Seabed environments and shallow subsurface geology of the Vlaming Sub-basin,
offshore Perth Basin
Summary results from marine survey GA0334
GEOSCIENCE AUSTRALIA
RECORD 2014/49
Nicholas, W. A., Howard, F., Carroll, A., Siwabessy, J., Tran, M., Radke, L., Picard, K. and
Przeslawski, R.
Department of Industry
Minister for Industry: The Hon Ian Macfarlane MP
Parliamentary Secretary: The Hon Bob Baldwin MP
Secretary: Ms Glenys Beauchamp PSM
Geoscience Australia
Chief Executive Officer: Dr Chris Pigram
This paper is published with the permission of the CEO, Geoscience Australia
© Commonwealth of Australia (Geoscience Australia) 2014
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ISSN 2201-702X (PDF)
ISBN 978-1-925124-43-9 (PDF)
GeoCat 78846
Bibliographic reference: Nicholas, W. A., Howard, F. J. F., Carroll, A. G., Siwabessy, P. J. W.,
Tran, M., Radke, L., Picard, K. and Przeslawski, R. 2014. Seabed environments and shallow subsurface geology of the Vlaming Sub-basin, offshore Perth Basin: Summary results from marine survey
GA0334. Record 2014/49. Geoscience Australia, Canberra.
http://dx.doi.org/10.11636/Record.2014.049
Contents
1 Introduction ............................................................................................................................................3
1.1 Aims and objectives .........................................................................................................................3
1.2 Recent Geology of the Rottnest Shelf .............................................................................................4
2 Methods .................................................................................................................................................7
2.1 Multibeam bathymetry, backscatter and sidescan sonar .................................................................7
2.2 Seabed sediment samples ..............................................................................................................7
2.3 Sediment geochemistry ...................................................................................................................7
2.4 Geomorphology ...............................................................................................................................7
2.5 Towed-video ....................................................................................................................................8
2.6 Infauna .............................................................................................................................................8
2.7 Acoustic sub-bottom profiles ............................................................................................................8
3 Results and interpretation......................................................................................................................9
3.1 Data, samples and limitations ..........................................................................................................9
3.2 Seabed characteristics.....................................................................................................................9
3.2.1 Bathymetry and feature types ....................................................................................................9
3.2.2 Sedimentology ..........................................................................................................................13
3.2.3 Sediment (environmental) geochemistry..................................................................................15
3.2.4 Observations from towed-video ...............................................................................................15
3.2.5 Biophysical characterisation of geomorphic features ...............................................................20
3.2.6 Infaunal assemblages ..............................................................................................................21
3.3 Shallow sub-surface geology .........................................................................................................22
3.3.1 Acoustic facies overview ..........................................................................................................22
3.3.2 Area 1 .......................................................................................................................................24
3.3.3 Area 2 .......................................................................................................................................25
4 Interpretation and implications for CO2 storage ..................................................................................27
4.1 Seabed characteristics of the Rottnest Shelf, Vlaming Sub-basin ................................................27
4.1.1 Mounds on the mid shelf margin ..............................................................................................27
4.1.2 Features of the mid shelf plain .................................................................................................27
4.1.3 Shallow Sub-surface Geology ..................................................................................................27
4.2 Implications for fluid migration .......................................................................................................28
5 Summary .............................................................................................................................................30
6 Recommendations ...............................................................................................................................31
7 Acknowledgements .............................................................................................................................32
8 References ..........................................................................................................................................33
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
iii
List of figures
Figure 1.1 Location of survey areas (Area 1 and Area 2) on the Rottnest Shelf, with
