| Titre | The depositional signature of cyclic steps: a late Quaternary analogue compared to modern active delta slopes |
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| Auteur | Ghienne, J-F; Normandeau, A ; Dietrich, P; Bouysson, M; Lajeunesse, P; Schuster, M |
| Source | Sedimentology 2020 p. 1-37, https://doi.org/10.1111/sed.12806  Accès ouvert |
| Image |  |
| Année | 2020 |
| Séries alt. | Ressources naturelles Canada, Contribution externe 20190106 |
| Éditeur | Blackwell Publishing Ltd. |
| Document | publication en série |
| Lang. | anglais |
| DOI | https://doi.org/10.1111/sed.12806 |
| Media | papier; en ligne; numérique |
| Formats | pdf; html |
| Lat/Long OENS | -127.0000 -49.0000 74.8333 45.8333 |
| Sujets | configurations lit; antecedents de sedimentation; cycles sédimentaires; deltas; géologie marine; sédimentologie; Sciences et technologie; Nature et environnement; Phanérozoïque; Cénozoïque;
Quaternaire |
| Illustrations | cartes de localisation; photographies; coupes transversales; diagrammes; échelles stratigraphiques; tableaux |
| Programme | Géoscience pour la sécurité publique Évaluation des géorisques terrestres et marins |
| Diffusé | 2020 10 06 |
| Résumé | (disponible en anglais seulement)
Cyclic-step bedforms typifying a Froude-supercritical flow regime are a recurrent component of depositional/erosional turbiditic systems. Over modern delta
slopes, cyclic steps have been inferred from observations of upslope-migrating crescent-shaped bedforms. However, the recognition in the sedimentary record of the resulting stratal pattern and depositional facies remains challenging. In this study,
the depositional facies observed in exposed late Quaternary glaciofluvial upper delta-slope sands (Portneuf-Forestville, Québec) are compared to those cored from a modern analogue consisting of sediment waves interpreted as cyclic steps (Southwind
Fjord, Baffin Island). The fossil and modern delta slopes share similar context, morphology and stratigraphic record. The clinoform foresets dip 2 to 6° and consist of prevailing sand-sized material including subhorizontal to upslope-dipping
top-cut-out turbidites. Individually, the latter are 5 to 20 cm thick and massive to planar laminated (prevailing TA and TB subdivisions). In the fossil delta slope, related successions form relatively thick, well-bedded suites, which are truncated
downcurrent by, and onlap upstreamward on, inclined composite erosion surfaces here referred to as pseudo-foresets. Pseudo-foresets are regularly spaced (10 to 30 m) and have dips greater than the clinoform foresets (ca 20°). Large composite scours
form pseudo-channel structures filled in by structureless pebbly sand deformed by sheared flame structures, in association with coarse sand showing undulating lamination and rip-up clasts. Similar depositional facies are observed on the modern delta
slope. The stratal pattern is best compatible with upslope-migrating bedforms and structureless sand indicates hydraulic jump deposits typical of cyclic steps. Cyclic-step flow events, encompassing a succession of genetically linked erosional cyclic
steps, depositional cyclic steps and subsequent waning-flow conditions, were associated with the dense basal layer of high-density (stratified) turbidity currents. They are specifically associated with pseudo-channel incision and infill. The
deposition of well-bedded suites on the stoss side, occasionally also on the lee side, is interpreted to result from a different type of turbidity current, which decelerated over the bedform stoss and accelerated over the lee sides of pre-existing
cyclic-step morphologies. Antidunal flow conditions in expanded (non-stratified) turbidity currents are tentatively suggested, deposition being linked, in this case, to interactions between inherited bedform morphologies and a near-bed tractive
layer. In the resulting dual flow model, the bulk of delta-slope sands was mainly deposited from turbidity currents not developing a cyclic-step instability, yet cyclic steps were instrumental in shaping and/or re-organizing the delta-slope
morphology, bedforms and resulting stratal patterns. In some delta systems, the upslope migration of supercritical crescentic bedforms may not only be due to repetitive cyclic steps but could also result from antidunal turbidity current conditions
remobilizing an inherited cyclically-stepped morphology. |
| Sommaire | (Résumé en langage clair et simple, non publié)
La stratigraphie de la pente d'un delta moderne (fjord Southwind, sur l'île de Baffin) a été comparée à celle d'un affleurement deltaïque émergé
(delta de Portneuf, à Québec) afin de proposer une nouvelle vision des structures portant le nom de gradins cycliques observées dans un contexte deltaïque. |
| GEOSCAN ID | 327885 |
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