A synthesis and review of the geological evidence for palaeotsunamis along the coast of southeast Australia: The evidence, issues and potential ways forward

Quaternary Science Reviews 54 (2012) 99e125

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A synthesis and review of the geological evidence for palaeotsunamis along the coast of southeast Australia: The evidence, issues and potential ways forward Claire Courtney a, *, Dale Dominey-Howes a, James Goff a, Catherine Chagué-Goff a, b, Adam D. Switzer c, d, Bruce McFadgen e a Australia e Pacific Tsunami Research Centre and Natural Hazards Research Laboratory, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia b Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia c Earth Observatory of Singapore, Division of Earth Sciences, Nanyang Technological University N2-01C-29, Singapore 639798, Singapore d Division of Earth Sciences, Nanyang Technological University N2-01C-29, Singapore 639798, Singapore e ori Studies, Victoria University of Wellington, PO Box 600, Wellington, New Zealand School of Ma

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 November 2011 Received in revised form 11 June 2012 Accepted 18 June 2012 Available online 28 July 2012

In recent years the role of extreme events such as tsunamis and storms in shaping coastal evolution and change has been increasingly appreciated. Around the world, tsunami geologists are increasingly recognising the signatures of palaeotsunamis almost everywhere they look and in many cases, base their interpretations on similar evidence for Quaternary tsunamis first identified in Australia. Geological research suggests that the coast of south east Australia and others worldwide may have been impacted  hoku events. by palaeotsunamis many times larger than the catastrophic 2004 Indian Ocean and 2011 To In Australia, the debate centres on the hypothesis that the coast of south east Australia preserves evidence for repeated, large magnitude Quaternary tsunamis. If independently validated, this hypothesis has profound implications for risk. Despite the potential importance of this hypothesis, no synthesis or comprehensive review of the proposed geological evidence and chronology exists. As a result it is difficult to assess the evidence and to draw conclusions about the nature of the hazard and risk along the coast. This synthesis details the spatial distribution of reported palaeotsunami deposits along the coast of New South Wales, south east Australia and summarises the distribution of different types of sedimentary and erosional evidence. The age range of reported palaeotsunami deposits is identified and mapped before discussing ‘same age’ (chronologically correlated) deposits. These data are then used to draw broad conclusions about the evidence and identify future research questions to aid in the testing of the hypothesis for repeated tsunami inundation. We show that 60 sites are purported to contain evidence of tsunami inundation over 650 km of the south east Australian coast with a spatial concentration south of Sydney. Geomorphic evidence, distinctly different to that used elsewhere in global palaeotsunami studies, is reported at 54 sites, with erosional features described as the most frequent indication of inundation. Proposed tsunami deposits are evident at 44 sites, with the dominant deposit type being imbricated boulder stacks. Radiocarbon dating at 39 of the sites led to a proposition of nine events during the Quaternary, eight of which occurred during the Holocene. Interestingly, 18 sites have no chronological data associated with them. Alternative interpretations are offered at six type field sites purported to contain palaeotsunami evidence. Attention is drawn to the disjunct between historical and geological scales of tsunami inundation in the region in addition to the contrast between the scale of reported palaeotsunamis and the robust evidence of smaller events. A synthesis of research into the nature of the evidence is offered, including critiques of evidence type and mechanisms. A critical review of the chronological data is also presented, in addition to the recalibration and analysis of published radiocarbon data. The paper concludes with an outline of research questions for further work on proposed palaeotsunami sites in Australia as well as a statement about likely risk in south east Australia. It also advocates the need for caution when interpreting evidence for palaeotsunamis elsewhere around the world when those interpretations are based on signatures originally reported in south east Australia. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Palaeotsunami Sediments Geomorphology Australia Holocene

* Corresponding author. Tel.: þ61 2 9385 8251. E-mail addresses: [email protected], [email protected] (C. Courtney). 0277-3791/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.quascirev.2012.06.018

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1. Introduction and aims Coastal evolution is thought to be the result of gradual dynamic processes that result from the interaction between geophysical variables such as currents, waves, sediment supply and sea level change (Carter and Woodroffe, 1994). These gradual processes may be interrupted by extreme events such as storms and tsunamis (e.g. Goff et al., 2003; Donnelly and Woodruff, 2007; Switzer and Jones, 2008a,b). The role of extreme events in coastal evolution has become a significant research focus because they often leave behind sedimentary and/or geomorphological evidence (e.g. Hall et al., 2008; Sawai et al., 2008; Goff et al., 2009, 2012a,b; Switzer and Burston, 2010). This evidence allows (1) the recurrence intervals of such events to be determined, and (2) an improved understanding of the role extreme events play in coastal evolution (Hansom, 2001). In recent years, a body of work has emerged (e.g. Moore and Moore, 1984; Breger et al., 2006; McGuire, 2006; Abbott et al., 2007; Bunch et al., 2008; Pinter and Ishman, 2008) that reports ‘megatsunamis’ associated with bolide ocean impacts or submarine sediment slides that have occurred, triggering immense coastal change over wide areas. Examples of a range of places around the world affected by these reported megatsunamis include SE Australia (Bryant et al., 2007), northern Australia (Abbott et al., 2007; Tester et al., 2007; Meyers et al., 2007; von Subt et al., 2010), Madagascar (Abbott et al., 2006a,b), the NE Atlantic e specifically Scotland and Ireland (Scheffers et al., 2009, 2010), Hawaii (Seymour, 1981; Moore and Moore, 1984, 1988; Bryan and Moore, 1994; McMurtry et al., 2008), Tunisia (Frébourg et al., 2010) and various sites in the Mediterranean (Sironi and Rimoldi, 2005). The occurrence and determination of potential future recurrence of such events is significant for two reasons. First, such megatsunamis lie outside the realm of human experience and consequently, we have little idea of how contemporary coastal systems would react. Secondly, given the increase of modern coastal populations and infrastructure, future events would be catastrophic e going well beyond the effects of the 2004 Indian hoku-oki tsunamis that resulted in Ocean (2004 IOT) and 2011 To 250,000 and 19,000 deaths respectively (Paris et al., 2007; Srinivas and Nagakawa, 2008; The Tohoku Earthquake Tsunami Joint Survey Group, 2012; National Police Agency of Japan, 2012). Analysis of coastal sites in regions at risk of tsunami indicates that such events, although rare, are not unique (Jankaew et al., 2008; Monecke et al., 2008). Furthermore, the 2004 IOT highlighted that while geologically stable countries such as Australia may not generate tsunamis, there may be devastating consequences as a result of distally generated teletsunamis. Much of the developing concept of megatsunami impacts on coastal evolution and change is scaffolded upon the work by Bryant et al. (1992a) and later studies that focused on Late Quaternary palaeotsunamis affecting the coast of SE Australia e specifically the coast of the state of New South Wales (NSW). While research by Bryant and co-authors focuses primarily on the coast of NSW, this body of research underpins palaeo-megatsunami literature (Dominey-Howes, 2007) with hundreds of citations of key studies (Young and Bryant, 1992; Bryant et al., 1992a, 1996b; Young et al., 1996; Bryant and Nott, 2001) in palaeotsunami studies globally (Fig. 1). The impact of the concepts, research methods and influences on global studies is highlighted when research supported by five of the key publications listed in Table 1 are tracked (Fig. 1). These publications have been adopted by the international tsunami research community and used to underpin interpretations of palaeo- and megatsunami deposits for different time periods,

ranging from the Triassic to Holocene and in different geographic locations. It can be clearly seen that NSW Quaternary palaeotsunami research has had a profound impact on supporting interpretations made by coastal geologists working in this area of research. The concept and interpretations of NSW Quaternary palaeotsunamis are, nevertheless, not without their detractors. Critiques of the megatsunami hypothesis in Australia have been presented by Dominey-Howes et al. (2006) and Switzer and Burston (2010) and globally, such as that of Felton et al. (2000) and Felton and Crook (2003, 2006) in Hawaii; McKenna et al. (2011) in Ireland and Hall et al. (2008) in Scotland. However, the concept of ‘megatsunamis’ affecting and driving change at the world’s coasts is alluring and deserves careful consideration; to reject this work as some have without rigorous examination is both unfair and unwise (DomineyHowes, 2007). Validation or rejection of the hypothesis also has significant implications for understanding and managing risk of the vulnerable coasts. Given the importance of the work of Bryant et al. (1992a) and others in underpinning the concept of megatsunamis as drivers of coastal evolution and environmental change (Goff and McFadgen, 2002; Switzer et al., 2005), a careful and systematic review of the previously reported geological, geomorphological and chronological evidence for palaeo-mega-tsunamis affecting SE Australia e the geographic place where this concept was first proposed, is warranted. Subsequently, the aims of this paper are to (1) identify and map the spatial distribution of reported Quaternary palaeotsunami deposits, including rebuttals, along the NSW coast of SE Australia and to summarise the distribution of different types of geological evidence (depositional and erosional); (2) identify and map the temporal distribution of ‘same age’ (contemporaneous) palaeotsunami deposits along the SE Australian coast to fully understand the chronology of proposed events; (3) critically evaluate the geological and chronological evidence and interpretations in terms of the nature of the evidence, disjunct of scale and the relationship between cause and effect; (4) to explore alternative interpretations of evidence through analysis of type sites, and finally (5) to draw general conclusions regarding the tsunami risk to SE Australia and the implications for palaeotsunami studies globally. Before proceeding, there is an important distinction to be made between ‘megatsunamis’ and ‘regular’ tsunamis. Differentiating between megatsunami and tsunami however is difficult because there is no strict definition of the term ‘megatsunami’ (McGuire, 2006). McGuire (2006, p.1898) defines ‘megatsunamis’ as “waves that are in excess of 100 m in height at source, and which remain destructive at oceanic distances”. Waves at the coast do not necessarily need to have extreme run-up heights but they may do. Where we use terms such as ‘tsunami’, ‘large tsunami’, or ‘megatsunami’ these relate specifically to the wording used in the original publication we referencing. We do not ourselves adopt any specific objective magnitude for tsunamis of different sizes.

2. Spatial distribution of previously reported palaeotsunamis along the coast of NSW, SE Australia and the types of geological evidence Before actually summarising the spatial distribution of reported palaeotsunami deposits and features along the NSW coast of SE Australia, it is necessary to first provide a short overview of the geology, physical geography, climate and sea level history of the region and then describe our methods of analysis. This should provide a context for further discussion and analysis.

Fig. 1. Identification of palaeotsunami evidence supported by research by Bryant et al. (1992a) and subsequent key studies (Young and Bryant, 1992; Young et al., 1996; Bryant et al., 1996b; Bryant and Nott, 2001). The coloured stars mark individual countries where suspected palaeotsunami evidence has been reported. The bar chart indicates the number of studies per geological period where these key references are used to support interpretation of field data. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table 1 List of publications presenting evidence for multiple Quaternary palaeotsunamis along the NSW coast of SE Australia (1992e2010). Author

Year

Title

Journal

Volume

Issue

Pages

Bryant, E.A., Young, R.W., Price, D.M. Bryant, E. A., Young, R.W., Price, D.M., Short, S.A. Young, R.W., Bryant, E.A.

1992a

Evidence of tsunami sedimentation on the southeastern coast of Australia Evidence for Pleistocene and Holocene raised marine deposits, Sandon Point, New South Wales Catastrophic wave erosion on the southeastern coast of Australia: impact of the Lanai tsunamis ca 105 ka? Coastal rock platforms and ramps of Pleistocene and Tertiary age in Southern New South Wales, Australia Theoretical constraints and chronological evidence of Holocene coastal development in central and southern New South Wales, Australia

Journal of Geology

100

n/a

753e765

Australian Journal of Earth Sciences Geology

39

n/a

481e493

20

3

199e202

Zeitschrift fur Geomorphologie

37

n/a

257e272

Geomorphology

7

4

317e329

Bulgarian Geophysical Journal

XXI

4

24e32

Journal of Geology

104

n/a

565e582

International Geological Correlation Program Journal of Coastal Research

367

n/a

1e228

12

4

831e840

Zeitschrift fur Geomorphologie

40

n/a

191e208

Physical Geography

18

n/a

440e459

Young, R.W., Bryant, E.A., Price, D.M. Young, R.W., Bryant, E.A., Price, D.M., Wirth, L.M., Pease, M. Young, R.W., Bryant, E.A., Price, D.M. Bryant, E.A., Young, R.W. Bryant, E.A., Jones, B.G., Yassini, I., Young, R.W. Bryant, E.A., Young, R.W., Price, D.M. Young, R.W., Bryant, E.A., Price, D.M Bryant, E.A., Young, R.W., Price, D.M., Wheeler, D.J., Pease, M.I. Young, R.W., Bryant, E.A., Price, D.M., Dilek, S.Y., Wheeler, D.J. Price, D.M., Bryant, E.A., Young, R.W. Fullagar, R., Head, L., Bryant, E.A., Beaman, S. Bryant, E. Bryant, E.A., Nott, J. Bryant, E.A., Walsh, G., Abbott, D. Bryant, E. Bryant, E., Haslett, S.K., Scheffers, S., Scheffers, A., Kelletat, D.

