Geology and landforms of Two Peoples Bay Nature Reserve, Western Australia

Two Peoples Bay at the heart of east Gondwana

The tectonic history of the Albany−Fraser Orogen records the period of amalgamation and stabilisation of the Columbia−Nuna Supercontinent from 2200 to 1450 Ma, before its break-up and re-assembly into the Rodinian Supercontinent from 1400 to 900 Ma (Johnson 2013, 2021; Gorczyk et al. 2024). The West Australian Craton is an amalgamation of the Archean Yilgarn and Pilbara cratons along the intervening Proterozoic Capricorn Orogen during the assembly of Columbia−Nuna (Fig. 4). Along the West Australian Craton’s south-eastern edge, NW−SE-directed crustal extension prevailed from ~1800 Ma, reworking the Archean crust of the Yilgarn Craton as basement within the Albany−Fraser Orogen (Munglinup Gneiss on Fig. 2; Spaggiari et al. 2014; Smithies et al. 2015a). After 1600 Ma, this event first formed the Barren Basin, and then the succeeding Arid Basin (Spaggiari et al. 2015). A switch to convergence brought together the West Australian Craton (including the Albany−Fraser Orogen) with the combined South Australian Craton and the Mawson Craton, which forms the opposing part of the Antarctic continent to Australia, across the Southern Ocean (Fig. 4). Greater India was attached to Australia along the Pinjarra Orogen on Australia’s western coast, the eastern edge of which is marked by the Darling Fault.

The 1200−1140 Ma Maralinga Event, with very high crustal temperatures, was centred on the Musgrave Province in central Australia (Smithies et al. 2015b). It produced melting of the lower crust and the underlying mantle, generating large volumes of predominantly felsic magma that intruded into the upper crust across the Musgrave Province, and the Rudall Province, the Madura and Coompana provinces and the Albany−Fraser Orogen. This effectively cratonised that region and froze into place the craton configuration at that time. Subduction had started around 1415 Ma with the development of the Loongana arc and continued until ~1260 Ma, by which time ‘soft’ docking, rather than a full continent−continent collision between the West Australian Craton and the South Australian Craton (Gorczyk et al. 2024) had put the Albany−Fraser Orogen at the heart of eastern Gondwana (Fig. 4).

Mesoproterozoic metamorphic and igneous rocks in and around the Two Peoples Bay Nature Reserve

The oldest rocks exposed in Two Peoples Bay Nature Reserve and its surrounding area are within the western Nornalup Zone of the Albany−Fraser Orogen (Fig. 2). Deformed layered granitic orthogneiss have been mapped including coarse-grained microcline augen gneiss, fine- to medium-grained granoblastic-textured granitic gneiss, and quartz−feldspar−biotite(−garnet−hypersthene) gneiss (Fig. 6a) (Stephenson 1973a, 1973b; Muhling and Brakel 1985). The presence of hypersthene is indicative of high-grade, granulite facies metamorphic conditions, and amphibolite to mafic granulite layers are present.

Fig. 6.

(a) Outcrop (1.5 m high) of layered granitic gneiss on Normans Beach; Maardjitup Gurlin/Mount Gardner can be seen in the right background, across Two Peoples Bay. (b) Typical rounded tor-like outcrop of porphyritic monzogranite east of Yilberup/Mount Manypeaks, looking towards Mermaid Point. Potassium feldspar phenocrysts can be seen as rectangular, white patches in weathered rock surface. Grass tree is 2.0 m high. (c) Coastal outcrop at Little Beach of undeformed porphyritic monzogranite intruding deformed and metamorphosed, gneissic inclusions of country rock, people for scale. (d) Close up of contact between porphyritic monzogranite and layered gneiss; penknife is 9 cm in length (photographs IM Tyler).