representative bathymetric contours, and the location of the portion of bathymetry obtained
during survey GA2434 directly north of Area 1. .......................................................................................6
Figure 3.1 Hill-shaded bathymetry of Area 1 showing location of sampling stations, video
transects, side scan area, and a probable fault scarp; Inset a, location of towed-video transects
and sampling stations adjacent to the probable fault; and Inset b, location of sampling station
37. ...........................................................................................................................................................11
Figure 3.2 Hill-shaded bathymetry of Area 2 overlain with locations of side scan sonar, sampling
stations, towed-video transects and principal geomorphic features; Inset a, location of sampling
stations and towed-video transects on the mid-shelf margin in the vicinity of mounds; and Inset
b, location of grab sample 17GR37 on a soft sediment feature. ............................................................12
Figure 3.3 Sediment characteristics for Areas 1 and 2 with a) carbonate content in the bulk
samples plotted against water depth; b) mean sediment grain size plotted against water depth
as determined by the laser method, and c) sediment sorting against water depth. Not shown in
c) is a muddy gravel sample with phi = 101.4 from sample 12GR25. ....................................................14
Figure 3.4 Plots of a) light rare elements and b) silver against aluminium concentrations
indicating higher concentrations of LREE and Ag at station 17. ............................................................15
Figure 3.5 Representative towed-video images from a transect of the mid shelf margin across
mounds in Area 2 (12cam02), with: a) the edge of a mound with a veneer of sand covering hard
substratum with epifaunal growth (sponges); b) hard consolidated rock with sponge growth; c)
hard consolidated rock supporting massive barrel sponge; d) edge of mound supporting laminar
sponge growth; and; e) sand ripples. .....................................................................................................16
Figure 3.6 Representative images of habitats showing: a) mixed sponge and octocoral gardens
at 22CAM06 (~43 m); b) massive barrel sponges on consolidated hard grounds (CHG) at
23CAM04 (~41 m); c) unconsolidated rhodolith beds over a sand base at 14CAM03 (~43 m); d)
bryozoan and algal communities on CHG at 26CAM08 (~45 m); e) reef-building scleractinian
corals on CHG at 23CAM04 (~41 m); f) reef-building scleractinian corals on CHG at 22CAM06
(~43 m); g) unconsolidated sand waves with small coralline encrusted rubble in troughs at
25CAM07 (~53 m); and h) unconsolidated sand ripples at 13CAM01 (~50 m). ....................................17
Figure 3.7 False colour hill-shaded bathymetry of Area 1 showing dominant substrates and
biota derived from towed-video characterisations parallel (Inset a) and perpendicular (Inset b) to
the probable fault. ...................................................................................................................................18
Figure 3.8 False colour hill-shaded bathymetry with locations of towed-video transects showing
dominant substrates and biota (Inset a) in the vicinity of mounds on the mid shelf margin, and
(Inset b) across an annular ridge, located within the parabolic ridge complex ......................................19
Figure 3.9 Representative infauna from grab samples dominated by crustaceans: (a) gammarid
sp. 55 (30GR61); (b) tanaid sp. 6 (30GR61); (c) gammarid sp. 20 (20GR43); (d) isopod sp. 2
(20GR43); (e) gammarid sp. 25 (29GR56); and (f) decapod sp. 10 (30GR61). ....................................22
Figure 3.10 Representative acoustic sub-bottom line GA334_229 from Area 1 located across
the fault (centre of image) visible in Figure 3.1. .....................................................................................25
Figure 3.11 a) Sub-bottom profile GA334_039 characterised by echo-type 1B-2, which
represent bedding planes dipping from the seafloor at different angles. b) Sub-bottom profile
GA334_055 characterised by echo-type 1B-1 and 2, which potentially may represent bedding
planes in shallow marine limestones. Vertical exaggeration = 20x ........................................................26
iv
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
List of tables
Table 3.1 Summary statistics for geomorphic features. .........................................................................13
Table 3.2 Parameters on habitats and associated bedforms recorded in towed-video
characterisations. ...................................................................................................................................20
Table 3.3 Geomorphic feature types in the survey areas, their average characteristics, and
number of associated towed-video characterisations. ...........................................................................21
Table 3.4 Average parameters for ecological habitats by geomorphic feature type. .............................21
Table 3.5 Descriptions of acoustic facies recognised and interpretations. ............................................23
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
v
vi
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
Executive summary
Background
As part of the Australian Government’s National CO2 Infrastructure Plan (NCIP), Geoscience Australia
is acquiring pre-competitive data to support industry assessments of offshore acreage release areas
for potential CO2 storage. The Vlaming Sub-basin was identified by the Carbon Storage Taskforce
(2009) as being potentially suitable for CO2 storage. In March and April 2012 Geoscience Australia
(GA) conducted a marine survey (GA0334) to provide information to contribute to an assessment of
the Vlaming Sub-basin for CO2 storage potential. GA examined physical, chemical and biological
evidence for fluid seepage at the seabed, and within the shallow sub-surface geology, and
investigated potential connectivity between the deep geological basin and the seabed.
