Abstract
Stratigraphic investigations of three coastal waterbodies in southeastern Tasmania reveal major paleoenvironmental phases related to sea level change and anomalous deposits consistent with tsunami inundation. Twenty-two short sediment cores were examined for their sedimentology and fossil diatom, foraminifera and macrofossil assemblages; nine radiocarbon ages were obtained. Despite diverse Holocene histories at each site, four common phases of Holocene paleoenvironmental evolution can be distinguished. In Phase I (pre-8000 yr BP) terrestrial environments existed. During Phase II (8000–6500 yr BP) ponded freshwater environments formed behind transgressive coastal barriers. In Phase III (6500–2000 yr BP) the sites were subject to varying degrees of marine influence, resulting in environments ranging from current-swept tidal inlets to sheltered brackish-marine lagoons. In Phase IV (2000 yr BP to present) there was a decrease in marine influence, one site changed to a freshwater wetland environment while the other two changed to ephemeral salt pans. This study suggests that postglacial sea level rise culminated after c. 7300 cal. yr BP in southeastern Tasmania and that there was probably a late-Holocene fall in sea level. These paleoenvironmental histories provide a framework within which to identify anomalous deposits and assess them for likely causes. Five anomalous deposits are identified, three of which are considered likely to have been deposited by tsunami occurring at c. 4000 cal. yr BP, c. 2000 cal. yr BP and <2000 cal. yr BP, although deposition by large storms cannot be ruled out.
Introduction
Recent major tsunami (for example the Indian Ocean tsunami of 2004) have highlighted the vulnerability to tsunami of coastlines that are otherwise considered relatively remote and tectonically stable. Australia is one example of a landmass that is not very tectonically active but has a long length of coastline that faces subduction-type plate boundaries – common source areas of large tsunami. In particular, the island of Tasmania is exposed to tsunami from a variety of sources. For example the eastern coast is particularly vulnerable to tsunamis generated at the Puysegur subduction zone off the southwestern coast of New Zealand (Figure 1A, Lebrun et al., 2000; Melhuish et al., 1999). Ten tsunami are known to have reached Tasmania in historic times (post
(A) Regional plate tectonic setting of Tasmania with major nearby subduction zones (SZ) shown. (B) Map of southeastern Tasmania and locations of the three study sites. Small inset shows a map of Tasmania and the area covered by the larger map
Whilst there is presently no record of paleotsunami in Tasmania there is a diverse record of paleotsunami evidence along the southeastern coastline of New South Wales, 600–1500 km northeast of Hobart (e.g. Bryant and Nott, 2001; Bryant and Young, 1996; Bryant et al., 1992, 1996). Much of the evidence for ‘mega-tsunami’ along this coastline remains debated (see review in Hutchinson and Attenbrow, 2009) but if such mega-tsunami did occur one might expect widespread effects because typically the tsunamigenic source is thought to be far field. Obtaining paleotsunami records from sites further afield along the Australian coast, such as in Tasmania, may help to resolve some debate.
One widely used method of establishing a pre-historic tsunami record, and thereby improving tsunami risk assessment for a particular region, is to document sedimentary evidence for tsunami inundation in the geologic record of coastal wetlands (e.g. Clague, 1997; Cochran et al., 2005; Nanayama et al., 2003; Nelson et al., 1996). The southeastern coast of Tasmania has a wealth of coastal waterbodies, yet comparatively few studies have been undertaken at such sites. Lacking a baseline understanding of the Holocene evolution of such sites makes it difficult to distinguish anomalous or extreme events from the geologic record. Therefore, in the initiation of a paleotsunami record for southeastern Tasmania, emphasis was placed on deriving Holocene paleoenvironmental histories for three coastal sites. The paleoenvironmental histories were used to determine which sites and timeframes are most suitable for paleotsunami preservation and what, if any, evidence exists for paleotsunami.
Methods
Core collection
This study focuses on sites in the vicinity of Hobart – as the main population centre of Tasmania it poses the greatest area of concern for any future damaging tsunami event. Sites were chosen to encompass a range of water body types with a variety of coastal aspects in order to maximise the likelihood of finding a paleotsunami record. At the three study sites, core transects were collected both parallel to and perpendicular to the line of the beach (Figure 2). At all sites, penetration to 2–3 m depth was sufficient for sampling back through to the early Holocene, 8000–9000 years before present (yr BP). Cores were collected using a double-tube, hand-operated coring device modified from Tratt and Burne (1980). Gouge coring was used to extend the total depth of penetration in some cases.
