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 />
Lithostratigraphic Units<br />
Six distinct sub-surface units were identified in the cores <strong>and</strong> are described below in their<br />
stratigraphic order based upon the visual logs <strong>and</strong> the results <strong>of</strong> the sediment analyses (Figs. 16 <strong>and</strong><br />
17). Additionally, in core CSV05 (Fig. 16; Appendix III), water-filled sub-surface cavities were<br />
observed in the basal limestone unit.<br />
1) Gravely shelly carbonate/quartz s<strong>and</strong>. This unit varies in colour from light grey-brown, pink-grey<br />
to green-grey (Fig. 17a) <strong>and</strong> is poorly sorted with fine to very coarse grained s<strong>and</strong> (mean grain size:<br />
0.48 mm; st<strong>and</strong>ard deviation: 0.39 mm). <strong>The</strong> deposit includes scattered to abundant carbonate gravel<br />
that contains fragments <strong>of</strong> bivalve shells, some gastropods, echinoids <strong>and</strong> bryozoan, <strong>and</strong> abundant<br />
whole foraminifera. Calcium carbonate within the combined s<strong>and</strong> <strong>and</strong> mud fraction ranges from 40<br />
to 60%. This unit was encountered in the southeastern margin <strong>of</strong> the sound near Kwinana Beach <strong>and</strong><br />
likely extends to the shoreline.<br />
2) Carbonate S<strong>and</strong> to Muddy S<strong>and</strong>. <strong>The</strong>se sediments are light grey to grey, moderately to very<br />
poorly sorted, very fine to very coarse grained s<strong>and</strong> <strong>and</strong> muddy s<strong>and</strong> with varying proportions <strong>of</strong><br />
carbonate gravel (fragments <strong>and</strong> whole shell, diatoms), plant fibers <strong>and</strong> decaying seagrass fragments<br />
(Fig. 17b). <strong>The</strong>re was no evidence <strong>of</strong> any depositional structures in this unit. Total carbonate content<br />
for the s<strong>and</strong> is approximately 90% but is less for the more muddy s<strong>and</strong> (~80%). <strong>The</strong> s<strong>and</strong> facies is<br />
confined to the banks at the northern <strong>and</strong> southern margins <strong>of</strong> <strong>Cockburn</strong> <strong>Sound</strong>, the eastern shore <strong>of</strong><br />
Garden Isl<strong>and</strong> <strong>and</strong> a sheet that covers the area immediately north <strong>of</strong> Garden Isl<strong>and</strong>. <strong>The</strong> unit<br />
becomes conformably muddier with depth (Fig. 17c), <strong>and</strong> the muddy s<strong>and</strong> is exposed on the bank<br />
slopes around the perimeter <strong>of</strong> the central basin.<br />
3) Muddy Quartz S<strong>and</strong>. This deposit comprises grey, loose, muddy (silt), moderately sorted, medium<br />
to very coarse grained, sub-rounded to rounded, quartz s<strong>and</strong> with scattered shell fragments (Fig.<br />
17d). This facies was only encountered in one core (CSV07; Appendix III) <strong>and</strong> is less than 1 m<br />
thick. Calcium carbonate averages 17% in the top <strong>of</strong> the unit, but decreases abruptly to 5% below a<br />
depth <strong>of</strong> 2 m. <strong>The</strong> proportion <strong>of</strong> mud also decreases below 2 m, from 30% to less than 10%, while<br />
the mean grain size (0.4-0.65 mm) increases with depth. <strong>The</strong> unit also includes interbedded lenses <strong>of</strong><br />
dark grey to reddish brown plant material <strong>and</strong> fine organic fibers. This deposit appears to be filling a<br />
depression within the basal limestone (Cross section B-B’, Fig. 18).<br />
4) Basin Mud <strong>and</strong> S<strong>and</strong>y Mud. This deposit consists <strong>of</strong> cohesive, greenish-grey, structureless, poorly<br />
sorted, very fine grained, s<strong>and</strong>y carbonate muds (clayey silt), gravelly in places with fine, hair-like<br />
fibres <strong>and</strong> coarse grained shell fragments scattered throughout (Fig. 17e). <strong>The</strong>re are occasional large<br />
(up to 5cm) whole bivalves <strong>and</strong> smaller (up to 1 cm) gastropods. In two cores (CSV05 <strong>and</strong> CSV08)<br />
the bottom 10 cm <strong>of</strong> this unit is composed <strong>of</strong> dark grey, cohesive, s<strong>and</strong>y mud with abundant whole<br />
shells (some articulated) <strong>and</strong> shell fragments (Fig. 17f). This deposit forms the majority <strong>of</strong> the<br />
central basin sediment fill.<br />
5) S<strong>and</strong>y Clay. This unit comprises oxidised, yellowish red with brown mottles (Fig. 17f, g), very<br />
fined grained (average mean grain size: 0.04 mm; st<strong>and</strong>ard deviation: 0.11 mm) firm to stiff s<strong>and</strong>y<br />
clay with remnant organic material, most likely roots. <strong>The</strong> sediment becomes s<strong>and</strong>ier with depth<br />
(Core CSV05), <strong>and</strong> contains sparse pieces <strong>of</strong> organic material (possibly fossil rootlets). It forms a<br />
cap on the Pleistocene limestone that underlies the central basin muds.<br />
6) Calcarenite Limestone Gravel: This deposit is light grey with irregular pieces <strong>of</strong> calcarenite<br />
gravel in a matrix <strong>of</strong> greenish grey, very poorly sorted s<strong>and</strong>y mud (Fig. 17h). <strong>The</strong> s<strong>and</strong> is very fine to<br />
very coarse grained calcareous quartzose s<strong>and</strong>. <strong>The</strong> limestone forms the bedrock <strong>of</strong> the study area,<br />
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