Coastal geomorphology and geology of the Gippsland Lakes region: a review and future directions

Contemporary shoreline

The oldest units found exposed on the modern coast are the Middle Devonian (380 Ma) Mount Singapore Granite on Wilsons Promontory (Fig. 1). These lithologies are an S-type monzogranite (Rossiter 2003) which are relatively high in Rb and low in Ba and Sr with occasional concentrations of garnet (Chappell et al. 1988). Weathering of the granite during the Mesozoic occurred to depths of over 300 m with much of this regolith being subsequently eroded, especially at the coast (Hill and Joyce 1995; Hill et al. 1995). The topography of Wilsons Promontory is strongly influenced by tectonic activity dating after the early-Cretaceous with vertical fault movement contributing to the elevation of palaeoerosion surfaces (Hill et al. 1995).

Fig. 1.

A simplified surficial geological map of the Gippsland Basin Bioregion, located in the southeastern corner of mainland Australia (Data sourced from Bioregional Assessments 2015).

The next oldest sediments to be exposed on the open-ocean coast are older limestones observed to outcrop on the beachface and surf zone at McGaurans Beach following storms (N Rosengren unpubl. data). This represents the uppermost and younger parts of the Seaspray Group of carbonates described by Holdgate et al. (2003) elsewhere in the basin.

The remaining units, which account for nearly all of the Ninety Mile Beach coast are composed of late Neogene deposits which form a coastal plain up to tens of kilometres wide, behind which low-energy swamp deposits are found. The width of the Holocene sequences is determined by the presence of Tertiary age terraces which are closest to the modern-day coast at McGaurans Beach. Where the coastal plain is wider, from approximately Seaspray north to Lakes Entrance, the lacustrine environment of the Gippsland Lakes region has formed where beach-barrier sequences of Last Interglacial and Penultimate Interglacial age are also found (Bird 1961, 1965; Thom 1984; Oliver et al. 2018; Kennedy et al. 2020).

Shelf environment

The southern extent of the contemporary offshore sedimentary environment is the Bassian Rise which extends from Wilsons Promontory to Flinders Island in Bass Strait. This rise separates the Gippsland and Bass basins and forms a morphological divide between the study area and the contemporary sedimentary environments further south. The sea floor of the inner shelf, adjacent to the study area, is broadly composed of gravelly sand with isolated patches of sand and gravelly muddy sand (James et al. 2008) with a mean grain size of 1–2 ɸ (ɸ units = −Log₂(mm units)) coarsening to greater than −1 ɸ offshore in an easterly direction (Blom and Alsop 1988) (Fig. 2). The surface sediments of the inner shelf are broadly Holocene in age and are composed of biogenic gravel, sand and mud (James et al. 2008), with a mineralogy of over 70% carbonate (Blom and Alsop 1988). Closer to shore in the upper shoreface, in the zone of principal sediment transport, the carbonate content is lower (<50%) (Blom and Alsop 1988). In some locations, such as the bedrock defined bays of Wilsons Promontory, the sandy sediments are observed to be entirely quartzose, representing a local source of sand from the surrounding granitic hills (Ierodiaconou et al. 2018). On the inner shelf, bryozoan-derived sediment dominate the carbonate fraction with this relict biogenic material becoming dominant at the shoreface offshore of Ninety Mile Beach (James et al. 2008) and also coinciding with the observed outcrops of carbonate rocks of the Seaspray Group in the surf zone.

Fig. 2.

The different grain sizes of surface sediment on the continental shelf and in Bass Strait (left map) and the distribution of surface sediment facies, based on study of samples indicated, on the continental shelf off Tasmania and in Bass Strait (right map) (Source: James et al. (2008); reproduced with permission – Elsevier licence # 5659610806113).

The contemporary inner shelf environment is not a major sediment source for the shorelines of Ninety Mile Beach north of Corner Inlet. It may have contributed some sediment to onshore barrier construction as sea level transgressed across its surface, drowned beach-barrier sequences are present on its outer edges (Brooke et al. 2017), but is considered to be a ravinement surface (Holdgate et al. 2003) with little connection to the modern coast. During the early to mid-Holocene, as sea level stabilised at modern elevations after rising from its lowstand during the last Glacial Maximum, the beach barriers of the contemporary coast were initially built from sediment transported from the inner shelf (Bird 1961, 1965, 1971; Thom 1984; Kennedy et al. 2020); however, by the late Holocene this source appears to have been exhausted with sediment supply being primarily sourced longshore from Corner Inlet in the south (Kennedy et al. 2020). There is currently a dearth of information on the degree of connectiveness between the modern littoral and shallow marine sedimentary systems. Investigations of the degree to which the modern shelf is a source or sink for the contemporary beaches is critical for understanding the resilience of the coast to future sea level rise and changed wave conditions in a warmer world.

