Wolfgang Preiss
Visiting Honorary Research Fellow, University of Adelaide, and Geological Survey of South Australia, Department for Energy and Mining

Download this article as a PDF (31.8 MB); cite as MESA Journal 91, pages 33–50
Published December 2019

The Wilmington Formation is overlain by the Elatina Formation. At the base is a reddish brown, fine- to medium-grained sandstone containing trains of red lithic granules. This is overlain by the Reynella Siltstone Member, a dark red, poorly bedded siltstone containing scattered red granules and rare small pebbles, and well-bedded siltstone intercalations, some of which show rhythmic laminations. There is limited exposure of these beds in Waterfall Creek (Fig. 10), but much better outcrops are found in the coastal cliffs north of the map area.

The Elatina Formation and its Reynella Siltstone Member, though only very sparsely pebbly, represent the Elatina glaciation (Williams et al. 2011) in the Adelaide region.

Wilpena Group

The Reynella Siltstone Member is overlain with a sharp, erosive contact by massive, reddish grey and brown sandstone and quartzite of the Seacliff Sandstone. This boundary is correlated with the base of the Wilpena Group in the Flinders Ranges (Thomson 1966; Dyson 1992). The sand was rapidly deposited under post-glacial transgressive conditions, and contains dewatering structures. Local thin red siltstone and shale interbeds are lenticular and intermittent.

Thinly bedded, pink to buff-coloured, fine-grained dolomite of the Nuccaleena Formation, commonly 1–10 m thick, occurs at the base of the Wilpena Group in the Flinders Ranges. At Hallett Cove, however, the dolomite occurs as lenses interfingering with the Seacliff Sandstone. The thickest of these dolomite units conformably overlies the Seacliff Sandstone at the north end of Black Cliff, where it grades up into thinly bedded, dark red siltstone of the Brachina Formation (Fig. 11).

The Brachina Formation was deposited as silt on the outer continental shelf at a time of highest sea level after post-glacial transgression. Interbedded, fine-grained sandstone beds up to 15 cm thick displaying flute casts on the soles of beds are interpreted as storm-modified turbidites (Fig. 12).

From these deepest water conditions, the basin gradually shallowed, culminating in the deltaic ABC Range Quartzite that crops out at O’Sullivan Beach, south of Hallett Cove (Fig. 1). These are the youngest Ediacaran rocks exposed near Hallett Cove, but red mudstone of the Bunyeroo Formation overlies the ABC Range Quartzite near Ochre Point. There are erratics of the quartzite in the late Paleozoic Cape Jervis Formation. The Wonoka Formation, Bonney Sandstone and Rawnsley Quartzite of the uppermost Wilpena Group in the Flinders Ranges are not represented near Adelaide.

After a major hiatus in deposition, represented by a major unconformity, sedimentation resumed in the Early Cambrian and continued until the start of Middle Cambrian time (c. 510 Ma), but these sediments have all been eroded from the Hallett Cove area.

Late Paleozoic: Carboniferous–Permian

In the latest Carboniferous to Early Permian, the supercontinent Gondwana lay in the south polar region; South Australia was covered by a continental ice sheet – the state’s third major glaciation. The unconformity between the Neoproterozoic rocks and the base of the Carboniferous–Permian glacial sediments represents a time gap of approximately 300 million years.

Cape Jervis Formation

Glacial sediments of the Cape Jervis Formation are preserved within a steep-sided, north-northwest-trending glacially eroded valley cut into the bedrock. The deepest part of the valley is filled with massive diamictite interpreted by Bourman and Alley (1990) as lodgement tillite (Fig. 13). It is exposed on Hallett Cove Beach where beach sand cover is very thin to absent, but its thickness is unknown. It has a matrix of greenish grey, sandy mudstone and a wide variety of pebbles and cobbles derived from bedrock outcrops both locally and further south. Two large erratics of fine-grained, cross-bedded sandstone (probably Wilmington Formation) have been eroded out of the tillite (Fig. 14). Other erratics include granite from Encounter Bay (Fig. 15).

