Changing Alluvial Style in Response to Changing Accommodation Rate in a Proximal Foreland Basin Setting: Upper Cretaceous Dunvegan Formation, North‐East British Columbia, Canada
Bibliographic record
Abstract
The early–mid-Cenomanian Dunvegan Formation represents a major deltaic complex that prograded > 400 km from the north-west to the south-east, parallel to the rising Rocky Mountain Cordillera, over about 2 Myr. The formation comprises at least ten regressive–transgressive allomembers, separated by regional marine transgressive surfaces and, further up-dip, by lacustrine flooding surfaces. The lower five allomembers (J–F) have a sigmoidal geometry, thinning up-dip towards the north-west. Allomember E differs in showing in the north-west a pronounced isopach ‘moat’, into which strata thicken from 35 to 80 m over 120 km, suggestive of renewed subsidence along the basin margin. Collectively, allomembers D–A show an even more pronounced thickening towards the active basin margin, from 30 to 100 m over 250 km, and this geometry persists in the lower units of the overlying Kaskapau Formation. Outcrop sections of the Dunvegan Formation in the Chetwynd–Pine River area are located within the thickened isopach ‘moats’ of allomembers E–A, and are dominated by alluvial deposits about 160 m thick. Non-channelized alluvial facies represent lake, mouth-bar, levee and crevasse splay environments, and organic histosol (coal), poorly drained, intermediate redox and better drained palaeosols. Siderite, organic debris and dinosaur tracks are ubiquitous elements of these facies. Channelized facies are dominated by non-migrating (probably anastomosed) sandstone channel-fills. Non-channelized facies form two associations: A, which is dominated by crevasse splays, levees, intermediate redox and better drained palaeosols; B, which is dominated by lakes, mouth-bars, organic and poorly drained palaeosols. Facies association A, indicating relatively well-drained conditions, dominates allomembers G to the lower part of E. From near the middle to near the top, allomember E is progressively dominated by facies association B, but the uppermost 5 m shows an abrupt return to better drained palaeosols. Allomembers D to A show alternations between associations A and B, although overall, association B predominates upward. The basal units of the overlying Kaskapau Formation comprise a transitional succession 40–50 m thick dominated by facies of association B, but including brackish-water lagoonal deposits, overlain by marine deposits. The thinning of allomembers H–F towards the north-west suggests minimal accommodation in that direction, and this is reflected in their constituent alluvial facies (association A), dominated by relatively well-drained environments. The upward change within allomember E, from association A to relatively poorly drained association B, is interpreted to record an upward increase in accommodation rate. This change is independently suggested by the isopach ‘moat’ in allomember E, attributed to renewed flexural subsidence. A pedocomplex 5 m thick at the top of allomember E suggests a final phase of very low accommodation rate. Allomembers D–A are again dominated by poorly drained freshwater environments, which pass upward into rocks of the basal Kaskapau Formation which show progressively increasing marine influence. This facies succession reflects an increasing accommodation rate, also suggested by regional isopach patterns.
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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 | 0.001 |
| 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) | 0.013 | 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".