The Geomorphology and Sediments of Cockburn Sound

More documents

Recommendations

Info

The Geomorphology and Sediments of Cockburn Sound Contents Executive Summary.................................................................................................iv Introduction..............................................................................................................1 Aims................................................................................................................................................ 1 Study Participants and Collaboration.............................................................................................. 1 Regional Setting.............................................................................................................................. 1 Climate and Oceanography............................................................................................................. 3 Bathymetry...................................................................................................................................... 3 Geology and Geomorphology......................................................................................................... 4 Anthropogenic Changes.................................................................................................................. 6 Methods .........................................................................................................8 Field Approach ............................................................................................................................... 8 Sample Analysis ............................................................................................................................. 8 Geochemical Analysis .................................................................................................................. 11 Data Analysis................................................................................................................................ 11 Results......................................................................................................... 12 Surficial Sediments of Cockburn Sound....................................................................................... 12 Calcium Carbonate .................................................................................................................. 12 Sediment Composition and Underwater Video ....................................................................... 12 Sediment Facies....................................................................................................................... 13 Principal Components Analysis of Sediment and Geochemical Data..................................... 19 Fine-Fraction Metal Concentrations........................................................................................ 24 Sub-Surface Sediments ................................................................................................................. 29 Lithostratigraphic Units........................................................................................................... 33 Downcore Geochemical Data.................................................................................................. 34 Discussion ................................................................................................... 39 Surface Sediment Facies............................................................................................................... 39 Geochemical Relationships ..................................................................................................... 40 Sediment Metal Concentrations............................................................................................... 40 Quaternary Stratigraphy................................................................................................................ 41 Holocene.................................................................................................................................. 41 Pleistocene............................................................................................................................... 42 Sub-Surface Geochemical Characteristics............................................................................... 45 Geological Evolution of Cockburn Sound.................................................................................... 45 Conclusions ................................................................................................. 46 References ................................................................................................... 47 Appendices Appendix I .................................................................................................................................... 50 Appendix II................................................................................................................................... 60 Appendix III.................................................................................................................................. 64 iii
The Geomorphology and Sediments of Cockburn Sound Executive Summary Cockburn Sound is a large, low-energy coastal waterway located on a moderate to high-energy coast near Fremantle, Western Australia. It has formed in an elongate depression in the lee of a remnant Pleistocene shore-parallel dune ridge. This study, undertaken as part of the Coastal CRC Project at Geoscience Australia, examined the geomorphology of the sea bed, the spatial distribution of the various sediment types and the geomorphic evolution of Cockburn Sound. Sediment grab samples and 3–6 m long vibracores were collected. Samples dominantly comprise biogenic carbonates, with sandy mud and mud in the large relatively deep (15–20 m) central basin; the marginal banks (2–10 m) are composed of carbonate sand; and there is mixed carbonate and quartz sand in the eastern nearshore zone. The stratigraphic analysis of Cockburn Sound reveals that much of the clay soil that formed on the original calcarenite land surface prior to the Holocene rise in sea level is preserved below the marine carbonate mud that has been deposited in the central basin. The central basin is partially infilled and considerable accommodation space remains for the further accumulation of muddy sediment. Up to four distinct lithostratigraphic units comprise the sediment fill identified in vibracores. The spatial extent of these units will be better defined with future acoustic sub-bottom profiling of the sound. Submarine groundwater discharge in the central and northeastern sections of Cockburn Sound is suggested by surface sediment geochemical data and the presence of water-filled cavities within a vibracore that penetrated the Tamala Limestone that underlies the central basin sediments. A higher proportion of fine terrestrial sediment with elevated levels of some trace metals was identified in surface sediments from the eastern side of the sound. Previously reported background metal concentrations in surface sediments are consistently higher than pre-industrial background values which have been determined from the new down-core geochemical data. Metal contamination in surface sediments, therefore, may have been greater than previously reported but there is evidence for recently improved conditions at some sites; mainly due to large reductions in discharges from industry. Importantly, the spatial distributions of trace metals in the surface sediments identified in this study need to be compared to maps of changes in the extent of benthic habitats in the sound to help identify any long-term ecological impacts of the sediment contamination, as well as their impacts on benthic habitat composition. Our maps of surface sediment types and their characteristics provide new insights into the physical controls on the distribution of benthic habitats in the sound. These maps can now also be compared with maps of acoustic backscatter to help explain patterns in the acoustic data that has been collected as part of the Coastal CRC Coastal Water Habitat Mapping Project. iv
Page 1 and 2: G E O S C I E N C E A U S T R A L I
Page 3: The Geomorphology and Sediments of
Page 7 and 8: The Geomorphology and Sediments of
Page 55 and 56:
The Geomorphology and Sediments of
Page 57 and 58:
Page 59 and 60:
Page 61 and 62:
Page 63 and 64:
Page 65 and 66:
Page 67 and 68:
Page 69 and 70:
Page 71 and 72:
Page 73 and 74:
Page 75 and 76:
Page 77 and 78:
Page 79 and 80:
Page 81 and 82:
Page 83 and 84:
Page 85 and 86:
Page 87 and 88:
Page 89 and 90:
Page 91 and 92:
Page 93:
show all

The Geomorphology and Sediments of Cockburn Sound

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?