Portland, Oregon, geology by tram, train, and foot by Ian P. Madin Oregon Department of Geology and Mineral Industries, 800 NE Oregon Street #28, Suite 965, Portland, Oregon 97232-2162 Overview: This field trip provides an introduction to the geology of the Portland, Oregon, area. Five field trip stops, all acc transportation and walks of easy to moderate difficulty, provide opportunities to see outcrops of Columbia River Basalt, Troutdale Forma- tion, Boring volcanic field flow and vents, Portland Hills Silt, and a small landslide. INTRODUCTION Residents of the Portland, Oregon, metropolitan area are proud of its scenery and its public transit system (also known as TriMet or MAX). Beneath both lies some fascinating geology, including Miocene flood basalt flows, Quaternary volcanoes, and deposits and landforms left by the great Bretz/Missoula Floods. This field trip provides an opportunity to see some of the geology of the Portland area by walking and using public transportation, starting from the Oregon Convention Center (777 NE Martin Luther King, Jr. Boulevard). A guided version of the trip will be offered during the October 2009 GSA meeting, but this guide is also intended for those who want to self-guide at their own convenience. Bus, streetcar, light rail, and tram fares can all be readily purchased at the Rose Garden Transit Center adjacent to the Oregon Conven- tion Center. provide a fun introduction to the geol- ogy of PortlandThis area. field Ittrip is notguide meant is intended to to be a definitive and comprehensive review of the subject. A list of references is provided at the end of this guide for those interested in more details. REGIONAL SETTING The tectonic and geologic setting of the Pacific Northwest (Figure 1) is domi- nated by the Cascadia Subduction Zone, where the Juan De Fuca plate is actively subducting toward the northeast be- neath the North American plate at the rate of almost 4 cm/yr. Portland is locat- ed in the Portland Basin, a forearc basin that lies between the Coast Ranges and the active volcanic arc of the Cascade Range. The Coast Ranges are composed of Eocene to Miocene marine sedimen- tary rocks and Eocene intrusive and ex- trusive basalt deposited on the accreted Siletz terrane. The Cascade Range is composed of a sequence of Eocene through Holocene arc volcanic rocks, decorated with a Preprint, OREGON GEOLOGY, VOLUME 69, NUMBER 1, FALL 2009 chain of Quaternary stratovolcanoes. Range and on the west by the Tualatin Mountains, a faulted an- ticline that hasThe been Portland growing basin since is thebounded Miocene. on theThe eastPortland by the Cascade Basin is traversed by the Columbia River which, at 2,000 km in length and with a drainage basin of 670,000 km major influence on the geology of the basin. 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Regional tectonicMM Msetting of the Portland Basin within the Pacific Northwest. e Zo kilometers Z kilometers Z Z kilometerskilometers Zo kilometers Zo Zo kilometerskilometers Zo S kilometers Z S kilometers S S S S S S eeee S e u u u e u o u o o u u u o u b b b b n b b n b 100 b 100 nnn b 100100100 n 100 n n n n n n n n n d d d d d d d d d d u d u u u d u d e e u d u e e dd u e d u e e d e e c c c c c c c c oo c ti oo ti ti o ti oo ti ti o ti o ti ti o o o o o o o o ccc o cc n c n n c n n n n n iiiiii n nnn o ooo JJuuaann DDeeFeF F uuccaa Riiiddggee 200200200 FFF rrrr aaaa cccc ttttt uuuu CanadaCanada rrrrr 2 Z Z Z ee Z e Z Z e Z eee Z ee Z o o , has had a o o o o o o o n n n n n n Z n n ZZZ n e e e e e e e e ooo e ggeses nnnn RanRan gee eeee Ran nngg ee ooastast RRaann CCoo PortlandPortland