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SUMMARY OF THE GEOLOGY OF THE SNOQUALMIE PASS 30-MINUTE BY 60-MINUTE QUADRANGLE, WASHINGTON

Pre-Tertiary accreted terranes

The pre-Tertiary rocks of the Snoqualmie Pass quadrangle crop out in several isolated tracts. In the northeast corner of the quadrangle at the edge of continuous outcrops of uplifted metamorphic and plutonic rocks of the North Cascades crystalline core (fig. 1), a tectonic melange of serpentinite, serpentinized peridotite, gabbro, diabase, and greenstone is continuous with the Ingalls Tectonic Complex, which is exposed to the east and northeast (Tabor and others, 1982b; Tabor and others, 1993). The Ingalls Tectonic Complex is a dismembered ophiolite containing mostly Late Jurassic components. In the mid-Cretaceous, it was thrust over the protolith of the Chiwaukum Schist (Miller, 1980a) of the North Cascade crystalline core, prior to the Late Cretaceous metamorphic event documented by Mattinson (1972). The relation of the Ingalls Tectonic Complex to the other rocks of the Northwest Cascade System is unknown.

 Regionl setting of the Snoqualmie Pass quadrangle
Fig. 1. Regional setting of the Snoqualmie Pass quadrangle. Four 30 x 60 minute quadrangles make up the Wenatchee 2-degree quadrangle.

Definite rocks of the Northwest Cascade System crop out adjacent to and east of the Straight Creek Fault Zone. The Easton Metamorphic Suite and structurally underlying unnamed metavolcanic rocks on North Peak are exposed in anticlinal cores of Eocene rocks. The North Peak rocks are probably correlative with the Chilliwack Group of Cairnes (1944), an extensive component of the Northwest Cascade System exposed across the Straight Creek Fault Zone to the northwest of quadrangle. The Easton, composed of phyllite, greenschist, and blue-amphibole schist, previously referred to as the Shuksan Metamorphic Suite by Misch (1966), is thought to have a protolith age of Middle and Late Jurassic and a metamorphic age of Early Cretaceous (Brown and others, 1982). Just south of Easton, the greenschist grades into fine-grained amphibolite in a tectonic zone, and the amphibolite appears to be derived from hornblende tonalite orthogneiss that yields 150-Ma (Late Jurassic) zircons.

Near Manastash Ridge the rocks of the Northwest Cascade System are truncated by a northwest-trending zone of high-angle faults that bound slivers of metavolcanic and metaplutonic rocks, as well as metasedimentary rocks and serpentinite in a tectonic melange referred to as the tectonic complex by Stout (1964). Tabor (1987; 1994) correlated the tectonic complex with the Helena-Haystack melange and the coincident Darrington-Devils Mountain Fault Zone exposed to the northwest. Northwest of the quadrangle, the Helena-Haystack melange and Darrington-Devils Mountain Fault Zone separate the Northwest Cascade System from the western and eastern melange belts. In the Snoqualmie Pass quadrangle, Tertiary intrusions and eruptive rocks isolate outcrops of western and eastern melange belts from the Helena Haystack melange (tectonic complex of Stout, 1964) and the Darrington-Devils Mountain Fault Zone. On Manastash Ridge, the Lookout Mountain Formation of Stout (1964) lies adjacent to, and is incorporated in, the tectonic complex. The Lookout Mountain Formation is mostly aluminium-rich staurolite-garnet mica schist containing lenses of gneissic amphibolite. It was intruded by the 157-Ma Quartz Mountain stock prior to metamorphism.

The western and eastern melange belts crop out in limited exposures along the north margin of the quadrangle. We correlate thermally metamorphosed chert, basalt, and marble north of Snoqualmie Pass with the eastern melange belt more widely exposed for about 80 km to the north (Frizzell and others, 1982, 1987; Tabor and others, 1988, 1993). In the northwest part of the quadrangle, exposures of graywacke, argillite, chert, and metagabbro represent the western melange belt which is also extensively exposed to the north. In those more northern areas, the melange belts contain Permian marbles as well as Late Jurassic to Early Cretaceous chert, argillite, and gabbro components, all of which were accreted to North America, probably after the Early Cretaceous and before the middle Eocene (Tabor, 1987).

Paleogene transtensional deposits

The most complete geologic record in the Snoqualmie Pass quadrangle was recorded during the Tertiary, a period marked in the early Paleogene by the development of local fluviatile basins filled with feldspathic sandstone and conglomerate, and subordinate volcanic rocks. The region was undergoing extension, cut by right-lateral, north-trending strike-slip faults (Tabor and others, 1984; Johnson, 1985; Heller and others, 1987), although earlier workers referred the volcanic rocks to the Challis Volcanic Arc (Armstrong, 1978; Vance, 1982). By the late Paleogene, the north trending Cascade Arc dominated the region (Frizzell and Vance, 1983; Vance and others, 1987).

