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TitreArchitecture of cyclic-step structures from a Holocene delta
AuteurDietrich, P; Ghienne, J -F; Normandeau, A; Bouysson, M; Schuster, M; Lajeunesse, P
SourceISC2018 - 20th International Sedimentological Congress, abstract volume; 2018 p. 1
Séries alt.Ressources naturelles Canada, Contribution externe 20180226
RéunionISC2018 - 20th International Sedimentological Congress; Québec, QC; CA; août 13-17, 2018
Documentsite Web
Mediapapier; numérique
Lat/Long OENS -68.0000 -66.0000 51.0000 49.5000
Sujetsdeltas; dépôts proglaciaires; sables; boues; graviers; structures sédimentaires; processus cycliques; configurations lit; bathymétrie; faciès; laminations; surfaces d'érosion; levés photogrammétriques; interprétation de photos aériennes; analyses granulométriques; Holocène; géologie des dépôts meubles/géomorphologie; sédimentologie; Phanérozoïque; Cénozoïque; Quaternaire
ProgrammeGéo-risques marins, Géoscience pour la sécurité publique
Diffusé2018 08 01
Résumé(disponible en anglais seulement)
It has been acknowledged that the frequency in the stratigraphic record of cyclic steps and other related supercritical bedforms has been largely underestimated. Identification at outcrop of the structures originating from the upslope migration of long-wavelength, fine-grained bedforms characterizing the deep sea remains problematic. However, our knowledge of coarser-grained and less extensive (m's to tens of m) supercritical structures recently profited from an increasing number of studies (bathymetric surveys, depositional facies, seafloor monitoring) that have essentially focused on sandy deltaic settings. Here, we present a case study from a Holocene proglacial delta succession, lying on the Québec North Shore of the St Lawrence Estuary. Large outcrops, owing to glacio-isostatic uplift and ensuing coastal erosion, have allowed the full architecture of beds and laminations deposited by the migration of cyclic steps along delta foresets to be determined.
The structures, developed in sand-sized material with subordinate muds and gravels, are observed in the upper segment of relatively low-angle (2-4°) delta foresets. Gravelly sands characterize the topsets, while fine sand and mud were exported downslope. Undulating 'beds', 5-20 cm thick, showing minimum wavelengths in the 10-20 m range and internal faint lamination, are truncated downstreamward by, and are onlapping upstreamward on, inclined composite erosional surfaces. These erosional surfaces have dips greater (10-20°) than the foresets and are regularly spaced and hence appear as pseudo-foresets. Scours, especially in the lower reaches of these pseudo-foresets, are filled in by the coarsest material. Higher up, heavy mineral concentrations are observed underlining these erosional surfaces.
Our multiscale survey (aerial photo cover, outcrop, laser particle size analysis) allow (i) the reconstruction of the parent bedforms that corresponded to three-dimensional, upstream-migrating structures; (ii) the distinction between 'laminae' vs. 'beds' in cyclic step complexes, the latter resembling individual turbiditic beds; (iii) the inventory of associated second-order sedimentary structures (load, flame, ripples and mud drapes...); (iv) the comparison with present-day active delta surfaces; (v) a better understanding of the processes tied to cyclic step migrations in deltaic settings.
Résumé(Résumé en langage clair et simple, non publié)
Des coupes stratigraphiques nous ont permis d'identifier les faciès associés à des structures sédimentaires appelées cyclic steps. Ces formes de terrain sont présentes en milieu marin, mais sont rarement identifiées dans les coupes stratigraphiques anciennes dues à un manque de connaissances quant à leurs critères d'identification.