The depositional signature of cyclic steps: A late Quaternary analogue compared to modern active delta slopes
Résumé
Cyclic‐step bedforms typifying a Froude‐supercritical flow regime are a recurrent component of depositional/erosional turbiditic systems. Over modern delta slopes, cyclic steps have been inferred from observations of upslope‐migrating crescent‐shaped bedforms. However, the recognition in the sedimentary record of the resulting stratal pattern and depositional facies remains challenging. In this study, the depositional facies observed in exposed late Quaternary glaciofluvial upper delta‐slope sands (Portneuf‐Forestville, Québec) are compared to those cored from a modern analogue consisting of sediment waves interpreted as cyclic steps (Southwind Fjord, Baffin Island). The fossil and modern delta slopes share similar context, morphology and stratigraphic record. The clinoform foresets dip 2 to 6° and consist of prevailing sand‐sized material including subhorizontal to upslope‐dipping top‐cut‐out turbidites. Individually, the latter are 5 to 20 cm thick and massive to planar laminated (prevailing TA and TB subdivisions). In the fossil delta slope, related successions form relatively thick, well‐bedded suites, which are truncated downcurrent by, and onlap upstreamward on, inclined composite erosion surfaces here referred to as pseudo‐foresets. Pseudo‐foresets are regularly spaced (10 to 30 m) and have dips greater than the clinoform foresets (ca 20°). Large composite scours form pseudo‐channel structures filled in by structureless pebbly sand deformed by sheared flame structures, in association with coarse sand showing undulating lamination and rip‐up clasts. Similar depositional facies are observed on the modern delta slope. The stratal pattern is best compatible with upslope‐migrating bedforms and structureless sand indicates hydraulic jump deposits typical of cyclic steps. Cyclic‐step flow events, encompassing a succession of genetically linked erosional cyclic steps, depositional cyclic steps and subsequent waning‐flow conditions, were associated with the dense basal layer of high‐density (stratified) turbidity currents. They are specifically associated with pseudo‐channel incision and infill. The deposition of well‐bedded suites on the stoss side, occasionally also on the lee side, is interpreted to result from a different type of turbidity current, which decelerated over the bedform stoss and accelerated over the lee sides of pre‐existing cyclic‐step morphologies. Antidunal flow conditions in expanded (non‐stratified) turbidity currents are tentatively suggested, deposition being linked, in this case, to interactions between inherited bedform morphologies and a near‐bed tractive layer. In the resulting dual flow model, the bulk of delta‐slope sands was mainly deposited from turbidity currents not developing a cyclic‐step instability, yet cyclic steps were instrumental in shaping and/or re‐organizing the delta‐slope morphology, bedforms and resulting stratal patterns. In some delta systems, the upslope migration of supercritical crescentic bedforms may not be only due to repetitive cyclic steps but could also result from antidunal turbidity current conditions remobilizing an inherited cyclically‐stepped morphology.
Domaines
Stratigraphie
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Ghienne_Normandeau_EtAl_Sedimentology_CyclicSteps.pdf (19.72 Mo)
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