1992b 1992 1993 1993

1995 1996 1996a 1996b 1996 1997

The imprint of tsunami in quaternary coastal sediments of Southeastern Australia Bedrock-sculpturing by tsunami, south coast of New South Wales, Australia Guidebook. South coast pre-conference field trip Tsunami as a major control on coastal evolution, southeastern Australia Catastrophic wave (tsunami?) transport of boulders in southern New South Wales, Australia The impact of tsunami on the coastline of Jervis Bay, southeastern Australia

1997

Chronology of Holocene tsunamis on the southeastern coast of Australia

Transactions, Japanese Geomorpholgical Union

18

1

1999

Thermoluminescence evidence for the deposition of coastal sediments by tsunami wave action Tsunami disturbance on coastal middens: McCauleys Beach, New South Wales

Quaternary International

56

1

Australian Coastal Archaeology, Research Papers in Archaeology and Natural History Cambridge University Press Natural Hazards Special Publication e Geological Society of London Springer Praxis Journal of Siberian Federal University

31

1999

2001 2001 2007 2008 2010

Tsunami: The Underrated Hazard (1st edition) Geological indicators of large tsunami in Australia Cosmogenic mega-tsunami in the Australia region: are they supported by Aboriginal and Maori legends? Tsunami: The Underrated Hazard (2nd edition) Tsunami chronology supporting late Holocene impacts

2.1. Overview of the coast of NSW, SE Australia The state of NSW covers w800,000 km2 and possesses a coast of approximately 2000 km (Short and Woodroffe, 2009). It stretches from latitude 28 10e37 30 S. State population is estimated at over seven million (Australian Bureau of Statistics, 2009) with 86% living in the coastal zone (Young, 2000) resulting in a high exposure to the negative effects of coastal processes and extreme events. The eastern continental margin of Australia consists of a late Cretaceouseearly Tertiary rifted plate boundary (Hayes and Ringis, 1973). The continental shelf is deep and narrow, extending 17e35 km offshore (Zann, 2000) with a mean slope of 5e6 (Roy, 1997). Over 80% of the shelf is more than 50 m deep (Davies, 1975). The southern coast of NSW is located within the strongly folded Lachlan Fold Belt province (Giordano and Cas, 2001). Volcanism is believed to have been continuous for over 30 million years (Branagan and Packham, 2000). Overlying deep marine arenites, cherts, shales and volcanics are fluvial arenites that are in turn overlain by unconsolidated Quaternary sediments. The southern coast of NSW experiences a cool temperate climate (Zann, 2000) with a mean annual temperature of 12e15  C (Bureau of Meteorology, 2011) and mean annual sea surface temperature of 18e20  C (Short and Woodroffe, 2009). Weather is driven by maritime air masses and is dominated by

1e19

123e128 235e238

n/a 24 273

n/a 3 n/a

1e320 231e249 203e214

n/a 1

n/a n/a

1e330 63e71

East Coast and mid-latitude cyclones. Annual rainfall along the coast ranges from 600 to 1200 mm (Bureau of Meteorology, 2001) and is fairly evenly distributed throughout the year. However, the region is also subjected to intense local storms (Roy, 1997). Rarely subjected to tropical cyclones, the coast of NSW is affected by intense low-pressure systems known as East Coast Lows that result in storm conditions, heavy rainfall, high winds and storm surges. The coast of SE Australia is subjected to the Southern Ocean swell approximately 200 days per year (Roy and Thom, 1981; Short and Woodroffe, 2009) although cyclones, high-pressure systems and sea breezes also influence wave regimes (Roy, 1997). Storm waves frequently exceed heights of 4 m and Shand et al. (2010) estimate significant wave heights in 100-year storms on the far South Coast of 9.3 m while waves of up to 16 m have been recorded (Thom, 1983) and Roy (1997) notes that NSW experiences a slightly higher storm wave climate than elsewhere. The tectonically stable, wave dominated and sediment deficient NSW coast (Roy and Boyd, 1996) geomorphologically consists of an embayed bedrock margin with drowned valleys. Eroded bedrock is partially filled by sandy barriers and beaches, tidal flats, lagoons and deltas (Roy et al., 1980; Roy and Thom, 1981). The steeply dipping deformed rocks of southern NSW result in extremely angular headlands and a lack of rock platforms (Johnson, 2004). The

C. Courtney et al. / Quaternary Science Reviews 54 (2012) 99e125

seaward margin of the narrow discontinuous coastal plain consists of largely Pleistocene and Holocene coastal barriers (Roy et al., 1980) with a total of 278 barriers found in NSW, occupying 58% of the coast (Short and Woodroffe, 2009). Prior to the last glacial phase, a highstand occurred 120, 000 years ago with sea levels estimated at 2e5 m higher than at present (Roy and Thom, 1981; Short and Woodroffe, 2009). Sea levels fell during glaciation to an estimated 130 m below present mean sea level (PMSL) (Johnson, 2004). Following this phase, Sloss et al. (2007) determined that sea levels in NSW rose until PMSL was attained 7900e7700 years BP. The mid-Holocene highstand ensued, with sea levels up to 1.5 m above PMSL 7700e2000 years BP. 2.2. Methodology Table 1 lists those publications that present evidence for Quaternary palaeotsunamis along the NSW coast of SE Australia, while publications critiquing or opposing the evidence for Quaternary palaeotsunamis in NSW are listed in Table 2. Other authors, such as Curtis (2002), Satake (2002), Dominey-Howes (2007) have offered critiques of various aspects of the studies underpinning interpretations of NSW Quaternary palaeotsunamis, only authors presenting direct scientific analysis of the proposed geological evidence are included in Table 2. All publications describing evidence for Quaternary palaeotsunamis in NSW were identified (Tables 1 and 2), analysed, and key information (e.g. impact locations, type of geological evidence identified, chronological data, and the proposed event source and date) was collated into a single comprehensive database. All sites were plotted on to a map (Fig. 2). Field sites described in the critiques listed in Table 2 were also identified and marked on Fig. 2. The maximum elevation of reported evidence at each site was obtained and reported on Fig. 3a. Where no data were available this was also recorded and the highest value used where multiple data were presented. Information concerning site-specific depositional and erosional evidence was obtained via content analysis of publications and is summarised in Fig. 3b. Analysis of studies critiquing the evidence for Quaternary palaeotsunamis was conducted based upon the themes of disjuncts with historical data, the nature of evidence and potentially erroneous interpretations. Event chronologies shown in Fig. 3c are based upon the most recent interpretations provided in the relevant publications (Table 1). To simplify event chronologies for general analysis, approximate

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median values were used in line with the method adopted by authors listed in Table 1. 2.3. Synthesis of reported palaeotsunami evidence along the coast of NSW 2.3.1. Spatial distribution The NSW coast is divided into three regions: ‘North Coast’, ‘Central Coast’ and ‘South Coast’ (Fig. 2B). Fig. 2 shows the distribution of 60 sites reported as possessing geological evidence for Quaternary palaeotsunamis. The most northerly site is Lord Howe Island (Site 1), located approximately 500 km east of Port Macquarie in the SW Pacific. The most southerly site is Haycock Point (Site 60). The sites span 650 km, from 31 S to 36 S and cover ten degrees of longitude. Mainland sites are distributed between Seal Rocks (Site 2) and Haycock Point (Site 60), with sites predominately located along a 90 km stretch of coast between Wollongong and Jervis Bay (Fig. 2B). There are a greater number of sites to the south of the state. Of the 60 sites, three are located on the southern part of the North Coast and 17 on the Central Coast. The remaining 40 sites are all located on the South Coast, with a moderately regular distribution. The studies that critique the reported evidence for Quaternary palaeotsunamis focus on six field sites concentrated entirely within the South Coast (Fig. 2) and spanning approximately 270 km. The most northerly site is Bass Point (Site 25), while the most southerly is Pambula Lake, located near Tura Point (Site 58). Five of these sites, however, are concentrated between Bass Point (Site 25) and Little Beecroft Head (Site 34) e just 50 km of coast. This results in significant spatial gaps both south of the Jervis Bay area (Fig. 2C) and north of Wollongong. Data about the maximum elevation of evidence identified as associated with Quaternary palaeotsunamis is provided for 38 of the 60 sites (Fig. 3a). Elevations range from 2 m above PMSL at Thirroul (site 15) and Sandon Point (Site 17) to 552 m at Lord Howe Island (Site 1). Steamers Beach (Site 42) has evidence reported at up to 130 m above PMSL, while nearby Crocodile Head (Site 38) has evidence at 80 m. There are 13 sites with evidence reported at more than 20 m above PMSL and five of these sites are located within Jervis Bay (Sites 32e46). There are ten sites with evidence reported at or below 5 m above PMSL, six of which are located in the 16 km between Coledale (Site 14) and Wollongong (Site 20). Sites in Sydney Harbour and the Sydney metropolitan area (Sites 6e13) have the least elevation data provided, with elevation data presented for only Little Bay (Site 12).

Table 2 List of publications that critique the idea of NSW Quaternary palaeotsunamis (2003e2010). Author Felton, E.A., Crook, K.A.W

Year

Title

2003 Evaluating the impacts of huge waves on rocky shorelines: an essay review of the book ‘Tsunami e the Underrated Hazard’ Goff, J., Hulme, K., McFadgen, B. 2003 “Mystic Fires of Tamatea”: Attempts to creatively rewrite New Zealand’s cultural and tectonic past Saintilan, N., Rogers, K. 2005 Recent Storm Boulder Deposits on the Beecroft Peninsula, New South Wales, Australia Switzer, A.D., Pucillo, K., Haredi, 2005 Sea level, storm, or tsunami: enigmatic sand sheet deposits in a R.A., Jones, B.G., Bryant, E.A. sheltered coastal embayment from Southeastern New South Wales, Australia Dominey-Howes, D.T.M., 2006 Tsunami and palaeotsunami depositional signatures and their Humphreys, G. S., Hesse, P.P. potential value in understanding the late-Holocene tsunami record Goff, J., Dominey-Howes, D. 2009 Australasian palaeotsunamis e Do Australia and New Zealand have a shared trans-Tasman prehistory? Hutchinson, I., Attenbrow, V. 2009 Late-Holocene mega-tsunamis in the Tasman Sea: an assessment of the coastal archaeological record of New South Wales Goff, J., Dominey-Howes, D., 2010 Analysis of the Mahuika comet impact tsunami hypothesis Chagué-Goff, C., Courtney, C. Goff, J., Dominey-Howes, D. 2010 Does the Eltanin asteroid tsunami provide an alternative explanation for the Australian megatsunami hypothesis?

Journal

Volume Issue Pages

Marine Geology

197

n/a

1e12

Journal of the Royal Society of New Zealand Geographical Research

33

4

795e809

43

4

429e432

Journal of Coastal Research

21

4

655e663

The Holocene

16

8

1095e1107

Earth Science Reviews

97

n/a

147e154

The Holocene

19

4

599e609

271

n/a

292e296

10

n/a

713e715

Marine Geology Natural Hazards and Earth System Sciences

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Fig. 2. A) Map of Australia showing study area of NSW, SE Australia; B) spatial distribution of Quaternary palaeotsunami deposits on the NSW coast reported in the publications listed in Table 1 and questioned by those listed in Table 2, with inset, C) detailing sites in the Jervis Bay area.

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‘large palaeotsunamis’ along the (SE) Australian coast. The thirteen classes of indicators are divided into depositional and erosional features (Table 3). The spatial distribution of the main signature types is shown in Fig. 2A.

Fig. 3. a. Distribution of maximum elevations of features attributed to palaeotsunami where identified within references listed in Table 1. b. Spatial distribution of the main depositional and erosional features defined by Bryant and Nott (2001). c. Temporal distribution of events defined by the authors of references listed in Table 1.

2.4. Types of geological evidence Bryant and Nott (2001) published a comprehensive review of the diagnostic criteria used as evidence to identify inundation by

2.4.1. Depositional signatures Depositional signatures of NSW Quaternary palaeotsunamis are classified as either sedimentary deposits or geomorphic forms (Table 3). While nine different signature types have been described, we primarily review five types of generic sedimentary deposit (sand laminae, sand layers with boulder floaters, chaotic sediment mixes or “dump” deposits, imbricated boulder stacks, smear deposits) and one geomorphic form most commonly reported within the studies listed in Table 1. Geomorphic forms consist of ridges, mounds and dunes but are commonly discussed under the generalised term ‘megaripples’ within the literature and are therefore classified as a single type. The remaining signatures are less common, with only limited evidence presented and subsequently they are only briefly discussed here. The most commonly recognised sand laminae signature types include estuarine run-up deposits, anomalous sand sheets or landward-tapering sand units in coastal muds or peat (Bryant, 2001). This signature type is observed at multiple locations along the NSW coast and is concentrated particularly in the Minnamurra to Shoalhaven region (Sites 26, 27, 28, 29, 30 and 31) (Fig. 3b). These take the form of barrier complexes within embayments (Bryant et al., 1992a) or anomalous landward thinning sands up to 0.5 m thick overlying estuarine sediments (Bryant et al., 1992a; Young et al., 1995, 1997; Bryant and Young, 1996; Bryant, 2008). The sand deposits are located up to a maximum elevation of w6 m above mean sea level (Young et al., 1997) and are reported to extend approximately 10 km inland at one location (Site 31) (Bryant, 2001). Individual lamina are thought to represent individual waves within a wave train (Bryant, 2008). The lower contact with estuarine clays in some cases contains cobble-sized sediments that could be interpreted as rip-up clasts. Sand layers with boulder floaters are similar to anomalous sand sheets, typically consisting of up to 1.5 m of sand that contain isolated or ‘floating’ clasts (Bryant et al., 1996b). This signature type is evident at four sites, primarily south of Wollongong (Fig. 2). Isolated clasts range from oblate pebbles to boulders at MacCauleys Beach (Site 17) (Bryant et al., 1992a), while only boulder-sized clasts are found at Bellambi Beach (Site 18) (Bryant et al., 1992a, 1996b). Large clasts are typically more common close to the upper and lower contacts of sand sheets (Bryant et al., 1992a). While the source of the clasts is readily available (usually cliffs located close to the deposits), the contemporaneous transport and deposition of boulders and sand are considered highly unusual (Bryant et al., 1992a). The presence of boulders within a sand matrix is ‘suggestive of rapid, isolated transport under high-energy conditions’ (Bryant, 2008, p. 59). Storm waves overwashing the platforms on which these deposits are located is discarded as a causal mechanism due to the elevation of the platforms and preferential transport of finer materials by such processes (Bryant et al., 1992a; Bryant, 2001). Chaotic sediment mixes or dump deposits are described as ‘prominent coastal features composed of either boulders or mixtures of well-sorted cobble and shell stacked into isolated ridges, terraces, or mounds’ (Bryant et al., 1992a, p. 757). There are seven sites where these are described between Sydney and Mystery Bay (Fig. 3b, sites 7e57). All ridges and mounds are described as containing sediments distinctly different from the surrounding coastal material, although data are not presented to quantify these descriptions. These deposits are described as being similar to features formed by wind-waves, solifluction, ice push, human disturbance or slope wash (Bryant et al., 1996b; Bryant and Nott,

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Table 3 Diagnostic criteria used as evidence for inundation by large tsunamis along the Australian coast (after Bryant and Nott, 2001). Depositional features Sedimentary Sand laminae deposits

(Sand layers with boulder floaters)

(Chaotic sediment mixes or “dump” deposits)

