The Geomorphology and Sediments of Cockburn Sound

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