IDENTIFICATION AND MAPPING OF SUBMERGED WAVE-CUT PLATFORMS USING HIGH-RESOLUTION BATHYMETRY, A METHODOLOGY

Wave-cut platforms and their associated shoreline angles have long been used as indicators of tectonic deformation in coastal environments. Historically, studies have focused on subaerial wave-cut platforms exposed along coastlines, accessible for topographic surveying and age-dating. With the recent availability of high-resolution bathymetric surveys, analysis and correlation of submerged wave-cut cut platforms will become an important component to future near-shore neotectonic studies. For this study, we analyzed ~440 km² of 1-m-resolution bathymetry data between Morro Bay and San Luis Bay, California.

As with emergent marine terraces, submerged shoreline angles can be used as tectonic strain gauges; however, accurate mapping and correlation of these offshore features is challenging and presents new issues to resolve given their inaccessibility. As our evaluation proceeded, we recognized a need to follow a consistent methodology for identification and mapping of submerged paleo-shoreline features. To this end, we assigned confidence levels for mapped shoreline angles based on their strength of geomorphic expression, continuity, uncertainty in location, and uncertainty in origin. For example, understanding the spatial relationship of mapped shoreline angles to bedrock structure became a fundamental component of our analysis. Where postulated strandlines parallel bedding or structure, differential erosion may obscure the location of the shoreline or result in morphology similar to a strandline but which is unrelated to periods of sea-level stability. Where strandlines cross bedding or structure, their origin is more confidently correlated to paleo sea-level still stands. The duration of sea-level occupation of these submerged paleoshorelines is estimated by comparison of the width of their wave-cut platforms to the well-developed Holocene wave-cut platform at the modern shoreline. Our preliminary results on wave-cut platform morphology suggest similar geometric relationships to emergent wave-cut platforms described by Bradley and Griggs (1976) near Santa Cruz, California. Upon completion of mapping and morphologic analyses, paleoshoreline depths are plotted on a longitudinal profile for correlation and evaluation of potential tectonic deformation.