The upper part of the glacial valley broadens out but did not extend beyond the artificial lake on Waterfall Creek and the Adelaide to Seaford railway line, where bedrock is exposed. This wider valley is floored by a polished and striated bedrock surface extending north along the clifftop from Black Cliff. The orientation of striae, asymmetric features such as crescentic gouges, and the inferred sources of glacial erratics, all confirm the general direction and sense of ice movement toward the north-northwest. The valley is filled with glaciolacustrine, thinly bedded, locally varved, weakly lithified, grey-green shale overlying the basal lodgement till. Bourman and Alley (1990) envisaged deposition in a lake dammed by ice against a bedrock slope to the east. Abundant dropstones in these sediments indicate the presence of floating ice. Prominent erratics include boulders of ABC Range Quartzite, Seacliff Sandstone, calcareous Marino Arkose Member, Wilmington Formation sandstone and Sturt Tillite, as shown on the geological map.

Sandstone interbeds up to 15 cm thick are commonly graded, suggesting deposition by turbidity currents (Fig. 16). Locally the shales display slump folds (Fig. 17). A medium- to coarse-grained massive sandstone, possibly aeolian, is associated with lenses of diamictite interpreted as flow tills (Fig. 18) within the glaciolacustrine shales (Bourman and Alley 1990). Some large boulders have been eroded from the flow till, e.g. limestone from the Marino Arkose Member (Fig. 19).

Normington et al. (2018) summarised the glaciation and deposition of the Cape Jervis Formation in terms of five stages: (i) formation of glaciated surfaces, (ii) deposition of lodgement till from ice advancing from the south, (iii) stagnation and retreat of ice and formation of glacial lakes, (iv) formation of a flow-till complex by further down-wasting and southerly retreat of ice, and (v) sea-level rise and marine incursion recorded at Cape Jervis.

Permian to Cenozoic

During the almost 300 million year time gap in the rock record at Hallett Cove after the late Paleozoic glaciation, major sedimentary successions of Permian to Cenozoic age were deposited in basins in the north of South Australia. The bedrock and Carboniferous–Permian sediments of the Mount Lofty Ranges underwent deep weathering. During the Late Jurassic rifting commenced along what is now Australia’s southern continental margin and at c. 96 Ma (Late Cretaceous), Australia separated from Antarctica. At c. 45 Ma (Middle Eocene), Australia began its more rapid northward motion. Eocene to Miocene sediments were deposited in the Adelaide – Golden Grove Embayment of the St Vincent Basin to the north and Noarlunga and Willunga embayments to the south. Hallett Cove, situated at the southwestern end of the Mount Osmond Fault Block that was uplifted during the Cenozoic along the Eden Burnside Fault (Preiss 2019b), occupied a peninsula between these embayments.

Pliocene

Hallett Cove Sandstone

It is uncertain whether the Eocene to Miocene sediments of the Noarlunga Embayment originally extended as far north as Hallett Cove. If they did, they were certainly removed by erosion by the time the sea transgressed onto the eroded fault block in the Pliocene, when the shelly, calcareous Hallett Cove Sandstone was deposited in the littoral zone. The sandstone contains fossils including bivalves such as oysters, and the large, multi-chambered foraminifer Marginopora. Many of the shells are preserved only as external moulds. The sandstone rests unconformably on the eroded surface of the Cape Jervis Formation (Figs 20, 21), but probably did not extend past the eastward limit of these easily eroded and partly exhumed glaciolacustrine sediments. The sandstone is not seen in contact with the bedrock high to the east.

Quaternary

The Quaternary Period is marked by repeated climatic cycles of glacial and interglacial episodes, but glaciation was restricted to relatively high latitudes. In South Australia, sediments are dominated by clayey, sandy and gravelly alluvial fan and floodplain deposits, slope deposits, and wind-blown sands and silts.

