The Geomorphology and Sediments of Cockburn Sound

The Geomorphology and Sediments of Cockburn Sound
Discussion
SURFACE SEDIMENT FACIES
Based on the surface sediment laboratory data, field descriptions and video transects of the seabed,
four distinct surface facies were identified in Cockburn Sound (Table 7). The areal distribution of
these deposits is shown in Figure 10. Because these and the associated sediment data (Figs. 6–11)
are georeferenced and incorporated in ArcInfo shapefiles they represent reliable maps of surface
sediment properties and can now be compared with the acoustic multi-beam backscatter data that
was recently collected in the Cockburn Sound. A summary of the various facies characteristics is
provided in Table 7.
Table 7: Summary of the physical and chemical characteristic of the surficial sediment types in
Cockburn Sound.
FACIES
Eastern
Nearshore
Eastern
Shoal
Carbonate
Banks
Central
Basin
SEDIMENT
COMPOSITION
Gravelly shelly
mixed
carbonate
quartz sand
Carbonate
Muddy Sand
Well sorted
Carbonate
Sand
Carbonate
Sandy Mud
MEAN
GRAIN SIZE
& SD (mm)
%
CaCO3
FINE FRACTION
GEOCHEMISTRY
(55% High Nd/Sr; High Zn,
Ni; Low Cu
0.12 - 0.43 >80% Low Nd/Sr; Mod Zn;
Low Cu, Ni
0.25 - 0.43 ~90% Low to mod Nd/Sr;
Mod Zn; High Cu,
Ni
0.023 - 0.12 ~80% Low to mod Nd/Sr;
Low Cu, Zn, Ni
LITHOSTRATIGRAPHIC
UNIT
Safety Bay Sand
Becher Sand (Semeniuk
and Searle, 1985)
Parmelia Bank and
Southern Flat Shellsand
Bridport Calcilutite
(Semeniuk and Searle,
1987)
The terrigenous component of the Eastern Nearshore quartz sand (gravely shelly mixed
carbonate/quartz sand) is probably reworked from older deposits while the carbonate component is
composed of the skeletal remnants of contemporary marine carbonate producing organisms. These
sediments are part of the beach unit of this region known as the Safety Bay Sand (Passmore, 1970).
The exact extent of this sediment type is unknown as it was only recovered in one sample (top of
core CSV3) off Kwinana Beach (Fig. 10). Passmore (1970) reported that almost the entire land
surface of the Rockingham and Peron Peninsulas, as well as the modern shallow-marine and dune
sand of this area, was composed of Safety Bay Sand. A core collected off James Point by France
(1977) included a similar sediment unit. Likewise, Semeniuk and Searle (1985) described the
occurrence of this unit at Woodman Point, at the northern end of the sound.
The carbonate sand and muddy sand that forms Parmelia and Success Banks, the ‘shellsand’
resource of Cockburn Sound, has long been dredged commercially for the production of quicklime
and cement. These Carbonate Banks areas (Fig. 10) are largely covered with seagrass and swept by
tidal and wind-induced currents as well as oceanic swells. These currents winnow the fines from the
banks, leaving behind the predominantly sand-size carbonate grains.
The distribution of Eastern Shoal sediment (carbonate muddy sand) indicates that the muddy sand is
being transported off the northern carbonate banks into Cockburn Sound and is likely also being
produced on the Eastern Shoal (Fig. 10). The combined carbonate sand and muddy sand facies
mapped in this study are equivalent to the Becher Sand Unit of Semeniuk and Searle (1985). The
Becher Sand was formerly part of the Safety Bay Sand which was divided into two distinct
39