Western Cordillera and Adjacent Areas

Edited by Terry W. Swanson


This volume includes guides for 15 of the field trips held in conjunction with the 2003 GSA Annual Meeting in Seattle. Topics covered include Glacial Lake Missoula and the Clark Fork Ice Dam; the Sauk Sequence in western Utah; the geology of wine in Washington state; the Columbia River basalt and Yakima Fold Belt; Alpine glaciation of the North Cascades; and recent geoarchaeological discoveries in central Washington. Quaternary geology of Seattle, engineering geology in the central Columbia Valley, and the tephrostratigraphy and paleogeography of southern Puget Sound also are covered, as are trips to central Cascade Range and the White River.

  1. Page 1
    • Clark Fork ice dam
    • current ripples
    • glacial
    • glacial lake
    • Lake Missoula
    • floods
    • scabland
    • pingo

    The first day begins in Spokane and follows Interstate 90 to Missoula, Montana. Stops are at the Rathdrum Prairie ice-dam outburst area east of Spokane; flood out-wash deposits in and around Coeur d’Alene, Idaho; Glacial Lake Missoula deposits west of Missoula, Montana; and Glacial Lake Missoula shorelines that are etched into the hillsides surrounding the city of Missoula. The second day’s travel includes sites in the Missoula Valley that display features associated with the catastrophic emptying of the lake and the subsequent refilling. The remainder of the day’s travel will be north and west of Missoula to sites along U.S. Highway 93 and State Routes 28, 382, and 200 to view the relationship of valley glaciers and Glacial Lake Missoula, pingo scar terrain, large terminal moraines, Joseph T. Pardee’s classic giant current ripple forms and gulch fills in the Camas Prairie and along the Flathead River, and Eddy Narrows, ending in Thompson Falls. The third day’s route continues west on Montana State Route 200 with stops to study glacial gravels associated with ice-dam advances up the Clark Fork River and the features associated with the ice dam that repeatedly occupied the Purcell Trench, dammed the Clark Fork River, and created multiple Glacial Lakes Missoula. Lastly, we will stop at the outburst area in Farragut State Park and the Rathdrum Prairie breakout area to view notable examples of current ripples and outburst features.

  2. Page 17
    • sequence stratigraphy
    • Sauk Sequence
    • Cambrian
    • Ordovician
    • Utah
    • tectonics

    The Sauk Sequence comprises more than 5 km of mixed carbonate and siliciclastic strata on the Paleozoic miogeocline of the eastern Great Basin. Rapid, post-rifting subsidence was the single most important factor for providing accommodation for accumulation of sediments. Despite the enormous thickness of strata and the tendency for unconformities to die out toward the margin of the continent, bounding surfaces of the Sauk Sequence and several sequence boundaries within this interval are preserved in mountain ranges of western Utah. The base and top of the Sauk Sequence are thick sandstones. The development of microkarst or truncation surfaces associated with major facies disclocations and deposition of major influxes of siliciclastics are the hallmarks of sequence boundaries and correlative conformities in this setting. The style of sequence boundary development was mostly a function of magnitude and duration of sea-level fall but was also influenced by tectonic features such as the House Range Embayment.

  3. Page 37
    • geomorphology
    • active tectonics
    • subduction wedges
    • glacial geology
    • soils
    • terraces

