Pacific Northwest geologic mapping and urban hazards project presents research results at GSA 2002, Corvallis BASALT OF HUNTZINGER, ASOTIN MEMBER OF SADDLE MOUNTAINS BASALT, COLUMBIA RIVER BASALT GROUP (CRBG) IDENTIFIED IN WESTERN WASHINGTON BEESON, Marvin H., Geology Department, Portland State Univ, P.O. Box 751, Portland, OR 97207, m.h.beeson@att.net and TOLAN, Terry L., Kennedy/Jenks Consultants, 1020 N.Ceter Parkway, Suite F, Kennewick, WA 99336 Geologic mapping in the Abernathy Creek and Germany Creek valleys, adjacent to the Columbia River, in western Washington has identified a CRBG flow as the basalt of Huntzinger on the basis of chemical composition (8 analyses), magnetic polarity, stratigraphic position and lithology. Preliminary field mapping indicates that the Huntzinger flow is up to 30 m-thick and it originally covered at least 75 km2in this area. This flow appears to pinch out near the Columbia River and several kilometers west of Abernathy Creek, but its extent to the north, and east of Germany Creek has not been determined. The Huntzinger flow occurs between two fluvial sedimentary interbeds that thicken north of the Columbia River for about 6 kilometers. The lower interbed (up to 60 m-thick) overlies a Sand Hollow flow of the Frenchman Springs Member (Wanapum Basalt). The upper interbed (up to 25 m-thick) underlies the Pomona Member (Saddle Mountains Basalt). Published radiometric ages for Pomona, Huntzinger, and Sand Hollow units are 12, 13, and 15.3 Ma respectively. Interbed and flow thicknesses suggest that this area was subsiding during Saddle Mountain time. This discovery represents the only known occurrence of the Huntzinger flow west of the Cascade Range. The Huntzinger flow followed an ancestral Columbia River canyon through the Cascade Range and spilled out onto a lowland underlain by fluvial sediments accumulated in a structural depression active at least between 12 and 15 m.y ago.

THE BUMP AND GRIND OF CASCADIA FOREARC BLOCKS: EVIDENCE FROM GRAVITY AND MAGNETIC ANOMALIES BLAKELY, Richard J.1, WELLS, Ray E.2, WEAVER, Craig S.3, MEAGHER, Karen L.3, and LUDWIN, Ruth4, (1) MS 989, U.S. Geol Survey, 345 Middlefield Rd, Menlo Park, CA 94025, blakely@usgs.gov, (2) US Geol Survey, 345 Middlefield Rd, Menlo Park, CA 94025-3561, (3) U.S. Geol Survey, Box 351310, University of Washington, Seattle, Seattle, WA 98195, (4) Dept. of Earth and Space Sciences, Box 351310, University of Washington, Seattle, Seattle, WA 98195 Arc-parallel migration of the Cascadia forearc causes relative motion between discrete crustal blocks, and we hypothesize that this motion generates upper-plate earthquakes.The high density and magnetization of mafic rocks (Siletzia) underlying the forearc produce high-amplitude, regional-scale gravity and magnetic anomalies well suited for mapping forearc blocks. We have used simple graphics software to jointly analyze gravity and magnetic data in order to estimate the lateral limits of Siletzia and delineate boundaries between discrete forearc blocks. With the possible exception of disruption by the Corvallis fault, Siletzia in Oregon appears to behave as a continuous block, extending offshore along the Oregon coast and eastward to underlie the entire Willamette Valley. At the Columbia River, Siletzia is offset along a northwest line that includes the Portland Hills-Clackamas River structural zone. Beneath Washington and Vancouver Island, Siletzia is segmented into eight to ten blocks, variously uplifted or down-dropped, reflecting compression of the Washington forearc ahead of the northward-advancing Oregon block. Block boundaries coincide with major faults, notably the Devils Mountain, Seattle, Tacoma, Olympia, and Doty faults. Earthquake locations and crustal blocks sometimes coincide in Puget Sound, but the relation remains problematic. For example, nearly all crustal earthquakes in Puget Sound occur northeast of the Olympia fault and east of the Hood Canal fault, but almost no earthquakes are known between the Doty and Olympia faults. Thus, block boundaries do not always control seismicity. The uncertainty between geophysical anomalies and crustal seismicity extends to the region between the Siletzia forearc blocks and the Cascade arc. Based on gravity and magnetic anomalies, we hypothesize that the Doty and Olympia faults are connected to the St. Helens seismic zone, but this is not apparent from the seismicity. The lack of correlation between earthquakes and block boundaries may reflect vastly different time spans: The historic record of earthquakes is brief compared with the geologic record represented in potential-field anomalies. Reconciling the geophysical observations with the earthquake distribution is needed before the block model can be incorporated into regional hazard assessments.

THE CANBY-MOLALLA FAULT, OREGON BLAKELY, Richard J., MS 989, U.S. Geol Survey, 345 Middlefield Rd, Menlo Park, CA 94025, blakely@usgs.gov, MADIN, Ian P., Oregon Department of Geology and Mineral Industries, 800 NE Oregon Street #28, Suite 965, Portland, OR 97232, STEPHENSON, William J., U.S. Geological Survey, Box 25046, MS 966, Denver, CO 80225, and POPOWSKI, Thomas, Northwest Geophysical Associates, Inc, 1600 Western Blvd, Corvallis, OR 97333 We have conducted a variety of geologic and geophysical investigations to determine the offset on an inferred fault marked by a remarkably linear aeromagnetic anomaly near the towns of Canby and Molalla, Oregon. The aeromagnetic lineament strikes north-northwest, is 60 km long, and is on strike at its northern end (about 25 km south of Portland) with faults mapped in Columbia River basalt (CRB). We have measured four detailed ground-magnetic profiles, collected 1000 additional ground-magnetic stations, conducted a seismic-reflection profile, and compiled logs from more than 70 water wells to further define the cause of the aeromagnetic lineament. A geologic cross-section, based on a detailed ground-magnetic profile east of Canby, suggests that the aeromagnetic lineament is caused by at least 150 m of near vertical, down-to-the-west offset of CRB, the only significant magnetic lithology near the surface in this area. This model is consistent with water wells from the area: Two wells immediately east of the fault penetrated CRB at 50 m depth, whereas CRB was not encountered in any wells (150 m maximum depth) west of the fault. In addition to vertical displacement, lateral offset of a small magnetic anomaly at the northern end of the aeromagnetic lineament indicates approximately 4 km of right-lateral displacement. We also conducted a high-resolution seismic-reflection transect across the aeromagnetic anomaly east of Canby. The transect was 600 m in length, was acquired with 2 m shot and receiver spacings, and provided 30-fold stacked coverage. The seismic cross section shows near-vertical truncation of CRB, consistent with the ground magnetic model. Youngest deformed sediments in the footwall are probably Missoula flood deposits, raising the possibility of Holocene deformation. A small, laterally restricted berm lies within 100m of our estimated location of the fault and also suggests young deformation. The linearity of the aeromagnetic anomaly, its association at its northern end with mapped faults in CRB, and the abrupt offset demanded by ground-magnetic, seismic, and well data indicate that the anomaly is a 60-km-long oblique strike-slip fault. Given its proximity to Portland and Salem, additional studies are warranted to assess possible Holocene deformation.

