U.S. Department of the Interior Prepared in cooperation with U.S. Geological Survey Portland State University, Shannon and Wilson, Inc., and the Tri-County Metropolitan Transportation District of Oregon Scientific Investigations Map 3144 122°52.5' 122°45' 122°37.5' 124° 122° 123° 122° 30' 47° 45°37.5' 0 INTRODUCTION A B Co AN INSIDE VIEW OF THE TUALATIN MOUNTAINS STRATIGRAPHIC FRAMEWORK The uppermost unit, mapped as basaltic andesite of Elk Point (Qbep), is a Table 1. 40Ar/39Ar ages from Pleistocene basalt flows in the Light Rail Tunnel, Portland Hills, Portland, Oregon. lum A C The Tualatin Mountains form a northwest-striking ridge about 350 m high that bia To improve our understanding of Quaternary faulting and folding in the Portland Hills fault The rock units exposed in the tunnel and exploratory borings consist of basaltic andesite flow that fills a channel along the scarp formed by the West 2 Washington Tualatin MountainsW R Sylvan Fault X Qbep CORRELATION OF MAP UNITS EXPLANATION Total -Gas Age Figure 2. Tualatin Mountains, location of ive Sylvan Fault. Unit Qbep crops out at the surface a few kilometers west and 1 3 4 5 separates Portland, Oregon, from the cities of the Tualatin Valley to the west. Known il r Portland Hills, we have compiled a geologic cross section (below) of the three main units: (1) Flood-basalt flows of the 15- to 16- million-year-old Sample Polarity (Ma) Plateau Age (Ma) Isochron Age (Ma) Indicated age (Ma) Indicated age (ka) c CP la SURFICIAL DEPOSITS r Portland Hills light-rail tunnel (red line), and selected south of the tunnel. Elk Point, a prominent conical hill directly north of the 6 6 m 0.2 Bruhnes Normal Approximate informally as the Portland Hills, the ridge is a late Cenozoic anticline, bounded by cean Co Tualatin Mountains. The cross section was compiled from several sources: Columbia River Basalt Group, which make up most of the rocks exposed in Number Age ±1s Age ±1s MSWD Age ±1s MSWD Intercept A lu mbia River e Portland Alluvial, eolian, and mass wastage deposits Contact—Dashed where approximate, short dash faults (dashed lines where approximately age in Ma P t tunnel, is inferred to be the vent for this flow. The flow has normal magnetic reverse faults that dip toward the anticlinal axis (figs. 1–3; Beeson and others, 1991; O t (1) Expected tunnel geology based on 90 borings totaling 15,000 linear the tunnel (green, blue, and orange on the cross sections); (2) younger basalt T e where inferred, queried where uncertain 781+84 N 0.17 0.0 3 0.138 0.004 1.4 0.120 0.015 1.02 299.1 ± 2.7 0.120 ± 0.015 120 ± 15 located, dotted where concealed). Numbers light rail tunnel 0 CG polarity and yielded an Ar/Ar plateau age of 120±15 ka, making it one of the Blakely and others, 1995, 2004; Madin and others, 2008; Peterson and Squier Vancouver R Basin feet (4572 m; at 0.3048 m per foot) along the tunnel route prior to flows of the Boring Volcanic Field of Evarts and others (2009), about 1.2 af i 802+53 N 1.05 0.01 none 1.027 0.031 43 295.6 ± 4.6 1.027 ± 0.032 1027 ± 32 along tunnel represent fault zones v youngest flows of the Boring Volcanic Field (table 1). 45° NW e Internal contact Associates, 1993). The anticline is a broad, open fold consisting chiefly of Miocene construction (Peterson and Squier Associates, 1993); million years old to 120,000 years old, exposed in the western third of the Ran g e r 0.4 804+46 R 1.06 0.04 1.173 0.021 2.1 1.22 0.040 3.4 294.9 ± 5.7 1.17 ± 0.02 1170 ± 20 e observed in tunnel and on profile below: Holocene ic The other Pleistocene flows in the tunnel are older than the basaltic andesite Qs Columbia River Basalt Group, with remnants of Miocene-Pliocene Troutdale d (2) Unpublished geologic mapping of the tunnel bores by Ken Walsh, tunnel (shades of pink on the cross sections); and (3) fine-grained sand, silt 807+73 R 2.98 0.02 none 1.41 0.12 51 328 ± 40 1.41 ± 0.12 1410 ± 12 SW a Oregon GALES CREEK FAULT ZONE (1) West Sylvan, (2) Central Sylvan, (3) of Elk Point, and do not correspond to any flows exposed at the surface fb Formation and Pleistocene basalt of the Boring Volcanic Field on the flanks of the c Portland Sylvan Fault who was a geologist with Parsons Brinkerhoff Construction and graduate and clay interbedded with the Boring basalts and deposited by local streams Qly Flow top breccia s SYLVAN Portland Hills Fault 809+44 N 1.39 0.02 0.971 0.018 0.52 0.952 0.024 0.8 296 ± 1.4 0.971 ± 0.018 971 ± 18 a East Sylvan, (4) Brickworks, (5) unnamed Qbsh FAULT IA R. nearby. They are mapped as correlative to the basalt of Sunset Hill ( ) anticline (Beeson and others, 1991, Madin and others, 2008). C COL UM B student working on the tunnel at Portland State University with Professor and as loess (yellow on the cross sections). 