Abstract

In the central Cascade Range of Washington, three structural blocks of early Tertiary sedimentary and volcanic rocks help to define the position and history of the southern segment of the Straight Creek fault. East of the fault, in the Teanaway River block, the early Eocene fluviatile feld-spathic sandstone of the Swauk Formation is interbedded with largely dacitic volcanic rocks of the Silver Pass Volcanic Member. Zircon fission-track ages on the Silver Pass are about 52 m.y. Overlying the tightly folded Swauk and Silver Pass is the Teanaway Formation, a gently dipping accumulation of basalt, andesite, rare dacite, and rhyolite that yields middle Eocene (about 47 m.y.) whole-rock K-Ar ages. Conformably overlying the Teanaway is the Roslyn Formation, a coal-bearing fluviatile feld-spathic sandstone of probable middle to late Eocene age.

The late Eocene and Oligocene(?) Naches Formation, exposed west of the Straight Creek fault in the Cabin Creek block, is rich in fluviatile feldspathic sandstone and rhyolite flows and tuffs in its lower part but grades upward into a sequence dominated by basalt but with some andesite. The basal Guye Sedimentary Member of the Naches Formation underlies and is interbedded with the Mount Catherine Rhyolite Member in the Snoqualmie Pass area. On the basis of zircon fission-track ages on silicic tuffs and whole-rock K-Ar ages on basalt, the basal part of the Naches is about 40 to 44 m.y. old. The formation is tightly folded and complexly faulted along the Straight Creek fault.

The Straight Creek fault intersects the Olympic-Wallowa lineament in the strongly deformed Manastash River block. In this block, fluviatile coal-bearing feldspathic sandstone of the Manastash Formation is overlain by principally dacitic volcanic rocks of the Taneum Formation. Overlying the Taneum is the basalt of Frost Mountain. We correlate this threefold sequence, feldspathic sandstone-dacite-basalt, with the sequence in the Teanaway River block.

Small patches of rhyolitic ash-flow tuff, dated at 33 m.y. (late Oligocene) and interbedded feldspathic sandstone overlie the Roslyn and Teanaway Formations in the Teanaway River block and are probably correlative with the late Oligocene Wenatchee Formation exposed in the Chiwaukum graben east of the Teanaway River block as well as with the late Oligocene volcanic rocks (30 m.y.) along the Cascade crest to the southwest. These latter rocks are little deformed and unconformably overlie the Naches Formation.

The ages and depositional record in the three blocks indicate a Tertiary history of dominantly vertical movement along the Straight Creek fault and its southeasterly splays that merge with the Olympic-Wallowa lineament. The horst-and-graben structure of the blocks, as well as enechelon fold axes in the Swauk Formation, suggests some post–early Eocene, right-lateral shear along the fault, although there is no direct evidence of lateral offset. Vertical movement, significant since early Eocene Swauk deposition, followed late Eocene Naches deposition but tapered off by late Oligocene time and ceased by Miocene time, when the fault was intruded by the 25-m.y.-old Snoqualmie batholith and other plutons. However, structures with Olympic-Wallowa lineament trends appear to have influenced folding of the Miocene Yakima Basalt Subgroup.

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