Regional Tertiary sequence stratigraphy and structure on the eastern flank of the central Cascade Range, Washington
Published:January 01, 2007
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Eric S. Cheney, Nicholas W. Hayman, 2007. "Regional Tertiary sequence stratigraphy and structure on the eastern flank of the central Cascade Range, Washington", Floods, Faults, and Fire, Pete Stelling, David S. Tucker
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Eocene sedimentary and volcanic rocks on the eastern flank of the Cascade Range consist of five regional, unconformity-bounded formations of the Challis synthem. These formations define a series of northwesterly striking folds. Five anticlines are 9 to 28 km apart, have pre-Tertiary crystalline rocks in their cores, high-angle reverse faults on their steeper northeastern limbs, and pass down-plunge into more gentle folds in the Neogene Columbia River Basalt Group (CRBG). Such northwesterly trending folds extend from east of the Columbia River across the Cascade Range to the Puget Lowland.
The Chiwaukum graben and Swauk basin, which heretofore were thought to be local, extensional, depositional basins, are, instead, the major northwesterly trending synclines in this series of folds. The Eocene formations were preserved, not deposited, in these synclines. Dextral, N-S faults cut the reverse faults and the pre-CRBG portion of some of the folds. The post-CRBG folds control the regional distribution of the Eocene formations.
The Cascade Range is a southerly plunging, post-CRBG anticline. Clasts in the Thorp Gravel indicate that this anticline began to rise ca. 4 Ma. The anticline has an amplitude of ∼3.5 km, and it causes the plunges of the northwesterly striking post-CRBG folds. The northerly and northwesterly post-CRBG folds form a regional interference pattern, or “egg-crate,” that dominates the present topography of Washington State.
As a result of new 1:24,000 mapping, we test two popular beliefs about the regional geology of the eastern flank of the central Cascade Range. The first is that the thick, nonmarine, arkosic Eocene formations of central Washington were deposited in pull-apart (transtensional) basins. Reputed examples are (1) the Swauk Formation in the Swauk basin, bounded by the Leavenworth and Straight Creek faults (Johnson, 1985; Taylor et al., 1988), and (2) the so-called Chumstick Formation in the Chiwaukum graben, bounded by the Entiat and Leavenworth faults (Gresens et al., 1981; Gresens, 1982; Tabor et al., 1982, 1984, 1987; Evans and Johnson, 1989; Evans, 1988, 1994). The second is the belief (Beeson et al. 1989) that Cascade Range has existed as a topographic entity since at least the Oligocene.
The first day of this trip examines Eocene strata in the Leavenworth area near U.S. highway 2 (US-2) and the Blushastin area on U.S. highway 97 (US-97) to determine if the strata were deposited in local extensional basins. To determine age of the present Cascade Range, the second day concentrates on Neo gene rocks and structures between Cle Elum, Ellensburg, and Yakima and ends in pre-Tertiary rocks near Easton on Interstate 90 (I-90). Pre-Cenozoic crystalline rocks and Quaternary deposits are virtually ignored on this trip.
We make four important points. First, the Eocene formations are of regional extent and have laterally persistent stratigraphies that indicate that they were not deposited in local, fault-bounded basins. Second, the reputed pull-apart basins are northwesterly striking regional synclines. Third, the Eocene through Oligocene was marked by two sets of faulting; the Eocene one accommodated northeast-southwest shortening, the Oligocene one accommodated north-south dextral strike slip. Fourth, two sets of folds are late Neogene; they are bigger and younger than commonly supposed, caused the rise of the Cascade Range, and dominate the topography of Washington.
Tabor et al. (1982, 1984, 1987, 2000) mapped the eastern flank of the Cascade Range at 1:100,000. After their mapping in the 1970s and early 1980s, 1:24,000 topographic maps and Global Positioning System (GPS) became available, and access via logging roads improved substantially. Mapping near Easton (Cheney, 1999), Blushastin (Cheney, 2003) and Leavenworth since 2005 benefited from these changes.
Geologic mapping in the three areas is somewhat challenging. The three areas are forested and have about a kilometer of topographic relief. The network of logging roads is deteriorating. The major valleys are filled with glacial and alluvial sediments, and the Blushastin and Leavenworth areas are mantled with loess. The formations are thick and have dips ranging from moderate to vertical. As a result of these conditions, marker units must be tens of meters thick, and to be mappable, units must be at least 100 m thick.
We first review the geology of the central Cascade Range. We then describe the geology of the Blushastin and Leavenworth areas and of the Yakima fold belt. These descriptions provide the basis for a discussion of regional structures and for the road logs of the trip. In the road logs and figures, field trip stops of Day 1 are numbered 1-1 through 1-21, and those of Day 2 are numbered 2-1 through 2-14. Based on weather and road conditions, some stops may have to be omitted.
Geology of the Cascade Range
The Cascade Range is a late Neogene, southerly plunging anticline (Cheney and Sherrod, 1999), which is locally capped by late Pleistocene volcanoes, such as Mount Rainier. Thus, pre-Cenozoic crystalline rocks dominate the northern part of the range, and unconformably overlying Cenozoic sedimentary and volcanic rocks tend to be progressively younger and more extensive southward. The central part of the range consists of both pre-Cenozoic crystalline rocks and Eocene sedimentary and volcanic rocks (Fig. 1).
The pre-Cenozoic crystalline rocks consist of terranes that accreted during the Cretaceous and post-accretionary tonalitic plutons (Dragovich et al., 2002; Paterson et al. 2004). The terranes extended North America westward, so that the Cenozoic rocks of the Cascade Range were deposited upon the western edge of a craton-like basement. The crystalline and Eocene rocks are cut by northwesterly to northerly trending, regional, dextral strike-slip faults (Tabor et al., 2000; Dragovich et al., 2002). The southern ends of some of these faults are shown in Figures 1 and 2.
The Cenozoic sedimentary and volcanic rocks consist of four inter-regional unconformity-bounded sequences of tectonic origin, so-called synthems (Fig. 1). Each synthem rests upon crystalline basement somewhere in Washington, and each extends beyond Washington (Cheney, 1994).
On the eastern flank of the Cascade Range, the Eocene Challis synthem contains five unconformity-bounded formations (Table 1). Table 1 lists the maximum thickness of each formation, but at some localities in the area of Figure 2, the thickness of each of the older three formations thins to zero beneath various unconformities. Local (synonymous) formational names have obscured the fairly simple regional stratigraphy and structure shown in Table 1 and Figure 2.
In central Washington, the predominantly basaltic Teanaway Formation is the marker unit in the Challis synthem. Where this formation is absent, the arkosic Swauk and Roslyn formations stratigraphically below and above the Teanaway Formation, respectively, are difficult to distinguish, but criteria do exist for doing so (Table 2). A swarm of dikes compositionally similar to basalt in the Teanaway Formation (here called Teanaway dikes) extends at least 15 km beyond the present northern limit of the formation (Tabor et al. 1982; Dragovich et al., 2002).
The sub-Teanaway unconformity is an important structural datum and may be an important thermal one. Formations below the unconformity are more intensely folded than formations above it (Tabor et al., 1982; Doran and Miller, 2006). Published fission-track dates on zircons in the Swauk Formation are younger than those in the overlying Taneum Formation (Cheney, 1994, Fig. 8 therein).
The most voluminous lithostratigraphic unit of the Walpapi synthem, the Miocene Columbia River Basalt Group (CRBG), underlies the southeastern limb of the Cascade Range anticline and its nose along the Columbia River (Figs. 2 and 3). The High Cascade synthem includes the andesitic rocks of the present Cascade magmatic arc and widespread glacial and alluvial sediments.
The best known structures on the eastern flank of the Cascade Range are the northwesterly and northerly strike-slip faults (Figs. 1 and 2). Less well known is a series of northwesterly trending folds and faults that involve the formations of the Challis synthem (Fig. 2). The folds are cut by the northwesterly and northerly trending strike-slip faults; whereas, the Oligo-Miocene Kittitas synthem is not (Cheney, 1999). A still younger period of folding caused northwesterly trending folds in CRBG, which refolded the northwesterly trending folds in the formations of the Challis synthem. The Cascade Range anticline is ≤4 Ma; it causes the northwesterly trending folds on its eastern limb to plunge to the southeast.
The Leavenworth fault, the southwestern bounding fault of Chiwaukumgraben, isamajor structure in the Blushastin and Leavenworth areas. Whetten (1980b) and Tabor et al. (1987) showed that it has northwesterly and northerly striking segments. Our mapping shows that the Leavenworth fault zone consists of north-westerly trending reverse faults (stops 1-6, 1-8, 1-12, and 1-13) and younger, northerly strike-slip faults (stops 1-4 and 1-12), both of which cut the Eocene Chumstick Formation. Because such evidence indicates that the Chumstick Formation is preserved in a structural, not a depositional, basin, hereafter we use the term Chiwaukum structural low instead of Chiwaukum graben.
Another major unrecognized feature is a belt of northwesterly trending, mostly southeasterly plunging, regional folds that have high-angle reverse faults on their steeper northeastern limbs. The folds are steepest in Eocene rocks but pass down plunge into more gently folded Neogene CRBG. The concept of small Eocene depositional basins (reinforced by synonymous formational names) hindered recognition of these folds. We call this belt, which extends from east of the Columbia River across the Cascade Range to the Puget Lowland, the Seattle-Wenatchee-Kittitas Fold and Thrust belt, or the SWIFT. The Blushastin area is our type example of SWIFT structure.
The Blushastin area is bounded on the west by pre-Tertiary crystalline basement (pT of Figs. 4 and 5). This basement consists of the Jurassic greenschist-grade metamorphic and ophiolitic rocks of Ingalls Tectonic Complex and the 91–96 Ma tonalitic Mount Stuart batholith (Tabor et al., 1982; Harper et al., 2003). The Swauk Formation of the Challis synthem is unconformable on, and locally in fault contact with, the crystalline basement (stop 1-14).
Units of the Swauk Formation in the Blushastin area have varying amounts of nonmarine conglomerate, sandstone, and black to olive siltstone (Table 3). The stratigraphy of the formation is displayed in a previously unrecognized syncline that underlies Tronsen Ridge (Fig. 4). Major criteria (Table 3) for distinguishing the members of the formation are the amount of conglomerate in the unit, the thickness of the conglomeratic beds, and the size of the clasts in the conglomerates. Other criteria are the style and thickness of bedding, the presence of map-scale unconformities, and stratigraphic position with respect to other members in the syncline. The thickness of members of the Swauk Formation is measured from cross sections (Fig. 5); because intraformational faults and folding are impossible to map, the true thicknesses may be less than those shown on the cross sections.
The most important unit for understanding the structure and geologic history is the basal conglomeratic member, the conglomerate of Tronsen Creek (Tsc in Fig. 4). This thickest (∼2 km) unit of the Swauk Formation contains polymict conglomerate (stops 1-9, and 1-14). Lithologies of the clasts are present in basement rocks to the west. The only mappable unit in this conglomeratic unit is the diamictite of Devils Gulch (Tscd in Figures 4 and 5 and Table 1). The diamictite is unsorted, unstratified, and usually monomict, with rounded clasts ≤1 m in diameter of tonalite (stop 1-12), ultramafic rock, o., rarely, phyllite. The granule-sized to microscopic matrix is composed of mineral fragments of the dominant type of clast. The origin of the diamictite is unknown; it may be a rock-avalanche deposit.
Gresens et al. (1981) and Tabor et al. (1982) believed that the diamictite is part of the Chumstick Formation and was deposited along the margin of Chiwaukum graben when the Leavenworth fault was active. However, Figure 4 shows that the diamictite is in the Swauk Formation and is cut by the northwesterly Leavenworth fault system.
The uppermost members of the Swauk Formation contain three unconformities (Table 3). So, despite having lithologies in common, the members of the Swauk Formation do not interfinger at a scale of 1:24,000; in other words, they are not stratigraphic equivalents or facies of each other, as commonly supposed (Tabor et al., 1982; Taylor et al., 1988).
In the Blushastin area, a polymict conglomerate (Ttb in Fig. 4) is similar to the conglomeratic member of the Swauk Formation, except that it contains angular to subrounded clasts of basalt (stop 1-10). Although this unit was previously mapped as a member of the Chumstick Formation (Tabor et al., 1982; Evans, 1994), we assign it to the Teanaway Formation because of its basaltic clasts and stratigraphic position.