The survey was undertaken on the continental shelf, offshore Perth, in two targeted areas: Area 1 to the
north of Rottnest Island and Area 2 to the south. These survey areas and sampling locations were chosen
on the basis of seismic interpretation indicating potential seepage close to the seabed. The survey aims,
methods and preliminary results are detailed in the post-survey report (Nicholas et al., 2013).
Features and datasets investigated
Surface and sub-surface geology and seabed habitats were investigated using acoustic sub-bottom
profiles, bathymetry, backscatter, side-scan sonar, sedimentology, chemistry and ecological data
obtained during two survey periods.
Interpreted local-scale geomorphic maps were produced for each survey area based on reprocessed
bathymetry data. Towed-video, sediment, geochemistry and biology samples provided information on
seabed characteristics (sedimentology, geology), habitats and ecology. Acoustic sub-bottom profiles
were investigated with the aim of determining if faulting and/or seepage was present in the shallow
sub-surface geology.
Key observations on the seabed and shallow geology
Seabed geomorphic features identified include plains, ridges, palaeo-dunes, mounds, shallow
depressions, sediment waves, and probable fault scarps. The seabed is comprised of:

areas with hard, lithified carbonate sedimentary rock; and

areas of flat and raised topography that are thinly veneered with unconsolidated carbonate-rich
sediments. These sediments are dominantly gravelly carbonate sands.
A large number of moderate relief, hard mounds exist on the mid shelf margin (Area 2), mostly in
water depths of 80-100 m, and support localised areas of sponges and octocorals (e.g. gorgonians).
Adjacent to these the seabed is characterised by homogeneous flat sediments. Large growth forms of
sponges and octocorals are rare in this environment, which may suggest either unsuitable substrate,
or a regularly disturbed seabed.
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
1
On the mid shelf, relict parabolic and crescent shaped ridges interpreted as palaeo coastal dunes rise
up to 10 m above the seabed in water depths of 40–50 m. These support high densities of biota, with
rare occurrences of reef-forming hard coral.
East of the palaeo-dunes the generally flat to low-relief mid shelf plains (Areas 1 and 2) contain both hard
and soft substrate (rock outcropping at the seabed, and seabed sediment). Rhodoliths were common
across large fields of flat relief, while rocky outcrops support macroalgae and red algae. Rare occurrences
of reef-building hard corals were also present. One probable fault scarp is present at the seabed in Area 1.
Indicators of fluid flow (chemical signatures, or salinity anomalies, carbonate chimneys) at the seabed or
features formed by sudden fluid release (pockmarks or plumes of anomalous water) were not identified.
Observations on the shallow sub-surface geology were limited to the uppermost ~ 30 m (though
generally less), equivalent to approximately the depth of the known uppermost stratum on this shelf at
Rottnest Island; the Tamala Limestone. The sub-surface geology is characterised in places by subaerially exposed surfaces, and potentially by shallow-marine limestones. Underlying the mid shelf
plains, the bedrock dips gently westward, whilst in contrast, underneath the palaeo-dunes, either no
sub-surface features were evident, or generally flat sub-surface layers were visible in the acoustic subbottom profiles.
Though probable faults are present at the seabed in Area 1, it was not possible to recognise these in
the acoustic sub-bottom profiles, in part due to poor signal penetration, and perhaps due to scale
differences between features at the seabed and those resolvable in the immediate sub-surface. Faults
and potential seepage indicators within the deeper sedimentary basin, visible in seismic profiles, have
no obvious linkages to seabed features in the acoustic sub-bottom profiles.
Implications for recognising seepage at the seabed
The apparent lack of fluid-related features at the seabed within the study areas, despite previous
studies indicating the existence of hydrocarbon and groundwater seeps in the region, may be due to
hard layers (e.g. calcretised crusts) and high hydraulic conductivity within the Quaternary limestones
of the Rottnest Shelf. These would potentially impede focused vertical fluid flow to the seabed, and
mask seepage by causing fluids to be transported laterally by diffuse and focused flow.
There are no soft sediment indicators of fluid release (i.e. pockmarks) in the study areas because the
sediment type and shallow geology (potentially karst in places) preclude their formation on this
carbonate shelf.
That no evidence of seepage was observed does not necessarily imply that it does not exist, or has not
occurred. If seepage is occurring, it may not have been sampled due to the difficulty in predicting
possible locations of point seepage. The spacing of sampling and towed-video may have been too large
to make any meaningful correlation among seabed and sub-surface features and samples. Furthermore,
seepage may be present, but at levels lower than the detection limits of the methods used.