Pidgeon (1990) obtained a U−Pb zircon age of 1289 ± 10 Ma from a sample referred to as ‘enderbitic gneiss’ that was collected near Middleton Beach, east of Mount Clarence in the town of Albany. The rock was described by Stephenson (1974) as a coarse-grained, dark green-grey, granoblastic, orthopyroxene-bearing granodiorite, and is therefore a charno-enderbite of the charnockitic series (Le Maitre et al. 2002). This sample is regarded as equivalent to the gneisses mapped around Two Peoples Bay Nature Reserve (Muhling and Brakel 1985), and the age is consistent with orthogneiss intruded as a granitic magma into granulite facies metamorphic conditions during the 1330−1260 Ma Albany−Fraser Orogeny Stage I.

Porphyritic monzogranite forms the Maardjitup Gurlin/Mount Gardner intrusion within the Reserve, which was referred to by Stephenson (1973a) as the ‘Mount Gardiner Adamellite’ (the granite rock name ‘adamellite’ is now obsolete, with the term ‘monzogranite’ preferred; Le Maitre et al. 2002). Muhling and Brakel (1985) did not adopt Stephenson’s name for the intrusion, or for its continuation along the coast, and its usage remains informal. The porphyritic monzogranite has abundant megacrysts of microcline in a groundmass of quartz, microcline, plagioclase (oligoclase/andesine), and biotite (Fig. 6b). The rock is generally homogeneous, without layering, but it does grade into foliated to gneissic zones, consistent with localised, post-intrusion shearing and local flattening. Minor micromonzogranite dykes up to several metres wide are present, locally with microcline megacrysts. Some minor pegmatite and quartz veins intrude the granite at several localities (Stephenson 1973a). The earlier gneisses occur as country rock xenoliths, well seen at the north-west end of Little Beach (Fig. 6c, d). Muhling and Brakel (1985; see also Stephenson 1973a) also noted wide zones along the intrusion margins they referred to as ‘migmatite’ where the gneisses are intruded by veins, dykes, patches, and lit-par-lit sheets of porphyritic monzogranite.

Pidgeon (1990) dated a sample of the related ‘Albany Adamellite’ taken from a quarry developed in biotite monzogranite with aligned K-felspar megacrysts (Stephenson 1972) on Mount Melville, west of Albany. A U−Pb zircon age of 1177 ± 4 Ma was obtained and is interpreted as the age of crystallisation of the zircon and is therefore the age of intrusion of the porphyritic monzogranite. Black et al. (1992) obtained similar ages (1180 ± 12 Ma; 1184 ± 11 Ma) from samples of the ‘late tectonic’ Porongurup granite (Fig. 5). This indicates that the granitic intrusions are members of the Esperance Supersuite, intruded during the 1200−1140 Ma Albany−Fraser Orogeny Stage II. There is a time gap of over 110 million years between the formation of the early, deformed and layered granitic gneisses and the later, intrusive porphyritic granite, which is consistent with the described intrusive margins to the intrusions and the presence of the gneisses as xenoliths in the porphyritic granite, rather than in situ melting in a single event as suggested by Stephenson (1973a).

From the aeromagnetic geophysical imagery (Fig. 5), the distinctly magnetic porphyritic granite intrusion can be interpreted as a series of linked bodies forming a discontinuous, sinuous sheet extending along the coast from Bald Island south of Cheynes Beach at least to Torbay, east of Denmark. The Porongurup granite is similarly magnetic (Fig. 5).

Cenozoic regolith landforms and landscape evolution in and around Two Peoples Bay Nature Reserve

During the Oligocene and Miocene, the western Esperance Shelf was emergent above sea level due to differential uplift produced by Australia’s tilt. In the area around Two Peoples Bay, laterite formed during deep weathering and is massive to pisolitic, sandy, and ferruginous. It is exposed principally in the north-eastern part of the mapped area (Fig. 1), overlying Proterozoic granite and sandy soils over the Plantagenet Group. This period of laterite formation is coincident with the warming climate of the ‘Miocene Climatic Optimum’.