    We use Quaternary stratigraphy to reconstruct landscape evolution and measure tectonic deformation of the Olympic Mountains section of the Pacific Northwest Coast Range. An important motivation for understanding orogenesis here, and throughout the Coast Range, is the concern about the relationship of active deformation to seismic hazards associated with the Cascadia subduction zone. There is also much interest in apportioning the nature of the deformation, whether cyclic or permanent, whether it involves mainly shortening parallel or normal to the margin, and how the deformation on the pro- versus retrowedge sides of the orogen compare. Pre-Holocene stratigraphy and structure provide the only records of sufficient duration to separate long-term permanent deformation from earthquake-cycle elastic deformation. For this reason, active-tectonic studies have focused on deformation of Quaternary deposits and land-forms, which are best preserved along the Pacific Coast and offshore on the continental shelf. At least four major glacial advances are recorded in the valley and coastal deposits along the western margin of the Olympic Peninsula. Both numeric and relative dating, including soils of these deposits, establish a stratigraphic anchor that is used to document the relationship between margin parallel and margin normal deformation in the Olympic Mountains, which, on a geologic time scale (>103 yr), seems to be the fastest deforming part of the Cascadia forearc high. The glacial stratigraphic framework is extended to fluvial terraces of the Clearwater drainage, which remained unglaciated during the late Pleistocene and Holocene, preserving a record of river incision, with each terrace recording the shape and height of past long profiles. We assess how fluvial terraces are formed in this tectonically active setting and then use features of the terraces to estimate incision rates along the Clearwater long profile. The long fluvial history preserved in the Clearwater ensures that the unsteady deformation associated with the earthquake cycle is averaged out, leaving us with a record of long-term rock uplift as well as horizontal shortening. We show, however, that the earthquake cycle may play an important role in terrace genesis at the millennial time scale.

  4. Page 69
    • wine
    • terroir
    • wine grapes
    • loess
    • soils
    • outburst floods
    E-mail: Busacca-busacca@wsu.eduE-mail:

    Washington State is second only to California in terms of wine produced in the United States, and some of its vineyards and wines are among the world’s best. Most Washington vineyards are situated east of the Cascades on soils formed from Quaternary sediments that overlie Miocene basaltic rocks of the Columbia River Flood Basalt Province. Pleistocene fluvial sediments were deposited during cataclysmic glacial outburst floods that formed the spectacular Channeled Scabland. Late Pleistocene and Holocene sand sheets and loess form a variable mantle over outburst sediments. Rainfall for wine grape production ranges from ~6-18 in (150-450 mm) annually with a pronounced winter maximum and warm, dry summers. This field trip will examine the terroir of some of Washington’s best vineyards. Terroir involves the complex interplay of climate, soil, geology, and other physical factors that influence the character and quality of wine. These factors underpin the substantial contribution of good viticultural practice and expert winemaking. We will travel by bus over the Cascade Mountains to the Yakima Valley appellation to see the effects of rain shadow, bedrock variation, sediment and soil characteristics, and air drainage on vineyard siting; we will visit the Red Mountain appellation to examine sites with warm mesoclimate and soils from back-eddy glacial flood and eolian sediments; the next stop will be the Walla Walla Valley appellation with excellent exposures of glacial slackwater sediments (which underlie the best vineyards) as well as the United States’ largest wind energy facility. Finally, we will visit the very creatively sited Wallula Vineyard in the Columbia Valley appellation overlooking the Columbia River before returning to Seattle.

  5. Page 87

    This field trip guide covers a two-day trip to examine the characteristics of Columbia River Basalt Group flows and the Yakima fold belt. This field trip focuses on the main physical characteristics of the lavas, compositional variations, and evidence for their emplacement, and on the geometry of the anticlinal ridges and synclinal valleys of the fold belt and deformational features in the basalts.

  6. Page 107
    • North Cascades
    • arcs
    • timescales
    • exhumation
    • pluton
    • rheology

    The crystalline core of the North Cascades (Cascades core) consists largely of oceanic and arc terranes that were metamorphosed to amphibolite facies and intruded by 96-45 Ma arc plutons. A crustal section recording paleodepths of ~5-40 km is preserved in the southern part of the core and facilitates evaluation of processes at different levels of the arc. After terrane juxtaposition, plutons were intruded during major arc-normal crustal shortening dominated by early recumbent folds and subsequent upright folds. Structural patterns emphasize the heterogeneous vertical partitioning of deformation and complex rheological stratification of arc crust at all scales. Metamorphic and geochronologic data indicate rapid burial of plutons and host rocks during the middle Cretaceous shortening. A subsequent major, cryptic event in the evolution of the Cascades core was the rapid underthrusting of Cretaceous sedimentary protoliths of the Swakane Gneiss to depths of >40 km between 73 and 68 Ma. The emplacement of the gneiss, which lacks arc magmas, may have removed the roots of many of the arc plutons. Plutonic rocks record both focused and unfocused, largely tonalitic magmatism that resulted in large plutons and abundant narrow sheets, respectively. Individual large-volume plutons were constructed over intervals of up to 5.5 m.y. Magmas probably ascended as visco-elastic diapirs, and emplacement was aided primarily by vertical material transfer, including ductile flow and stoping, and possibly regional folding. The magmatic, metamorphic, and structural processes recorded in the Cascades core exemplify the dynamic evolution of arcs and the large vertical and lateral displacements during arc construction.

