Integrating millennial and interdecadal shoreline changes: Morpho-sedimentary investigation of two prograded barriers in southeastern Australia

Abstract

Prograded barriers are distinctive coastal landforms preserving the position of past shorelines as low relief, shore-parallel ridges composed of beach sediments and commonly adorned with variable amounts of dune sand. Prograded barriers have been valued as coastal archives which contain palaeoenvironmental information, however integrating the millennial timescale geological history of barriers with observed interdecadal modern beach processes has proved difficult. Technologies such as airborne LiDAR, ground penetrating radar (GPR) and optically stimulated luminescence dating (OSL) were utilised at Boydtown and Wonboyn, in southeastern Australia, and combined with previously reported radiocarbon dates and offshore seismic and sedimentological data to reconstruct the morpho-sedimentary history of prograded barrier systems. These technologies enabled reconstruction of geological timescale processes integrated with an interdecadal model of ridge formation explaining the GPR-imaged subsurface character of the barriers. Both the Boydtown and Wonboyn barriers began prograding ~ 7500¿8000 years ago when sea level attained at or near present height along this coastline and continued prograding until the present-day with an initially slower rate of shoreline advancement. Sources of sediment for progradation appear to be the inner shelf and shoreface with a large shelf sand body likely contributing to progradation at Wonboyn. The Towamba River seems to have delivered sediment to Twofold Bay during flood events after transitioning to a mature estuarine system sometime after ~ 4000 cal. yr BP. Some of this material appears to have been reworked onto the Boydtown barrier, increasing the rate of progradation in the seaward 50% of the barrier deposited over the past ~ 1500 years. The GPR imaged beachfaces are shown to have similar geometry to beach profiles following recent storm events and a model of ridge formation involving cut and fill of the beachface, and dune building in the backshore, explains the character of the preserved beachface record and the morphology of the ridges. This model is applicable to future management of individual beaches where such beaches are subject to ongoing cut and fill, dune building processes and inherited sediment budget conditions.

Application of Ground Penetrating Radar Supported by Mineralogical-Geochemical Methods for Mapping Unroofed Cave Sediments

“…Where LiDAR data are not available, aerial photos can also be an indispensable tool in planning the proper location of GPR profiles [13]. Nowadays, remotely gathered data (e.g., LiDAR) in conjunction with the ground penetrating radar method and field observations enable easier and comprehensive studying of various geomorphological issues [50][51][52][53][54][55][56][57][58]. Previously unrecognized geomorphological features can now be revealed by LiDAR data in combination with different geophysical methods.…”

Technical note: Optimizing the utility of combined GPR, OSL, and LiDAR (GOaL) to extract paleoenvironmental records and decipher shoreline evolution

“…Figure 7c). Consequently, Oliver et al (2017b) overestimate the recurrence and impact of storms, without discussing variation in intensity/magnitude. Neither of the sites studied are optimal for extracting a Holocene storm record (or sea-level curve) as Wonboyn has vegetation that inhibits the collection of GPR across the entire barrier and Boydtown has a tidal creek running across the middle, eroding and modifying part of the record.…”

“…Over roughly a year, three studies have utilized GOaL on prograded systems to: 1. reconstruct sea level (Coastas et al, 2016(Costas et al, 2016, 2. determine the impact of storms (Oliver et al, 2017b), and 3. decipher barrier evolution and 15 sediment supply (Oliver et al, 2017a). These studies are used here as a framework to discuss the significance of GOaL and potential pitfalls.…”

“…Results showed good agreement with known sea-level curves in southwest Europe, demonstrating the potential of applying this method to regions where mid-to late-Holocene records are not as well 30 documented and/or are debated. Additionally, this GOaL data set could also be used to determine a storm and sediment Oliver et al (2017b) used GOaL on two proximal prograded barriers (Wonboyn and Boydtown) along the southeast 10 coast of Australia. The crux of the study focused using topographic profiles of the present-day beach spanning days to years to interpret shoreline evolution over centuries to millennia.…”

“…Figure 7a). Oliver et al (2017b) concluded that all of the paleobeachfaces in the geophysical record were stacked post-storm profiles with no berm stratigraphy preserved. However, this interpretation is likely skewed because the gain control is high in the GPR data (e.g.…”