Northwesterly trending folds and faults are typical of the Blushastin area (Fig. 4). The northwesterly trending Camas Creek fault (stop 1-13) is subparallel to the axial trace of the Blushastin anticline, which implies a genetic relationship between the two. Although the fault is not exposed, deflections of its trace up valleys suggests that it dips ≥70°SW. The fault places topographically higher and older Swauk and Teanaway formations in the Blushastin anticline over a syncline in the younger Roslyn Formation (Fig. 5); thus, it is a reverse fault. Because the fault obliquely truncates more than the 1.5-km-wide limb of the syncline in the Roslyn Formation (Fig. 4), it must have an offset of >4 km. Strain analysis of slickensided joints in the diamictite of Devils Gulch of the Swauk Formation in the hanging-wall of the Camas Creek fault (stop 1-12) provides independent evidence of reverse displacement (see inset in Fig. 4).
Units of the Swauk Formation (especially the diamictite) extend 13 km to the southeast where they pass unconformably below CRBG (Tabor et al. 1982). The diamictite is bounded on the northeast by the Roslyn Formation (Tabor et al., 1982). We assume that this contact is the Camas Creek fault (Fig. 2), as it is at Blushastin.
In the vicinity of Tronsen Ridge, the Swauk Formation is folded about hinges with sinuous axial traces (stop 1-15). Tean-away dikes are perpendicular to the general trend of these traces, a relationship that indicates that the dikes are syn- to post-shortening (e.g., the dikes track stretch along the fold hinges). Thus, the sinuous folds are older than or the same age as the Teanaway Formation, older than the Roslyn Formation, and older than the folding that generated the Blushastin anticline and Camas Creek fault.
A third generation of folding occurs in the area. The Blushastin area is part of the core of the regional Naneum Ridge anticline. This anticline passes down-plunge to the southeast into less folded CRBG (Tabor et al. 1982). To the northwest, pre-Tertiary crystalline rocks in the core of the anticline are a southeasterly projecting prong (Fig. 4). In the vicinity of this prong, Eocene rocks within the Naneum Ridge anticline are more steeply folded than CRBG to the southeast (see Tabor et al., 1982). In summary, two pre-CRBG episodes and one post-CRBG episode of folding occur.
The Leavenworth area (Fig. 6) is astride the west-central portion of the Chiwaukum structural low. The area is bounded on the west by amphibolite-facies rocks of the Nason terrane and by the 91–96 Ma tonalitic Mount Stuart batholith (Tabor et al., 1987; Paterson et al., 1994). The most widespread Tertiary formation in the Chiwaukum structural low is the Chumstick Formation of Gresens et al. (1981), which we correlate with the Roslyn Formation. The Roslyn Formation in the Chiwaukum structural low is similar to the Swauk Formation, but usually the two can be distinguished (Table 2). The major structures in the Eocene rocks of the Chiwaukum structural low are the northwesterly plunging Peshastin syncline on the southwest and the doubly plunging Eagle Creek anticline on the northeast (Whetten, 1980a; Whetten and Laravie, 1976; Tabor et al., 1987). Swakane biotite gneiss crops out in the core of the Eagle Creek anticline and northeast of the Entiat fault.
Previous workers (Gresens et al., 1981; Evans, 1988, 1994; Evans and Johnson, 1989) believed that facies changes dominate the entire vertical section of the Roslyn Formation. However, a previously unrecognized robust marker unit of conglomerate (Trci and Trcu of Fig. 6, stops 1-2 and 1-3) indicates a laterally extensive stratigraphy (Table 4), which is subdivided by three unconformities. The most obvious unconformity is the angular discordance between Trci and underlying Tss in cross section L–M of Figure 7.
The thickness of the Roslyn Formation in the Chiwaukum structural low is controversial. Estimates range from a maximum of 12 km based on stratigraphy (Evans, 1994) to a minimum of 2 km based on a gravity survey near Leavenworth (Silling, 1979). The thicknesses of the units in Table 4 are from cross sections (Fig. 7) or from Figure 6. Table 4 suggests that the composite thickness of the formation in the Chiwaukum structural low is ∼6.5 km. Because intraformational faults and folding are impossible to map, the true thicknesses of some units may be considerably less than shown in the table. Moreover, because the formation has three mappable internal unconformities (and, perhaps, other unmapped ones), the actual thickness at a given point may be less than any composite thickness of the unconformity-bounded units. Thus, 6.5 km probably is an upper limit.
Felsic tuffs are a minor component of the Roslyn Formation in the Chiwaukum structural low (Table 2; Gresens et al., 1981). However, because most tuffs are <1 to ≤5 m thick, they are not useful markers or mappable units in the rugged and forested western portion of the Chiwaukum structural low. McClincy (1986) recognized 19 tuffs. Recent mapping in the Leavenworth area has discovered another six outcrops not previously mapped by Whetten and Laravie (1976) or by Whetten (1980a, 1980b); so, the total number of tuffs might exceed 19. Two tuffs are ≥10 m thick and were useful markers for Whetten and Laravie (1976) and Gresens et al. (1981) southeast of Figure 6. The nearest possible source of the tuffs is the Duncan Hill pluton, which is ∼20 km northeast of Plain (Fig. 2).
The Deadhorse Point Member (Trd in Table 4) is the youngest unit of the Roslyn Formation in the Chiwaukum structural low. Its stratigraphic position and locally high organic contents (four of eight samples have 1.5%–4.3% total carbon [Evans, 1988, Table 3.2 therein]) suggest that it may be correlative with the coal-bearing upper part of the Roslyn Formation near Cle Elum. However, no coal beds are known in the Roslyn Formation in the Chiwaukum structural low (Huntting, 1943).
The common perception is that only the Roslyn (Chumstick) Formation is in the Chiwaukum graben (Evans, 1988, 1994). However, the distinctive diamictite of Devils Gulch occurs near Mill Creek (near stop 1-8) and on the northern end of Tumwater Mountain (cross section L–M of Fig. 7). Moreover, a conglomerate on Tumwater Mountain is so texturally and compositionally similar to the conglomerate of Tronsen Creek (stop 1-5) that Evans (1994) correlated the two. Unfortunately, no Teanaway dikes, which would independently verify this correlation (Table 2), are known in the conglomerate on Tumwater Mountain.
Whetten (1980a, 1980b) mapped the Leavenworth fault as a northwesterly trending feature on the northeastern side of Tumwater Mountain northwest of Leavenworth and at Mill Creek 9 km south of Leavenworth. At both locations, he mapped pre-Tertiary basement rocks on the southwest side of the fault and sedimentary rocks, which he believed to be Chumstick Formation, on the northeastern side. We adopt this as the strict definition of the Leavenworth fault: basement rocks against sedimentary rocks. Thus, the Camas Creek reverse fault at Blushastin is not the same fault. At Blushastin the fault that places basement rocks against sedimentary rock is 1–2 km southwest of the Camas Creek fault (Fig. 4). Nonetheless, the Camas Creek fault is a component of the Leavenworth fault system or zone.
The dip of the Leavenworth fault is difficult to determine. Along Tumwater Mountain, outcrops are sufficiently sparse (and the dip of the fault sufficiently steep) that deflections of the trace of the fault across valleys cannot be demonstrated. However, adjacent basement rocks at the southeastern end of Tumwater Mountain (stop 1-6) and at Mill Creek (stop 1-8) have fabrics that dip ≥70°SW, which suggests that the fault dips steeply to the southwest. That is, the Leavenworth fault probably is a reverse fault; therefore, the Chiwaukum structural low is not a graben in the conventional sense. A brecciated Teanaway dike adjacent to the fault at Mill Creek (stop 1-8) implies that the Leavenworth fault is essentially the same age as the Camas Creek fault.
On the northeastern side of Tumwater Mountain the conglomerate of Tronsen Creek of the Swauk Formation dips north-eastward toward nonconglomeratic Roslyn (Chumstick) Formation, which dips southwestward. Whetten (1980a, 1980b) mapped these two units as restricted to opposite limbs of a syncline in the Roslyn Formation.
However, the contact is the Camas Creek fault, which places Swauk Formation over Roslyn Formation. In the headwaters of Freund Creek, the fault dips as shallow as 45°SW, with synclinally folded conglomerate of Tronsen Creek over homoclinally dipping nonconglomeratic Roslyn Formation.
An outcrop of the diamictite of Devils Gulch of the Swauk Formation is northeast of the Leavenworth fault near Mill Creek (Fig. 6). By analogy to the Blushastin area, the Camas Creek fault is between this diamictite and the Roslyn Formation northeast of it (cross section J–K of Fig. 7).
Diamictite also caps a hill on the northwestern part of Tumwater Mountain. Below the diamictite is nonconglomeratic Roslyn Formation (Whetten, 1980a). Therefore, the hill is a klippe of the Camas Creek fault (cross section L–M of Fig. 7). Hills 0.3 km to the west (Whetten, 1980a) and 0.7 km to the southeast have no outcrops of diamictite but have ≤0.5 m boulders of tonalite below their summits. Tumwater Mountain was not glaciated; so, the presence of these bounders indicates that the klippe was once more extensive.
The other important features of the Leavenworth area are three previously unrecognized north-south faults that dextrally displace the Leavenworth fault and other features 0.5–10 km (Fig. 6). The most obvious of the three is the Chumstick Creek fault. This fault truncates the Swauk Formation and the Camas Creek fault at Blushastin (stop 1-12). It dextrally offsets the Leavenworth fault at Mill Creek, the axial trace of the Peshastin syncline, the axial trace of the Eagle Creek anticline, and the upper planar-bedded conglomerate, T.c., of the Roslyn Formation (Fig. 6). North of Mill Creek, it also juxtaposes southwesterly dipping sandstones of the Roslyn Formation against easterly dipping conglomerate (cross section J–K of Fig. 7). North of US-2, it accounts for the anomalous north-south course of the lower portion of Chumstick Creek. Outcrops of the upper planar-bedded conglomerate, T.c., along Chumstick Creek have fractured cobbles, white veinlets, and outcrop-scale north-south faults (stop 1-4).
North of Leavenworth, displacement on the Icicle Creek fault is about a kilometer, but the offset of the Leavenworth fault south of Leavenworth is ∼10 km. This disparity implies some sort of transfer fault beneath the Quaternary cover between the Chumstick Creek and Icicle Creek faults (Fig. 6). To accommodate several kilometers of offset, the fault must have several strands or be a zone of distributed shear.
Yakima Fold Belt
The Yakima fold belt in CRBG extends from east of the Columbia River northwestward to Cle Elum. The following summary is from Bentley (1977) and Reidel et al. (2003). The belt consists of narrow anticlinal ridges and broad synclinal valleys. The length of folds varies from 1 to 150 km, but most of the major folds are >60 km in length. The wavelengths of the folds vary from 1 to 20 km. Structural relief of the folds is typically <600 m. The anticlines are asymmetric, with imbricate thrust faults on the steeper northeastern limb. At the surface the thrusts dip shallowly, but in deeper exposures they dip as high as 60°.
Some deformation in the fold belt is demonstrably young. The post-CRBG Ringold Formation, which is younger than 6 Ma (Fig. 3), is involved in folds (Reidel et al., 1994, Fig. 7 therein). Scarps in the ca. 4 Ma Thorp Formation may be faults (Waitt, 1979) but do not appear to have been active during the Holocene (B.L. Sherrod, 2007, personal commun.). However, some faults in the fold belt do cut Pleistocene and Holocene sediments (Reidel et al. 1994, Table 4 therein).
The descriptions of the Blushastin and Leavenworth areas and of the Yakima fold belt indicate that two periods of Eocene folding were followed by Oligocene strike slip faulting and post-CRBG folding. The Eocene and post-CRBG folds have a common northwesterly strike. In the discussion that follows, we first show that the Blushastin anticline is just one example of a series under-recognized, northwesterly trending folds in Eocene rocks and in the CRBG, which we call the Seattle-Wenatchee-Kittitas Fold and Thrust belt (SWIFT).
Our recognition of the style and timing of folding stems from two realizations: (1) The formations of the Eocene Challis sequence extend beyond the major faults that cut them. (2) Regional folds that involve the Miocene CRBG also affected the formations of the underlying Challis sequence. This second realization requires that the hinges of major folds in the CRBG identified by Tabor et al. (1982) be extended into the formations of the Challis sequence and into the basement rocks (Fig. 2). As a result, lithologically similar formations of the Challis sequence occur in the same stratigraphic order on opposite limbs of the regional Naneum Ridge and Ainsley Canyon anticlines (Fig. 2; Cheney, 1994, 2003). Specifically, the basaltic conglomerate of the Blushastin area (Ttb of Fig. 4) and the Chumstick Formation of the Chiwaukum structural low occur in the same stratigraphic order on the northeastern limb of the Naneum Ridge anticline as do the Teanaway Formation and Roslyn Formation, respectively, on the southwestern limb (Fig. 2).
