Pyrite and Marcasite Coated Grains in the Ordovician Winnipeg Formation, Canada: An Intertwined Record of Surface Conditions, Stratigraphic Condensation, Geochemical "Reworking," and Microbial Activity
Bibliographic record
Abstract
Abstract Sulfide-coated grains are enigmatic sedimentary particles that occur in sediments ranging in age from Archean to Jurassic. Although their oolitic and pisoidal texture suggests energetic and agitated conditions, their mineralogy seems to require a reducing and stagnant environment. Because of this conflict, they have long been considered diagenetic replacements of precursor iron oxide, chamosite, or carbonate ooids. Coated iron sulfide (CIS) grains from the Ordovician Winnipeg Formation of Saskatchewan, Canada, formed as primary diagenetic precipitates under shallow burial conditions, involving intermittent episodes of tractional reworking, grain abrasion, reburial, and precipitation of further concentric laminae. These iron sulfide grains are found in shallow marine sandstones with indications of episodic wave reworking, and indicate sufficient organic-matter burial to promote reducing pore waters and iron sulfide formation. Horizons with strong enrichment of iron sulfide grains (up to 40%) indicate negative net sedimentation to facilitate geochemical "reworking" of iron. CIS grains formed during intervals of maximum sediment starvation, when organic matter was enriched in surface sediments, and rare storm waves intermittently reworked the surface sediment. Alternating laminae of pyrite and marcasite within these grains point to substantial fluctuations in pore-water pH, driven by intermittent oxidation of previously formed iron sulfides. Filamentous and coccoid structures within cortical laminae suggest microbial involvement in lamina formation through providing a growth medium for sulfate-reducing bacteria and potentially a template for mineral growth. Coated iron sulfide grains of the Winnipeg Formation preserve a combined record of condensation, seafloor reworking, fluctuating chemical conditions in the surface sediment, and high surface productivity. This information-rich record is made even more valuable by the fact that the host sediment consists largely of heavily bioturbated monotonous sandstones that could not provide these insights on their own merit.
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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 | 0.001 | 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".