SPATIAL DATABASE FOR ENHANCED GEOLOGIC MAPPING IN SEATTLE, WA BLOUKE, Kristen J.1, SHIMEL, Scott A.2, 2, O'NEAL, Michael A.2, and BOOTH, Derek B.2, (1) Earth and Space Sciences, Univ of Washington, Box 351310, Seattle, WA 98195-1310, blouke@u.washington.edu, (2) Department of Earth and Space Sciences, Univ of Washington, Box 351310, Seattle, WA 98195-1310, ktroost@u.washington.edu Geologic mapping in urban areas poses a conundrum: little bare ground is available for inspection, but subsurface data, primarily in the form of geotechnical explorations, are abundant. Most of these data, however, are widely scattered and poorly organized in building and utility departments, transportation agencies, and private consulting firms. The Seattle-Area Geologic Mapping Project has developed, and is now populating, a GIS-based relational database to efficiently store, manipulate, and display the vast amount of existing subsurface geologic data across the Seattle area to enhance and facilitate the creation of geologic maps.The database is designed to accommodate geologic data from a variety of sources and formats, to create a common interface for entering and displaying data, and to support current and future scientific and engineering studies. Geologic information from tens of thousands of field explorations, exposures, and excavations have been entered into the database and include such information as location, geologic layer type and depth, and material density and type. The database contains"raw" data as well as fields for geologic interpretation and for the metadata on original source documents, original scale, and data quality. The data, currently stored in ArcView shapefile format and an Access database, are entered and accessed through customized ArcView interfaces. The database is now being converted to an ArcSDE geodatabase with an Oracle database backend. Customized tools have been developed to facilitate the visualization and interpretation of raw data by interactively viewing the subsurface geologic data through a series of cross sections, stick logs, and fence diagrams at user-selected locations. Partnerships have been formed with a number of local public agencies (such as building departments, public utilities, and transportation agencies) both to acquire the raw data from geologic and geotechnical studies and to return the populated database and GIS interface to those agencies. The information will then be readily available to engineers, planners, and the public, identifying locations where non-proprietary geologic data can be reviewed to improve subsequent investigations in the vicinity of existing studies.

GEOLOGIC MAPPING AT 1:12, 000 SCALE ACROSS THE CITY OF SEATTLE BOOTH, Derek B., , and SHIMEL, Scott A., Department of Earth and Space Sciences, Univ of Washington, Box 351310, Seattle, WA 98195-1310, dbooth@u.washington.edu Detailed geologic maps of the City of Seattle are being prepared to provide both the raw data and the geologic interpretations needed to characterize the material properties and topography of geologic materials within the most densely populated part of the Puget Lowland of western Washington.The same urban development that has obscured much of the ground surface in this urban environment has also created a tremendous source of near-surface and subsurface data, permitting abundant verification of observed or inferred surficial deposits. We are compiling a GIS/Oracle database that houses raw data, metadata, and interpreted geologic information for many thousands of data points. From these data we are producing a set of "traditional" surficial geologic maps, but with a level of detail and supporting data that are unprecedented in the region. Using as our template the four 7.5-minute quadrangles that almost entirely cover Seattle, map production is proceeding at a rate of about one quadrangle per year. The first two quadrangles, Seattle SW and Seattle NW, demonstrate the utilization and display of surface exposures and subsurface data sources; the integration of analytic methods (on these maps, radiometric dating, thermoluminescence dating, and diatom and pollen analyses); and the incorporation of geophysical information, most critically here in the delineation of the Seattle fault zone as it traverses the populated areas of southwest Seattle. From this variety of data sources, we are producing 1:12, 000-scale geologic maps, digitized both for GIS display and for incorporation in subsequent 3-D representations. Collaboration with other geoscientists in the USGS, UW, WA DNR, private consultants, and other universities and state geological surveys has been invaluable, as has been the integration of this detailed mapping within the context of a regional geologic framework for the central Puget Sound region.

THE LANDSCAPE OF WESTERN WASHINGTON--OVERBURDEN AND UNDERBURDEN BOOTH, Derek B., Department of Earth and Space Sciences, Univ of Washington, Box 351310, Seattle, WA 98195-1310, dbooth@u.washington.edu. The landscape of western Washington owes its modern form to a varied tapestry of geologic processes and geologic materials. Elucidating the form of that landscape has been similarly complex--the myriad pieces of our current understanding continue to be contributed, piece by piece, by a tremendous range of geoscientists. Glacial geologists owe much to the tectonic processes that built the mountain ranges of the Cascades and Olympics, and which also have left an intervening lowland to allow for multiple invasions of glacial ice. That lowland environment has proven to be sufficiently polar to result in a rich glacial record, spanning much (if not all) of the Quaternary, but sufficiently equatorial to permit abundant meltwater and the resulting variety of landforms and deposits characteristic of temperate ice sheets. Great topographic relief and abundant moisture have also accelerated the postglacial processes of fluvial erosion and deposition, superimposing a postglacial fluvial landscape that belies the relatively brief period in which these processes have been active. The effects of this unique geologic history and suite of geologic deposits are expressed not only by the form of the modern landscape but also by the conditions in that landscape, affecting geomorphic processes, biological systems, and human activity alike. Riverine systems, for example, have emerged as a preeminent concern of the entire Pacific Northwest. Their response to both the unintentional consequences of human populations and the intentional manipulations of engineering works, commonly in the name of economic or ecosystem enhancement, are intimately tied to the geologic setting and associated geomorphic activity. Geologic hazards, as another example, are arrayed very unevenly across this region; our success in recognizing, preparing, and recovering from their effects depends on our understanding of their underlying causes and distribution. Finally, the region as a whole is a place of great variety and great beauty. It has enticed both scientists and non-scientists for well over a century, and it will always sustain our collective interest and enthusiasm for working here.

A SIMPLE ALGORITHM FOR SEQUENTIALLY INCORPORATING GRAVITY TO ENHANCE SEISMIC TRAVELTIME TOMOGRAPHY: APPLICATION TO THE UPPER-CRUSTAL STRUCTURE IN PUGET LOWLAND, WASHINGTON BROCHER, Thomas M., MS 977, US Geol Survey, 345 Middlefield Rd, Menlo Park, CA 94025-3561, brocher@andreas.wr.usgs.gov, PARSONS, Tom, MS 999, US Geol Survey, 345 Middlefield Rd, Menlo Park, CA 94025-3591, and BLAKELY, Richard J., MS 989, US Geol Survey, 345 Middlefield Rd, Menlo park, CA 94025 The earth's three-dimensional upper-crustal structure can be revealed by modeling variations in seismic first-arrival travel times and in potential field measurements, which is particularly useful in regions having sparse borehole-control and limited outcrops. In the Puget Lowland, Washington State, USA, we demonstrate a simple method, designed to improve our resolution of the 3-D geometry of Cenozoic basins, for sequentially satisfying seismic first-arrival traveltime and gravity residuals in an iterative 3-D inversion.The algorithm is portable to any seismic analysis method that uses a grided representation of velocity structure.Our technique calculates the gravity anomaly resulting from the velocity model by converting to density with Gardner's rule. The residual between calculated and observed gravity is minimized by weighted adjustments to the model velocity-depth gradient where the gradient is steepest and where seismic coverage is least. The adjustments are scaled by the sign and magnitude of the gravity residuals, and a smoothing step is performed to minimize vertical streaking. The adjusted model is then used as a starting model in the next seismic traveltime iteration. The process is repeated until one velocity model can simultaneously satisfy both the gravity anomaly and seismic traveltime observations within user-defined acceptable misfits. We test our algorithm with data gathered in the Seismic Hazards Investigation in Puget Sound (SHIPS) experiment. We perform resolution tests with synthetic traveltime and gravity observations calculated with a checkerboard velocity model using the SHIPS experiment geometry and show that the addition of gravity significantly enhances resolution.We calculate a new velocity model for the region using SHIPS traveltimes and observed gravity, and we show examples where correlation between surface geology and modeled subsurface velocity structure is enhanced. These improvements include enhanced resolution of the Everett basin, of the Olympic accretionary core complex, and of northwestern end of the Southern Whidbey Island fault. This technique can be readily applied to other areas having both tomography and gravity data, including the Georgia basin, the San Francisco Bay area, and the Los Angeles region in the Cordilleran forearc.

