Age and volcanic stratigraphy of the Eocene Siletzia oceanic plateau in Washington and on Vancouver Island
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Bibliographic record
Abstract
Geophysical, geochemical, geochronologic, and stratigraphic observations all suggest that the basalts that underlie western Oregon and Washington (USA), and southern Vancouver Island (Canada) form a coherent terrane of Eocene age, named Siletzia. The total volume of basalt within Siletzia is comparable to that observed in large igneous provinces and several lines of evidence point toward the terrane's origin as an accreted oceanic plateau. However, a thick sequence of continentally derived turbidites, named the Blue Mountain unit, has long been considered to floor the northern part of the terrane and its presence has led to alternative hypotheses in which Siletzia was built on the continental margin. We present new high-precision U-Pb zircon dates from silicic tuffs and intrusive rocks throughout the basaltic basement of northern Siletzia, as well as detrital zircon age spectra and maximum depositional ages for the Blue Mountain unit to help clarify the volcanic stratigraphy of this part of the terrane. These dates show that northern Siletzia was emplaced between 53.18 0.17 Ma and 48.364 0.036 Ma, similar to the age and duration of magmatism in the central and southern parts of the terrane. Turbidites in the basal Blue Mountain unit have maximum depositional ages as young as 44.72 0.21 Ma and are distinctly younger than the basaltic basement that forms Siletzia. This age relationship implies that they were thrust under the terrane after 44.72 0.21 Ma along one or more enigmatic faults. The younger age for these sedimentary rocks no longer requires construction of Siletzia on the continental margin, and we consider our revised stratigraphy to provide further support for the origin of the terrane as an accreted oceanic plateau.
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Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
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Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
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