  7. Page 137
    • Pleistocene
    • glaciation
    • Younger Dryas
    • Puget Lowland
    • Cascade Range
    • Columbia Plateau

    The advance of the Cordilleran Ice Sheet (CIS) during the Vashon Stade is limited by 14C dates from sediments beneath Vashon till, which indicate that ice advanced southward across the Canadian border sometime after ca. 18 ka 14C yr B.P. and reached the Seattle area soon after 14.5 ka 14C yr B.P. The Puget lobe underwent sudden, large-scale terminus recession and downwasting not long after 14.5 ka 14C yr B.P., and backwasted northward from its southern terminus past the Seattle area by ca. 14 ka 14C yr B.P. Rapid thinning of Vashon ice after the terminus had receded north of Seattle allowed marine water from the Strait of Juan de Fuca to flood the lowland, floating the remaining ice and disintegrating the remaining CIS northward all the way to Canada, except for a narrow band along the eastern margin of the lowland.

    Everson glaciomarine drift (gmd), consisting mostly of poorly sorted stony clay deposited from floating ice, was deposited essentially contemporaneously over the central and northern Puget Lowland. Unbroken, articulated, marine shells, some in growth positions, indicate that the gmd represents in situ deposition. More than 150 14C dates from Washington and British Columbia fix the age of the Everson gmd at 11,500 to ca. 12,500 14C yr B.P., making it a valuable stratigraphic marker over the central and northern Puget Lowland.

    Ice-contact marine deltas and shorelines were produced on Whidbey Island as the CIS thinned and disintegrated in the central Puget Lowland, allowing marine water from the Strait of Juan de Fuca to penetrate beneath the ice. During this time, the CIS had disintegrated in the deeper water of the inland waterways, but grounded ice remained along the eastern side of the mainland, changing the ice flow direction from N-S to NE-SW, from the grounded ice on the mainland toward the open deep water to the west at the Strait of Juan de Fuca. A well-defined, marine ice-margin existed along the south and west sides of Penn Cove and isostatically raised shorelines and marine deltas were formed at elevations up to ~33 m on southern Whidbey Island and up to ~88 m on northern Whidbey. The shorelines are best preserved along the sides of marine embayments on the island.

    Following the deposition of Everson gmd and the emergence of the northern Puget Lowland, the CIS readvanced several times, defining four phases of the Sumas Stade: Sumas I represents grounding of the CIS and deposition of till in the western Fraser Lowland. Sumas II consists of a well-defined moraine and meltwater channels deeply incised into Everson gmd. A series of Sumas III moraines that occur in British Columbia shed meltwater that built a broad outwash plain behind the Sumas II moraine. A Sumas IV moraine occurs across a Sumas III meltwater channel at the eastern margin of the Fraser Lowland.

