Greenschist-facies metamorphism of the Burgess Shale and its implications for models of fossil formation and preservation
Bibliographic record
Abstract
Metamorphosed silty mudstones of the Burgess Shale and Stephen Shale formations record a poly metamorphic history. An early greenschist-facies event associated with burial by Paleozoic strata produced a nearly ubiquitous bedding-parallel cleavage (S1). Tectonic exhumation during the formation of the southern Canadian Rocky Mountains produced a domainal subgreenschist-facies retrograde overprint in which a high-angle crenulation cleavage (S2) was developed. Whereas all rocks have experienced these two events, the degree of deformation and fossil pre servation varies with position relative to the Cathedral Escarpment. This paleosubmarine cliff resulted in a zone of reduced deformation within adjacent strata by buttressing them during burial and deflecting deformation during orogenesis. Fossil-bearing strata are composed of a typical greenschist assemblage of muscovitechloritequartzalbite, are devoid of clays, and contain an average of 0.28% organic carbon. This typical metamudstone assemblage is consistent with the typical whole-rock composition of these rocks which tends to be richer in K and Al and poorer in Fe relative to the Post-Archean Average Shale. These mineralogicalcompositional characteristics suggest that the premetamorphic clay assemblage was likely illitesmectitekaolinite, with no evidence of highly reactive species such as nontronite or Na-montmorillonite. This is contrary to the required conditions for taphonomic models involving organic preservation due to clay-related suppression of decomposition-related reactions. Metamorphism of the Burgess Shale has also re duced the total organic carbon content to <20% of initial values. This must be considered in any models that involve interpretation of organic carbon in diagenetic processes (e.g., fossil formation and determination of paleoredox conditions).
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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 | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.002 |
| Scholarly communication | 0.002 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.000 |
| Insufficient payload (model declined to judge) | 0.004 | 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".