Most rocks of Tertiary age can be assigned to three structural blocks distinguished in part by stratigraphy and in part by differing degrees of Tertiary deformation. East of the Straight Creek Fault Zone, the oldest Paleogene rocks crop out in two uplifted structural blocks: the Teanaway River Block and the Manastash River Block (fig. 2; Tabor and others, 1984). In the Teanaway River Block, the tightly folded and faulted early and middle Eocene Swauk Formation consists of fluvial feldspathic subquartzose sandstone, siltstone, and conglomerate and the interbedded and overlying dacite and andesite flows, breccia, and tuff of its early Eocene Silver Pass Volcanic Member. The relatively undeformed middle Eocene Teanaway Formation of andesite and basalt flows, tuff, and breccia with minor rhyolite unconformably overlies the Swauk. The fluvial middle and late Eocene Roslyn Formation conformably overlies the Teanaway and is composed of subquartzose feldspathic sandstone, is conglomerate rich in its lowermost part, and contains extensively mined coal beds in the uppermost of its three subunits.

The lower part of the sequence in the Teanaway River Block-clastic rocks, silicic and intermediate volcanic rocks, and basalt--is repeated in the Manastash River Block where all the rocks are tightly folded and faulted along structures that help define the intersection of the Straight Creek Fault Zone with the Olympic-Wallowa lineament (Tabor and Frizzell, 1979; Tabor and others, 1984). In the Manastash River Block, the feldspathic subquartzose to quartzose sandstone and lesser amounts of siltstone, conglomerate, and bituminous coal of the early Eocene Manastash Formation are partly correlative with the Swauk Formation. Andesite, dacite, and rhyolite in flows, tuff, and breccia of the early Eocene Taneum Formation conformably overlie the Manastash Formation and are correlative with the Silver Pass Volcanic Member. Above the Taneum, the conformable, middle Eocece basalt of Frost Mountain correlates with the Teanaway Formation (fig. 5).

West of the Straight Creek Fault Zone, the early Eocene nonmarine part of the section is missing. In the eastern part of the Green River-BlockCabin Creek, near the Straight Creek Fault Zone, the strongly deformed Naches Formation consists of middle Eocene to early Oligocene(?) volcanic rocks and interbedded fluvial feldspathic subquartzose sandstone. The Naches, in part, correlates with the relatively undeformed Roslyn Formation of the Teanaway River Block (fig. 5).
correlation chart for Tertiary units in the Snoqualmie Pass quadrangle
Figure 5 Correlation of selected Tertiary units in Snoqualmie Pass quadrangle, Wash. Absolute time scale from Berggren and others (1985). Modified from Tabor and others (1984, fig.7).

In the western foothills of the Cascade Range, on the west margin of the quadrangle, the mildly deformed rocks of the middle and late Eocene Puget Group are lithologically similar to the Naches, but separated from that unit by a cover of younger Tertiary volcanic rocks. Both the Naches Formation and the Puget Group are recognized by their distinct bimodal volcanic constituent, consisting prominently of basaltic and rhyolitic rocks. Whereas, the Naches overlies pre-Tertiary metamorphic and granitic rocks, the Puget Group conformably overlies middle Eocene shallow marine volcanic lithic subquartzose sandstone of the Raging River Formation. The Raging River Formation is somewhat younger than the terriginous Swauk and Manastash Formations, east of the Straight Creek Fault. The Naches Formation and the Puget Group are nowhere seen in contact with each other, but they may be continuous under the younger volcanic cover. Both sequences may have been deposited by a single middle to late Eocene fluvial-deltaic system along the Eocene continental margin (see Johnson, 1985, p. 292).

The volcanic rocks of Mount Persis, mostly exposed north of the quadrangle (Tabor and others, 1993), are in fault contact with the Puget Group on Rattlesnake Mountain, and the mild deformation of the former unit contrasts with the pronounced deformation of the Puget Group rocks suggesting an appreciable age difference. The volcanic rocks of Mount Persis appear to be late(?) Eocene in age and thus be at least partly contemporary with volcanic rocks in the middle and late Eocene Puget Group. Cascade Arc

A thick sequence of Oligocene and Miocene volcanic rocks of the Cascade Arc underlies most of the Snoqualmie Pass quadrangle (fig. 1 and generalized geologic map) and forms the bulk of the southern Cascade Range in Washington and Oregon. In the quadrangle, these rocks crop out mostly in the Green River-Cabin Creek Block, but this block is traversed by the northwest-trending White River Fault, which separates the volcanic rocks into two slightly different sequences.

