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

The rocks and deposits within the Wenatchee quadrangle can be grouped into six generalized units (Generalized geologic map): (1) Precambrian(?) Swakane Biotite Gneiss in the northeastern part of the quadrangle and the probable Jurassic low-grade metamorphic suite, mostly composed of the Easton Schist, in the southwestern part; (2) the Mesozoic Ingalls Tectonic Complex; (3) the Mesozoic Mount Stuart batholith; (4) lower and middle Tertiary nonmarine sedimentary and volcanic rocks; (5) Miocene basalt flows and interbedded epiclastic rocks constituting part of the Columbia River Basalt Group and interbedded silicic volcaniclastic rocks of the Ellensburg Formation; and (6) Pliocene to Holocene alluvium, glacial, flood, and mass-wastage deposits.

An old terrane of eroded metamorphic and igneous rocks forms the basement for the Tertiary sedimentary and volcanic rocks. The Swakane Biotite Gneiss may be the oldest rock in the area (see below). The low-grade metamorphic suite of phyllite and greenschist was metamorphosed at least as long ago as the Early Cretaceous (Armstrong, 1980). The Ingalls Tectonic Complex is mostly serpentine and serpentinized peridotite but includes tectonic slices of Upper Jurassic metasedimentary and metavolcanic rocks, gabbro, and diabase. The Ingalls was thermally metamorphosed to varying degrees by the intrusion of the Mount Stuart batholith in the Late Cretaceous, about 93 million years ago (Engels and Crowder, 1971; age adjusted for hew constants).

In the early Tertiary, differential uplift and erosion of the older rocks produced basins and graben (The term "graben" is used in the structural sense, meaning an elongate fault-bounded block downdropped with respect to its neighbors. The term does not imply that the infacing scarps are fault scarps.) rapidly filled with fluvial arkose, shale, and conglomerate of the Swauk and Manastash Formations, Chumstick Formation (see Whetten in Gresens and others, 1977, p. 100-108), and basaltic to rhyolitic volcanic rocks including the Silver Pass volcanic rocks of Foster (1960), the Teanaway Basalt, the Taneum Andesite, and the basalt of Frost Mountain. The Wenatchee Formation (Gresens, Whetten, and Naeser, 1981) and possible correlatives lies with angular unconformity on the deformed earlier Tertiary rocks.

Deformation and erosion continued prior to, and perhaps during, eruption of the Miocene Grande Ronde Basalt. The continental tholeiitic basalt flows erupted southeast of the Wenatchee area and lapped up onto the higher Cascade Range. Dacitic debris spread southward and eastward from contemporaneous volcanoes in the southern Cascade Range, and feldspathic sand washed down from the northern Cascades and Okanogan Highlands to interfinger with the growing pile of basalt and form the Ellensburg Formation. The Grande Ronde Basalt and interbedded and overlying sedimentary rocks were tilted southeastward and were folded and faulted (fig. 3) as the modern Cascade Range rose and the Columbia Plateau differentially subsided. The basalt pile displays south- to southeast-trending anticlines and synclines conspicuously shown by the topography, the larger anticlines forming ridges, the synclines, valleys.

Map showing structure contours drawn on the Grande Ronde Basalt
Fig. 3. Structure contour map of the Grande Ronde Basalt. Contours drawn on top of flows of reversed magnetic polarity (unit Tgr2)

Faulting and possibly folding continued at least into the Pliocene, producing fault scarps in the Thorp Gravel in the Kittitas Valley. Growth of the Cascade Range uplifted the west margin of the Grande Ronde Basalt. Major rivers and their tributaries incised, carving a regional, generally northwestward-facing erosional escarpment. Landslides, block slides, and debris flows of basalt descended into the incised valleys below. The oldest debris flows now cap divides between the Columbia River tributaries near Wenatchee, but extensive landsliding continues into the present. Moraines, outwash, lacustrine deposits, and loess record at least three glaciations in the upper Yakima River valley. During the Pleistocene Epoch glaciers intermittently advanced down the major western tributaries of the Columbia River. Quaternary deposits along the Columbia River valley record mainly the sweep of many catastrophic late Pleistocene floods.

Generalized geologic map of the Wenatchee Quadrangle
Click here for enlarged version with explanation (28K).

Thumbnail of generalized geololgic map of the Wenatchee quadrangle
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