Portland DDU Feasibility Study: The Spatial and Temporal Evolution of the Portland and Tualatin Basins, Oregon, USA Scanlon2019EvolutionPortlandTualatinBasinsTHESIS.pdf

The Portland and Tualatin basins are part of the Puget-Willamette Lowland in the Cascadia forearc of Oregon and Washington. The Coast Range to the west has undergone Paleogene transtension and Neogene transpression, which is reflected in basin stratigraphy. To better understand the tectonic evolution of the region, Darby Scanlon modeled three key stratigraphic horizons and their associated depocenters (areas of maximum sediment accumulation) through space and time using well log, seismic, outcrop, aeromagnetic, and gravity data. Three isochore maps were created to constrain the location of Portland and Tualatin basin depocenters during 1) Pleistocene to mid-Miocene (0-15 Ma), 2) eruption of the Columbia River Basalt Group (CRBG, 15.5-16.5 Ma), and 3) MidMiocene to late Eocene time (~17-35 Ma).

Results show that the two basins each have distinct mid-Miocene to Pleistocene depocenters. The depth to CRBG in the Portland basin reaches a maximum of ~1,640 ft, 160 ft deeper than the Tualatin basin. Although the Portland basin is separated from the Tualatin basin by the Portland Hills, inversion of gravity data suggests that the two were connected as one continuous basin prior to CRBG deposition. Local thickening of CRBG flows over a gravity low coincident with the Portland Hills suggests that Neogene transpression in the forearc reactivated the SylvanOatfield and Portland Hills faults as high angle reverse faults. This structural inversion separated the once continuous Portland and Tualatin basins in the mid-late Miocene. A change in the stress regime at that time marks the transition from Paleogene forearc extension to deformation dominated by north-south shortening due to collision of the forearc against the Canadian Coast Mountains. An eastward shift of the forearc basin ii depocenter over the Neogene likely reflects uplift of the Coast Range to the west. A change in regional stress in the mid to late-Miocene, along with uplift of the Oregon Coast Range, caused a 10-fold decrease in sediment accumulation rates across the Portland and Tualatin basins. Transpressional oblique-slip faulting continues to deform the region as the forearc undergoes clockwise rotation and collides with the rigid Canadian Coast Mountains to the north. Darby Scanlon Master's Thesis which contains methods and products used in the study fo the Tualatin Basin .

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Field Value
Citation Date 2019-07-29T00:00:00-06:00

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Original ID f0000000-58cc-4372-a567-000000001167
Index Date 2019-08-08T16:09:48-06:00
Original Format ISO-USGIN
Original Version 1.2


Name Darby Scanlon
Position primary contact
Organization Portland State University
Email scanlon.darby3@gmail.com

Geographic Extent

North Bound 45.977309536237
South Bound 45.07371741078
East Bound -122.21050625
West Bound -123.4298875