Les Sillons: a relict foredune plain

Abstract

`Les Sillons' (meaning `furrows') is the local name given to an outstanding example of a relict foredune plain on Les Iles de la Madeleine, Quebec. This series of shore-parallel dune ridges was built seaward during a period of post-glacial sea level rise, an indication of moderate to high sediment supply. Sea level has continued to rise; on the seaward side of the complex, wave erosion is now reactivating sand and active dunes are burying relict foredune ridges. Other similar features exist in Atlantic Canada (King 2000), but Les Sillons is the largest, having a maximum width of 2250 m and covering an area of 10.6 [km.sup.2]. The objectives of this article are to describe the setting and morphology of Les Sillons, and to explain its development based on regional Holocene history. Relict Foredune Plains Prograding coastlines are often associated with conditions of falling or stable sea level; however, progradation may also occur with rising sea level if there is a sufficient rate of sediment supply (Thompson and Baedke 1995; Hesp and Short 1999). Repeated construction of new foredune ridges at the back of the beach causes the shoreline to advance, or prograde, seaward. More widely spaced, lower ridges indicate more rapid progradation (Hesp 1984; Shepherd 1987). Examined closely, the `parallel' nature within a relict foredune plain is usually an approximation, as individual ridges are frequently a few degrees off parallel and merge into adjacent ridges. Genesis of foredune ridges is a subject of some controversy (Hesp 1984; Trenhaile 1997). The terms `beach ridge' and `foredune ridge' have been used both interchangeably and with differing meanings (Hesp 1984). The distinction is preferred- foredune ridges are characterized by the establishment of pioneer vegeration and can attain greater heights (swale to crest) than beach ridges. A series of arcuate shore-parallel progradational foredune ridges forms in a low wave energy sediment sink that is protected from significant prevailing longshore currents; an ideal location for these conditions is between two headlands (Komar 1998). In a higher energy coastal environment, the preservation potential of long, continuous ridges is lower; and with prevailing currents a prograding spit is more likely to form, such as Buctouche Spit, N.B. (Ollerhead and Davidson-Arnott 1995) or Long Point, Ontario (Davidson-Arnott and Conliffe Reid 1994). Regional Setting Les lies de la Madeleine are located in the southern Gulf of St. Lawrence (Figure 1). Two effects of the Laurentide ice sheet in the region during the last glacial period were depression of the crust, and provision of large quantities of sediment that were later moved onshore and used in the development of coastal landforms (Loring and Nora 1973). Deglaciation was accompanied by isostatic rebound that varied according to the thickness of ice and the passing of a forebulge, and by concurrent rising eustatic sea level (Quinlan and Beaumont 1981). Thus, the timing and magnitude of relative sea level changes in Atlantic Canada was variable. Evidence of past sea levels on Les Iles de la Madeleine is limited, and a specific sea level curve for the islands has not been produced. However, the general pattern for the southern Gulf of St. Lawrence is that the combination of isostatic rebound and eustatic sea level rise resulted in coastal emergence followed by submergence (Quinlan and Beaumont 1981; Scott et al. 1987; Grant 1989). Observations of modern sea level rise in the region are approximately 0.35 m per century (Shaw and Forbes 1990). [FIGURE 1 OMITTED] Barriers on Les Iles de la Madeleine Les Iles de la Madeleine consist of seven major bedrock islands and six major sandy depositional barriers connected to the islands (Figure 1). Two of the sandy barriers are terminal spits, and the other four comprise two double tombolo systems, separated by shallow lagoons, that extend between six of the islands. …