(A) Map of the South Neck Beach Main Wetland and cores sites SNB 1–6. (B) Map of the South Arm locality and core sites. (C) Map of the Primrose Beach locality and core sites. (D) Survey profiles from the mean high water (MHW) mark, over the dune crest and into the back-barrier basins at the three study sites, A–A’, B–B’ and C–C’ are marked on maps (A), (B) and (C), respectively. Core sites that do not lie on the profile line have been projected onto the graph at their correct elevation
Surveying
The core collar elevations and dune heights were surveyed with reference to mean tidal level (MTL). An uncertainty value is calculated for each elevation value that takes into account the uncertainties associated with measurement and MTL corrections as follows: Elevation uncertainty = √(L2 + M2 + S2) where L is levelling uncertainty (± 0.1 m), M is uncertainty on correction estimation of the mean high water mark (± 0.1 to 0.28 m, site dependent), and S is uncertainty on correction from MHW to MTL (± 0.1 m). For all elevation values relative to MTL the total uncertainty is less than ± 0.31 m.
Sample elevations have not been corrected for sediment compaction or tectonic uplift. It is assumed that sediment compaction will have been negligible because the cores are generally short, and bottom out in hard, incompressible sands. Tectonic uplift is also considered to be negligible because the area shows little evidence of such movement but it cannot be completely eliminated. There is one inferred last interglacial (~125 ka) shoreline at Mary Ann Bay, 10 km SSE of Hobart, which implies a low rate of uplift of 0.15 mm/yr (Murray-Wallace and Goede, 1991, 1995) but a tectonic mechanism for the uplift has not been advanced.
Stratigraphic and microfossil analysis
The sedimentology of all cores was logged by visual assessment and described at centimeter-scale detail, however, only core summaries are presented here. The fossil groups used in this study were molluscs, foraminifera and diatoms. A total of 29 samples of 8–15 g were selected for benthic foraminifera analysis. Where foraminifera were present in adequate abundances approximately 100 tests from the >63 µm grain size fraction were identified (Appendix 1, available online; Albani et al., 2001). A total of 35 samples were examined for their diatom content, samples were examined on temporary water mounts without chemical or physical processing. Diatom valves were identified for each sample using standard reference floras (e.g. Hartley, 1996; Krammer and Lange-Bertalot, 1991, 1999a, b, 2000; Witkowski et al., 2000). Qualitative assessments of paleoenvironment were developed, the diatom assemblages were used to supplement a broad understanding of the paleoenvironment rather than being used for quantitative estimates of salinity or tidal elevation (Appendix 2, available online).
Radiocarbon dating
Radiocarbon dating was carried out on peat units, charcoal fragments and well-preserved shell fragments or whole shells. Conventional radiocarbon ages were calibrated using the SHCal04 calibration curve of McCormac et al. (2004) or the Marine04 calibration curve of Hughen et al. (2004). No average ΔR values have been calculated for Tasmanian marine water (Ulm, 2006) so the default laboratory value of −30 ± 13 years was used for calibration of shell samples. All ages in the text are presented as calibrated years before present (cal. yr BP).
Results
South Neck Beach
South Neck Beach is located on Bruny Island at the southern end of a 5 km long tombolo. The beach faces east and is fully exposed to the ocean (Figure 1B). On the western side of the tombolo is Simpsons Bay which is a sheltered, shallow bay. There are several wetland areas, separated by relict sand dunes, between Simpsons Bay and the modern dunes at South Neck Beach. The modern dunes are several hundred meters wide and 2.3–3.2 m above MTL. Field studies were concentrated in the southern and largest wetland (Main Wetland, Figures 2 and 3). During site visits in spring the wetland had a water depth of approximately 10–15 cm, with freshwater wetland plant species growing at the surface.
