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Kittitas Valley Field Trip The Structure and Stratigraphy of the Eastern Central Cascades J Figge, 2009 This document is designed to display in book format on the Adobe Reader® platform. To format this document properly, select the “view” category, open the “page view” listing, and select the “two page view” option. This will format the document properly for your viewing. Kittitas Valley Field Trip The Structure and Stratigraphy of the Eastern Central Cascades J. Figge 2009 North Seatt;e Community College This document was prepared for the exclusive use of students enrolled in Geology 101 at North Seattle Community College, to whom it is provided as part of that body of academic coursework.This document is not available for commercial distribution, and should not be made available for purchase in any form or under any circumstances. By accepting this document, students agree to the conditions for its use, as detailed on this page. Neither the author nor the publisher receive any profit or other compensation from the limited distribution of this document. All Parts of this work, as an electronic file or as a paper document, including all text, illustrations and diagrams, except where otherwise noted, are protected under US and International Copyright Laws. No part of this Book may be copied, by electronic or other means, including the posting of parts of this work on the Internet, without the expressed written permission of the author and publisher. All Rights Reserved. © John Figge 2009 Kittitas Valley Field Trip The Structure and Stratigraphy of the Eastern Central Cascades John Figge 2009 Published by The Northwest Geological Institute P.O. Box Seattle, Washington USA 98115 Kittitas Valley Field Trip The Structure and Stratigraphy of the Eastern Central Cascades J. Figge 2009 Kittitas Valley Field Trip The Structure and Stratigraphy of the Eastern Central Cascades Introduction Of all the geologic field trips offered out of the Seattle area, none is more popular than the trip over the mountains to the Kittitas Valley around the town of Cle Elum. This east-side setting offers a refreshing break from the damp climate of the Puget Sound region, and is popular in the spring and fall when the weather is less dependable to the west. The region features an instructive mix of igneous, sedimentary and metamorphic rocks, in a setting which illustrates both the principles of stratigraphy and the fundamentals of structural geology. It is an excellent trip for introductory students, yet offers enough variety to be interesting to those with a greater depth of experience. Of no minor significance, it also includes a stop at the renowned Cle Elum Bakery, one of the oldest and best bakeries in the northwest. Given the variety of rocks and the generally good outcrop locations across this region, it would be possible to engineer a number of possible field trips to suit a range of different instructional purposes. Introductory trips are usually characterized by a larger number of stops, designed to illustrate large-scale features and relationships. More advanced trips usually involve fewer stops, with a greater attention to the details offered in the rock record. This version is designed as an introductory – level trip, suitable for students in the latter half of a college-level introductory course. It serves to illustrate the variety of rock types, the basics of stratigraphy and structure, and how the course of geologic evolution can be traced through the rock record. It consists of a dozen stops, making it about a 12-hour round trip from Seattle. Two hours are invested at the last stop, which can be considered optional, however desirable. This trip is broadly centered around the town of Cle Elum, some 80 miles east of Seattle on Interstate 90. It includes stops at Lake Easton State Park, along the shores of Lake Cle Elum, along the Yakima River, along Taneum Creek, and at the Peoh Point lookout. It also includes one stop in the town of Roslyn, and one in Cle Elum, the latter conveniently close to the bakery. Restrooms are available at Lake Easton, in Cle Elum (City Park or Safeway), at Indian John Hill (I-90) and (in season) at Taneum camp in Taneum Canyon. Perhaps the best time to take this trip is early May, when the wildflowers are in full bloom. Yellow arrowroot (above) covers the slopes of Taneum Canyon, while blue lupine carpets the prairies of the Kittitas Valley. It is a refreshing break from the gray skies of the west side, and a beautiful setting for examining the local geology. Image (Left) Yellow arrowroot colors slopes above the Yakima River, below the town of Cle Elum. Here, the Yakima cuts into flows of the Columbia River Basalts, forming a distinct canyon. In the summer, this is a very popular stretch for rafting. The rail lines serve the Burlington Northern - Santa Fe Railroad, and are the main east-west route across the state. Trains are a common sight along this stretch of the river. Images: (Above) Rocks of the Roslyn Formation outcrop above the town of Cle Elum. These distinctly white slopes can be seen from the freeway. (Avove Right) Peaks of the Stuart Range rise above the meadows of the Thorp Prairie. Table of Contents Geology of the Kittitas Valley .........................................................................1 A Brief History of the Kittitas Valley................................................................4 The Kittitas Valley Field Trip Stop 1 The Easton Greenschist, Lake Easton........................................................9 Stop 2 The Swauk and Teanaway Formations, Lake Cle Elum............................12 Stop 3 The Swauk Formation, Lake Cle Elum.......................................................15 Stop 4 The Roslyn Coal Mines, Roslyn................................................................17 Stop 5 The Roslyn Formation, Cle Elum..............................................................20 Stop 6 The Ellensburg Formation, Yakima River..................................................21 Stop 7 The Columbia River Basalt Flows, Yakima River......................................25 Stop 8 The Thorp Gravel, Taneum Canyon............................................................27 Stop 9 The Columbia River Basalts and Ellensburg Formations, Taneum Cyn.....30 Stop 10 The Roslyn Formation, Taneum Canyon...................................................32 Stop 11 The Swauk Formation, Taneum Canyon....................................................33 Stop 12 The Darrington Phyllite, Taneum Canyon.................................................37 Stop 13 Peoh Point.................................................................................................39 A Final Last Word From Your Instructor............................................................................44 Kittitas Swauk Formation Syncline Teanaway Formation 2 3 Roslyn Formation 4 Fraser Fault 5 6 Cle Elum 13 12 11 Ainsley Canyon 7 10 Anticline 9 8 Columbia River BAsalts Figure 1 Simplified geologic map of the Kittitas Valley area. The two main structural features are the Kittitas Syncline and the Ainsley Canyon Anticline, both of which plunge to the southeast. The Columbia River Basalts erupted after these features were initially formed, but have been similarly deformed along these lines. The various formations along Taneum Canyon and the Yakima River are not detailed, as the outcrops are too small at this scale. See figure 59 for additional details. Red dots indicate field stops 1 The Geology of the Kittitas Valley The geology of the Kittitas Valley includes metamorphic basement rocks which were added to the continental margin in Late Cretaceous time, perhaps something like 90 – 95 million years ago. Those older rocks are overlain by a sequence of sedimentary and volcanic rocks which date from Eocene time, and which extend discontinuously to as recently as 4 million years ago. Those sedimentary and volcanic rocks can be assigned to two distinct sequences. The first dates from Eocene time (~53 – 38 Ma), and is known as the “Challis” sequence. The second dates from 37 – 4 Ma, and is known as the “Cascade” sequence. The two sequences are separated by a major regional unconformity, and represent two distinct regimes of regional plate-tectonic relationships. Map area to the left Figure 2 Regional map showing the locale of the Kittitas Valley The basement rocks in this area are a suite of phyllite and greenschist which represent the metamorphosed equivalent of oceanic crust and ocean-floor sediments. The greenschist is properly known as the Easton Greenschist, but the two rocks are better known as the Shuksan Greenschist and the Darrington Phyllite, for their occurrence on the west side of the Cascades. Together, they are commonly referred to as the Shuksan Metamorphic Suite. They are part of the Northwest Cascades Belt of terranes, which was added to the continental margin in Late Cretaceous time. The sedimentary and volcanic “Challis” sequence of formations probably dates from an early horizon of perhaps 53 million years ago. The oldest of these is a localized basal section of felsic volcanic rocks and arkose sandstones known as the Taneum Formation. These may date from as early as 53 Ma. They are overlain by a regionally thick section of fluvial arkose sandstone, siltstone and conglomerate known as the Swauk Formation. The Swauk Formation is unconformably overlain by the largely basaltic Teanaway Formation, dated at about 47 Ma. The Teanaway basalts are in turn overlain by the Roslyn Formation of arkose sandstone, noted for the local abundance of coal beds, the