GEOLOGIC CROSS SECTIONS THROUGH THE ROSEBURG 30’ X 60’ QUADRANGLE, OREGON: NEW CONSTRAINTS FROM POTENTIAL FIELD MODELING DUROSS, Christopher B., Geology and Geophysics, Univ of Utah, Salt Lake City, UT 84105, cbduross@hotmail.com, BLAKELY, Richard J., US Geol Survey, 345 Middlefield Rd, Menlo Park, CA 94025-3561, and WELLS, Ray E., US Geol Survey, 345 Middlefield Rd, Menlo Park, CA 94025-3561 Geologic and potential-field data were examined to address fold and thrust structures in the Roseburg 30’ x 60’ quadrangle. Suturing of the Paleocene-Eocene oceanic Siletz terrane to the continent during early Eocene produced a fold and thrust belt in which the Jurassic and lower Cretaceous Dothan accretionary complex and Rogue volcanic arc were thrust NW over the Coast Range basalt basement. To elucidate the subsurface structure of the fold and thrust belt and the nature of the terrane boundary at depth, we constructed three geologic cross sections constrained by new geologic mapping, potential-field data, and deep exploratory wells. Restorable geologic cross sections were used as starting points for simultaneous modeling of aeromagnetic and gravity profiles in which structurally and genetically distinguishable units were characterized by their density, remnant magnetization, and magnetic susceptibility. Restoration of our cross sections indicates that the Siletz terrane and Umpqua Group have been shortened 26 km, or 37 percent. The detachment below the fold and thrustbelt deepens SE, from 1.8 km beneath the Umpqua basin to about 3.5 km near the suture. Geophysical profiles derived from the geologic models are in good agreement with observed gravity and magnetic anomalies over the Mesozoic terranes and the broad Umpqua arch of the Coast Range basement. On the Umpqua arch, we infer reversely magnetized subaerial basalt flows overlying an intrusive complex in order to satisfy the gravity high, aeromagnetic low, and depth to the Coast Range basalt basement based on well data. Near the suture, geophysical modeling suggests sediments (Paleocene-Eocene strata?) are thicker in the lower plate than previously recognized. The terrane boundary dips southeast 30° at depth (> 900 m) and steepens to 70° at the surface. The considerable thickness and depth of the Umpqua Group clastic sedimentary rocks and the nature of the complex syn-depositional structures found throughout them may have significant implications for hydrocarbon exploration.

COMPLEX RIGHT-LATERAL FAULTING AT THE NORTHERN END OF THE PORTLAND BASIN: GEOLOGIC, AEROMAGNETIC, AND PALEOMAGNETIC EVIDENCE EVARTS, Russell C., HAGSTRUM, Jonathan T., BLAKELY, Richard J., FLECK, Robert J., BLOCK, Jessica L., and DINTERMAN, Philip A., US Geol Survey, 345 Middlefield Rd, Menlo Park, CA 94025, revarts@usgs.gov New geologic mapping in and adjacent to the northern Portland basin, in concert with aeromagnetic and paleomagnetic studies and 40Ar/39Ar dating, is revealing new details of the complex geologic structure. Late Eocene (c. 37-35 Ma) volcanogenic strata of the Cascade Range in southwestern Washington are regionally deformed into large-amplitude SE-plunging folds, and bedding generally dips E and S at about 20°. A few km E of the Columbia River near Kalama, WA, however, bedding swings abruptly to the NNW. Across the river in Oregon, sparse measurements indicate that strikes return to a roughly E-W orientation.This pattern suggests the Paleogene rocks are deformed into a large drag fold by right-lateral movement on a fault beneath the NNW-trending Columbia River valley. This structure is also reflected in a complex, NW-striking aeromagnetic anomaly that crosses the Columbia River about midway between Kalama and Saint Helens, OR. South of Kalama, exposures of middle Miocene Grande Ronde Basalt are plastered against valley walls on both sides of the Columbia River. Near Saint Helens, basalt outcrops exhibiting scabland morphology poke through alluvium of the valley floor. Chemical data indicate that many of these outcrops are erosional remnants of a single Grande Ronde flow that once filled the ancestral valley. Paleomagnetic measurements from 8 sites in this flow indicate that the flat-lying rocks on the valley floor are rotated 20-30° clockwise relative to outcrops on the valley walls. We interpret the valley-floor rocks as microblocks caught up in a broad right-lateral shear couple. The geologic and paleomagnetic results reflect deformation within a complex system of N- to NW-striking faults at the northern end of the Portland basin inferred from linear aeromagnetic anomalies. Motion on these structures is predominantly right-lateral strike-slip, although high-angle reverse offset is inferred for some of them. Collectively, these dextral oblique faults may have accommodated a significant part of the northward translation of the Oregon Coast Range crustal block relative to interior North America that is recorded by transpressive structures in the Washington Coast Range. Sparse geodetic data indicate that this motion is continuing. Thus some or all these faults should be considered as potentially active.

THE BORING VOLCANIC FIELD OF THE PORTLAND, OREGON AREA: GEOCHRONOLOGY AND NEOTECTONIC SIGNIFICANCE FLECK, R. J.1, EVARTS, R. C.1, HAGSTRUM, J. T.1, and VALENTINE, M. J.2, (1) U.S. Geol Survey, 345 Middlefield Rd, Menlo Park, CA 94025, fleck@usgs.gov, (2) Geology Dept, Univ. Puget Sound, Tacoma, WA 98416 Dozens of young monogenetic volcanoes lie scattered west of the axis of the Cascade volcanic arc in NW Oregon and SW Washington. They are most abundant near Portland, where they have been informally called the Boring Lava. Here we refer to this collection of vents as the Boring volcanic field (BVF). Most flows are relatively primitive olivine-phyric basalts and basaltic andesites. Compositions range widely from low-K tholeiitic to high-K calcalkaline. New 40Ar/39Ar incremental-heating whole-rock age determinations from the northern part of the BVF generally confirm the late Pliocene and Quaternary age of volcanism as indicated by previous K-Ar dating. Paleomagnetic polarities from dated flows corroborate the 40Ar/39Ar ages. However, 40Ar/39Ar results also show that many of the basalts contain excess 40Ar, yielding eruptive ages younger than K-Ar ages from the same flows. Patterns of Ar release suggest at least three discrete"reservoirs" of 40Ar in these samples, with the excess component resulting in U-shaped age spectra. The new results demonstrate that volcanic activity is more recent than previously thought. Three eruptive centers (BattleGround Lake, Rocky Butte, and in the Portland Hills) yield ages of about 100-125 ka. Combined with available K-Ar ages, these ages suggest that the area of active volcanism is expanding westward and northward with time, with the three youngest ages being obtained from vents near the NW margin of the BVF. The BVF occupies an anomalously near-trench, fore-arc setting within the Cascadia subduction system. Vents are not evenly distributed within the fore-arc but are concentrated within and along the periphery of the Portland basin, which has been interpreted as a Neogene pull-apart structure.We suggest that the distribution of vents in the BVF demarks a zone of localized crustal extension related to northward translation and clockwise rotation of the Oregon Coast Range microplate relative to interior North America. South of the BVF, extension is concentrated along the axis of the volcanic arc, whereas to the north tectonism is compressional.The BVF occurs in a transitional zone in which extension becomes less intense and more widely distributed. The age pattern in the BVF suggests that this transitional regime is slowly propagating to the NW with time.

