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TitleUpslope migrating bedforms in a proglacial sandur delta: cyclic steps from river-derived underflows?
AuthorDietrich, P; Ghienne, J F; Normandeau, AORCID logo; Lajeunesse, P
SourceEuropean Geosciences Union General Assembly 2017 abstracts; 2017.
Alt SeriesEarth Sciences Sector, Contribution Series 20160431
PublisherEuropean Geoscience Union
MeetingEuropean Geosciences Union General Assembly 2017; Vienna; AT; April 23-28, 2017
Mediapaper; on-line; digital
File formatpdf
Lat/Long WENS -66.5000 -66.0000 50.2500 50.0000
Subjectssedimentology; surficial geology/geomorphology; proglacial lakes; proglacial deposits; glacial deposits; bathymetry; deltas; deltaic sediments; deltaic deposits; glaciation; deglaciation; isostatic rebound; sedimentary structures; turbidite sedimentation; flow structures; cross-stratification; hummocks; erosional surfaces; Pleistocene; sandstones
Released2017 01 01
AbstractBackstepping 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.
Summary(Plain Language Summary, not published)
This paper discusses backstepping cross-strata identified in a proglacial fluviodeltaic system resulting from sediment-laden supercritical flows plunging and evolving into an underflow that generated cyclic steps

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