Shore platform and cliff notch transitions along the La Paz Peninsula, southern Baja, Mexico

Abstract

Increasing exposure to wave action produces a northerly transition from high tidal notches to shore platforms inthe andesitic lahar deposits of the La Paz Peninsula, southern Baja, Mexico. Twenty-four notches were surveyedand wear pins were cemented into the apex of each notch. Thirty-six transverse micro-erosion meter (TMEM)stations were installed on three surveyed platforms. Field measurements were made over a 2.5 year period.The wear pins suggested notch backwearing (horizontal erosion) is <2mm yr^-1. The shore platforms were fairlynarrow (a few tens of metres) and steeper (1º) than most platforms in similar microtidal environments, reflectinga weak wave environment and resistant rocks. Mean TMEM downwearing (vertical erosion) rates for each of thethree platforms ranged from 0.14mm yr^-1 to 0.42mm yr^-1. There was a good relationship between notch height(difference in elevation between the floor and the roof at the front of the notch) and exposure to wave action, butnotch depth is time-dependent and the relationship with exposure was not statistically significant. Notch heightwas also related to the orientation and wave fetch of the site. Field evidence suggested that the notches were notproduced by bioerosion or chemical weathering but by alternate wetting and drying or salt weathering from hightidal immersion and wave-generated splash and spray. Coastal morphology is fairly well adjusted to present sealevel although notch occurrence in the upper portion of the high tidal zone suggests that there is slow tectonic uplift in this region.