Shore platform downwearing in eastern Canada: The mega-tidal Bay of Fundy

Abstract

Laboratory and field work was conducted to determine rates of surface downwearing on shore platforms at two sites in the Bay of Fundy, Canada; these data are needed to assess and model the effect of downwearing mechanisms on platform evolution. Sandstone and basalt samples (900) were exposed to semidiurnal tidal cycles over 3 years, using de-ionized water or artificial sea water. They were immersed for 1, 6, or 11 h over each 12 h tidal cycle, representing the lowest high (LHT), mid-, and highest low (HLT) tidal levels, respectively. A further 600 samples were immersed in de-ionized water or artificial sea water for 90 min every 1, 2, or 3 weeks, representing increasingly higher elevations above the LHT level, or exposed for 90 min every 1, 2, or 3 weeks, representing increasingly lower elevations below the HLT level; these experiments ran for 12 months. In the field, surface downwearing was measured at 108 transverse micro-erosion meter (TMEM) stations over 2 to 6 year periods. In the laboratory, mean downwearing rates between the LHT and HLT levels were 0.61–1.80 mm yr − 1 in sandstones and 0.01–0.18 mm yr − 1 in basalts; rates were highest at the LHT level. Rates decreased with elevation above the LHT level and were uniformly low below the HLT level. Salt weathering was dominant above the LHT level. Salt weathering and wetting and drying were important in sandstones between the LHT and HLT levels, but salts inhibited rock breakdown in basalts. In the field, the mean downwearing rate was 1.254 mm yr − 1 in sandstones, which was similar to the experimental data, and 0.722 mm yr − 1 in basalts, which was much higher than in the experiments with artificial sea water but similar to the experiments with de-ionized water. There was no relationship in the field between downwearing rate and rock hardness or TMEM station elevation.