former economic mainstay of this region. The uppermost elements of the “Challis” sequence are absent in this area, simplifying the picture. Three formations of the “Cascade” sequence are preserved overlying the Roslyn Formation in this area. They include the <25 Ma Ellensburg Formation of tuffaceous sandstone and conglomerate, the 17-15 Ma Grand Ronde Member of the Columbia River Basalt Group, and the ~4 Ma Thorp Gravel. The informally-designated Ellensburg Formation includes all sedimentary interbeds below and between the various flows of the Grand Ronde Basalts. These are tuffaceous sandstones, siltstones and conglomerate, derived from the Cascade Arc volcanoes to the west. They are andesitic to dacitic in character, and include significant lahar deposits in their mix. 2 Palouse Loess 1 MA Thorp Formation 4 MA Columbia River Basalts Ellensburg Formation Cascade Episode 17 MA 25 MA 40 MA Roslyn Formation 46 MA Teanaway Formation Challis Episode 48 MA Swauk Formation 53MA Darrington Phyllite The flows of the Grand Ronde member of the Columbia River Basalts are basalt. Locally, flows include a pillow-palagonite complex at their base, a reflection of the wet landscape which persisted between eruptive events. Elsewhere these flows display classic columnar structure, a reflection of the cooling process in lava flows. The youngest rocks here are a section of gravel known as the Thorp Formation. The Thorp gravels date from about 4 million years ago, and form a thick belt in the middle portion of the Kittitas Valley. This accumulation broadly dates from the onset of the uplift which has produced the modern Cascade Mountains. The rocks in this area are preserved in a northwest trending fold with a broad syncline (the Kittitas Syncline) on the north side, and a narrower anticline (the Ainsley Canyon Anticline) on the south side. This is part of the Yakima Fold Belt, which extends southeast along this strike. These folds are accommodating northeasterly compression produced by the northward shearing of California by the Pacific Plate, and the eastward compression produced by the local Juan De Fuca Plate. This regime first developed between 25 and 20 million years ago, and persists into the present. While these rocks are fairly abundantly exposed in the Kittitas Syncline to the north, many of the important contacts are not particularly well illustrated. The first half of this field trip illustrates the structure as progressively younger rocks appear toward the center of the syncline, but those relationships are not particularly evident in the field. The latter part of this trip ascends the Ainsley Canyon Anticline up Taneum Canyon, which more clearly delineates the local stratigraphy. Along the southern margin of the Kittitas Valley there is a low-angle thrust fault which occupies the inflection point between the Ainsley Canyon Anticline and the Kittitas Valley Syncline. This is known as the Easton Thrust Fault. Like much of the rest of the Yakima Fold Belt, this is a fold-and-thrust belt, where folding accumulates stress which is periodically relieved by lowangle faulting. Not coincidentally these are the same characteristics as are found in the Seattle Fault, which lies broadly on strike to the west. Between these two locales, uplift of the the north-south striking Cascade Anticline over the last five million years has served to obscure the original connections across this region. Figure 3 (Above) Simplified stratigraphic column for the Kittitas Valley region. All contacts are unconformable. Not to scale. The Tertiary formations here can be divided into two groups, based on two distinct episodes of regional plate tectonic relationships. 3 A Brief History of the Kittitas Valley Figure 4 The town of Roslyn, circa 1889. Early mining operations are pictured here. Over the ensuing years, they were expanded considerably. Prior to the 1870’s, the Kittitas Valley was the exclusive domain of native tribes which had inhabited the region for millennia. Army patrols and prospecting parties passed through the region, but found little of particular interest. This changed in 1873 when gold was discovered on the Swauk River. This caused a modest rush to the region, and by 1879, a rough road extended over Blewett Pass from modern-day Cle Elum. Mining was a going concern here into the 1890’s, when returns began to diminish. While gold proved a profitable venture for a number of local interests, the larger history of this area centered on a lessglamorous commodity: coal. Coal was fuel for the railroads, which determined the course of development over the last half of the 19th Century. Absent the coal fields in Centralia, Renton, Bellingham and Cle Elum, the history of this state would have taken a much different course. Explorers for the Great Northern Railroad discovered the extensive coal deposits in the Roslyn Formation north of Cle Elum in the mid 1880’s, and developed it as a major fueling station for their regional network. The railroad established the town of Roslyn to develop the mines, named for Roslyn, New York – the home town of the mining superintendent. The mines were producing for the railroad by 1886, boosting production with the opening of the Stampede Pass tunnel in 1888. Roslyn grew to a population of several thousand and the mines continued to expand. Labor unrest resulted in a general strike in 1888, an event of regional significance. The owners brought in 300 young African-American men from the south to serve as strike breakers, and employed a private militia to maintain the peace. The governor took exception to the notion of private law enforcement, with the effect that an article in the State Constitution now prohibits it. The strike was resolved and the strike-breakers were absorbed into the workforce. This was a huge increase in the African-American population of the state at that date, and was an important historical event in that context. As a thriving mining town, Roslyn had a reputation for respecting ethnic and cultural differences. The local cemetery is said to have names from some 24 different countries. 4 Figure 5 Coal mining in Roslyn in the early 1900’s. The mineshafts extended some 2700 feet beneath the town, in seven levels. One set of tracks brought miners and ore carts down, the other brought full carts up. The work was dirty, hazardous and strenuous, but provided steady employment for over half a century. Historical images from the Kittitas County Historical Society. The mines were also the site of the state’s worst mining accident, when 45 died in a gas explosion on the lowest level of the mine in 1892. This was seven levels down, 2700 feet directly beneath the town. Production continued into the 1950’s, but shut down when the railroads converted to diesel fuel. While huge amounts were mined, it was only 20% of the amount available. The town went on to be the set for a 1990’s-era sitcom called “Northern Exposure”, where it played the role of the fictitious town Cicely, Alaska. In appreciation, the production company furnished new metal roofs for the town residents. The town is home to The Brick Tavern, the oldest continuously-operating saloon in the state under the same name. It dates from 1898. The town of Cle Elum is a few years younger than Roslyn, situated along the main east-west rail line. It was founded as a more refined community than the rough-and-tumble mining camp that was Roslyn. It featured quality hotels, eating establishments and general stores, and catered to travelers along this major cross-country route. Its commercial district was decorated in ornate woodwork, and many of the buildings were quite opulent in their architecture. Unfortunately it was wooden architecture, and 30 blocks of the downtown business district burned to the ground in 1918. The town never really recovered from that disaster. By the 1930’s automobile travel was becoming popular, and Cle Elum enjoyed a position along Highway 10, the major east-west route over Snoqualmie Pass to Seattle. The Kittitas Valley was developed as agricultural land, based on water supplied by the Yakima River, and local artesian wells. The Yakima, Keechelus and 5 Figure 6 A saloon in the town of Ronald, just north of Roslyn, circa 1898. The Roslyn mines stretched from Ronald to Cle Elum, the # 3 mine being in the town of Ronald. Ronald took its name from the mining superintendant, with the thought that it might gain some advantage from that patronage. Ronald’s current claim to fame is the “Old # 3 Tavern”, advertised on bumper stickers seen around the state. It stands to benefit from increased development underway in the region. Figure 7: The hotel in Cle Elum, circa 1910. Cle Elum distinguished itself as a more refined and cultured setting than the mining town of Roslyn. Located on the main rail line, it hoped to prosper by providing services to travelers. The town adorned its central business district with opulent architecture, as pictured here. Unfortunately they were of wooden construction, and most of the business district burned to the ground in a fire in 1918. Cle Elum valleys were dammed to form lakes, as a water supply for irrigation. In post-war time, the local economy also benefited from the expansion of the ski areas at Snoqualmie Pass, and other recreational opportunities in the area. As automobile and truck traffic increased, Cle Elum became a major stop along the main east-west corridor. That status declined in 1968, when the Interstate 90 by-pass route was finally completed. For many years prior, it had the distinction of being the only stoplight on the interstate between Seattle and Boston. Figure 8: Cle Elum today. Main street here hasn’t changed much since 1968, when the freeway by-pass was completed. The faded patchwork of buildings, gas stations and truck service yards gives it a somewhat utilitarian sense of decor. 