  8. Page 159
    • geologic hazards
    • engineering geology
    • dams
    • landslides
    • Columbia River basin
    E-mail: Badger-badgert@wsdot.wa.govE-mail:

    The deeply incised central Columbia River valley of Washington State and its tributaries expose mid to late Tertiary basalt flows and clastic sedimentary rocks, pre-Tertiary crystalline bedrock outcrops where the river flows along the eastern slope of the Cascade Mountains between Wenatchee and Chelan. River incision has primarily been driven by the uplift of the Cascades, deposition of the voluminous Columbia River basalts, and the formation of the Yakima fold belt. Glaciation during the Pleistocene, the terminus of which reached Chelan and the northern Waterville Plateau, infused large quantities of sediment into the valley. Concurrently, catastrophic glacial outburst floods, unprecedented in size, repeatedly swept down the river from the north and over the Quincy Basin in the south. Trip stops include some of the early engineering works, principally the dams, where much of the regional stratigraphy was developed and challenging engineering solutions were required for difficult geologic conditions. Stops also exemplify the pervasive large-scale landsliding, common where basalts overlie weak sedimentary rocks. Due to the steep topography, transportation corridors and other developments are widely threatened by rockfall and debris flow hazards. Seismicity is also a regional hazard; the largest historic earthquake in eastern Washington, moment magnitude 6.5-7.0, was sited near Chelan.

  9. Page 177
    • unconformities
    • synthem
    • fold-and-thrust belt
    • egg-crate structure
    • Cascade Range

    The Tertiary sedimentary and volcanic rocks of the Cascade Range unconformably overlie a crystalline basement of previously accreted terranes. The Tertiary strata are parts of four synthems, or interregional unconformity-bounded sequences of tectonic origin. Thus, the formations in these synthems were not deposited in local basins.

    The 55-38 Ma Challis synthem has five regional unconformity-bounded formations; the names with precedence are (from the base up) Swauk, Taneum, Teanaway, Roslyn, and Naches. Near Blewett Pass (nee Swauk Pass), the Challis fluvial and arkosic Swauk Formation is ~5 km thick and has several members in a generally upward-fining succession. The members of the Swauk do not interfinger, and some are separated by unconformities.

    The Oligocene to mid-Miocene andesitic and rhyolitic Kittitas synthem is almost absent in the area. The most voluminous lithostratigraphic unit in the mid-Miocene to Pliocene Walpapi synthem is the Columbia River Basalt Group. Clasts of Columbia River Basalt Group and older rocks in the ca. 4 Ma Thorp Formation of the High Cascade synthem record initial uplift of the Cascade Range to the west.

    North of Blewett Pass, the northwesterly segment of the Leavenworth fault is the Camas Creek reverse fault that places Swauk and Teanaway in the Blushastin anticline over a syncline in the Roslyn Formation. Northerly striking faults in the Leaven-worth fault zone are parts of a younger system that cuts the Camas Creek thrust and northwest-striking folds in Challis rocks.

    In style, scale, and age, the Camas Creek fault resembles the Easton Ridge thrust south of Cle Elum, the Eagle Creek fault in the Chiwaukum graben, and the Seattle fault in the Puget Lowland. These faults are on the steeper northeastern limbs of major anticlines in Challis rocks. Down plunge, these folds are more gentle in Walpapi rocks. These folds and faults are part of the regional Seattle-Wentachee-Kittitas fold-and-thrust belt.

    The Straight Creek fault is a major, north-south, dextral fault in the northern Cascade Range. The fault offsets all five of the Challis unconformity-bounded formations. The southeasterly curving discontinuity along which it was mapped east of Easton is due to unconformities at the base of the Taneum and Teanaway, not a fault. The Straight Creek fault is 2.7 km west of Easton and passes southward beneath Kittitas rocks. Although the fault dextrally displaces pre-Tertiary units >90 km, Tertiary displacement is &55 km. This may indicate two (or more) periods of displacement. Perhaps the displaced portion of the fault underlies Puget Sound.

    Two sets of post-Walpapi folds deform the Tertiary synthems. The Seattle-Wentachee-Kittitas fold-and-thrust belt is part of a set of northwest-striking folds. One of several north-trending regional anticlines causes the Cascade Range. The Cascade Range anticline, with an amplitude of ~3.5 km, has risen in approximately the past 3.5 m.y. This anticline causes the plunges of the Seattle-Wentachee-Kittitas fold-and-thrust belt folds. The two sets of folds cause a regional interference, or “egg-crate,” pattern that dominates the present topography of the Pacific Northwest.

