ABSTRACT Seattle’s geologic record begins with Eocene deposition of fluvial arkosic sandstone and associated volcanic rocks of the Puget Group, perhaps during a time of regional strike-slip faulting, followed by late Eocene and Oligocene marine deposition of the Blakeley Formation in the Cascadia forearc. Older Quaternary deposits are locally exposed. Most of the city is underlain by up to 100 m of glacial drift deposited during the Vashon stade of Fraser glaciation, 18–15 cal k.y. B.P. Vashon Drift includes lacustrine clay and silt of the Lawton Clay, lacustrine and fluvial sand of the Esperance Sand, and concrete-like Vashon till. Mappable till is absent over much of the area of the Vashon Drift. Peak local ice thickness was 900 m. Isostatic response to this brief ice loading was significant. Upon deglaciation, global ice-equivalent sea level was about −100 m and local RSL (relative sea level) was 15–20 m, suggesting a total isostatic depression of ~115–120 m at Seattle. Subsequent rapid rebound outstripped global sea-level rise to result in a newly recognized marine low-stand shoreline at −50 m. The Seattle fault is a north-verging thrust or reverse fault with ~7.5 km of throw. Conglomeratic Miocene strata may record initiation of shortening. Field relations indicate that fault geometry has evolved through three phases. At present, the north-verging master fault is blind, whereas several surface-rupturing faults above the master fault are south verging. The 900–930 A.D. Restoration Point earthquake raised a 5 km × 35 km (or larger) area as much as 7 m. The marine low-stand shoreline is offset by a similar amount, thus there has been only one such earthquake in the last ~11 k.y. Geomorphology is largely glacial: an outwash plain decorated with ice-molded flutes and large, anastomosing tunnel valleys carved by water flowing beneath the ice sheet. Euro-Americans initially settled here because of landscape features formed by uplift in the Restoration Point earthquake. But steep slopes and tide flats were not conducive to commerce: starting in the 1890s and ending in the 1920s, extensive regrading removed hills, decreased slopes, and filled low areas. In steep slopes the glacial stratigraphy is prone to landslides when saturated by unusually wet winters. Seismic hazards comprise moderately large (M 7) earthquakes in the Benioff zone 50 km and more beneath the city, demi-millennial M 9 events on the subduction zone to the west, and infrequent local crustal earthquakes (M 7) that are likely to be devastating because of their proximity. Seismic shaking and consequent liquefaction are of particular concern in Pioneer Square, SoDo, and lower Duwamish neighborhoods, which are largely built on unengineered fill that was placed over estuarine mud. Debris from past Mount Rainier lahars has reached the lower Duwamish valley and a future large lahar could pose a sedimentation hazard.

ABSTRACT With a thick and highly variable mixture of glacial and nonglacial soils overlying bedrock, punctuated by seismically active fault zones, Seattle is a challenging arena for geologists, engineering geologists, and geotechnical engineers. Because of this geologically complex stratigraphy, Seattle has a higher density of geoprofessionals and subsurface explorations than other cities of equal size. Even so, the subsurface always delivers surprises when construction begins. By visiting three major civil works, SR 520 floating bridge, Alaskan Way Viaduct/SR 99 tunnel, and the Beacon Hill Transit tunnel, you will discover the interaction between Seattle geology and the engineering that made these projects successful.

ABSTRACT The northern Puget Lowland of Washington State, USA, provides an exceptional opportunity not only to examine grounding line processes associated with marine-based ice sheets, but also to relate subaerial outcrop to marine geological observations of grounding line landforms and sedimentary processes in Antarctica and the deglaciated Northern Hemisphere. During this trip, we visit outcrops that record the interaction of the Cordilleran Ice Sheet and its bed, starting with locations where the ice sheet slowly flowed across crystalline bedrock. We also visit locations where the ice flowed across unconsolidated deposits, allowing discussions of subglacial bed deformation and grounding zone wedge development. Evidence shows that grounding line retreat across Whidbey Island was punctuated by periods of grounding line position stability and local ice advance during the growth of multiple grounding zone wedges. We will discuss the criteria for identifying grounding zone wedges, including diamicton units with foreset bedding that downlap onto a regional glacial unconformity at the base, and are truncated at the top by localized unconformities indicative of ice advance across the foreset beds. Grounding zone wedge foreset beds are composed of debris flows sourced from a deformation till and from sediment transported to the grounding line by subglacial meltwater. The overlying surface unconformity is associated with a laterally discontinuous till and pervasive glacial lineations. Other field stops focus on iceberg scouring and evidence of subglacial meltwater drainage, as well as the transition from marine to subaerial conditions during retreat of the Cordilleran Ice Sheet from the northern Puget Lowland.

