter were accreted to form the Olympic core.

Limited data from the Yakutat terrane suggest that it lay offshore of southern British Columbia sometime during middle Eocene to early Oligocene time, consistent with paleomagnetic and some paleontologic interpretations, and subsequently migrated northward by plate motions.

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