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Tectonic Evolution of the Tualatin Basin, Northwest Oregon, As Revealed by Inversion of Gravity Data

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Tectonic Evolution of the Tualatin Basin, Northwest Oregon, As Revealed by Inversion of Gravity Data Tectonic evolution of the Tualatin basin, northwest Oregon, as revealed by inversion of gravity data D.K. McPhee, V.E. Langenheim, R.E. Wells, and R.J. Blakely U.S. Geological Survey, 345 Middlefi eld Road, Menlo Park, California 94025, USA ABSTRACT basin edges, particularly along the concealed formation. In Blakely et al. (2000), high-reso- northeast edge of the Tualatin basin beneath lution aeromagnetic data were used to infer that The Tualatin basin, west of Portland the greater Portland area. the Tualatin basin is bounded by northwest- (Oregon, USA), coincides with a 110 mGal trending dextral strike-slip faults. gravity low along the Puget-Willamette low- INTRODUCTION Here we describe a three-dimensional (3-D) land. New gravity measurements (n = 3000) model of the Tualatin basin based on more than reveal a three-dimensional (3-D) subsurface The Tualatin basin, west of Portland (Oregon, 3000 newly acquired gravity measurements in geometry suggesting early development as USA), is one of several structural basins within the Tualatin, Portland, and northern Willamette a fault-bounded pull-apart basin. A strong the Puget-Willamette lowland, a 600-km-long basins (Morin et al., 2007). We show that basin northwest-trending gravity gradient coin- structural and topographic trough between the geometry is consistent with an early history of cides with the Gales Creek fault, which forms Coast Range and Cascade volcanic arc (Fig. 1). transtensional extension between two overlap- the southwestern boundary of the Tualatin The Puget-Willamette lowland is seismically ping, northwest-striking strike-slip fault zones. basin. Faults along the northeastern mar- active and home to most of the Oregon and gin in the Portland Hills and the northeast- Washington State populations and major cities. GEOLOGIC SETTING trending Sherwood fault along the southeast- In Puget Sound, active faulting accommodates ern basin margin are also associated with margin-parallel shortening (Nelson et al., 2003; The Tualatin basin is between the Cascade gravity gradients, but of smaller magnitude. Brocher et al., 2004) driven by the northward volcanic arc and the Cascadia subduction zone The gravity low refl ects the large density con- motion of Oregon and Washington against Can- (Fig. 1A). The basement beneath the basin is trast between basin fi ll and the mafi c crust ada (Wells et al., 1998; McCaffrey et al., 2007). not well known, but likely consists mostly of of the Siletz terrane composing basement. Although the level of seismicity is much lower the Siletz terrane, an oceanic plateau or sea- Inversions of gravity data indicate that the in northwest Oregon, the Tualatin basin and mount chain accreted to the continental mar- Tualatin basin is ~6 km deep, therefore 6 northern Willamette basin appear to be respond- gin ca. 50 Ma (Duncan, 1982). The Siletz ter- times deeper than the 1 km maximum depth ing to the same north-south compression. Bee- rane is exposed in the Coast Range west of the of the Miocene Columba River Basalt Group son et al. (1985) argued that northwest-striking, Tualatin basin (Fig. 1B), where it is composed (CRBG) in the basin, implying that the basin dextral strike-slip faults played an important of late Paleocene to middle Eocene oceanic contains several kilometers of low-density role in the tectonic evolution of the northern basalt fl ows, intrusions, and breccias of the pre-CRBG sediments and so formed pri- Willamette basin. These structures are diffi cult Siletz River Volcanics (Snavely et al., 1968; marily before the 15 Ma emplacement of the to map, however, because much of the region Wells et al., 1995, 2000). Seismic data show CRBG. The shape of the basin and the loca- is covered with Missoula fl ood deposits that that the Siletz terrane consists of high-velocity tion of parallel, linear basin-bounding faults largely conceal geomorphic and geologic evi- (and likely dense) mafi c crust 20–35 km thick along the southwest and northeast margins dence of tectonism older than 15,000 yr. Yeats beneath the Coast Range and Willamette basin suggest that the Tualatin basin originated et al. (1996), Popowski (1996), and Wilson ~75 km south of the Tualatin basin (Tréhu et as a pull-apart rhombochasm. Pre-CRBG (1998) used seismic refl ection and well data to al., 1994). extension in the Tualatin basin is consistent interpret shallow (upper few kilometers) struc- East of the Tualatin basin, basement is more with an episode of late Eocene extension doc- ture within the Tualatin basin, particularly faults diffi cult to defi ne due to the lack of exposures umented elsewhere in the Coast Ranges. The and folds affecting the Columbia River Basalt and overlying volcanics of the Cascades volca- present fold and thrust geometry of the Tual- Group (CRBG) and younger fl uvial deposits. nic arc. Our best candidate for pre-Oligocene atin