6 Snoqualmie Pass Stuart Range Keechelus Lake Kachess Lake 2 Cle Elum Lake 3 1 Lake Easton Yakima River 4 Roslyn Cle Elum 5 13 6 7 Taneum Creek 12 11 10 9 8 Figure 9 Area map showing location of the field stops. Heavy red line is Interstate 90. Seattle lies roughly 60 miles (100 km) west of Snoqualmie Pass 7 Kittitas Valley Field Trip A Few Notes on Field Guide Organization: Travel directions are printed flush to the margin, in italic type Field stops and other geologic notes are printed flush to the margin, in regular type Other notes of interest are intented, and printed in this type. Travel distances are given in miles General Description: This trip starts out at Lake Easton State Park, about 70 miles east of Seattle. From there it makes its way north to the shores of Lake Cle Elum, for two stops in the Swauk Formation. Returning to the south, it makes one stop in the town of Roslyn, and one in the town of Cle Elum, to consider the Roslyn Formation. It then continues east to examine outcrops of the Ellensburg Formation, the Thorp Formation, and the Columbia River Basalt flows. To route then crosses to the south side of the valley, and heads up the canyon of Taneum Creek. Here are included stops in the Thorp Formation, the Columbia River Basalts, the Ellensburg Formation, the Roslyn Formation, the Swauk Formation, and finally the Darrington Phyllite. The trip concludes with a visit to Peoh Point, a spectacular viewpoint on the Kittitas Valley, and an excellent setting for considering the structure of the region. Figure 10 (Above) Rocks of the Roslyn Formation, outcropping above the town of Cle Elum. Note the distinctly white color to the rocks 8 Figure 11 Lake Easton This is an artificial lake, impounded to provide irrigation water for the Yakima Basin. It takes its name from the small community of Easton, just to the east. The town was so named because it was at the eastern end of the Great Northern Railway tunnel under Stampede Pass. Just 70 miles east of Seattle, this is a popular recreation site over the summer. The lake is a state park, and features camping, boating, hiking, and a host of other activities. Stop 1: The Easton Greenschist, Lake Easton Take the Lake Easton State Park exit (70) from Interstate 90 and follow the signs to Lake Easton State Park. Inside the park, take a right turn at the intersection and continue .9 miles to the swimming beach. Lake Easton is a man-made feature designed to provide water for irrigation in the Yakima Valley. It is impounded by a dam, and the level of the lake varies seasonally. It is a popular recreation area, just an hour east of Seattle. From the swimming beach, the outcrop lies along the shoreline of the lake about 100 feet to the west. When the water is low you can walk the shoreline, but a trail leads to the area above, and generally provides easier access. Leave the trail at the switchback, and continue 100 feet to a point above the outcrop. A short path leads down to the lake here. The outcrop is modest, but provides good exposures. The rock here is greenschist, a metamorphic variety. It is the metamorphic equivalent of basalt, where the original olivine, pyroxene and feldspar minerals have been changed to chlorite, actinolite and epidote. This happens at temperatures of ~250 C, and under several thousand atmospheres of pressure. It is a fine-grained species with a well-developed foliation, here dipping almost vertically. On a clean exposure, you can see a distinctive banding in the rock. This unit is called the Easton Greenschist, named after exposures in this area. It is however part of a larger belt of rocks better preserved in the Mt. Baker area well to the northeast. There, the rock is known as the Shuksan Greenschist (after Mt. Shuksan). The “Easton” name is more proper, but the “Shuksan” name is more common. When the lake level is low (late fall), exposures of phyllite outcrop south of the greenschist. Phyllite is a somewhat silvery fine-grained metamorphic rock characterized by microscopic muscovite (mica) minerals. It develops from a mudstone, under conditions similar to those which produce greenschist from basalt. Again, this rock is best known for its occurrences well to the northeast, where it is known as the Darrington Phyllite (Darrington is a town on the Stilliguamish River). Because both rocks were produced at the same metamorphic grade, they were likely part of a common “package” of rocks, a “suite” as they are known. The most common “suite” of rocks on our planet is basalt with a section of mud on top. This is the character of our ocean floors, but is an uncommon combination on the continents. Accordingly, it 9 Figure 12 (Right) Outcrop of the Easton (Shuksan) greenschist along the shore of Lake Easton. Hammer provides scale. In late season the lake level is lower, exposing sections of phyllite below. Figure 13 (Below) Detail from the outcrop above. Here you can see the distinctly green color of the rock, and the near-vertical foliation. Hammer provides scale. is most likely that this represents a section of oceanic crust, which has been suitably metamorphosed and somehow ended up as part of the continent. The Shuksan and Darrington rocks are known collectively as the “Shuksan Metamorphic Suite.” These are the “basement” rocks in this area, the deepest levels exposed. All of the “basement” rocks of Washington (west of Spokane) consist of sections of oceanic crust and the remains of Pacific island groups which have been added (“accreted”) to the margin of the continent over the last 200 million years. This has happened under an evolving set of plate-tectonic relationships, which have developed as North America has progressively moved to the west over this expanse of time. Much of this has happened under convergent margin conditions, much as exist today. In this setting, North America effectively collided with two large islands chains at about 170 and 115 million years ago, which added distinctive belts of rock which are the “basement” to most of British Columbia and northern Washington east of the Puget Sound. The rocks of the Shuksan Suite are part of a larger group of similar rocks known as the “Melange Belts”. The term “mélange” is French for a “mix” of rocks, as is their general character. While the Shuksan suite 10 Melange Belt Teranes New (~120 Ma) Kula-Farallon spreading center Older (>120 Ma) Subduction Zone Columbia Embayment Figure 14 Simplified diagram illustrating the suspected origins of the Melange Belt Terranes. These appear to have have been accretionary deposits accumulated above a subduction zone along the coast south of here. That subduction zone and the associated coastal rocks were rifted off the margin as a new spreading center (the Kula-Farallon Ridge) formed here. The northward sense of plate motion north of that new spreading center transported these rocks to our area. preserves coherent sections of oceanic crust, most of the rocks of the Melange Belts have been disrupted, fragmented, sheared, jumbled and juxtaposed into an indecipherable mix. Rock lithologies like this typically accumulate in an “accretionary wedge” of material scraped off the descending oceanic plate as it is subducted beneath the continent. Such accumulations are common in areas well south of here, and we suspect that these rocks have their origins in what is now southern Oregon or northern California. Along with the rocks of the Shuksan Suite (the subducting oceanic plate), these rocks appear to have been rifted off that southern coastline during a change in regional plate-tectonics, marked by the inception of a new east-west trending spreading center intersecting the continent in what is now northern California. This new spreading center produced a northerly sense of plate motion to the north, transporting these rocks toward our region. In our area, that northerly plate motion appears to have concluded along an east-west trending subduction zone, across what is now the southern half of the state. As these sections of displaced mélange rocks arrived at the subduction zone, they were obducted (thrust over) across the top of the continent along low-angle thrust faults. Some of these great “thrust sheets” may have been thrust for hundreds of kilometers. We suspect that this happened about 90 million years ago. These rocks overthrust the southern end of the Insular Belt, which had been accreted about 25 million years earlier. In the end, these rocks are probably a displaced section of southern Oregon or northern California coastline, the remains of an old subduction zone which was rifted off the coastal margin, exhumed from depth and transported northward, then thrust across the edge of the continent here about 90 million years ago. Along with the rocks of the Intermontane and Insular Belts, and those of the Olympic Coast Belt, they comprise the deepest “basement” rocks of Washington. 