Recognition of the Sedimentary Architecture of Dryland Anabranching (Anastomosing) Rivers
Notice bibliographique
Résumé
Abstract The application of rigorous criteria for identifying fluvial style from sedimentary deposits is crucial if environmental reconstructions from stratigraphy are to be reliable. From modern observations, it appears that anabranching (anastomosing) rivers can be divided into separate humid and dryland classes. Each class has dissimilar characteristics to the other, necessitating different criteria for their recognition in the rock record. The criteria being used by sedimentologists to recognize deposits of anabranching rivers are derived solely from modern examples in humid regions, heavily biased to a few specific cases in Canada. These criteria are dominated by the concept of ribbon sandstone bodies encased in fine-grained floodplain sediment, with an overall low sand-to-mud ratio in the succession. There has been a general failure to appreciate that dryland anabranching rivers have characteristics significantly different from these and almost certainly leave a sedimentary record quite dissimilar to that previously expected. Avulsion or obtrusion (the gradual formation of an alternative channel) are the characteristic dynamic of anabranching rivers, forming and maintaining coexisting multiple anabranch channels. Avulsion or obtrusion require stream banks to have a high resistance to erosion, usually achieved by cohesive bank sediments or the reinforcement of banks by vegetative root mats. In some drylands, riparian vegetation can be sparse and bank resistance may depend in large part on a high clay content. Here, channel fills are mud dominated and floodplains have a uniform fine-grained character which is usually structureless because of intense pedoturbation and some bioturbation. The evidence from such present-day dryland anabranching rivers suggests that their sedimentary record is likely to be one of laterally extensive mud-rich beds with weak or enigmatic fluvial indicators. Elsewhere, dryland anabranching systems are sand-dominated, their banks reinforced with riparian gallery forests sustained by infrequent flow and a shallow water table. Though the stratigraphy of these has barely been investigated, they are clearly different from humid-region anabranching systems, for their channels and islands are predominantly sandy. Anabranching rivers with prominent channel sandbodies separated by muddy islands have never been described from contemporary drylands. The interpretation of a dryland origin for ancient stratigraphic successions with this form is made, therefore, without evidence that they can actually exist. In the light of this, such interpretations should be reevaluated.
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Comment cette classification a été obtenuedéplier
Prédiction distillée sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Apprise à partir de 10 348 étiquettes directes de Codex et de 10 348 étiquettes directes de Gemma. Le mode candidate est l'union des têtes enseignantes seuillées; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont ni des étiquettes humaines ni des étiquettes directes de modèles de pointe.
Scores Codex et Gemma par catégorie
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,004 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,000 | 0,001 |
| Études des sciences et des technologies | 0,000 | 0,001 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,001 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,001 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,002 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule tête enseignante, pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».