Another important characteristic of the SWIFT folds is, like the Camas Creek fault on the northeastern limb of the Blushastin anticline, the anticlines have high-angle reverse faults in Eocene rocks on their steeper northeastern limbs and have thrust faults in CRBG on their northeastern limbs. The Eagle Creek fault, which cuts the northeastern limb of the Eagle Creek anticline, is high-angle northwest of Wenatchee (Whetten and Laravie, 1976) but is a thrust fault in rocks of the Challis synthem down plunge at Wenatchee (Patton and Cheney, 1971). A rhyodacitic dike, with K-Ar dates varying from 41 to 45 Ma, intrudes the Eagle Creek fault at Wenatchee (Cameron, 1996) and is offset by north-south faults (Patton and Cheney, 1971).
In the Ainsley Canyon anticline, the Easton Ridge reverse fault dips 70° to 80° SW in former underground coal mines (Walker, 1980). It places the Teanaway and Taneum formations over the Roslyn Formation in the syncline at Cle Elum, and truncates the entire 3 km thickness of the Roslyn Formation (Walker, 1980; Tabor et al., 1982). In contrast, the structural relief on the CRBG in the Taneum monocline on strike with the Easton Ridge fault to the southeast is only ∼300 m (Tabor et al., 1982, Fig. 3 therein).
Other pairs of folds and reverse faults likely exist between the Ainsley Canyon anticline and the Puget Lowland. Rocks of the Kittitas synthem underlie this area of the central Cascade Range; they outline northwesterly trending, broad synclines and asymmetric anticlines with steeper northeastern limbs. No faults have been recognized that cut the steeper limbs, but mapping is only 1:100,000 (Tabor et al., 2000).
In the Puget Lowland the westerly trending Newport Hills anticline and the active Seattle fault involve both Eocene and Miocene rocks. The Seattle fault dips steeply to the south and is parallel to the steeper northern limb of the anticline (Blakely et al., 2002). The Seattle fault bounds the southern end of the Seattle basin, a syncline containing strata of the Challis, Kittitas, Walpapi, and High Cascade synthems (Blakely et al., 2002).
Swift and Owl
The northwesterly Olympic-Wallowa lineament (OWL) of Raisz (1945) extends from northwestern Washington to northeastern Oregon. The portion from Cle Elum southeast to the Columbia River is coincident with the Yakima fold and thrust belt in CRBG. The segment of OWL from Cle Elum northwest to the Puget Lowland is in pre-CRBG rocks in the previously unrecognized belt of northwesterly trending SWIFT folds.
Egg Crate of the Pacific Northwest
Northerly and northwesterly trending regional folds in CRBG produce an interference, or egg-crate pattern, in Pacific Northwest. This pattern is most obvious once the Cenozoic sedimentary and volcanic rocks are compiled into the four synthems on 1:250,000 maps or smaller (Cheney and Sherrod, 1999). It is expressed on the eastern flank of the Cascade Range as regional dips and folds in CRBG.
The southeastern limb and nose of the northerly trending Cascade Range anticline is outlined by the CRBG of the Walpapi synthem (Fig. 1), but the continuity of the synthem on the southwestern limb is obscured by the interference pattern of westerly trending folds. The Pasco basin to the east and the Puget Lowland to the west are post-CRBG synclines (Cheney and Sherrod, 1999). Because the synclinal pattern of the formations of the CRBG in the Pasco Basin is regional (Cheney, 1997, Fig. 3 therein), it is not apparent at the scale of a quadrangle.
The paleontology of the Walpapi synthem provides independent proof of post-CRBG uplift of the Cascade Range. The Vantage, Ellensburg, and Ringold units (Fig, 3) contain flora and fauna that are typical of a warm-temperate, summer-wet climate, unlike the present steppe and local grassland caused by the rain shadow of the present Cascade Range (Leopold and Denton, 1987). Additionally, fission-track dating shows that the Cascade Range has been uplifted since 12 Ma (Reiners et al., 2002).
Some of the local egg-crate patterns are worth noting. Mount Stuart in the pre-Tertiary rocks northwest of Blewett Pass is the highest mountain in the central Cascade Range; it is near the intersection of the northerly Cascade Range anticline and the northwesterly trending Naneum Ridge anticline. Uplift of the Cascade Range anticline explains the southeasterly plunge of the Naneum Ridge, Kittitas Valley, and Ainsley Canyon folds. The southward plunge of the Cascade Range anticline at the Washington/Oregon border is caused by the westerly trending The Dalles-Umatilla syncline (Wheeler and Mallory, 1970; Cheney and Sherrod, 1999), which demonstrates that some westerly folds are even younger than the Cascade Range anticline.
The egg crate and the paleontological evidence refute the popular notion (e.g., Beeson et al., 1989) that the CRBG ponded in the Pasco basin against the Cascade Range and flowed down the ancestral gorge of the Columbia River (and other paleovalleys) to the sea. The relief on the base of the CRBG along its western margin and in outliers in the area of Figure 2 is ∼1250 m (Table 5). It seems unlikely that basalt flowed 1250 m uphill. Moreover, as noted above, the map pattern of the Pasco basin indicates that the basin is a syncline, not a depositional, basin. Finally, the Columbia River crosses the Cascade Range by flowing along a structure in CRBG that is younger than Cascade Range anticline, The Dalles-Umatilla syncline (Wheeler and Mallory, 1970). This syncline is younger than any pre-CRBG valley.
A common perception is that crustal extension and/or transtension generated local basins, in which the thick arkosic successions of the Eocene Swauk and Roslyn Formations and their correlatives were deposited (Gresens, 1982; Tabor et al., 1982, 1984, 1987; Johnson, 1985; Taylor et al., 1988; Evans and Johnson, 1989). This concept continues to pervade discussions of the geology of Washington (see Paterson et al., 2004; Doran and Miller, 2006).
A central argument for the existence of pull-apart basins is the position of conglomerates near faults. Two important reputed examples for syntectonic deposition in the Chiwaukum graben are the diamictite at Blushastin (Gresens et al. 1981; Tabor et al., 1982; Johnson, 1985) and the conglomerate at Tumwater Mountain northwest of Leavenworth (Evans 1988, 1994; Evans and Johnson, 1989). However, both units are the conglomerate of Tronsen Creek of the Swauk Formation, not the Roslyn Formation (Figs. 4 and 6, respectively). Additionally, conglomerates adjacent to the Leavenworth fault in the Blushastin and Leavenworth areas and on the flanks of the Eagle Creek anticline have high dips characteristic of postdepositional deformation, not deposition.
Another problem is the nature of the bounding faults. Gresens et al. (1981) pointed out that evidence for strike slip movement on the Entiat and Leavenworth faults of the same age as the Roslyn Formation is lacking; no such evidence has surfaced since 1981. Moreover, the Camas Creek and Leavenworth faults that cut the Roslyn Formation are reverse faults, not normal (extensional) or strike-slip (transtensional) faults.
The Straight Creek fault also is important in interpretations of transtension. Previous interpretations hold that the Straight Creek fault bounded the Chuckanut-Puget-Naches basin to the west and the Swauk basin to the east (Johnson, 1985; Taylor et al., 1988). However, the Swauk, Teanaway, and Roslyn formations occur on both sides of the Straight Creek fault, and dextral displacement of ∼55 km on the fault postdates these formations (Cheney, 1999). All other slip on the Straight Creek and related faulting is pre–55 Ma (Tabor et al., 1984, 2000). Additionally, the putative Chuckanut-Puget-Naches and Swauk pull-apart basins are 90–50 km wide, respectively, which is significantly wider than most modern and ancient pull-apart basins (Johnson. 1985).
Other data are difficult to reconcile with pull-apart basins. Pull-apart basins generally have complexly interfingering sedimentary facies (Taylor et al., 1988), but the unconformities within the Swauk and Roslyn Formations (Tables 3 and 4) indicate that they have laterally extensive stratigraphies. Except for the basal conglomerate-bearing unit of the Swauk Formation, the conglomerate of Tronsen Creek (Evans, 1994, Fig. 8 therein), the paleocurrent directions in the Swauk and Roslyn formations are from the east or northeast (Johnson, 1985, Evans, 1994). This lack of diverse paleocurrent directions suggests a regional control, not control by local faults.
The important conclusion to be drawn from the above observations and from the SWIFT folds in Figure 2 and Table 5 is that the reputed extensional basins are northwesterly striking regional synclines. The so-called Swauk basin consists of regional Eocene formations preserved in the Kittitas Valley syncline. The so-called Chiwaukum graben is a compound structural low composed predominantly of the Peshastin syncline and the unnamed syncline between the Eagle Creek and Swakane anticlines; these synclines preserve the Roslyn Formation. The southwest limb of each syncline is bounded by one or more high-angle reverse faults, not an extensional fault.
Folding in Time and Space
The eastern flank of the Cascade Range records multiple episodes of folding. Northwesterly trending folds with kink-shaped crests and sinuous traces in the Swauk Formation near Tronsen Ridge (Fig. 7) record the earliest period of folding. Because the northeasterly trending Teanaway dike swarm intrudes these folds, and because the Teanaway Formation is less intensely folded than the underlying Swauk and Taneum formations, these folds are pre-Teanaway in age.
The initiation of shortening in the central Cascade region (marked by the unconformity below the Teanaway Formation) seems to have occurred after regional extension. Extension ended in the Priest River metamorphic complex on the Washington/Idaho border by 46 Ma (Doughty and Price, 1999), in the Okanogan metamorphic complex of north-central Washington by 47 Ma (Kruckenberg et al., 2006), in the upper crustal rocks of the Cascade area by 55 Ma (Paterson et al., 2004), and in the lower crustal rocks of the Cascade region by 45 Ma (Paterson et al., 2004). Thus, extension may have ended before deposition of the arkosic Roslyn Formation. This hypothesis might be tested by U-Pb dating of zircons in the tuffs in the Roslyn (Chumstick) Formation.
Another alternative is that extension may not have been important in the area of the Cascade Range. For example, the results of Paterson et al. (2004) constrain the timing of exhumation of middle crustal rocks, not of extension, as such. Wernicke and Getty (1997, Fig. 9D therein) suggested that the cooling ages and uplift could have been caused by compression (folding), not extension.
A second period of folding along northwesterly axes created steep dips in rocks as young as the Teanaway and Roslyn (Chumstick) formations, but not in the Naches Formation and the Walpapi synthem (Cheney, 1999). Examples are the Eagle Creek, Naneum Ridge, and Ainsley Canyon anticlines and the Kittitas Valley syncline on the east flank of the range and the Newport Hills anticline near Seattle. This period of folding may have imparted the sinuous traces to the pre-Teanaway folds.
The Yakima fold belt in CRBG records a third period of folding. The Eagle Creek, Naneum Ridge, and Ainsley Canyon, and Kittitas Valley folks in Challis rocks pass down plunge to the southeast as more gentle folds or monoclines in CRBG (Fig. 2). As noted above, the structural relief of the Eocene rocks in the Ainsley Canyon anticline is an order of magnitude greater than the amount of deformation in the CRBG on the same anticline. This post-CRBG folding disrupted the continuity of the Eocene formations and thereby fostered the concept of small depositional basins.
The second and third periods of folding, which may have had unrelated origins, coincided in space to form the northwesterly trending folds we call the SWIFT. That is, post-CRBG folding reactivated existing folds in the Challis rocks.
A fourth period of folding caused the Cascade Range anticline (and uplift of the Cascade Range) after 4 Ma. This north-south anticline causes the plunges of the SWIFT folds. This young north-south fold and the SWIFT folds are the components of the egg-crate pattern of the Pacific Northwest.
Plate Tectonic Models
Here we offer plate tectonic models for deposition of the Eocene Challis synthem and for the Neogene egg-crate pattern of folds. The sedimentary formations of the Challis sequence are thick, regionally extensive, unconformity-bounded, arkosic, mostly derived from the east, and do contain marine fossils west of the Cascade Range. Thus, these formations resemble successions deposited on the trailing edge or margin of a continent, not in local basins.
