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##### U.S. Geological Survey   Scientific Investigations Map 2834 

## Geologic Map of the Saint Helens Quadrangle, Columbia County, Oregon, and Clark and Cowlitz Counties, Washington

### *By* Russell C. Evarts

### 2004

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The Saint Helens 7.5' quadrangle is situated in the Puget-Willamette Lowland approximately 35 km north 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, Cascade Range has been the locus of a discontinuously 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 Saint Helens quadrangle lies in the northern part of the Portland Basin, a roughly 2000-km2 topographic and structural depression. It is the northernmost of several sediment-filled structural basins that        collectively constitute the Willamette Valley segment of the Puget-Willamette Lowland (Beeson and others, 1989;        Swanson and others, 1993; Yeats and others, 1996). The rhomboidal basin is approximately 70 km long and 30 km        wide, with its long dimension oriented northwest. The Columbia River flows west and north through the Portland        Basin at an elevation near sea level and exits through a confined bedrock valley less than 2.5 km wide about 16 km        north of Saint Helens. The flanks of the basin consist of 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 contributions from streams        draining the adjacent highlands are locally important.        

The Portland Basin has been interpreted as a pull-apart basin located in the releasing stepover between two        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 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        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 fieldanomalies,        and reconnaissance geologic mapping (Beeson and others, 1989; Beeson and Tolan, 1990; Yelin and        Patton, 1991; Blakely and others, 1995). Another dextral structure, the Kalama Structural Zone of Evarts (2002),        may underlie the north-northwest-trending reach of the Columbia River north of Woodland (Blakely and others,        1995).        

This map is a contribution to a U.S. Geological Survey (USGS) program designed to improve the geologic        database for the Portland Basin region of the Pacific Northwest urban corridor, the populated 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/2004/2834 |
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| This illustration       is a .jpg (JPEG) nonnavigable image of the USGS geologic-map plot of the       Mount Saint Helens quadrangle. A full-size, navigable map graphic can       be viewed on-screen or plotted (StHelens_map.pdf; 20.1 MB).       Note: the full-size plot requires a large-format plotter, and is best reproduced       at 600 (or greater) dpi. |

 

| **File         Name** | ****File         Type and Description****File         Size**README**readme.txtText-only readme file that explains how to use the digital database         12 Kb **DATA**SIM2834_shp.zip Package  containing shapefiles exported from the personal geodatabase. Includes FGDC-compliant metadata 4.7 MB SIM2834_db.zipCompressed       package of the digital database for this map. Includes FGDC-compliant metadata.          8.2 MB**FILES         for VIEWING and PLOTTING**STH_geol_text.pdfPDF file of the descriptive geologic pamphlet that accompanies the map 548 KB StHelens_map.pdfPDF file       of map sheet that can be used for viewing map in a browser, as well as for       plotting20.1         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/2004/2834Maintained by: Mike Diggles Last modified: July 1, 2005 (mfd)
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