Detrital zircon provenance of Lower Cretaceous conglomerate beds in western Canada and the United States
Bibliographic record
Abstract
Lower Cretaceous deposits in western North America are characterized by a relatively thin, but widespread, interval of conglomerate beds that overlie a sub-Cretaceous unconformity. Although well documented, the tectonic and basinal significance of these units remains unclear. Provenance investigations have shed light on some aspects of these deposits (Schultheis and Mountjoy, 1978), however standard provenance methods are hindered by the fact that the conglomerate beds are composed largely of chert and quartzite clasts and their derivatives. We employed detrital zircon uranium-lead (U-Pb) geochronology to better understand the provenance of Lower Cretaceous conglomerate units, constrain their depositional ages, and elucidate the large-scale depositional history of the region during Early Cretaceous time. Our interpretations are based on 900 new detrital zircon U-Pb ages from samples of Lower Cretaceous conglomerate units exposed in British Columbia, Alberta, and Montana. Lower Cretaceous samples from the Cadomin Conglomerate in British Columbia and Alberta contain populations of detrital zircons with ages between 115-170 Ma and large populations with ages of ca. 1800 Ma. We postulate these sediments were derived from the erosion of Mesozoic arc-related rocks and Proterozoic through Lower Paleozoic strata exposed in thrust sheets in the Cordilleran Orogen. Maximum depositional age for these units varies, but is generally ca. 116-120 Ma. The Lower Cretaceous Kootenai Conglomerate from northern Montana contains similar age populations, although the majority of the zircons have ages between 120 and 180 Ma (Fuentes et al., 2009).
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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 | 0.001 | 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".