11 Figure 15: Xenolith (inclusion) of a block of Swauk sandstone in the Teanaway Basalt. Person gives scale. By the stratigraphic principle of inclusion, the Swauk must be older than the Teanway. Stop 2 The Swauk and Teanaway Formations, Lake Cle Elum Return to Interstate 90 and continue east to the exit for the town of Roslyn (Exit 80). Take the exit and continue north through the towns of Roslyn and Ronald. From downtown Ronald (the # 3 Tavern) continue about 7 miles north, eventually along the shores of Lake Cle Elum, to the distinctive outcrop. There is abundant parking on the west side of the road. The location is 47 20 19.88 N by 121 06 17.42 W Elevation 2261 feet Lake Cle Elum is an artificial lake, impounded by a dam on the south end. The lake stores water for irrigation, and thus its shoreline varies seasonally. The dam was built in 1933, enlarging a small lake which used to mark the valley here. This is a very popular recreation area. The road extends north into the heart of the Alpine Lakes Wilderness Area. There are two rocks exposed here, the dark basalt of the Teanaway Formation and the tan-colored sandstone of the Swauk Formation. A large block of the Swauk sandstone is preserved as a prominent xenolith (inclusion) in a flow of Teanaway Basalt. By the stratigraphic principle of inclusion, the sandstone is thus older than the basalt. The Swauk Formation is the dominant unit for some distance north of this location, on the border of the Teanaway Basalt. It takes its name from the Swauk (Blewett) Pass area, well to the northeast of here. Much of the formation is sandstone, as seen here, but significant proportions are conglomerate and finer-grained rocks. Collectively, they reflect deposition in a river-basin setting, probably by a substantial river system. Based on the minerals which make up these sediments, their source area was likely well to the east or northeast, along the front of the Rocky Mountains. It is a uniquely thick assemblage, accumulating to as much as 5 km of sediment. It contains significant coal beds and is notably fossiliferous, the most common species being palm trees. The fossil assemblages reflect a significantly warmer and more equitable climate than exists now. It likely dates from ~53 - 48 million years ago. 12 Figure 16: Cross-bedded sandstone, just north of the locale pictured in figure 15. These have been somewhat metamorphosed by the adjacent basalt. Note that these beds dip steeply to the rear. Pipe gives scale. Figure 17: The Swauk sandstone, at an outcrop well up the road from the field stop. These are planar and crossbedded sandstones, typical of a floodplain setting. The Swauk is a voluminous formation, encompassing up to 5 km of strata. This makes it one of the thickest non-marine sedimentary formations in North America. These sediments were likely transported by a large river system heading on the Rocky Mountains. 13 Figure 18: The Swauk Formation with an intruding dike of the Teanaway Basalt, at a location near Blewett Pass. This illustrates that the basalts intruded vertically through previously-tilted strata. The Swauk Formation was folded into a series of NW-SE striking folds, and then eroded back to a relatively flat landscape prior to the eruption of the Teanaway Basalts. Adjacent to the Teanaway Basalt, these sandstones have been contact metamorphosed to quartzite. Up the road (to the north), this character diminishes. Bedding is well-displayed in some of these rocks, ranging from millimeter to meter scale. While much of the bedding is planar, it is possible to discern crossbeding in some sections, a reflection of deposition in a river system. The sandstone is largely comprised of moderately well-sorted medium-grade sand of quartzofeldspathic composition. Quartz grains are typically sub-rounded in aspect. The rock also contains beds of distinctly finer material, comprising mudstones of various composition. Basalt is the dominant rock of the Teanaway Formation, although there are minor sedimentary beds to be found. The basalt is black, aphanitic, and phenocrysts are relatively rare. Chemically, they are closest to MORB (mid-ocean ridge) basalts, those which erupt from mid-ocean spreading centers. Here, these rocks have erupted through fissures, which can be seen cutting the Swauk Formation. Above the Swauk Formation, they accumulated to several hundred meters of lava. It covered an area at least twenty miles in diameter, in an event dated at about 47 million years ago. The Teanaway Basalt is probably related to the Crescent Basalts, which make up much of the Olympic Peninsula. Collectively they represent an immense outpouring of basalt, erupted through fissures in the crust. These may have erupted off a “stalled” spreading center, along transform faults which riddled the plate to the west. The Teanaway Basalts appear to be the easternmost expression of this regime, erupting along faults which extended to the west. There is evidence for multiple eruptive events, but all appear to have been voluminous extrusions. It is important to recognize that the Teanaway Basalts intruded Swauk Formation rocks which had been folded prior to that event. Between the ~50 - 48 Ma conclusion of deposition in the Swauk Formation, and the ~47 Ma eruption of the Teanaway Basalts, these rocks were folded into a series of NW-SE striking folds. Those folded rocks were eroded back to a relatively flat landscape prior to the eruption of the Teanaway Basalts. 14 Figure 19 Stop 3 outcrop, Dr. Furutani lecturing. Note the mixed character of this rock, including both fine-grained and coarse-grained components. This combinaton reflects the actions of a river meandering across its floodplain, some 50 million years ago. Students look on in absolute amazement. Stop 3: The Swauk Formation, Lake Cle Elum Turn around and return south on the road toward Roslyn. After about 2 miles, pull off at the driveway to a gated real-estate development and park along side the road. The outcrop is just north of the driveway. The location is 47 19 46.82 N by 121 06 20.48 W Elevation 2307 feet The rock here is part of the Swauk Formation, but with more varied lithologies than were present at the last stop. The outcrop here is a mix of sandstone and conglomerate, all of which dip to the southeast. The lowermost strata are largely sandstone, consisting of floodplain and riverbank deposits. The conglomerate layer above lies unconformably on the sandstone, and represents channel deposits which have cut into those older layers. Consistent with those seen before, these sediments reflect deposition by a river system migrating across its floodplain. The clasts in the conglomerate are polymictic, accumulated from a variety of source areas. Most are sub-rounded to rounded, but local varieties (e.g. the Stuart granodiorite, the Ingalls peridotite) are more angular in aspect. These rocks are exposed to the north of this area. The character of the Swauk Formation isn’t consistent with much local relief on the landscape, so these probably came from modest outcrops rather than large exposures. Notable in the mix are angular fragments of coal, clearly of local origins. Coal layers are not uncommon in the Swauk 15 Crevasse Splay sandstone Floodplain siltstone, shale, coal Main Channel conglomerate Levee sandstone, siltstone Oxbow Lake siltstone, shale Point Bar cross-bedded sandstone Figure 20 (Above) Depositional setting for the Swauk Formation, showing where various sediments are deposited in a river basin setting. The rocks of stop 3 are river-channel deposits, incised into older riverbank and floodplain sediments. These are common lithologies in the Swauk Formation. Adapted from Mustoe, 1997. Figure 21: (Right) Outcrop detail. The black line highlights the unconformable contact between the conglomerate above and the finer sediments below. The conglomerate is a channel deposit, while the fine-grained materials are levee and floodplain deposits. The unconformable contact is created as the river channel migrates across its floodplain over time. The vretical features are drill holes, used to blast rock during highway construction. They are not, contrary to popular belief, the tunnels of hortas. Hortas are a silicon-based lifeform capable of boring through rocks by secreting a strong acid. Hortas only live on Janus VI. formation. Looking in the prominent cleft in the outcrop, you can see a coalified section of tree trunk. This is the source of part of this material. The wood has undergone fossilization to a degree, preserving enough detail to see annular rings in its cross-section. As noted earlier, the Swauk Formation is locally fossiliferous, containing abundant leaf impressions and woody material in fine-grained sediments. This tree trunk fell into the river channel here some fifty million years ago, and was subsequently buried by accumulated sediments. 16 Figure 22: The modern-day town of Roslyn. Stop 4: The Roslyn Coal Mines Roslyn, WA Return down the road to the town of Roslyn. Turn left on Railroad Avenue and continue to its end at the site of the old coal mines. An interpretive trail runs though this site. The town of Roslyn was founded by the Great Northern Railway, to provide coal for its railroad empire. It is the only significant coal deposit in Eastern Washington, and was thus a strategic coup for the railroad. Coal from the Roslyn mines powered locomotives over Stampede Pass to the Puget Sound, and across Eastern Washington to Spokane and points beyond. This stop is located at the head of the main shafts, at the end of the main rail spur. From here, the mines extended some 2700 feet beneath the town. The Roslyn mines produced coal for the railroad from the mid 1880’s through the 1950’s, when locomotives changed to diesel fuel. At its height, the mines supported a town of several thousand people. As always, the work was dirty, strenuous and dangerous. Labor unrest sparked a general strike in 1888, and some 300 African-American men were brought in from the south as strikebreakers. After the strike those men were absorbed into the workforce, a significant event in regional African-American history. The strike was resolved, but safety concerns persisted as tunneling progressed to deeper depths. Worker’s concerns about methane gas accumulations were verified in 1892, when a large explosion ripped through the lowest level. Some 45 men died in the explosion, the worst mining accident in state history. The closure of the mines was a major economic blow to the town, which had few other sources of employment. To a certain degree, expanding recreational interests provided some measure of relief. The town went on to be the set for a 1990’s-era sitcom called “Northern Exposure”, where it played the role of the fictitious town of Cicely, Alaska. In appreciation, the production company furnished new metal roofs for the town residents. 