Accretion & Mountain Building in Central Washington, Physical Geology Field Trip, July 14-15, 2007

Cassandra Strickland, Instructor

Physical geology students spent two days in the Central Cascades exploring Washington’s tectonic history. This is the second trip in a series of two. During their first trip (June 26), students visited Mt. Rainier and explored modern Cascade volcanism. This second trip focused on the processes of terrane accretion, crustal deformation and intrusion. Students had the opportunity to see a wide variety of rock types and even collect fossils.

Day One. Yakima Fold Belt, Swauk & Teanaway Formations.

Our first stop was to discuss uplift of the Miocene-age Columbia River Basalts (CRBGs) in the Yakima Fold and Thrust Belt. These anticlines are generally east-west trending and asymmetric in cross-section. To the west they are bounded by the Cascade Range, and die out in the Palouse. The stresses producing the Yakima Fold belt may result from oblique subduction at the convergent plate margin. This creates a clockwise rotation of western and central Washington, and causes both compressional and shear stresses. After leaving the CRBG province behind, we headed up north of Cle Elem to the sedimentary formations of the Eocene-age Swauk Basin (Figure 1).

The Swauk Formation consists of arkosic sandstones (angular & coarse grained with local cross-stratification) interbedded with minor shales and conglomerates. The shales and the sandstones are fossiliferous; the shales contain abundant leaf fossils with beautiful coloration and I have found abundant (poorly preserved) wood in certain sections of the sandstone (Figure2). One specimen included a gingko leaf. The sediments accumulated during the Eocene (~ 54-49 Ma) in a non-marine basin. Sediments reach thicknesses of >26,000 ft of sediments from, and came from nearby uplifting sources, like the Mt. Stuart batholith (predominant source rock ) and the Nason terrane. The rocks have been folded and faulted and intruded by dikes (Figures 3 & 4).

The Swauk is heavily intruded by basaltic-to-diabasic dikes of the Eocene Teanaway Group. Thousands of the dikes cross-cut the sedimentary rock. The dikes were conduits for lava to reach the surface, and Teanaway lavas flowed over the top of the Swauk formation ~ 47 Ma. The basalts of the Teanaway Group comprise the 3^rd largest volcanic province in Washington.

At our first stop, we discussed subsidence of the Swauk basin during Eocene time, the formation of the sedimentary rock and intrusion of the Teanaway Volcanics. We followed Old Blewett Pass road, looking at various formations and creating simple cross-sections (Figure 5).

The day concluded at the Rock Island Campground, Wenatchee National Forest, Cascade Mountains, near Leavenworth, WA (Figure 6). The campground is in the heart of the Nason Terrane, an accreted volcanic arc that has been highly metamorphosed. Chiwaukum schist float littered the campground; some blocks were the size of cars. Icicle Creek, which runs through the campground, cuts through Chiwaukum schist and has polished it to a countertop-worthy gleam. This schist displays gneissic texture and boudinage in places, and contains abundant almandine garnet.

Day Two: Chiwaukum Graben and Ingalls Complex

The day started with a hike through Chiwaukum schist, garnet collecting part of our goal (Figures 7,8,9.) After a morning of hiking through metamorphic rocks along Icicle Creek, we headed out of the Stuart Range and down into the Chiwaukum Graben. A ‘graben’ described a fault-bounded, down-dropped basin. The Chiwaukum Graben was active during the Mid-Eocene, and during this time accumulated deep sediments (~5,800 ft) called the Chumstick Formation. During the Oligocene Period, however, the Chiwaukem Graben experienced compression and the arkosic (quartz) sandstones of the Chumstick formed were deformed and steeply inclined. At Peshastin Pinnacles State Park (a place normally visited mostly by climbers), we examined the sandstones of the Chumstick Fm. (Figure 10.) The students went on a ‘find the cross-bedding’ hunt (Figure 8.) This excursion was cut short by the soaring temperatures, which climbed above 100+ Fahrenheit.

We decided to head back to school, following highway 97 south. This took us right through the ophiolitic Ingalls Complex, which represents old sea floor that was accreted to the western margin of North America during the Jurassic Period. ‘Ophiolite’ refers to rocks that were once part of the sea floor but are now part of a continental landmass. During the LateJurassic, subduction brought a volcanic island arc to the North American continent and collision ensued. The rocks of the Ingalls Complex are mainly metamorphosed mafic and ultramafic igneous rocks, and represent not only volcanic arc basalts but also oceanic plate basalts and gabbros and mid-ocean ridge basalts (Metzger, E.P., Miller, R.B., and Harper, G.D., 2001, Geochemistry and Tectonic Setting of the Ophiolitic Ingalls Complex, North Cascades, Washington: Implications for Correlations of Jurassic Cordilleran Ophiolites, The Journal of Geology, 2002, volume 110, p. 543–560.) Although metamorphosed mafic/ultramafic rocks are common, other, more ‘interesting’ metamorphic rocks can also be found, representing metamorphosed sedimentary rock and also more than one stage of metamorphism in places. After rock collecting in a road cut…….after the instructor took home enough phyllite to stock two geology classrooms……we headed back to campus.

This trip is only offered summer quarter, due to weather constraints in the Cascade mountains.

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