Core logs from the South Neck Beach Main Wetland. The logs show the grain size and major stratigraphic features of the cores. Columns to the left of each core log show the location of foraminifera (F) and diatom (D) samples and a broad paleoenvironmental interpretation (P). Grey dashed lines indicate where certain correlations exist between cores. Radiocarbon ages and shell species are shown when available; stars denote anomalous units which are outlined further in Table 2A. The legend also applies to Figures 4 and 5
A dark brown to black very compact sand layer occurs at the base of all cores in the Main Wetland (Figure 3). Charcoal near the top of this unit in core SNB-2 has an age of 8404–8293 cal. yr BP (Table 1). The black sand does not contain shells or foraminifera (Appendix 1, available online) but a diatom sample showed partially dissolved valves of Paralia sulcata (Appendix 2, available online). P. sulcata is a thick-walled, robust diatom species that is often the only remaining trace of an assemblage when sediments have undergone transport or prolonged weathering (Denys and de Wolf, 1999). This unit is interpreted as a terrestrial deposit – probably weathered dune sands; its variable elevation reflects the pre-existing dune topography. The presence of the partially dissolved brackish-marine diatom P. sulcata was likely due to transportation from a nearby marine environment. In cores SNB-2, SNB-3 and SNB-5 the black sand is overlain by a silty fine sand unit, with occasional organic fragments, that is interpreted also as terrestrial sediment.
Radiocarbon ages of samples from coastal field sites around Hobart
All radiocarbon dating was undertaken by the Rafter Radiocarbon Laboratory, New Zealand. The Marine04 calibration curve of Hughen et al. (2004) and SHCal04 calibration curve of McCormac et al. (2004) were used.
Overlying the terrestrial sediments and extending up to approximately 0.4 m depth in all cores are silt, clay and sand units containing shells or shell fragments. Most of the units contain abundant, well-preserved brackish-marine diatom assemblage and intertidal foraminifera. These shelly units represent a period of marine sedimentation within a tidal inlet. The finer-grained sediments probably represent more sheltered parts of the inlet while the well-sorted sand represented more current-swept areas or tidal channels. Radiocarbon ages indicate the period of tidal inlet sedimentation started around 6314–6155 cal. yr BP and lasted until at least 2776–2675 cal. yr BP.
Within the 0.5–1.5 m thick sequence of marine sediments there are several notable units. At 0.65–0.55 m depth in SNB-6 is an upward fining medium sand with shells, brackish micromolluscs, brackish-marine diatoms and terrestrial organic fragments. It represents a higher energy depositional environment than the fine-grained sediments above and below. In cores SNB-2, SNB-3 and SNB-5 a distinctive layer of Ostrea angasi (southern mud oyster) can be correlated at a depth of ~0.7 m between the three cores (Figure 3). A radiocarbon age of 4218–3970 cal. yr BP was obtained from the oyster layer in core SNB-2 (Figure 3; Table 1). The microfossil assemblages from the sand surrounding the oyster shells imply deposition in an intertidal environment. This is reasonably consistent with the living depth for the oysters as they typically live at and below low tide in low- to medium-energy environments. However, the unusual features of the oyster layer are that the shells occur as fragments, the grainsize of the sediments is different in each core, and the unit is coarsest at the site considered to be in the most sheltered part of the inlet (SNB-5). These features suggest that the oyster layer (shells and sediment) may have been transported into the inlet rather than forming in situ. Another unusual layer is a shell hash unit in core SNB-2 at 0.6–0.45 m depth. The shell hash has a molluscan assemblage characteristic of a relatively cool, southern Australian tidal inlet or the head of an enclosed bay, with very shallow water, soft sediment flats and salinity significantly lower than oceanic (A. Beu, personal communication, 2007). This shell hash unit may have accumulated on a bar within the inlet or it may be a coarse tidal channel lag deposit. In cores SNB-2 and SNB-3 a 0.15 m thick, coarse, light-grey well-sorted sand unit without shells overlies the tidal inlet sediments. This coarse unit may represent either a high-energy influx or a tidal channel (Figure 3).
At the top of all the Main Wetland cores is a 0.4 to 0.2 m thick layer of peat; the peat contains soil diatoms which confirm a freshwater environment. Radiocarbon ages indicate the peat started to accumulate some time between 2776–2675 cal. yr BP and 1307–1242 cal. yr BP (Figure 3, Table 1).
South Arm
South Arm Beach is located at the head of Storm Bay; the beach faces to the southwest and opens to Half Moon Bay (Figure 1B). At this site there are two depositional basins (salt pans) behind a narrow barrier of sand dunes that reach elevations between 2.6 and 5.3 m above MTL (Figure 2B). Seven cores of lengths between 1.5 and 2.5 m were collected from the South Arm salt pans, five from SAS (SAS-1–SAS-5) and two from SAG (SAG-1 and SAG-2; Figures 2B and 4).
