Revision of the position of the Ordovician–Silurian boundary in southern Ontario: regional chronostratigraphic implications of δ<sup>13</sup>C chemostratigraphy of the Manitoulin Formation and associated strata
Bibliographic record
Abstract
δ13C values of 142 samples from the Manitoulin Formation and subjacent strata collected from 14 exposures and two drill-cores on Manitoulin Island, Bruce Peninsula, and the region south of Georgian Bay suggest that the Manitoulin Formation is latest Ordovician (Hirnantian) rather than earliest Silurian in age. A δ13C excursion identified as the Hirnantian isotope carbon excursion (HICE), which has a magnitude of nearly 2.5‰ above baseline values, is present in an interval from the upper Queenston Formation to the lower to middle part of the Manitoulin Formation in most of Bruce Peninsula and in the area south of Georgian Bay, whereas on Manitoulin Island the HICE appears to be absent. This indicates that a significant part of the Manitoulin Formation is older on the Bruce Peninsula and in its adjacent region than on Manitoulin Island. The chemostratigraphically based conclusions are consistent with biostratigraphic data from conodonts and brachiopods. The Hirnantian δ13C curve from Anticosti Island, Quebec is closely similar to those of southern Ontario. Traditionally, the Ordovician–Silurian boundary has been placed at the base of the Manitoulin Formation, but the new results suggest that it is more likely to be at, or near, the base of the overlying Cabot Head Formation. These new results have major implications for the interpretation of the geologic history and marine depositional patterns of the latest Ordovician of a large part of the North American Midcontinent.
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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 | 0.002 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.001 |
| 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".