  10. Page 201

    The main objective of this one-day trip will be to visit the late Eocene Bear River cold-methane-seep deposit in Pacific County, southwestern Washington, providing an opportunity to collect samples from the richly fossiliferous deep-water limestone. To and from the Bear River deposit we will traverse marine Tertiary basalts and volcaniclastic and siliciclastic strata. If weather and road conditions permit, we intend to make brief stops at other seep deposits, rock outcrops, and sites of local historic importance. The trip is intended to not only provide a geographic context for the complex geology of southwestern Washington, but also some sense of the difficulties, posed by both climate and vegetation, that are encountered by geologists and paleontologists when working in this region.

    Two of the stops on this field trip (Fig. 1), the Naselle River bridge and the Bear River site, were also visited as part of a previous Geological Society of America field trip (Nesbitt et al., 1994). The Menlo site has not received any study subsequent to brief analyses in the early 1990s (Goedert and Squires, 1990; Squires and Goedert, 1991; Campbell and Bottjer, 1993). In Pacific County, megapaleontological studies (other than those dealing with cold-seep assemblages) have concentrated mainly on a limited number of localities in only a few rock units (e.g., Moore, 1984); for some rock units, published megapaleontological data is nonexistent. One late Eocene and newly recognized cold-seep limestone near Knappton is introduced for the first

  11. Page 209
    • lahar
    • delta
    • Holocene
    • Mount Rainier
    • Seattle

    Clay-poor lahars of late Holocene age from Mount Rainier change down the White River drainage into lahar-derived fluvial and deltaic deposits that filled an arm of Puget Sound between the sites of Auburn and Seattle, 110–150 km downvalley from the volcano’s summit. Lahars in the debris-flow phase left cobbly and bouldery deposits on the walls of valleys within 70 km of the summit. At distances of 80–110 km, transitional (hyperconcentrated) flows deposited pebbles and sand that coat terraces in a gorge incised into glacial drift and the mid-Holocene Osceola Mudflow. On the broad, level floor of the Kent Valley at 110–130 km, lahars in the runout or streamflow phase deposited mostly sand-sized particles that locally include the trunks of trees probably entrained by the flows. Beyond 130 km, in the Duwamish Valley of Tukwila and Seattle, laminated andestic sand derived from Mount Rainier built a delta northward across the Seattle fault. This distal facies, warped during an earthquake in A.D. 900–930, rests on estuarine mud at depths as great as 20 m.

    The deltaic filling occurred in episodes that appear to overlap in time with the lahars. As judged from radiocarbon ages of twigs and logs, at least three episodes of distal deposition postdate the Osceola Mudflow. One of these episodes occurred ca. 2200–2800 cal. yr B.P., and two others occurred ca. 1700–1000 cal. yr B.P. The most recent episode ended by about the time of the earthquake of A.D. 900–930. The delta’s northward march to Seattle averaged between 6 and 14 m/yr in the late Holocene.

  12. Page 225
    • Quaternary stratigraphy
    • tephrochronolgy
    • Puget Lowland
    • glacial geology

    Our detailed mapping in the south Puget Sound basin has identified two tephras that are tentatively correlated to tephras from Mount St. Helens and Mount Rainier dated ca. 100-200 ka and 200 ka, respectively. This, plus the observation that fluvial and lacustrine sediments immediately underlying the Vashon Drift of latest Wisconsin age are nearly everywhere radiocarbon infinite, suggests that glacial and nonglacial sediments of more than the past five oxygen-isotope stages are exposed above sea level. Distal lacustrine advance outwash equivalent to the Lawton Clay in the Seattle area is conspicuously absent. Instead, a thick (>120 ft) glaciolacustrine silt below the Vashon sediments contains dropstones and is radiocarbon infinite. Elsewhere, coarsegrained advance Vashon outwash rests unconformably on radiocarbon-infinite non-glacial sediments. These relationships may imply that late Pleistocene tectonic activity has modified the paleotopography and stratigraphy of the south Puget Sound area.

