From outwash to coastal systems in the Portneuf–Forestville deltaic complex (Québec North Shore): Anatomy of a forced regressive deglacial sequence

Abstract

Deglacial sequences typically include backstepping grounding zone wedges and prevailing glaciomarine depositional facies. However, in coastal domains, deglacial sequences are dominated by depositional systems ranging from turbiditic to fluvial facies. Such deglacial sequences are strongly impacted by glacio‐isostatic rebound, the rate and amplitude of which commonly outpaces those of post‐glacial eustatic sea‐level rise. This results in a sustained relative sea‐level fall covering the entire depositional time interval. This paper examines a Late Quaternary, forced regressive, deglacial sequence located on the North Shore of the St. Lawrence Estuary (Portneuf Peninsula, Québec, Canada) and aims to decipher the main controls that governed its stratigraphic architecture. The forced regressive deglacial sequence forms a thick (>100 m) and extensive (>100 km2) multiphased deltaic complex emplaced after the retreat of the Laurentide Ice Sheet margin from the study area ca 12 500 years ago. The sedimentary succession is composed of ice‐contact, glaciomarine, turbiditic, deltaic, fluvial and coastal depositional units. A four‐stage development is recognized: (i) an early ice‐contact stage (esker, glaciomarine mud and outwash fan); (ii) an in‐valley progradational stage (fjord head or moraine‐dammed lacustrine deltas) fed by glacigenics; (iii) an open‐coast deltaic progradation, when proglacial depositional systems expanded beyond the valley outlets and merged together; and (iv) a final stage of river entrenchment and shallow marine reworking that affected the previously emplaced deltaic complex. Most of the sedimentary volume (10 to 15 km3) was emplaced during the three‐first stages over a ca 2 kyr interval. In spite of sustained high rates of relative sea‐level fall (50 to 30 mm·year−1), delta plain accretion occurred up to the end of the proglacial open‐coast progradational stage. River entrenchment only occurred later, after a significant decrease in the relative sea‐level fall rates (<30 mm·year−1), and was concurrent with the formation and preservation of extensive coastal deposits (raised beaches, spit platform and barrier sands). The turnaround from delta plain accretion to river entrenchment and coastal erosion is interpreted to be a consequence of the retreat of the ice margin from the river drainage basins that led to the drastic drop of sediment supply and the abrupt decrease in progradation rates. The main internal stratigraphic discontinuity within the forced regressive deglacial sequence does not reflect changes in relative sea‐level variations.

Can sediment supply variations create sequences? Insights from stratigraphic forward modelling

“…For example, topset aggradation within the period of falling sea level is also observed in Model 1.2. Similar observations have been made in various mathematical modelling and flume experiments (Petter & Muto, ; Prince & Burgess, ; Swenson & Muto, ) and also from study of Holocene strata (Dietrich et al, ; Nijhuis et al, ). The time and length scale of the topset aggradation during falling relative sea level is affected by rate of relative sea‐level change, sediment discharge, water discharge and shelf gradient (Swenson & Muto, ).…”

Ice‐marginal forced regressive deltas in glacial lake basins: geomorphology, facies variability and large‐scale depositional architecture

“…These deltas commonly evolve from ice-contact systems to glacifluvial deltas during icemargin stillstand and retreat (Lønne 1995;Dietrich et al 2017) and delta foreset-topset contacts can be used as water-level indicators if shoreline features are poorly developed or became eroded by later peri-and paraglacial processes (Winsemann et al 2009Perkins & Brennand 2015;Lang et al 2018). These deltas commonly evolve from ice-contact systems to glacifluvial deltas during icemargin stillstand and retreat (Lønne 1995;Dietrich et al 2017) and delta foreset-topset contacts can be used as water-level indicators if shoreline features are poorly developed or became eroded by later peri-and paraglacial processes (Winsemann et al 2009Perkins & Brennand 2015;Lang et al 2018).…”

Ice‐margin fluctuation sequences and grounding zone wedges: The record of the Late Palaeozoic Ice Age in the eastern Karoo Basin (Dwyka Group, South Africa)

“…Wave and interference ripples reworking in place the GS indicate that the ice retreated after the construction of GZW. It is thought that the GZW was then abandoned and subsequently winnowed and washed‐out of the fine fraction of the diamictite by nearshore processes, either immediately or after a fall of RSL that exposed the summit of the GZW – the effective GS – to wave action (Dietrich et al , , ; Demet et al , ). Furthermore, the wedging of wave‐rippled sediments or the pebble/cobble lag against the basement slopes indicate that by the time basement highs situated above this horizon emerged only palaeotopographic lows were inundated.…”

“…Altogether, the boulder pavement, the undulating geometry of the sandbeds making up the top surface of SU2, as well as the microscopic grain bands that could be interpreted as boudins are evidence of overriding ice (Visser and Hall, ; Buechi et al , ; Busfield and Le Heron, and references therein). The rhythmic climbing ripples seem to indicate a tidal influence on deposition and might have resulted from the interaction of tides with glaciofluvial inputs to generate and/or support tide‐influenced sediment gravity flows (Smith et al , ; Cowan et al , ; Dietrich et al , ). Observed inverse grading (Figure B and C) may represent waxing flow of hyperpycnal events (Mulder et al , ), whereas sand clasts puncturing the laminae are interpreted as dropstones derived from floating ice (sea ice, drifting icebergs or ice shelf; Dowdeswell et al , ).…”