Geochemistry, geology, and isotopic (Sr, S, and B) composition of evaporites in the Lake St. Martin impact structure: New constraints on the age of melt rock formation
Bibliographic record
Abstract
We report new Sr, S, and B isotopic data for evaporites (gypsum, anhydrite), carbonates, melt rocks, gneisses, and groundwaters recovered in and around the Lake St. Martin (LSM) impact structure, Interlake Region, Manitoba, Canada. The LSM meteorite impacted Devonian to Ordovician carbonates and sandstones of the eastern Williston Basin, resulting in partial melting of underlying Superior Province (∼2.5 Ga) gneisses of the Canadian Shield. Overlying the LSM melt rocks are red beds and evaporites (anhydrite/gypsum/glauberite) previously inferred to have been deposited during the Jurassic. The 87 Sr/ 86 Sr (lowest values cluster at 0.70836) and δ 34 S CDT (+23.7 ± 0.9‰) of the evaporites, combined with B isotope compositions of associated groundwaters ( δ 11 B NBS951 = +25 to +28‰), are consistent with evaporite deposition within the impact structure near the edge of an ocean‐connected salina. The establishment of a marine origin for the evaporites offers a method of age assignment using the secular variation of S and Sr isotopes in seawater. Comparison of Sr and S isotope results with the seawater curves precludes Jurassic deposition for the evaporites or correlation with Watrous and Amaranth formation evaporites, previously considered correlative with those at LSM. The lowest Sr and mean of S isotope values from the LSM evaporites are similar to seawater in the latest Devonian, consistent with conodonts recovered from carbonate breccia overlying melt rocks, and we suggest this as an alternative age of the evaporites. Data presented here preclude a Jurassic age for the evaporites and therefore for the impact event.
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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) | 0.000 | 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".