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SUMMARY OF THE GEOLOGY OF THE SKYKOMISH RIVER 30-MINUTE BY 60-MINUTE QUADRANGLE, WASHINGTON

The Skykomish River quadrangle is almost bisected by the Straight Creek fault (see fig. 1 below) and contains evidence of the fault's Tertiary history. This major structure extends from central Washington into Canada and has been interpreted to have from 80 to 192 km of right-lateral strike-slip offset (Misch, 1977; Vance and Miller, 1981; Monger, 1985). Within the quadrangle, Neogene plutons have intruded the fault and have obscured its exact location, but a complex of smaller faults cutting Tertiary rocks as well as fault-bounded pre-Tertiary units suggest the Tertiary influence of this major structure. The Evergreen fault bounds the east side of this fault complex, and it may represent a late en echelon strand of the Straight Creek fault.

Regional location of the Skykomish River quadrangle
Figure 1. Regional setting of the Skykomish River Quadrangle

In general, the Straight Creek fault separates unmetamorphosed and low-grade metamorphic Paleozoic and Mesozoic oceanic rocks on the west from medium- to high-grade metamorphic rocks on the east. Within the Skykomish River quadrangle, this contrast is less distinct, and low-grade metamorphic rocks assigned to the herein-revised Early Cretaceous Easton Metamorphic Suite (in part equivalent to the Shuksan Metamorphic Suite of Misch, 1966) that crop out only west of the fault north of the quadrangle occur on both sides of the fault within the quadrangle and continue on the east side south of the quadrangle. The offset of the Easton Metamorphic Suite reflects the dextral strike-slip movement. On the south margin of the quadrangle and beyond to the south, the fault separates the lower Eocene Swauk Formation on the east from the upper Eocene and Oligocene(?) Naches Formation on the west. The clearly identified Swauk Formation or its correlatives does not crop out in Washington north of the Skykomish area on the east side of the fault, but west of the fault, the upper Eocene and Oligocene(?) Barlow Pass Volcanics of Vance (1957b), correlative with the Naches Formation, continues to the north. The Barlow Pass and questionably correlative strata appear to lie across a major strand of the fault in the aforementioned complex of faults, suggesting that major strike-slip movement was concluded by middle Eocene time. Predominantly vertical movement with the east side up could account for the distribution of the Eocene and Oligocene rocks seen today.

West of the Straight Creek fault, the oldest rocks are relatively unmetamorphosed Paleozoic and Mesozoic melanges. A western belt is predominantly argillite and graywacke; sedimentary and gabbroic components yield Late Jurassic and Early Cretaceous ages, and marble phacoids are late Paleozoic in age. An eastern belt is predominantly chert and greenstone but appears also to have both Paleozoic and Mesozoic components. The melanges may have undergone both sedimentary and tectonic mixing. Their origin appears to be accretionary. Marble in the eastern belt contains Permian fusulinids with Tethyan affinities, which led Danner (1970; 1977, p. 500) to propose that the rocks including the marble did not become part of North America until middle Mesozoic time.

North of the quadrangle, a widespread unit west of the Straight Creek fault is the Easton Metamorphic Suite. This unit of phyllite, greenschist and blue-amphibole schist is thought to have a protolith age of Middle and (or) Late Jurassic and to have been metamorphosed in the Early Cretaceous (Brown and others, 1982; Brown, 1986, p. 146). In the Skykomish River quadrangle, the Easton is sparsely represented by the Darrington Phyllite and the Shuksan Greenschist that are exposed near and within the Straight Creek fault zone.

Between probable en echelon faults within the fault zone are the low- to medium-grade metamorphosed pelites and scarce metaigneous rocks of the Tonga Formation of Yeats (1958b). The Tonga has a late kinematic to static metamorphic overprint that locally has increased its grade from greenschist on the south to amphibolite facies on the north. The Tonga appears to grade into the amphibolite-grade Chiwaukum Schist of the Nason terrane although the exact gradation is interrupted by faults and intrusive rocks.