Submarine groundwater discharge (SGD) is known to occur up to several kilometres from the coast on
the Rottnest Shelf. If SGD exists in the surveyed areas, the interaction of sub-surface groundwater
flow with any potential seepage sourced from depth may also reduce the influence and detectability of
seepage from the underlying geology. Thus the potential for recognising hydrocarbon-bearing
seepage from the geological strata of interest in this shelf environment may be low.
2
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
1 Introduction
As part of the Australian Government’s National CO2 Infrastructure Plan (NCIP), Geoscience Australia
has undertaken integrated assessments of selected offshore sedimentary basins for their CO 2 storage
potential. In March and April 2012, Geoscience Australia completed a seabed survey (GA0334) over
two targeted areas (Area 1 and Area 2) of the Vlaming Sub-basin (Figure 1.1), as part of a larger study
investigating the suitability of the Vlaming Sub-basin for geological storage of CO2.
This document summarises the results and interpretation of seabed and shallow geological (to 30 m
below the seabed) data acquired during survey GA0334 in the Vlaming Sub-basin. These data and
their interpretations are being used to support the investigation of the Vlaming Sub-basin for CO2
storage potential.
1.1 Aims and objectives
The aim of the survey was to collect seabed and shallow sub-surface (< 100 m) data to inform an
assessment of the potential connectivity between the deep geology of the Vlaming Sub-basin and
seabed. This data also forms a baseline reference for marine environments, providing information on
possible biological indicators of seepage, and characterisation of marine habitats overlying the
potential CO2 reservoir.
Specific objectives of this study were:

the identification of seabed geomorphology and shallow sub-surface geologic features that
potentially indicate fluid seepage from the underlying sedimentary basin (e.g. fault scarps,
bioherms);

the characterisation of seabed sediment types, texture and geochemistry;

the characterisation of benthic habitats and species, particularly relationships to fluid seepage;
and
the identification of potential seabed hazards to CO2 storage activities and associated

infrastructure.
Details of the survey activities (GA0334), data collected, and preliminary observations are presented
in Nicholas et al. (2013). This document presents the summary results of the analysis and
interpretation of data and samples acquired on these surveys.
Bathymetric data analysed in this study has been supplemented with data collected on the R.V.
Southern Surveyor survey GA2434 (SS042007) over the continental shelf 3 km north of Area 1
(Figure 1.1).
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
3
1.2 Recent Geology of the Rottnest Shelf
The recent geology (mid to late Quaternary) of the Rottnest Shelf, underlain by the Vlaming Subbasin, is represented by Rottnest Island, the largest of several limestone islands and reefs that extend
from the coast onto the shelf, close to the survey areas of this study (Figure 1.1). Rottnest Island is
composed primarily of mid to late Pleistocene carbonate aeolianite, the Tamala Limestone (Playford,
1988; Murray-Wallace and Kimber, 1989; Price et al., 2001; Hearty, 2003; Mylroie and Mylroie, 2010).
A coral reef limestone, the Rottnest Limestone, occurs intercalated within the Tamala Limestone as a
regional marine stratum (Playford, 1997). Fossiliferous calcarenite and marl (Herschell Limestone)
occur adjacent to the prominent saline lakes of the island (Teichert, 1950; Glenister et al., 1959;
Playford and Leech, 1977; Playford, 1997; Price et al., 2001; Hearty, 2003; Mylroie and Mylroie, 2010,
Gouramanis et al., 2012).
Rottnest Shelf is divided by Rottnest Island into northern and southern sections. The whole of Rottnest
Shelf is a wave-dominated and open shelf, set within the passive continental margin of southwest
Australia (Collins, 1988; James et al., 1999). The dominant wave-swell direction on the southern
Rottnest Shelf is from the southwest, with an average significant wave height of 1.5 m (Collins, 1988).
On the shelf, ridges and algal hardground were noted between 50 and 60 m water depths, inshore of
which the seabed forms an unconformity hardground with wave rippled sand (Collins, 1988; James et
al., 1999).