basin, the result of Neogene compres- Popowski (1996) used potential fi eld and seis- basement east of the Tualatin basin is the mid- sion, is superimposed on the ancestral pull- mic refl ection data to interpret a two-stage his- dle to late Eocene basalt of Waverly Heights apart basin. The present 3-D basin geometry tory of the Tualatin basin, with early, left-lateral exposed in a small area at the southeast end of may imply stronger ground shaking along shear accommodating block rotation and basin the Portland Hills, between Portland and Oregon Geosphere; April 2014; v. 10; no. 2; p. 264–275; doi:10.1130/GES00929.1; 6 fi gures; 3 tables. Received 14 March 2013 ♦ Revision received 6 January 2014 ♦ Accepted 22 January 2014 ♦ Published online 21 February 2014 264 For permission to copy, contact [email protected] © 2014 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/2/264/3332695/264.pdf by guest on 26 September 2021 3D gravity inversion of t he Tualatin basin 124° 122° W City (Fig. 1B). Waverly Heights basalts are also A Olympic Puget thought to have been part of an oceanic island Sound Mtns. accreted to western Oregon during the Eocene (Wilson, 1998; Beeson et al., 1989a, 1989b) and 0 KM 100 .Seattle may be part of the Siletz terrane. We also include the voluminous, middle to 47° N late Eocene Tillamook Volcanics (Fig. 1B) in our basement assemblage because of their com- positional similarity to underlying Siletz terrane Arc (Wells et al., 1995). The Tillamook Volcanics form a >3-km-thick sequence of dominantly e d basalt fl ows and breccias with minor siltstone Puget-Willamette Lowland a and conglomerate (Wells et al., 1995) and are WASHINGTON c Pacific locally separated from the underlying Siletz ter- s Ocean Portland . a PB OREGON rane by thin sedimentary interbeds. C Cascadia Subduction Zone Subduction Cascadia TB As a group, the Eocene Siletz terrane, Waverly Heights basalt, and Tillamook Vol- NWB 45° canics are considerably denser than overlying Ra n g e Co ast Eocene and younger sediments that compose the Tualatin basin fi ll. Several hydrocarbon explora- tion wells and industry seismic data show that at least 2 km of Eocene and Oligocene marine sed- 123° 122° 30′ W imentary rocks fi ll the Tualatin basin (Oregon B Q B Department of Geology and Mineral Industries, C 2013; Popowski, 1996). Two of the deeper wells PORTLAND E in the Tualatin basin, the Texaco Cooper Moun- V BASIN Eo tain #1 and the Nahama and Weagant Klohs F #1 (Table 1), show ~700 to 1100 m of marine A’ sedimentary rocks overlying tens to hundreds of meters of Eocene mudstone and siltstone, and Tillamook Dixie Volcanics Mountain Vancouver then an additional 500 to 1300 m of tuffaceous mudstone, siltstone, and minor sandstone of the COLUMBIA Yamhill Formation (Yeats et al., 1996; Oregon Po RI rtland Hills VER Department of Geology and Mineral Industries, Gale s Cre e k Portland Fau lt Sylvan-Oatfield 2013). The Texaco Cooper Mountain #1 well Fault COLU East encountered Siletz River Volcanics at ~2600 m TUALATIN BASIN Bank (Table 1). CRBG fl ows entered northwestern ′ Beaverton Fault Fault 45° 30 Port lan d Oregon between 16 and 14.5 Ma (Yeats et al., Hills 1996; Beeson et al., 1989a) and range in thick- Fault ness from a few meters to 300 m (Newton, 1969; Cooper Waverly Oregon Water Science Center, 2013) within the Coast Range Heights Mountain basalt Tualatin basin and surrounding uplands (Fig. Chehalem Mountains 1). In the Tualatin basin, CRBG fl ows occupy CLA Sherwood Fau lt only a small fraction (maximum of 300 m) of A Oregon the total vertical section of post-Eocene basin Parrett City fi ll (minimum of 2 km). Mountain Canby Late Miocene and younger sediments NORTHERN Boring Lava (Pleistocene) WILLAMETTE Columbia River Basalt (Miocene) Canby- Figure 1. (A) Index map showing basins Eocene-Oligocene sedimentary rocks BASIN Molalla Fault Volcanic rocks of the Western Cascades Woodburn (tan) within the southern Puget-Willa- Eocene oceanic basement rocks R mette lowland. PB—Portland basin; TB— E Well V I Tualatin basin; NWB—Northern Willa- Fault Anticline Homocline R Mount Angel Fault E mette basin. (B) Simplifi ed geologic map T 0 25 KM T E of the Tualatin basin and surrounding M A Mount Angel area (modifi ed from Blakely et al., 2000). L L ′ I Red line is profi le A–A (Fig. 5A). Wells are W labeled in Figure 3 and listed in Tables 1 45° and 3. White squares are cities. Salem Geosphere, April 2014 265 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/2/264/3332695/264.pdf by guest on 26 September 2021 McPhee et al. TABLE 1. WELLS USED TO CONSTRAIN DEPTH TO BASEMENT Lat† Long† Depth to basement ID* Well (N) (W) (km) (NAD27) (NAD27) 1 Texaco Cooper Mountain Well #1 45°27′13.21″ 122°52’13.51” 2.635§ 2 Reichhold Finn #1 45°02′46.68″ 123°12’29.52” 1.084§ 3 Reserve Bruer #1 45°00′29.16″ 123°13’19.56” 1.387§ 4 CLAC 3107 45°23′44.74″ 122°41’31.85” >0.277 5 Nehama and Weagant Klohs #1 45°20′46.50″ 122°58’22.51” >1.789 6 Richfield Barber #1 45°33′17.86″ 122°46’41.52” Unknown (see text) Note: References: Oregon Department of Geology and Mineral Industries (2013); Orzol et al.
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