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

Abstract

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