Core logs from the SAS and SAG basins at South Arm. See Figure 3 for the legend. Grey dashed lines indicate where certain correlations exist between cores. Note that the SAG cores are at a smaller scale than the SAS cores
Homogenous fine to medium-fine sand units occur at the base of all cores across the SAS basin (Figure 4). The sand units contain occasional fragments of organic matter or pieces of wood and there are no marine indicators. The sands are probably terrestrial in nature, either of fluvial or aeolian origin. At the top of these sands is a distinct organic-rich, black layer that can be correlated between all the cores of the SAS basin (Figure 4). Wood fragments and terrestrial snail shells indicate this is a terrestrial unit, it occurs as peat at its lowest elevation position suggesting that a freshwater pond or wetland existed near the centre of the basin. The correlative of this peat layer in the SAG basin was dated at 7256–7424 cal. yr BP (Figure 4).
Above the black organic horizon in the most seaward cores (SAS-5, SAS-4 and SAS-1) are 0.1–0.3 m units of silt or sand (Figure 4). These units contain the only occurrence of marine shell fragments in the cores and probably represent the only time in the basin’s history when it had a direct connection with the open sea.
Above the black organic horizon in cores SAS-2 and SAS-3, and above the marine silt and sand units in the other cores, are distinctive basin-wide units of finely (millimeter-scale) laminated clays. Scattered micromolluscs within the clay are Coxiella badgerensis, a common salt lake and brackish lagoon species. The finely laminated nature of the clays implies a calm and sheltered depositional environment, consistent with a lagoon without tidal currents. Two microfossil samples from the laminated clays in SAS-4 and SAS-1 yielded high concentration diatom assemblages indicative of brackish-marine salinities but without any evidence for an open marine component (Appendix 2, available online). A radiocarbon age of 4412–4145 cal. yr BP was obtained from a peaty clay lamination in the SAS-1 core (Figure 4; Table 1).
Above the laminated clays in cores SAS-2 and SAS-4, and above the silty fine sands in cores SAS-5, SAS-3 and SAS-1, there is a basin-wide transition to very light grey, almost white, non-laminated clays and silts. C. badgerensis micromolluscs, and a high concentration, monospecific diatom assemblage (Appendix 2, available online) indicate there was brackish-marine water in the basin at this time. The white colour is thought to be the result of a high salt content so these pale units are interpreted as salt pan deposits.
Within the salt pan deposits in SAS-4 and SAS-1 there are fine and medium-coarse sand units of <0.15 thickness containing abundant C. badgerensis and an intertidal foraminifera assemblage (Appendix 1, available online). The sand layers are anomalous for a salt pan environment and may indicate a tidal channel or an influx of coarse material to the site.
At the top of all the SAS basin cores is a transition from white-grey clay to grey-brown clay. This more organic clay may relate to recent human-induced changes in the catchment such as forest clearance and changes in drainage.
The paleoenvironmental record of the SAG basin is similar to the SAS basin except the sediments of SAG are generally coarser. It is likely that the SAG basin had a more direct connection with the sea. It was probably a brackish-marine lagoon with an intermittently active tidal channel. At 0.5 m depth in both SAG cores is a fining-upward medium-fine sand unit of c. 0.3 m thickness. These sands contain millimeter-scale shell fragments, numerous C. badgerensis and towards the top are small intraclasts of clay including some with laminae intact. If this is an in situ deposit, it represents a much higher-energy and more marine environment than that seen elsewhere in the South Arm basins – for example a permanently open tidal inlet. However, there are two features of this unit that suggest it may have been transported. First, the fining-upward structure is characteristic of one-off deposition through a water column; second, the intraclasts of clay towards the top of the unit are most simply explained as rip-up clasts requiring a transport mechanism to erode them from source and deposit them with the sand.