A problem with such a “trailing edge” model is that the arkosic formations are intercalated with thick volcanic successions that are atypical of such a setting. The thick bimodal volcanic units, such as the Taneum, Teanaway, and Naches formations, likely were generated during subduction of the ridge and gap between the Kula and Farallon plates (Breitsprecher et al., 2003). The unconformities between the volcanic and arkosic formations suggest significant tectonic changes and indicate that volcanism was episodic. Perhaps episodic arkosic sedimentation and bimodal volcanism were caused by subduction oblique to the contemporary margin of North America. In such a model, periods of subduction dominated by volcanism would alternate with periods of strike-slip parallel to the continental margin. During periods of strike-slip, sedimentation on the edge of the continent might mimic a trailing edge.
Presumably, the two sets of post-CRBG folds of the egg crate did not form simultaneously. The northerly striking set include the Coast range anticline, the Puget Lowland syncline, the Cascade Range anticline, the Pasco Basin, and possibly, reactivation of the antiformal metamorphic core complexes of northeastern Washington (Cheney and Sherrod, 1999). Because these folds generally decrease in amplitude eastward, they may be an “edge-effect” on the continent related to the offshore Cascadia subduction zone.
The northwesterly to southwesterly set of folds extends from central Washington (Fig. 2) to central Oregon (Cheney and Sherrod, 1999). This set is consistent with the northwestward drift of the Sierra Nevada block into Oregon and the consequent clockwise rotation of Oregon into Washington. This model was originally proposed on the basis of the present northerly to north-easterly GPS velocities in western Washington and adjacent Oregon relative to North America (Wells et al., 1998). By providing geological structures associated with both long-lived and recent shortening, the SWIFT may provide an independent test of specific geodetic inversions.
East-west folds in the penultimate glacial sediments (>45 ka) of the Puget Lowland (Booth et al. 2004), and continuing activity in the Yakima fold belt (Reidel et al., 1994) and the Seattle fault record recent deformation in the SWIFT. The seismic hazards of the Seattle basin and Seattle fault have been intensely studied in recent years (Blakely et al., 2002; Sherrod et al., 2004) but have lacked a regional context. We suggest that the context is the SWIFT.
Geodetic studies (Wells et al., 1998; Wang et al., 2003) show that most of Washington west of the Columbia River currently is undergoing northeast-southwest compression. Thus, in addition to the Seattle fault, other northwesterly trending, southwesterly dipping reverse faults associated with other recognized and unrecognized SWIFT anticlines in the Puget Lowland and in central Washington pose seismic hazards. The northeast-southwest compression could also reactivate high-angle, northwesterly striking faults that are unrelated to the SWIFT.
Mapping at 1:24,000 on the eastern flank of the Cascade Range challenges previous concepts of local, extensional, Eocene depositional basins. The recognition of Cenozoic synthems leads to the identification of a series of major, north-westerly striking folds that have been episodically reactivated. The reputed Swauk and Chiwaukum extensional basins are regional, unconformity-bounded formations of the Eocene Challis synthem preserved in the regional, northwesterly striking synclines. High-angle reverse faults, not extensional faults, bound the southwestern limbs of the synclines.
The recognition of Cenozoic synthems and the regional northwesterly striking folds facilitates identification of an interference pattern (or egg crate) of intersecting, late Neogene north-south and northwesterly folds. These folds are bigger and younger than commonly appreciated. For example, the present topography of the Cascade Range is due to a north-south anticline that began rising ca. 4 Ma.
Road Log for DAY 1
Introduction to Day 1
In the text, stops are numbered 1-1 to 1-21 for Day 1 and 2-1 to 2-14 for Day 2. Good weather and driving conditions will allow visits to scenic overlooks and outcrops that emphasize structures. Poor conditions will restrict stops mostly to paved roads, and for Day 1 these, coincidentally, emphasize lithologies and stratigraphy. The stops for the alternative itineraries of Day 1 are:
Poor weather: 1, 2, 3, 4, 5, 9, 10, 11, 12, 13, 14, 16, 17, 18, 19, 20, and 21.
Good weather: 5, 6, 7, 8, 9, 10, 11, 12, 13, 15, 16, 17, 19, 20, and 21.
The longest hike (stop 1-8) is ∼0.5 km. Unlike Day 2, the opportunity for toilet stops is limited.
The locations of stops are keyed to the mileage posts (MP) located every mile along the highways. When instructed to stop at a decimal MP, such as 10.2, look for MP 10, and use the odometer to estimate 0.2 miles. Cumulative mileage can be estimated by the MPs between the stops.
The road log for Day 1 begins at the junction of US-2 (MP 104.7) and US-97 (MP 185.0) about five miles east of Leaven worth. From the junction proceed westbound on US-2 toward Leavenworth to either Stop 1-1 or 1-5.
Stop 1-1. Typical, Massive, White-Weathering Roslyn Sandstone
MP 102.5 of US-2
As noted above, many of the rocks from stops 1-1 to 1-12 previously were thought to be Chumstick Formation and to have been deposited in the Chumstick graben. Most are here assigned to the Roslyn Formation. The sandstones here are white weathering, cross-bedded, have pebble lags, scattered pebbles ≤7 cm, but almost no conglomerate. The white sandstones are 2–15 m thick and grade upward into thinner, gray to black fine-grained sandstone. The sandstones dip northeasterly and are on the on southwestern limb of the regional Peshastin syncline.
Continue northwesterly on US-2.
MP 100.3 of US-2
At the traffic light near the eastern end of Leavenworth proceed either straight ahead for stop 1-5 or turn northward (right) on the Chumstick Highway (SR-209) toward Plain for stop 1-2. SR-209 follows the valley of Chumstick Creek, which is the type area of the Chumstick Formation (Gresens et al., 1981). The first few miles of SR-209 are in the southwestern limb of the Peshastin syncline, that is, the drainage cuts across the stratigraphy. This anomaly is caused by the north-south Chumstick Creek fault.
MP 5.4 of SR-209
Turn west (left) on Sunitsch Canyon Road and proceed 0.2 miles to first road cut beyond the railroad tracks.
Stop 1-2. Irregularly Bedded Conglomerate of the Roslyn Formation
Return to SR-209 and proceed northward.
MP 8.2 of SR-209
Take the side road to the east (right) to the large parking area adjacent to the railroad tracks. After parking, walk north on SR-209 ∼300 m past the railroad overpass.
Stop 1-3. Tuffs and Upper Planar Bedded Conglomerate of the Roslyn Formation
This stop examines potential marker units in the Roslyn Formation. This is the upper planar-bedded conglomerate (Trcu of Fig. 6 and Table 4), which is ∼0.85 km thick. Conglomerate, sandstone and tuff are intercalated on a 1–6 m scale (for a measured section of this locality see Evans, 1988, Fig. A.6 therein). Clasts in the conglomerates are felsic volcanic rocks (which, according to Table 2, are characteristic of Roslyn conglomerates), granitic rocks, felsic to intermediate gneiss, and bull to cherty quartz.
Two felsic tuffs range from aphanitic to barely recognizable lapilli. Rounded grains of quartz are minor and generally <1mm. Compared to tan conglomerate and sandstone, the tuffs weather gray and into angular blocks. These two tuffs continue at least 0.5 km to the north and 1.3 km to the south.
Heretofore, the only known marker units in the Roslyn Formation were such felsic tuffs. The rugged topography, extensive Quaternary cover, and forests make such thin tuffs as these unreliable as marker units (Gresens et al., 1981). However, Gresens et al. (1981) did map two tuffs 7–20 m thick as marker units in the central part of the Chiwaukum structural low. Correlation of tuffs by trace-element geochemistry is not straightforward (McClincy, 1986). The thicker tuff here is geochemically similar to a tuff in Clark Canyon 3.3 km to the southeast, but that tuff is in the lower planar-bedded conglomerate unit (Trcl); whereas, this one is in the upper-planar bedded conglomerate.
The irregularly bedded conglomerate (Trci) and the upper planar-bedded conglomerate (Trcu) occupy the same stratigraphic position in the Peshastin syncline. For 3 km north of stop 1-2, Trci progressively thins, and the underlying Trcu progressively thickens (Fig. 6). Trci and Trcu could be facies equivalents, or Trci could be unconformable on Trcu. In either case, Trcu plus Trci are a robust marker unit in the Roslyn Formation, which is 0.4–1.4 km thick and has a strike length of 28 km (Fig. 6).
Another feature to notice here is minor, irregular white veinlets <1 mm thick in the thickest conglomerate. Northeasterly tending white veinlets are more common in the railroad cut immediately to the east. Such veinlets are proximal to faults (stop 1-4). The Chumstick Creek fault is in the topographic low east of the railroad cut. Return to the vehicles and continue north on SR-209.
MP 9.8 of SR-209
Highway passes beneath transmission lines.
MP 10.1 of SR-209
Pull out on the western (left) shoulder of the highway.
Stop 1-4. Satellite of the Chumstick Creek Fault
The northeasterly dips of the strata indicate that now we are on the northeastern limb of the Eagle Creek anticline. Multiple beds of conglomerate in the upper planar bedded conglomerate here are thinner and finer grained than at stop 1-3. Note that some clasts are fractured. The strata are cut by white veinlets and by a north-south fault with ≥6 m of dextral displacement. This fault is a satellite or a splay of the north-south Chumstick Creek fault, which is under cover ∼0.1 km to the east. From north to south the Chumstick Creek fault dextrally offsets the following steeply southwesterly dipping features: the Leavenworth fault at Mill Creek (stop 1-8) by 2.0 km, the axis of the regional Peshastin syncline by 1.9 km, and the axis of the Eagle Creek anticline near here by 3.0 km.
Before we turn around, continue northward on SR-209 to the junction with Little Chumstick Creek Road at MP 11.3. At MP 10.7 is the covered contact with the Deadhorse Canyon Member (Trd of Fig. 6 and Table 3) of massive sandstone with intercalated black and olive fine-grained sandstones. The 5–15 m thick, planar beds of massive, white-weathering sandstone of this member are conspicuous on the hills to the northwest and north.
At the road junction at MP 11.3 turn around and return southward toward Leavenworth.
MP 5.2 of SR-209
The upper planar-bedded conglomerate is in a road cut on the east side of the valley 0.2 miles south of the junction with the Sunitsch Canyon Road. It is east of the Chumstick Creek fault (Fig. 6); whereas, at stops 1-3 and 1-4 upper planar bedded conglomerate is west of the fault. The exact amount of dextral displacement of the conglomerate is unknown.
MP 0.0 of SR-209 and MP 100.3 of US-2
At the junction with US-2 in Leavenworth turn west and proceed 0.8 miles west. At MP 99.5 at the Wells Fargo Bank turn right (northeast) on Ski Hill Drive. Follow Ski Hill Drive 1.2 miles northward and turn left (west) on Maple Street.
At the end of Maple Street (0.2 miles) turn right and then stay left to the residence of John Anderson. Walk 0.1 mile west to a quarry.
Stop 1-5. Conglomerate of Tronsen Creek of the Swauk Formation
Cobble and boulder conglomerate is interstratified with sandstone on a 1–5 m scale. Rounded to subangular clasts in the conglomerate are up to 1 m in diameter. Evans and Johnson (1989, stop 7) examined this conglomerate 1.3 km to the northwest. Their major observations were: (1) The lithologies of the clasts, (53% biotite-quartz schist, 42% granodiorite, 4% quartz or quartzite, 1% rhyodacite porphyry clasts; 102 clasts counted) indicate that they were derived from just southwest of the Leavenworth fault. (2) The conglomerates represent fanhead channels 3–5 m thick and 40–80 wide. (3) Paleocurrent directions from trough cross-beds in the sandstones are to the NNE.
All previous workers (Whetten, 1980b; Evans, 1988, 1994; Evans and Johnson, 1989) included this section in the Roslyn (Chumstick) Formation. However, lithologically and texturally it is similar to the conglomerate of Tronsen Creek of the Swauk Formation (see Table 2) in the Blushastin area (Evans, 1994, Fig. 2 therein; Evans and Johnson, 1989), which we will see at stops 1-9 and 1-14. Unfortunately, no Teanaway dikes occur here to confirm that this is the conglomerate of Tronsen Creek.
Return to Ski Hill Drive; then, proceed 0.5 miles southward and turn right (west) on Ranger Road. Proceed 3.4 miles to a pass with a microwave tower. The pavement ends at 0.6 miles (but continue straight). At the first fork in the road stay left; at the second fork stay right.
Stop 1-6. Overview of the Chiwaukum Structural Low and Fabric Related to the Leavenworth Fault
We shall first enjoy the view. We are at an altitude of 920 m; Leavenworth is 3.5 km to the southeast and ∼360 m below.
