Paleoenvironmental implications of an expanded microfossil assemblage from the Chamberlain Formation, Belt Supergroup, Montana
Bibliographic record
Abstract
The Chamberlain Formation, one of the lower members of the early Mesoproterozoic Belt Supergroup, has previously yielded low-diversity assemblages of microfossils but the reported fossils were of limited utility for inferring paleoenvironmental conditions. Here, we describe substantially more diverse microfossil assemblages from drill core of the Chamberlain Formation obtained from the Black Butte mine locality near White Sulphur Springs, Montana. The Chamberlain Formation biota contains abundant Valeria, Leiosphaeridia, Synsphaeridium, and Lineaforma, with lesser amounts of Satka, Symplassosphaeridium, andConiunctiophycus. The assemblages partially overlap with, but are distinct from, microfossils recently reported from the Greyson Formation, another unit from the Helena embayment of the Belt Supergroup. Since the overlapping taxa exhibit similar states of preservation but dissimilar relative abundances, we interpret the assemblages as reflective of distinct paleoenvironmental conditions of the sampled sections of the Chamberlain and Greyson Formations. The Chamberlain Formation assemblages are most comparable to microfossil groupings reported from the Bylot Supergroup of Canada and the Roper Group of Australia from sediments from very shallow-water (supratidal to lower shoreface) marine environments. This comparison corroborates previous hypotheses on the basis of sedimentological data that the lower Chamberlain Formation sediments were formed in a lagoonal or mud-flat environment. By contrast, the Greyson Formation assemblages are most comparable to microfossil groupings associated with sediments from shallow-shelf marine environments. The fidelity of comparisons among the 1.2 Ga Bylot Supergroup, 1.49 Ga Roper Group, and 1.45 Ga Belt Supergroup assemblages indicates that the groups of microorganisms that produced these assemblages, and their associations with the paleoenvironments that they inhabited, may have been characteristic of the littoral marine biosphere throughout much of the Mesoproterozoic.
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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 | 0.000 |
| 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 | 0.000 |
| Open science | 0.000 | 0.001 |
| 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".