17 Figure 23 Roslyn, circa 1900. Stop 4 is at the entrance to the mines, the terminus of the railroad spur. Note the well-ordered appearance of the town, with boardwalks, telegraph poles and a baseball field (front left). Paint seems to be a scarce commodity. Over the last two decades the community has adopted something of an artisan image, building on its popularity from the television series. It is a charming community displaying over a century of history, and draws a healthy stream of tourists over the summer months. The town is home to The Brick Tavern, the oldest continuously-operating saloon in the state under the same name. It dates from 1898. The Roslyn Mines were, appropriately, in the Roslyn Formation. The Roslyn Formation is largely sandstone and siltstone, and (obviously) includes extensive coal beds. Here, the strata of the Roslyn Formation dip to the east, as do those in the Swauk Formation. The Swauk, the Teanaway and the Roslyn Formations are all folded into a broad northwest-southeast striking syncline, dipping to the southeast. This is known as the Kittitas Syncline. At this latitude, the Roslyn Formation occupies the central portion of that syncline, which probably formed between 40 and 38 million years ago. By the map distribution of these rocks, one can conclude that the Roslyn Formation overlies the Teanaway Formation. Figure 24 The historic Brick Tavern, the oldest continuously operating saloon under the same name in the State of Washington. It was completed in 1889, and still stands today. It is said that construction required some 20,000 bricks. Unfortunately, the brickwork has since been painted over. 18 Return to the highway and head south to the town of Cle Elum “Suncadia” is a residential / recreational development which was approved by the town of Cle Elum some years ago, in hopes that it would boost the local economy. It is a secured private community featuring a championship golf course, private river access, private hiking and biking trails, tennis courts, swimming pools, and a host of other amenities. Modest vacation homes start in the low $400’s. Ultimately, it will be larger than the town itself. Rather than spuring development in Cle Elum, this project has fostered rapidly-increasing development of the corridor through Roslyn to Lake Cle Elum. Vacation lots have been platted across the landscape, many of them gated developments designed to appeal to security-conscious west-siders. They are being marketed to “coasties” from the Puget Sound region, and to retirees across the country. The residents of Cle Elum haven’t seen any benefit, and those in Roslyn see this as a serious threat to their historic community. The advisability of such development is dubious. The entire region is ponderosa pine forest, a landscape periodically subject to fires. At the round-about, turn left (east) toward Cle Elum Between the towns of Roslyn and Cle Elum are a whole string of coal mines, the talings piles can be seen along the north side of the road. Continue through the town of Cle Elum to Peoh Street (Bakery) or Columbia Avenue (Stop 5) Cle Elum is a somewhat non-descript town, a patchwork of faded buildings from different eras marking the main street of commercial activity. The number of vacant store-fronts suggests a less-than-thriving local economy. The rail yards, sas stations, truck service yards and other utilitarian enterprises give it a unique ambiance. The town is none-the-less the commercial center of the upper Kittitas Valley, and provides services to a broad region. It’s most redeeming quality is that it hosts the Cle Elum Bakery, widely acknowledged as one of the best in the state. Figure 25 The historic Cle Elum Bakery. The brick ovens here have not gone cold since first fired up in 1906. It is a full-service bakery, providing a range of breads, cookies, pastries and other products. Cross-state travelers often stop here to stock up on baked goods. 19 Figure 26: Exposures of the Roslyn Formation, above the town of Cle Elum. Unfortunately, these outcrops are extremely weathered, and show few details of the rock. They are also on private property, and not available for public access. What can be seen here is the distinctly white color of the rock, in contrast to the tan or buff color of the Swauk Formation. Note also that these rocks are not cut by dikes of the Teanaway Basalt, which helps to distinguish the Roslyn from the Swauk where the rock types are similar. Stop 5 The Roslyn Formation Cle Elum, WA Near the east end of town, turn left on Columbia Avenue. Columbia turns right onto a dirt road just past a church. This dirt road rises and then reaches a fork. Take the right fork and park at the prominent switchback in the road. While the Roslyn Formation is abundantly exposed in the slopes above town to the north (see above), these are on private property, and are heavily weathered exposures. The outcrop here is small, but provides an convenient opportunity to examine fresh samples. Figure 27 The Roslyn Formation, stop 5. This is not much of an outcrop, but provides an opportunity to examine fresh samples of the rock. This is on one of the many new roads being cut into the hills behind the town of Cle Elum, for new housing developments and vacation communities. Development is happening all across the area, except in the town of Cle Elum itself. 20 Figure 28 The Roslyn Formation, Stop 5. The scale of bedding in the Roslyn Formation is too large to be seen at this outcrop, but it is a good site for examining the rock itself. In contrast to the tan and buff-colored sandstones of the Swauk Formation, this is a distinctly white rock. The white color comes from an alteration product (laumontite). The Roslyn Formation is largely a sandstone unit, similar to the sandstones of the Swauk Formation in its general character. It is frequently a bit finer-grained, but the major difference is in the color. While the Swauk Sandstone is tan to buff in color, the Roslyn is distinctly white. This reflects the presence of an alteration product (laumonite) in the rock. Of equal significance, the Roslyn displays much larger-scale bedding than does the Swauk. The beds in the Swauk Formation are typically millimeter to centimeter in scale, reflecting events of limited duration. By contrast, the bedding in the Roslyn is typically meter-scale, reflecting events of longer duration. During Roslyn time, depositional conditions did not change as often as seen in Swauk time. Rocks of the Roslyn Formation were deposited in a river basin setting, but an environment featuring more boggy conditions and a more stable depositional regime. The scale of bedding cannot be seen here, but can be observed at the old Red Bridge Crossing off the Teanaway River Road, to the northeast. More advanced students will want to make this side-trip. For introductory purposes, this outcrop illustrates the fundamental character of the rock itself. Figure 29 (Right) A hand-sample of sandstone from the Roslyn Formation. Note the distinctly white color of the rock. Dime provides scale. 21 Figure 30 The Ellensburg Formation, along the Yakima River (Stop 6). The Ellensburg is almost exclusively volcanic sediments, shed from the ancestral Cascade volcanoes to the west. Note the prominent fault in the center of the image. Stop 6: The Ellensburg Formaton Yakima River Return to main street (SR 970) and continue east out of town. The road is SR 970, heading north. After a short distance, turn right onto US 10, signed for Ellensburg. Take this road for about 5 miles to a prominent turn-out above the Yakima River. These are rocks of the Ellensburg Formation, which locally overlie the Roslyn Formation. Here, they occupy the center of the Kittitas Syncline. The Ellensburg is an informally-designated formation, consisting of more than one mappable unit of rock. This is a good representative exposure of these rocks, illustrating their essential character. The rocks here vary from very fine-grained sediments to conglomerates, but all share in one characteristic – they are all volcanic (largely, andesitic) rocks. The finest material is volcanic ash (tuff), while the sandier material is a tuffaceous sandstone. The conglomerates consist of poorly-sorted, matrix-supported, well-rounded cobbles of andesite and dacite, often in meter-scale beds. These deposits likely date from 20-25 Ma. They are sediments which were eroded off the ancestral Cascade Volcanoes to the west, carried by rivers flowing to the east. Most of the finer sediments were deposited by rivers, but some of the conglomerates represent debris flows and lahars which periodically coursed down the river valleys. The rivers carrying these sediments were overburdened in their capacity, rapidly accumulating sediment and constantly changing course. Elsewhere, the formation displays a classic pattern of trough cross-bedding, characteristic of heavily overburdened rivers. 