  13. Page 237
    • geoarchaeology
    • Columbia Basin
    • forensic geology
    • Channeled Scablands
    • Paleo-Indian
    • Sentinel Gap
    E-mail: Huckleberry-ghuck@wsu.eduE-mail: Lenz-blenz@geoscientists.orgE-mail: Galm-jgalm@ewu.eduE-mail:

    Geoarchaeological research in the mid-Columbia region of central Washington over the past 10 yr has produced new information regarding Paleo-Indian archaeology and environmental change in the inland Northwest. Stratigraphic, sedimentological, and geomorphic studies provide important contextual information for locating and interpreting Washington’s earliest archaeological sites and human remains. Recent discoveries increasingly point toward human occupation of the region during a time of post-glacial warming and reduced effective moisture 11,200–9000 14C yr B.P. This field guide presents recent research focusing on geoarchaeological studies at the Kennewick Man discovery site, at latest Pleistocene relict Channeled Scabland marsh sites, and at the recently excavated Sentinel Gap Paleo-Indian site.

  14. Page 251
    • ophiolite
    • Ingalls
    • Jurassic
    • Washington
    • sedimentary serpentinite
    • ophiolite breccia

    The Ingalls Ophiolite Complex is a suprasubduction-zone ophiolite formed largely in a fracture-zone setting. Mantle tectonites are cut by a large, high-T shear zone overprinted by sheared serpentinite. Mafic complexes of ca. 161 Ma gabbro, sheeted dikes, and pillow lava occur as large blocks in the sheared serpentinite. An overlying Late Jurassic argillite unit contains minor chert, graywacke, and pebble conglomerate, along with lenses of ophiolite breccias. Detrital serpentinite forms some of these breccias, and mafic blocks in other breccias range up to hundreds of meters in diameter. Older basement is locally present in the ophiolite complex, includ ing (1) phyllite, metachert, and pillow basalt of the undated De Roux unit overlain by (2) Early Jurassic pillow lava, basalt breccia, and minor chert and oolitic limestone of the Iron Mountain unit (fossil seamount). This older basement indicates that at least part of the ophiolite is polygenetic. The presence of a high-T mantle shear zone and ophiolitic breccias containing clasts derived from the lower crust and upper mantle suggest formation of the ophiolite in a fracture zone setting. Late calc-alkaline dikes cut the various units, some of which are related to the middle Cretaceous Mount Stu art batholith, and others of which are probably Late Jurassic.

  15. Page 267
    • Quaternary
    • Puget Lowland
    • Seattle
    • tectonics
    E-mail: Troost-ktroost@u.washington.eduE-mail: Booth-dbooth@u.washingon.eduE-mail:

    Seattle lies within the Puget Sound Lowland, an elongate structural and topographic basin bordered by the Cascade and Olympic Mountains. The geology of the Seattle area is dominated by a complex, alternating, and incomplete sequence of glacial and interglacial deposits that rest upon an irregular bedrock surface. The depth to bedrock varies from zero to several kilometers below the ground surface. Bedrock outcrops in an east-west band across the lowland at the latitude of south Seattle and also around the perimeter of the lowland. Numerous faults and folds have deformed both the bedrock and overlying Quaternary sediments across the lowland, most notably the Seattle fault. During an earthquake on the Seattle fault ca. 980 A.D., 8 m of vertical offset occurred.

    The Seattle area has been glaciated at least seven times during the Quaternary Period by glaciers coalescing from British Columbia. In an area where each glacial and interglacial depositional sequence looks like its predecessor, accurate stratigraphic identification requires laboratory analyses and age determinations. The modern landscape is largely a result of repeated cycles of glacial scouring and deposition, and recent processes such as landsliding and river action. The north-south ridges of the lowland are the result of glacial scouring and subglacial stream erosion. The last glacier reached the central Puget Sound region ca. 15,000 years ago and retreated past this area by 13,650 14C yr B.P. Post-glacial sediments are poorly consolidated, as much as 300 m thick in deep alluvial valleys, and susceptible to ground failure during earthquakes.

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