Modern sediments and stratigraphy of Cockburn Sound, Western Australia

Summary

This study assessed the geological evolution and the modern sedimentary environments of Cockburn Sound based on a suite of surface samples, cores and sub-bottom profiles. Sediments are predominantly made up of calcium carbonate, with minor quantities of quartz sand and mud in the deeper main basin. Seabed sub-surface data show that during the last ice age, when sea level was lower, the bed of Cockburn Sound was exposed, and soil formed on the surface of the Tamala Limestone bedrock. A river also flowed through the area at this time. Around 9500 years ago the basin that forms the sound was submerged by the rising sea and marine biogenic carbonate sediments started to accumulate. Sediment accumulation was initially rapid and most of the sound was covered in several metres of sediment. Trace element analysis of the surface sediments show modern terrestrial sediment input and industrial contamination in the southeast of the sound near Kwinana Beach.

Cockburn Sound

Cockburn Sound is a large low-energy coastal waterway located south of Fremantle, Western Australia. It sits in a depression between the mainland and the lee of Garden Island. The sound is 15 km long and ranges from 9 km to 6.5 km wide, an area of approximately 124 km2. The large low gradient central basin slopes from a depth of 17 m to a maximum of 22 m, and is flanked by relatively steep slopes. The Eastern Shoal is a relatively planar feature with an average depth of 8 m and several isolated reefs on its western margin. In summer circulation in the sound is wind driven by the onshore southwesterly winds, while in the winter and spring it is driven by atmospheric pressure systems with infrequent storms. The exchange of water between the sound and the open ocean is restricted by Parmelia Bank to the north and a narrow inlet in the south, consequently flushing times are between 22 days in winter, and 44 days in summer. Cockburn Sound has contrasting shoreline environments with heavy industry along much of its eastern coast, whereas the western shoreline of Garden Island is largely undeveloped, apart from the naval base at Careening Bay.

Detailed bathymetry and topography for Cockburn Sound.

Figure 1. Detailed bathymetry and topography for the Cockburn Sound region. The mouth of the Swan River and Fremantle can be seen in the north. Dredged channels are clearly evident in Parmelia and Success Banks to the north of the sound.

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Seabed sediments and environments

There are three main sedimentary environments in Cockburn Sound. In the central basin the sediments are mainly fine with a mud content that increases in the south and southwest. Video transects show localised isolated shell deposits and patches of unidentified macroalgae in this relatively deep environment. The sand banks and shoals on the margin of the sound support sea grass meadows, which comprise relatively coarse sediment with 80-90% calcium carbonate. Shell gravel (fragments and whole shells) is common within this sea grass habitat. Small patches of exposed limestone reef are evident on the western edge of the Eastern Shoal (Figure 4).

Grainsize and calcium carbonate content for surface samples in Cockburn Sound.

Figure 2. Grainsize and calcium carbonate (CaCO3) content of surface samples in Cockburn Sound. Muds dominate the Central Basin, while sand and gravel are found on the margins of the basin.

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Trace elements

In Cockburn Sound proportions of trace elements (e.g. the Nd/Sr ratio) can be used to identify terrestrially-derived sediment (Figure 3). Neodymium (Nd) is derived almost exclusively from terrestrial sources, whereas strontium (Sr) is abundant in seawater and found in appreciable concentrations in marine carbonates. The pattern of Nd/Sr ratios in the seabed suggests the input of fine terrestrial sediment from industrial sources at Kwinana Beach, adjacent to the Eastern Shoal, and possibly Jervoise Bay. These sediments also have relatively high concentrations of some metals (e.g. Zinc), and their distribution pattern suggests a possible anticlockwise transportation pathway for sediment (Figure 3). Isolated sites where metal concentrations were elevated above environmental guidelines were also identified in the study (Copper, Nickel and Zinc) and could represent sites where dredge spoil or other debris has been dumped.

Figure showing Nd/Sr ratio and the distribution of Zn in parts per million (ppm).

Figure 3. Left: Nd/Sr ratio in the surface sediments indicates the input of terrestrial sediment from Kwinana Beach. The location of the surface samples analysed is shown by black triangles. Right: the distribution of Zn in parts per million (ppm) shows a similar pattern to the Nd/Sr ratio.

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Sub-surface sediments and geological evolution

Twelve cores were collected from Cockburn Sound from which cross-sections were constructed (Figure 5). The Tamala Limestone is present at the base of several cores and forms the bedrock beneath the sound. Overlying this limestone is sandy clay, yellowish-red in colour. This is probably the soil that formed on top of the limestone when Cockburn Sound was exposed during the last ice age, when sea level was lower. Immediately overlying the red soil is a sediment unit that contains many shells, both whole and fragmented, in a matrix of sandy carbonate mud. These shells are up to 9500 years old and record the initial flooding of Cockburn Sound in the early Holocene as sea level was rising. Radiocarbon ages of shells in the cores were used to calculate sediment accumulation rates. There was relatively rapid accumulation immediately after the initial marine inundation, approximately 0.06 cm per year. This rate of accumulation has decreased over time to as little as 0.008 cm per year in the central basin. 

Acoustic sub-seafloor profiles show the ancient weathered surface of the Tamala Limestone (Figure 4) and a buried channel structure in the middle of the sound. The profiles show that during the last 9500 years an average of 2.5 m of sediment has accumulated in the centre of the sound, while over 5 m of sediment has been deposited as carbonate banks on the northern and southern margins.

Two sub-bottom profiles of Cockburn Sound. A) The Central Basin showing the ancient river channel;  B) Small reef on the western margin of the Eastern Shoal.

Figure 4. A sub-bottom profile of Cockburn Sound showing a small limestone reef on the western margin of the Eastern Shoal, and Holocene sediment above the Pleistocene reflector.

 

Stratigraphic cross sections for northern (A-A') and central (B-B') Cockburn Sound. Sample 1: carbonate sand with seagrass fibre; Sample 2:  basin sandy mud; Sample 3:  the contact between gravelly shelly mud and the Pleistocene clay unit; and Sample 4:  Tamala Limestone calcarenite gravel.

Figure 5. Stratigraphic cross sections for northern (A-A') and central (B-B') Cockburn Sound. Sample 1: carbonate sand with seagrass fibre; Sample 2: basin sandy mud; Sample 3: the contact between gravelly shelly mud and the Pleistocene clay unit; and Sample 4: Tamala Limestone calcarenite gravel.

View the 3D model of Cockburn Sound

Further information from the coastal CRC.

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