STRATIGRAPHIC RECORD OF CRETACEOUS TECTONICS IN THE METHOW BLOCK, NORTH CASCADES, WASHINGTON HAUGERUD, Ralph A., U.S. Geol Survey, Dept. Earth & Space sciences, University of Washington, Box 351310, Seattle, WA 98195, rhaugerud@usgs.gov, TABOR, Rowland W., U.S. GeolSurvey MS 975, 345 Middlefield Road, Menlo Park, CA 94025, and MAHONEY, J. Brian, Univ Wisconsin - Eau Claire, 105Garfield Ave, Eau Claire, WI 54701-4811 We divide strata of the Methow block south of 49º N into 5 Cretaceous-Tertiary sequences. (1) Fossiliferous, volcaniclastic, shallow marine to fluvial Lower Cretaceous strata include rocks at Dead Lake (Albian), the Patterson Lake unit (Albian), the type Panther Creek unit of Barksdale (1975)(Valanginian?), and sedimentary rocks on Buck Mtn (Barremian or older to Albian). We found no proximal volcanic rocks of this age. In the Lightning Creek drainage this unit is in part Jurassic. (2) Overlying Three Fools sequence comprises east-derived, quartzofeldspathic, mid-Albian, proximal turbidites of the Harts Pass Formation, overlain by--and inferred to interfinger with--the west-derived chert-clast-rich Jackita Ridge unit, also turbiditic but thinner bedded and siltier than the Harts Pass. Rare ammonites in the Harts Pass are mid-Albian; inoceramids from the Jackita Ridge are Albian to Cenomanian. (3) Pasayten Group overlies the Three Fools sequence, locally on an angular unconformity. Constituents of the Pasayten Group are west-derived, chert-clast-rich, and largely fluvial Virginian Ridge Fm; east-derived, arkosic, and largely fluvial Winthrop Fm; and redbeds and volcanic rocks of the Goat Wall unit. We abandon the Midnight Peak Fm of Barksdale (1975): volcanic rocks at its type locality are Three AM member of the Winthrop Fm; stratigraphically higher beds near Mazama are Goat Wall unit. (4) Campanian-Maastrichtian conglomerate of the Pipestone Canyon Formation is fluvial, largely proximal, and rich in rhyolite debris. It is overlain by undated basalt. (5) Isolated, mountain-capping, hornblende-phyric volcanic rocks (e.g. Storey Peak, Last Chance Point) may correlate with dated middle Eocene rocks farther east. Basin deepening at the transition between sequences 1and 2 dates the onset of contraction in this part of the North Cascades as early to mid-Albian. Three Fools sequence (2) and Pasayten Group (3) are upwards-shallowing foreland-basin fill developed in front of, and cannibalized by, the east-vergent Hozameen-Chuwanteen thrust system. After Goat Wall time (~87 Ma) this region was within the Coast Belt hinterland. Pipestone Canyon Fm (4) records subsequent wrench faulting that in part defines the Methow block.

HIGH-RESOLUTION PUBLIC-DOMAIN TOPOGRAPHY FOR WESTERN WASHINGTON: THE PUGET SOUND LIDAR CONSORTIUM HAUGERUD, Ralph A., U.S. Geological Survey, Dept. Earth& Space Sciences, University of Washington, Box 351310, Seattle, WA 98195, rhaugerud@usgs.gov. Lidar surveys contracted by the Puget Sound Lidar Consortium (PSLC, includes local government staff and USGS and NASA researchers, http://pugetsoundlidar.org) will have covered 8, 000 km2 of the central, western, and southern Puget Lowland by May 2002, barring bad weather and equipment failure. Topography with 6 ft (2 m) X-Y resolution and Z accuracy of ~30 cm, except in areas of extremely dense second-growth forest, is calculated from data obtained with a small-footprint, discrete-return airborne laser scanner that generates 1 pulse/m2 and records up to 4 returns/pulse. We began late in 1999 with a shared goal of identifying Holocene fault scarps to be trenched for seismic hazard studies. To date we have identified 5probable Holocene fault scarps. Harding and others (this meeting) describe details of Holocene deformation extracted from lidar topography of the Seattle fault zone. Lidar topography has other geologic uses, including: 1) Landslide inventory: large deep-seated slides can be mapped consistently. Small shallow slides are not mappable, but the steep slopes and contributing areas that control their occurrence are. 2) Geomorphic mapping that assigns origins and relative ages to all topographic surfaces. Where constructional landforms dominate, such a map is an excellent proxy for a geologic map. 3) A topographic base for detailed conventional geologic mapping at scales as large as 1:6, 000. 4) Quantitative geomorphology: one can extrapolate to end-Vashon (15 ka) landforms and quantify changes since then. Long-term rates of stream erosion, landsliding, and other processes can be evaluated. Lidar data are also being used to address non-geologic concerns including hydrology, transportation planning, and endangered species habitat. Characterization of forest canopy from vegetation returns (2/3 of the total) may eventually prove to be the most valuable use of the data. We hope to cover the entire Puget Lowland, but face several challenges. Funding is not sustained. Strategies for surveying high-relief areas need to be developed. Tools to monitor data completeness, consistency, and accuracy are incomplete. Procedures for classifying returns as forest, structure, ground, or blunder need to be improved and automated. Delivery of voluminous lidar data to the public is daunting.

FOLDING AND RUPTURE OF AN UPLIFTED HOLOCENE MARINE PLATFORM IN THE SEATTLE FAULT ZONE, WASHINGTON, REVEALED BY AIRBORNE LASER SWATH MAPPING HARDING, David J., Geodynamics Branch, NASA's Goddard Space Flight Ctr, Mail Code 921, Greenbelt, MD 20771, harding@core2.gsfc.nasa.gov, JOHNSON, Sam Y., USGS, MS 966, Box 25046, Denver, CO 80225, and HAUGERUD, Ralph A., USGS, Box 25046, Dept. of Geological Sciences, University of Washington, Seattle, WA 98195 The Seattle Fault zone (SFZ) is a 5 to 7 km-wide, east-west trending zone of south-dipping thrust faults, north-dipping back thrusts, and folds. Puget Sound marine terraces record uplift of a wave-cut platform about A.D. 900 (Bucknam et al., 1992) during one, or probably several, earthquakes in the SFZ (Nelson et al., 2000). Airborne Laser Swath Mapping (ALSM) surveys of Kitsap County and part of King County document spatial patterns of terrace uplift, as measured from shoreline angles at the landward edge of the uplifted platform. The shoreline angles are mapped using profiles and slope images interactively generated from ALSM "bald earth" DEMs with a 1.8-m grid spacing and decimeter-level vertical accuracy. On the east side of Puget Sound, terraces (up to 8 m above MHHW) from Williams Point to north of Alki Point define a 5-km-wide, north-vergent anticline with planar limbs. The fold amplitude is 6 m, and the limbs dip more steeply north (0.25°) than south (0.10°). The hinge, located at Alki Point, is above the SFZ Frontal thrust suggesting slip on it during the ~ A.D. 900 earthquakes produced the fold. Three 1 to 2 m-high south-side-up terrace offsets on the south limb suggest lesser slip occurred on more southerly thrusts during, or after, the Frontal thrust events. The offsets align with the projection of three thrusts mapped offshore in seismic-reflection profiles. Western Puget Sound terraces also define a broad anticline extending 13 km west from southern Bainbridge Island to Dyes Inlet, but the structure is more complex with significant along-strike heterogeneity over short distances. The anticline hinge is locally modified by back thrusts producing a tightened, south-vergent fold with an amplitude as large as 8 m. The hinge in the Bainbridge Island terrace coincides with the Toe Jam Hill back thrust scarp in the island interior. On the south limb a terrace around Waterman Point, southwest of Bainbridge Island, is ruptured by two backthrusts expressed as3-m-high, south-facing scarps. Terrace rupture indicates that slip occurred on the backthrusts during the ~ A.D. 900earthquakes, consistent with dated surface-faulting events along the Toe Jam Hill fault scarp. Farther south, localized terrace uplifts as large as 3 m suggest slip may have also occurred on two unnamed thrusts within the SFZ during, or after, the ~ A.D. 900 events.