Primrose Beach
Primrose Beach lies at the head of Frederick Henry Bay and faces southwest (Figure 1B). At this location there are two back barrier basins, PS and PSS (Figure 2C). The basins are separated by an elongate, shore-parallel 0.5 m topographic high which existed naturally prior to road construction and the basins were formerly joined by a small channel at the northwestern end. There is a single line of dunes with a steep seaward face between the PSS basin and Primrose Beach. The dune crest reaches an average elevation of 6.98 m above MTL. Three cores of 1–2 m length were collected from the PS basin and two from PSS (Figures 2C and 5).
Core logs from the PS and PSS basins at Primrose Beach. See Figure 3 for the legend. Grey dashed lines indicate where certain correlations exist between cores
The basal 0.3 m of PS-2 (Figure 5) is organic-rich silt that grades upward into a 1 m thickness of fine sand with significantly less organic matter; these lower units are probably fluvial or aeolian. The terrestrial sediments are overlain by a peat layer that yielded an age of 9888–9541 cal. yr BP (Table 1). Above the peat are finely (millimeter to centimeter scale) laminated clays containing abundant C. badgerensis micromolluscs, brackish-marine diatoms and intertidal foraminifera (Appendices 1 and 2, available online). The depositional environment of the laminated clays was probably a saline lagoon; the laminated and fine-grained nature of the sediment suggests a sheltered depositional environment protected from wave and current activity by a barrier. The top 0.1 m of PS-2 is a clay unit with micromolluscs distinguished by an absence of laminations. This change may reflect human disturbance effects to the lagoon or a decrease in the seasonal water fluctuations that had previously caused the laminations to form.
The general sequence of terrestrial environments transitioning to a saline lagoon environment is also observed in the other two PS basin cores (Figure 5). The lagoonal sediments in PS-3 and PS-1 are not laminated and are not as thick as those in PS-2 probably because the two former cores were taken at more marginal positions in the coastal site (Figure 2C). In PS-1 the occurrence of medium and fine sand units (e.g. between 0.5 and 0.2 m depth) is anomalous for an enclosed saline lagoon. However, PS-1 is the most seaward of the cores in the PS basin so it may have received more aeolian sand than the central basin.
The PSS basin cores are generally sandier than those from the PS basin (probably reflecting the more seaward position of the PSS cores), but the paleoenvironmental sequence is similar. Terrestrial sands are overlain by brackish-marine sediments. The presence of shell fragments in PSS-1 suggests an inlet rather than a lagoon environment but the shells occur only as small fragments and are not in situ. The upper 0.5 m of PSS-1 is homogeneous medium sand that is probably aeolian and related to the recent dune blow-out in the area (Figure 2C).
Discussion
Holocene evolution of coastal waterbodies of southeastern Tasmania
Detailed paleoenvironmental histories of the South Neck Beach, South Arm and Primrose Beach study sites are relatively diverse with site elevation, freshwater and marine inundation, dune barrier development and sediment supply all playing a part in local variation. However, broader-scale changes that correlate between sites can also be identified. Four phases of coastal development are inferred from the individual paleoenvironmental histories (Figure 6) and these are related to changes in Holocene relative sea level (Figure 7).
Schematic illustration of the Holocene paleoenvironmental evolution of coastal wetlands of southeastern Tasmania
Radiocarbon ages obtained from coastal wetlands of southeastern Tasmania plotted relative to present mean tidal level. Arrows indicate the relative position of relative SL at the time of sample deposition, i.e. SL is inferred to be lower than the elevation of terrestrial deposits and higher than the elevation of marine deposits. Also shown is the SL curve developed by Sloss et al. (2007) for the southeast of mainland Australia and the paleoenvironmental phases identified during this study
Phase I: Terrestrial environments, pre-8000 cal. yr BP
At the base of all three wetlands are terrestrial sediments most commonly of aeolian or fluvial origin. These were deposited when relative sea level was below its present elevation (Figure 6). Age control comes from charcoal and wood within aeolian sediments at South Neck Beach and Primrose Beach, where dates of 8404–8293 cal. yr BP and 9888–9541 cal. yr BP at −2.2 ± 0.31 and −1.04 ± 0.31 m above mean tidal level (AMTL) respectively, were obtained (Figure 7, Table 1).
