Shore platform downwearing in eastern Canada: Micro-tidal Gaspé, Québec

Work has been conducted, in the laboratory and field, on a micro-tidal, subhorizontal shore platform in argillaceous rocks at Mont Louis, in Gaspé, Québec. Surface downwearing rates for 450 rock samples were measured over 3 years in the laboratory at the neap low, mid-, and high tidal levels, under semi-diurnal tidal conditions. De-ionized water was used to determine the role of wetting and drying, and artificial sea water for the additional effect of salt and possibly chemical weathering. Another 300 samples were immersed in de-ionized water or artificial sea water for 90 min every 1, 2, or 3 weeks, representing conditions between the neap and spring high tidal levels, and exposed in air for 90 min every 1, 2, or 3 weeks to represent conditions between the neap and spring low tidal levels. Surface downwearing was recorded in the field at 34 transverse micro-erosion meter (TMEM) stations, most of which were installed along 4 shore-normal profiles in 2004 and 2005. Mean downwearing rates in the laboratory ranged from 1.25 mm yr− 1 at the neap high tidal level to 0.63 mm yr− 1 at the neap low tidal level. Downwearing was much slower between the neap and spring tidal levels and it was uniformly slow below the neap low tidal level. The presence of salts inhibited downwearing within the neap tidal range, where wetting and drying was dominant, and promoted it between the neap and spring high and low tidal levels. Surface swelling was fairly common in the field, and the mean downwearing rate (0.242 mm yr− 1) was much lower than in the laboratory, possibly because the TMEM stations were installed in harder rocks. Modeling supports the contention that the Mont Louis platform was cut initially by waves at a higher elevation, and then lowered by weathering in the last few thousand years as relative sea level fell to its present elevation.