Imbricated boulder Smear deposits stacks

Geomorphic forms

(Ridges, mounds and dunes)

(Coastal barriers)

Carseland

Splayed landward tapering sand units

Erosional features Bedrock Cavitation sculpturing features

Geomorphic forms

(Impact marks)

(Drill holes)

(Sinuous grooves)

S-Forms

(Flutes)

(Cavettos)

(Transverse troughs) (Muschelbrüche)

(Sichelwannen)

(Potholes)

Sculptured headland features

(Whirlpool and plugs)

(Canyon drainage channels)

(Rapids, cascades and falls)

(Roche Moutonnées) (sic)

(Toothbrushshaped headlands)

(Fluted promontories)

Landscape features

(Truncated cliffs)

(Raised platforms)

(Ramps)

(Headlands)

(Eroded barrier remnants)

2001). These alternative depositional mechanisms are considered unlikely by the reporting authors, leading to the hypothesis that these sediments are the result of catastrophic tsunami (Bryant and Nott, 2001). Dump deposits are described as a result of large tsunami wave trains eroding back beach material, which is reworked and redeposited by subsequent waves (Bryant et al., 1992a). These deposits are interpreted as behaving similarly to pyroclastic flows, with layers deposited from downward turbulent pulses of water (Bryant, 2001). Spatial variations in the internal fabric of the deposits as well as the differential extent of erosion are considered to be the result of turbulent vortices (Bryant, 2008). Imbricated boulder stack (Table 3) deposits are, after erosional features, the most common diagnostic criterion used for identifying NSW Quaternary palaeotsunamis. Twenty sites are reported to contain boulder deposits (Fig. 3b) that are imbricated and sometimes deposited in ridges, trains or en echelon. These deposits are described as occurring well above the modern limits of storm waves (Bryant and Nott, 2001), with boulders as large as 30 m3 observed (Young et al., 1997). The transport, alignment and stacking of boulders are considered analogous to boulder movement in large scale, high velocity unidirectional flow (Bryant et al., 1996b). In some circumstances, imbricated boulders are described as ‘a single-grained swash line at the upper limit of tsunami run-up’ (Bryant, 2001, p. 76). Calculations of the velocities required to transport boulders based upon Costa’s (1983) flow velocity and Nott (1997) bedload transport equations are used by Bryant and Nott (2001) to support the hypothesis that windwaves of sufficient height could not be generated to account for the boulders. As a result, ‘megatsunamis’ are considered to be the most likely mechanism for the transport and stacking of boulders (Bryant and Nott, 2001). The mechanism for the transport of large boulders to elevations in excess of 30 m is described as ‘jetting across the top of the cliffs and developing a roller vortex in front of the cliff edge’ (Bryant, 2001, p. 81). These jets are calculated to exceed velocities of 200 m s1, capable of eroding surfaces and exposing bedrock to large lift forces (Bryant, 2008). It is worth noting however, that the boulders are frequently described as being transported in suspension rather than as bedload (Bryant and Young, 1996). Smear deposits (Table 3) are the rarest of the depositional signatures, occurring only at Flagstaff Point (Site 21) and Kiama Head (Site 28). These deposits are ‘often spread in a continuous layer less than 30 cm thick over the steep sides and flatter tops of headlands’ (Bryant, 2001, p. 74). Smear deposits are described as rich in quartz sand, shell and gravel and are deemed not to be the products of weathering as they are found on volcanic sandstone and basalt (Bryant, 2008). These coarse sediments are described by Bryant (2001, 2008) as

(Inverted keel-like ridges)

‘mud-rich sediment transported by tsunamis, which allows for the material to be spread smoothly over the ground under pressure and for the good preservation of tsunami sediments during overwash’. The geomorphic forms of ridges, mounds and dunes (Table 3) are more commonly referred to as megaripples or chevrons. These megaripples are noted at Lord Howe Island (Site 1), Crocodile Head (Site 38) and at Steamers Beach (Site 42) (Fig. 3b), where they are described as reaching a maximum elevation of >130 m above mean sea level (Young et al., 1997; Bryant, 2001; Bryant and Nott, 2001). According to Bryant et al. (1997), these features have been erroneously reported as aeolian dunes and consist of sand interbedded with coarser clasts that fine upwards. These features ‘form as bedforms under catastrophic unidirectional flow’ (Bryant, 2001, p. 73). The deposits are thought to have been produced by large sedimentladen tsunamis overtopping high elevations, depositing sediment into bedforms, then forming splayed landward tapering sand units (Table 3) as the water drains (Bryant, 2001, 2008). Megaripples display cross-bedding and contain clasts that ‘leave no doubt that this deposit. was emplaced by waves rather than wind’ (Bryant et al., 1997, p. 452). Deposits at Lord Howe Island (Site 1) are reported to contain fragments of fragile coral that would have been broken during storm overwash (Bryant and Nott, 2001). Carseland ‘represents the planing or smoothing of raised estuarine surfaces by tsunami run-up’ (Bryant and Nott, 2001, p. 234). Although this geomorphic form is listed, no examples from NSW are actually given. Furthermore, in Bryant (2008) carseland is no longer included in the descriptions of depositional signatures of tsunamis. Coastal barriers (Table 3) are initially categorised as geomorphic features (Bryant and Nott, 2001) but are later defined as a type of coastal landscape created by tsunamis (Bryant, 2008). Tsunamis are described as a possible mechanism for shifting coastal barriers landward and providing additional sediments to these landforms, with Bellambi Beach (Site 18) reported as the only example in SE Australia. 2.4.2. Erosional signatures The erosional signature types listed in Table 3 are used as diagnostic criteria at 26 sites (Fig. 3b) located between Sydney Harbour and Haycock Point (Sites 7e60), with concentrations around Jervis and Batemans Bay (Sites 32e46 and 52) (Fig. 2). The most significant erosional feature is noted at Lord Howe Island, where inverted keel-like ridges are reported at 552 m above PMSL. On the mainland, erosional features are reported at elevations up to 45 m above PMSL at Bass Point (Bryant and Young, 1996) and 50 m above PMSL at an unspecified Jervis Bay location (Bryant et al., 1996a,b). There are a large variety of erosional signatures used as diagnostic criteria

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for NSW Quaternary palaeotsunamis. While this paper focuses on megatsunami deposits, it is prudent to at least briefly discuss the main types of erosional signatures, particularly given that erosional features occur in conjunction with depositional features at eight of the 60 sites. Cavitation features are likened to s-form signatures but formed under extremely high velocity conditions (Bryant and Young, 1996). S-forms are interpreted as being analogous to bedrock sculpturing observed from subglacial flows (Bryant and Young, 1996) and many of the features are described using nomenclature typically associated with glacial features. Sculptured headland features are reported to have been formed as a result of gravity-accelerated flow down the steep sides of headlands during inundation. The resulting seaward butte-like structures are separated from the shore by eroded depressions thought to have been caused by erosive channelisation. This signature type is also used to identify flow direction. Bryant et al. (1996b) remarks that no other explanation exists to explain these features. Landscape features are large-scale geomorphic alterations, often with entire headlands undergoing significant erosion. The scale of these features is attributed to the effects of inundation by multiple tsunamis separated by “substantial intervals of time” (Bryant and Young, 1996). 3. Chronology of NSW Quaternary palaeotsunamis 3.1. General discussion of dating Analysis of the thermoluminescence (TL) and radiocarbon age determinations presented within the publications listed in Table 1 indicate that nine events are reported to have occurred during the Quaternary. Each of the events is discussed below. Our analysis was based on all of the 29 cited radiocarbon age determinations listed in Table 4. Chronological analysis in this synthesis focuses on radiocarbon age determinations only due to concerns over the reliability of the thermoluminescence dating technique (Kojo, 1991; Radtke et al., 2001) and the limited information available concerning specific sampling techniques used in the original publications. We note, however, that in several cases TL ages were crosscorrelated with radiocarbon data to infer an events age (Bryant et al., 1997). Fig. 3c displays the event chronologies site by site reported in the publications listed in Table 1, while Fig. 4 shows the correlation between presented radiocarbon data and proposed events. Of the 60 palaeotsunami sites, 39 have chronological data (TL or radiocarbon) associated with them (Fig. 3c). Cullendulla Creek (Site 51) (Bryant et al., 1992b; Young et al., 1993b, 1997; Bryant, 2001, 2008) is reported to have been affected by four events and separate dating evidence is presented for each event. This site, therefore, contains the most chronological evidence for individual events. Bryant (2008) stated that there are 29 radiocarbon samples obtained from marine shells on the NSW coast that produce ages less than two thousand years old. Analysis of the studies in Table 1 however, identifies only 22 cited radiocarbon samples within this age range (Table 4). Six of these radiocarbon samples are specifically identified as marine shell species, while information for the remainder is equivocal (Table 4). Between 1992 and 1997, 26 radiocarbon age determinations were reported in five publications (Bryant et al., 1992a, 1996b, 1997; Young et al., 1995, 1997). The first and second editions of Tsunami: The Underrated Hazard (Bryant, 2001, 2008) utilises only dates from Young et al. (1997), with collations of radiocarbon data interpreted via schematic representations with aggregated results plotted as age distributions over time.

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3.2. Undated events It is stated that a ‘substantiated’ chronology for NSW reveals that tsunamis have occurred at w6500 years BP, w2900 years BP, 1500 years BP, 900 years BP and 220e250 years BP (Bryant and Nott, 2001, p. 244). This chronology appears to provide the basis for a cyclical pattern, with recent events described as occurring every 600 years (Bryant and Nott, 2001, p. 246). Undated evidence infers that these events impacted Bass Point (Site 25), Cathedral Rocks (Site 30), Haycock Point (Site 60) Tura Point (Site 58), Steamers Beach (Site 42) (Young et al., 1997; Bryant, 2001; Bryant and Nott, 2001). Furthermore, Crocodile Head (Site 38), Steamers Beach (Site 42) and Gumgetters Inlet (Site 43) (Bryant et al., 1997; Young et al., 1997; Bryant, 2001, 2008), being located within Jervis Bay, are reported as being affected by six separate events but there is no dated evidence provided at these sites for any of the six events. 3.3. The dates of NSW Quaternary palaeotsunamis 3.3.1. The 105,000 years BP event While Bryant (2001, 2008) focuses primarily on NSW events during the Holocene, Bryant et al. (1992a, 1996b); Young and Bryant (1992); Young et al. (1993a, 1995); and Price et al. (1999) report an event 105,000 years BP that they link to the proposed, but controversial, Lanai volcanic flank collapse in Hawai’i (Felton, 2002; Keating and Helsley, 2002; Noormets et al., 2002, 2004; Crook and Felton, 2008). Bedrock sculpturing specifically at Red Point (Site 22), Tura Head (Site 58) and Wollongong (Sites 20 and 21), and more generally along the NSW coast has been attributed to this event (Bryant et al., 1992a, 1996b). However, as shown in Figs. 3c and 4, no chronological data from any NSW site have ever been presented to support this. 3.3.2. The 9000 years BP event The oldest Holocene palaeotsunami is linked with the Deluge Comet Impact of 8200  200 years BP (Bryant, 2008). This appears to be a continuation of the impact hypothesis linked to global catastrophes evident in multiple cultural traditions such as the Biblical flood (Kristan-Tollmann and Tollmann, 1994), which has been critiqued for insufficient and ambiguous data (Deutsch et al., 1994). Wood overlying tektites ‘associated with the Tasman Sea’ is reported to date from 8200  250 years BP, although no site details or laboratory reference are given (Bryant, 2008, p. 252). In the absence of an alternative event cited around this time, we assume that this palaeotsunami is matched with the 8700e9000 years BP event reported by Young et al. (1997). Of seven sites purported to contain evidence of this event, three sets of chronological evidence are presented. These include a single radiocarbon date (Beta82245) of 8740  70 years BP (uncorrected) from shell hash at Steamers Beach (Site 42) (Bryant et al., 1997; Young et al., 1997), an undated imbricated boulder train at Little Beecroft Head (Site 34) (Young et al., 1997) and TL (thermoluminescence) dated material from Kioloa Beach (Site 48) (Young et al., 1997) and Mermaids Inlet (Site 35) (Young et al., 1996), that have an age range between 8700 and 27,000 years old. 3.3.3. The 6500 years BP event An event at 6500 years BP is reported throughout the South Coast of NSW. Evidence includes chaotic dump deposits at Bass Point (Site 25) (Bryant and Nott, 2001), estuarine sand sheets at Bellambi (Site 18) and Callala Beaches (Site 36) (Young et al., 1997; Price et al., 1999; Bryant, 2008), erosional features at Cathedral Rocks (Site 30) and Tura Head (Site 58) (Bryant, 2001, 2008), imbricated boulders at Gum Getters Inlet (Site 43) (Bryant, 2001)

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Table 4 Radiocarbon data cited in references listed in Table 1 (dates presented in alpha-numerical order of laboratory numbers; Beta ¼ Beta Analytic; SUA ¼ Sydney University). Refer to Fig. 4 for plotted relationships between CARs. Indicator type