LATE JURASSIC TERRANE LINKAGES IN THE NORTH CASCADES: NEWBY GROUP IS QUESNELLIA? MAHONEY, J. Brian, Univ Wisconsin - Eau Claire, 105 Garfield Ave, Eau Claire, WI 54701-4811, mahonej@uwec.edu, HAUGERUD, Ralph A., U.S.Geol Survey, Dept. Earth & Space Sciences, University of Washington, Box 351310, Seattle, WA 98195, FRIEDMAN, Richard M., Univ British Columbia, 6270 University Blvd, Vancouver, BC V6T 1Z4, Canada, and TABOR, Rowland W., U.S. Geol Survey MS 975, 345 Middlefield Road, Menlo Park, CA 94025 The Newby Group is a Late Jurassic volcanic arc assemblage at the base of the southern Methow terrane in the eastern North Cascades. The Newby underlies thick clastic strata of the Methow basin, an overlap assemblage that links Methow, Bridge River and Cadwallader terranes by mid-Cretaceous time. Our work suggests the Newby Group represents a volcanic arc carapace to mid-crustal metamorphic and plutonic assemblages of the western Intermontane superterrane. We divide the Newby into several lithostratigraphic units, including: (1) black argillite, volcanic sandstone and minor calcarenite of the Twisp Formation of Barksdale (1975);(2) laumontite-bearing andesitic breccias, tuff, pelite, and minor flows of the Bear Creek unit; (3) extensive silicic to intermediate volcanic breccia, tuff, and flows with intercalated volcaniclastic strata of the Lookout Mountain unit, which grade SE into (4) low-grade, mostly silicic, phyllite and schist of the McClure Mountain unit. The base of the Newby is not exposed and the underlying basement is unknown. Isotopic (eNd @ 6-7) and geochemical data (e.g. LaN/YbN @ 1.0-2.5) suggest the Newby represents an uncontaminated island arc assemblage.Fine tuff intercalated with black argillite in the upper Twisp Formation, immediately below a gradational contact with overlying Bear Creek unit, yields a U/Pb zircon age of 151.0 +8.7/-0.3 Ma (Late Jurassic). A rhyolite dike in tuff of the Lookout Mountain unit yields a U/Pb zircon age of 152.8 ±0.9 Ma. Mylonitic quartz diorite within the McClure Mountain unit has a U/Pb age of 142.8 +0.9/-0.3 Ma. Steep, N-S foliation of the McClure Mountain unit is cut by the syn- to post-kinematic Alder Creek stock (141.6 +1.0/-0.3 Ma; U/Pb zircon). Subjacent tonalite to granodiorite plutons include the Button Creek stock, Alder Creek stock, Golden Doe Ranch stock and granodiorite of Tice Ranch. Ages from these bodies effectively date magmatism and intra-arc deformation as circa 140-155 Ma. The granodiorite of Tice Ranch (152.8 ±1.6Ma, U/Pb zircon) intrudes the eastern edge of the Newby Group and gabbro and amphibolite at the west edge of the Okanogan batholith. This intrusive relationship ties the Methow terrane to the Intermontane superterrane by Late Jurassic time.

AN EXAMPLE OF LATE EOCENE/EARLY OLIGOCENE SEA FLOOR TOPOGRAPHY IMAGED BY 3D SEISMIC DATA, MIST GAS FIELD, AND IN OUTCROP, N. COAST RANGE, OREGON MEYER, H. Jack, Gas Storage Department, NW Nat, 220 NW Second Ave, Portland, OR 97209, h2m@nwnatural.com and NIEM, Alan R., Geosciences, Oregon State Univ, 104 Wilkinson Hall, Corvallis, OR 97331 In the Mist Gas Field major unconformities are present on top of the middle to late Eocene Cowlitz, u. Eocene/l. Oligocene Keasey, and Sager Creek Fms. A 10 sq. km, high frequency, small bin (12 m) 3D seismic survey in the Mist Gas Field in Columbia Co., Oregon imaged the contact between the Keasey Fm. and the overlying Sager Creek Fm. The Keasey/Sager Creek surface resembles a deeply incised submarine channel system on the continental slope. The major channel in the 3D map area is 200 m deep and 1400 m wide. Smaller channels feed into the main channel. In the Mist Gas Field, the Keasey Fm. is composed of massive to laminated, deep marine (bathyal) tuffaceous clay rich silt st. with some glauconite ss stringers. The overlying channel-filling Sager Creek Fm. consists of deep marine (bathyal) thin, well bedded laminated micaceous ss, siltst. and mudst. In outcrop, w. of the gas field, nested channels of Sager Creek Fm. contain concretionary rhythmically thin bedded to laminated, v. f. gr. micaceous, carbonaceous lithic arkosic turbidite ss (Bouma cde sequences), and foram-bearing burrowed-laminated micaceous mudst. with flame and load structures. The 35 km long wedge-shaped turbidite unit (400 m thick) is channelized into the Keasey Fm. and may have been fed from the east by the Pebble Ck. delta front facies of the Pittsburg Bluff Fm. On well bore image logs the formational contact is sharp and distinct. Immediately above the contact a zone of mud-chip conglomerate or chaotic bedding of variable thickness (1m to 4 m) is overlain by laminated beds. Well bore radioactive logs suggest the mineralogy of the micaceous Sager Creek Fm. has an affinity to the older micaceous Cowlitz Fm. As the tuffaceous forearc strata of the Keasey Fm. were sourced from initial Western Cascade arc volcanism, the change in mineralogy may represent cessation of volcanism and reestablishment of regional drainage to the granitic/meta.source rocks east of the Cascades. L. Eocene normal faulting is the primary trapping mechanism in the Mist Gas Field.Seismic data clearly show that the trapping faults terminate before reaching the Keasey/Sager Creek surface. Reactivated post-m. Miocene conjugate oblique-slip faults have minor offsets in outcrop. The 3D imaged surface therefore is close in appearance to the original l. Eocene/e. Oligo. sea floor topography.