Phase II: Freshwater environments, ~8000–6500 cal. yr BP
In many of the cores of all three sites there are fluvial or freshwater peat deposits between the terrestrial sediments of Phase I and overlying marine sediments. These organic-rich deposits may represent ponding landward of a transgressive barrier (Figure 6). As relative sea level was rising, sand barriers were probably being driven landward and the presence of freshwater environments suggests that the migrating sand barriers were able to keep pace with the rate of relative sea level rise, rather than becoming overwhelmed. There is not adequate age data to constrain the timing of relative sea level rise culmination. A date from woody peat at the base of the SAG-1 core yielded an age of 7424–7256 cal. yr BP (Figure 7). The peat has a present elevation of −0.95 ± 0.31 m AMTL and if it is assumed the peat accumulated above sea level it implies relative sea level was below this elevation at ~ 7300 cal. yr BP.
Phase III: Intertidal lagoons and inlets, ~6500–2000 cal. yr BP
All three coastal waterbodies display various degrees of marine inundation during the mid–late Holocene, probably representing the time of highest Holocene sea level (Sloss et al., 2007; Switzer et al., 2010). Conditions varied from a current-swept intertidal inlet at South Neck Beach to a sheltered, brackish-marine lagoon without direct marine connection at South Arm and with a marine connection at Primrose Beach (Figure 6). The oldest dated shell at South Neck Beach has an age of 6314–6155 cal. yr BP and occurs at an elevation of −1.7 ± 0.31 m AMTL, implying relative sea level had reached above that elevation by ~6200 cal. yr BP (Figure 7). The youngest age constraining the end of Phase III is a shell date of 2776–2675 cal. yr BP obtained from the tidal inlet shell hash at an elevation of −0.85 ± 0.31 m AMTL in the SNB-2 core.
Phase IV: Marine to freshwater transition, ~ 2000 cal. yr BP to present.
At South Neck Beach there is a pronounced transition from an intertidal inlet to a freshwater wetland that occurred at approximately 2000 ± 400 yr BP. At South Arm and Primrose Beach a transition to freshwater peat is not seen but there are late-Holocene paleoenvironmental changes suggestive of decreasing marine influence. At South Arm, particularly in the SAS basin, there is a transition from a permanent brackish marine lagoon to an ephemeral salt pan. At Primrose Beach there is a change in the upper 0.2 m of the PS basin that may relate to a decrease in seasonal brackish-marine water inundation (as inferred from the absence of laminations). Late-Holocene ages are inferred for both transitions but they have not been directly dated.
There are three possible reasons for the late-Holocene paleoenvironmental changes at all three coastal waterbodies. The first is that there was a late-Holocene fall in relative sea level. Primary evidence for this is the decreasing marine influence at all three sites. If the transitions at South Arm and Primrose Beach were accurately dated and corresponded in time to the South Neck beach transition then this scenario would be the most likely because a relative sea level change is the best mechanism to produce a synchronous change at all three sites. However, in the absence of age data this remains ambiguous. The second possible reason is that the barriers built up sufficiently to block the tidal inlets of the wetlands. Third, basin infilling may have raised the depositional environments above the level of marine influence. These last two scenarios could have occurred at different times across at each location.
Implications for SL records in southeast Tasmania
In Figure 7 the radiocarbon dates collected for this study are compared with the SL curve developed by Sloss et al. (2007) for southeastern mainland Australia. The curve of Sloss et al. (2007) indicates sea level culminated between 7900 and 7700 cal. yr BP. However, according to the data presented here terrestrial deposits were still accumulating at South Arm ~7300 cal. yr BP indicating that postglacial relative sea level rise culminated after this time in the Hobart region. A post ~7300 cal. yr BP culmination age is consistent with other records from southern mainland Australia which suggest an age of approximately 7000 cal. yr BP (Belperio et al., 2002; Thom and Roy, 1985). A sea level high stand between 6300 and 5800 cal. yr BP has been inferred from the pollen record of a core taken from an interdune depression at Macquarie Harbour on the west coast of Tasmania (Fletcher and Thomas, 2010). No correlative evidence for this sea level highstand is found at these eastern Tasmania sites.
The change from paleoenvironmental Phase III to Phase IV in the wetlands of southeastern Tasmania approximately correlates with the fall in relative sea level from the Holocene highstand (1–2 m AMTL) to present mean SL in the Sloss et al. (2007) curve (Figure 7). A fall in relative sea level at c. 2000–2500 cal. yr BP was also inferred along the southeastern Australia coastline by Switzer et al. (2010). It cannot be unequivocally proved that there was a relative sea level fall between ~2700 and 1300 cal. yr BP from the data presented here, but it is the simplest explanation for why there is a decrease in marine influence at all sites. Previously it was thought that the mid-Holocene highstand, commonly seen elsewhere in Australia, was absent in Tasmania (Lambeck, 2002).
