The Geomorphology and Sediments of Cockburn Sound

The Geomorphology and Sediments of Cockburn Sound
Introduction
Cockburn Sound is a key study site for the Coastal CRC Coastal Water Habitat Mapping (CWHM)
Project. At this site, benthic habitats have been mapped at a range of scales and levels of detail using
a suite of acoustic and video techniques. The focus of the Coastal Geomorphology subproject in
Cockburn Sound is to examine the geomorphology of the sea bed, the spatial distribution of the
various sediment types and the geomorphic evolution of this basin. These new geological data will
be used to ground-truth acoustic data collected in Cockburn Sound, such as the multibeam
backscatter data collected using the Coastal CRC’s Reson 8125 Seabat system. This study will also
provide a better understanding of the physical controls on the distribution of benthic habitats in the
sound and substrate data that can be incorporated into a benthic habitat classification scheme.
Several environmental surveys on the carbonate sand resources of Parmelia and Success Banks have
been conducted over the last two decades. However, relatively little attention has been paid to the
broader nature of the sediments and stratigraphy of Cockburn Sound, particularly in the deeper
central basin. Therefore, another focus of this study is to provide a more detailed level of knowledge
of the benthic environments and sediment fill in the deeper reaches of the sound, and a linkage of
these to more thoroughly studied shallow habitats.
AIMS
This report examines the geomorphology and sediments of Cockburn Sound through the analysis of
a suite of sediment grab samples and vibracores as well as existing data in published reports and
scientific papers. The specific aims of this study are to: 1) Characterise the surface and sub-surface
sediments and geochemistry of Cockburn Sound from a representative set of sediment grab samples
and vibracores; and 2) Develop a conceptual model of the recent evolution of Cockburn Sound
(spanning the Late Pleistocene and Holocene), based on the new and existing data. Importantly,
these geological data are required to enable better interpretation of acoustic sub-bottom profiles and
multibeam sonar datasets that have or will be collected in Cockburn Sound as part of the Coastal
CRC’s CHWM Project.
STUDY PARTICIPANTS AND COLLABORATION
The field survey and sample analysis was undertaken by Darren Skene, David Ryan and Brendan
Brooke (Geoscience Australia (GA)). Assistance in the field was provided by the crew of the vessel
FP Response, Jamie Strickland (Fremantle Ports) and Mark Small (Fremantle Ports), Lee Woolhouse
(hydrographic surveyor, Fremantle Ports) and Rob McCauley (Curtin University of Technology).
Lynda Radke (GA) undertook the analysis of the surface sediment geochemical data and Jodie Smith
(GA) interpreted the vibracore geochemical data and completed the editing and formatting of the
report. Helen Bostock (GA) assisted with editing the report. Laboratory analyses at GA were
undertaken by Alex McLachlan, Neal Ramsey and Richard Brown (sedimentology) and Bill Pappas,
John Pike and Liz Webber (geochemistry).
REGIONAL SETTING
Cockburn Sound is an elongate, shallow, partially enclosed coastal basin with an area of
approximately 124 km 2 , located immediately south of Fremantle, Western Australia (Fig. 1). The
sound is approximately 22 km long and ranges from 15 km wide in the north to 9 km wide in the
south and sits between the mainland and a remnant Pleistocene dune ridge that forms Garden Island.
This relatively quiescent coastal water body has contrasting shoreline environments with heavy
industry along much of its eastern coast whereas the western shoreline of Garden Island, apart from
the naval base at Careening Bay, is largely undeveloped.
1