Palaeotsunami indicator

Geological

Particle/grain sizes range from boulders to fine mud e palaeotsunami sediment grain sizes are source-dependent Sediments generally fine inland and upwards, rise in altitude inland and can extend for kilometres inland and alongshore Each wave can form a distinct sedimentary unit and/or there may be laminated sub-units Distinct upper and lower sub-units representing runup and backwash can sometimes be identified Lower contact is unconformable or erosional Can contain intraclasts (rip-up clasts) of reworked (natural and anthropogenic) material Sometimes associated with loading structures at base of deposit and can be associated with liquefaction features on the ground surface Micro-scale features (e.g. micro-rip-up clasts, organic entrainment, erosive contacts) that may be visible in thin section but not in field stratigraphy Measurement of anisotropy magnetic susceptibility (AMS) combined with grain size analysis provides information on hydrodynamic conditions ‘typical’ during tsunami deposition. Essential when no sedimentary structures are visible. Heavy mineral laminations often present but source-dependent e normally near base of unit/sub-unit but not always. Increases in elemental concentrations palaeosalinity indicators, including element ratios relative to under- and overlying sediments. Possible contamination by heavy metals and metalloids (source-dependent, inc. water depth source) Geochemical (saltwater signature) and microfossil evidence often extends further inland than landward maximum extent of sedimentary deposit Individual shells and shell-rich units are often present. Often more intact shells as opposed to shell hash. Small, fragile shells and shellfish can be found near the upper surface of more recent palaeotsunami deposits Shell, wood and less dense debris often found “rafted” near top of sequence Often associated with buried vascular plant material and/or buried soil and/or skeletal (non-human) remains Marked changes in foraminifera and other marine microfossils assemblages occur. Deeper water species are introduced e this is unlikely in storm or anthropogenic deposits, and/or increase in foraminifera abundance and breakage of tests. Pollen concentrations are often lower (diluted) in the deposit because of the marine origin and/or include high percentage of coastal pollen (e.g. mangroves). Pollen changes above and below the deposit are often indicative of sustained environmental change Archaeological sites e a sediment layer separating, underlying or overlying anthropogenic deposits/occupation layers Archaeological middens: changes in shellfish species/absence of expected species indicate sudden change in palaeoenvironmental conditions Archaeological structures show structural damage by water to buildings/foundations at a site Archaeological burial sites have been reworked, often recognisable as “culturally inappropriate” burials Replication e coastal archaeological occupation layers and shell middens are often separated or extensively reworked at several sites along coastline giving a regional/national signal of inundation Traditional Environmental Knowledge (oral traditions) from the locality/region Acquired palaeogeomorphology indicates tsunami inundation e a tsunami geomorphology is present that could include evidence of: i) uplift or subsidence/compaction of site/locality, ii) scour/erosion/reworking of sediments at site/locality e altered dune morphology, iii) sand sheet or other similar deposits such as gravel deposition/gravel pavements Palaeogeomorphology at the time of inundation indicates low likelihood of storm inundation Known local or distant tsunamigenic sources can be postulated or identified Known local and regional palaeoenvironmental drivers indicate low likelihood of storm inundation Replication e similar contemporaneous coastal deposits are found regionally giving a regional signal of inundation

Chemical

Biological

Archaeological

Anthropological Geomorphological

Contextual

and megaripples at Crocodile Head (Site 38) and Steamers Beach (Site 42) (Bryant et al., 1997; Bryant, 2001). Three radiocarbon age determinations are used to provide chronological evidence (Beta43951, Beta-54207 and Wk8659). It is interesting to note however, that the one paper listed in Table 1 that does not specifically deal with tsunamis attributes an event dated to around 6500 years BP at Thirroul Beach (Site 15) to the ‘final stages of the Holocene transgression’ (Bryant et al., 1992b, p. 490). 3.3.4. The 5000 years BP event The geological evidence for this event is reported at three sites and includes isolated boulders in a sand matrix at Bellambi Beach (Site 18) (Bryant, 2001, 2008) and tightly packed boulders at McMasters Beach (Site 5) (Bryant et al., 1992a). No chronological data however, are presented at these two sites. At Shoalhaven estuary (Site 31) a shell-rich sand sheet extending 10 km inland is also reported as evidence for this event (Bryant, 2001, 2008). The age for the Shoalhaven sand sheet is based upon the radiocarbon date of an ‘oyster shell’, although no data are provided. Fig. 4 shows the lack of correlation between radiocarbon age determinations and this proposed event. The only cited chronological information for the sand sheet are two thermoluminescence dates ranging in age between 15,500  1500 and 25,600  3200 years BP (Price et al., 1999). 3.3.5. The 2900 years BP event Two radiocarbon age determinations (SUA 2901, Beta-36434) from shells within the Cullendulla Creek (Site 51) sand ridges

(Young et al., 1997) and a single radiocarbon value (Beta-107033) of 2810  70 14C years BP from an unspecified source in a shelly sand deposit in Batemans Bay (Site 52) (Bryant et al., 1997; Bryant and Nott, 2001) are used to date an event at approximately 2,900 years BP. Based upon the nature and extent of the dated tsunami evidence, this is believed to be a local event, despite evidence reported at ten locations from Lord Howe Island (Site 1) to Tura Point (Site 58). As shown in Fig. 3, the presented radiocarbon data correlate well with this proposed event. However, despite this correlation, the spatial extent of evidence and number of sites reported to contain geological evidence, the last mention of this event is in Bryant and Nott (2001). 3.3.6. The 1500 years BP event The remaining events in the “substantiated chronology” are specifically tied to comet activity. The 1500 years BP event is the most widely postulated, with 16 sites reported to contain geological evidence. Spatially, geological evidence extends from the most northerly to most southerly field sites. Seven dated samples from the Central and South Coasts are used as evidence for this event. The dated samples form part of a radiocarbon age obtained from an aggregated distribution of multiple data by Bryant (2008: p. 236) that is linked to comet observations from Asian and European historical records between 401 and 500 AD. 3.3.7. The 900 years BP event As with the 1,500, 500 and 250 years BP events, the 900 years BP event has good correlation with comet observations and event

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107034 and Beta-79070) (Sites 7e12), Atcheson Rock (Beta-78894) (Site 23), Crookhaven (Beta-84641) (Site 31) and an unspecified date from Jervis Bay (Fig. 1). At Lord Howe Island, an ‘oyster shell’ yielded a date of 350  60 years BP (Beta-87100) (Bryant and Nott, 2001). We presume that this is linked to the “500 year BP event”, although this is not explicitly stated. 3.3.9. The 250 years BP event This event is reported at 14 sites and is linked to eight radiocarbon age determinations. These dates, such as Beta-43948, a shell from Mystery Bay (Site 57), which was dated at 106.2  0.8%, are generally modern age material. However, sites are linked to a 250 years BP event because it is argued that since the geological evidence indicates ‘megatsunami’ inundation, these dates must be older than calibrated. It is argued that an event must have occurred immediately prior to European settlement of the region in 1788AD since no large tsunami has inundated the coast since then (Young et al., 1997). As a result, all radiocarbon ages younger than 250 years BP are linked to this pre-European event. The proposed age for this event therefore provides the justification for not applying an oceanic reservoir correction, because to do so yields a modern date that must be incorrect. 4. A critical evaluation of the geological and chronological evidence for NSW Quaternary palaeotsunamis

Fig. 4. Plotted CARs for samples listed in Table 4 (Years BP count back from 1950AD; for simplicity, modern dates are plotted at 1950AD). Green bars show age ranges, red bars indicate where a second, equally statistically significant CAR exists for the same sample. Blue boxes or lines represent the Holocene ages for AMH events discussed in the text (? ¼ assumed to be a CAR because the date is presented as a value in years AD in Bryant et al. (2007)). Grey shading indicates area of no apparent clustering from 1800 years BP to 1950AD, light yellow shading outlines the timeframe of the Holocene sea level highstand in Australia (after Sloss et al., 2007). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

chronologies produced from bulked radiocarbon age distributions based upon 29 radiocarbon age determinations (Bryant and Nott, 2001). This event is reported at 15 sites, concentrated between Atcheson Rock (Site 23) and Cullendulla Creek (Site 51), with five radiocarbon age determinations used as chronological evidence (Beta 36435, Beta 75519, Beta 78896, Beta 84641 and SUA 2872). Three of these dates (Beta 36435, Beta 78896 and SUA 2872) (Table 4) correlate well with the proposed event as shown in Fig. 4. 3.3.8. The 500 years BP event According to Bryant (2008), analysis of the bulked radiocarbon data shows several peaks in the combined distribution of dates, indicating that there are several probable events at circa 1100, 1400 and 1600 AD. In addition to the 900 and 250 years BP events, there is a prominent peak in the combined distribution centred on 1500 AD  85 (an event not noted earlier in Bryant and Nott, 2001). This led Bryant (2008) to conclude that ‘there is a 95% probability that a cosmogenically induced mega-tsunami event occurred in the Tasman Sea between 1200 and 1730’ (Bryant, 2008: p. 261). Bryant (2008) states that this major event was caused by the deep ocean impact of comet X/1491 B1 impacting 250 km south of New Zealand at 8.00 pm, Australian Eastern Standard Time (Summer) on February 13th, 1491 (E. Bryant, personal communication, August 30, 2009). Named the Mahuika Comet Impact, this event is purported to be responsible for the ‘megatsunami’ that formed most of the erosional and depositional features at twelve sites south of Sydney (Bryant, 2008). This is the “500 year BP event” linked by Bryant (2008) to two dates from six sites in Sydney Harbour (Beta-

A complete evaluation of the previously reported geological and chronological evidence for NSW Quaternary palaeotsunamis was hampered by two factors. First, there was no specific synthesis of palaeotsunami research in SE Australia (something we have addressed here) and second, our understanding of palaeotsunami geology is constrained by the fact that palaeotsunami deposits remain one of the most poorly understood aspects of marine science. Early work by Bourgeois et al. (1988), Atwater and Moore (1992), Clague and Bobrowsky (1994), Dawson (1994), Minoura et al. (1996) and Goff et al. (1998) and others has resulted in a baseline understanding of the nature and character of palaeotsunami deposits. This has been further enhanced by post-tsunami field surveys such as those by Satake et al. (1993), Shi et al. (1995), Goldsmith et al. (1999), Paris et al. (2007), Okal et al. (2010) and Goto et al. (2011) that explore, describe and explain modern tsunami deposits and their geomorphological effects. Given that the study of (palaeo-) tsunami geology is a relatively young discipline, it is of little surprise that the first challenge to the geological evidence for Quaternary palaeotsunamis in SE Australia did not occur until 2003. The critique by Felton and Crook (2003) was a reaction to the publication of the monograph ‘Tsunami e The Underrated Hazard’ and provided an analysis of some of the geological evidence presented in support of the hypothesis for repeated Quaternary palaeotsunamis, thereby surpassing the largely literary critiques of Synolakis and Fryer (2001); Curtis (2002); Dengler (2002) and Satake (2002). The majority of critiques of the evidence for NSW Quaternary palaeotsunamis focus on the possible erroneous interpretation of site-specific evidence, creating a piecemeal approach to validating or refuting the previous work. However, of the ten publications critiquing the geological evidence for Quaternary palaeotsunamis affecting NSW (Table 2), several themes are repeated. These themes consist of the disjunct between the size of the palaeotsunamis and their smaller historical counterparts, the possible erroneous interpretation of geomorphological and sedimentary features and processes, the very nature of the evidence with reference to widely accepted palaeotsunami identification criteria, the cause and effect

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deductions used and the unknown source events or mechanisms and each of these will be discussed in turn.

‘robust’ evidence for tsunami(s) of much smaller magnitude than those previously presented.

4.1. Disjuncts in size between palaeotsunamis and modern events

4.2. Evaluation of the geological evidence

As noted by Dominey-Howes et al. (2006) and Dominey-Howes (2007), the historical Australian tsunami record extends back only as far as 1858AD and comprised 47 events between 1858AD and 2006AD. These records show that the Australian coast has not experienced large events (Felton and Crook, 2003; DomineyHowes, 2007; Goff and Dominey-Howes, 2009). The largest historical tsunami runup in Australia was 6 m at Cape Leveque in 1977, generated as a result of an earthquake in the Sunda Islands, Indonesia (Dominey-Howes, 2007). On the NSW coast, the 1960 Chile earthquakeetsunami resulted in widespread tsunami observations, but the maximum wave height was just 5.5 m (Beccari, 2009). The magnitude of these historic events are at odds with the scale of the events suggested by the reports of Quaternary palaeotsunamis, where inundations of up to 552 m are reported at Lord Howe Island (Site 1) (Bryant and Nott, 2001) and 130 m above PMSL at Steamers Beach (Site 42) (Bryant et al., 1997; Young et al., 1997; Bryant, 2001, 2008). As stated by Dominey-Howes (2007, p.102) ‘the palaeo record is dominated by a trend of low-frequency high magnitude events and the historic record is characterised by the reverse’. While this is not entirely unexpected since only large tsunamis appear to leave long-term physical records (Goff and Dominey-Howes, 2009), the magnitude of the disjunct is rather marked. However, the possibility exists that large-magnitude events have occurred since 1788AD but that they went unnoticed and recorded. A further disjunct occurs between the nature and character of the physical evidence presented for ‘large’ to ‘very large’ versus much ‘smaller’ Quaternary palaeotsunamis along the NSW coast. For example, at Minnamurra Head (Site 27), Bryant (2001, 2008) describes evidence for ‘dump deposits’ at elevations of up to 40 m above PMSL (indicative of a tsunami significantly larger than anything recorded in the historic record). In contrast, research by Switzer et al. (2005, 2006) and Switzer and Jones (2008a,b) identified evidence of tsunami inundation of the back barrier at Minnamurra (Site 26) proposed as the result of an event 4300 years BP (Switzer et al., 2005) and the adjacent Killalea Lagoon more recently at 1500 years BP (Switzer et al., 2006). The evidence reported by Switzer et al., is in the form of anomalous large laterally extensive sand sheets containing rip-up clasts, cobbles and organic matter overlying poorly developed sand sheets. These features described by Switzer et al., are consistent with palaeotsunami diagnostic criteria described in Table 5 and the research presents evidence for inundation likely associated with tsunamis. However, the maximum elevation recorded for these deposits is just 3 m above PMSL and as such, may be storm driven. Likewise, Switzer et al. (2010, 2011) identified an anomalous coarse shelly unit at Old Punt Bay, which is adjacent to Cullendulla Creek (Site 51) (Fig. 2). While Young et al. (1997) note that deposits at Cullendulla Creek are present at less than 5 m above PMSL, they are described as extending 1.5 km inland (Young et al., 1997; Bryant, 2001, 2008). The anomalous unit identified by Switzer et al. (2010, 2011) was also found to contain palaeotsunami evidence using criteria described in Table 5 that was localised and present at elevations of up to 2.25 m above PMSL. It is observed that the evidence presented can be attributed to either storm or tsunami events. The authors state that this research ‘could add to a considerable bank of geomorphic evidence for tsunami reported for this coast’ (Switzer et al., 2005, p. 662). The ‘take home message’ here is that Switzer et al. (2005, 2006, 2010, 2011), using a suite of widely cited and known evidence for tsunami and palaeotsunami deposits, identify

