The Geomorphology and Sediments of Cockburn Sound

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The Geomorphology and Sediments of Cockburn Sound Figure 5: Sediment sampling and underwater video equipment used in this study. A) Van-veen type grab sampler; B) Submersible vibracorer; C) Towed underwater video camera used by the CWHM team from University of Western Australia. GEOCHEMICAL ANALYSIS Sub-samples were wet sieved using a 63 µm nylon sieve and deionised water to separate the fine (
The Geomorphology and Sediments of Cockburn Sound Results SURFICIAL SEDIMENTS OF COCKBURN SOUND Surface sediment grain size characteristics for Cockburn Sound are presented in maps of mean grain size (Fig. 6), relative sediment grain sorting as defined by grain size standard deviation normalised to mean grain size (Fig. 7), and pie charts showing relative percentages of gravel, sand and mud for each sample site (Fig. 8). The results of all sedimentological analyses are provided in Appendix I (Table A1.1, A1.2). Sediments within the central basin are dominantly fine with the mud content increasing in the south and southwest of the sound (Figs. 6 and 8). Coarser, mainly sandy sediments are confined to the sand banks and shoals as well as the margins of the sound. Samples from Parmelia and Three Fathom Banks in the north and Southern Flats to the south are almost exclusively sand with a low proportion or absence of mud (Figs. 6 and 8). Gravel deposits are confined to the Eastern Shoal and the western margin of the sound, adjacent to Garden Island (Fig. 8). Figure 7 indicates that the coarser sediments located on Three Fathom Bank, Eastern Shoal and the Southern Flats tend to be better sorted whereas the central basin sediments show a greater range in grain size. Calcium Carbonate The highest proportions of calcium carbonate (CaCO 3 ) occur on the banks at the northern and southwestern margins of Cockburn Sound, typically 80 to 90% CaCO 3 (Fig. 9). The proportion of CaCO 3 in the sediment samples appears to be related to mean sediment grain size (Figs. 6 and 9). CaCO 3 concentration in the muddy sand ranges between 77 and 87% (average 82%). Lower percentages, from 60 to 66%, were found in a number of sand samples collected on the Eastern Shoal and may be due to the mixing of the carbonate sand with quartz sand derived from the erosion of the underlying Tamala Limestone or the reworking of relict sand deposits. The lowest proportion (53%) is associated with gravely sand collected in the southeastern section of the sound, offshore from Kwinana Beach. Sediment Composition and Underwater Video On the shallow northern banks and southern flats of Cockburn Sound the seabed consists of light grey, relatively uniform, moderately sorted, fine to coarse grained sand with varying proportions of shell gravel (fragments and whole shell), seagrass fiber and decaying seagrass fragments. Video footage from Parmelia Bank shows these sediments form the substrate of extensive seagrass meadows and patches of seagrass which alternates with bare sand flats, some covered with decaying seagrass debris. The seabed of the Eastern Shoal comprises grey, very poorly sorted, very fine to very coarse grained muddy sand with scattered carbonate gravel (0 to 18% shell and foraminifera) and occasional blackened seagrass fragments. Gravels include in situ mollusc shells usually associated with seagrass and eroded fragments of limestone reefs that form talus slopes. Video footage across parts of the Eastern Shoal reveals patches of exposed limestone reef, especially along the western edge of the shoal. The adjoining slope that grades down to the central basin is strewn with calcarenite gravel in places. The central basin is characterised by fine grained sediment in water depths greater than approximately 15 m. These deposits are cohesive, greenish-grey, poorly sorted, very fine grained, sandy carbonate muds (clayey silt) with varying proportions of shell fragments and whole shell. Localised high proportions of gravel in the central basin are typically related to isolated shell 12
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The Geomorphology and Sediments of Cockburn Sound

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