22 Figure 31 Normal (extensional) fault in the Ellensburg Formation. Image on the right shows pre-fault configuration. This fault probably developed as the Columbia Basin to the east sank under the accumulated weight of up to 5 km of basalt flows from the Columbia River Basalts. The “Ellensburg” formation refers to all tuffaceous sediments which accumulated below and in-between flows of the Columbia River Basalts along the east slopes of the Cascades. They reflect active volcanism in the Cascade Arc to the west, and rapid erosion and transport of that material east by local rivers. Plant fossils can often be found in the finer sediments, reflecting a temperate climate, but a much wetter setting than exists today. While the ancestral Cascade Volcanoes rose to the west, the modern Cascade Range did not start to rise until much more recently, over the last five million years. Prior to that date, the moisture from Pacific weather systems flowed unimpeded into what is now Eastern Washington. The other feature of note here is a prominent high-angle fault. It can be seen cutting and offsetting beds of fine-grained sediments and conglomerate. This is what is known as a “normal” fault, which results from extending or depressing the crust. The diagnostic characteristic of this sense of motion is that the “hanging wall” of rock lying above the fault line has been down-dropped, relative to the “footwall” of rock lying below the fault. This is characteristic of an extensional tectonic regime or a subsiding setting. In this case, it likely reflects the subsidence of the Columbia Basin to the east, as it accumulated up to 5 km of the Columbia River Basalts, largely between 19 and 15 million years ago. This is one of many faults which developed as that basin sank under the massive weight of these lava flows. Continue east on Highway 10, toward Ellensburg. Continue about 2.5 miles to the next stop. In short order, the route enters the province of the Columbia River Basalt Flows. These are a regional-scale feature covering much of eastern Washington and Oregon. 23 Figure 32 (Above) The Thorp Gravels, along Highway 10. These overlie the Columbia River Basalt Flows. Above the gravel can be seen a thin layer of the Palouse Loess. Both of these units become thicker to the east. Figure 33 (Below right) The Yakima River, as it cuts into the Columbia River Basalts. From near stop 7. In 2 miles, note the layer of gravel which lies on top of the Columbia River Basalt Flows. This is gravel of the Thorp Formation, known as the Thorp Gravels. The Thorp Gravels accumulated about 4 million years ago. Atop the Thorp Gravels lies the Palouse Loess, the dominant soil of Eastern Washington. The Palouse Loess is a glacially-produced silt, and was deposited over a million years ago, during the early ice ages of the Pleistocene. It is an outstanding soil for agricultural purposes, and is the economic foundation of Eastern Washington. Continue a total of miles to a slight turnout on the right side of the road. The rocks here are a distinctly yellow color. 24 Figure 34 Pillow-palagonite complex, along US 10 (Stop 7). These developed as basalt flowed over moist ground or water. They are relatively common appearances across the basalt province of Eastern Washington, illustrating that this region enjoyed a wet climate prior to the rise of the modern Cascade Range. Stop 7: The Columbia River Basalts Yakima River These are flows of the Columbia River Basalts. These flood basalts erupted from fissures, principally in southeastern Washington and northeastern Oregon. They erupted between 17 and 6 million years ago, but most of the material was erupted about 16 million years ago, as part of the Grand Ronde series. These rocks date from that episode. The Columbia River Basalts appear to have erupted as the continent moved over the Yellowstone Hot Spot. The area in which they erupted is a zone along which the continent is spreading apart, creating a weakness in the crust. While now restricted to eastern Washington, Oregon and the Columbia River Gorge, these flows were probably much more extensive when they first erupted. Since that time, the modern Cascade Range has risen, likely resulting in the loss of considerable exposure. In this area, an east-dipping paleoscope is reflected in the flow of the rivers depositing the Ellensburg Formation, so at least a modest topographic high existed to the west when the flows first erupted. Their original western margin in this area remains uncertain. Erupting prior to the uplift of the modern Cascade Range, the Columbia River Basalts flowed over a landscape which enjoyed a damp maritime climate, marked by temperate forests and numerous small lakes and boggy areas. This setting is reflected in the fossils from the Gingko Petrified Forest, near Vantage. In the Grand Coulee area, the cast of a small rhinoceros was preserved at the base of a flow, known as the “Blue Lake Rhino”. Its habitat is consistent with such a setting. 25 Figure 35 Clay layer (arrow) at the base of the palagonite complex, stop 7. This clay accumulated in the bottom of a small lake, a lake which the basalt subsequently flowed over. Clay Layer The rocks at this field stop are also consistent with such a setting. In contrast to the clean (often columnar) appearance of the surrounding flows, these are a lumpy mix of basalt and a yellow mineral known as palagonite. Palagonite is a clay mineral is formed as basalt flows over water or damp areas. It is interspersed with large “pillows” of dark basalt, the characteristic form of lavas erupted in a submarine setting. Together, they are known as a “pillow-palagonite” complex, and it develops where hot lava contacts water. There is additional evidence for this at this stop. At the base of the pillow-palagonite complex, there is a bed of white clay. This clay accumulated in the bed of a small lake which existed here, a lake which the lava subsequently flowed over. Figure 37 (Below) Fossilized tree trunks, at the Gingko Petrified Forest Interpretive Center. These trees reflect a moist and temperate setting here some 16 million years ago, before the rise of the modern Cascade Range. Figure 36 (below) A model of the Blue Lake Rhino. The body cast of this animal was found at the base of basalt flows near Blue Lake, in the Grand Coulee. This model is on display at the Dry Falls Interpretive Center 26 Swauk Formation Teanaway Formaton 2 1 3 Roslyn Formation Kittitas Syncline 4 5 Cle Elum 13 12 Ainsley Canyon Anticline 11 6 7 10 9 . Columbia River Basalts 8 Figure 38 Geologic map of the Kittias Valley region, showing field trip stops. Rocks of the Ellensburg Formation, along with the outcrops along Taneum Creek, are not detailed at this scale. See figure 9 for regional view. Continue east on US 10 to the Thorp Mill Road, and turn right on that road. Continue 2.5 miles to the intersection with the Taneum Road. Turn right on the Taneum road and continue across the Thorp Prairie to where it crosses Interstate 90. On the far side, park at the entrance to Taneum Canyon. Reset your trip odometer here. Stop 8 Taneum Canyon and the Thorp Gravel Taneum Creek broadly parallels the course of the Yakima River to the north, separated from that drainage by the South Cle Elum Ridge. The remainder of the trip ascends Taneum Canyon, to a point where one can cut across the intervening ridge, and descend back down to Cle Elum. The high point of the trip is Peoh Point, a spectacular vantage on the Kittitas Valley below. Taneum Canyon is cut into a structural feature called the Ainsely Canyon Anticline. This is a southeasterly-plunging anticline which strikes northwest – southeast. It is the anticlinal counterpart to the Kittitas Valley Syncline to the north. In contrast to the relatively gentle character of the latter, the Ainsley Canyon Anticline is a more steeply-folded structure. These features reflect a regime of north-directed compression which has been in effect for the last twenty 27 Taneum Canyon Thorp Gravel Figure 39 Looking west into the entrance of Taneum Canyon. The hillside on the right is mantled in the Thorp Gravels, which accumulated starting about 4 million years ago. They likely reflect the rise of the modern Cascade Range. million years. On a larger scale, these are northern elements in a structural zone known as the Yakima Fold Belt. Because this feature plunges to the southeast, the route up the canyon exposes progressively older strata as one makes their way up the valley. For whatever ambiguity persists over the order of the rocks we have visited, this section serves to remove it. The Thorp Gravel The Thorp Gravel is the highest member in this package (not including the Pleistocene Palouse Loess or local glacial tills), and is thus the first exposed in this section. The large rounded hill to the west is comprised of the Thorp Gravel. In this part of the Kittitas Valley, these coarse-grained sediments have accumulated to appreciable thicknesses. Topographically, they give a rounded aspect to a landscape otherwise largely cut from blocky flows of the Columbia River Basalts. The Thorp Gravel has been dated at about 4 million years in age, based on radiometric dating of ash layers found near its base. This makes its accumulation coeval with the rise of the modern Cascade Range to the west. Paleocurrent indicators in the unit reflect a western source area for this gravel. It may well have been eroded off the rising Cascades. It is a polymictic assemblage, displaying a broad diversity of rock types. 28 Figure 40 (Right) Elk graze in the meadows of Taneum Canyon. Part of this region is an Elk reserve. Figure 41 (below) Near the entrance to the canyon. The rock outcrops are basalt. The prairie at the entrance to Taneum Canyon is part of an elk reserve, managed by the Rocky Mountain Elk Foundation. Elk are the largest members of the deer family, and can often be seen grazing here. Continue up the road to the next stop at 2.3 miles. 