Timing and controls on the delivery of coarse sediment to deltas and submarine fans on a formerly glaciated coast and shelf

Abstract

24The evolution of deltas and submarine fans is often envisioned as largely controlled by 25 relative sea-level (RSL) variations. However, in some cases, RSL can have less effect on 26 delta and submarine fan activity than sediment supply and shelf geomorphology. In order 27to document the relative importance of these three factors on deltaic and submarine fan 28 evolution in a former glaciated environment, this paper documents the delivery of coarse 29 sediment to the Laurentian Channel (eastern Canada). The well-constrained stratigraphic 30 and geomorphological framework of both the glacio-isostatically uplifted deltas and the 31 modern Laurentian Channel fans allow us to document and contrast the evolution of 32 river-fed deltas, river-fed canyon/fan systems and longshore drift-fed fans during 33 deglacial and postglacial times. The evolution of these different types of fans can be 34 divided into three phases. The first phase is characterized by delta progradation on the 35 shelf while RSL was at its maximum, although already falling, and the ice-margin 36 gradually retreated inland. The second phase is characterized by the delivery of deltaic 37 sediment in the deep realm of the Laurentian Channel, permitted by the supply of large 38 amounts of glaciogenic sediments derived from the retreating ice margin and the 39 lowering of the RSL. At the same time, sediment instability along the steep Laurentian 40 3 Channel formed small incisions that evolved into submarine canyons where the narrow 41 shelf allowed the trapping of longshore sediment. The third phase is characterized by the 42 withdrawal of the ice-margin from the watershed of the main rivers and the drastic 43 decrease in sediment supply to the deltas. Consequently, the delta fronts experienced 44 strong coastal erosion, even though RSL was still lowering in some cases, and the eroded 45 sediments were transferred onto the shelf and to adjacent bays. This transfer of coastal 46 sediments allowed the continued activity of longshore drift-fed canyons. The retreat of 47 the ice margin from the watersheds thus controlled the supply of sediment and induced a 48 change in delta type, passing from river-dominated to wave-dominated. This paper 49 highlights the role of the type of sediment supply (ice-contact, glaciofluvial and 50 longshore drift) in the timing and activity of submarine fans in high-latitude 51 environments. It proposes a conceptual model for high-latitude shelves where sediment 52 delivery to submarine fans is mostly controlled by structural inheritance (watershed area 53 and shelf geomorphology) rather than RSL fluctuations. Therefore, although RSL fell 54 during delta progradation, this study demonstrates that it was not the main contributor to 55 delta and submarine fan growth. This has wider implications for the extraction of sea-56 level information from stratigraphic successions. 57

Deglacial sequences and glacio-isostatic adjustment: Quaternary compared with Ordovician glaciations

“…Both processes are interpreted as the result of a dramatic decrease of the fluvial input vs. shallow-marine processes ratio, in relation with the retreat of the ice margin from the drainage basin. As already recognized in other case studies, this is inferred to have a profound impact on downstream depositional patterns (Church, 1972(Church, , 2006Syvitski, 1989;Forbes & Syvitski, 1994;Corner, 2006;Eilertsen et al 2011;Hein et al 2014;Nutz et al 2015;Normandeau et al 2017).…”

“…2017a; Normandeau et al 2017), and (iv) delta-front channels or canyons are today essentially inactive, their abandonment being dated back to the retreat of the ice-margin out of the river drainage basins (Normandeau et al 2015;. Therefore, a continuum of depositional systems is identified in the deltaic complexes -rather than deltas-of the Québec North Shore, these sediment bodies being essentially deglacial relicts incorporating ice-contact deposits and glaciofluvial deltas.…”

“…Meltwater and glaciogenic sediment discharge delivered by the retreating ice margin and carried by proglacial rivers fed coastal glaciofluvial deltas affected by rapid RSL fall owing to the active GIA (1-10 cm.year -1 , Shaw et al 2006;Tarasov et al 2012;Peltier et al 2015). The connection between the glaciofluvial deltas and the LIS margins ceased between 10 to 6 ka, depending on the northward extent of the drainage basins as the ice margin retreated from them (Normandeau et al 2017;Dietrich et al 2017b). Ensuing sedimentation essentially reflected a paraglacial development which took place under slower rates of RSL fall.…”

“…Based upon the Sept-Îles case study, as well as upon other deltaic complexes analysed along the Québec North Shore (e.g., Duchesne et al 2010;Normandeau et al 2017;Dietrich et al, 2017a;2017b), a generic model encompassing the entire deglacial evolution is here proposed, distinguishing three stages of development (Fig. 6).…”

Post‐Last Glacial Maximum evolution of a “fjord‐type” lake based on high‐resolution seismic data: the Lago Roca/Acigami (southern Tierra del Fuego, Argentina/Chile)

“…3) is interpreted as the transition from the proximal gravel-dominated subaqueous fan to a sand-andmud-dominated distal subaqueous fan (Thomas and Chiverrell, 2006). The channel shape with onlapping reflectors point to debris flows or density currents occupying scoured channels (Thomas and Chiverrell, 2006;Normandeau et al, 2017).…”