A SYNTHESIS OF THE ROSS LAKE FAULT SYSTEM AND ITS BEARING ON THE BAJA BRITISH COLUMBIA MODEL MILLER, Robert B., Dept. of Geology, San Jose State Univ, San Jose, CA 95192-0102, rmiller@geosun.sjsu.edu, HAUGERUD, Ralph A., U.S. Geol Survey, Dept. Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195, and DRAGOVICH, Joe D., Washington Div. Geology& Earth Resources, Olympia, WA 98504 The > 10-km-wide Ross Lake fault system (RLFS) is part of a 500-km-long zone of high-angle faults in the northern Cordillera. The RLFS consists of two major sets of faults. The eastern set of the Hozameen and Slate Creek faults and more southerly North Creek fault form the western boundary of the Jura-Cretaceous Methow basin and in part separate it from metamorphic equivalents of Methow strata.Minor structures along the North Creek fault record dextral strike slip bracketed between ~ 88 and 50 Ma. The same formations lie on both sides of the faults implying modest slip (10s of km?). The northernmost strand of the western fault set, the Ross Lake fault (s.s.), is a vertical zone of horizontally-lineated mylonite that separates upper-amphibolite-facies rocks of the Cascades crystalline core from sub-greenschist-facies rocks to the east. Some dextral shear and 6-12km of NE-side down normal slip occurred from 50 (?) to post-45 Ma. At Elijah Ridge, the Ross Lake fault steps westward across a gently dipping extensional zone to the Gabriel Peak tectonic belt. This ~ 100-km-long, NE-dipping mylonite zone is dominated by flattening, but kinematic indicators record dextral shear in the north and reverse shear farther south. This transpressional deformation occurred from 65 Ma (and earlier?) to 58 Ma when at least 7-24 km of dextral slip was probably transferred to the eastern faults by ENE-striking shear zones. Younger (< 50 Ma) ENE-striking sinistral faults at least locally accommodated 5-10 km of dextral strike slip by vertical axis rotation. The fault sets merge southward to form the Foggy Dew fault zone where mylonites record oblique dextral-normal slip (down-to-E).Slip is bracketed between 65- to 48 Ma; some occurred after 60 Ma and the zone records the regional transition from~65-58 Ma transpression to ~57-45 Ma transtension. The fault zone is truncated to the SE by the 48 Ma Cooper Mtn.batholith, which also obliterates its intersection with the southern continuation of the Pasayten fault. South of this batholith, only a narrow, discontinuous shear zone is on strike with the Foggy Dew fault and similar units lie on both sides of this projection of the RLFS. We conclude that total dextral slip on the RLFS is less than several hundred kilometers and the system could not have been the major contributor to postulated large translation of Baja B.C.

BEDROCK ARCHITECTURE, NATURAL RESOURCES, AND GEOLOGIC HAZARDS OF NE PART OF THE OREGON COAST RANGE FOREARC NIEM, Alan R., Geosciences, Oregon State Univ, 104 Wilkinson Hall, Corvallis, OR 97331, niema@geo.orst.edu and NIEM, Wendy A., Dept.of Geosciences, Oregon State Univ, 104 Wilkinson Hall, Corvallis, OR 97331-5506, niema@geo.orst.edu Recent USGS-sponsored mapping in Columbia Co., OR (Clear Ck., Vernonia, Pittsburg, Birkenfeld, Bacona 7.5-min. quads) shows that the m. Eocene Tillamook Volcanics (economic basement) is a complex of overlapping tholeiitic to subalkalic shield volcanoes that formed an oceanic island in the forearc (42-46 Ma, 40Ar/39Ar). The basalt is unconformably overlain by rocky shoreline basalt conglomerate, shelfal arkosic ss and deep marine mudst of the transgressive Hamlet Fm. Natural gas is produced in the Mist gas field from the overlying delta front micaceous arkosic Cowlitz C&W ss capped by a deep marine mudst seal (upper Cowlitz mbr.).Locally, Grays River basalt and W. Cascade arc-derived Goble Volcanics (subsurface) and equivalent Cole Mtn. basalt interfinger with these sedimentary units. Late Eocene extension created horst and graben fault traps (e.g., Nehalem graben) on the Nehalem arch (a gravity volcanic high). These m. to u. Eocene units and faults are truncated by deepmarine Eo/Oligocene Keasey Fm. tuffaceous siltst derived from the W. Cascade arc. Sager Creek Fm. arkosic turbidites filled slope channels incised into the Keasey and are overlain by deltaic arkosic ss, coals, and shelfal tuffaceous sandy siltst of the Oligocene Pittsburg Bluff Fm. The Scappoose Fm. (u. Oligo. to l. Miocene) represents continued deposition of continental micaceous arkosic detritus and influx of W. Cascade ashes. Paleovalleys incised into the Eo/Oligocene units were filled with a basalt conglomerate of Wapshilla Ridge clasts of the l. Mio. Mist mbr. (Scappoose Fm.) and with subaerial plateau-derived l. to m. Mio. Columbia River flood basalts (R2 Wapshilla Ridge to N2 Winter Water flow units). Starting in late m. Mio. wrench faulting between the right-lateral Gales Creek and Clatskanie-Scappoose fault zones created conjugate sets of oblique-slip right-lateral and normal NW to W faults and subordinate left-lateral oblique-slip to normal NE faults. These small offset faults mimic older Eocene faults, continuing small block clockwise rotation. The NW faults are on strike with the Portland Hills fault zone and pull apart basins. The antecedent Nehalem River incised a narrow bedrock valley around the uplifting north-plunging Nehalem arch and in the graben in the late Neogene. Geologic hazards include landslides and floods (e.g., 1996).

SEQUENCE STRATIGRAPHY, TECTONIC SETTING, AND NATURAL GAS POTENTIAL AND PETROLEUM SYSTEMS OF EOCENE UMPQUA ACCRETIONARY COMPLEX AND TYEE FOREARC BASIN, S. OREGON COAST RANGE NIEM, Alan R.1, NIEM, Wendy A.1, and RYU, In Chang2, (1) Dept. of Geosciences, Oregon State Univ, 104 Wilkinson Hall, Corvallis, OR 97331-5506, niema@geo.orst.edu, (2) Dept. of Earth and Environmental Sciences, Korea Univ, Seoul, 136-701, South Korea Four depositional sequences (seq. I-IV) fill the 8, 000m thick Umpqua/Tyee basins. Syntectonic lower Eocene fan delta, slope, and submarine fan lithic (meta) lithofacies of seq. I (lower Umpqua Gp) represent a partially subducted accretionary wedge derived from the Klamath Mts. (Kmts) during a soft collision. Deltaic lithic arkosic ss, congl., coal, and shelf-slope mdst of seq. II (u. Umpqua Gp) filled irregular lows and thin over submarine highs of seq. I created by imbricate thrust faults (e.g., Reston high). Farther basinward, seq. I and II on lap the Umpqua arch, an older constructional high of l. Eocene Siletz River Volcanics (economic basement) and then thicken into the Smith River subbasin. By m. Eocene, continental micaceous arkosic/volc. arc-derived deltas and deep-sea sandy fans of seq. III and IV prograded down the Tyee forearc basin, unconformably across the Umpqua basin structural trend. Tertiary convergence continued with clockwise basin rotation, minor l. Eocene thrusting, Neogene extension/oblique slip faulting and gentle Coast Range folding. Diagenesis has diminished most primary ss porosity and permeability with smectite-corrensite clay, zeolite and qtz cement and compaction. Reservoir-quality porosity and permeability occur in a few seq. II, III, and IV delta front and turbidite ss. Organic geochem. indicates that most mdst and ss units are thermally immature with lean, gas-prone Type III/IV kerogen. However, coals, carbonaceous mdst and some adjacent Mesozoic KMts mélange units may be organic-rich sources of microbial and meta/thermogenic methane in numerous seeps and wells. Computer models predict some deeply buried seq. I and II units had matured by the Oligo/Miocene. Petroleum system 1 (PS-1) is related to proposed subductionzone maturation with KMts and gas/fluid migration along thrust faults into seq. III reservoirs (e.g., White Tail Ridge Fm. [WTR]) with mdst seals. PS-2 is unconventional plays associated with overpressured basin-center gas and secondary porosity in turbidites (Tyee Mtn. Mbr.) or coal bed methane in the deltaic units of seq. III and IV (e.g., WTR). PS-3occurs due to thermal maturation of seq. III and IV units (e.g., WTR, Spencer Fm) by local sills and mid-Tert. W.Cascade arc volc. Plays include Coast Range anticlines, fault propagation folds, and strat. pinchouts.