Evidence for paleotsunami in southeastern Tasmania
All three sites investigated in this study appear to have had the potential for receiving and preserving evidence of paleotsunami for the last 8000 years because they have been sheltered basins immediately adjacent to the coastline throughout that time. However, a major factor in the ability of these sites to receive tsunami is the presence of coastal dune systems or barriers which appear to have persisted in some form between 8000 cal. yr BP and the present. The past height and continuity of dunes or coastal barriers is unknown and is a key determinant in the size of tsunami or storms that could have inundated these sites.
Within the framework of paleoenvironmental evolution at each site, five units that are anomalous in some way and could potentially represent extreme marine inundations events, such as large storms or paleotsunami, are identified (see starred units in Figures 3–5). In recent years many studies have been undertaken to characterise modern tsunami and paleotsunami deposits, particularly with the aim of determining features that can distinguish between tsunami and storm deposits (e.g. Cochran et al., 2005; Dominey-Howes et al., 2006; Goff et al., 2004; Kortekaas and Dawson, 2007; Morton et al., 2007; Nanayama et al., 2000; Sawai et al., 2009; Switzer and Jones, 2008). There are no stratigraphic or micropaleontological characteristics that unequivocally differentiate between storm and tsunami deposits but in some settings there have been shown to be differences in unit thickness, grain size sorting, heterogeneity of the deposit, how the deposit mantles or infills topography and presence of intraclasts (e.g. Kortekaas and Dawson, 2007; Morton et al., 2007). Many of these characteristics are difficult to assess from core samples. In this section the anomalous deposits recorded in the three Tasmanian study sites are discussed. The anomalous units are compared with several widely recognised characteristics of tsunami deposits to determine the likelihood that the units represent paleotsunami (Table 2A and B).
Derived from many publications, e.g. Clague and Bobrowsky (1994); Cochran et al. (2005); Dominey-Howes et al. (2006); Gelfenbaum et al. (2001); Hemphill-Haley (1996); Minoura et al. (1996, 1997); Pinegina and Bourgeois (2001); Sawai et al. (2009); Switzer and Jones (2008).
(B) Summary of anomalous units identified in this study and their tsunami characteristics. Numbers in the fifth column refer to the tsunami characteristics listed in Table 2A
At South Neck Beach two units contain at least four of the five key tsunami deposit characteristics and so are considered likely to represent paleotsunami. One consists of an oyster shell hash with an age of ~4000 cal. yr BP and the other is a coarse to medium sand with shells and micromolluscs with an age between ~2700 and ~1270 cal. yr BP. Both occur within a background of sheltered brackish-marine intertidal environments but it is unknown how far away the open coast was at the time of their deposition. In the present-day setting a tsunami is the most likely mechanism capable of transporting sediment from the ocean to the core sites. However, if the Simpsons Bay shoreline has prograded extensively over time it is possible that the core sites were much closer to the head of a sheltered bay earlier in the Holocene – storm waves could have deposited the oyster shell hash and the upper unit could represent a regressive beach deposited as the bay infilled.
One anomalous deposit at South Arm Beach displays four of the five key tsunami deposit characteristics. It occurs in the four eastern-most cores and although it is not directly dated, its position in the stratigraphy suggests it is younger than 2000 cal. yr BP. It is a fine-coarse sand unit that occurs within a salt pan environment in the SAS basin and on top of aeolian deposits in the SAG basin. Characteristics include clay rip-up clasts, an unusual foraminiferal assemblage and numerous Coxiella badgerensis micromolluscs. The unit definitely represents a higher energy environment than was pre-exisiting in the basins and/or transport into the basins by a one-off high energy event. If it is assumed that a dune similar that of the present day was in place (3.5 m above MTL and continuous) then tsunami is the most likely mechanism. However, if there was a breach in the dune near these basins then there may have been a brief tidal connection to these sites and/or the possibility of transport during a large storm.
There are two anomalous units at Primrose Beach but both are considered unlikely to be tsunami deposits. The units occur in only one of five cores collected at this locality so they are more simply explained by local processes such as the existence of a tidal channel or high aeolian sand input at one core site.