ABSTRACT A tidal marsh at the head of Discovery Bay contains the longest record of tsunami deposits in Washington State. At least nine tsunami deposits dating back 2500 yr are preserved as fine sand layers in peaty tidal marsh deposits. Discovery Bay is a setting that amplifies tsunami waves, has an abundant sediment source, and a tidal marsh that traps and preserves tsunami deposits. The youngest deposit, bed 1, is probably from the 1700 A.D. Cascadia earthquake. Bed 2 has a newly revised age of 630–560 cal yr B.P. (1320–1390 A.D.), an age range that overlaps with the ages of tsunami deposits from Vancouver, British Columbia, and northern Oregon, as well as evidence for strong shaking in the region including submarine and sublacustrine slope failures. However, there is no geologic evidence for a late fourteenth-century earthquake or tsunami in any of the southwest Washington estuaries that record seven Cascadia earthquakes in the last 3500 yr. Discovery Bay bed 2 and similar-aged evidence in the region may represent a short rupture on the Cascadia subduction thrust, possibly centered west of the Strait of Juan de Fuca, that did not cause significant coastal subsidence. Other possible sources considered for bed 2 include a crustal fault earthquake, a tsunamigenic slope failure, or a transoceanic tsunami. Older tsunami deposits beds 3–9, which outnumber the number of Cascadia earthquakes in the last 2500 yr, are likely from a combination of Cascadia and non-Cascadia sources. Additional radiocarbon dating of beds 3–9 will improve age ranges and constrain potential sources.

ABSTRACT This paper reviews the Mesozoic terranes in the central Cascades, south of the Windy Pass thrust and east of the Straight Creek–Fraser River fault, and provides a guide to field locations for these units. These include the Easton Metamorphic Suite, Hicks Butte complex and higher-grade tectonic zone, the Peshastin Formation, and the Ingalls ophiolite complex (also known as the Ingalls terrane). Age data, whole rock and mineral chemistry, and structural data are reviewed. These oceanic- and arcaffinity terranes formed outboard of the North American craton during the Jurassic and accretion likely occurred during the Late Jurassic or Early Cretaceous. They were then dextrally translated north and emplaced in Washington State during the Late Cretaceous. A better understanding of these Mesozoic terranes will more closely constrain the tectonic development of the North American Cordillera.

