Thomas S.N. Oliver

Towards more robust chronologies of coastal progradation: Optically stimulated luminescence ages for the coastal plain at Moruya, south-eastern Australia

Accurate chronologies are fundamental for detailed analysis of palaeoenvironmental conditions, archaeological reconstructions and investigations of Holocene coastal morphological changes. Chronological data enable estimation of rates of shoreline progradation, and provide appropriate context for forecasting future coastal changes. A previously reported radiocarbon chronology for the Moruya coastal plain in south-eastern Australia indicated a decelerating overall rate of progradation with minimal net seaward shoreline movement in the past ~2500 years. Single-grain and multi-grain aliquot optically stimulated luminescence (OSL) analyses demonstrate that marine sands from this region have excellent luminescence characteristics. A series of OSL ages across this coastal barrier indicates a remarkably linear trend of Holocene shoreline progradation. The linear trend of seaward shoreline movement indicates that the barrier has grown at an average rate of 0.27 m/yr with successive ridge formation every ~110 years. The oldest ridge on the barrier appears to correspond to cessation of rapid post-glacial sea-level rise, and the large foredune at the seaward margin of the barrier is <400 years old. The contrast between the existing radiocarbon chronology and the OSL ages reported in this study implies the need for a more cautious interpretation of coastal barrier chronologies, in Australia and around the world, where they have been based on radiocarbon dating of shell hash.

Measuring, mapping and modelling: an integrated approach to the management of mangrove and saltmarsh in the Minnamurra River estuary, southeast Australia

Mangrove and saltmarsh ecosystems appear particularly vulnerable to the impacts of climate change, and their effective management will require forecasts of how these wetland habitats are likely to respond to sea-level rise through the twenty-first century. We describe a preliminary study of a small stand of mangrove and saltmarsh that involves measuring of elevation change and accretion, mapping of wetland communities, and modelling of their potential response to sea-level rise. The wetland occurs on the banks of the Minnamurra River estuary in southern New South Wales and has been the focus of several studies over recent decades. The research includes empirical measurements of sedimentation at sites in both mangrove and saltmarsh vegetation using the surface elevation table-marker horizon technique. This is a site at which mapping has been undertaken to delineate the extent of each vegetation community from a time-series of aerial photographs using geographical information systems; the gradual incursion of the mangrove, Avicennia marina, into more landward saltmarsh communities, observed over past decades for many systems in southeastern Australia, has continued into the twenty-first century. The observed patterns of change are compared with simulations of how this wetland system might respond to future sea-level rise, adopting several different approaches and the upper and lower bounds of Intergovernmental Panel on Climate Change sea-level rise projections. The model results show considerable variability in response depending on the parameters adopted. We advocate the need for the integration of these three approaches, measuring, mapping and modelling, as a basis for future management and adaptation. Our study demonstrates the considerable opportunities to refine the data input and model outputs as part of adaptive management, as more sophisticated technologies and data become available.

Chronology, Morphology and GPR-imaged Internal Structure of the Callala Beach Prograded Barrier in Southeastern Australia

ABSTRACT Oliver, T.S.N., and Woodroffe, C.D., 2016. Chronology, Morphology and GPR-imaged Internal Structure of the Callala Beach Prograded Barrier in Southeastern Australia. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 318–322. Coconut Creek (Florida), ISSN 0749-0208. Holocene prograded coastal barriers, comprising a sequence of relict foredune ridges, are depositional environments, which have been used to reconstruct coastal processes. Such reconstructions benefit from new techniques and technologies now available in coastal studies. This study investigated the Callala Beach prograded barrier deposit situated within Jervis Bay on the NSW south coast. This prograded barrier, composed of a series of low-relief, shore-parallel ridges, formed after sea level stabilised on this coastline in the mid Holocene. The approach involved analysis of Light Detection and Ranging (LiDAR) topographic data, ground-penetrating radar (GPR) collection and processing, and dating of ridge deposits using the optically-stimulated luminescence (OSL) dating technique. These data sets demonstrate that the most landward ridge of the Callala Beach barrier was deposited ∼7500 years ago, closely aligning with the best estimates for the timing of sea-level stabilisation in southeastern Australia. Progradation continued throughout the late Holocene at a steady rate of ∼0.1 m/yr until near the present time, as shown by an age of ∼400 years immediately behind the modern foredune. GPR-imaged subsurface structures captured the beachface and dune facies; a regular series of reflectors indicated incremental accumulations of sediment over the late Holocene. Volumes of sand accumulated during barrier growth indicated an average sediment supply for the entire embayment of ∼1600 m3/yr or ∼0.3 m3/yr per metre of beach. The long term trend of sediment supply has implications for coastal management as the local council is commencing a beach nourishment program at Callala Beach.

