| Titre | Upslope migrating bedforms in a proglacial sandur delta: cyclic steps from river-derived underflows? |
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| Auteur | Dietrich, P; Ghienne, J F; Normandeau, A ; Lajeunesse, P |
| Source | European Geosciences Union General Assembly 2017 abstracts; 2017. |
| Image |  |
| Année | 2017 |
| Séries alt. | Secteur des sciences de la Terre, Contribution externe 20160431 |
| Éditeur | European Geoscience Union |
| Réunion | European Geosciences Union General Assembly 2017; Vienna; AT; avril 23-28, 2017 |
| Document | livre |
| Lang. | anglais |
| Media | papier; en ligne; numérique |
| Formats | pdf |
| Province | Québec |
| SNRC | 22J/01 |
| Lat/Long OENS | -66.5000 -66.0000 50.2500 50.0000 |
| Sujets | lacs proglaciaires; dépôts proglaciaires; dépôts glaciaires; bathymétrie; deltas; sédiments deltaïques; dépôt deltaïque; glaciation; déglaciation; relèvement isostatique; structures sédimentaires;
sédimentation turbiditique; structures flui dales; stratification entrecroisée; buttes; surfaces d'érosion; Pléistocène; grès; sédimentologie; géologie des dépôts meubles/géomorphologie |
| Diffusé | 2017 01 01 |
| Résumé | (disponible en anglais seulement)
Backstepping cross-strata on steep foresets of a Gilbert-type proglacial fluviodeltaic system are ascribed to cyclic steps and other associated supercritical
bedforms. They provide insight into how sandur river flows transition into the marine realm. These sedimentary structures are located on steep foresets (up to 17°) with corresponding top-lying, flat-based topsets in an upper Pleistocene delta on the
North Shore of the Gulf of St. Lawrence, Québec, Canada. Packages of backstepping cross strata of sand and gravel, lying in the lower part of the delta front outcrop, are organized in 10-20 m spaced pseudo-foresets with a mean slope of 11-12°
seawards. Backstepping strata include frequent internal erosion surfaces that onlap upslope on pseudo-foresets and are interpreted as cyclic steps. Narrow, deep, and asymmetrical scours and upslope-climbing cross beds are interpreted as
chutes-and-pools and antidunes respectively. Very shallow (less than 15 m) depositional paleo-bathymetry is inferred from the preservation of the delta brink. The well-organized stratal pattern in cyclic step to antidune deposits indicates relatively
steady and uniform flow patterns. There is insufficient distance for a headscarp large enough to transform to the volume of observed accreted sands or for a flow transformation from a gravitational collapse to net-depositional cyclic steps. These
deposits are sandier than the topsets beds and are thus not derived from them, but rather correspond to topset erosional surfaces. The development of cyclic steps from hyperpycnal flows was likely enhanced by tidal drawdown processes. The resulting
sediment-laden supercritical flows plunged inertially and evolved into an underflow that generated the cyclic steps on the upper foresets. The cyclic steps have a high aspect ratio and represent an end member of coarse-grained sediment deposited on
steep slopes, in contrast to low-gradient, low-aspect-ratio muddy deposits. |
| Sommaire | (Résumé en langage clair et simple, non publié)
Cet article discute des strates croisées rétrogrades identifiées dans un système fluviodeltaïque proglacial résultant d'écoulements
supercritiques chargés de sédiments plongeant et évoluant en un courant sous-glaciaire qui a produit des gradins cycliques. |
| GEOSCAN ID | 299869 |
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