The depositional and erosional features reported along the coast of SE Australia and linked to palaeotsunamis form part of a distinctive suite of indicators. Evaluation of the geological evidence requires analysis by signature type and critically, by comparison with the developing literature focused on palaeotsunami deposits and their associated indicators. 4.2.1. Erosional impacts Proxies for palaeotsunamis reported in the international literature generally focus on the deposits resulting from inundation or on soft sediment erosion features. In contrast, the proxies reported by authors describing SE Australian Quaternary palaeotsunamis centre on erosional impacts of inundation on solid bedrock. This in itself is not a problem. However, erosional signatures commonly reported in the international literature are distinctly different to those reported for Quaternary palaeotsunamis in SE Australia, such as the presence of erosional or unconformable basal contacts, noted in numerous post-tsunami surveys such as McSaveney et al. (2000), Nanayama and Shigeno (2006), Morton et al. (2007) and Goto et al. (2007). As shown in Table 3, erosional features comprise the bulk of diagnostic criteria used to identify Quaternary palaeotsunamis in SE Australia, with sub-categories of bedrock sculpting and geomorphic forms such as sculptured headlands and landscape features. These [erosional] diagnostic criteria lack appropriate sedimentary facies models and systematic descriptive schemes (Felton, 2002). While posttsunami surveys do observe such basal contacts, they are exclusively in soft-sediment environments such as beaches, wetlands and coastal plains. Furthermore, there is a question of scale. Events reported for SE Australia are associated with tsunamis an order of magnitude larger than their historically recorded counterparts and it is possible that these palaeotsunamis were highly erosional in nature. However, if this is indeed the case, it raises the question of the location of the eroded material as no significant quantities of eroded sediments have yet been identified. 4.2.2. Sand layers Sand laminae, frequently described for NSW Quaternary palaeotsunamis, appear broadly similar to diagnostic criteria of fine sediment deposits described by Williams and Hutchinson (2000), Kelsey et al. (2002), Kontopoulos and Avramidis (2003), Goff et al. (2004), Schulte et al. (2006), de Lange and Moon (2007), Haflidason et al. (2007), Morton et al. (2007) and Paris et al. (2007) amongst others. However, descriptions of NSW Quaternary palaeotsunami deposits do not use other sedimentological characteristics such as fining upward/inland sequences, distinctive layering and loading structures. Bryant et al. (1992b) describe the presence ‘of at least one shell-rich layer analogous to the type of tsunami sand deposits described in estuarine environments in Scotland (Dawson et al. 1988) and the Columbia River estuary (Atwater, 1987)’ but do not quantify these findings in terms of grain size, sorting or mineralogy. This is a key and recurrent issue with the research describing NSW Quaternary palaeotsunamis, as with the exception of boulder measurements and calculations, there are no quantitative data presented but rather predominantly descriptive interpretations of deposits. Similarly, Bryant and Nott (2001) state that ‘sand laminae appear as 20e30 cm thick layers sandwiched within deltaic or lagoonal muds. They are a commonly recognised signature of tsunami’ again citing work by Atwater (1987) and Dawson et al. (1988). However, Bryant and Nott (2001) do not specify precisely

Table 5 Modern and palaeotsunami signature types (after Goff et al., 2012a,b). ORCa applied?

CRAb (14C yr BP)

vC13 (ppm)

Material dated

Elevation (masl)

Locationc

Context/interpretation

Referenced (first citation)

CARe(yr BP)

Beta 36434

No

1850  75

n.r.

Whole shell

n.r.

Cullendulla Creek (Site No. 51)

Bryant et al., 1992a

1169e1655

Beta 36435

No

1420  75

n.r.

Whole shell

n.r.

Cullendulla Creek (Site No. 51)

Bryant et al., 1992a

723e1189

Beta 43948

No

106.2  0.8% mod

n.r.

Whole shell

4.85

Mystery Bay (Site No. 57)

Bryant et al., 1992a

Modern

Beta 43949

No

1580  60

n.r.

Whole shell

2.00

Mystery Bay (Site No. 57)

Bryant et al., 1992a

912e1306

Beta 43950

No

106.4  0.8% mod

n.r.

Galeolaria (intertidal filter feeder)

1.70

Haycock Point (Site No. 60)

Bryant et al., 1992a

Modern

Beta 43951

Yes

6600  80

1%

Tapes watlingi, Plebidonax deltoides

n.r.

Bellambi Beach (Site No. 18)

Bryant et al., 1996a,b

6775e7300 6827e7353

No

6560  80

Shells

n.r.

6560

Estuarine sediments

Date ‘13C Adjusted Age (BP)’ with no oceanic reservoir correction applied. No context provided e assume surface sample from sand ridge 6. Also cited as Beta 26434 (Young et al., 1997). CAR not stated in text. Date ‘13C Adjusted Age (BP)’ with no oceanic reservoir correction applied. No context provided e assume surface sample from sand ridge 2. CAR not stated in text. Date ‘13C Adjusted Age (BP)’ with no oceanic reservoir correction applied. From “seaward ridge” e since no tsunami has occurred since European arrival in 1788, this date MUST be older than 200 years. CAR not stated in text. Date ‘13C Adjusted Age (BP)’ with no oceanic reservoir correction applied. From “landward ridge” e no correction applied because uncorrected age matches TL date at MacCauleys Beach. CAR not stated in text. Date ‘13C Adjusted Age (BP)’ with no oceanic reservoir correction applied. Sample from boulder linked with others apparently tossed over 7 m high ramp e since no tsunami occurred since European arrival in 1788, this MUST be older than 200 years BP. CAR not stated in text. Date ‘Conventional 14C Age’. Sample from estuarine clay overlain by Holocene beach sands. Ages ‘ environmentally adjusted for ocean reservoir effect by 0.56  0.08 ka’. CAR not stated in text. Date ‘Conventional C14 age years (C13 adjusted)’ with no oceanic reservoir correction. Sample in estuarine clay overlain by thick orange sand with boulders and pumice. Rapid deposition of sand assumed to be result of tsunami. CAR not stated in text. Date ‘radiocarbon dated’. CAR not stated in text.

C. Courtney et al. / Quaternary Science Reviews 54 (2012) 99e125

Laboratory number

Young et al., 1997

Bryant and Nott, 2001 (continued on next page)

111

112

Table 5 (continued ) ORCa applied?

CRAb (14C yr BP)

vC13 (ppm)

Material dated

Elevation (masl)

Locationc

Context/interpretation

Referenced (first citation)

CARe(yr BP)

Beta 54207

No

6340  59

n.r.

Estuarine shell

n.r.

Callalla Beach (Site No. 36)

Young et al., 1997

6541e7067

Beta 56985

No

180  60

n.r.

Unbroken shell

Unclear

North Beach (Port Kembla) (Site No. 19)

Young et al., 1997

Modern

Beta 62723

No

1590  70

n.r.

Shell

w5.00 Unclear

Coledale (Site No. 14)

Young et al., 1997

911e1330

Beta 70926

No

590  70

n.r.

Shell

w4.00 Unclear

Kioloa Beach (Site No. 48)

Young et al., 1997

5e407

Beta 75519

No

800  60

n.r.

Shell

>3.00? Unclear

Narawallee (Site No. 47)

Young et al., 1997

244e612

Beta 76596

No

550  70

n.r.

Shell

<4.00 Unclear

Mermaids Inlet (Site No. 35)

Young et al., 1997

5e372 Modern

Yesf

110  60

1.4

Date ‘Conventional C14 age years (C13 adjusted)’ with no oceanic reservoir correction. Sample from mud. No context provided e date also cited as 6340  50. CAR not stated in text. Date ‘Conventional C14 age years (C13 adjusted)’ with no oceanic reservoir correction. Sample taken from base of a sand, clay and shell deposit that rises to <2 m above modern beach. Chaotic mix ¼ tsunami dump deposit. CAR not stated in text. Date ‘Conventional C14 age years (C13 adjusted)’ with no oceanic reservoir correction. Sample from shell, sand, and rounded pebbles on 5 m bench e the lack of rounding of flat cobble clasts suggests they were moved as suspended load by a tsunami. CAR not stated in text. Date ‘Conventional C14 age years (C13 adjusted)’ with no oceanic reservoir correction. Sample from near base of cobble/pebble layer at Shelly Point. Not sorted, not midden, shell unbroken so probably tsunami suspended load. CAR not stated in text. Date ‘Conventional C14 age years (C13 adjusted)’ with no oceanic reservoir correction. Shell from midden on top of boulder apparently moved by tsunami. Other boulders present. Boulder is 3 m high. CAR not stated in text. Date ‘Conventional C14 age years (C13 adjusted)’ with no oceanic reservoir correction. Shell from beneath boulders overlying sand. The highest boulder is at 4.0 masl. Date apparently “statistically identical” to Beta 70926 (Beta 76596 cited in text incorrectly as 500  70?). CAR not stated in text. Date ‘Conventional 14C Age’. Marine reservoir correction applied and then removed. Cited in text as 550  70 (p. 455). Context is ‘shell trapped under a boulder at the back of the stranded “recent” beach’. CAR not stated in text.

Bryant et al., 1997

C. Courtney et al. / Quaternary Science Reviews 54 (2012) 99e125

Laboratory number

Yes (0.56  0.08 kag)

770  60

1.7

Cabestana spengleri (shallow marine)

>20.00 Unclear

Atcheson Rock (Site No. 23)

Beta 78895

No

160  60

n.r.

Shells

n.r.

Greenfields Beach (Site No. 39)

Yesf

103.3  0.7% mod

0.6

No

1760  60

n.r.

Yesf

1300  60

3.0

Beta 79070

No

170  60

n.r.

Shells

w2.00? Unclear

Little Bay (Site No. 12)

Beta 79071

No

240  50

n.r.

Shells

n.r.

Short Point (Site No. 59)

Beta 82245

No

8740  70

n.r.

Shell hash

n.r.

Steamers Beach (Site No. 42)

Beta 78896

Galeolaria

w1.00 Unclear

Cape St George (Site No. 44)

Date ‘Conventional 14C Age’. Used corrected age. Shell within “tsunami overwash” deposit e a chaotically sorted boulder, sand, shell layer 0.5e2 m thick draped over the headland. Matched with uncorrected ages for Beta 43498 and Beta 43950 to suggest an event occurred “in the last 500e700 yr”. CAR not stated in text. Date ‘Conventional C14 age years (C13 adjusted)’ with no oceanic reservoir correction. Sample from low elevation boulder, but boulders extend to w8.0 masl. This is a sheltered bay, so cannot be storm emplaced, must be tsunami. CAR not stated in text. Date ‘Conventional 14C Age’. Marine reservoir correction applied and then removed. CAR not stated in text. Date ‘Conventional C14 age years (C13 adjusted)’ with no oceanic reservoir correction. A boulder dropped on Galeolaria worm tubes, presumed alive at time e boulder purported to have been carried over 5.0 m high shore platform to get there (Called Stoney Creek in Bryant et al., 1997). CAR not stated in text. Date ‘Conventional 14C Age’. Marine reservoir correction applied and then removed. Cited in text as 1760  60 (p.456). CAR not stated in text. Date ‘Conventional C14 age years (C13 adjusted)’ with no oceanic reservoir correction. Shells on boulder in carved bedrock channels that “extends above storm wave limits”. CAR not stated in text. Date ‘Conventional C14 age years (C13 adjusted)’ with no oceanic reservoir correction. Shells on boulder apparently transported over 10 m high cliff by tsunami then dumped back in sea. CAR not stated in text. Date ‘Conventional C14 age years (C13 adjusted)’ with no oceanic reservoir correction. No context for sample e part of sand, shell hash, scattered pebbles, and deposit mud lenses þ flat bedding rise up to 130 masl. “Largest tsunami detected”. CAR not stated in text.

Bryant et al., 1996a,b

145e553

Young et al., 1997

Modern

Bryant et al., 1997

Young et al., 1997

650e1043 1077e1519

Bryant et al., 1997

Young et al., 1997

Modern

Young et al., 1997

Modern

Young et al., 1997

8645e9260 9098e9610

C. Courtney et al. / Quaternary Science Reviews 54 (2012) 99e125

Beta 78894

(continued on next page)

113

114

Table 5 (continued ) CRAb (14C yr BP)

vC13 (ppm)

Yesf

8400  60

5.3

Beta 83930

No

115  1.0% mod

n.r.

Oyster shell

w5.00 Unclear

Bass Point (Site No. 25)

Beta 84641

No

970  90

n.r.

Shells

2.50

Crookhaven (Site No. 31)

Beta 87100

No

350  60

n.r.

Oyster shells

n.r.

Lord Howe Is. (Site No. 1)

Beta 93326

No

390  60

n.r.

Cabestana spengleri

n.r.

Green Island (Site No. 46)

Beta 107032

No

770ñ  0

n.r.

Shell

16.00? Unclear

n.r.

Beta 107033

No

2810  70

n.r.

n.r.

n.r.

Batemans Bay (Site No. 52)

Beta 107034

No

1020  50

n.r.

n.r.

4.00? Unclear

Gogerlys Point (Site No. 7)

Material dated

Elevation (masl)

Locationc

Context/interpretation

Referenced (first citation)

Date ‘Conventional 14C Age’. Marine reservoir correction applied and then removed. Cited in text as 8740  70 (p.454) and 8700  70 (p.455). CAR not stated in text. Date ‘Conventional C14 age years (C13 adjusted)’ with no oceanic reservoir correction. Shell on boulder in a group assumed to be emplaced by tsunami. No other details provided. CAR not stated in text. Date ‘Conventional C14 age years (C13 adjusted)’ with no oceanic reservoir correction. Shell attached to top of boulder. CAR not stated in text. ‘A C13/C12 correction has been applied. a local reservoir correction has not been used’. Shells taken from highly irregular calcarenite surface at Searle Point. Apparently the age of this prominent event cannot be expressed as a calendar year however, it probably occurred in the 18th century. CAR not stated in text. ‘A C13/C12 correction has been applied. a local reservoir correction has not been used’. A whole shell in a crag and tail “dump” deposit on SW side of island. No further sedimentological details. CAR not stated in text. ‘A C13/C12 correction has been applied. a local reservoir correction has not been used’. A cliff in NSW? No information provided about sample position or characteristics of deposit. CRA cited here as reported by authors. Date used as evidence for a 900 BP event. CAR not stated in text. ‘A C13/C12 correction has been applied. a local reservoir correction has not been used’. Sample from a “layered, shelly sand deposit raised along the shoreline of the estuary” e no sample context provided. CAR not stated in text. ‘A C13/C12 correction has been applied. a local reservoir correction has not been used’. Unknown sample material associated with boulders on a terrace 4 m above mean sea-level within the sheltered confines of Port Hacking. CAR not stated in text.