QUATERNARY STRATIGRAPHIC FRAMEWORK FOR THE WILLAMETTE VALLEY, OREGON O'CONNOR, Jim E., US Geol Survey, 10615 SE Cherry Blossom Dr, Portland, OR 97216-3103, oconnor@usgs.gov, SARNA-WOJCICKI, Andrei M., U.S. Geol Survey, 345 Middlefield Rd, Menlo Park, CA 94025, WOZNIAK, Karl C., Oregon Water Rscs Dept, 158 12th St.NE, Salem, OR 97310, and GANNETT, Marshall W., USGS, 10615 SE Cherry Blossom D, Portland, OR 97216 We propose a modified late Quaternary stratigraphic framework for the central and southern parts of the Willamette Valley, Oregon, based on geologic mapping, examination of stratigraphic exposures and well logs, and new radiometric age and tephrochronology. This framework consists of six episodes of distinct depositional environments: (1) Since at least 400 ka, large and thick (locally greater than 120 m) gravel fans have formed where major Cascade Range tributaries to the Willamette River enter the valley, and have forced the northward-flowing mainstem Willamette to the western margin of the valley. Locally exposed in the fan stratigraphy are debris flow deposits from a ca. 75 ka eruption of Mt. Jefferson, and a 430-400 ka obsidian-rich lahar that came down the McKenzie or Middle Fork Willamette Rivers. (2) Between about 30 and 22 ka (radiocarbon yr), widespread sand and gravel transport by braided river systems resulted in 5-20 m of sand and gravel deposition on the fans and across much of the valley bottom. (3) A period of local surface stability and soil formation between about 22 and 15 ka (radio carbon yr), indicating a somewhat incised river system, ended when (4) much of the Willamette Valley was repeatedly back flooded by cataclysmic releases of ice-dammed Glacial Lake Missoula into the Columbia River drainage between 15 and 12.5 ka (radiocarbon yr). These floods left up to 35 m of gravel, sand, silt, and clay that are locally mappable to an altitude of 105 m. (5) Subsequent to deposition of Missoula Flood sediment in the valley, but prior to about 12.0 ka (radio carbon yr), there was renewed aggradation along the major Cascade Range tributaries, resulting in several-kilometer-wide swaths of 3-to-15 m thick gravel deposits within locally incised Missoula flood deposits. (6) The Willamette River and its major tributaries have subsequently incised into these gravels, forming multilevel Holocene floodplains composed of gravel, sand, and silt.

GEOLOGIC CONTROLS ON SITE RESPONSE AND GROUND FAILURES IN SEATTLE DURING THE 2001 NISQUALLY EARTHQUAKE SHIMEL, Scott A.1, 1, BOOTH, Derek B.1, FRYER, Jacob M.1, and FRANKEL, Arthur D.2, (1) Department of Earth and Space Sciences, Univ of Washington, Box 351310, Seattle, WA 98195-1310, shimel@u.washington.edu, (2) U.S. Geol Survey, Denver, CO 80225 Ground failures during the February 28, 2001 Nisqually earthquake were similar to those reported after the previous Benioff-zone earthquakes in 1965 and 1949, and they included landslides, settlement, and lateral spreads. Existing subsurface geologic data were reviewed to evaluate if the measured site response variations could be explained by corresponding variations in the underlying Quaternary deposits. Despite limited data, clear spatial patterns have emerged. Although the observed ground-failure patterns do not always correlate with the distribution of surficial geologic materials, some of the site response variations are consistent with subsurface geologic variability within the upper 100 m. Evaluation of existing borehole data provides a means to study non-linear site responses in select areas, such as where there are marked differences in the thickness of certain extensive subsurface units like the overconsolidated Esperance sand. Ground failures and high amplification were noted in weak surficial geologic materials. Liquefaction-related settlement and lateral spreading, by far the most common forms of ground failure in these soils, were concentrated in the northern Duwamish valley, and in the northern half of Boeing Field. Smaller and more widely scattered features were recognized elsewhere in the Duwamish valley, on the now-exposed pre-1911 shorelines of Lake Washington and Lake Union, and in a few filled wetlands in north Seattle. Virtually all observed liquefaction features occurred within deposits of loose Holocene alluvium or artificial fill; perhaps more remarkable, however, are the many areas of similar deposits within the city that did not liquefy.

GEOLOGIC MAP OF THE NORTH CASCADES, WASHINGTON: PROGRESS REPORT ON A COMPILATION TABOR, Rowland W., U.S. Geol Survey MS 975, 345 Middlefield Road, Menlo Park, CA 94025, rtabor@usgs.gov and HAUGERUD, Ralph A., U.S.Geol Survey, Dept. Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195 Comprehending the geology of a large and complex area has always been problematic for the earth scientist as well as the layman. As our detailed knowledge increases for specific areas, the integration of geologic units and structural interpretations over a broad region becomes more and more confusing to all but the connoisseur. Smaller scale regional compilations can help. With the coming of the digital age, compilation of geology has become less onerous. We have compiled digital data available for 8 1:100, 000-scale geologic maps between 120° and 122° west longitude and 47° and 48° north latitude for portrayal at 1:200, 000. We constructed a regional stratigraphic scheme that is expressed in a database with two tables: source-map units and regional units.Correlations are expressed as the relation (in a database sense) between the two tables. The regional stratigraphy is strongly hierarchical; by moving to a higher level in the hierarchy we have grouped related units, doing most of the generalization work required by the scale change. We have draped the geology over a shaded-relief model of the North Cascades providing a useful visual base which we have enhanced with digital cultural data edited and adjusted for the scale. In order to make this map of interest to the specialist and non-specialist, we are writing two map explanations. A conventional description of map units will be lithologically concise, but weighted with references to the original data sources. A second explanation will expand the description of map units, simplifying geologic jargon and explaining the formation of the units and their place in the geologic story.

SOME GEOLOGIC FACTORS THAT INFLUENCE GROUNDWATER AVAILABILITY IN THE COLUMBIA RIVER BASALT GROUP (CRBG) AQUIFER SYSTEM IN THE WILLAMETTE VALLEY (WV), OREGON TOLAN, Terry L., Kennedy/Jenks Consultants, 1020N. Ceter Parkway, Suite F, Kennewick, WA 99336, TerryTolan@KennedyJenks.com and BEESON, Marvin H., Geology Department, Portland State Univ, P.O. Box 751, Portland, OR 97207 The aquifers in the Miocene flood-basalt flows of the CRBG represent a significant potable groundwater resource in the WV. CRBG aquifers result from the interplay of intrinsic physical characteristics of the basalt flows and external pre-/post-emplacement factors. CRBG flows are giant sheetflows (flow volumes of 500 to >1, 000 km3) which spread 300 to >500 km from their vents to reach the WV. Flows of the Grande Ronde Basalt comprise most of the CRBG section (up to 16 flows with a thickness of >300 m in northern WV and up to 9 flows with a thickness of <180 m in central WV) although locally the Frenchman Springs Member (Wanapum Basalt) can also be significant. Interflow zones typically host CRBG aquifers. Paleoenvironmental conditions during flow emplacement (e.g., presence of lakes/streams, general paleotopography, ground surface material (sediment vs.volcanic rock)) influenced the formation and extent of flow-bottom structures (e.g., pillow complexes, flow lobes)that can host aquifers. Flow dense interiors are essentially impermeable and act as aquitards. A sequence of CRBG flows can contain a series of individual confined aquifers. Post-emplacement geologic processes/events can significantly modified CRBG aquifer properties in the WV. Tectonic deformation in post-CRBG time has created faults that transect the CRBG section. Faults can be either barriers to, or pathways for, groundwater movement. Faults with stratigraphic offsets (>10 m) usually inhibit horizontal groundwater and in some areas (e.g., Stayton/Silverton/Salem) multiple faults create a series of hydraulically isolated CRBG aquifers. Tectonic fractures cutting the interior of CRBG flows can create limited secondary permeability with small yields (generally <10 gpm). Deep weathering of the CRBG in some areas (e.g., Salem, Eola, and Waldo Hills) has effectively destroyed the permeability in interflow zones, reducing the number of potential aquifers in the CRBG section. Hydrothermal alteration of CRBG interflow zones (e.g., Oregon City/Gladstone) associated with post-CRBG volcanism can locally eliminate permeability. Knowledge of both intrinsic physical characteristics of CRBG flows and external pre-/post-emplacement factors that modify them is essential for understanding the hydrogeology of the CRBG aquifer system in the WV.

