Age and biostratigraphy of Early Cambrian tuffs from SE Australia and southern China
Bibliographic record
Abstract
Reassessment of faunal discoveries in the Fleurieu Peninsula, South Australia, shows that the Sellick Hill tuff is late Atdabanian or early Botomian in age and is preceded by Tommotian to Atdabanian equivalents. Previous and new sensitive high-resolution ion microprobe (SHRIMP) zircon ages from the tuff have been adjusted for Pb loss in the reference zircon, and mixture-modelling has been applied to resolve inheritance and Pb loss. The revised tuff age is now 522.0 ± 2.0 Ma ( σ ), 0.7% younger than the original estimate. Undocumented SHRIMP ages for zircons from a middle to upper Botomian tuff near Cymbric Vale were cited in 1994 as a single 525 ± 4 Ma population, but reprocessing using the present approach shows instead that the correct age of volcanism is 517.8 ± 2.1 Ma ( σ ). Reprocessed and new SHRIMP ages for zircons from tuff within Bed 5 of the Lower Cambrian Meishucun section, South China, give a revised age for the earlier Tommotian of 538.2 ± 1.5 Ma ( σ ). We agree with US workers that the stratotypic lowest Cambrian sequence of Newfoundland correlates within the Nemakit–Daldynian of Siberia. These results agree broadly with the Early Cambrian time scale based on zircon U–Pb isotope dilution data, but differences remain that arise more from interpretations of biostratigraphy and from the preservation and origin of the sampled zircon grains than from instrumental accuracy. In particular, the increased duration for the Tommotian indicated by our results supports notions of a more gradualistic evolution attending the origins of animal skeletons.
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How this classification was reachedexpand
Full frame distilled prediction
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) | 0.004 | 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 teacher head, 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".