With continued tilting extending into the eastern Eucla Basin, uplift raised the late Miocene sea floor to ~150 m to form the emergent Nullarbor Plain. In the western Esperance Shelf and Scadden Embayment, prominent late Eocene wave-cut benches are seen at 260−280 m around hills that would have formed islands, including Mount Ragged and Mount Barren (fig. 14 in Cockbain 2014).

At a smaller scale, the tilting of the Australian continent also produced an intermediate wavelength (~100 km) deformation. This is identified in the western part of the Eucla Basin where Eocene sediments lie at significantly lower elevations than equivalent sequences in the central Eucla Basin and in paleodrainages in the Norseman−Kalgoorlie region. This reflects down warping southwards from the east–west Jarrahwood Axis of the associated Ravensthorpe Ramp (fig. 29 in Cockbain (2014)). In the Two Peoples Bay area the effect is that the base of the Plantagenet Group, and the equivalent Eocene sea floor remained close to present sea level, as is seen at Herring Beach. Immediately inland of Two Peoples Bay, the upper lateritized surface of the Pallinup Formation is around 50 m above sea level and up to 100 m around granite outcrops (Fig. 1). This is the lower end of a slope identified by Muhling and Brakel (1985) that rises gently to the base of the Stirling Range (elevation ~200 m). It comprises residual white sand and laterite overlying the Plantagenet Group, on which lie numerous lakes and swamps. Monadnocks of fresh granite and gneiss include the Porongurup Range (up to 670 m). The local Kalgan River has incised into the surface (Fig. 5).

The late Pliocene and the Pleistocene were marked by alternating periods of aridity and humidity due to the cycles of glacial and interglacial periods. Since the Last Glacial Maximum at 21−19 ka (thousands of years ago), sea level has risen ~125 m to its present level (Lewis et al. 2013).

The middle and late Pleistocene Tamala Limestone (‘coastal limestone’) fringes the Western Australian coast from Shark Bay round to Esperance as lithified shoreline and dune sands, typically associated with inland swamp and lacustrine deposits, and by cave formation (Playford et al. 1976; Cockbain 2014).

In Two Peoples Bay Nature Reserve, calcareous quartz sandstone of Pleistocene to Holocene age outcrops prominently along the coast from Sinker Reef to west of Rocky Point (Figs 1 and 7a). It is an eolian deposit, strongly cross-bedded, and is interpreted here as the stratigraphic equivalent to the Tamala Limestone. The unit includes many root structures (rhizoliths) formed by calcification around roots of trees and shrubs that grew on the original dunes. Fossil land snails (Bothriembryon) are common in the sandstone.

Fig. 7.

(a) Sinker Reef, looking east to Maardjitup Gurlin/Mount Gardner (408 m) and False Island (see Fig. 1). An elevated, probably Holocene shoreline platform is about 3.5 m above the adjoining wave-washed, modern platform. The photographer is standing on the cliff above, which is composed of calcareous quartz sandstone, a lithified Pleistocene eolianite deposit equivalent stratigraphically to the Tamala Limestone. This represents the accumulation of sand shoals and dune sands that ‘tied’ Maardjitup Gurlin/Mount Gardner to the mainland during the Pleistocene and Holocene. (b) Kaiupgidjukgidj/Moates Lake, glimpsed through fringing vegetation in swampy peaty sands, has a large blow-out with mobile dunes to the south (see Fig. 1). Holocene parabolic sand dunes, previously stabilised by vegetation are present further to the south and east (photographs KA Evans).

The maximum exposed thickness of the unit is about 60 m at the eastern end of the Sinker Reef cliff. Here the sandstone can be seen to abut a steeply sloping unconformity on the granite of Maardjitup Gurlin/Mount Gardner (Fig. 7a). The sandstone extends below sea level; therefore, its total thickness is unknown.

Residual quartz sand of late Pleistocene to Holocene age overlies the calcareous sandstone over a large part of the area west of Maardjitup Gurlin/Mount Gardner, extending along valleys in the granite to elevations of up to 200 m. The sand is interpreted to be a product of decalcification through weathering of the Pleistocene calcareous sandstone (W. M. McArthur, G. A. Bartle, K. A. Evans, I. M. Tyler, unpubl. data). Small patches of the original sandstone can still be seen in places below the sand (e.g. in the valley behind Waterfall Beach, Fig. 1).

