Origin of shore‐normal channels from the shoreface of Sable Island, Canada
Notice bibliographique
Résumé
A suite of modern, shore‐normal channels was discovered on the southern shoreface of Sable Island, Canada. Repetitive multibeam and side scan surveys of these features, as well as box coring and sampling, showed that the depressions were perennial features of the shoreface. They occurred in fine and medium sand and were most abundant in‐depths less than 20 m. They were up to 1 m deep and 50 m wide, increased in size seawards, and generally followed the seabed slope. The channels dominated the inner shoreface becoming wider and deeper seawards to depths in excess of 40 m. There is a continuum of scales in width and depth of these channels, the smallest of which are equated with gutters preserved in the geological record. The internal structure of the shoreface sediments where the gutters are found is characterized by normally graded tempestites composed of cut‐and‐fill structures overlying an erosion surface. Thus, the environmental setting, internal structure, and scale are similar to ancient counterparts. Gutters have been interpreted to result from downwelling over palaeo‐shorefaces during ancient storms [ Myrow, 1992b ]. The composition of gutter infill, as well as the stability, evolution, associated bed forms, and hydrodynamic conditions under which they formed, were examined to verify genetic interpretations of the ancient counterparts. The multisensor benthic lander RALPH was deployed within a field of shore normal channels for 17 days during late winter 1998. RALPH burst‐sampled flow at hourly intervals while imaging the seabed. Resulting sequential imagery revealed the genesis of a group of five gutters during storm spin‐down that was coincident with the formation of large‐scale wave ripples. This was followed by infilling of the gutters and wave ripple destruction within 20 hours by longshore sand transport. The gutters were located within, and parallel to, a larger channel (50 m wide and 0.50 m deep). They were formed by coastal set‐up and subsequent downwelling that was triggered by the rotation of storm winds from seawards (onshore wind) to landwards (offshore wind). The gutters formed within 2–3 hours during spin‐down of a winter storm under “live‐bed” conditions of sand transport. Wave height and mean tidal flows were well below those of the storm peak and so they were not directly responsible for creating the gutters. The mechanism we propose for the evolution of gutters supports the geological interpretations; that is, a storm‐driven, downwelling event as described by Swift and Niedoroda [1985] . However, the downwelling event appeared to take place suddenly (rapid acceleration in flow), briefly (few hours duration), and within narrow corridors guided by wider and deeper channels of the shoreface. High turbidity during the time of gutter formation leads us to believe that the phenomenon of inner shelf turbidity currents [as suggested by Walker, 1985 ] exists and may be instrumental in channel formation.
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Comment cette classification a été obtenuedéplier
Prédiction machine sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Le volet Gemma est une étiquette directe du modèle pour chaque travail de la base, lue sur la notice réduite au titre. Le volet Codex est un classifieur appris des 10 348 étiquettes directes de Codex et calibré sur les taux pondérés de l'échantillon; les champs sans appui suffisant ne portent aucun appel Codex. Le mode candidate est l'union des deux volets; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont pas des étiquettes humaines.
Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,001 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,001 | 0,001 |
| Études des sciences et des technologies | 0,002 | 0,001 |
| Communication savante | 0,001 | 0,000 |
| Science ouverte | 0,000 | 0,001 |
| Intégrité de la recherche | 0,000 | 0,000 |
| 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 source (Gemma direct ou Codex distillé), 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 ».