The Quaternary stratigraphy of Hallett Cove is well displayed in erosional features and comprises both Pleistocene and Holocene sediments. The earliest Quaternary is represented by lenticular beds of weakly iron-cemented pebble conglomerate of the Ochre Cove Formation. At Marino Rocks, these lenses have been observed overlying the Plio-Pleistocene Burnham Limestone, while on the clifftops at Hallett Cove, they overlie the Hallett Cove Sandstone or the Cape Jervis Formation where the Hallett Cove Sandstone is missing. The pebbles are typically hematite-stained siltstone or fine-grained sandstone derived from the Wilmington Formation. The base is not exposed, but is inferred to be erosional. Outcrops of Ochre Cove Formation lenses have been observed as far east as the lower parts of The Amphitheatre (Fig. 22), where they are overlain by red mottled sandy clay of the Hindmarsh Clay. Southeast of the Sugarloaf, sandy clay directly overlies the Cape Jervis Formation.

The Hindmarsh Clay contains a prominent, weakly indurated, white and red iron-mottled sandstone interbed forming an erosional ledge on the face of The Amphitheatre, and is overlain by the upper part of the Hindmarsh Clay (Fig. 23).

A subtle but persistent colour change in the clays is suggested as the boundary between the Hindmarsh Clay and the overlying Keswick Clay (Firman 1969; Sheard and Bowman 1998; Sheard et al. 2016). However, confirmation that the uppermost clay unit correlates with the Keswick Clay will require mineralogical analysis.

Ward (1966) defined a weakly consolidated, orange-brown, medium-grained, well-rounded sand at the Hallett Cove Beach railway station as the Ngankipari Sand. The extent of this unit is uncertain; although Ward claimed an extent as distant as Lonsdale, sands at these locations are completely unconsolidated, unlike the type Ngankipari Sand which has been affected by calcrete formation in the form of nodules, crusts and rhizomorphs. The Ngankipari Sand is tentatively considered to be Late Pleistocene.

The Late Pleistocene Pooraka Formation, which forms prominent alluvial fans along the Eden–Burnside and Para fault scarps, is represented at Hallett Cove only by local channel fills of sandy and pebbly clay with rubbly, nodular calcrete paleosols in the north of the map area. In 1992 fossil remains of the world's largest marsupial, Diprotodon, were discovered in similar alluvial sediments of the Field River, located inland from the southern end of Hallett Cove beach. In the latter part of the Late Pleistocene humans arrived in Australia, about 65,000 years ago (Clarkson, Jacobs and Pardoe 2017), but the age of the Aboriginal implements found at Hallett Cove is uncertain.

Calcrete paleosols are widely developed in South Australia (Dart et al. 2007). At Hallett Cove, a prominent calcrete profile occurs above the Pleistocene succession exposed in The Amphitheatre. The calcrete includes nodules and laminar sheets developed at the top of off-white calcareous silt of presumed aeolian origin. During cold, arid climatic episodes when sea levels were much lower, the continental shelves, including Gulf St Vincent, were largely exposed. Wind erosion of the Cenozoic marine carbonates on the shelves produced lime dust which was spread over the landscape, and subsequently underwent soil processes of dissolution and re-precipitation to form calcrete (Dart et al. 2007). The calcrete at Hallett Cove also caps sections of older units at lower elevations between The Amphitheatre and the coast, suggesting the formation of a seaward erosional slope after deposition of the Keswick Clay but before calcrete formation. East of the Carboniferous–Permian glacial valley, calcrete directly overlies bedrock.

Over large areas of the seaward-facing slope of Hallett Cove, the calcrete surface is covered with thin, patchy deposits of loose, well-sorted pale brown, fine-grained sand of wind-blown origin. Although there is no dating for this surficial sand, it is completely unconsolidated and not affected by calcrete formation; it may therefore be early Holocene. These surficial sands are incised by a number of gullies, the deepest of which contains Waterfall Creek. Holocene clayey, sandy and pebbly alluvium in these gullies is itself incised by currently active channels and their gravelly bedload. On steeper slopes, talus deposits of angular bedrock clasts are set in poorly sorted sandy and silty clay, some clast rich and some matrix dominated.