813+73 acif fault. Figure modified from Blakely and 0.6 Coas t Qbcm ent Anticlinal structures similar to the Tualatin Mountains are characteristic of the 0 100 KM Marvin Beeson (both now deceased; shown in fig. 4, right); and Flows of the Columbia River Basalt Group were erupted from fissure vents and the basalt of Cornell Mountain ( ) (R.C. Evarts, unpub. data; Madin Entablature jointing (3/13/98) N 1.35 0.02 none 1.070 0.080 65 310 ± 61 1.07 ± 0.08 1070 ± 80 P PORTLAND HILLS 45° 30' BEAVERTON FAULT others (2004), after Beeson and others and others, 2008). The tunnel slices through the center of the basalt of Sunset 813+73 northern Willamette Valley (fig. 1), where the structures accommodate (3) Stratigraphy, geochronology, geochemistry, and magnetic polarity of in eastern Oregon and Washington and covered large parts of Oregon, VOLCANIC ROCKS OF THE FAULT (1989b, 1991) and Madin (1990). Portland (9/4/98) N 1.19 0.01 none 1.048 0.026 5.9 316 ± 11 1.048 ± 0.026 1048 ± 26 margin-parallel shortening of the Cascadia fore arc. Global Positioning System (GPS) Boring basalt flows, structural mapping of the Sylvan Fault, and video of Washington, and Idaho. A detailed stratigraphy of the Columbia River Basalt Hill vent complex, which consists of a pile of breccia and cinders 65 m high, BORING VOLCANIC FIELD col Colonnade jointing Cooper which is intruded by a large dike and interfingers with flows thinning away 817+31 R 1.16 0.01 1.152 0.010 1.03 1.160 0.038 0.95 295 ± 11 1.152 ± 0.010 1152 ± 10 results indicate that the shortening is due to the northward motion of Oregon at Mountain tunnel geology, all gathered by USGS scientists in the tunnel. Group has been defined on the basis of geologic mapping, geochemistry, and 0.8 0.12 120± 15 ka Qbep br 1 several millimeters per year with respect to stable North America (McCaffrey and Chehalem Mountains Geologic units encountered in the tunnel and boreholes were described paleomagnetism (Swanson and others, 1979; Beeson and others, 1985; Reidel from the center. Flows and breccias of the basalt of Sunset Hill have greater Flow breccia Paleomagnetic polarity of basaltic unit, N = Normal, R = Reversed Cornell Mountain Qbsh QUATERNARY 2 40 39 Tualatin Age (Ma) Total-Gas age is the age calculated from the sum of all radiogenic Ar divided by the sum of all potassium derived Ar in an age spectrum (incremental-heating) experiment; CLACK abundances of large-ion-lithophile elements (LILE) K O, Ba, and Sr than the 0.60 others, 2007). Some of the uplifts may contain active faults, but the structures are A in the tunnel report (Peterson and Squier Associates, 1993) and the maps and others, 1989; Reidel, 2005). Many of these flows entered the northern 2 Oregon City M Qlo "Ma" is the age in millions of years before the present. A younger basaltic andesite (table 2). Two of the Qbsh flows are dated at X Matuyama Reversed Diamict poorly exposed and are overlain by thick Pleistocene loess and Missoula flood S of Beeson and others (1991, 1989b) and Madin and others (2008). A core Willamette Valley, where two formations, the Grande Ronde Basalt and the 0.97± 0.02 Ma 3 40 39 39 Quaternary sediments Valley Elk Point 1.0 Pleistocene An Ar/ Ar plateau age is the weighted mean age of contiguous steps representing at least 50% of the potassium derived Ar released in an incremental-heating experiment 5 slightly older than 1 Ma, have normal magnetic polarity, and evidently erupted X 1.0 deposits. R 3 4 from the exploratory borings is on display at the Washington Park MAX overlying Wanapum Basalt are recognized. Both formations are encountered X Jaramillo 1.05± 0.03 Ma Qbsh Qbshi for which ages are concordant at the 95% level of confidence (Fleck and others, 1977). IV Estacada 1 2 Figure 3. View of Portland, Oregon, looking NW; white dotted lines Qbcm E Qbshi 4 40 36 39 36 Between 1993 and 1998, construction of the 3-mile-long (4500-m-long) TriMet Pleistocene and Pliocene Boring R 45°30' Light Rail Station. This is the deepest station in North America, and it is in the tunnel and boreholes.