The creek below and to the southeast is the trace of the Leavenworth fault. Note that because the conglomerate on the northeast side of the valley (stop 1-5) does not project above the crystalline rocks on the southwest side of valley, a fault (with the southwest side up) is required. The strike of the valley indicates that our viewpoint is adjacent to the fault.
The mountain in the middle distance that just rises above the far horizon at N 170, is Tip Top, 21 km away. The Leavenworth fault passes about 2.5 km south of Tip Top. The total dextral displacement of the fault from south of Tip Top to here is ∼11 km.
The ridge on the horizon at N 160 (42 km from here) is Mission Ridge. There, at an altitude of 2000 m, the Roslyn (Chumstick) and Swauk formations of the Chiwaukum structural low are unconformably overlain by the Columbia River Basalt Group (CRBG). The northeasterly dip of the CRBG off the Naneum Ridge anticline is well displayed by Mission Ridge, which descends gradually for 20 km north-eastward to the Columbia River. Thus, the structural relief is 2000 m in 20 km.
The lateral moraines of the Pleistocene Icicle Creek glaciers are well displayed on the ridge southeast of Leavenworth. The terminal moraines are in the northern suburbs of Leavenworth.
The north-south valley of Chumstick Creek (along the Chumstick Creek fault) is N 100 and 3 km away.
The ridge at N 090 is underlain by the Roslyn Formation. Because the strata of the Roslyn Formation farther west on this ridge dip toward the older Swauk conglomerate, the two must be separated by a fault. That fault is the offset segment of the southwesterly dipping Camas Creek high-angle, reverse fault.
The highest ridge ∼16 km to the northeast is the fault-line scarp of the Entiat fault. The Entiat is the northeastern bounding fault of the Chiwaukum structural low; so, we are looking across the entire width of the Chiwaukum structural low. Pre-Cenozoic, amphibolite-facies metamorphic rocks are in the scarp and beyond (Whetten and Laravie, 1976; Tabor et al., 1987).
The highest nubbin (1772 m) 20 km distant at N 045 atop the scarp is Sugarloaf Peak (Fig. 2). Sugarloaf Peak is an erosional remnant of an undated Tertiary welded tuff, with a preserved thickness of ∼70 m.
The ridge at N 020 (6 km distant) with prominent northwest dipping Chumstick strata is along the axis of the northwest plunging Peshastin syncline.
The peak 14 km away at N 000 is Natapoc Mountain (1281 m). The white outcrops below the summit are steeply dipping Chumstick Formation. With binoculars, it is possible to see that the summit of Natapoc Mountain is underlain horizontal beds, the Summit Conglomerate of Page (1939). Because the conglomerate is predominantly composed of clasts of porphyritic andesite and various crystalline rocks (Whetten, 1980a), perhaps, it correlates with the ca. 4 Ma Thorp Gravel (stop 2-4) of the Yakima Valley 80 km to the south. Despite the uncertainties of the ages of the Sugarloaf Peak tuff and the Summit Conglomerate, originally they must have been more extensive across the Chiwaukum structural low. Thus, the important conclusion is that the topography of the Chiwaukum structural low post-dates them and is erosional, not depositional.
The tan ridge in the foreground at N 000 is underlain by Swauk conglomerate; whereas, the higher ground to the left is underlain by pre-Cenozoic ultramafic and metamorphic rocks. Thus, the Leavenworth fault continues to the northwest.
The outcrop at this stop is as important as the scenery. It consists of pre-Cenozoic serpentinite and talc-bearing rock bounded on the south by biotitic (pelitic) metamorphic rocks. The youngest of the pre-Cenozoic rocks in this area is the tonalitic Mount Stuart batholith. However, here the serpentinite contains a 36 by 50 cm inclusion of tonalite, which suggests that the tonalite was tectonically incorporated into originally older serpentinite in post–Mount Stuart time. The serpentinite also contains phacoids of serpentinite bounded by zones of more foliated serpentinite. The foliations vary from N 295 to N 335 and define one zone that dips vertically and another that dips 75° to 85 ° SW. The two zones impart an s-c–like fabric to the serpentinite with a sense of top-to-the-northeast. Fabrics in serpentinite can be problematic, and true s-c fabrics occur in rocks with ductile deformation. However, the s-c–like fabrics here may indicate that the adjacent Leavenworth fault is a reverse fault, dipping steeply to the southwest.
Return downhill to Leavenworth and turn east (left) on US-2. Continue past the junction with SR-209 and cross the bridge over the Wenatchee River.
MP 100.5 of US-2
Just east of the eastern abutment of the bridge, turn south (right) on East Leavenworth Road. Continue 0.1 miles and turn east (left) on Mountain Home Road. Follow Mountain Home Road (which becomes unpaved) 4.8 miles past second homes and large erratics of Mount Stuart tonalite to the lip of a hanging valley.
Stop 1-7. Overview of Icicle Creek
We are at 735 m at with a magnificent view to the north of Leavenworth and the glaciated valley of Icicle Creek. Features to note (other than the numerous erratics and the narrow-gauge railroad tracks) are:
Several lateral moraines of Pleistocene glaciers (or still-stands of thereof) that issued from the valley of Icicle Creek, the upper part of which is at N 270. The highest and most weathered (i.e., oldest) moraine is ∼125 m above to the southeast. That is, the Icicle Creek glaciers moved up what is now the hanging valley.
At N 015 in the far distance is the Entiat fault-line scarp. In the middle distance is the mouth of Chumstick Creek in the north-south Chumstick Creek fault, which passes to the east of us.
The ridge of conglomerate of the Swauk Formation adjacent to Ranger Road (stops 1-5 and 1-6) is N 000 and 8 km distant. From here the conglomerate appears to have a nearly horizontal dip. Note the abrupt eastern termination of the ridge.
The mountainous topography from the northwest to the southwest has bold outcrops, most of which are rocks of the Mount Stuart batholith. The outcrop across the valley at N 250 (0.4 km distant) also is Mount Stuart tonalite. Outcrops at the end of the hanging valley (to the south) and below it (to the north) indicate that bedrock at this stop is Roslyn Formation (Fig. 6). Thus, a fault occurs between us and the outcrop of Mount Stuart tonalite at N 250. The apparent displacement of the Leavenworth fault by this north-south Icicle Creek fault (from Tumwater Mountain, stop 1-5) to Mill Creek (stop 1-8) is ∼10 km; this large displacement requires some sort of concealed transfer fault in the valley below between the Chum-stick Creek and Icicle Creek faults.
Continue southward 1.7 miles on Mountain Home Road. At 1.2 miles is a road junction and a pass, beyond and below which the road becomes Mill Creek Road. Stop 0.5 miles south of the pass at an abandoned (and bermed) logging road on the uphill (west) side of road.
Stop 1-8. Mill Creek Segment of the Leavenworth Fault
This is the approximate trace of the Leavenworth fault east of the Icicle Creek fault. Walk about 0.25 mile along the abandoned and brushy logging road to view rocks adjacent to the fault.
The eastern outcrops along the road are intensely cleaved felsic metavolcanic rock. Centimeter- to meter-scale phacoids of more massive felsic volcanic rock have mm-scale veinlets in various orientations. Cleavage or incipient foliation in the metavolcanic rocks varies from vertical to 70° SW.
The western outcrop is a dike of basalt ∼40 m thick, which is megascopically and geochemically similar to Teanaway dikes (Peters, 2006). Along its margins the dike is intensely jointed and brecciated. The northeastern sheared contact of the dike with felsic metavolcanic rock dips 75° SW. The southwestern contact of the dike is not exposed. From these outcrops we infer that the Leavenworth fault dips steeply to the southwest (is a reverse fault), is either syn- or post-Teanaway in age and was active after the dike was emplaced.
Return to the vehicles and continue down Mill Creek Road 2.9 miles to US-97.
At 0.3–0.7 miles from stop 1-8, the road crosses a landslide on the intersection of the Leavenworth and Chumstick Creek faults.
At 1.0–1.7 miles from stop 1-8 are views (through trees) to the north of the valley that contains the north-south Chumstick Creek fault. Strata on the eastern side are typical white-weathering Roslyn sandstones and dip to the southwest. Strata on the skyline to the northwest are the upper planar-bedded conglomerate, which dips easterly.
MP 181.1 on US-97
Mill Creek Road enters US-97 at MP 181.1. Turn southbound (right) on US-97.
MP 179.7 of US-97
At Camas Creek Road turn eastward (left) and drive past massive, white sandstone of the Roslyn Formation.
At 2.8 miles on the right (south) is a quarry in the columnarly jointed Camas Land diabase in the Roslyn Formation. Major- and trace-element geochemistry indicates that the diabase is geochemically similar to CRBG (Peters, 2006), not Teanaway basalt; so, this may be an invasive sill of CRBG. Although the nearest outcrop of CRBG is 18 km to the south (Tabor et al., 1987), the top of the diabase might be only 10s to 100s of meters below the original contact of CRBG with the underlying Roslyn Formation.
At the end of the pavement at 3.1 miles bear right. Proceed another 3.5 miles to an outward curve in the road.
Stop 1-9. Conglomerate of Tronsen Creek of the Swauk Formation
Stops 1-9, 1-10, and 1-11 are in the Swauk, Teanaway, and Roslyn (Chumstick) formations, respectively. The strata at these stops have northeasterly dips. Stops 1-5, 1-9, and 1-14 are the distinctive conglomerate of Tronsen Creek of the Swauk Formation (Tsc). However, most outcrops in Tsc are fluvial, feldspathic to lithofeldspathic sandstone and black siltstone, with minor or no conglomerate (Table 3).
At stop 1-9, upward fining intervals are bounded by minor unconformities at the base of conglomeratic beds. Because many of the conglomerates are matrix-supported and seem to have a bimodal distribution of sizes of clasts, they may have been mudflows. The most conspicuous clasts are dioritic to granitic. Taylor et al. (1988) reported that the average composition from six localities is 42.9% plutonic, 26.2% volcanic, 20.5% quartz, 5.4% metamorphic, 2.8% chert, and 2.2% sedimentary. Very rare clasts of coal occur at stop 1-9. Some of the finer grained strata here are unlaminated and unbedded, suggesting that they may be paleosols.
Turn the vehicles around (toward US-97) and drive 0.8 miles back to the next major outward curve.
Stop 1-10. Basalt-Bearing Conglomerate (Teanaway Formation)
Unlike stop 1-9, this polymictic conglomerate (Ttb of Fig. 7) also contains clasts of basalt. Elsewhere, Ttb contains <1% basaltic clasts, some basalt flows, and black siltstone. Tabor et al. (1982) recognized that the clasts are Teanaway basalt but placed this conglomerate in the Roslyn (Chumstick) Formation. However, they did note (1982, p. 10) that this conglomerate could be Teanaway Formation.
Tabor et al. (1982) believed that the contact of this conglomerate with the conglomerate of Tronsen Creek of stop 1-9 is a fault in the Leavenworth fault zone. Figure 4 shows that along strike to the southeast, Ttb cuts through Tsc into the underlying diamictite (Tscd); that is, the contact probably is an unconformity.
Continue 0.6 miles toward US 97.
Stop 1-11. Roslyn (Chumstick) Sandstone
This is typical, white Roslyn (Chumstick) sandstone (Tabor et al., 1982). However, in the absence of Teanaway dikes this could be either the Swauk or the Roslyn Formation. To the northwest and southeast, this sandstone also truncates the basalt-bearing conglomerate and comes in contact with Tsc (Fig. 7). Therefore, the Roslyn Formation could be unconformable upon the Teanaway Formation, and stops 1-9, 1-10 and 1-11 could be a homoclinal succession of unconformable formations, Swauk, Teanaway, and Roslyn formations respectively. Subsequent stops test this hypothesis.
Continue toward US-97. Camas Creek Road enters US-97 at MP 179.7. Turn left and proceed southbound on US-97.
MP 178.7 of US-97
Turn left (southwest) on Old Blewett Road and park.
Stop 1-12. Diamictite of Devils Gulch
This virtually monomict diamictite (Tscd in Tsc of Table 3) is composed of rounded clasts of tonalite <1 m in diameter, which could have been derived from the Mount Stuart batholith. The sand-sized to microscopic matrix consists of mineral grains of the same tonalitic rock. At present, the Mount Stuart batholith crops out ≥4 km to the west, but similar plutons might be unconformably under the Chumstick Formation to the east or beneath more distant CRBG to the southeast. On strike to the southeast, parts of the diamictite are 0.1%–100% rounded clasts of ultramafic rocks. The ultramafic clasts typically are ellipsoidal and impart a crude foliation to Tscd. One outcrop of Tscd east of Peshastin Creek is composed almost entirely of clasts of pelitic phyllite.