29 Figure 42 The Columbia River Basalt Flows and the Ellensburg Formation, in lower Taneum Canyon. This view illustrates that the basalts consist of two distinct flows. Stop 9 The Ellensburg Formation and the Columbia River Basalts There are two rock types here, recognizable as the Columbia River Basalts and the Ellensburg Formation. The Ellensburg is easily distinguished by its tuffaceous character, and the presence of andesite pebbles in the mix. The position of the basalt above the Ellensburg eliminates the possibility that this is the Teanaway Basalts. Above, it can be seen that this is two distinct flows of the Columbia River Basalts, with the contact showing the effects of weathering. These are the Grand Ronde flows, dating from about 16 million years ago. 30 Figure 43 The Ellensburg Formation, below the Columbia River Basalt Flows. The Ellensburg is easily discerned by the tuffaceious character of the fine sediments and the volcanic character of larger clasts. The texture of the basalt flows here is described as a “hackely” one. It develops near the top of such flows, in the part called the “entablature.” Because of its well-broken texture, water percolates down through flows like this, and often emanates at contact points. For engineers building roadways, this is the preferred material for road-beds. It breaks relatively easily, and produces smaller fragments which are angular and durable. It is considerably easier to process than massive basalt. These flows lie beneath the Thorp Gravels, and over the Ellensburg Formation. The contact with the Ellensburg has been eroded, by water draining down through the rock, and emanating at the contact point. Because it will be seen that the basalts do not occur below this section of the Ellensburg, this section is older than the basalt flows. The earliest Ellensburg sediments may date from about 25 Ma, about 9 million years before the Grand Ronde flows. Elsewhere, these sediments also occur as interbeds between various flows. Looking up the road, one can discern than the contact between these two rocks dips to the southeast. This is consistent with the southeast-plunging character of the Ainsely Canyon Anticline. Accordingly, we will be advancing through progressively older rocks as we progress up the valley. Continue to the next stop at 4.2 miles. 31 Figure 44 The Roslyn Formation, exposed by a landslide on a prominent bend in the road. Note the distinctly white color. Stop 10 The Roslyn Formation A recent slide on this bend in the road has exposed the outcrop above. On inspection, these are found to be rocks of the Roslyn Formation. While not as well lithified as found elsewhere, the distinctive white color to the sediments easily identifies it as the Roslyn. A few smaller outcrops down the road are also consistent with this interpretation. They also confirm that the previous stop was the basal section of the Ellensburg Formation, and that no lower basalt flows intervene between the Ellensburg and the Roslyn Formations in this area. The contact between the Ellensburg and the Roslyn FormaFigure 45 Detail of the site above. These sediments are not tions is an unconformable one. A major regional unconfor- as well lithified as those seen earlier, but are clearly of the mity at about 38 Ma marks the division beween the “Chal- Roslyn Formation. Hammer provides scale. lis” episode rocks of the Swauk, Teanaway and Roslyn Formations and the “Cascade” episode rocks of the Ellensburg, Columbia River and Thorp Formations. Continue to the next stop at 5.2 miles. 32 Figure 44 Outcrop of the Swauk Formation, along the Taneum Road. The outcrop is not exactly obvious, so it is worth watching your mileage to see that you don’t pass it. It is a fairly productive site for fossil hunting, given a continuing supply of new material. Stop 11 The Swauk Formation The rocks here are the Swauk Formation, easily identified by their tan to buff color. Based on our earlier stops, we are left to wonder: what happened to the Teanaway Basalts? The answer is that a small normal fault cuts the valley east of here, and the area to the east has been down-dropped relative to that on the west. In this process, the Teanaway Basalts have been faulted out of the sequence here. They are present on the slopes above, but do not appear at the river level where the road is. This is a fairly good location to dig for fossils, if enough fresh material is available. Over the years we have found fossils of palm fronds, woody debris, and the leaves of a variety of species at this outcrop. Collectively, they reflect a lowland river-basin setting, a paratropical environment where the average temperature was about 70 F, and varied no more than 5 degrees year - round. The dominant species were palm trees, but a wide variety of other foliage shared that setting. 33 Figure 45 Leaf imprints from the Swauk Formation.. These appear to be a cinnamonium species, thought to be an extant member of the Laural family. Hammer provides scale. The fossil foliage of the Swauk Formation is typical of a warm paratropical environment, dominated by medium to large-leaved species suited for a warm damp climate. The west-side equivalent of the Swauk Formation is the Chuckanut Formation around Bellingham, some 90 miles north of Seattle. These sediments are identical in all respects, and share the same fossil assemblages. They represent the deposits of a continuous river basin between ~53 and 48 Ma. Starting at about 48 Ma, a large north-south striking fault developed not far west of here, extending north to the Canadian arctic. Over the next ten million years, the rocks Figure 46 Leaf imprints from the Swauk Formation. This appears to be a maple leaf, a common species. Hammer provides scale. 34 Figure 47 Leaf imprints (Palm fronds) in the Chuckanut Formation, east of Bellingham. The Swauk and the Chuckanut Formations were originally coextensive. Based on the fossil record, palm trees were the dominant vegetation during this period, reflecting a warmer and more equitable climate. on the west side of that fault (known as the Fraser Fault) were displaced some 90 miles (145 km) to the north. The Chuckanut formation, now at the latitude of Bellingham, was originally deposited at the latitude of Seattle. There, it was the downstream equivalent of the Swauk Formation. This same tectonic event was responsible for emplacing the Olympic Coast Belt into the continental margin. Figure 48 Generalized paleogeography of the northwest over Eocene time (ca 50 Ma). Note the large river system draining from the east. Note also the absence of the Olympic Peninsula, and the large embayment in the coastline to the south. Dashed line is state outline. * Columbia Embayment 35 * locates Swauk Formation Fraser Fault Leech River Fault C C S S Fraser Fault 50 Ma 40 Ma Figure 49 (Above) Really complicated-looking map depicting the Mid - Eocene (50-40 Ma) tectonic setting in this region. The different-colored areas represent different “terrane” units which make up the basement rocks. The figure on the left reflects conditions at about 50 Ma, while the figure on the right reflects conditions at about 40 Ma, after faulting on the Fraser and Leech River Faults. In the figure on the left, the red dots represent the Chuckanut Formation (C) around Bellingham, and the Swauk Formation (S) around Cle Elum. These were originally a coextensive formation. In the figure on the right, note how right-lateral movement on the Fraser Fault has displaced the west side to the north by about 150 km. This same regime was responsible for transporting the oceanic rocks of the Olympic Coast Belt to the north, underthrusting the southern end of Vancouver Island along the Leech River Fault. In this process, this last “terrane” was implaced on the continental margin. These events happened between 50 and ~40 Ma, and reflect the north-directed transform tectonics which was characteristic of the 58 - 38 Ma “Challis” Episode. By 37 Ma, a convergent margin had been re-established along the coast here. That tectonic setting is characteristic of the modern (37 Ma - present) “Cascade” Episode. Continue up the road to a major fork, and take the right fork signed for Peoh Point. Stop a few hundred feet up the road at this outcrop. Total mileage 9.6 miles. 36 Figure 50 An outcrop of the Darrington Phyllite, above Icewater Creek Camp. On a sunny day, it is easy to spot as it glitters in the sun. Stop 12 The Darrington Phyllite These rocks are the Darrington Phyllite, part of the Shuksan Suite, as mentioned at stop 1. Late in the season, when the water is low, you can see these rocks at Lake Easton. Along with the Easton Greenschist, these rocks comprise the “basement” rocks of this region, and occupy the lowest stratigraphic position. Phyllite is a low-grade metamorphic rock which develops from a mudstone. In this case, it was the mud covering a section of oceanic crust. The minerals are microscopic, but the distinctive sheen on the rock results from microscopic crystals of muscovite, a mica mineral. While homogeneous, the rock has a distinctive structural sense of layering, or foliation, to it. The Darrington Phyllite is distinctive because it contains abundant graphite. This gives the rock a decidedly silvery appearance. You will find that it will make a visible mark on a piece of paper (graphite is the material used to make pencil lead). Graphite is a carbon compound, and prompts the question of how so much carbon was deposited in this mud. Carbon comes from organic sources, and this quantity suggests that it was probably derived from a terrestrial source. Large amounts of organic material are brought by rivers draining into the ocean, particularly after fires and other events. This suggests that this section of oceanic plate, as it was being subducted, lay not far from the shoreline of the continent. Like the Swauk Formation (but not the younger formations), the Shuksan Suite here has been displaced from its western counterpart to the northwest. These rocks are typical of the region north of the Stilliguamish River, and have been displaced from their western counterparts along the Fraser Fault. As discussed at the last stop, this happened between 48 and 38 Ma, during the Challis Episode. 