The Geomorphology and Sediments of Cockburn Sound

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