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##### U.S. Geological Survey   Scientific Investigations Map 2826

## Geologic Map of the Ariel Quadrangle, Clark and Cowlitz Counties, Washington

### *By* Russell C. Evarts

### 2004

| The Ariel 7.5' quadrangle is situated along the Puget-Willamette Lowland approximately 50 km north of Portland, Oregon. The lowland, which extends from Puget Sound into west-central Oregon, is a complex structural and topographic trough that lies between the Coast Range and the Cascade Range. Since late Eocene time, the Cascade Range has been the locus of an active volcanic arc associated with underthrusting of oceanic lithosphere beneath the North American continent along the Cascadia Subduction Zone. The Coast Range occupies the forearc position within the Cascadia arc-trench system and consists of a complex assemblage of Eocene to Miocene volcanic and marine sedimentary rocks. 

The Ariel quadrangle lies along the northeastern margin of the Portland Basin, a roughly 2000-km2 topographic and structural depression in the central Puget-Willamette Lowland (Beeson and others, 1989; Swanson and others, 1993; Yeats and others, 1996). 

The Portland Basin is approximately 70 km long and 30 km wide; its long dimension is oriented northwest. Its northern boundary coincides, in part, with the lower Lewis River, which flows southwestward through the map area. The Lewis drains a large area in the southern Washington Cascade Range, including the southern flank of Mount St. Helens approximately 25 km upstream from the quadrangle, and joins the Columbia River about 12 km west of the quadrangle. The flanks of the Portland Basin consist of deformed and eroded Eocene through Miocene volcanic and sedimentary rocks that rise to elevations exceeding 2000 ft (610 m). Seismic-reflection profiles (L.M. Liberty, written commun., 2003) and lithologic logs of water wells (Swanson and others, 1993; Mabey and Madin, 1995) indicate that as much as 550 m of late Miocene and younger sediments have accumulated in the deepest part of the basin near Vancouver. Most of this basin-fill material was carried in from the east by the Columbia River, but sediment deposited by streams draining the adjacent highlands is locally important. The Portland Basin has been interpreted as a pull-apart basin located in the releasing stepover between two en echelon, northwest-striking, right-lateral fault zones (Beeson and others, 1985, 1989; Beeson and Tolan, 1990; Yelin and Patton, 1991; Blakely and others, 1995). These fault zones are thought to reflect regional transpression and dextral shear within the forearc in response to oblique subduction of the Pacific Plate along the Cascadia Subduction Zone (Pezzopane and Weldon, 1993; Wells and others, 1998). The southwestern margin of the Portland Basin is a well-defined topographic break along the base of the Tualatin Mountains, an asymmetric anticlinal ridge that is bounded on its northeast flank by the Portland Hills Fault Zone (Balsillie and Benson, 1971; Beeson and others, 1989; Blakely and others, 1995), which is probably an active structure (Wong and others, 2001; Liberty and others, 2003). The nature of the corresponding northeastern margin of the basin is less clear, but a poorly defined and partially buried dextral extensional fault zone has been hypothesized from topography, microseismicity, potential field-anomalies, and reconnaissance geologic mapping (Beeson and others, 1989; Beeson and Tolan, 1990; Yelin and Patton, 1991; Blakely and others, 1995). 

This map is a contribution to a program designed to improve the geologic database for the Portland Basin region of the Pacific Northwest urban corridor, the densely populated Cascadia forearc region of western Washington and Oregon. Better and more detailed information on the bedrock and surficial geology of the basin and its surrounding area is needed to refine assessments of seismic risk (Yelin and Patton, 1991; Bott and Wong, 1993), ground-failure hazards (Madin and Wang, 1999; Wegmann and Walsh, 2001) and resource availability in this rapidly growing region. The digital database for this publication is available on the World Wide Web at https://pubs.usgs.gov/sim/2004/2826 |
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| **File         Name** | ****File         Type and Description****File         Size**README**SIM2826readme.txtText-only readme file that explains how to use the digital database         12 Kb **DATA**SIM2826_shp.zip Package  containing shapefiles exported from the personal geodatabase. Includes FGDC-compliant metadata 11.6 MB SIM2826_db.zipCompressed       package of the digital database for this map. Includes FGDC-compliant metadata.          8.1 MB**FILES         for VIEWING and PLOTTING**Arl_geol_text.pdfPDF file of the 35-page descriptive geologic pamphlet that accompanies the map 3.1 MB Ariel_map.pdfPDF file       of the ~49" x 35" map sheet that can be used for viewing map in a browser, as well as for       plotting30.5         MB
For questions about the content of this report, contact Russ Evarts

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| Department of the Interior | U.S. Geological Survey |URL of this page: https://pubs.usgs.gov/sim/2004/2826Maintained by: Mike DigglesLast modified: May 10, 2007 (mfd)******** |
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