Figure 24 Beachrock formed by lime cementation of well-rounded gravel (a coarse facies of the Semaphore Sand Member) on the shore platform, overlying Cape Jervis Formation tillite. Staff 1.0 m long. (Photo 417991)

The present shoreline results from sea level rise after the last Pleistocene glaciation. In South Australia, shoreline deposits of this Holocene transgression are termed Saint Kilda Formation, represented at Hallett Cove by the Semaphore Sand Member. These well-sorted quartz sands patchily cover the shore platform of the actual cove, itself developed on softer tillite of the Cape Jervis Formation while the harder Neoproterozoic bedrock forms cliffs and shore platforms to the north and south. Pebbles, cobbles and small boulders derived from the cliffs cover the shore platform adjacent to the cliffs and are undergoing rounding by wave action. These gravels even extend beyond the cliffs some distance south of Black Cliff, and here a small patch has been cemented with lime to form beachrock, overlying the Cape Jervis Formation tillite (Fig. 24).

Most of the gentler slopes in the map area that are not undergoing active erosion have thin soil cover. These soils vary from pale brown sandy loam where developed on the surficial sands to clayey loams further inland, where they lie upon weathered bedrock or calcrete.

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Structure and tectonics

Delamerian Orogeny

At the end of Early Cambrian sedimentation at c. 510 Ma, plate convergence well to the east of South Australia led to the Delamerian Orogeny, which deformed the Neoproterozoic and Cambrian rocks of the Mount Lofty and Flinders ranges.

The strongly folded, faulted and cleaved Neoproterozoic strata at Hallett Cove occupy the west limb of a large-scale regional, south-southwest-plunging anticline with superimposed smaller scale folds. Overall younging is toward the west, as confirmed by cross-bedding in sandy layers. Folds are generally asymmetric, with axial planes and axial plane cleavage dipping to the east.

Minor folds also plunge to the south-southwest, so that the stratigraphy exposed in the cliffs also youngs to the south. The thick-bedded sandstones and quartzites of the Seacliff Sandstone are highly competent, and have responded to the compressive stresses of the Delamerian Orogeny by forming open, concentric folds and minor brittle faults. In contrast, the thinly bedded siltstones of the Brachina Formation formed tighter, chevron-style folds, as seen at the foot of Black Cliff (Fig. 25).

Several faults are exposed in the map area. The most obvious can be seen from the wooden stairs north of Black Cliff (Figs 26, 27), where a band of pink dolomite with red shale interbeds is faulted against quartzite of the Seacliff Sandstone to the west. This steeply dipping fault is difficult to trace laterally, as it passes northward under shale of the Cape Jervis Formation, while to the south it is largely covered by cobbles on the shore platform; beyond this point the movement appears to have been taken up on bedding planes within the Seacliff Sandstone. Two more faults exposed within the Seacliff Sandstone at the mouth of Waterfall Creek (Fig. 28) are brittle structures, characterised by narrow brecciated zones and abrupt changes of bedding orientation.

Belperio et al. (1990) used shallow marine seismic surveys to map the extension of the Eden–Burnside Fault offshore from Hallett Cove.

The presence of slickensided surfaces along some bedding planes in the Brachina Formation indicates bedding-plane slip during folding. There are also local discordances in bedding orientation exposed on the shore platform north of Black Cliff due to minor bedding-parallel faulting (Fig. 29). Other minor slip surfaces with thin quartz–chlorite veins along them cut across bedding (Fig. 30).

In contrast to rocks in the eastern Mount Lofty Ranges, which were highly metamorphosed in the Delamerian Orogeny, the Neoproterozoic strata at Hallett Cove, though strongly lithified, have been metamorphosed only at very low grade, probably reaching a maximum temperature of only about 300° C. Slaty cleavage is developed in the fine-grained siltstones of the Wilmington Formation and Brachina Formation, while the sandstone and quartzite units display only a crude spaced cleavage or no tectonic fabric.