Some laterite formation must also have taken place in the Pleistocene or Holocene, as it is present within the Pleistocene dune sandstones in the southwestern part of the Reserve (Fig. 1).

The Two Peoples Bay area is dominated by Maardjitup Gurlin/Mount Gardner, a mass of granite which rises to 408 m above sea level (Fig. 1). The granite coastline flanking the east, north, and south sides of Maardjitup Gurlin/Mount Gardner is very steep, with deeply incised streams descending to the sea. It shows elements of a drowned shoreline produced by sea level rise from melting ice over the 20 ka since the Last Glacial Maximum, with indentations on the coast controlled by the inundated stream valleys, and small offshore islands marking the crests of drowned hills. A similar coastal topography is found in the Djilliyiranup/North Point area, north of Two Peoples Bay.

The main granite mass around Maardjitup Gurlin/Mount Gardner is interpreted to have formed an island, and it was ‘tied’ to the mainland during the late Pleistocene and Holocene by the accumulation of sand shoals and dune sands, now largely lithified as calcareous sandstone.

The coastline west of the Maardjitup Gurlin/Mount Gardner granite mass is backed by cliffs up to 50 m high, composed of calcareous quartz sandstone, which is a lithified Pleistocene eolianite deposit regarded here as the stratigraphic equivalent to the Tamala Limestone. West of Rocky Point the cliff is fronted by a sandy beach, whereas at Sinker Reef there is little beach sand, and the cliff is fronted by an elevated shoreline platform that is about 3.5 m above the adjoining modern platform (Fig. 7a). The elevated platform is relatively little eroded and is presumably of Holocene age. Emergence of this platform is likely to be a result of ongoing tectonic uplift related to continental tilting, rather than a real drop in sea level. It is possible that the platform correlates with elevated platforms extending along the coast to the 2.4 m elevated platform at Rottnest Island, which emerged about 4.8 ka ago (Playford 1988).

Two Peoples Bay itself is bounded to the west by a wide beach, backed by beach ridges, dune sands, and some intervening swamps (Fig. 7b; W. M. McArthur, G. A. Bartle, K. A. Evans, I. M. Tyler, unpubl. data). Miialyiin/Angove, Tyiurrtmiirit^y/Gardner, and Kaiupgidjukgidj/Moates Lakes have been dammed behind late Pleistocene, Holocene and modern dunes. The lakes are fringed by peaty sands marking earlier extensions of the lakes and their associated swamps. Most of the Holocene sand dunes can be seen to be parabolic in form in aerial photography (viewed from the Landgate Aerial Imagery layer on GSWA’s GeoVIEW.WA) and are oriented consistent with a prevailing wind from the W-SW, along the coast (Hesp 2024). They are now stabilised by vegetation, but there is a large blow-out with mobile dunes south of Kaiupgidjukgidj/Moates Lake (Fig. 7b; W. M. McArthur, G. A. Bartle, K. A. Evans, I. M. Tyler, unpubl. data), and some coastal dunes are active near Rocky Point. Several Holocene deposits have been mapped as beach sand, beach ridges, mobile and fixed dune sand, and lake and swamp deposits. Most consist of calcareous quartz sands, except for some swamp and lake deposits, which are commonly peaty and non-calcareous.

The residual sands formed by leaching of the Pleistocene eolian calcareous quartz sandstones that overlie Proterozoic granite on the flanks of Maardjitup Gurlin/Mount Gardner are effective reservoirs for groundwater, feeding small permanent springs and seepages. Consequently, many of the creeks draining Maardjitup Gurlin/Mount Gardner flow throughout the year, although the rate of flow is generally very small during the summer. The presence of this permanent water, with consequent dense vegetation, is perhaps one of the reasons why the Djimaalap/noisy scrub-bird (Atrichornis clamosus) was able to survive in this small area.