Tabor et al. (1982) assigned the diamictite to the Roslyn (Chumstick) Formation. To the southwest the diamictite is in contact with the Leavenworth fault (Fig. 6; Tabor et al., 1982). This led Tabor et al. (1982, p. 8) to state that the “abundance [of diamictites] in the Leavenworth fault zone, especially those on the now downthrown side of the faults, suggests that they flowed off rapidly rising fault-block mountains.” However, Figure 4 shows that to the southeast the diamictite is bounded above and below by the conglomerate of Tronsen Creek of the Swauk Formation. Furthermore, elsewhere, Teanaway dikes occur in the diamictite. Because the diamictite is in the Swauk Formation, not the Roslyn Formation, it does not indicate that the Chiwaukum graben was a syndepositional basin.
This outcrop also has numerous slickensided joints. The slick-ensides cut across the tonalitic clasts and matrix of the diamic-tite without deflection. All striae are of the asperity-abrasion or groove type (cf. Means, 1987). We measured 129 striae on multiple surfaces. The surfaces have a mean northeast strike and dip steeply. Most striae have a moderate plunge to the southwest. Some shallowly plunging striae have right-lateral steps on the surfaces. Faces with steeply plunging striae are adjacent to faces with shallow plunging striae, and some of the slickensided surfaces are curved.
The net regional strain produced by slip on these multiple surfaces is best described with a momenttensor inversion (Cladouhos and Allmendinger, 1993). By symbolically representing the bulk strain as a focal mechanism of an earthquake, the trends and plunges of the striae and sense of slip on the faces is best interpreted as indicating reverse slip on the adjacent Camas Creek fault (see inset in Fig. 4). The preponderance of moderately plunging slickensides virtually precludes the normal fault proposed by Tabor et al. (1982).
A minor component of strike-slip motion exists in the focal mechanism. The presence of crystalline rocks above the diamic-tite on the wall of the canyon west of this stop shows that the northerly trending Chumstick Creek strike-slip fault truncates the diamictite and the Camas Creek fault (Figs. 2, 4, and 6).
Continue north on Old Blewett Pass highway about one mile through outcrops of diamictite on the east and tonalitic glacial erratics on the west.
Stop 1-13. Camas Creek Thrust
On the valley wall west of US-97, thick beds of Roslyn sandstone dip southwesterly toward the diamictite of the Swauk Formation of stop 1-12. This relationship requires a fault between the Roslyn and the Swauk formations. This fault barely “Vs” upstream, indicating that it dips steeply southwestward, but to the southeast (Fig. 6), “Vs” imply dips ∼70°. This fault places a topographically high anticline of Swauk and Teanaway formations over a syncline in the Roslyn Formation (Fig. 5). This is the Camas Creek reverse fault. Because the fault truncates the Roslyn (Chumstick) Formation, that formation was not deposited syntectonically in the Chiwaukum graben.
Continue northward on Old Blewett Highway to the intersection of US-97 and turn left (southbound) on US 97. Pass stop 1-12 at MP 178.7 and continue southbound.
MP 178.1 of US-97
The highway crosses the covered northwesterly trace of the Leavenworth fault. One mile to the south the highway enters the gorge of Peshastin Creek in pelitic rocks of the Ingalls Tectonic Complex. Note that columnarly jointed Teanaway dikes intrude the Ingalls Tectonic Complex.
MP 174.0 of US-97
This is Blewett! The Historical Marker relives the glory days of gold mining between 1877 and 1910 in Culver Gulch west of the highway (Margolis, 1994; Woodhouse et al., 2002). Continue southbound on US-97 through rocks of the Ingalls Tectonic Complex.
MP 172.1 of US-97
Stop at the large parking area on the southwestern (left) side of the US-97.
Stop 1-14. Conglomerate of Tronsen Creek and the Magnet Creek Fault Zone
Walk ∼200 m south on US-97. On the eastern side of the highway is a photogenic exposure: a small normal fault in the conglomerate of Tronsen Creek is occupied by a Teanaway dikelet. The polymict conglomerate contains clasts of greenschist-facies rocks of the Ingalls Tectonic Complex and of tonalite.
Walk back to the parking area. The northeasterly striking Magnet Creek fault passes just north of the parking area. It juxtaposes the pre-Tertiary Ingalls Tectonic Complex on the northwest against the Swauk Formation to the southeast. Sheep Mountain above and northeast of the highway is mostly greenstone of the Ingalls Tectonic Complex. Across the highway, black serpentinite is against strata of the Swauk Formation, which are intruded by variably fractured and altered Teanaway dikes.
The Magnet Creek fault zone is part of the contact of the Swauk Formation with the Ingalls Tectonic Complex that extends for 30 km west of here (Fig. 2), and which is generally considered to be the edge of the Swauk basin (Taylor et al. 1988; Evans and Johnson, 1989). Truncation of the southeast-erly dipping strata in the Magnet Creek fault zone indicates that this is a faulted, not a depositional, margin of the basin. To the west, the contact is unfaulted, is discontinuously marked by paleolaterite on ultramafic rocks of the Ingalls Tectonic Complex (Tabor et al. 1982, 2000; and Fig. 8), and, locally, is steeply overturned (Tabor et al., 1987; Doran and Miller, 2006). Therefore, this unfaulted unconformity was the floor, not the edge, of the Swauk depository.
MP 168.8 of US-97
Turn left (northeast) on unpaved Five Mile Road. Up the road 0.45 miles (past the second major curve) are poorly exposed lithologies of the conglomerate of Tronsen Creek. An abrupt reversal in dip of the strata (see cross section E–F of Fig. 5) indicates a syncline with a sharp hinge. Such kink-like folds are common in thrust belts.
Continue up Five Mile Road almost 3.0 miles to the fifth major curve.
Stop 1-15. Tronsen Ridge Member of the Swauk Formation and Folds
This is the shale of Tronsen Ridge (Tsr in Table 3). The three planar-bedded lithologies of this unit are massive white sandstone, black siltstone, and olive siltstone. The white sandstone may be due to failures of levees adjacent to flood plains represented by the black and olive siltstones.
The northeasterly dip indicates that an anticlinal axis exists between here and the syncline 3 miles down the road—but visi ble <0.6 km to the southwest (cross section E–F of Fig. 5). The anticline strikes through the ridge to the northwest, which is underlain by thickly bedded conglomerate of Tronsen Ridge and a conspicuously rusty weathering Teanaway dike. Incidentally, the conglomerates on this ridge and in the hogbacks across Tronsen Creek to the southwest are darker weathering than interbedded sandstone because lichens grow on the clasts in conglomerate but not on friable sandstone.
The syncline and anticline are parts of a set of folds with previously unrecognized sinuous axial traces (Fig. 4). Dikes of the Teanaway swarm generally strike northeasterly (perpendicular to the folds). Sinuous folds also occur east of Tronsen Ridge (Fig. 4). The sinuous traces probably indicate that these folds were later refolded.
Juxtaposition of cross section A–B with C–D of Figure 5 reveals the asymmetry of the Blushastin anticline. Northwest of here the southwestern limb of the syncline under Tronsen Ridge is mostly missing (Fig. 4). The contact with the conglomerate of Tronsen Ridge is interpreted (Figs. 4 and 5) to be a southwest-ward dipping thrust fault, but the dip of the fault is unknown. Farther to the northwest at Windmill Point (at a deeper structural level), this fault cuts the pre-Tertiary basement (Fig. 4, cross section C–D of Fig. 5). However, near stop 1-15 and to the southeast, this fault appears to die out in an anticline in the shale of Tronsen Ridge (i.e., is a blind thrust).
On a clear day, this stop has scenic views. To the WNW is the Mount Stuart Range (≥l8 km distant) underlain by the 91–96 Ma Mount Stuart batholith. To the SW (4 km) is flat-topped Diamond Head, which is nearly horizontal CRBG unconformably overlying Swauk strata near stops 1-16 and 1-17.
Return downhill and turn southbound (left) on US-97.
MP 163.9 of US 97 (Blewett Pass)
At Blewett Pass (formerly Swauk Pass) turn east (left) and proceed 0.4 miles (past the end of the pavement) to the U.S. Forest Service (USFS) toilet on the right side of the road.
Stop 1-16. Scenic View of Tronsen Ridge
One hundred meters down the road from the toilet is a view of much of the Swauk Formation. To the southeast (far right) are well forested, flat-topped ridges capped by nearly flat lying CRBG. The sub-Walpapi angular unconformity can be visualized by extrapolating the CRBG northward over strata of the Swauk Formation, which dip ∼40° to the northeast. The thick-bedded strata on the southern part of the ridge are the conglomerate of Tronsen Creek (Tsc in Table 1 and Fig. 4). Farther north, the middle portion of the ridge is underlain by the sandstone of Swauk Pass, which also has thick beds in it (and which is indistinguishable from the conglomerate of Tronsen Creek at this distance). On the barren slopes to the north are the thinner and planar-bedded siltstone and sandstone of the shale of Tronsen Ridge (Tsr of Table 1 and Fig. 4). The shale of Tronsen Ridge dips ≥50° northeastward and is unconformable on the sandstone of Swauk Pass and the conglomerate of Tronsen Ridge (Fig. 4). Nearly vertical Teanaway dikes are conspicuous ribs in the shale of Tronsen Ridge.
Return downhill to Blewett Pass (MP 163.9 on US-97) and turn left (southbound but here westward) on US-97.
MP 163 of US-97
Park in the wide pull-out on the left (south) side of the highway.
Stop 1-17. Sandstone of the Swauk Pass Member of the Swauk Formation
The sandstone and black siltstone (Tss of Table 3 and of Figs. 4 and 5) are typical of the sandstone of Swauk Pass (Evans and Johnson, 1989, Fig. 4 therein). The sandstone is the areally most extensive member of the Swauk Formation (Tabor et al., 1982). Here, the three thick beds of massive sandstone in the generally upward thinning successions may be crevasse splays on a flood plain represented by the black siltstone. Fossil palm fronds have been collected here.
Continue southbound on US 97.
MP 161.2 of US-97
Pull over on the very narrow shoulder and climb to the quarry on the right (north) side of the highway.
Stop 1-18. Teanaway Dike Cutting Volcaniclastic Unit in Sandstone of Swauk Pass
An intermediate volcaniclastic rock with felsic clasts occurs here in the sandstone of Swauk Pass. Tabor et al. (1982) and Taylor et al. (1988) mapped the volcaniclastic rock as the Silver Pass Member of the Swauk Formation. However, the Silver Pass at its type locality northeast of Easton is unconformable upon the Swauk Formation and is equivalent to the Taneum Formation (Cheney, 1994). Volcaniclastic rocks such as this are minor interbeds in the sandstone of Swauk Pass. This one has a strike length of 8 km (Taylor et al., 1988).
Remnants of a Teanaway dike occur on the wall of the quarry. The back-fill includes serpentinite of the Ingalls Tectonic Complex.
At the eastern end of the quarry, beds of sandstone are nearly vertical and somewhat boudinaged. Given these dips and intensity of deformation, the original geometry and extent of the “Swauk basin” obviously are not preserved.
Continue southbound on US-97. Note that most of the strata dip southerly, and some are intruded by rusty weathering Teanaway dikes.
MP 152.5 of US-97
On the left is the road to Liberty. Placer gold was discovered along Swauk Creek in 1873. Production from the Liberty district was from placers, “hard-rock mining,” and dredging (Jordan, 1967; Woodhouse et al., 2002). “Boom time” was 1891–1901. Some production continues today. The source of the gold is quartz veinlets in the Swauk and Taneum formations and the contacts of Teanaway dikes. The sandstone of Swauk Pass is hydrothermally altered and weakly mineralized for >9 km along a WNW-trending anticline (Margolis, 1994).
MP 151.9 of US-97
Notice the “windrows” of gravel from dredging for gold in 1926 or 1940 (Jordan, 1967). “Volunteer” vegetation makes these windrows barely noticeable today.
MP 15l.2 of US-97
Park on the western side of the highway at the Liberty Café.
Stop 1-19. Teanaway Formation
This stop illustrates that the Teanaway Formation is not solely composed of basalt. The basalt commonly is rusty weathering, black, and vesicular or amygdaloidal. Opposite the northern end of the parking lot, sparsely amygdaloidal basalt occurs below bedded, felsic volcaniclastic rock. Note that the regional dip is still southward.