Post-Delamerian tectonics and geomorphology

After the Delamerian Orogeny, the Delamerian mountains were actively eroded and, by the time of the Carboniferous–Permian, the present crustal level was already exposed at the surface. The Carboniferous–Permian glaciation affected large areas of South Australia and left widespread glacial deposits, but these have been preserved only in favourable sites such as tectonically depressed basins (Alley et al. 1995). The deposits at Hallett Cove form part of the Troubridge Basin, but they occupy only a minute remnant of the former extent of this basin. Much more extensive occurrences of the glacial sediments are found on Fleurieu Peninsula and southern Yorke Peninsula, and they have been mapped using seismic data across Gulf St Vincent (Party and Weste 1998). The preservation of glacial sediments at Hallett Cove, which is on an uplifted fault block elsewhere devoid of sediments of this age, indicates that they must occupy a deeply scoured glacial valley. Preiss (2019b) argued that the more extensive glacial deposits of southern Fleurieu Peninsula have been preserved due to post-Permian extensional movement on the Willunga Fault, prior to more recent compressional uplift on this structure.

The St Vincent Basin evolved during the Cenozoic at the same time, and by the same process of east–west compression of the Australian continent, as the Mount Lofty Ranges were being uplifted; the Mount Osmond Fault Block upon which Hallett Cove is situated was tilted southward (Preiss 2019b). The sea transgressed into the Noarlunga Embayment to the south and the Adelaide – Golden Grove Embayment to the north during the Eocene. Despite the uplift of this fault block, there was a coastline near Hallett Cove in the Pliocene when the Hallett Cove Sandstone was deposited in a littoral environment that occupied the partially exhumed late Paleozoic glacial valley, but did not extend far upslope inland. Post-Pliocene uplift raised the fault block, including Hallett Cove, to its present elevation.

The present undulating landscape inland from Hallett Cove resulted from fluvial incision of the Mount Osmond Fault Block during the Quaternary. Material eroded from the valleys was deposited as alluvial fans of the Pooraka Formation north of the Eden–Burnside Fault scarp. During times of low sea level in the Pleistocene cold, arid episodes, wind-blown lime silt blanketed the landscape and was largely converted to calcrete by soil processes. During the milder conditions of the Holocene, further gullies were eroded, cutting into the calcrete and older deposits, while sea level rose dramatically, reaching a maximum at c. 6 thousand years before present (ka) (Lambeck and Nakada 1990). This caused erosion of the coastline to its present position, forming high cliffs in the hard Neoproterozoic bedrock and forming coves where Carboniferous–Permian glacial valleys were partially exhumed. Hallett Cove is but one of these – others are found further south on Fleurieu Peninsula. Erosion of the Pleistocene clays and overlying calcrete crust formed The Amphitheatre and its steep-sided gullies, and continues to the present day.

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Geological history

The geological history of Hallett Cove began before the formation of its oldest exposed rocks. Initial rifting of the South Australian Precambrian Craton at c. 830 Ma led to the birth of the Adelaide Geosyncline, a complex of rift and sag basins in which a total thickness of 10–15 km of sedimentary and minor volcanic rocks, exposed throughout the Flinders and Mount Lofty ranges, accumulated over the next 320 million years.

The outcrops at Hallett Cove represent sedimentation only during the latter half of this period, commencing during the interglacial period between the two great Cryogenian glaciations of South Australia: the Sturt and Elatina glaciations. The preserved record of events is not continuous – there are major gaps between the mid-Ediacaran and the Early Cambrian, and from the Cambrian Delamerian Orogeny until the Late Carboniferous. Post-Permian sedimentation and tectonic events are recorded in other parts of South Australia, but can only be inferred at Hallett Cove, where the record resumes in the Pliocene, and continues, more or less punctuated, through the Quaternary Period. A summary of the key recorded events in earth history and the time periods they represent follows.

