Log Jams and Flood Sediment Buildup Caused Channel Abandonment and Avulsion in the Pennsylvanian of Atlantic Canada
Bibliographic record
Abstract
Abstract Accumulations of logs and flood sediment frequently block modern channels and may trigger avulsion, but these effects are difficult to demonstrate for the ancient record. Braided-fluvial channels in the Pennsylvanian South Bar Formation of Atlantic Canada contain sandstone successions up to 6 m thick of sigmoidal cross-beds, plane beds, and antidunes, deposited rapidly at high-flow-stage. These strata are commonly capped by accumulations up to 2.5 m thick of flattened, coalified logs and coal intraclasts (originally peat fragments), many of which are overlain by mudstone laid down in abandoned channels. The logs include lycopsids, calamiteans, tree ferns, pteridosperms and cordaitaleans, inferred to have grown on inactive braided tracts near the channels. A compaction estimate suggests that one log accumulation was originally more than four times its present thickness. Most accumulations are interpreted as stable “transport log jams” formed during floods, although some may have been “unstable jams” stranded on bars during peak-flow recession. Associated with the logs are extrabasinal gravel and intraclasts of mudstone and coal, which suggest that floods in sediment-choked channels undercut banks of gravelly sand capped by mud and forested peat, widened the channels, and toppled riparian vegetation. An estimated blockage ratio of 8% for one accumulation (ratio of the cross-sectional areas of the log jam and host channel) is close to the 10% value considered to cause substantial blockage in some modern rivers. In two instances, a radical change in paleoflow between pre- and post-abandonment channels is consistent with an interpretation that log jams and flood sediment buildup promoted channel-belt avulsion. Although large trees had evolved by Middle to Late Devonian times, it is unlikely that riparian plants occurred in stands that were sufficiently dense to exert a major influence on river dynamics until the Pennsylvanian. Thus, we report some of the earliest evidence for the effects of woody debris on ancient fluvial systems.
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
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 | 0.003 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 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".