Electron microprobe geochronology and chemical variation of detrital monazite from the Lower Cretaceous sandstones of the Scotian basin and the Chaswood Formation, eastern Canada
Bibliographic record
Abstract
Detrital petrology of sandstones of the Lower Cretaceous Chaswood Formation onshore Nova Scotia and New Brunswick and from the Missisauga and Logan Canyon formations of the Scotian basin was studied to understand sediment dispersal patterns, syn-sedimentary tectonic uplift, and paleogeography. Electron-microprobe geochronology on sparse detrital monazite grains complements classical petrographic work. Laboratory accuracy is conservatively estimated to be 5%. After removing grains showing Th or U mobility and single analyses that were extreme outliers, 401 analyses of 75 monazite grains were interpreted. In the Chaswood Formation, 98 analyses from Vinegar Hill gave a unimodal distribution of 426 ±15.3 Ma. In the Elmsvale basin, the distribution of 102 analyses was bimodal, with 433±4.2 Ma and 468±7.1 Ma for the base and 448±8.9 Ma and 498±7.3 Ma for the top of the formation. Only 38 analyses from Diogenes Brook showed a trimodal distribution of 439±8.2 Ma, 480±8.6 Ma and 543±14.5 Ma. At Brierly Brook, 49 analyses gave a mean age of 495±14 Ma, with 2 outlying grains with 614±15 Ma and 723±13.5 Ma. At Belmont, most grains were altered, but one grain gave an apparently reliable age of 527±8.9 Ma. In offshore wells from the Scotian basin, one grain from the MicMac Formation yielded an age of 1201±1.8 Ma. Five grains from the Missisauga Formation clustered around 499±10.5 Ma, with one grain 628±22.6 Ma. Five grains (40 analyses) from the Logan Canyon Formation gave an age of 504±9 Ma, two grains clustered at 1051±30 Ma and two at 1879±40 Ma. Monazite and other detrital petrographic data suggest four principal sources to the Lower Cretaceous sands. The ancestral St Lawrence river drained a hinterland that included Taconic ophiolites and metamorphic rocks and Proterozoic rocks of the Shield, with principal coarse sediment supply from western Newfoundland, uplifted as a result of Cretaceous rifting of the North Atlantic. An ancestral Saint John River was supplied with metamorphic detritus of Silurian age. Between these two rivers were two other river systems that lacked a Shield source and received principally Taconic metamorphic detritus, with lesser amounts of Silurian age.
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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 | 0.002 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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".