BasinBasin RR F F F F F F F F F A A A A A A A A A a a a a a a a a a n n n n n n n n n u u u u u u u u u d d d d d d d d d l l l l l l l l l t t t t t t t r S t r S t r r S S r S r r S S r S r S e e e e e e e e e a a a a a a a a a ee a e a a a ee a e a a e a d a dd n dd n d n n ad n n n a n aa n aa s ca s s c s s cc s s c s s cc CaCassc WashingtonWashington CaliforniaCalifornia NorthNorth AmericanAmerican PlatePlate OregonOregon NevadaNevada 1 Figure 2. Lidar image of the Portland, Oregon, area, showing field trip stops (stars) and major geographic features. Image consists of an elevation color gradient over slopeshaded lidar. The peak elevation for Bretz/Missoula floodwaters was about 115 m, corresponding to the base of the brick red color. Elevation shown by color gradient: Brick red color indicates higher elevations up to 390 m; white color is sea level. THE GEOLOGY OF PORTLAND The City of Portland sits at the confluence of the Willamette ing. The Tualatin Mountains, a straight and narrow range with a and Columbia rivers and occupies the western half of the Port- sharp, fault-bounded eastern edge, separate the Portland Basin land Basin and much of the adjacent Tualatin Mountains (Fig- from the Tualatin Basin to the west. ures 2 and 3). The flat floor of the Portland Basin is punctuated The Tualatin Basin is generally flat, with a few faulted and by several small buttes and the Boring Hills, a complex region folded highs in the center of the basin. where small volcanic cones mix with blocks uplifted by fault- 2 Portland, Oregon, geology by tram, train, and foot — Preprint, OREGON GEOLOGY, VOLUME 69, NUMBER 1, FALL 2009 012 CoCC 000 111 222 Coo 000 111 222 ollluu kilometers uum kilometerskilometers 5 mmbb kilometerskilometers bbiiiaa RR iiivv eerr PP Portland Basin orooro rrr tlatltltl TT aa uu nn uu dd aa dd lll HH aa IIIlll llllll tititi lllss nn FF MoMo aa MoMo uululu WW Faults ll lllttt iiillllll approximate uu lllaa buried nn mm tatata ee thrust tatata tttttt iii tttee iiinn ss Portland Rocky Butte OCC SS OCC yy lllvv aa nn FF aa 1 uu lllttt RRiRRRi iii eelll 5 vvv 4 nnee 5 vvv Tu eee AX T eee MA rrr M 2 rrr Mt Tabor Tualatin Basin 3 Kelly Butte BoringBoring Cooper Hills Mountain Mt Scott PP ooro rrr tltltlatl aa nn dd HH IIIlll llllll Bull OO lllss aa FF aattt FF kee tttfff aa Mountain Laakk iiiee uluulu o L lll lll eeg dd ttt ssw FF O aa uu lllttt Figure 3. Generalized geologic map of Portland, Oregon. Map area is identical to Figure 2. Data from Oregon Geologic Data Compilation, release 5. PORTLAND STRATIGRAPHY t Hillsboro Formation — Fine-grained fluvial sedimentary deposits of Coast Range provenance deposited in the t Alluvium — Holocene sand, silt, and gravel deposited in Tualatin Basin the channels and on the floodplains of the Columbia and t Troutdale Formation — Fine- to coarse-grained fluvial Willamette rivers and their tributaries sediments of Cascade Range and Columbia River prov- t Landslides—Quaternary landslide deposits enance deposited in the Portland basin t Bretz/Missoula flood deposits — Pleistocene (15–23 ka t Columbia River Basalt — Miocene flood basalt flows [thousands of years]) sand and silt (fine facies) and pebble that originated in western Idaho and eastern Oregon and to boulder gravel (coarse facies) deposits from repeated Washington and entered the Portland Basin via an ances- glacial outburst floods that originated in Montana tral Columbia River channel (see Table 1, after References) t Portland Hills Silt — Pleistocene loess deposited in the t Scappoose Formation — Miocene marine tuffaceous Tualatin Mountains (Portland Hills) during recent ice ages.