Research papersThe evolution of sea cliffs over multiple eustatic cycles in high energy, temperate environments
Introduction
Sea cliffs are steep coast features with slopes greater than 20° that are formed in bedrock or clay by erosive forces at the land-water interface, especially in areas of high relief (Inman and Nordstrom, 1971; Emery and Kuhn, 1982; Hampton and Griggs, 2004). The cross-sectional morphology of cliffs is commonly subdivided into a steep scarp and a less steep deposit of talus and other debris at the base (Hooke, 1999; Lim, 2014). The shape and rate of erosion of sea cliffs is primarily determined by the geology in which they are formed (Trenhaile, 1987; Lim, 2014; Naylor and Stephenson, 2010). In general, the more resistant the lithology, the lower the rates of erosion and the greater the potential elevation of the cliff (Griggs and Trenhaile, 1994). For example, in southern Italy, an inverse relationship was found to occur between coastal cliff erosion and compressive strength (Budetta et al., 2000). However, erosion is not always a steady, continuous process but rather can result from episodic failures (Lee et al., 2001). This poses a difficulty in the study of sea cliffs in that there is often a mismatch in the temporal scale of observed events and that of landform evolution (Kennedy et al., 2017).
Marine and terrestrial climates are an important determinate of the erosive forces acting on the coast (Griggs and Trenhaile, 1994; Trenhaile, 2001). All contemporary cliffs are to some degree influenced by subaerial weathering processes, such as wind and rain (Lee and Clark, 2002; Hutchinson, 2002), water runoff (Ross and Morgan, 1986), bioerosion (Naylor and Viles, 2002), chemical weathering, and groundwater seepage (Griggs and Savoy, 1985). Any section of a cliff that sits close to sea level is also impacted by marine processes through tidally-modulated wave action. Marine processes both remove fallen debris as well as directly erode the cliff edifice (Hills, 1971). In addition, direct wave impact also transfers energy from the waves to the cliff, which can weaken the rock face through microseisms (Lollino et al., 2008; Norman et al., 2013; Young et al., 2013, Kennedy et al., 2018). As a result, sea cliffs are generally classified as either being dominated by subaerial or marine erosional processes (Emery and Khun, 1982; Kennedy and Dickson, 2007).
As the evolution of sea cliffs is closely linked to sea level (Pluet and Pirazzoli, 1991; Woodroffe and Murray-Wallace, 2012), their base is often used to reconstruct past eustatic cycles. The accuracy of such reconstructions is dependent on the proxy used, but can reach decimeter scale, especially when using features such as notches (Pirazzoli, 1986; Kershaw and Guo, 2001; Sisma-Ventura et al., 2017; Trenhaile, 2015). While this is not as accurate or precise as scleractinian coral proxies found in tropical settings (Siddall, 2003; Woodroffe and Murray-Wallace, 2012; Woodroffe and Webster, 2014), sea cliff evolution can span multiple eustatic cycles and can occur in temperate erosive environments which may not favour depositional proxies. Thus, they are often used for identifying lower sea levels from bathymetric data (Kennedy et al., 2002; Sivkov et al., 2011; Westley, 2011; Niedzielski et al., 2013; Cawthra, 2016; Brooke, 2017).
Investigations of the precise morphology of sea cliffs found at lower sea levels are, however, lacking. While the link between coastal geomorphology and sea-level variations has been well known for decades, the reliability of sea cliffs as sea level proxies has not been quantifiably tested. This study, therefore, sets out to test the utility of subaerial and submarine sea cliffs as sea level proxies both at present and past lower elevations by quantifying the morphology of sea cliffs formed along the coast of Victoria, Australia from modern sea level to over 50 m depth.
Section snippets
Regional setting
Victoria is located in south-eastern Australia between 38° - 39°S and 141° - 147°E (Fig. 1). The coast is classified as a Mediterranean climate, having wet winters with average minimum temperatures between 12 - 15 °C and dry summers with average maximum temperatures between 24 - 27 °C (Bureau of Meteorolgy, 2017). Waves with a mean average height of 2–3 m impact the southern margins (Flocard et al., 2016), while the eastern section of coast has an average height of 2 m. Winter swells reaching
Methods
Bathymetric and terrestrial LiDAR data were collected using a LADS Mk II system with a GEC-Marconi FIN3110 intertidal motion sensing system and a dual frequency kinematic geographic positioning system (kGPS). The resulting dataset includes seamless terrestrial-marine mosaics from elevations of 10 m above sea level to depths of 25 m below sea level (Quadros and Rigby, 2010). Multibeam sonar data were acquired using a using a Reson Seabat 101 multibeam sonar (Ierodiaconou et al., 2007) and a
Modern coast
A total of 261 km of cliffs were identified and analyzed across all five regions. Cliff height ranged from 3.9 to 152.5 m with an average of 27.4±17.7 m. The mudstone cliffs had the greatest mean heights, at 54.9±16.5 m. The granites had the largest range of heights, from 15.7 to 92.5 m, but were the shortest cliffs overall with an average height of 18.5± 8.0 m (Fig. 3a). The Port Campbell Limestone (measured at Gibson Steps) showed the lowest rebound (r) values varying between 10 and 24 with
Discussion
The morphology of a cliff is controlled by the geological constraints (e.g. bedding, jointing, thickness, and compressive strength) that act either to enhance its strength and allow it to maintain a more vertical profile or to weaken its integrity and limit its height (Stephenson and Naylor, 2011). For example, the Port Campbell Limestone in Victoria is a large homogenous unit with horizontal layering. This allows for cliffs to be eroded by wave activity at the base while still having the
Conclusions
Sea cliffs are erosional features that are constantly exposed to weathering processes both from the sea and the atmosphere. This study provides quantitative evidence that the morphology and erosional patterns of cliffs depends heavily on their lithologic characteristics. Cliffs with the higher slopes tended to be the taller cliffs, which can be attributed to softer, more easily eroded rock types. Cliffs composed of rocks such as Port Campbell Limestone were easily eroded by waves but possessed
Acknowledgements
We thank Parks Victoria for funding the capture of the multibeam sonar data used in this study. We thank members of the crew Sean Blake and Dr Alex Rattray of Deakin University's research vessel Yolla for assistance in the collection of the multibeam sonar data. We thank the Department of Environment and Primary Industries Department of Environment and Primary Industries coordinated imagery program for access to the georegistered aerial photography and the Future Coasts Program for access to
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