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TitleArchitecture of cyclic-step structures from a Holocene delta
AuthorDietrich, P; Ghienne, J -F; Normandeau, AORCID logo; Bouysson, M; Schuster, M; Lajeunesse, P
SourceISC2018 - 20th International Sedimentological Congress, abstract volume; 2018 p. 1
Alt SeriesNatural Resources Canada, Contribution Series 20180226
MeetingISC2018 - 20th International Sedimentological Congress; Québec, QC; CA; August 13-17, 2018
DocumentWeb site
Mediapaper; digital
File formatpdf
AreaSt. Lawrence Estuary; Quebec North Shore
Lat/Long WENS -68.0000 -66.0000 51.0000 49.5000
Subjectssurficial geology/geomorphology; sedimentology; deltas; proglacial deposits; sands; muds; gravels; sedimentary structures; cyclic processes; bedforms; bathymetry; facies; laminations; erosional surfaces; photogrammetric surveys; airphoto interpretation; grain size analyses; Holocene; Phanerozoic; Cenozoic; Quaternary
ProgramPublic Safety Geoscience Marine Geohazards
Released2018 08 01
AbstractIt 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.
Summary(Plain Language Summary, not published)
Trough the examination of sedimentary outcrops, we identified the facies on a type of bedform called cyclic steps. These bedforms are widespread in the marine environment but are hardly ever identified in outcrops due to lack of recognition criteria.

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