The Nason terrane is composed mostly of metapelite assigned to the Chiwaukum Schist and banded gneiss derived from the schist. The protolith age of the Chiwaukum Schist is uncertain, but the Chiwaukum may contain sedimentary inclusions of the Late Jurassic or Early Cretaceous Ingalls Tectonic Complex, indicating that its age could lie between the Late Jurassic and the Late Cretaceous. However, some workers (Evans and Berti, 1986, p. 698; Magloughlin, 1986, p. 263-264) assign a Triassic and (or) Jurassic protolithic age to the Chiwaukum on the basis of Rb-Sr isochrons established for the Chiwaukum and its probable correlative in Canada.

The Late Jurassic or Early Cretaceous Ingalls Tectonic Complex is mostly an unmetamorphosed melange that crops out east of the Straight Creek fault, although in the Skykomish River quadrangle the complex is thermally metamorphosed by the Late Cretaceous Mount Stuart batholith. The Ingalls appears to be an ophiolite complex (Southwick, 1974; Hopson and Mattinson, 1973; Miller, 1977; Miller and Frost, 1977, p. 287) thrust over the Chiwaukum Schist (Miller, 1980a, b, 1985).

During the Late Cretaceous, the Ingalls Tectonic Complex, the Chiwaukum Schist, and the Tonga Formation of Yeats (1958b) were intruded by the Mount Stuart batholith and Beckler River stocks. The intrusions produced hornfels in the Ingalls and late- to post-kinematic mineral growth in the Tonga Formation, but the timing of metamorphic events and intrusion of the Mount Stuart into the Chiwaukum Schist is still under debate. The deeper seated, Late Cretaceous Sloan Creek and Tenpeak Mountain plutons also invaded the Chiwaukum Schist, and these plutons, although retaining igneous textures and structures, are predominantly metamorphic in fabric and structure (see Tabor and others, 1980, 1982a).

By early Tertiary time the Cretaceous and older rocks had been uplifted and partially eroded. Lithofeldspathic subquartzose sandstone and conglomerate of the lower Eocene Swauk Formation and its intercalated Silver Pass Volcanic Member were deposited unconformably on the pre-Tertiary rocks. Following deformation, uplift, and erosion of the lower Eocene rocks, various volcanic rocks and subquartzose sand of the upper Eocene and Oligocene(?) Naches Formation, and the correlative Barlow Pass Volcanics of Vance (1957b) were deposited. Similar sedimentation and volcanism is expressed farther west in the quadrangle in the volcanic rocks of Mount Persis and in the Puget Group.

Strong deformation continued in the mountainous region of the North Cascades. The folded Barlow Pass and Naches are overlain unconformably by less deformed upper Oligocene and Miocene calc-alkalic volcanic rocks laterally equivalent to the Ohanapecosh and Stevens Ridge Formations near Mt. Rainier (see Frizzell and others, 1984). In a preliminary report (Tabor and others, 1982a), we stated that the Oligocene and Miocene rocks on the west flank of the Cascades conformably overlay the lower Tertiary Puget Group. Further mapping and reevaluation of the structural differences lead us to think that the Oligocene and Miocene rocks unconformably overlie the Puget Group, although with less angular discordance than that of the corresponding unconformity in the mountains to the east (Frizzell and others, 1984).

Roughly contemporaneous with the volcanism, tonalite and granodiorite batholiths invaded the crust of the area. The Index batholith cooled about 34 Ma during the Oligocene and the Snoqualmie and Grotto batholiths about 25 Ma in the Oligocene and (or) Miocene.

Alpine river valleys in the quadrangle record multiple advances and retreats of alpine glaciers. Multiple advances of the Cordilleran ice sheet, originating in the mountains of British Columbia, Canada, have left an even more complex sequence of outwash and till along the western mountain front, up these same alpine river valleys, and over the Puget Lowland.

Generalized geologic map of the Skykomish River Quadrangle.
Click here for enlarged version with explanation (60K).

Thumbnail of generalized geologic map of the Skykomish River quadrangle

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