Potential tsunami deposits occur within all three of the coastal wetlands studied here, but only three units are considered likely to represent paleotsunami. Further knowledge of local coastal geomorphology and dune histories is required to confirm a tsunami origin for these deposits rather than storm or tidal channel changes. From the broad chronological framework developed for these sites, ages for these three units of ~4000 cal. yr BP, ~2000 cal. yr BP and <2000 cal. yr BP are estimated. The latter two units are not directly dated so it is possible that they coincide in which case a region-wide event could be inferred. Within an 8000 year window of opportunity for preservation of tsunami deposits this represents a recurrence interval of 3000–4000 years. Such an estimate must be considered a very preliminary average for the region considering the small number of sites investigated to date. It also represents only moderate to large events given that smaller tsunami are unlikely to breach or overtop dune systems and leave evidence of their occurrence in the geological record.
The source events of the potential Tasmanian paleotsunami deposits described in this study are unknown. Ten paleotsunami events have been identified Australia-wide (Dominey-Howes, 2007) but none of these have confirmed sources and only one of them correlates in age to the events identified in this study. A large-scale sand washover sheet at Killalea Lagoon on the southeast coast of Australia has an age of <800 years (Switzer and Jones, 2008) and may correlate to the <2000 year anomalous deposit identified at South Arm Beach although the dating of both units is extremely poor. Goff and Dominey-Howes (2009) suggest a correlation between the <800 year event of Killalea Lagoon and multiple paleotsunami deposits of the New Zealand coastline deposited between 725 and 300 yr BP. Most of the New Zealand events in this time frame are relatively small and had local sources that are unlikely to have affected Tasmania, the only exception is a possible, but poorly documented, Puysegur subduction zone event at ~540–620 yr BP (Goff and Dominey-Howes, 2009). A proposed tsunami event recorded by a sandsheet on the southern New South Wales Coast and dated at approximately 4300 ± 500 yr BP (Switzer et al., 2005) potentially correlates with the ~4000 cal. yr BP event identified in Tasmania by this study, although more precise dating of both events would be required to confirm this. Evidence for two large paleotsunami (runup up to 16 m) along the southeastern Australian coastline at approximately 6500 and 900 cal. yr BP has been proposed by Bryant and Nott (2001). No paleotsunami evidence is seen in this study for a 6500 yr BP event in Tasmania. The 900 yr BP event potentially correlates with the <2000 year anomalous deposit identified at South Arm Beach in this study but the absence of widespread evidence for this event in all three study sites would imply this was not a megatsunami.
Conclusions
The stratigraphy preserved within wetlands of southeastern Tasmania contains a wealth of information about coastal evolution through the Holocene. The three coastal wetlands studied here have diverse sedimentary records but common stages of paleoenvironmental evolution are distinguished. Terrestrial and freshwater environments are recorded early in the Holocene (low and rising sea level). These transition to brackish marine lagoons and inlets at some time between ~7300 and 6200 cal. yr BP (sea level high stand) and then revert to salt pans and freshwater wetlands at ~2000 cal. yr BP providing evidence for a late-Holocene fall in sea level. These paleoenvironmental histories provide a framework within which five anomalous deposits that require relatively high energy mechanisms of deposition are identified. Three of these are considered likely to have been deposited by tsunami occurring at ~4000, ~2000 and <2000 cal. yr BP. However, further work on dune and barrier histories in the region is required to confirm these events as tsunami.
Footnotes
Acknowledgements
Mineral Resources Tasmania led this project that was funded by the Tasmanian Government as part of their Natural Disaster Mitigation Program. Members of the project steering group Chris Beattie, Chris Sharples, Amy Prendergast, David Burbidge and Joanna Ellison are thanked for their initial input and guidance. Chris Sharples provided a wealth of local knowledge that helped greatly with site selection. Brigid Morrison provided field support. David Shaw ensured excellent core retrieval. Alan Beu identified molluscs and provided environmental preferences for them. Kelvin Berryman assisted with project design, site selection and report reviewing. Nicola Litchfield, Marcus Vandergoes, Alastair Dawson and three anonymous reviewers are thanked for their reviews of the manuscript. This manuscript was completed under New Zealand’s Foundation for Research Science and Technology Geological Hazards and Society programme.
References
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