Stratigraphy and megaflora of a K-T boundary section in the eastern Denver Basin, Colorado
Bibliographic record
Abstract
A 50-m-thick section of the Denver Formation (D1 sequence of Raynolds) exposed at the West Bijou Site of the Plains Conservation Center, Arapahoe County, Colorado, contains the first complete Cretaceous-Tertiary (K-T) boundary section in the Denver Basin. The 3-cm-thick boundary claystone coincides with a 21 percent palynological extinction, contains iridium and shocked-mineral anomalies, and is immediately overlain by a 7-cm-thick, fern-spore anomaly interval. The entire 50-m section is of reversed magnetic polarity and is interpreted to be subchron C29r because of paleontological data, the presence of a tuff with an 40 Ar/39 Ar date of 65.73 ± 0.13 Ma, and by correlation to the Kiowa cored well. A diagnostic basal Puercan (Pu1) mammal jaw was found 12 m above the boundry clay, and ceratopsian and hadrosaurian dinosaur fragments occur 4 m below the boundary clay. Estimates of the basin sedimentation rate derived from the duration of C29r based on marine cyclostratigraphy suggest that the 28 m of basal Paleocene strata represent approximately 200,000 years. Abundant fossil leaves found within the Paleocene sedimentary rocks at the West Bijou Site K-T boundary section allow assessment of early Paleocene patterns of plant diversity. Nine leaf localities at eight stratigraphic levels in the basal 22 m of the Paleocene section were sampled and analyzed to better understand the flora that survived the global K-T catastrophe. The Paleocene flora of this site is taxonomically dominated by dicotyledonous angiosperms (74%), with lesser numbers of monocotyledons (10%), ferns and allies (11%), and conifers (5%). By number of specimens, angiosperms comprise greater than 95 percent of the flora. Within the sampled section, there were no recognizable directional trends in diversity or abundance, suggesting that earliest Paleocene vegetation was stable, although not particularly diverse. The West Bijou Megaflora is strikingly similar in composition and relative abundance to basal Paleocene floras from the northern Great Plains. This K-T boundary disaster-recovery flora, also known as the FUI flora, is shown to have been widespread, ranging from central Colorado to southern Canada, a distance of at least 1,100 km. The West Bijou Site megaflora is markedly different from coeval floras along the western margin of the Denver Basin, supporting the hypothesis that orography and elevation provided greater influence on basal Paleocene floral composition and diversity than did latitude.
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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 | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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".