New UPb and Ar/Ar isotopic age constraints on the timing of Eocene magmatism, Fort Fraser and Nechako River map areas, central British Columbia
Bibliographic record
Abstract
Twenty-three new, precise, Eocene UPb and 40Ar/39Ar age determinations for calc-alkaline volcanic rocks of the Ootsa Lake Group and associated intrusive rocks, widespread in the Nechako Plateau in central British Columbia, constrain the timing of the Eocene magmatism to 53.247.6 Ma, with a local duration of as little as 23 million years. The new dates show that magmatism in the study area is partly coeval with that responsible for the Babine Igneous Suite Newman Volcanic Suite (5350 Ma) to the north, and for the Endako Group (5145 Ma), which overlies the Ootsa Lake Group; however, locally the three magmatic suites are distinct in age and (or) the strata record a magmatic hiatus of as much as 7.5 million years. The ages generally young from north to south (5247 Ma) along the western portion of the study area. The Babine Igneous Suite Newman Volcanic Suite represents the oldest member of this series. However, in the east, the Ootsa Lake Group volcanic rocks are generally older (5351 Ma). The anomalously older ages may be related to the interaction of magmatism and formation of a nearby and coeval core complex, which ongoing studies show was uplifted at about the same time during the Eocene. Felsic plutonism associated with the Ootsa Lake Group occurred between 50.5 and 47.3 Ma. These plutons were emplaced in an extensional setting along north-northeast-trending faults. The new dates, stratigraphic relationships, and suggested correlations of Eocene strata in the study area with that to the west and north require a revision of the stratigraphic nomenclature for the Ootsa Lake and Endako groups.
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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 | 0.002 | 0.002 |
| Science and technology studies | 0.001 | 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) | 0.004 | 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".