Bryant et al., 1997

CARe(yr BP)

Young et al., 1997

Modern

Young et al., 1997

316e748

Bryant and Nott, 2001

5e199

Bryant and Nott, 2001

5e229

Bryant and Nott, 2001

n.r.

Bryant and Nott, 2001

2284e2773

Bryant and Nott, 2001

448e757

C. Courtney et al. / Quaternary Science Reviews 54 (2012) 99e125

ORCa applied?

Laboratory number

Yesf

n.r.

n.r.

Paphies australis (subtidal bivalve)

30.00

Mason Bay, Stewart Is, NZ (Site not shown)

SUA 2872

Yes (450 yrsi)

1520  120

n.r.

Anadara trapezia (intertidal filter feeder)

>2.50

Sandon Point (Site No. 16)

No

1970  60

Shell

n.r.

SUA 2900

No

3200  60

n.r.

Whole shell

1.50

Cullendulla Cr (Site No. 51)

SUA 2901

No

2950  70

n.r.

Whole shell

1.20

Cullendulla Cr (Site No. 51)

a

Context unclear. Sample taken about 500 m inland and 30 masl. No depositional context, surface sample? CAR not stated in text. Sample from a shell midden. The disturbed deposit was probably caused by higher sea levels from 6900 to1520 BP. Nature of fabric of Holocene beach unusual and for this reason catastrophic phenomena such as tsunami cannot be ruled out. CAR not stated in text. A C13/C12 correction has been applied. a local reservoir correction has not been used’. According to this citation, the previous reference (Bryant et al., 1992b) described the material as being in a probable tsunami deposit containing sand, shell, and boulders. The uncorrected age fits with a 1600e1900 BP event. CAR not stated in text. Date ‘13C Adjusted Age (BP)’ with no oceanic reservoir correction applied. No context provided e assume surface sample from sand ridge 8, west of main transect. CAR not stated in text. Date ‘13C Adjusted Age (BP)’ with no oceanic reservoir correction applied. No context provided e assume surface sample from sand ridge 6, west of main transect. CAR not stated in text.

Bryant et al., 2007

1301  36ADh

Bryant et al., 1992b

751e1330 1295e1751

Young et al., 1997

Bryant et al., 1992a

2751e3253

Bryant et al., 1992a

2368e2948

ORC: The Oceanic Reservoir Correction for New South Wales is around 319 years, delta R 11  85 (http://intcal.qub.ac.uk/marine/). CRA: Conventional Radiocarbon Age e the reported result of a radiocarbon measurement expressed as a time in years. c Refer to Fig. 1 for location of Site No. d References (first citation): Where additional data has been added in a subsequent publication this follows beneath the first citation. e CAR: Calibrated Age Range e the age of a dated sample or event relative to 1950AD, determined by calibrating the CRA. Date ranges represent the 95% confidence interval. With the exception of one CAR, all have been calibrated as part of this review. As described within the text, calibration was based upon a Delta-R marine correction of 11  85 using Winscal08 and expressed at a 95% confidence level. f The authors do not state a specific ORC, but state that “ages are adjusted for a global ocean reservoir effect i”. g The authors state a specific ORC but no reference is cited. The state that “ages are environmentally adjusted for ocean reservoir effect by 0.56  0.08 ka”. h This is assumed to be a CAR e it is the only age cited for this sample. i Result corrected using an assumed oceanic reservoir effect of 450 years after Gillespie and Temple (1977). n.r. ¼ Not Reported. b

C. Courtney et al. / Quaternary Science Reviews 54 (2012) 99e125

Not cited

115

116

C. Courtney et al. / Quaternary Science Reviews 54 (2012) 99e125

why these sands may be considered palaeotsunami deposits, or provide any quantitative analysis. 4.2.3. Imbricated boulders The presence of imbricated boulders is the most widely found diagnostic feature (located at 20 sites) of NSW Quaternary palaeotsunamis. As previously stated, boulder imbrication analysis often represents the only quantitative analysis of sediments at NSW Quaternary palaeotsunami sites. Stacked boulders are reported at elevations up to 33 m above PMSL (Bryant et al., 1992b) while boulders also grade upwards into gravels and coarse sands at 6.5 m above PMSL (Bryant et al., 1997). As is often noted within publications about NSW Quaternary palaeotsunamis, Bryant et al. (1992b) state that large boulder assemblages move little during storm events. However, Saintilan and Rogers (2005) reported that following a storm event in October 1999, large boulders were transported on to the rock platform on the point adjacent to Little Beecroft Head. A larger boulder of 9.7 tonnes was emplaced 2 m asl and 4 m from the seaward edge of the platform. Saintilan and Rogers (2005) concluded that constructive interference of incoming and reflecting waves was able to amplify wave heights and that the emplacement of high elevation boulder fields could be achieved solely by storm swells. 4.3. Comparison between NSW palaeotsunami indicators and the wider literature A full description and analysis of the increasing number of indicators and features used to identify palaeotsunami deposits around the world would add significant, unnecessary length to this paper. However, in recognising this important emerging area of research, we refer to readers to excellent summaries and discussions by a variety of authors including Chagué-Goff et al. (2002, 2011), Dominey-Howes (2002), Goff et al. (2001, 2012a,b), Nichol et al. (2003), Pinegina et al. (2003), Switzer and Jones (2008a,b) amongst others. However, in Table 5, we summarise the diagnostic criteria reported by these authors and taken from posttsunami surveys following modern events. Consequently, the suite of indicators currently employed for the identification of suspected palaeotsunamis has arisen from intensive peer-reviewed multidisciplinary research. Given the onus on researchers to unequivocally demonstrate the tsunami origins of deposits (Dominey-Howes, 2007) it is imperative that researchers use as many of the reported proxies as possible when interpreting suspected palaeotsunami deposits (Goff et al., 2010c). In light of the proceeding comment, it is conspicuous that most of the reports of NSW Quaternary palaeotsunamis by researchers listed in Table 1 describe criteria that are different to those used in other parts of the world (Dominey-Howes, 2007; Goff et al., 2012a,b). A direct comparison between the synthesis of Table and the diagnostic criteria in Table 5 demonstrates these differences. One possible explanation for the variation in proxies is that Australian palaeotsunamis may produce unique signatures that are dominated by erosion morphologies. Unfortunately, research along the NSW coast has not as yet, been able to identify physical evidence of deposits left by historic and modern tsunamis (Goff and Dominey-Howes, 2009), something vital to corroborate or refute this possibility. Direct comparison between the NSW Quaternary palaeotsunami evidence and descriptions of palaeotsunamis from other parts of the world may also be hindered by terminology. For example, descriptive labels such as ‘Muschelbrüche’ and ‘Sichelwanne’ are typically used to describe glaciated landscapes (Young et al., 1995; Bryant and Young, 1996). Authors describing these features as evidence for NSW Quaternary palaeotsunamis have

observed that they are difficult to conceptualise in the coastal environment as ‘there is no other reference to (s-forms) presence in the coastal literature’ (Bryant and Young, 1996, p. 566). This fails to address the difficulties of comparing impacts taking place in different geomorphic environments. 4.4. Cause and effect Young and Bryant (1992) postulated that certain features in the coastal morphology of NSW were the ‘distal’ effects of the tsunami wave train resulting from the Lanai collapse-tsunami first proposed by Moore and Moore (1984). Research was conducted at Tura Head (site 58) (Fig. 2), where the authors identified erosional features consisting of quarried joint-bounded blocks, removal of debris from the cliff base, an array of clefts, asymmetrical Sichelwannen (shallow curved depressions superimposed on clefts) and flutes. These features are cited and expanded upon by Bryant et al. (1992b). References to features resulting from the Lanai event subsequently appear in Young et al. (1993a), Bryant and Young (1996), Bryant et al. (1996b), Bryant and Nott (2001), Bryant et al. (2007) and Bryant (2008). Jones and Mader (1996) numerically modelled the proposed Lanai event. They stated that even a debris slide ten times larger than that required to generate the inundation suggested by Moore and Moore (1984) would not result in a significant tsunami on the NSW coast. Subsequent publications by Felton et al. (2000), Rubin et al. (2000) and Keating and Helsley (2002) all cast doubt on the interpretation of Moore and Moore (1984) of tsunami deposits on Hawai’i (the supposed source). In essence the deposits on Hawai’i did not represent a single event but rather one of either a sequence of normal littoral and alluvial deposits uplifted in the late Quaternary (Rubin et al., 2000), a punctuated deposition associated with a rocky shoreline (Felton et al., 2000), or shorelines resulting from uplift of the island associated with lithospheric deformation (Keating and Helsley, 2002). While the exact processes associated with the Lanai gravels continue to be a matter of debate, there appears to be consensus that they do not represent inundation by a tsunami 105,000 years before present in the near field. As such, there can be no far field tsunami deposits in NSW. Given that the proposed Lanai tsunami led to the classification of several coastal features in NSW as palaeotsunami indicators, rejection of the event in its supposed source region casts serious doubt as to the validity of these features as signatures of anything catastrophic in the far field e namely in NSW. 4.5. Unknown sources Dominey-Howes (2007) notes that 23% of historical Australian tsunamis were generated by unknown causes from unknown source areas. The issues surrounding source areas and mechanisms are highlighted by the controversy surrounding the proposed 500 years BP event, the Mahuika impact and associated megatsunami. Goff et al. (2010a) conducted an assessment of the evidence for the Mahuika event and noted that comet impact with the Earth was unsubstantiated due to a virtually indeterminate orbit. All lines of evidence proposed by Bryant (2008) for the Mahuika comet impact were categorically refuted and Goff et al. (2010a, p. 295) concluded that ‘no comet, “Mahuika” or otherwise, struck the Earth on February 13, 1491 AD’. That is not to say that impacts are disregarded as a causal mechanism for Australian tsunamis; the country is recognised as a significant target for asteroid strikes, although impacts are generally older than Tertiary and not widespread in NSW (Haines, 2005; Dominey-Howes, 2007). Furthermore, Goff and Dominey-Howes (2010) postulate that in the absence of a reliable

C. Courtney et al. / Quaternary Science Reviews 54 (2012) 99e125

event chronology, all the purported evidence for multiple Holocene megatsunamis may actually relate to a single event, the Eltanin asteroid impact tsunami about 2.5 million years ago. Proponents for NSW Quaternary palaeotsunamis may have the correct mechanism if not the correct events. Goff et al. (2010a, p. 295) pose the question, ‘if the comet impact did not happen, then what caused the apparent megatsunami deposits along the eastern coast of Australia that have been so intimately linked to the “Mahuika” event?’ 5. Alternative interpretations on the origins of sediment deposits and landforms along the coast of NSW, SE Australia Felton (2002) notes that the varied interpretations of the Lanai gravel deposits represented a clash of paradigms, with new paradigms being applied to existing problems regardless of suitability. This is a key issue within geomorphological interpretation and further complicated by the difficulties inherent in identifying evidence in the field, particularly features associated with the recent topic of palaeotsunami research. Furthermore, interpretation is hindered by continuing processes such as erosion and weathering, while evidence may also be disturbed by human activity. Given the sheer number of variables, unequivocal evidence is difficult to present, although both Dominey-Howes (2007) and Goff et al. (2012a,b) emphasise that researchers have a duty to do so. The equivocal evidence cited by proponents of repeated Quaternary palaeotsunamis in NSW, SE Australia has resulted in a number of reassessments, giving rise to alternative interpretations at several sites. We do not intend to present ‘alternative interpretations’ for the previously reported evidence at all 60 sites along the NSW coast. Little value exists in doing this. However, we select key sites (‘type sites’) that represent specific types of previously reported evidence and offer alternative interpretations for these (Table 6). Future researchers can then apply test our alternative interpretations at other similar sites along the NSW coast using the recommendations suggested in Table 6 among others. Alternative interpretations may be also be analysed in terms of the processes or mechanisms responsible. While there is a large degree of interaction between alternative processes, with multiple mechanisms at play in each location, these alternatives warrant individual attention. Interpretations have been separated based upon themes of sea level change, high energy events and weathering/erosion, with each discussed below. 5.1. Higher sea levels in the past (last glacial and postglacial highstands) It is imperative to factor in effects of sea level change when reconstructing any palaeoenvironment. As previously noted, NSW has experienced significant sea level changes throughout the Quaternary with two distinct highstands. The earliest at 120, 000 years BP resulted in sea levels 2e5 m higher than present (Roy and Thom, 1981). The most recent highstand as calculated by Sloss et al. (2007) at 7700e2000 years BP produced sea levels on the NSW coast of 1.5 m above PMSL. These fluctuations have significant impacts on the analysis and interpretation of NSW Quaternary palaeotsunami evidence, with eleven sites reported as containing evidence at 5 m or less above PMSL (Fig. 3a). Furthermore, with the paucity of elevation data in NSW Quaternary palaeotsunami literature, less than half of the sites (27 as shown in Fig. 3a) contain evidence at more than 5 m above PMSL. Adjusting the maximum elevation of deposits to 3.5 m above Holocene Highstand Sea Levels (HHSL) allows for the possibility of smaller magnitude events and mechanisms. Such elevations are well within the range of wind waves under storm conditions as calculated by Shand et al. (2010) and at locations such as Thirroul