Formation of beach-ridge plains: an appreciation of the contribution by Jack L. Davies

A robust debate amongst coastal geomorphologists as to the processes by which beach-ridge plains around Australia have formed was initiated by a former President of the Institute of Australian Geographers. This review gives special consideration to the work of Jack L. Davies, whose academic contributions to coastal geomorphology in Australia have not always been appropriately acknowledged when explaining how similar plains have evolved elsewhere in the world. Davies recognised that relatively steep storm waves caused erosion (cut) on beaches, whereas less steep long-period swell waves returned sand (fill). He considered the beach berm to be the nucleus on which a beach ridge formed, which could subsequently develop into a foredune, in contrast to cobble ridges that were deposited during storms. Offshore conditions regulate supply of sand to the shoreline, partly through effects on wave refraction, with higher rates of supply where the nearshore is shallow. It was apparent to Davies that the elevation of successive ridges might, but not necessarily, provide evidence of past changes of sea level, despite adornment by variable amounts of windblown dune sand. Morphodynamic understanding of long-term coastal evolution, based on radiocarbon dating chronologies, has demonstrated that Australian coastal plains formed over the past ~6000 years when sea level has been close to its present level, in contrast to several documented locations in the northern hemisphere where the sea has been rising for the past few millennia. Particularly insightful were observations by Davies that ridge formation could be influenced by a range of factors including changes in sea level, storminess, or sediment supply. These factors acting singly or in combination seem likely to change in the future. Understanding such responses remains a high priority and can be addressed by new technologies, such as light detection and ranging, optically stimulated luminescence dating, ground-penetrating radar, and computer simulation.

Sedimentology of Late Holocene fluvial levee and point-bar deposits from the Cambodian tract of the Mekong River

The study of channel margin deposits is crucial to better understand fluvial systems and has significant social and economic implications. This paper describes sedimentological features of Late Holocene levee and point-bar deposits from five excellent outcrop exposures along the Cambodian tract of the Mekong River. Point-bar deposits show a typical fining-upward trend with low-angle inclined bedding and consist of gravelly sand, sand and mud beds. Levee deposits show a typical coarsening-upward or no clear vertical grain-size trend and consist of mud and very fine sand beds. Channel margin deposits formed in the outer river bend show a simple levee element, whereas inner bend deposits show more complex architecture with combinations of point-bar and levee elements. Water-level fluctuations play a fundamental role in the construction of point-bar and levee architecture; the resulting deposits show evidence of strong river currents with rapid sedimentation alternating with evidence of subaerial exposure with no sedimentation. This combination of sedimentary features can also form in other environments, but is distinctive of channel margin deposits forming in climates with a pronounced seasonality, including large and perennial rivers in tropical savannah climate areas.

Rapid shoreline progradation followed by vertical foredune building at Pedro Beach, southeastern Australia: Rapid shoreline progradation followed by vertical foredune building

Prograded coastal barriers are accumulations of marine and aeolian sands configured into shore-parallel ridges. A variety of ridge morphologies described around the world reflect differences in origin as a consequence of differing prevailing coastal morphodynamics. The ‘morphodynamic approach’ described by Wright and Thom (1977) expounds the coastal environmental conditions, hydrodynamic and morphodynamic processes and inheritance of evolutionary sequences over varying temporal scales which interdependently operate to produce an assemblage of coastal landforms adjusted, or adjusting to, a dynamic equilibrium. At Pedro Beach on the southeastern coast of Australia a large sandy deposit of foredune ridges provides an opportunity to explore the morphodynamic paradigm as it applies to coastal barrier systems using optically stimulated luminescence (OSL) dating, ground penetrating radar (GPR) and airborne LiDAR topography. The prograded barrier at Pedro Beach has formed following the stabilisation of the sea level at its present height on the southeast Australian coastline. A series of dune–capped ridges, increasing in height seawards, formed from ∼6000 years ago to ∼4000 years ago. During this time the shoreline straightened as bedrock accommodation space for Holocene sediments diminished. Calculation of Holocene sediment volumes utilising airborne LiDAR topography shows a decline in sediment volume over this time period coupled with a decrease in shoreline progradation rate from 0.75 m/yr to 0.49 m/yr. The average ridge ‘lifetime’ during this period increases resulting in higher ridges as dune-forming processes have longer to operate. Greater exposure to wave and wind energy also appears to have resulted in higher ridges as the sheltering effect of marginal headlands has diminished. A high outer foredune has formed through vertical accretion in the past 700 years, evidenced by GPR subsurface structures and upward younging of OSL ages, with a sample from 1 m deep within the crest of this dune returning an age of 90 ± 10. An inherited disequilibrium shoreface profile will drive onshore accumulation of sandy sediments forming a prograded barrier; however, if there is no longer ‘accommodation space’ for sediment, this will be an overriding factor causing the cessation of progradation as occurred ∼4000 years ago at Pedro Beach. Following progradation cessation, excess sediment in the disequilibrium shoreface profile will be moved alongshore as barrier progradation (embayment filling) has diminished the potential of headlands to act as impediments to sediment bypassing in the nearshore. It is hypothesised that the chronology and geomorphology of the Pedro Beach barrier system typifies the changing ‘strength of influence’ in the interaction between geologically inherited accommodation space, sediment delivery and beach/dune/shoreface dynamics over the mid-late Holocene.