Penultimate Interglacial (Prior Barrier), ~220,000 years ago (MIS 7)

This earliest phase of consolidated beach-barrier development is found in the Prior Barrier sequences (Fig. 3). These are located at the inland limit of the Gippsland Lakes, and the sequence is best represented at Banksia Peninsula. The sediments are fine to medium grained and white, with the colour representing bleaching that has occurred in the extended period between their deposition and today.

Apart from the descriptive work of Bird and Jenkin, there has been little detailed investigation of this sequence. At present there is no direct evidence of its age, and this remains a significant gap in the understanding of how the coast of Gippsland behaves during periods of high sea level such as today and into the future. The age of ~220,000 years ago is an estimate based on the assumption that sea level at that time was at or near present elevations in this region. It is also estimated based on its geomorphology and position relative to the well-dated sequences of younger age located seaward (see below).

Last Interglacial (Inner Barrier), 80,000–125,000 years ago (MIS 5a–5e)

The Inner Barrier is typified by the sand sequence between Lake Victoria and Lake Reeve, on which Loch Sport is located (Fig. 4). The sequence was described by Jenkin (1968) who classified it as Phase Iva age under his stratigraphic timeline. The dating in Oliver et al. (2018) provides the most accurate and precise age of the sequence. This most recent work, using optically stimulated luminescence dating, confirms and refines many of the previous qualitative inferences and finalises the pre-existing debates on sea level change and landform response in the region. It is a composite feature involving deposition of marine, estuarine and aeolian deposits.

Fig. 4.

Topographic LiDAR images of the three barrier sequences in which the Gippsland Lakes are formed. The luminescence ages on the Inner Barrier and adjacent to the Prior Barrier are derived from Oliver et al. (2018).

The Inner Barrier was deposited primarily around 120,000–130,000 years ago (Fig. 3). This period is known as the Last Interglacial (MIS 5e) when global sea level was 3–5 m higher than present elevations (Murray-Wallace and Woodroffe 2014). The barrier has formed as a series of parallel ridges which are elevated around 7 m above present sea level. The ridges are former foredunes formed on top of beach deposits. The size and shape of the dunes, as well as the sediment they contain, are very similar to the modern beach (Oliver et al. 2018) (Fig. 4). We can therefore assume that this sequence formed, and behaved, in a similar manner to the modern-day beach.

One enigmatic age (77 ± 3.5 thousand years, MIS 5a) (Oliver et al. 2018) has been recorded at its inland edge near Loch Sport (Fig. 4). This area has a distinctly different morphological character to the rest of the dune ridges and occurs on a ‘terrace’ 2 m lower in elevation (at 5.5 m AHD) than the main foredune ridges. The area contains less distinct ridges which appear to spread out in a fan shape in a northerly direction. The timing of deposition of this sequence occurs at the end of the Last Interglacial when sea level was 20‒30 m lower than the peak of the highstand. It has been preliminarily interpreted to be an estuarine shoreline forming in a lake environment. This phase of deposition has only recently been discovered and not previously described elsewhere in the Gippsland Lakes.

Dune reworking during the Last Glacial Period, 60,000–20,000 years ago (MIS 3–2)

Between the interglacial periods, the climate of Gippsland was colder, drier and windier than today. Winds blew from a mostly westerly direction. As sea level was up to 120 m lower than present – during the coldest peak of the Last Ice Age – a land bridge connected the Gippsland region to Tasmania with Bass Strait being dry and above sea level (Fig. 3).

The beaches and dunes of the Prior and Inner Barrier were stranded above sea level during this time and subjected to reworking by wind. A lower cover of vegetation occurred due to the harsher climate (Hill and Bowler 1995). This meant the dunes formed previously at the coast could now be remobilised. This led to the creation of blow-outs and parabolic dunes, U-shaped dunes features that are oriented along the average wind direction (generally westerly). These dunes are now stable and are most prominent along the Inner Barrier at Sperm Whale Head (Fig. 4). These dunes were most active around 18,000–20,000 years ago, with additional evidence of dune activity in the Prior Barrier occurring at around 40,000–50,000 years ago (Bryant and Price 1997; Oliver et al. 2018).