ABSTRACT The incorporation of metasedimentary rocks into the mid- to deep crust of continental magmatic arcs has significant mechanical and geochemical consequences for arc systems. The Late Cretaceous–Eocene North Cascades arc is one of the few continental magmatic arcs in the world that exposes a large amount of exhumed deep-crustal metasedimentary rocks. Here, we investigate a range of processes that may have been important in transferring sediment into the arc by combining field mapping with bulk-rock Nd analyses, U-Pb and Hf-isotopic study of detrital zircons, and U-Pb dating of zircon and monazite to determine the timing of metamorphism and melt crystallization from metasedimentary samples collected in two deep-crustal domains of the North Cascades (the Skagit Gneiss and Swakane Gneiss). We also use these data to examine provenance links between the metasedimentary rocks and potential sediment sources in the accretionary wedge (western mélange belt), the forearc (Nooksack Formation), and the present-day backarc (Methow terrane) to the North Cascades arc. Jurassic strata of the Methow terrane and the Nooksack Formation have unimodal detrital zircon age peaks and near-depleted mantle ε Ηfi values, whereas zircons from the middle Cretaceous strata of the Methow terrane have a bimodal age distribution and less radiogenic ε Ηfi values. In comparison, the accretionary western mélange belt (WMB) has Jurassic to Upper Cretaceous sandstones characterized by multiple Mesozoic age peaks, and the Upper Cretaceous sandstones also reveal distinct Proterozoic zircon populations and unradiogenic Late Cretaceous zircons. The Skagit metasedimentary rocks yield zircon-age signatures that fall into two groups: (1) a wide range of zircon dates from Proterozoic to latest Cretaceous and (2) a more limited range of Late Triassic to latest Cretaceous grains with no Proterozoic zircons. Both groups reveal a mix of ε Ηfi values. The Swakane metasedimentary rocks have similar detrital zircon age signatures to Group 1 Skagit metasediments. For Swakane rocks, >100 Ma zircons have radiogenic ε Ηfi values, whereas younger zircons plot between near-depleted mantle to unradiogenic values. Overall, the data are most consistent with some metasedimentary rocks of the Swakane and Skagit Gneisses being sourced from either the forearc or the accretionary wedge. This sedimentary material was buried to mid-crustal depths by ca. 75–65 Ma, coeval with major magmatism within the North Cascades arc. Moreover, the distinct combination of unradiogenic Late Cretaceous detrital zircons and ca. 1.4–1.3 and 1.8–1.6 Ga Proterozoic peaks is documented in many of the forearc and accretionary-wedge units exposed along western North America. The Proterozoic peaks likely reflect zircon derived from southwestern Laurentian crust, equivalent to the latitude of the present-day Mojave Desert. Therefore, the detrital-zircon results from both the Swakane and Skagit Gneisses, as well as parts of the accretionary wedge, support at least moderate translation of sedimentary material along the margin of western North America during the Late Cretaceous.

ABSTRACT This guide describes a three-day field trip to the Paleogene sedimentary and volcanic rocks exposed between the Straight Creek–Fraser River and Entiat faults in the central Washington Cascades. These rocks record a history of deposition, deformation, and magmatism that can be linked to tectonic events along the North American margin using a robust chronology coupled with detailed sedimentological, stratigraphic, and structural studies. These events include deposition in a large sedimentary basin (Swauk basin) that formed in the forearc from <59.9–50 Ma; disruption and deformation of this basin related to the accretion of the Siletzia oceanic plateau between 51 and 49 Ma; the initiation, or acceleration of right-lateral, strike-slip faulting and the development of at least one strike-slip sedimentary basin (Chumstick basin) starting ca. 49 Ma; and the re-establishment of a regional depositional system after ca. 45–44 Ma (Roslyn basin) as strike-slip faulting was localized on the Straight Creek–Fraser River fault. These events are compatible with the presence of the Kula-Farallon ridge near the latitude of Washington ca. 50 Ma and its southward movement, or jump, following the accretion of Siletzia. This trip visits key outcrops that highlight this history and links them to regional studies of sedimentation, faulting, and magmatism to better understand the geologic record of this tectonic setting.

ABSTRACT The Methow, Chelan, Wenatchee, and other terrane blocks accreted in late Mesozoic to Eocene times. Methow valley is excavated in an exotic terrane of folded Mesozoic sedimentary and volcanic rocks faulted between crystalline blocks. Repeated floods of Columbia River Basalt ca. 16 Ma drowned a backarc basin to the southeast. Cirques, arêtes, and U-shaped hanging troughs brand the Methow, Skagit, and Chelan headwaters. The late Wisconsin Cordilleran ice sheet beveled the alpine topography and deposited drift. Cordilleran ice flowed into the heads of Methow tributaries and overflowed from Skagit tributaries to greatly augment Chelan trough’s glacier. Joined Okanogan and Methow ice flowed down Columbia valley and up lower Chelan trough. This tongue met the ice-sheet tongue flowing southeast down Chelan valley. Successively lower ice-marginal channels and kame terraces show that the ice sheet withered away largely by downwasting. Immense late Wisconsin floods from glacial Lake Missoula occasionally swept the Chelan-Vantage reach of Columbia valley by different routes. The earliest debacles, nearly 19,000 cal yr B.P. (= 19.0 k.y.), raged 335 m deep down Columbia valley and built high Pangborn bar at Wenatchee. As Cordilleran ice blocked the northwest of Columbia valley, several giant floods descended Moses Coulee and backflooded up the Columbia. As advancing ice then blocked Moses Coulee, Grand Coulee to Quincy basin became the westmost floodway. From Quincy basin many Missoula floods back-flowed 50 km upvalley past Wenatchee 18–15.5 k.y. ago. Receding ice dammed glacial Lake Columbia centuries more—till it burst ~15 k.y. ago. After Glacier Peak ashfall ~13.6 k.y. ago, smaller great flood(s) swept down the Columbia from glacial Lake Kootenay in British Columbia. A cache of huge fluted Clovis points had been laid atop Pangborn bar (East Wenatchee) after the Glacier Peak ashfall. Clovis people came two and a half millennia after the last small Missoula flood, two millennia after the glacial Lake Columbia flood. This timing by radiocarbon methods is under review by newer exposure dating— 10 Be, 26 Al, and 36 Cl methods.

