Earth's earliest known extensive, thick carbonate platform suggested by new age constraints
Bibliographic record
Abstract
• Four areas of 2.86 Ga carbonate rock occur in Uchi subprovince, Superior Province. • New age data indicates they are chronostratigraphically equivalent. • This provides a chronologic framework for study of the Mesoarchean seafloor. • This recognizes Earth's oldest large carbonate platform. Proterozoic and Phanerozoic carbonate platforms have provided considerable information on how the flora, fauna and water chemistry of warm, shallow seas evolved through time. This contrasts with the relative scarcity of Archean examples of these extensive repositories of biochemical and chemical sediments. Until now the late Neoarchean Campbellrand-Malmani and Hamersley carbonate platforms have provided the only examples of extensive, thick Archean carbonate deposits. Scattered outcrop areas of Mesoarchean carbonate, up to 400 m thick, are present in western Superior Province, but past geochronology has assigned significantly different ages to them. A reappraisal of previously dated felsic volcanic rocks as sandstones, combined with new U-Pb zircon geochronology conducted on intermediate to felsic tuffs, determined that four of these carbonate occurrences, now scattered over 2300 km 2 , were deposited between 2.87 Ga and 2.85 Ga. The realization that an extensive, thick carbonate platform, and deeper water chronostratigraphic equivalents (Slate Bay Assemblage), are probably present in this area provides a basis for future comprehensive studies of the relationships between the various types of depositional processes and compositions of seawater chemistry developed on and above the Mesoarchean seafloor.
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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 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.006 | 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".