Neoproterozoic

c. 830–800 Ma

Initial rifting took place along northwest- to north-northwest-trending fractures; mafic dykes were intruded and lavas erupted; evaporitic mixed clastic–carbonate sediments of the Callanna Group were deposited in rift basins. The Adelaide region remained on the western rift shoulder during this initial stage and left no sedimentary record.

c. 800–700 Ma

North–south trending rift basins were formed; sand, silt and dolomite of the Burra Group of Torrensian age were deposited in fluvial, marginal marine and marine shelf settings; these are the oldest sediments in the Adelaide area.

c. 670–660 Ma

Sturt glaciation: glaciomarine diamictite (Sturt Tillite) and associated sand, silt and minor carbonates were deposited in a shelf setting.

c. 660–645 Ma

Post-glacial transgression led to deposition of late Sturtian carbonaceous and calcareous siltstone of the Tapley Hill Formation and the oolitic Brighton Limestone, followed by the early Marinoan Angepena Formation redbeds. These formations represent deposition under conditions of post-rift thermal subsidence.

c. 645–640 Ma

The beginning of events recorded by in situ rocks at Hallett Cove: an influx of coarse granitic detritus marks the onset of a transgressive–regressive sequence, the Wilmington Formation. Laminated grey-green shales above the uppermost arkose tongues of the basal Marino Arkose Member represent maximum flooding, while the overlying sediments coarsen upward to massive fine-grained sandstone, reflecting filling of the basin.

c. 640–635 Ma

The Elatina glaciation is well recorded in the Flinders Ranges. At Hallett Cove a thin, pale red-brown sandstone correlated with sandstone of the Elatina Formation contains lithic granule bands of possible fluvioglacial origin. This was followed by deposition of the dark red, massive to laminated Reynella Siltstone Member over an erosive surface. This unit contains tidal rhythmites, granules, irregularly dolomitised intervals and rare small glacial clasts, and broadly correlates with the glacial diamictites of the Elatina Formation in the Flinders Ranges.

c. 635–600 Ma

Post-glacial sea-level rise initiated the next transgressive–regressive sequence, the Sandison Subgroup. The base of the Seacliff Sandstone is a major sequence boundary, and correlates with the base of the Wilpena Group in the Flinders Ranges where dolomite and shale of the Nuccaleena Formation transgressively overlie the Marinoan-aged glacials. At Hallett Cove massive sands were deposited by grain flow in deepening water. During intervals of reduced sediment supply, lenticular silt and dolomite beds were deposited, the latter equivalent to the much more continuous Nuccaleena Formation. Maximum flooding is probably represented by the very fine grained, dark red siltstones at the gradational base of the Brachina Formation. The remainder of the Brachina Formation represents highstand deposits: siltstone with fine-grained sandstone event beds reflect renewed sand supply at or below storm wave base, passing up to shallow-water, cross-bedded, red sandstone seen south of Hallett Cove. The upward-shallowing succession is capped with clean, cross-bedded quartzite of the ABC Range Quartzite of deltaic origin. This is the origin of some quartzite erratics at Hallett Cove.

Cambrian

c. 530–510 Ma

Deposition of the carbonate-dominated Normanville Group and turbidite-dominated Kanmantoo Group on Fleurieu Peninsula. These deposits have been eroded from the Hallett Cove area.

c. 510–490 Ma

Northwest-directed thrusting and associated folding of the Delamerian Orogeny affected the Mount Lofty Ranges, Fleurieu Peninsula and Kangaroo Island, followed by sinistral transpression, folding and metamorphism in the eastern Mount Lofty Ranges where early- and late-syntectonic granite plutons and mafic dykes were intruded.

Ordovician to Late Carboniferous

c. 490–480 Ma

Intrusion of post-tectonic granites and extrusion or shallow intrusion of felsic porphyry (Mount Monster in the South-East). These, as well as the syntectonic granites, appear as erratics in the Carboniferous–Permian glacial deposits at Hallett Cove.

c. 480–305 Ma

No events are recorded at Hallett Cove. The Delamerian mountains were progressively eroded.

Late Carboniferous to Early Permian

c. 305–280 Ma

The Australian portion of Gondwana underwent continental glaciation as it passed through the south polar region. Major steep-sided glacial valleys were glacially sculpted into the landscape, including on Fleurieu Peninsula and Hallett Cove. The deepest parts of the valleys were filled with lodgement till. Glacial lakes formed in the upper, wider part of the glacial valley where varved clays with sand interbeds were deposited. Abundant dropstones indicate the presence of floating ice. Glacial erratics were derived from nearby outcrops as well as much further afield, e.g. granite from Encounter Bay and porphyry from Mount Monster.

