TRACKING THE SPATIAL AND TEMPORAL EVOLUTION OF THE PORTLAND AND TUALATIN BASINS, NORTHWEST OREGON

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, suggesting the basins have a complex history. To better understand basin evolution, we 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 time (0-15 Ma), 2) eruption of the Columbia River Basalt Group (CRBG, 15.5-16.5 Ma), and 3) mid-Miocene to late Eocene time (~17-35 Ma). Results show that the two basins each have distinct Pleistocene to mid-Miocene depocenters. The depth to CRBG in the Portland basin is about ~1640 ft, about ~160 ft more 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 sometime prior to CRBG deposition. Our mid-Miocene to Eocene isochore map shows a package of sedimentary rocks overlying Siletz basement with a maximum thickness of ~5.3 km, thinning eastward from the Tualatin into the Portland basin, corroborating the interpretation that the two basins were connected during that time. Isochore maps also show significant local thickening of Eocene to mid-Miocene rocks over a gravity low coincident with the Portland Hills, suggesting that the Portland Hills uplift was the product of Neogene structural inversion. A change in the stress regime at that time marks the transition from Paleogene forearc magmatism associated with extension to deformation dominated by north-south shortening due to collision of the forearc against the Canadian Coast Mountains. This work integrates Tualatin and Portland basin history into our understanding of Cascade forearc evolution, and it provides a foundation for ongoing seismic hazard research in the Portland metropolitan and surrounding areas.