Continue southbound on US-97.
MP 149.7 of US-97
At the junction of US-97 with SR-970, turn left on US-97 toward Ellensburg.
MP 147.1 of US 97
At this pass, park on the wide shoulder on the right.
Stop 1-20. Gently Dipping Thorp Gravel
Mostly basaltic conglomerate of the Thorp Gravel overlies Grande Ronde Basalt of CRBG (Tabor et al., 1982). Both formations project northwestward over the red weathering ridges of the Teanaway Formation, which here are the foothills of the Cascade Range. At stop 2-4 Thorp Gravel overlies the Ellensburg Formation, which is stratigraphically above the Grande Ronde Basalt (Fig. 3). Therefore, the sub-Thorp contact is an unconformity. It is the basal unconformity of the High Cascade synthem.
Return down hill to the junction of US-97 and SR-970 and turn left (westbound) on SR-970 toward Cle Elum.
MP 6.9 of SR-970
Turn right (north) on Teanaway Road. Proceed ∼0.4 miles and turn left on Old Bridge Road. Cross the Teanaway River, and stop at the western end of the bridge.
Stop 1-21. Roslyn Formation
This white arkosic sandstone dips southwesterly toward the axis of the Kittitas Valley syncline. The sandstone is like the Chumstick Formation at stops 1-1 and 1-11. Figure 2 shows that the Roslyn and the Chumstick formations occur above Teanaway Formation on opposite limbs of the Naneum Ridge anticline, which indicates that the Roslyn and Chumstick formations are equivalent.
Return to SR-970 and follow it into Cle Elum.
Road Log for DAY 2
Introduction to Day 2
Today we will examine Neogene rocks, their involvement in the rise of the present Cascade Range, the Yakima fold belt, and the egg crate of the Pacific Northwest. The route proceeds southeastward through Ellensburg to the Yakima fold belt. In the afternoon we will examine pre-Tertiary rocks and their relationship to the Straight Creek fault near Easton. The stops for the alternative itineraries of Day 2 are:
Poor weather stops: 1, 2, 3, 4, 5, 6, 7, 12, 13, and 14.
Good weather stops: 5, 6, 7, 8, 9, 10, 11, 12, 13, and 14.
The poor-weather option starts at the junction of SR-970 and SR-10 ∼4 miles southeast of Cle Elum. The good-weather option joins this road log 2 miles west of Ellensburg at Exit 106 of I-90.
Junction of SR-970 (MP 2.6) and SR-10 (MP 100.2)
Lookout Mountain to the east is the western edge of CRBG at this latitude. The lowest basalt is MSU R2 (Fig. 3) of the Grande Ronde Basalt (Tabor et al., 1982). The western slope of Lookout Mountain is a landslide, which is best visible later in the day from I-90 while approaching stop 2-7. By proceeding eastbound on SR-10, we will advance up-section through units of the Walpapi synthem (stops 2-1 to 2-4). Stop 2-1 is near the base of the landslide.
Stop 2-1. Bristol Unit at the Base of the Walpapi Synthem
MP 90.2 of SR-10
Just east of the junction with Taylor Road on the northern side of the highway are volcaniclastic strata of the Bristol unit of Bentley (1977). The most conspicuous clasts are hornblende-bearing dacite. After a brief inspection of the rocks at Taylor Road, the vehicles will move slowly eastward along SR-10. Note that the lithofacies, all of which contain hornblende-bearing dacite, vary from conglomerate to siltstone. Panels of outcrops, each with a somewhat different but shallow dip, are bounded by faults. The faults probably are related to the landslide on Lookout Mountain (Bentley, 1977, Fig. 4 therein).
MP 92.2 of SR-10
The quarry on the left (north) is in R2 of the Grande Ronde Basalt (Tabor et al., 1982).
MP 92.6 of SR-10
Outcrops of the volcaniclastic Coleman unit of Bentley (1977), which is between basalts of MSUs R2 and N2, occur for the next 0.5 miles. The outcrops are poor, and parking is impossible.
MP 96.0 of SR-10
Park on the south (right) side of the highway next to the Yakima River.
Stop 2-2. Anatomy of a Basalt Flow of CRBG
Note the three portions of this Grande Ronde flow: a basal pillow/palagonite unit, a middle portion of columns (colonnade), and an upper portion of closely spaced fractures (“brickbat” texture), known as the entablature. Radial columns within the colonnade do not seem to mark a former lava tube. They may represent nonplanar cooling of a single flow or another flow of magma within an inflated flow. Entablatures are thought to form by catastrophic cooling as lakes and fluvial systems that were disrupted by flows reestablished themselves on top of still hot flows. Many CRBG flows do not have a basal pillow/palagonite, but most have a colonnade and an entablature. The shallow southeasterly dip indicates that we still are proceeding up-section.
Continue eastward on SR-10.
MP 98.3 of SR-10
Pull off on the wide shoulder on the south (right) side of the highway.
Stop 2-3. Ellensburg Formation
This is a photogenic exposure of the Ellensburg Formation. Note the gentle southeasterly dip. The orange-weathering Thorp Formation in the cliffs above has enough southeasterly dip that it is at the level of the road at stop 2-4.
In the Ellensburg Formation in the lower part of the cliff, massive (unbedded and poorly sorted) volcaniclastic material is underlain and overlain by bedded units. Note the huge size of the clast on the upper contact of the massive unit, which probably was a lahar. Cross bedding in the bedded fluvial units implies a western source (the site of the present Cascade Range). Many clasts are hornblende dacite. The Ellensburg is noted for its abundance of pumice. Elsewhere, the upper part of the Ellensburg contains minor clasts of CRBG (Smith, 1988).
The “Ellensburg problem” is that, despite Bentley’s efforts (1977), all interbeds of hornblende-bearing dacitic volcaniclastic units at different stratigraphic positions, some of which are separated by unconformities (Fig. 3), are all called Ellensburg Formation (Smith, 1988). Giving all of the volcaniclastic units the same formational name obscures unconformities, structures, and tectonic events. This Ellensburg at stop 2-3 is stratigraphically above the Bristol and Coleman units. In the type area 30 km to the south, the “Ellensburg” is 280–350 m thick and overlies the Saddle Mountains Basalt Formation (Smith, 1988). The great thickness of this unit at stop 2-3 implies that it may be equivalent to the type section.
Continue east on SR-10
MP 100.8 of SR-10
Pull off on the wide shoulder on the southern side of the highway.
Stop 2-4. Thorp Gravel
Much of the Thorp Gravel weathers orange. Whiter units in the cliff above are tuffs that have fission-track dates of 3.8 and 4.4 Ma and a K-Ar date of 4.5 Ma (Tabor et al., 1982). These dates should be confirmed. The Thorp is younger than, or correlates with, the upper part of the Ringold of Figure 3 (Waitt, 1979).
In the talus at the side of the highway, the four major groups of clasts are (1) hornblende dacite and other felsic volcanic rocks, (2) granite, (3) metamorphic rocks (mostly greenschist facies), and (4) basalt. The basalt is both Teanaway and CRBG (Waitt, 1979). Clasts of sedimentary rocks from the Challis synthem are exceedingly rare. All of these lithologies currently crop out in the Cascade Range to the west.
Because the Thorp Gravel overlies Grande Ronde Basalt (stop 1-21) and the Ellensburg Formation (stop 2-3) is overlies an unconformity. This unconformity and the clasts in the Thorp Gravel document initiation of the last uplift and erosion of the Cascade Range. Stop 1-21 demonstrates that continued uplift imparted a tectonic dip to the Thorp Gravel.
Continue east on SR-10.
MP 105.3 of SR-10
Turn right (southeast) at the 4-way stop and follow the signs for southbound US-97 for 1.2 miles to the I-90 Interchange (Exit 6) west of Ellensburg. All of the topography (from horizon to horizon) visible during this drive is underlain by CRBG or younger formations. The ridges are anticlines in CRBG (stops 2-5 and 2-6).
Exit 106 of I-90
Enter I-90 eastbound toward Spokane.
Exit 109 of I-90
Exit I-90 southbound on SR-821, Canyon Road. In 4.2 miles, SR-821 begins to cross the northeastern limb of Manastash Ridge and to enter Yakima Canyon. Here, variable dips in basalt and the Vantage Sandstone mark the Thrall syncline just north of the much larger Manastash anticline (Bentley, 1977, Fig. 9 therein).
MP 12.4 of SR-821
Take the entrance to the Lmuma Creek Recreation Site and drive downhill to the parking lot and toilets.
Stop 2.5. Umtanum Ridge Anticline of the Yakima Fold Belt
Look southeastward down the canyon to Baldy, the hill topped with an antenna. The topographic relief from the Yakima River to the top of Baldy is 600 m. Most of the rock visible from here is Grande Ronde Basalt. Enjoy (possibly even sketch or photograph) the northeastward-verging, asymmetric Umtanum Ridge anticline in CRBG. Infer the thrust at the base of the nearly vertical northeastern limb. Bentley (1977, Figs. 20 and 25 therein) did.
The active Seattle fault and Newport Hills anticline on strike 150 km to the northwest (Blakely et al. 2002; Sherrod et al., 2004) are roughly the same age and orientation. They and the Yakima fold belt are parts of the SWIFT.
Across the valley, tephras are exposed in the terraces. Deposition and erosion of these indicate multiple periods of tectonism and/or climate change.
Return to SR-821 and turn left toward Ellensburg. Turn right 0.5 miles beyond the top of the canyon to Thrall and to the entrance (Exit 3) to Interstate 82 (I-82) a half mile to the east.
MP 3 of I-82
Enter I-82 southbound toward Yakima and proceed uphill.
MP 7.4 of I-82
Exit to the viewpoint on Manastash Ridge (and anticline) below Vanderbilt Gap.
Walk to the outcrop at the upper parking lot.
Stop 2.6. Walpapi Stratigraphy and Structure near Vanderbilt Gap
We are on the northeastern limb of the Manastash Ridge anticline, the northeasternmost large anticline of the Yakima fold belt. The anticline is 150 km long (Bentley, 19677).
First, we examine a photogenic example of the Squaw Creek Sandstone. The sandstone ranges in various other places from a few centimeters to 16 m thick (Smith, 1988, Fig. 4 therein). Beneath the Squaw Creek is the Frenchman Springs flow. Above it is the Roza lava flow with a well developed pillow-palagonite zone at its base. Both are units of the Wanapum Basalt Formation (Fig. 3). The composition of the Squaw Creek sandstone is remarkably similar to the arkoses of the Challis synthem. The cross bedding implies an easterly source, the opposite of that of the Ellensburg Formation at stop 2-3. The nearest possible source for the Squaw Creek must be beyond the present (and former) geographic extent of the CRBG in northeastern Washington and western Idaho.
The view from the lower parking lot, clouds permitting, is most instructive. The Kittitas valley is 400 m below. Lookout Mountain is the low wooded plateau in the Kittitas Valley 30 km to the northwest; there along SR-10 and farther east at stops 2-1 to 2-3, Walpapi rocks dip gently southeastward. The distant slope of the Kittitas Valley to the northeast is underlain by Grande Ronde Basalt, which dips several degrees to the southwest toward the steeper northeasterly dips of the units here at stop 2-6. Thus, the Kittitas Valley syncline is asymmetric (Bentley, 1977, Fig. 7 therein) and plunges southeasterly. The syncline does have smaller folds within it (Bentley, 1977, Fig. 6 therein).
Mount Stuart, which is the highest peak in the central part of the Cascade Range, may be visible to the northwest; it is underlain by pre-Tertiary batholithic rocks. CRBG on the skyline northeast of Kittitas Valley has a regional dip which, if extended northwestward, projects above Mount Stuart. That is, regionally CRBG dips easterly off the Cascade Range, as it does at stops 1-21 and 2-2. Thus, the age of uplift of the Cascade Range and of the southeasterly plunge of the Kittitas Valley syncline is post-CRBG. The eastward dip of the Thorp Gravel at stop 1-21 shows that some of the uplift is <4 Ma.
Regionally, intersecting folds in CRBG illustrate the egg crate of the Pacific Northwest. The Cascade Range is a northerly tending anticline. The Umtanum Ridge, Manastash Ridge, Ainsley Canyon Naneum Ridge, and Eagle Creek anticlines and the Kittitas Valley syncline are northwesterly trending folds. These folds plunge to the southeast because they were tilted by the rise of the Cascade Range anticline to the west.