Mid-Permian to Paleocene

c. 280–45 Ma

No events are recorded at Hallett Cove, but important deposits formed in Permian, Triassic, Jurassic and Cretaceous basins of northern South Australia. In the Jurassic and Cretaceous, thick sedimentary successions were deposited on Australia’s southern margin during and after separation from Antarctica. Deep weathering occurred and paleoplains were formed in the area of the present Mount Lofty Ranges.

Eocene to Miocene

c. 45–5 Ma

The St Vincent Basin evolved with concomitant uplift of the Mount Lofty Ranges; Hallett Cove was situated on a peninsula projecting into the St Vincent Basin, formed by upthrusting and southward tilting of the Mount Osmond Block, with the Adelaide – Golden Grove Embayment to the north and the Noarlunga Embayment to the south.

Pliocene

c. 5–2 Ma

With a rise in sea level in the Pliocene, the shoreline lay just east of Hallett Cove where the calcareous and fossiliferous Hallett Cove Sandstone was deposited in a littoral environment.

Pleistocene

c. 2 Ma – 130 ka

Continuing uplift on the Mount Osmond Block raised Hallett Cove above sea level; early to mid-Pleistocene gravels and sands of the Ochre Cove Formation, derived from the rising Mount Lofty Ranges, were deposited in fluvial channels. These were followed by floodplain and lacustrine clays and sands of the Hindmarsh Clay and the expansive Keswick Clay on the western slopes of the Mount Osmond Block.

c. 130–80 ka

The Late Pleistocene Pooraka Formation formed alluvial fans along the Eden–Burnside Fault Scarp and filled local erosional valleys within the uplifted Mount Osmond Block.

c. 80–30 ka

Humans arrived in Australia during this period.

c. 30–11 ka

During the last glacial phase, sea levels were very low. Cold, arid conditions prevailed and lime dust was swept from the exposed floor of Gulf St Vincent, blanketing the eroded landscape. Soil processes of dissolution and re-precipitation of lime formed a hard surficial calcrete crust.

Holocene

c. 11–0 ka

With deglaciation, sea level rose, flooding Gulf St Vincent, and wave action cut the sea cliffs and shore platforms we see today. Streams such as Waterfall Creek cut channels into the landscape, aeolian sand was blown over the calcrete surface, and talus covered eroding slopes. Finally, sandy and clayey loam soils formed on the gently undulating upper parts of the landscape.

The future

The long-term future of the remarkable geological features preserved at Hallett Cove is not secure. Erosive forces of nature have exposed the rocks and the record they contain. Even without further human interference, these same forces will continue their attack. The glaciated pavement exposed only along a narrow clifftop probably extends some distance beneath the cover of soft, easily eroded glaciolacustrine shales and sandstones, but is limited to the east by the eastern side of the glacial valley. At the same time, the cliffs are facing constant attack from storm waves, abetted by instabilities in the rock caused by bedding planes, cleavages, faults and fractures. The landform features known as The Amphitheatre and the Sugarloaf also owe their origin to erosion. The relatively hard calcrete capping protects the underlying soft Pleistocene clays and sands of The Amphitheatre, but continued erosion of the clays by gullying tends to undercut the capping, and eventually it will all give way. In a few thousand years, all the soft late Paleozoic and Quaternary sediments will likely be gone, leaving a cliffline cut well back into the highly indurated Neoproterozoic rocks. Renewed uplift on the Eden–Burnside Fault can only accelerate these entirely natural processes. Global warming will raise sea levels, further exacerbating the attack on the coastal cliffs.

We are fortunate to be living in this narrow time window, with enough erosion having occurred to expose the wonders of Hallett Cove, but not yet sufficient to wipe these fragile relicts from the face of the map.

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Acknowledgements

The many and varied contributions of all previous investigators of Hallett Cove geology are gratefully acknowledged, as they form the foundation upon which the present interpretations are based. I thank Carmen Krapf and Stacey Curtis for their helpful comments on the geological map, and Neville Alley and Elinor Alexander for their thoughtful reviews of the manuscript.

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