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(Site 15), Sandon Point (Site 17), Narrawallee (Site 47), Cullendulla Creek (Site 51) and Mystery Bay (Site 57) under normal swell conditions. 5.2. High energy events The imbricated boulder deposits of Little Beecroft Head (Site 34) have been presented as evidence for Quaternary palaeotsunamis by Bryant et al. (1996b, 1997) and Young et al. (1996). However, as previously stated, research by Saintilan and Rogers (2005) identified that storm swells were capable of moving boulders that were previously determined to be palaeotsunami emplaced as they were beyond the range and transport capacity of wind waves. Shand et al. (2010) calculate that the South Coast of NSW experiences 15e16.5 storm events each year with waves frequently exceeding heights of 4 m. Hundred year storms at Eden near Haycock Point (Site 60) have significant wave heights estimated by Shand et al. (2010) at 9.3 m while elsewhere in NSW waves of up to 16 m have been recorded (Thom, 1983). This storm climate, particularly in conjunction with sea level fluctuations makes storms seem one of the most plausible alternative mechanisms for the emplacement of boulders. It is also worth considering that tropical cyclones may well have impacted the coast of NSW under past climatic conditions. Less probable events such as large waves (freak or rogue waves) also cannot be discounted. Testing of the hypothesis that other high energy events (beyond palaeotsunamis) may have been responsible for the imbricated boulders will only be achieved through extensive analysis of boulders throughout the NSW coast. Table 6 provides recommendations for the resolution of this issue. Boulders are not the only feature of NSW Quaternary palaeotsunamis that may be attributable to storms as finer material such as anomalous sediment deposits may be transported and deposited by storm events. Differentiating between palaeostorms and palaeotsunamis has been a matter of much debate but has been advanced by comparative studies of known tsunami and storm deposits at individual sites (e.g. Goff et al., 2004; Nott, 2004; Kortekaas and Dawson, 2007; Morton et al., 2007; Goto et al., 2010). Broadly, the criteria for identifying both storm and tsunami events are those detailed in Table 4, with palaeotsunami deposits generally vertically and laterally more extensive (Tuttle et al., 2004; Kortekaas and Dawson, 2007; Goto et al., 2011). Additionally, Goff et al. (2004) determined that storm sediments were better sorted than tsunami deposits and did not fine inland, while Kortekaas and Dawson (2007) observed that storm deposits tended not to contain rip-up clasts and did not contain upward fining sediments (Tuttle et al., 2004). As has been previously stated, evidence of NSW Quaternary palaeotsunamis does not include quantitative fine sediment analysis that would allow comparisons of this nature. However, recommendations for future study in Table 6 provide a number of suggestions for how to resolve whether the fine anomalous sediments (Table 3) may have been storm emplaced. With differentiation between storm and tsunami events often a question of deposit extent (both vertically and landward), both inundation events share characteristics of general high energy marine conditions. Subsequently, the same criteria may be broadly applied to both storms and tsunami, with differentiation the second step in interpretation of an inundation event. 5.3. Weathering and erosion 5.3.1. In situ weathering Minnamurra Point or Head (Site 27) was described as a chaotically sorted dump deposit with rounded metamorphic pebbles and shell fragments in a mud matrix overlying volcanic bedrock at an elevation of over 40 m above PMSL (Bryant, 2001, 2008). This site

Features

Type site

Site description

Alternative interpretations

Methods to test alternative interpretations

Anomalous sediments

Anomalous sand laminae or sheets, frequently tapering landward. Laterally extensive, low elevations.

Shoalhaven (Site 31)

Large estuary in drowned river valley on low relief coastal plain dominated by Quaternary sediments. Lagoons and lakes present with multiple river channels. Limited exposure to Tasman Sea. Shoalhaven River provides terrestrial material, general estuarine low-energy processes.

Barrier transgression through marine and Aeolian transportation of material during marine transgressive sequences. Reworking of barrier material throughout low elevation sites during flooding or higher sea levels. Emplacement during a sequence of ‘small’ high energy events. Sediment is reworked terrestrial material from the sandstone dominated interior, transported by the Shoalhaven river, particularly during flood events.

Bimodal deposits

Anomalous sand sheets with boulder floaters; sand layers at low elevations containing isolated clasts of pebble size and above.

Bellambi Beach (Site 18)

Bay and headland intermediate beach approximately 2.5 km long. Situated on the low relief coastal plain dominated by Quaternary sediments. Fully exposed to Tasman Sea dominant south easterly swells.

Dump deposits

Boulders and cobble/shell mixtures stacked in isolated ridges, terraces or mounds.

Mystery Bay (Site 57)

Bay and headland reflective beach less than 200 m in length. Rocky shoreline with remnants of Pleistocene platforms. Fully exposed to Tasman Sea dominant south easterly swells.

Barrier transgression through marine and Aeolian transportation of material during marine transgressive sequences with reworked underlying sediments. Barrier transgression through marine and Aeolian transportation of material during marine transgressive sequences with reworking of surrounding sediments during storms. Barrier transgression through marine and Aeolian transportation of material during marine transgressive sequences with reworking of surrounding sediments during ongoing coastal processes. Coarse clast beaches; coarse sediments are reworked and deposited as part of ongoing coastal processes. Gradual onshore deposition of coarse sediments following significant changes in the sediment cell.

Quantitative analysis of sediments to characterise and determine origin and likely transport mechanisms (sedimentary, micropalaeontological and geochemical analysis to determine if the sediments are of marine origin). Sedimentary analysis to identify rapid, high-energy depositional environment features such as bedding, fining upward sequences and loading structures. Micropalaeontological analysis to identify oceanic microfossils within the sediment. Obtain chronological data and compare against sea level curves. Stratigraphic analysis to determine the extent of individual laminae to identify distinct depositional sequences. Quantitative analysis of sediments to characterise and determine origin and likely transport mechanisms (sedimentary, micropalaeontological and geochemical analysis to determine if the sediments are of marine origin). Comparison of coarse clasts with sediments underlying and surrounding the anomalous units to identify origins.

Imbricated boulders

Boulders imbricated, may be deposited in ridges. Boulders may be extremely large, transported beyond calculated limits of wind waves.

Mermaids Inlet (Site 35)

Joint controlled inlet among vertical sandstone cliffs approximately 40 m high. Situated on h tip, fully exposed to Tasman Sea and dominant south easterly swells.

Sediment clasts emplaced by multiple storm events. Sediment clasts emplaced by ‘small’ tsunamis or isolated high energy events. Clasts eroded from higher elevations.

Quantitative analysis of sediments to characterise and determine origin and likely transport mechanisms (sedimentary, micropalaeontological and geochemical analysis to determine if the sediments are of marine origin). Analysis of sediments available for transportation and deposition within the immediate area; sedimentary analysis to identify source material and model processes at the site. Monitoring of large clast movements at the sites. This could be achieved by tracking geospatially referenced boulders under a variety of storm conditions. Recalculate wind wave limits having adjusted elevations to allow for sea level change using existing calculations. Assessment of the calculations used to determine wind wave limits.

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Geological evidence type

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Table 6 Type sites for NSW Quaternary palaeotsunami diagnostic criteria, including notable features, alternative interpretations and suggestions for resolution of conflicting interpretations.

Continuous mud-rich layers containing sand, shell and gravel.

Kiama Head (Site 28)

Latite headland bounded by cliffs 15e20 m high. Situated on headland with multiple anthropogenic features. Fully exposed to Tasman Sea and dominant south easterly swells.

Reworked sediments. In situ soil horizon. Anthropogenic origins.

Megaripples

Ridges, mounds and dunes, often at extremely high elevations. Landward tapering with interbedded coarse clasts fining upwards. Sediments may be visibly different to beach sediments.

Steamers Beach (Site 42)

Bay and headland intermediate beach approximately 800 m long. Situated on a peninsular headland the beach is situated between sandstone cliffs approximately 40 m high that are near vertical to the east with a lower gradient to the west. Fully exposed to Tasman Sea and dominant south easterly swells.

Aeolian dunes with relict soils. Reworked sediments from higher elevations.

Erosional features

Includes both large scale (toothbrush shaped headlands) and small scale (ramp block stripping, sinuous offset flutes, plucking, clefts, cavettos, sichelwannen grooves) features.

Tura Point (Site 58)

Headland approximately 30 m high, separating two remnant platforms 2e4 m above PMSL (dipping to SE). Rocky shoreline of near-horizontally bedded Cambrian sandstones. Fully exposed to Tasman Sea and dominant south easterly swells.

Rocky shoreline exposed to long-term weathering processes in a high energy environment.

Identify the lateral extent of the deposit to identify the scale of any proposed event. Quantitative analysis of sediments to characterise and determine origin and likely transport mechanisms (sedimentary, micropalaeontological and geochemical analysis to determine if the sediments are of marine origin). Analysis of shell and any microfossils to determine source environment and degree of breakage to determine if damaged as in a high energy event. Examine land use at the site and identify past activities and land uses. Quantitative analysis of sediments to characterise and determine origin and likely transport mechanisms (sedimentary, micropalaeontological and geochemical analysis to determine if the sediments are of marine origin). Stratigraphic analysis to identify distinct depositional sequences as well as structures such as palaeosols. Stratigraphic analysis to identify rapid, high-energy depositional environment features such as bedding, fining upward sequences and loading structures. Analysis of sediments from higher elevations and surrounding area for comparative purposes. Examination of modern analogues demonstrating erosion to bedrock by tsunami (specifically Lituyya Bay, Alaska) and development of appropriate facies models.

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Smear deposits

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was re-examined by Dominey-Howes et al. (2006), who conducted an assessment of the nature of the marine evidence for the deposit and a comparison between deposit material and local lithology. Through clast analysis, thin sections, particle sizing, sediment chemistry and micro- and macro-fossil analysis, the researchers determined that there were ‘no unusual geomorphological forms or sedimentary deposits that might reasonably be interpreted as the product of catastrophic marine inundation’ (Dominey-Howes et al., 2006, p. 1103). The final conclusion was that the unconsolidated sediments represented in situ weathering of the underlying bedrock. 5.3.2. Long-term surface weathering The role of long-term erosion and weathering in the coastal environment cannot be ignored. Given the complex chemical and mechanical processes on rocky shores, it is highly possible that long-term weathering may well account for the majority of erosional features noted in Table 3. Few studies have directly challenged the erosional features of the NSW Quaternary palaeotsunami literature. Felton and Crook (2003) note that at Eden (35 km south of Tura Head (Site 58)), a sandstone monument that was erected in 1869 AD some 15 m from the beach displays significant incipient tafoni and spalling. The authors note that the sforms on the rock platform flutes at Tura Head ‘seem unlikely to survive even a few decades of weathering in a harsh coastal environment’ (Felton and Crook, 2003, p. 5). However, this statement needs to be treated with caution as it fails to take into account the differences between sub-aerial weathering and the physical and chemical dominated weathering processes on rock platforms. Felton and Crook (2003) also note that s-forms are typically associated with sustained high velocity flows and question the ability of ephemeral (although possibly recurrent) flows to produce these features. This confirmation of the role of long-term weathering in a specific erosional feature hints that gradual processes occurring over long periods may provide alternative interpretations of the erosional features. Validation or rejection of this alternative is dependent on a thorough investigation as indicated in Table 6. 5.4. Evaluations of chronological indicators The philosophy behind events identified within the sequence of NSW Quaternary palaeotsunamis is that there are a suite of unique depositional and erosional features that can only be explained by their linkages with catastrophic tsunamis, mega- or otherwise. To tie these features together as contemporaneous events in the landscape requires rigorous chronological control. This has been attempted through the extensive use of radiocarbon and thermoluminescence dating. This correlation of chronological data is complicated by the incorporation of older material as well as issues of inbuilt age and therefore discrepancies and wide error margins are a common issue. Dates from both techniques have been reported, but this paper examines only radiocarbon evidence in an attempt to determine the overall validity of the chronology used. A brief overview of event chronologies highlights the difficulties associated with interpreting the number of events identified. For example, widespread events in the last two millennia are listed as occurring at approximately 450, 1050 and 1700 AD (Bryant and Nott, 2001) and later, in approximately 1100, 1400 and 1600 AD (Bryant, 2008). However, the most prominent composite radiocarbon peak apparently occurs around 1500  85 AD (Bryant, 2008: p. 261), and it is proposed that probably only one megatsunami occurred somewhere between 1200 and 1730 AD, or more precisely a comet impact on 13 February 1491 AD. In addition, Bryant (2008) also states that the three most recent events occurred in 1700 AD, 1450 years BP and 1050 years BP. Given this somewhat irregular

approach to the use and interpretation of radiocarbon data for dating megatsunamis associated with the NSW Quaternary palaeotsunami record, we are currently unable to determine the precise number or age of apparent events under consideration. It is possible that three events may have been subsumed into one apparent megatsunami that occurred on 13 February 1491 AD. The validity of this impact scenario has already been discussed above and is detailed in Goff et al. (2010a). In an attempt to provide as robust a synthesis as possible of the SE Australian palaeotsunami data, we have used the most recent radiocarbon calibration tools to determine the Calibrated Age Range (CAR) for all the cited samples (Table 4). A detailed collation of all the relevant data related to individual radiocarbon samples has led us to believe that all of the ages cited in the literature listed in Table 1 are Conventional Radiocarbon Ages (CRAs) as opposed to CARs. This may not be entirely correct, but in the light of the fragmentary data provided in the literature, it is a reasonable conclusion. The full suite of CARs we have determined from these data is shown in Fig. 4. The CARs displayed in Fig. 4 show no significant clustering between about 1950AD and 1800 years BP. While the references cited in Table 1 come to the conclusion that there are events in 220e250 years BP, 1500 AD  85 (365e535 years BP), 900 years BP and 1500 years BP, there is no clear evidence for clustering in the radiocarbon data. Similar conclusions can be drawn about the possible 5,000, 6,500, and 9000 years BP events, except that in these instances no radiocarbon data directly correlate with the proposed event chronologies. The absence of recent discussions suggests that the authors may have discounted the evidence. To date, proponents of the evidence of NSW Quaternary palaeotsunamis are yet to place their proposed events between 6500 and 2900 years BP in the context of Australia’s Holocene sea level highstand. Sloss et al. (2007) place a highstand of þ1.0e1.5 m between 7200 and 2200 years BP (Fig. 3). Any possible evidence for Late Quaternary palaeotsunamis need to be disentangled from coastal processes that took place at this time and in particular, the role of sea level rise. Several publications have queried the interpretations of radiocarbon data used by proponents of NSW Quaternary palaeotsunamis. Goff et al. (2003) recalibrated radiocarbon data from publications prior to 2001 listed in Table 1 to reveal a characteristic Calibration Stochastic Distortion pattern of peaks and troughs. The researchers concluded that correlations between the generated chronology and the incidence of comets and meteorites are ‘purely fortuitous and misleading’ (Goff et al., 2003). In addition, research by Hutchinson and Attenbrow (2009) into the coastal archaeological record of NSW also critiques the proposed chronology of palaeotsunamis. The critique provides an archaeology-based alternative interpretation that is largely beyond the scope of this paper, although it is worth noting that shellfish exploitation patterns and changes in fishing techniques do not coincide with the proposed timing of Quaternary palaeotsunamis. Furthermore, the authors calculate a mean probability distribution of calibrated radiocarbon ages from 52 NSW coastal archaeological sites, including Macauleys Beach (Site 17) and Bass Point (Site 25), to reveal there is no difference to random expectations. Hutchinson and Attenbrow (2009) conclude that archaeological and chronological analysis offers no support for the occurrence of palaeotsunamis in NSW. 5.5. Additional factors for consideration The findings of Saintilan and Rogers (2005) that high elevation boulder fields could be achieved solely by storm swells are building on the assumption that wave climates have not differed during the

