GEOLOGIC CONTROLS ON SHORE PLATFORM MORPHOLOGY: IMPLICATIONS FOR BIODIVERSITY

The erosion of rocky coasts reflects the balance between wave force and the resisting force of the cliff material. On a kilometer scale these factors have been extensively studied, but there are fewer studies that assess morphology of rocky shore platforms at the meter to decimeter scale. At this scale, the complex topography of the shore platform provides habitat for invertebrates, algae, and small vertebrate fish. In this work, we focus on developing methods to quantify the relationships among rock hardness, topography, and biodiversity in the intertidal zone of coastal Oregon. We quantified rock hardness using measurements from a Schmidt hammer, where higher rebound values (units of R) indicate harder rocks. Topographic maps of several locations near Cape Arago and Yachats, Oregon, were derived from Structure-from-Motion of handheld photography. Despite their geographic proximity, there are strong differences in lithology and structure between sites. At Cape Arago, rocks are composed of deltaic sandstones and the shore platform is made up of both in situ bedrock and boulders derived from cliff failures. In contrast, the sites near Yachats are composed of both basalt and deltaic sandstones with basalt intrusions, with the shore platform primarily cut into in situ bedrock. Differences in lithology between each site were reflected in the rock hardness measurements: Cape Arago rocks had a mean value of 32.2 R compared to Yachats mean values of 39.9 R (basalt) and 31.4 R (deltaic sandstone). The structural differences between sites are reflected in the shore platform morphology. The Coos Bay sites have a flatter shore platform and tidal habitat zones are along a gradient from the shore to the cliff. In contrast, the sites near Yachats have variable shore platform topography. Rocks at these sites are generally taller, resulting in vertical tidal zonation from the bottom of the face of the rock to the top of the rock. Beyond the site-to-site comparisons, intra-site variance in rebound values show the role of small scale controls such as biological weathering, discontinuities, wetness, distance from shore, and geographical orientation. Observations of rock hardness and topography provide insight to the morphology of the intertidal zone and the geological contribution to biodiversity in these ecosystems.