The period of dune reworking is not unique to the Loch Sport region of the Gippsland Lakes. Dunes of this orientation have been described around the margins of Corner Inlet and along the Yanakie Peninsula (Hill and Bowler 1995). These have all been inferred to be active during the peak of the last Ice Age and therefore indicate regional aeolian reworking around this time. This phase of reworking ended when the climate began to warm, and sea level began to rise to present elevations (Oliver et al. 2018; Kennedy et al. 2020).

Holocene (Outer Barrier), 10,000 years ago – today (MIS 1)

The Outer Barrier encompasses Ninety Mile Beach and the dune systems immediately behind it (Fig. 3). It is part of a continuous littoral drift system that extends from the entrance to Corner Inlet northeast of Wilson’s Promontory all the way to Mallacoota near the New South Wales border. This discussion will, however, focus on that part of the barrier where dating has been undertaken and therefore the areas where the evolutionary history is most well constrained (Kennedy et al. 2020). This area extends from McLoughlins Beach to Lakes Entrance.

During the Holocene, sea level rose quickly from its lowest elevation of c. −120 m around 18,000 years ago reaching modern elevations by around 7000 years ago. Sea level history in the Gippsland region since then is best defined by an envelope as indicators of higher or lower positions can reflect a range of geomorphic processes (Kennedy et al. 2020). In other parts of Victoria there are indications that it may have been higher, e.g. estuarine muds (Kennedy et al. 2021) and raised notches (Baker et al. 2001; Gardner et al. 2006). Quantification of the presence, extent, and height of sea level within the Holocene remains an unanswered but fundamentally critical question to be solved for understanding the response of Victoria’s coast to global warming.

The first part of the outer barrier to form began as islands around 9500 years ago at Rotamah Island (Fig. 5). This represents the earliest known deposition on the coast during the Holocene. As sea level had not yet stabilised, the island was likely transgressive in nature, moving landward as the shoreline migrated inland. It likely looked similar to the active islands at the mouth of modern-day Corner Inlet. Soon after sea level stabilisation, further offshore islands formed in what is today the Golden Beach area. The source of sand for these offshore islands was likely the shallow marine area immediately seaward.

Fig. 5.

A schematic model for the evolution of the Holocene barrier. The first phase of evolution saw island accretion in the northern part followed by a long barrier island fronting the pre-Holocene barriers. A final phase of evolution was related to an input of sediment from the southwestern end of the barrier which infilled the tidal entrance in the north and created the Bunga Arm enclosing the earliest Holocene beach barrier within the Gippsland Lakes (Sourced from Kennedy et al. 2020 and reproduced with permission – Elsevier licence # 5660121063809).

These offshore islands continued to build vertically until around 6500 years ago, before starting to extend in a southerly direction until 3000 years ago. This extension led to the formation of the lake system seaward of Loch Sport. The lakes remained open at this time, with a free connection to the open ocean through a large tidal channel located at the present day Loch Grange. Rotomah Island and others in the vicinity were still isolated sandy islands exposed to the open ocean (Fig. 5).

The next major change occurred after 3000 years ago, it appears that infill of Corner Inlet (details of which are unknown) reached a stage where sand could be transported northwards along the Outer Barrier (Fig. 5). The beach ridges at McLoughlins Beach rapidly built in a 500-year window between 2000 and 2500 years ago. Sand moved quickly along Ninety Mile Beach, infilling the entrance at Loch Grange and then forming another barrier seaward of Rotomah Island leading to the creation of the Bunga Arm. This resulted in a natural entrance, near where Lakes Entrance is today, as the only connection between the Gippsland Lakes and the open ocean.

A final phase of dune mobility has occurred in the last 200 years. Reworking appears to have occurred along the seaward edge of the entire beach system with the barrier rolling over in a landward direction. Fresh sand is observed to bury older soil surfaces along the entire front section of the Outer Barrier. The timing of this phase of mobility, in the last 200 years, suggests it is a result of changed land management from 1850s up to the 1980s. All these areas are now broadly stable due to the removal of stock and introduction of Marram Grass (Ammophila sp.), as well as changed fire patterns.