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late Holocene; an inherent flaw in palaeoenvironmental reconstructions as noted by Sloss et al. (2007). Climate has fluctuated throughout the Holocene, with high resolution pollen analysis in the South Coast identifying a Holocene moist period (Hope et al., 2004) that resulted in a cooler, wetter climate 7500e3500 years BP (Donders et al., 2007). These wetter conditions may be reflective of increased storminess that would have dramatically impacted on the wave climates. The questionable role of the Mahuika impact in generating a tsunami has already been explored. However, the alternative interpretation of the geological evidence warrants further examination. Publications by Bryant et al. (2007) and Bryant (2008) are informed by the discovery by Abbott et al. (2003) of a 20 km diameter crater 250 km SW of New Zealand. Goff et al. (2010a) observed that Abbott et al. (2003) are the only research team to have identified this feature. This is not surprising given that the low resolution of existing bathymetric data makes the identification of such features challenging (Goff et al., 2010a). Additional support for the Mahuika impact hypothesis is in the form of marine sediments overlying tektites and other impact ejecta (Abbott et al., 2003). Goff et al. (2010a) offer a synthesis of alternative mechanisms causing this layer of ejecta, primarily the concentrating effects of bioturbation and bottom currents on sediments, as well as tectonic sieve-like processes that leave larger particles close to the surface. Furthermore, research by Kumar (2006) critically highlights the fact that the inferred tektites are microcrystalline rather than glass, and represent clays forming casts within foraminifera tests. The alternative interpretation for all of the physical evidence for the impact casts serious doubt as to the validity of the proposed Mahuika event and therefore the ensuing tsunami. 5.6. So what do we know with confidence? Having gone to considerable length to critique the existing geological and chronological evidence, the question that emerges is what do we actually know with any confidence? As previously noted, research by Switzer et al. (2005, 2006, 2010, 2011) utilises the widely accepted diagnostic criteria noted in Table 4. Using these criteria, reliable evidence has been obtained and attributed to tsunami events 4300 years BP at Minnamurra (Site 26) and 1500 years BP at the adjacent Killalea Lagoon. Additionally, an anomalous unit has been identified by Switzer et al. (2010, 2011) at Old Punt Bay in Batemans Bay (Site 52), situated on top of an elevated beach deposit emplaced 2500e5000 years BP. The polygenetic nature of the sediments, in conjunction with the micro and macrofaunal remains were interpreted by Switzer et al. (2011) as indicative of a laterally extensive high energy marine inundation 2500e2900 years BP. However, no effort is made at determining whether this event was a storm or tsunami. Similarly, Courtney (2012) identified evidence of high energy marine event at a further site in Batemans Bay (Site 52) using the diagnostic criteria described in Table 4. A discrete sand layer was located in an isolated coastal wetland with microfossil and geochemical evidence of widespread marine inundation. Dating was unable to refine the chronology beyond a broad range of 6000e4000 years BP. Insufficient evidence was available to differentiate between a storm or tsunami origin for the sediments and additional data collection is required to make a determination. From these studies, two deductions may be made. Firstly, that there is a body of robust evidence that suggests that the coast of NSW has been struck by several ‘small’ (as opposed to mega) palaeotsunamis or other high energy marine inundations. This suggests that proponents of NSW Quaternary palaeotsunamis are fundamentally correct; the coast of NSW is at risk of inundation events.

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The second deduction is that three events proposed for NSW Quaternary palaeotsunamis coincide with dating obtained by Switzer et al. (2005, 2006, 2010, 2011). The evidence from Killalea (Switzer et al., 2005, 2006) supports the NSW Quaternary palaeotsunami event proposed at 1500 years BP. While 16 widely distributed sites are linked to this event by researchers listed in Table 1, concerns with the chronology raised earlier suggest that the scale of the proposed event is unlikely. However, three sites (MacCauley’s Beach (Site 16), Wollongong (Site 20) and Flagstaff (Site 21)) near Killalea contain dated material linked to this date (Fig. 3c) that indicates a localised event may have occurred in the Wollongong area. Similarly, the Old Punt Bay chronology obtained by Switzer et al. (2010, 2011) coincides with the event proposed at 2900 years BP. Radiocarbon dating of sites within Bateman’s Bay by Young et al. (1997), Bryant et al. (1997) and Bryant and Nott (2001) proposed a localised event that agrees with the conclusions drawn by Switzer et al. (2010, 2011). However, additional testing of the anomalous sediments at Old Punt Bay is required to determine whether this evidence is indicative of a storm or tsunami event. 6. Potential ways forward So where to from here? In this last section of the paper, we address three key issues. First, how can we advance the research in NSW, SE Australia to resolve the difficulties we have identified; second, what are the implications of our analysis for the wider international tsunami and palaeotsunami research community for their interpretation of suspected palaeotsunami deposits and features that are scaffolded on the earlier work from SE Australia; and last, what do we see as the tsunami risk for the NSW coast of SE Australia? 6.1. Advancing the research in SE Australia As previously noted, re-examination of the 60 sites purported to contain evidence of NSW Quaternary palaeotsunamis is impractical. As a result, by analysing the main features of key type sites, it is possible to identify a range of recommendations that may assist in a more efficient and scientifically sound method of validating or rejecting the NSW Quaternary palaeotsunami evidence. Table 6 provides a summary of alternative interpretations of the dominant types of geological evidence associated with NSW Quaternary palaeotsunamis as well as recommendations for testing both the diagnostic criteria and the alternative interpretations. Recommendations for verifying alternative interpretations of evidence vary depending upon the nature of the evidence. For example, analysis of anomalous sand sheets focuses on the sediments by obtaining sedimentary, geochemical and micropalaeontological data and comparing them to known values associated with palaeotsunami deposits. This is markedly different to the analysis of imbricated boulders, where mechanisms are more likely based upon collection of new, site-specific data such as tracking individual boulders before and after high energy events. However, there are a number of recommendations for evaluating the diagnostic criteria that are applicable across all type sites. Given the absence of quantitative analysis reported for NSW Quaternary palaeotsunami sites, it is imperative that quantitative data is obtained to characterise and identify the characteristics of sediments, their source and likely transport/deposition mechanisms. The characterisation of Australian tsunamis is also of paramount importance. The variation between widely accepted palaeotsunami diagnostic criteria and Australian evidence has been proposed as a result of Australian palaeotsunamis producing unique signatures

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that are dominated by erosion morphologies. Goff and DomineyHowes (2009) suggest that this controversy can only be resolved by direct comparison to modern or historical tsunami evidence. With research to date only able to identify a small number of possible tsunami deposits, it is vital that research be undertaken to actively investigate potential modern and historical evidence. Furthermore, analogues need to be obtained from a variety of locations to provide diversity of characteristics as well as from as broad a spatial range as possible. These sites will act as comparative benchmarks against which proposed NSW Quaternary palaeotsunami deposits and features might be compared and contrasted. Any such investigation and analysis of new sites must utilise the widely accepted diagnostic criteria associated with palaeotsunamis. Similarly, future work should ensure that internationally accepted terminology is adopted to ensure standardisation of reporting and enable more direct comparisons between sites. Researchers must ensure careful recording of data and full provision of information relating to sites (such as georeferenced locations) and samples (such as radiocarbon attribute information) to prevent erroneous interpretations and to facilitate alternative analyses. By developing such benchmark sites, it may be possible to finally accept or reject the current research relating to NSW Quaternary palaeotsunamis once and for all. 6.2. Implications for the international research community Notwithstanding the partial validation of proposed sites and events through correlation with the robust evidence of smaller marine inundations, the NSW Quaternary palaeotsunamis research by authors in Table 1 cannot be used with complete confidence. The number of concerns regarding use of geological and chronological data as well as the likely erroneous interpretations mean that significant testing as suggested in Table 6 must be undertaken. While the NSW Quaternary palaeotsunami hypothesis cannot yet be finally validated or rejected, its flaws suggest that reliance upon the tools utilised by its proponents seems unwise. Subsequently, the research building upon the diagnostic criteria utilised in the study of NSW Quaternary palaeotsunamis (Fig. 1) is also cast into doubt. This suggests that the sites utilising the unique depositional and erosional features attributed to NSW Quaternary palaeotsunamis must be questioned and revisited to determine the validity of the findings. 6.3. Tsunami risk for the coast of NSW The NSW coast affected by the reported Quaternary palaeotsunamis now has a population of over 330,000 and 400,000 properties worth more than AUD $150 bn are located within 3 km of the shore (Bird and Dominey-Howes, 2006, 2008; Chen and McAneney, 2006). While smaller than some coastal populations such as those in Tokyo, Jakarta, New York, Guangzhou or Istanbul, the future repeat of events of the magnitude of those proposed during the Quaternary would result in significant losses within the socio-ecological system. However, our analysis suggests that there is little reliable evidence to support the hypothesis that repeated large scale (mega)tsunamis have occurred. This does not mean that there is no chance of future such events occurring. 7. Conclusions Evidence of large Quaternary palaeotsunamis reported on the SE Australian coast has been used as diagnostic criteria for the identification of palaeotsunamis in numerous studies globally. Furthermore, the implications of risk to such events on the vulnerable coast warrant careful re-examination and evaluation of

the reported evidence. A summary and graphical representation of the spatial distribution of geomorphic evidence, as well as distribution of elevation and event chronology data enabled a synthesis of existing literature. Critical evaluation of geomorphic and chronological evidence revealed a number of concerns, specifically regarding the erroneous interpretation of evidence. Alternative interpretations of evidence were offered with particular reference to the context of the sites and geomorphic influences throughout the Holocene. Ongoing coastal processes of weathering and deposition under both normal and storm conditions are noted as likely interpretations of most features. Evaluation of the evidence reveals that three of the reported nine events appear to indicate high energy marine inundation, although not on the scale proposed and possibly indicative of storms. A wide range of future research is required to address conflicting interpretations and recommendations presented emphasise the importance of standardised research techniques and terminology. Given the use of south east Australian palaeotsunami indicators to underpin studies globally, it is likely that a re-examination of sites beyond south east Australia is also required. Acknowledgements We would like to express sincere thanks to Jon Nott of the School of Earth and Environmental Sciences, James Cook University, for helpful discussions during the preparation of this paper. This research was supported by Australian Research Council Grant number DP0877572. We also thank the Editors and two referees for significant constructive feedback on earlier drafts of the manuscript. References Abbott, D., Bryant, E.A., Gusiakov, V., Masse, W.B., Raveloson, A., Razafi ndrakoto, H., 2006b. Report of International Tsunami Expedition to Madagascar. www.ldeo. columbia.edu/users/menke/slides/madagascar06/report.pdf (6 June 2008). Abbott, D., Bryant, T., Gusiakov, V., Masse, W., 2007. Megatsunami of the world ocean: did they occur in the recent past? EOS Trans. Am. Geophys. Union 88 (23), joint assembly supplement, abs. PP42A-04. Abbott, D., Martos, S., Elkinton, H., Bryant, E.F., Gusiakov, V., Breger, D., 2006a. Impact craters as sources of mega-tsunami generated chevron dunes. Geol. Soc. Am., Abstr. Progr. 38 (7), 299. Abbott, D.H., Matzen, A., Bryant, E., Pekar, S.F., 2003. Did a bolide impact cause catastrophic tsunamis in Australia and New Zealand? Geol. Soc. Am., Abstr. Progr. 35 (6), 168. Atwater, B.F., Moore, A.L., 1992. A tsunami about 1000 years ago in Puget Sound, Washington. Science 258 (5088), 1614. Atwater, B.F., 1987. Evidence for Great Holocene earthquakes along the outer coast of Washington State. Science 236, 942e944. Australian Bureau of Statistics, 2009. NSW Population: Summary of Findings (No 3101.0). Retrieved February 8, 2011, from AusStats. Database. Beccari, B., 2009. Measurements and Impacts of the Chilean Tsunami of May 1960 in New South Wales, Australia, vol. 35. NSW State Emergency Service. Bird, D., Dominey-Howes, D., 2006. Tsunami risk mitigation and the issue of public awareness. Aust. J. Emerg. Manage. 21, 29e35. Bird, D., Dominey-Howes, D., 2008. Testing the use of a ‘questionnaire survey instrument’ to investigate public perceptions of tsunami hazard and risk in Sydney, Australia. Nat. Hazards 45, 99e122. Bourgeois, J., Hansen, T.A., Wiberg, P.L., Kauffman, E.G., 1988. A tsunami deposit at the CretaceouseTertiary boundary in Texas. Science 241, 567e570. Branagan, D.F., Packham, G.H., 2000. Field Geology of New South Wales, third ed. New South Wales Department of Mineral Resources, Sydney. Breger, D., Abbott, D., Burckle, L., Gerard-Little, P., Elkinton, H., Martos, S., 2006. Plop plop fizz fizz: identifying and characterizing Holocene microejecta from two oceanic cosmic impacts using analytical scanning electron microscopy. EOS Trans. Am. Geophys. Union 87 (52). Fall Meet. Suppl., Abstract P51A1179. Bryan, W.B., Moore, J.G., 1994. Geologic effects of giant tsunami waves, Lanai and Molokai, Hawaii (Abstract). In: Geological Society of America, 1994 Annual Meeting, Seattle, WA, p. 378. Bryant, E.A., Young, R.W., Price, D.M., Short, S.A., 1992b. Evidence for Pleistocene and Holocene raised marine deposits, Sandon Point, New South Wales. Aust. J. Earth Sci. 39, 481e493. Bryant, E.A., Young, R.W., 1996. Bedrock-sculpturing by tsunami, South Coast of New South Wales, Australia. J. Geol. 104, 565e582.

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