Testing the potential of drowned shore platforms as sea level proxies in a temperate, microtidal environment: Victoria, Australia

Shore platforms are erosional benches, often found at the base sea cliffs, which form through erosion concentrated within the intertidal zone. In locations without suitable depositional proxies, these erosional landforms have been used to reconstruct past sea levels. The precision and accuracy of shore platforms as sea level proxies is highly variable as their morphology, particularly their vertical position in the intertidal zone, is modulated by boundary conditions especially those related to the geology in which they are formed. In this study the semi-horizontal shore platforms on the microtidal temperate coast of Victoria, Australia are examined to test the utility of these systems as sea level indicators. These results are then applied to a suite of submerged shore platforms found offshore using high-resolution LiDAR topographic and multi-beam sonar bathymetric data. The elevation of both the contemporary and drowned platforms are measured at both the cliff-platform junction and the mid-point of each profile line. Both methods are compared to assess their utility to represent sea level at the time of their formation. It is found that those platforms formed within resistant sandstone and basalt are the best proxies for sea level forming in the upper intertidal zone, while those formed in softer Tertiary carbonates are less precise being predominately subtidal features. The morphology of the drowned platforms is broadly similar to those at current sea level elevations, and they are concentrated at a depth 49.7 to 57.2 m below modern sea level. It is inferred that these drowned platforms likely formed during marine isotope stage 5a or 5d.