Sponsor
This research was supported by the Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE) office through the Portland Deep Direct-Use Thermal Energy Storage (DDU-TES) Feasibility Study, grant #DE-EE0008104. Publication of this article in an open access journal was funded by the Portland State University Library's Open Access Fund.
Published In
Geosphere
Document Type
Article
Publication Date
6-1-2021
Subjects
Geology -- Oregon -- Portland Region, Faults (Geology) -- Oregon
Abstract
The Portland and Tualatin basins are part of the Salish-Puget-Willamette Lowland, a 900-km-long, forearc depression lying between the volcanic arc and the Coast Ranges of the Cascadia convergent margin. Such inland seaways are characteristic of warm, young slab subduction. We analyzed the basins to better understand their evolution and relation to Coast Range history and to provide an improved tectonic framework for the Portland metropolitan area. We model three key horizons in the basins: (1) the top of the Columbia River Basalt Group (CRBG), (2) the bottom of the CRBG, and (3) the top of Eocene basement. Isochore maps constrain basin depocenters during (1) Pleistocene to mid-Miocene time (0–15 Ma), (2) CRBG (15.5–16.5 Ma), and (3) early Miocene to late Eocene (ca. 17–35 Ma) time. Results show that the Portland and Tualatin basins have distinct mid-Miocene to Quaternary depocenters but were one continuous basin from the Eocene until mid-Miocene time. A NW-striking gravity low coincident with the NW-striking, fault-bounded Portland Hills anticline is interpreted as an older graben coincident with observed thickening of CRBG flows and underlying sedimentary rocks. Neogene transpression in the forearc structurally inverted the Sylvan-Oatfield and Portland Hills normal faults as high-angle dextral-reverse faults, separating the Portland and Tualatin basins. An eastward shift of the forearc basin depocenter and ten-fold decrease in accommodation space provide temporal constraints on the emergence of the Coast Range to the west. Clockwise rotation and northward transport of the forearc is deforming the basins and producing local earthquakes beneath the metropolitan area.
Rights
Copyright (c) 2021 The Authors
This work is licensed under a Creative Commons Attribution 4.0 International License.
Locate the Document
DOI
10.1130/GES02298.1
Persistent Identifier
https://archives.pdx.edu/ds/psu/35964
Citation Details
Scanlon, D. P., Bershaw, J., Wells, R. E., & Streig, A. R. (2021). The spatial and temporal evolution of the Portland and Tualatin forearc basins, Oregon, USA. Geosphere, 17(3), 804–823. https://doi.org/10.1130/ges02298.1
Description
Supplemental Materials. Table S1 contains links to publicly available data sources used in this study. Table S2 contains well names, surface elevations, and stratigraphic picks for surfaces described in the text. Please visit https://doi.org/10.1130/GEOS.S.13697626 to access the supplemental material,