Shore Platform Processes in Eastern Canada

Abstract

This research is conducted on a mesotidal, argillite shore platform at Mont Louis in Gaspé, Québec, and on macrotidal platforms in the basalts of Scots Bay and the sandstones of Burncoat Head in Nova Scotia. Rock samples have been subjected to wetting and drying and to salt weathering cycles. The platforms were surveyed; rock hardness was determined with a Rock Test Hammer; waves were recorded in the field; and downwearing rates were measured at 56 micro-erosion meter stations over 1 to 3 years. Weathering is the dominant process at Mont Louis, although the horizontal platform may have been cut by waves at the high tidal level. Wave backwearing was much more important than downwearing by weathering during the Holocene at Scots Bay. Wave quarrying only occurs on a few scarps today, however, and without much abrasive material, slow downwearing now dominates over most of the platform surface. Waves probably help to remove loosened sand grains at Burncoat Head, thereby contributing to platform downwearing. Abrasion is also important in places, but the data suggest that backwearing by wave quarrying and probably frost has been a little more important than downwearing by abrasion and weathering during the Holocene.Processus des plates-formes littorales de l’est du Canada. Une plate-forme littorale d’argilites soumise à des marées modérées à Mont Louis en Gaspésie, Québec, et des plates-formes soumises à des fortes marées dans les basaltes de Scots Bay et les grès de Burncoat Head de la Nouvelle-Écosse furent étudiées. Des échantillons de roches ont été soumis à des cycles de mouillage et de séchage et d’haloclastie. Le profil des plates-formes a été mesuré, la dureté des roches a été établie par le test du marteau de Schmidt, les vagues ont été mesurées sur le terrain et le taux d’usure vertical a été quantifié à 56 stations avec des appareils détectant la micro-­érosion, le tout sur une période d’un à trois ans. L’altération est le processus dominant à Mont Louis, bien que la plate-forme horizontale ait été entaillée par les vagues au maximum des marées hautes. L’usure horizontale par les vagues était plus importante que l’usure verticale par altération durant l’Holocène à Scots Bay. L’extraction de grands blocs par les vagues ne se produit toutefois de nos jours que sur quelques abrupts et, en l’absence de matériel abrasif, une lente usure verticale domine maintenant sur la plupart des plates-formes. Les vagues ont probablement aidé à éliminer les grains de sable produits par l’altération à Burncoat Head, contribuant ainsi à l’usure verticale de la plate-forme. L’abrasion est également importante à certains endroits, mais les données indiquent que l’usure horizontale par les vagues, et aussi par le gel, a été un peu plus importante que l’usure verticale par l’abrasion et l’altération durant l’Holocène

Tidal wetting and drying on shore platforms: An experimental assessment

“…Micro-erosion meter data are also available from 41 stations installed along profiles perpendicular to the shore (shore normal) in the study areas in eastern Canada. (Trenhaile and Kanyaya, 2004), although, as they have presently been collected only for 1 year, they must be treated with caution. Downwearing rates are between 0.02 and 1.79 mm year À 1 on the sandstones of Burntcoat Head, between 0.37 and 1.42 mm year À 1 on the basalts of Scots Bay, and between 0.019 and 0.39 mm year À 1 on the argillites at Mont Louis.…”

Patterns of surface downwearing on shore platforms in eastern Canada

“…Most surface elevation (swelling) occurred in the argillaceous rocks at Mont Louis, where more than half the stations were affected, and at Brander where elevation occurred at about 40% of the stations ( Figures 5, 6 and 7). Surface elevation is a fairly common occurrence on shore platforms where it has been attributed generally to wetting and drying and salt crystallization, although apparent occurrences can also result from operator error (Kirk, 1977;Mottershead, 1989;Stephenson and Kirk, 2001;Taylor, 2003;Stephenson et al, 2004;Stephenson and Thornton, 2005;Foote et al, 2006;Trenhaile, 2006;Trenhaile et al, 2006;Gómez-Pujol et al, 2007;Porter and Trenhaile, 2007;Porter et al, 2010a,b;Stephenson and Finlayson, 2009). Surface elevation can persist for periods ranging from hours to years, but as platform surfaces can only be lowered over the long-term, it is a temporary condition and for this reason, swelling events were excluded when calculating mean downwearing rates.…”

“…The experimental apparatus and methods have been discussed previously (Kanyaya and Trenhaile, 2005;Trenhaile, 2006;Trenhaile et al, 2006;Porter and Trenhaile, 2007;Porter et al, 2010a, b), and to avoid repetition, only brief descriptions are provided here. There were two types of experiment (Table II): a) Pumps and timers were used to circulate water through a series of basins that contained 1219 rock samples, in the form of cores of basalt and argillite, 1·9 cm in diameter and 2 cm in height, and cubes of sandstone with 2 cm long sides.…”

“…The other three study sites were in the micro-to mesotidal St Lawrence Estuary and Gulf of St Lawrence (Figures 1 and 2, and Figure 5 in Porter et al, 2010b) The subhorizontal shore platform at Mont Louis consists primarily of Ordovician siltstones, mudstones, and shales (Enos, 1969;Trenhaile et al, 2006;Porter et al 2010b). The platform is from about 140 to 180 m in width, and it is at an elevation at, or slightly above, the mid-tidal level.…”

Schmidt hammer tests across a recently deglacierized rocky coastal zone in Spitsbergen - is there a "coastal amplification" of rock weathering in polar climates?

“…On the one hand during the last three decades several geomorphological works along Atlantic Canada's rocky shorelines (e.g. Dionne and Brodeur 1988;Dionne 1989;Trenhaile and Mercan 1984;Trenhaile et al 1998;Trenhaile 2001;Trenhaile et al 2006;Porter and Trenhaile 2007;Porter et al 2010) have realized fundamental advances in our understanding of ice and frost action on the morphology of intertidal zones as well as "freezing−thawing & wet− ting−drying" influence on shore platforms and cliffs relief. Relatively few investi− gations, however, have tested the efficiency of those processes in polar settings.…”