The Spatial and Temporal Evolution of the Portland and Tualatin Basins, Oregon, USA
Bibliographic record
Abstract
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, I 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) Mid-Miocene 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 Sylvan-Oatfield 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 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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.002 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".