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 />
is not clear, but may be related to spoil dumps. <strong>The</strong>se new data should be useful for the<br />
environmental management <strong>of</strong> the sound, especially if sites <strong>of</strong> sediment contamination are also areas<br />
<strong>of</strong> benthic habitat loss. As noted above, future analysis <strong>of</strong> swath coverages <strong>of</strong> the contaminated sites<br />
on the western margin <strong>of</strong> the sound should prove whether they are related to spoil dumps.<br />
QUATERNARY STRATIGRAPHY<br />
<strong>The</strong> results <strong>of</strong> the vibracoring program (Fig. 16 <strong>and</strong> Appendix III), <strong>and</strong> logs <strong>of</strong> cores previously<br />
collected to assess the shells<strong>and</strong> resource in the northern area <strong>of</strong> <strong>Cockburn</strong> <strong>Sound</strong> (Coastal <strong>and</strong><br />
Marine Geosciences, 1998b; Dames <strong>and</strong> Moore, 1979) have been integrated into a series <strong>of</strong> west-east<br />
<strong>and</strong> south-north stratigraphic cross sections (Fig. 21). <strong>The</strong>se sections provide new insights into the<br />
stratigraphy <strong>of</strong> the sound <strong>and</strong> will be ground-truthed through future sub-bottom pr<strong>of</strong>iling.<strong>The</strong> cross<br />
sections show that four Holocene lithostratigraphic units partially fill the depression in the<br />
Pleistocene calcarenite that forms the bedrock <strong>of</strong> <strong>Cockburn</strong> <strong>Sound</strong>. Previous studies <strong>of</strong> the<br />
stratigraphic relationships <strong>of</strong> coastal units on the Rottnest Shelf have reported a similar assemblage<br />
<strong>of</strong> Holocene deposits (Semeniuk <strong>and</strong> Searle 1985; Semeniuk <strong>and</strong> Searle 1987; Searle <strong>and</strong> Semeniuk<br />
1985; Searle et al. 1988; Semeniuk et al. 1988). In the following sections, the major units<br />
encountered in this study are related to the published regional lithostratigraphy.<br />
Holocene<br />
Gravely shelly carbonate/quartz s<strong>and</strong>. This facies is part <strong>of</strong> the Safety Bay S<strong>and</strong>, which is a 2 to 6 m<br />
thick sequence (tabular deposit) <strong>of</strong> laminated to structureless s<strong>and</strong> <strong>and</strong> shelly s<strong>and</strong>. It underlies dune<br />
deposits <strong>of</strong> the present coast <strong>and</strong> overlies the Becher S<strong>and</strong>. A carbonate <strong>and</strong> quartz s<strong>and</strong> unit (Safety<br />
Bay S<strong>and</strong>) is restricted to the eastern side <strong>of</strong> <strong>Cockburn</strong> <strong>Sound</strong>. This unit was encountered south <strong>of</strong><br />
James Point (section C-C’; Fig. 21) where this sediment was emplaced in a beach environment <strong>and</strong><br />
overlies the muddy s<strong>and</strong> facies <strong>of</strong> the Becher S<strong>and</strong>.<br />
Carbonate S<strong>and</strong>/Muddy S<strong>and</strong>. This s<strong>and</strong> <strong>and</strong> muddy s<strong>and</strong> facies is equivalent to the seagrass <strong>and</strong><br />
slope units <strong>of</strong> the Becher S<strong>and</strong> Unit (Semeniuk et al. 1988). <strong>The</strong> Becher S<strong>and</strong> is Holocene in age; all<br />
radiocarbon ages for shells obtained from within this unit have are less than 7,000 years BP (Woods<br />
<strong>and</strong> Searle, 1983; Semeniuk <strong>and</strong> Searle 1985). <strong>The</strong> unit overlies <strong>and</strong> interfingers with the basin<br />
s<strong>and</strong>y muds (section C – C’, Fig. 21). <strong>The</strong> Becher S<strong>and</strong> is thickest in Parmelia Bank to the north <strong>and</strong><br />
Southern Flats to the southwest <strong>of</strong> <strong>Cockburn</strong> <strong>Sound</strong> where this carbonate s<strong>and</strong> <strong>and</strong> muddy s<strong>and</strong> is at<br />
least 15 m thick. Previous drilling in Parmelia Bank (Coastal <strong>and</strong> Marine Geosciences, 1998b) shows<br />
this unit to be thickest in the centre <strong>of</strong> the north-south bedrock depression that underlies <strong>Cockburn</strong><br />
<strong>Sound</strong> <strong>and</strong> thins towards the eastern side <strong>of</strong> the Garden Isl<strong>and</strong> Ridge (section A-A’, Fig. 21). Cross<br />
sections A-A’, C-C’ <strong>and</strong> D-D’ show that the Becher S<strong>and</strong> overlies the basin muds (Fig. 21).<br />
Significantly, these results show that the bank sediments are being transported into <strong>Cockburn</strong> <strong>Sound</strong><br />
via Parmelia Bank <strong>and</strong> the Southern Flats <strong>and</strong> are infilling the northern <strong>and</strong> southern margins <strong>of</strong> the<br />
central basin.<br />
Muddy Quartz S<strong>and</strong>. This facies is less than 1 m thick <strong>and</strong> appears to be infilling a depression within<br />
the limestone that forms the Garden Isl<strong>and</strong> Ridge (section B-B’, Fig. 21). Possibly, this deposit is<br />
reworked sediment that marks the Holocene-Pleistocene unconformity. Alternatively, it may be a<br />
relict heavily leached surficial horizon <strong>of</strong> the Tamala Limestone, or a remnant deposit <strong>of</strong><br />
Cooloongup S<strong>and</strong> (Passmore, 1970).<br />
Basin s<strong>and</strong>y mud/mud. This deposit represents the Bridport Calcilutite (Semeniuk <strong>and</strong> Searle, 1987).<br />
It is wholly Holocene with reported radiocarbon ages for the unit <strong>of</strong> less than 7,000 years BP<br />
(Semeniuk <strong>and</strong> Searle, 1987). <strong>The</strong>re is a sharp, unconformable contact with the underlying<br />
Pleistocene clay unit (CSV05 <strong>and</strong> 08, Figs 17e, f). <strong>The</strong>se basin muds occupy former depressions in<br />
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