The sequence of rocks south of the White River Fault is in part equivalent to, and continuous with, rocks mapped south of the quadrangle near and around Mount Rainier (fig. 1; Fiske and others, 1963). These rocks underlie the youngest component of the Cascade Arc, the Quaternary volcanoes that give the southern Cascade Range its striking profile. The oldest unit, the Oligocene Ohanapecosh Formation, consists mostly of colorful, highly altered, well-bedded andesite to dacite breccia, volcaniclastic sedimentary rocks, and locally abundant altered basalt and andesite flows. Relatively fresh basalt and andesite flows, breccias, and subordinate rhyodacite ash-flow tuff and breccia, and volcanic sedimentary rocks of the early Miocene Fifes Peak Formation, as redefined by Vance and others (1987), unconformably overlie the Ohanapecosh and flank a structural high in the White River area. A prominent Fifes Peak member, herein called the Sun Top unit, was deposited in pronounced canyons cut into the Ohanapecosh Formation; another prominent interbed in the upper part of the Fifes Peak Formation, herein called the rhyolite unit of Clear West Peak, appears to be an extracaldera ash-flow tuff equivalent to rhyolite ash flow tuffs and intrusions filling a caldera on Clear West Peak (Fiske and others, 1963; Fischer, 1970; Mattinson, 1977; McCulla, 1986). We assign well-bedded tuff, breccia, and minor flows of highly altered andesite, basalt, and dacite north of the White River Fault to the Ohanapecosh Formation found south of the fault. Likewise, we assign the overlying flows of generally fresh porphyritic andesite, subordinate breccia, and locally conspicuous interbeds of mudflow breccia to the Fifes Peak Formation, but, as mapped north of the White River Fault, some flows assigned here to the lowermost part of the Fifes Peak may actually be deposits of Ohanapecosh eruptions.

Among the few outliers of the Oligocene to Miocene volcanic cover that crop out north of the extensive Cascade Arc rocks exposed south of Snoqualmie pass is a prominent accumulation of volcanic rocks east of the Straight Creek Fault Zone. Andesite and dacite breccia and tuff in the Goat Mountain area are continuous with the volcanic rocks of Mount Daniel just to the north (Tabor and others, 1993). A partial ring dike on Goat Mountain and nearby catastrophic breccias of probable landslide origin suggest that these volcanic materials were deposited in a tectonovolcanic depression (Hammond, 1965; Tabor and others, 1984).

The Miocene Grande Ronde Basalt of the Columbia River Basalt Group overlies the Fifes Peak Formation along the lower Little Naches River. Along the southwest side of the Manastash River Block, this flood basalt has been offset vertically more than 1,000 meters along north-northwest-trending high-angle faults that parallel the Olympic-Wallowa Lineament.

Although the isotopic ages obtained from mapped units of the Oligocene to Miocene volcanic rocks are imprecise, they suggest that these units have considerable temporal overlap, and that the overlap hampers exact correlation and reconstruction of eruptive events. The eruption of these volcanic rocks in the Snoqualmie Pass quadrangle appears to have begun about 35 Ma (early Oligocene) and ended about 20 Ma (early Miocene) (Vance, 1982; Frizzell and Vance, 1983; Turner and others, 1983).

The Oligocene and Miocene Snoqualmie batholith and related stocks intrude the volcanic rocks of the Cascade Arc north of the White River Fault. Rocks of the arc are also intruded by the Miocene Carbon River stock (a pluton in the Tatoosh volcanic-plutonic complex of Mattinson, 1977), which is exposed south of the quadrangle) and related plugs near the southwest corner of the quadrangle. Intrusive events at 25 and 17-20 Ma formed the Snoqualmie and Tatoosh bodies; a 14-Ma event is recorded in a Tatoosh pluton (Mattinson, 1977). Numerous smaller intrusive bodies invaded the volcanic pile between these large plutons and may be high-level apophyses connected in the subsurface to a continuous batholith.

An outlier of late Miocene volcanic rocks and associated dikes, the Howson Andesite, crops out on ridge tops on both sides of Cle Elum Lake. Pliocene and Pleistocene flows and pyroclastic deposits of olivine basalt overlie flows of the Fifes Peak Formation on ridge tops of Dalles Ridge and above Canyon Creek. A thick flow of early Pleistocene andesite, an early lava of the Mount Rainier volcano, caps a ridge near Huckleberry Creek. In the past 10,000 years, numerous mudflows swept down the White River valley from Mount Rainier (Crandell, 1969), and several eruptions have blanketed the southern part of the area with various thicknesses of tephra (Mullineaux, 1974).

Moraines and outwash deposits record as many as three alpine glaciations in the high Cascades. Late Pleistocene and Holocene alpine glacial deposits occupy many of the higher mountain valleys and cirques. In the west quarter of the quadrangle, multiple advances of the Puget lobe of the Cordilleran ice sheet left a record dominated by the most recent invasion of ice about 14,000 yr. B.P. (Crandell, 1963; Waitt and Thorson, 1983) during the Vashon stade of the Fraser glaciation (Armstrong and others, 1965). Marginal and submarginal drainage has blanketed much of this area with waterlain ice-contact deposits and recessional outwash, which, together with a sequence of channels, spillways, and terraces, express the progressive northwest retreat of the Vashon ice margin during deglaciation (Booth, 1984).

Generalized geologic map of the Snoqualmie Pass Quadrangle
Click here for enlarged version with explanation (64K).

Thumbnail of the generalized geololgic map of the Snoqualmie Pass quadrangle
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