U–Pb zircon age of an Ordovician tuff in southeast Yukon: implications for the age of the Cambrian–Ordovician boundary<sup>1</sup>Yukon Geological Survey Contribution 013.<sup>2</sup>Geological Survey of Canada Earth Science Sector (ESS) Contribution 20110362.
Bibliographic record
Abstract
Three conodont fossil occurrences stratigraphically above and below a rhyolitic tuff in the Lower Ordovician Crow Formation, southeast Yukon, constrain the tuff to the early Tremadocian Rossodus manitouensis Zone and possibly the Rossodus tenuis Zone. U–Pb dates were obtained from zircon from the tuff that was imaged with cathodoluminescence and chemically abraded to reduce the likelihood of dating grains that contain older components or suffered Pb loss, respectively. Six dates from grains with oscillatory parallel zoning are equivalent, with a weighted mean 206Pb/238U date of 491.04 ± 0.13 Ma. These grains are interpreted as being primary volcanic crystals, and the date is therefore taken as the depositional age of the tuff. Four other dates from grains with sector zoning are slightly older, up to 492.0 Ma, and are interpreted as inherited or recycled from earlier volcanics. The currently defined Cambrian–Ordovician boundary is inferred to be below the tuff and separated from it by at least two conodont biostratigraphic zones that are estimated to span at least 2.25 Ma based on the top of the Rhabdinopora flabelliformis parabola graptolite zone being older than the base of the Rossodus tenuis conodont zone. We interpret the evidence in southeast Yukon to suggest that the global Cambrian–Ordovician boundary is older (>493.3 Ma) than previous estimates of ∼488 and 491 Ma based upon legacy 207Pb/206Pb zircon ages.
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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 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| 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) | 0.002 | 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".