U-Pb zircon ages of volcanic ashes integrated with ammonite biostratigraphy, Fernie Formation (Jurassic), Western Canada, with implications for Cordilleran-Foreland basin connections and comments on the Jurassic time scale
Bibliographic record
Abstract
Abstract The Jurassic system of the Western Canada Sedimentary Basin records the transition in its tectonic setting from a “passive” back-arc platformal basin to a foreland basin at the western margin of ancient North America. We report new U-Pb zircon ages from bentonite layers and from probable volcanic ash components of clastic detritus in other strata of the Fernie Formation, which encompasses most of the Jurassic in the western portions of the basin and which is now deformed in the Rocky Mountain fold-and-thrust belt. The bentonite ages come from the lower Nordegg Member (Pliensbachian) and an equivalent ash layer in the Lower Fernie phosphatic shale. Detrital zircon spectra from the Bathonian Gryphaea Bed silty limestone and the zircon ages from the mainly Oxfordian Green Beds glauconitic sandstone also are likely indicative of contemporaneous ash-falls. In addition, we review previously published U-Pb bentonite ages from the Fernie Formation and comment on the Jurassic time scale as represented on the International Chronostratigraphic Chart. We have compiled an updated local stratigraphic correlation chart against a time scale that incorporates ages for some of the Middle and Upper Jurassic stage boundaries, from the literature, that differ from those on the current standard charts. The presence of multiple volcanic ashes throughout the Jurassic system in the Western Canada Sedimentary Basin supports tectonostratigraphic models with relatively nearby western magmatic activity. The southeastern Omineca crystalline belt and Quesnellia terrane contain magmatic rocks with ages that could account for all of the Fernie ashes, and are closest to the depositional basin, but source terranes farther afield cannot be ruled out.
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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 | 0.004 | 0.004 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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".