STATUS AND FINDINGS FROM 1:24, 000 SCALE GEOLOGIC MAPPING IN THE PUGET LOWLAND, WA TROOST, Kathy Goetz1, BOOTH, Derek B.1, HAUGERUD, Ralph A.2, BORDEN, Richard K.3, and BARNETT, Elizabeth2, (1) Department of Earth and Space Sciences, Univ of Washington, Box 351310, Seattle, WA 98195-1310, ktroost@u.washington.edu, (2) US Geological Survey, Dept. Earth & Space Sciences, University of Washington, Box 351310, Seattle, WA 98195, (3) 3644Yosemite Dr, Salt Lake City, UT 84109-2365 A major effort is underway to produce new or updated 1:24, 000-scale geologic quadrangle maps of the Puget Lowland. Goals of the mapping effort include 1) a revised Quaternary stratigraphy supported by modern dating techniques (including magnetostratigraphy, tephrochronology, fission track, and thermoluminescence analysis), 2) mapping with attention to hazards analyses and groundwater availability, 3) full use of digital topography for mapping young deposits and as a base for map presentation, and 4) digital compilation and publication of all map data in digital and analog (paper) form. We are using a regionally consistent stratigraphy and are edge-matching all quadrangles for seamless mosaics. Because of increased awareness of geologic hazards and a desire to pursue intelligent mitigation, municipalities and agencies have initiated and partly funded some of the new mapping, which has thus centered on major urban areas and trend of the Seattle fault. As an example of this work, we present the 9-quadrangle block centered on Tacoma WA. Particular features of these maps include: 1) abandonment of much previous pre-last-glacial stratigraphic nomenclature, subdividing older deposits primarily on the basis of radiocarbon age, depositional facies, and/or magnetostratigraphy rather than assigning them to named glacial and interglacial periods; 2) subdivision of the extensive late-glacial recessional outwash sequence south of Tacoma; 3) digitization and stratigraphic revision of the previously published map of Vashon Island (Booth, 1991); 4) recognition and documentation of tectonic structures: inclined beds, folds, and faults; 5) obtaining many new absolute and relative age dates; 6)recognition of multiple tephra and extensive lahar deposits in the pre-last-glacial units. The mapping is part of a collaborative effort involving the University of Washington, US Geological Survey, other universities, City of Seattle, Washington Dept. of Natural Resources, local agencies, and private businesses.

SUMMARY OF THE OLYMPIA NONGLACIAL INTERVAL (MIS 3) IN THE PUGET LOWLAND, WASHINGTON TROOST, Kathy Goetz, Department of Earth and Space Sciences, Univ of Washington, Box 351310, Seattle, WA 98195-1310, ktroost@u.washington.edu. Deposits of the Pleistocene Olympia nonglacial interval, correlated to marine isotope stage 3, from 15, 000 to 60, 000 years old, are widely distributed but discontinuous in the Puget Lowland. These deposits, herein called "Olympia beds", are most easily identified by provenance of clastic detritus, depositional environments, and age. Criteria for recognizing Olympia beds include presence of andesite-rich sand, lavender color, organic layers and paleosols, diatomites, and tephras. The topographic relief on the Olympia beds is likely greater than 230 m, ranging as high as 170 m above sea level and 60 m below modern sea level. Thickness, elevation, grain size, and composition all vary significantly over short lateral distances. The thickest Olympia beds (> 25 m), found near Tacoma, include multiple tephra, lahar, peat, and diatomite layers. The lack of Olympia-age lahar sand andesitic sand on the west side of Puget Sound suggest that Olympia-age equivalents of Colvos Passage and the Tacoma Narrows channels were present near their current locations during that time. Forty new radiocarbon dates on peat and paleosols confirm their correlation with the Olympia interval. At least five Olympia-age tephras and lahar shave been identified near Tacoma, with source areas including Mt. St. Helens and Mt. Rainier. Paleoecological analyses indicate a wide range of paleoenvironments for the Olympia interval. Many locations of Olympia beds yield excellent pollen preservation with a predominance of pine and spruce; freshwater diatomites suggesting clear, shallow lakes and large littoral areas; and macrofossils including mammoth teeth and tusks, Pinus sp.? cones and needles, branches, leaf prints, and in-situ tree roots. Olympia beds are locally deformed by folding and possible liquefaction, such as at Mee Kwa Mooks Park in Seattle and near the Tahlequah ferry terminal in Tacoma. Some of the deformation certainly has a tectonic origin. Since the Olympia beds are a mappable unit, they may provide limiting ages on tectonic deformation in the Puget Lowland, and on the timing of the subsequent Vashon glacial advance.

GREAT SLIP IN GREAT SUBDUCTION EARTHQUAKES OCCURS UNDER FOREARC BASINS WELLS, Ray E., US Geol Survey, 345 Middlefield Rd MS 975, Menlo Park, CA 94025, rwells@usgs.gov, BLAKELY, Richard, USGS, 345 Middlefield Rd, Menlo Park, CA 94025, and SUGIYAMA, Yuichi, National Institute of Advanced Industrial Sci and Technology, 1-1-3Higashi, Tsukuba, Ibaraki, 305-8567, Japan Asperities, or areas of greater seismic slip and moment release, have been recognized for many great subduction zone earthquakes. The origin of these areas of high slip and their role in earthquake recurrence are much debated. Asperities may have geologic significance, and they are commonly correlated with strong upper plate blocks. However, along the Nankai Trough of SW Japan, well constrained regions of maximum co-seismic slip in the 1923, 1944, 1946 and 1968 thrust events correlate with the bathymetric and gravity lows of forearc basins. This suggests a link between basin formation and the slip process. Asperities for other subduction zone earthquakes, usually determined by waveform inversion techniques, were compared to forearc structure revealed by global bathymetric and gravity data. Asperities for the 1938 Alaskan Peninsula (Mw 8.2), 1957 central Aleutians (Mw 8.6), 1965 Rat Island (Mw 8.7), and 1986 Andreanof (Mw 8.0) earthquakes coincide with the location and size of basin-centered forearc gravity lows along the Aleutian terrace. Off southern Chile, offshore basins have little bathymetric expression, but are visible in the free-air gravity and seismic profiles. Slip maxima in the 1960 earthquake (Mw 9.5) coincides well with the size and distribution of basin-centered forearc gravity lows. Along the Cascadia subduction zone, large offshore gravity lows coincide with the geodetically-defined coupled zone and suggest large-scale seismic segmentation similar to SW Japan. Basin-centered asperities suggest that basins grow by interseismic subsidence driven by tectonic erosion at depth. Subduction erosion has been well documented along sediment-starved subduction zones and is inferred to be an important process globally. Our data suggests that erosion is coincident with seismic slip.Once forearc lows are created, lower fault-normal stresses beneath them may at times help to localize slip. Along sediment-dominated subduction zones, the location of forearc basins may be a useful indicator of long-term seismic moment release.