AGE AND PROVENANCE OF EOCENE NONMARINE SANDSTONES IN THE CENTRAL AND NORTHWEST WASHINGTON CASCADES: FISSION TRACK EVIDENCE FROM DETRITAL ZIRCONS

Fission track (FT) ages were determined for detrital zircons from 24 sandstone samples of the major nonmarine Eocene sedimentary units in the Washington Cascades. The detrital zircon FT ages are broken down into age populations which can be correlated with specific nearby source domains characterized by distinctive zircon FT ages. A western domain, lying west of the Straight Creek fault (SCf), consists of Mesozoic accreted terranes with FT ages of 180-100 Ma. Three zircon FT sources have been identified in the crystalline plutonic-metamorphic rocks of the Cascades core, lying east of the SCf. The structural block between the SCf and the White River shear zone (WRsz) includes the Mt. Stuart domain (FT ages of 92-65 Ma) and the Nason domain (72-55 Ma). The Chelan domain, between the WRsz and the North Creek-Foggy Dew fault, has zircon FT ages in the range 57-37 Ma.

Detrital zircon FT age populations in nonmarine sandstones in the Central Washington Cascades can all be correlated with local sources. Sandstones of the early Middle Eocene Swauk Fm. (depositional age 54-49 Ma) have large Mt. Stuart, moderate Nason, and small western detrital zircon FT age populations. Late Middle Eocene sandstone units, the Roslyn, Naches and Chumstick Fms. (depositional ages of 45-36 Ma) have a variable mix of Mt. Stuart, Nason and Chelan FT age populations. Dated samples of the upper Roslyn and Naches Fms. have a 36 Ma age population, a maximum age for deposition. Zircon FT ages in the Swakane gneiss, the basement of the Chumstick Fm., are about 43 Ma, a maximum for deposition.

Sandstones of the early Middle Eocene Chuckanut Fm. of the NW Cascades foot-hills have a large Nason detrital zircon FT age population (~58 Ma), a moderate to large Mt. Stuart population (95-72 Ma), and a variable population of old zircons (180-100 Ma). The detrital zircon FT ages are similar to those of the coeval Swauk Fm. This points to a common source, and supports speculation that the two units were deposited in close proximity and were later separated by about 170 km of dextral offset on the SCf. Since this offset is nearly the same as the separation postulated for Mesozoic units in the Central Washington Cascades from similar units in southern B.C., it is clear that the SCf was not initiated until after Swauk deposition had ended.