The Geomorphology and Sediments of Cockburn Sound
The Geomorphology and Sediments of Cockburn Sound
The Geomorphology and Sediments of Cockburn Sound
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<strong>The</strong> <strong>Geomorphology</strong> <strong>and</strong> <strong>Sediments</strong> <strong>of</strong> <strong>Cockburn</strong> <strong>Sound</strong><br />
Discussion<br />
SURFACE SEDIMENT FACIES<br />
Based on the surface sediment laboratory data, field descriptions <strong>and</strong> video transects <strong>of</strong> the seabed,<br />
four distinct surface facies were identified in <strong>Cockburn</strong> <strong>Sound</strong> (Table 7). <strong>The</strong> areal distribution <strong>of</strong><br />
these deposits is shown in Figure 10. Because these <strong>and</strong> the associated sediment data (Figs. 6–11)<br />
are georeferenced <strong>and</strong> incorporated in ArcInfo shapefiles they represent reliable maps <strong>of</strong> surface<br />
sediment properties <strong>and</strong> can now be compared with the acoustic multi-beam backscatter data that<br />
was recently collected in the <strong>Cockburn</strong> <strong>Sound</strong>. A summary <strong>of</strong> the various facies characteristics is<br />
provided in Table 7.<br />
Table 7: Summary <strong>of</strong> the physical <strong>and</strong> chemical characteristic <strong>of</strong> the surficial sediment types in<br />
<strong>Cockburn</strong> <strong>Sound</strong>.<br />
FACIES<br />
Eastern<br />
Nearshore<br />
Eastern<br />
Shoal<br />
Carbonate<br />
Banks<br />
Central<br />
Basin<br />
SEDIMENT<br />
COMPOSITION<br />
Gravelly shelly<br />
mixed<br />
carbonate<br />
quartz s<strong>and</strong><br />
Carbonate<br />
Muddy S<strong>and</strong><br />
Well sorted<br />
Carbonate<br />
S<strong>and</strong><br />
Carbonate<br />
S<strong>and</strong>y Mud<br />
MEAN<br />
GRAIN SIZE<br />
& SD (mm)<br />
%<br />
CaCO3<br />
FINE FRACTION<br />
GEOCHEMISTRY<br />
(55% High Nd/Sr; High Zn,<br />
Ni; Low Cu<br />
0.12 - 0.43 >80% Low Nd/Sr; Mod Zn;<br />
Low Cu, Ni<br />
0.25 - 0.43 ~90% Low to mod Nd/Sr;<br />
Mod Zn; High Cu,<br />
Ni<br />
0.023 - 0.12 ~80% Low to mod Nd/Sr;<br />
Low Cu, Zn, Ni<br />
LITHOSTRATIGRAPHIC<br />
UNIT<br />
Safety Bay S<strong>and</strong><br />
Becher S<strong>and</strong> (Semeniuk<br />
<strong>and</strong> Searle, 1985)<br />
Parmelia Bank <strong>and</strong><br />
Southern Flat Shells<strong>and</strong><br />
Bridport Calcilutite<br />
(Semeniuk <strong>and</strong> Searle,<br />
1987)<br />
<strong>The</strong> terrigenous component <strong>of</strong> the Eastern Nearshore quartz s<strong>and</strong> (gravely shelly mixed<br />
carbonate/quartz s<strong>and</strong>) is probably reworked from older deposits while the carbonate component is<br />
composed <strong>of</strong> the skeletal remnants <strong>of</strong> contemporary marine carbonate producing organisms. <strong>The</strong>se<br />
sediments are part <strong>of</strong> the beach unit <strong>of</strong> this region known as the Safety Bay S<strong>and</strong> (Passmore, 1970).<br />
<strong>The</strong> exact extent <strong>of</strong> this sediment type is unknown as it was only recovered in one sample (top <strong>of</strong><br />
core CSV3) <strong>of</strong>f Kwinana Beach (Fig. 10). Passmore (1970) reported that almost the entire l<strong>and</strong><br />
surface <strong>of</strong> the Rockingham <strong>and</strong> Peron Peninsulas, as well as the modern shallow-marine <strong>and</strong> dune<br />
s<strong>and</strong> <strong>of</strong> this area, was composed <strong>of</strong> Safety Bay S<strong>and</strong>. A core collected <strong>of</strong>f James Point by France<br />
(1977) included a similar sediment unit. Likewise, Semeniuk <strong>and</strong> Searle (1985) described the<br />
occurrence <strong>of</strong> this unit at Woodman Point, at the northern end <strong>of</strong> the sound.<br />
<strong>The</strong> carbonate s<strong>and</strong> <strong>and</strong> muddy s<strong>and</strong> that forms Parmelia <strong>and</strong> Success Banks, the ‘shells<strong>and</strong>’<br />
resource <strong>of</strong> <strong>Cockburn</strong> <strong>Sound</strong>, has long been dredged commercially for the production <strong>of</strong> quicklime<br />
<strong>and</strong> cement. <strong>The</strong>se Carbonate Banks areas (Fig. 10) are largely covered with seagrass <strong>and</strong> swept by<br />
tidal <strong>and</strong> wind-induced currents as well as oceanic swells. <strong>The</strong>se currents winnow the fines from the<br />
banks, leaving behind the predominantly s<strong>and</strong>-size carbonate grains.<br />
<strong>The</strong> distribution <strong>of</strong> Eastern Shoal sediment (carbonate muddy s<strong>and</strong>) indicates that the muddy s<strong>and</strong> is<br />
being transported <strong>of</strong>f the northern carbonate banks into <strong>Cockburn</strong> <strong>Sound</strong> <strong>and</strong> is likely also being<br />
produced on the Eastern Shoal (Fig. 10). <strong>The</strong> combined carbonate s<strong>and</strong> <strong>and</strong> muddy s<strong>and</strong> facies<br />
mapped in this study are equivalent to the Becher S<strong>and</strong> Unit <strong>of</strong> Semeniuk <strong>and</strong> Searle (1985). <strong>The</strong><br />
Becher S<strong>and</strong> was formerly part <strong>of</strong> the Safety Bay S<strong>and</strong> which was divided into two distinct<br />
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