Upslope-Migrating Bedforms In A Proglacial Sandur Delta: Cyclic Steps From River-Derived Underflows?

Abstract : 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 176) 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 11-126 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 (, 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.
Document type :
Journal articles
Complete list of metadatas

Cited literature [47 references]  Display  Hide  Download

https://hal.archives-ouvertes.fr/hal-02067992
Contributor : Pierre Dietrich <>
Submitted on : Thursday, March 14, 2019 - 3:08:44 PM
Last modification on : Friday, April 5, 2019 - 8:16:13 PM
Long-term archiving on : Saturday, June 15, 2019 - 7:35:17 PM

File

Dietrich et al., 2016 cyclic s...
Publisher files allowed on an open archive

Identifiers

Collections

Citation

Pierre Dietrich, Jean-François Ghienne, Alexandre Normandeau, Patrick Lajeunesse. Upslope-Migrating Bedforms In A Proglacial Sandur Delta: Cyclic Steps From River-Derived Underflows?. Journal of Sedimentary Research, Society for Sedimentary Geology, 2016, 86 (2), pp.113-123. ⟨10.2110/jsr.2016.4⟩. ⟨hal-02067992⟩

Share

Metrics

Record views

19

Files downloads

42