ABSTRACT The rich Quaternary history of the Pacific Northwest showcases the important linkages between multiple geologic processes that have shaped its sedimentology and geomorphology. This field trip in eastern Washington explores the evolution of landforms that developed within the Palouse and the Channeled Scabland—geomorphically distinctive areas that were indirectly and directly influenced by multiple Quaternary glacial outburst megafloods. These floods produced expansive fine-grained sediments that were subsequently remobilized by the wind to generate sand dunes, sand sheets, and the thick loess of the Palouse. Landforms and deposits that date from the Last Glacial Maximum (LGM) include dramatically eroded scab-land features, coarse-to fine-grained flood slackwater deposits, sand dunes, loess, and paleosols. Sedimentary, paleopedologic, and geomorphic evidence for similar magnitude glacial outburst megafloods and loess accumulation that are related to the penultimate glaciation, during oxygen isotope stage 4, is recorded in several loess outcrops. This field trip traces the windblown sediments from source to sink and particularly focuses on Eureka Flat—the engine of the Palouse loess—and well-studied sections of thick loess farther downwind. A rich paleoclimate record is emerging from the loess stratigraphy and paleosols based on luminescence ages, tephrochronology, and paleoecologic analyses.

ABSTRACT The Salmon River suture zone is the boundary between the accreted (Blue Mountain) terranes and cratonic North America in western Idaho. This region was the focus of study by the EarthScope IDOR (IDaho-ORegon) project that integrated structural geology, geochemistry, geochronology, and seismology. This field trip traverses from western Idaho to eastern Oregon, covering the Atlanta lobe of the Idaho batholith, Blue Mountains terranes, and the middle Cretaceous western Idaho shear zone that separates these two domains. The main component of the Atlanta lobe is the Atlanta peraluminous suite, and it intruded from 83 to 65 Ma, was derived from crustal melting, and lacks a regionally consistent fabric. The crust below the Idaho batholith is relatively thick and seismic velocities are consistent with the entire crust being relatively felsic. The western Idaho shear zone overprints the Salmon River suture zone and obscures most evidence for the suturing. It is the present boundary between Blue Mountains terranes and cratonic North America. From studies along this transect, we have determined that the western Idaho shear zone exhibits dextral transpressional deformation, was active from ca. 103 to 90 Ma, and magmatism occurred during deformation; presently exposed levels on this transect record deformation conditions of 730 °C and 4.3 kbars. There is an ~7 km vertical step in the Moho at or slightly (<20 km) east of the current exposure of the western Idaho shear zone, separating thicker crust to the east from thinner crust to the west. Blue Mountains terranes immediately outboard of the western Idaho shear zone likely were located farther south during the middle Cretaceous and underwent strike-slip displacement during western Idaho shear zone deformation. The Olds Ferry terrane—the accreted terrane located immediately west of the western Idaho shear zone—was underplated by mafic magmatism, likely in the Miocene during eruption of the Columbia River basalt group. The field trip will utilize StraboSpot, a recently developed digital data system for structural geology and tectonics, so participants can investigate the relevant data associated with the IDOR EarthScope project.