The northerly set of folds of the egg crate may be related to the Cascadia subduction zone (an “edge-effect” on the continent). The northwesterly set may be geologic evidence for the suggestion of Wells et al. (1998) that California is drifting northward at millimeters per year and rotating Oregon “into the soft underbelly of Washington” (R.E. Wells, 1999, personal commun.).
Return to I-82 and continue southeastward toward Yakima. Note the variable dips of sandstone and of basalt along Vanderbilt Gap. Here, a small syncline is on the crest of the Manastash anticline, and is bounded by faults on the south (Bentley, 1977, Fig. 18 therein). Once through the gap, note Baldy, (stop 2-5), on the Umtanum Ridge anticline 8 km to the south.
MP 11.4 of I-82
Take Exit 11; pass under I-82, and take the northbound (here westbound) lanes of I-82 back toward Ellensburg.
MP 0.0 of I-82 and MP 110.1 of I-90
At the junction of I-82 and I-90, take I-90 westbound toward Seattle. Starting at about MP 91of I-90 are views to north of the landslide on the western slope of Lookout Mountain (above stop 2-1).
MP 89.5 of I-90
Exit into the Indian John Hill Rest Area.
Stop 2.7. Scenic View from Indian John Hill
The western end of the rest area has a splendid view of Mount Stuart to the northwest. To the west is the forested dip slope of the Roslyn Formation on the northern limb of the Kittitas Valley syncline.
Continue westbound on I-90, which follows the axial trace of the Kittitas Valley syncline (Tabor et al., 1982, Fig. 3 therein) and descends the western erosional edge of CRBG to the Roslyn Formation in the Yakima River valley.
MP 84.4 and Exit 84 of I-90
Note: In case of poor weather, omit stops 2-8, 2-9, 2-10, and 2-11 and proceed to stop 2-12 off Exit 78. If the weather is good, take Exit 84.
Exit 84 leads to Cle Elum. Cle Elum and the neighboring towns of Roslyn and Ronald mined coal (mostly underground) from the Roslyn Formation from 1882 to 1963. Total production was 64 million tons (Walker, 1980) and ceased when railroad locomotives converted to diesel.
Exit 84 also leads to spectacular Peoh Point (stop 2-11), the high rocky point (1225 m) on the southern side of the valley. To get there, follow the signs to South Cle Elum by going into Cle Elum and turn left (west) on 1st Street (the main street). After a few blocks, turn left on Rossetti Way, which crosses under I-90 and over the Yakima River to become South Cle Elum Way. Turn right on Lincoln Avenue and left on 5th Street, which feeds into Westside Road. Turn right on Westside Road and in ∼2 miles turn left (south) on USFS Road 3350 (gravel). Road 3550 eventually curves eastward up the side of valley and at ∼5 miles crosses extensive hummocky topography. At ∼6 miles, turn right on USFS 211 and proceed <0.2 miles to a quarry.
USFS Road 211
Stop 2-8. Grande Ronde Basalt
Here, the Grande Ronde Basalt is at an altitude of 1250 m. Notice fragments of tuffaceous material intermixed with the basalt. Nearby the tuffaceous material was mapped as Ellensburg Formation (Tabor et al., 1982), but it could be from a unit of the Kittitas synthem. The hummocky topography is due to a large landslide (Tabor et al., 1982), but for our purposes, we shall consider the basalt to be virtually in place.
Return to USFS 3550, turn right, and continue eastward for 0.5 miles. Turn left into USFS 115 and stop.
Stop 2-9. Darrington Phyllite
The pre-Tertiary Darrington Phyllite is in the core in the Ainsley Canyon anticline. The phyllite characteristically has numerous, predominantly concordant quartz veinlets. Stops 2-8 and 2-9 demonstrate that at least locally the Walpapi synthem unconformably overlies the crystalline basement.
Continue 0.5 miles northeastward on USFS 115.
Stop 2-10. Swauk (Manastash) Formation
USFS Road 115
Here, an underwhelming outcrop of northward dipping Swauk (Manastash) Formation demonstrates that the Challis synthem also nonconformably overlies Darrington Phyllite. An interesting exercise is to sketch these two unconformities on a single cross section.
Two synthemal basal unconformities may be wonderful, but the best is yet to come. Continue about a mile on USFS 115, but NOT if the road is covered with mud or snow because the vehicles may not be able to return uphill. Park at the locked gate to Peoh Point and prepare for a 0.5 km hike. Peoh Point has a stunning view; so, take Figure 2 and a camera. See features on the maps of Tabor et al. (1982, 2000). The road and Peoh Point are in hornblende-bearing dacite (Tabor et al., 1982) of the Taneum Formation.
Stop 2-11. Scenic View from Peoh Point
The structural relief of CRBG in the Kittitas Valley syncline and the Naneum Ridge anticline on the distant skyline to the northeast is 1200 m.
The long descending ridge to the west that strikes toward Peoh Point is Easton Ridge. It is underlain by northeasterly dipping Teanaway basalt in the hanging wall of the Easton Ridge thrust. The rusty weathering rib ∼2 km east of the Peoh Point is the same Teanaway basalt passing beneath CRBG. At the base of the cliff below Peoh Point, the Easton Ridge thrust virtually eliminates the Teanaway Formation. The Easton Ridge thrust is similar to the Seattle fault and to the Camas Creek thrust of stop 1-13.
Rusty weathering hills 15 km to the northeast are underlain by Teanaway basalt on the northeastern limb of the Kittitas Valley syncline. Mount Stuart dominates the skyline 23 km to the north; it is an analog for the Kingston arch north of Seattle. Roslyn and former coal-wash piles from the Roslyn Formation are 8 km to the northwest. Moraines dam Cle Elum Lake 15 km to the northwest. The hills that cross the Yakima River valley ∼25 km to the northwest mark the trace of the Straight Creek fault near stop 2-14.
Return to Cle Elum. To access I-90, turn left (west) and continue uphill on 1st Street (the main street) of Cle Elum. It feeds into the entrance ramp for the westbound lanes toward Seattle.
Exit 78 of I-90
Take Exit 78. Go left (south) on Golf Course Road for 0.8 miles to the T junction with West Side Road and turn left. Proceed 0.9 miles to the junction of West Side Road and Fawler Creek Road.
Stop 2-l2. Darrington Phyllite
This pelitic phyllite has folded and disrupted quartz veinlets. Please do not destroy (e.g., hammer) them. The protolith probably was Jurassic and the metamorphism was Cretaceous (Tabor et al., 2000; Tabor, 1994). The phyllite is in the core of the Ainsley Canyon anticline.
Return to I-90 and continue westward past Easton.
Exit 70 of I-90
Take Exit 70, turn left (south) over I-90, and follow the frontage road (southeastward) toward Easton <0.5 miles to the entrance of Easton State Park.
Stop 2-13. Sub-Teanaway Unconformity
Northeastward across I-90 and the Yakima River valley is a ridge with >600 m of relief. The brown weathering, bedded unit to the right is basalt of the Teanaway Formation, which dips moderately northeastward. At the base of the ridge and up valley (to the left) is a lighter colored and more steeply dipping unit, the felsic volcanic rocks of the Taneum Formation stratigraphically below the basalt. Rarely is an intra-Challis unconformity so obvious.
Enter the Easton State Park and turn right at the first stop sign. Proceed along the northern shore of Lake Easton for 1.0 miles past the picnic area (with toilets). Continue another 0.3 miles to the parking area at the end of the road. Walk to the outcrop near the southeastern abutment of the bridge of former US-10.
Stop 2-14. Shuksan Greenschist and Straight Creek Fault
The Shuksan is a handsome green phyllite. The foliation strikes northwesterly and is nearly vertical. Quartz veinlets are more numerous on the hill 100 m to the southeast.
The Shuksan Greenschist and the Darrington Phyllite are in the core of the Ainsley Canyon anticline. The Teanaway Formation on the northeastern side of the valley dips northeastward (stop 2-13). The railroad grade on the southern shore of Lake Easton is in arkosic sandstone of the Swauk Formation on the southwestern limb of the anticline.
Another reason for this stop is to contemplate the location of the Straight Creek fault. The Darrington Phyllite and the Shuksan Greenschist are distinctive and widespread units in the northern Cascade Range west of the Straight Creek fault. Dextral displacement of 90–190 km on the fault offset these units to the Easton–Peoh Point area (Tabor, 1994; Tabor et al., 2000).
Tabor (1994, Fig. 6 therein) and Tabor et al. (2000) mapped the trace of the Straight Creek fault in the Easton area as a southeasterly striking discontinuity that separates Naches Formation on the southwest from the Swauk, Taneum, Teanaway, and Roslyn formations on the northeast. More detailed mapping (Cheney, 1999) demonstrated that much of the Naches mapped by previous investigators is composed of the Naches Formation plus unrecognized portions of all other formations of the Challis synthem. Thus, the major criterion for locating the Straight Creek fault was invalid.
Furthermore, Tabor et al. (2000) placed the Shuksan Greenschist at stop 2-14 west of the Straight Creek fault and extended the fault southeastward on the northeastern side of the Yakima Valley along the discontinuity that eliminates the Swauk Formation. This discontinuity is caused by unconformities below the Taneum and Teanaway formations on the northeastern limb of the Ainsley Canyon anticline (Fig. 2; Cheney, 1999).
The greatest structural discontinuity is a southerly one 2.7 km west of Easton. This discontinuity is inferred to be the Straight Creek fault. Formations of the Challis synthem occur on both sides of it. The Shuksan Greenschist at stop 2-14 is east of this discontinuity. To the north, the Straight Creek fault truncates the axial trace of the Ainsley Canyon anticline (Fig. 2). To the south, the Straight Creek fault is unconformably overlain by the Kittitas synthem (Fig. 2; Cheney, 1999).
Cheney (1999) showed that the Straight Creek fault cuts the Taneum Lake fault (TLF in Fig. 8B). Tabor (1994) inferred that the Taneum Lake fault and the Darrington–Devils Mountain fault zone probably are the same dextral fault, with 112 km of dextral displacement. If so, the Taneum Lake fault and the Darrington–Devils Mountain fault zone, both of which cut Naches or equivalent rocks, are offset ∼55 km by the Straight Creek fault (Fig. 8B). In contrast, pre-Tertiary units are offset 90–190 km (Tabor, 1994). This difference implies that the Straight Creek fault had pre-Tertiary and post-Naches dextral movements (Fig. 8B–8D). The portion of the Straight Creek fault that is dextrally offset by the Darrington–Devils Mountain fault zone might be one of the northerly trending faults in the Puget Lowland (Fig. 8C).
Retrace the route back to the westbound-entrance ramp at Exit 70 of I-90 and proceed westbound on I-90. The trace of the Straight Creek fault is in a major covered interval at MP 68.8 (Cheney, 1999). Seattle is ∼68.8 min to the west.
End of Day Two and of the field trip.
We earnestly thank many people, including B.L. Sherrod, the late R.J. Stewart, and numerous students and other participants of field trips for numerous insightful discussions. T. Bush, B. Byers, I.R. Cheney, N.I. Chutas, R.B. Frost, M.W. Hawkes, S. Moon, S. Petrisor, C. Rhone, B.L. Sherrod, and D. Trippett kindly assisted in the fieldwork in the Blushastin area. I.R Cheney, L.A Gilmour, and C. Rhone kindly assisted in the fieldwork in Leavenworth area. M.T. Brandon, P.J. Umhoefer, J.P. Hibbard, and R.B Miller provided constructive criticisms of previous drafts. P. Stelling and B.L. Sherrod reviewed the present manuscript. Hayman thanks the University of Washington for Peter H. Misch and Joseph A. Vance graduate fellowships.
Figures & Tables
Floods, Faults, and Fire
The ten geological field guides presented in this volume explore key areas of the geologist’s Paradise that is Washington State and British Columbia. These trips investigate a wide variety of geologic and geographic terrains, from the dry steppe of the channeled scablands and Columbia River basalt group to the east, across the glaciated and forested Cascade arc and Coast Mountains, to the geologically complex islands in the west. This guidebook may be unique in that four of the trips utilize boats to reach remote field areas and are therefore rarely visited by geologists. Although these trips were guided during the 2007 GSA Cordilleran Section meeting, the guides were written to ensure that people can easily guide their own trips. The result provides an excellent source of exciting, thought-provoking geologic adventures for years to come.
- Cascade Range
- Chumstick Formation
- Columbia River Basalt Group
- cross sections
- crystalline rocks
- field studies
- North America
- plate tectonics
- road log
- sequence stratigraphy
- Swauk Formation
- United States
- Chiwaukum Graben
- Leavenworth Fault