37 Figure 51 The Darrington Phyllite, detail from the image to the left. Hammer provides scale. The luster of this rock is a combination of the microscopic muscovite (white mica) typical of phyllite, with graphite, which imparts a silvery color. Continue up the road toward Peoh Point. At the top of the ridge, there is a parking area where the road makes several forks. Park here and continue on foot. The road to Peoh Point can be traveled in 4-wheel drive vehicles under good conditions, but it is a somewhat nerve-wracking affair. It is easier to park the vehicles here, and walk the mile to the point. As you start up the road, note the rock it is cut into . This is the Swauk Formation. It is a silty, clayey section of the Swauk, and makes for a very slick road when wet. Occasionally, fossils can be found in the outcrops here. Follow the road, as signed, to the ridge crest and the turnoff for Peoh Point. Park here and walk this road for about a mile and a quarter to the Peoh Point Overlook. Figure 52 (Right) A hand-sample of Darrington Phyllite, a somewhat less graphitic example than seen in the field. Phyllite develops from mudstone, and is characterized by the development of microscopic muscovite. The Darrington Phyllite also includes knobs and ribbons of quartz, as seen here, 38 Figure 53 Looking north from Peoh Point. In the background are peaks of the Stuart Range, culminating in 9415 - foot Mt. Stuart. Below is Cle Elum Lake, an artifically-impounded reservoir which stores water for irrigation. The town of Roslyn can be seen in the foreground. This is a truly spectacular vantage on the Kittitas Valley, well-worth the trip when the weather is clear. Highly recommended on clear spring days, when snow still mantles the mountains. Stop 13 Peoh Point As you start up the road, you will notice that the road-cut is in fine-grained sediments of the Swauk Formation. When wet, this is a slick bit of roadway. This is the same belt of the Swauk Formation as was visited below. On reaching Peoh Point, you will notice that the rock is a light-colored volcanic rock (felsite). These are rocks of the Taneum Formation, which locally appears between the Swauk Formation and the Teanaway Formation. This magma was quite different from the Teanaway Basalt, and came from a different source. It reflects a continental - arc setting (the Challis Arc) which existed east of the Fraser Fault from 58 - 40 Ma. The Taneum Formation is not a wide-spread unit. Peoh Point provides a spectacular vantage on the Kittitas Valley, and on the dramatic Stuart Range to the north. The Stuart Range is composed of granodiorite, an intrusion dated at about 90 million years ago. This was about the same time that the Melange Belts were being added to the continent here. It was a time of great mountain-building along the coast, as the ancestral Coast Range Mountains developed over this period. The granodiorite that makes up Mt. Stuart was intruded over a dozen kilometers deep in the crust, at the root of an ancient volcano which rose on the surface at that time. The overlook provides a clear view of Cle Elum Lake, which is impounded by a dam for irrigation purposes. Prior to being artificially dammed, two smaller lakes occupied this valley, impounded by moraines. On careful inspection, you can discern moraine features on the landscape here. As discussed earlier, the Kittitas Valley area is a large southeast-plunging syncline, known as the Kittitas Valley Syncline. To the south, the field trip route has just ascended the Ainsley Canyon Anticline, the southern half of that fold system. As it turns out however, there is a break between the Kittitas syncline and the Ainsley Canyon Anticline. That break is a low-angle thrust fault, known as the Easton Thrust. 39 Figure 54 Looking east from Peoh Point. Note the light-colored volcanic rock in the foreground, part of the Taneum Formation. Consider the basalt flows which make up the top of the mountain in the center of the image. The tilt of these flows (down to the east) reflects uplift of the modern Cascade Range to the west. Projected westward, the top of this flow extends over the top of Mt. Stuart to the west (see figure 53, left). These rocks comprise what is known as a fold-and-thrust belt. Compressed from the south, the rocks here fold until they periodically yield along the fault, thrusting to the north. This is the character of most of the structures in the Yakima Fold Belt, as well as similar features to the west. While recent uplift of the Cascade Mountains has obscured connections between the two, these faults extend west to the Puget Sound region. The Seattle Fault, for example, shares these same characteristics. Stress accumulates in folding the Newcastle Anticline along the south end of the fault, and is periodically released by displacement on the Seattle Fault at its base. This fold-and-thrust regime has been going on for some twenty million years, driven by the northward shearing of the California coast along the transform margin in that region. As California moves north, it drives Oregon into Washington along a northeast-southwest axis. Because the northern half of Washington includes considerable granitic rock, it acts as a “backstop” against this northeastern compression. As a result, a northwest-southeast set of folds (locally, the Yakima Fold Belt) has developed to accommodate this compression. As noted, this is the same regime which causes periodic movement on the Seattle Fault to the west. 40 Figure 55 (Above) “Fixed” North American Block The Easton Thrust Fault, cutting across the southern end of the Cle Elum Valley. Rocks on the foreground side have been uplifted and moved northward along this feature. North-South Compression Yakima Fold Belt Figure 56 (Right) Diagram illusrating how the combination of northward shear along the California margin and eastward compression along the Washington-Oregon margin produce northeast-directed compression into Eastern Washington, and northward compression in Western Washington. Based on a diagram by the US Geologic Survey. The Yakima Fold Belt is a series of northweststriking folds which have accomodated this compression. It is a fold-and-thrust belt, where folding has yielded to low-angle thrust faulting along the base of the folds. In Western Washington, northward compression is accomodated on fold-and-thrust features like the Seattle Fault and the South Whidbey Island Fault, among others. These share the same architecture as the folds in the Yakima area. Rotating Coastal Block Juan De Fuca Plate Pacific Plate 41 Translating Sierra Block Figure 57 (Above) Northeast Southwest Kittitas Syncline The Easton Thrust Fault, cutting across the base of the Cle Elum Ridge. This view is looking east from Peoh Point. Rocks in the foreground have been moved up and north relative to those behind. Ainsley Canyon Anticline Figure 58 (Right) Easton Thrust Fault Diagram illustrating the development of a fold-and-thrust belt. Rocks are initially folded by compression, but eventually yield to lowangle thrust faulting along the base of the anticline. This is the dominant character of the Yakima Fold Belt. 42 Kittitas Valley Syncline Easton Thrust Fault Ainsley Canyon Anticline Figure 49 Simplified geologic map of the Kittitas Valley, showing the major structural features. The Easton Thrust Fault lies at the base of the Ainsley Canyon Anticline, moving those rocks up and northeast over rocks in the Kittitas Valley Syncline. This is the character of the Yakima Fald Belt, accomodating compression to the northeast. 43 A Last Final Word From Your Instructor: The Pacific Northwest is home to some of the most remarkable geology to be found anywhere on the planet. No region can claim to a greater variety of rock types, or features them in more spectacular settings. More significantly, no region affords such a remarkable venue on the truly collosal forces which drive the dynamics of our planet, or such graphic illustrations on the variety of geologic processes which they support. There is simply no better place on the planet to see how the Earth works. There is certainly no better place on Earth to learn and experience geology. If you are planning on living in this area, you should know that you are living in the midst of some of the most incredible geology in the world. You should know that the modern landscape that surrounds you is the product of a truly amazing course of geologic history, one that stretches back hundreds of millions of years. You should recognize that you occupy a unique point in time and space in that course of history, amidst the ongoing geologic processes which will continue to shape this region into the future. Finally, you should appreciate that you are the very first generation of people to enjoy the privilage of walking this landscape with a knowledge of the story behind it. In this respect, yours is a truly privilaged position in that course of Earth history. John 44