Quaternary deposits on the Rottnest Shelf coast are dominated by coastal dune systems, including
the Holocene Quindalup Dune System (marine and aeolian sands), the Pleistocene Spearwood Dune
System (Tamala Limestone), the Bassendean Dune System (Bassendean Sand) and the Ridge Hill
Shelf (Ridge Hill Sandstone). Holocene parabolic dunes are particularly evident onshore on the Swan
Coastal Plain, and overlie either earlier Holocene sand or Pleistocene aeolian limestone and yellow
sand (Semeniuk and Glassford, 1988).
While the late Pleistocene to Holocene strata on Rottnest Island (offshore) are representative of
similar deposits in the Perth region (onshore), and the sub-surface Quaternary deposits onshore are
reasonably well known, the shallow sub-surface strata of the offshore Perth Region, including Rottnest
Island, are less understood. For example, the Tamala Limestone on Rottnest Island was identified in a
borehole, extending to 70 m below present sea-level, underneath which is a Pleistocene, Miocene or
older sand similar to the onshore Cretaceous Rockingham Sand (Playford, 1988). The Rockingham
Sand is a shallow-marine, yellow-brown, medium to coarse-grained feldspathic quartz sand, previously
thought to be early Quaternary (Playford et al., 1976; Playford, 1988). Onshore, the Tamala Limestone
is commonly underlain by the Bassendean Sand (Smith et al., 2011), the latter unconformably
overlying Tertiary and Cretaceous sedimentary strata, but in places the Tamala limestone directly
overlies Cretaceous strata. While in the Rottnest Island borehole, as onshore, the Kings Park
Formation underlies the Rockingham Sand equivalent, for the rest of this shelf the precise relationship
is to a great extent unknown.
Faults have been noted within the Lower Cretaceous strata underlying the Rottnest Shelf, (Bale,
2004), with potentially reactivated faults close to the surface (Borissova et al., 2013; Langhi et al.,
2013). However, no major faulting has been identified in Quaternary strata (e.g. Playford, 1988). This
is perhaps due to the presence of extensive Quaternary carbonate sedimentary deposits, potentially
overlying recent faults, but masking them at the seafloor. The position of most faults has only been
determined by seismic surveys (Playford et al., 1976).
4
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
Coastal aquifers located onshore on the Swan Coastal Plain, have groundwater flow generally
directed towards the shoreline (Davidson, 1995). These include unconfined shallow aquifers and those
at depth, including the regionally important Leederville Formation that overlies the South Perth Shale.
Caves, dolines and solution pipes occur in the limestone of the onshore Swan Coastal Plain, and Inner
Shelf, with shallow dissolution hollows common in the Sepia Depression (water depths ~ 20 m)
(Collins, 1988). Groundwater discharge at the seabed has been noted within Cockburn Sound (Burnett
et al., 2006), outside Cockburn Sound (Loveless et al., 2008) and within reefs several kilometres
offshore at Marmion Marine Park (Greenwood et al., 2013).
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
5
Figure 1.1 Location of survey areas (Area 1 and Area 2) on the Rottnest Shelf, with representative bathymetric
contours, and the location of the portion of bathymetry obtained during survey GA2434 directly north of Area 1.
6
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
2 Methods
This section outlines the post-survey analytical and interpretation methodologies used in this study.
Details of data and sample acquisition methods employed during marine survey GA0334 are
presented in Nicholas et al. (2013).
2.1 Multibeam bathymetry, backscatter and sidescan sonar
Multibeam bathymetry, backscatter and sidescan sonar data acquired during the GA0334 survey by
Fugro Survey Pty Ltd were reprocessed at Geoscience Australia using Caris HIPS/SIPS v7.1 SP1
software. For bathymetry, this included: corrections for tide to mean sea level (MSL) and vessel pitch,
roll and heave; and filtering each swath line to remove any remaining artefacts and noisy data (e.g.
nadir noise and data outliers). Backscatter data were reprocessed using a Geocoder algorithm
(Fonseca and Calder, 2005), and subsequently corrected for transmission loss, ensonification area
and local slope. Bathymetry maps, and backscatter sidescan mosaics were created in Caris and then
exported as mosaic grids at 2 m resolution for display and analysis.
2.2 Seabed sediment samples
Grain size and carbonate content were determined using laser granulometry and sieving, and the
carbonate bomb method (Muller and Gastner, 1981), respectively. Grain size data were brought into
GRADISTAT (Blott and Pye, 2001) and statistics calculated on the merged grain size dataset.
2.3 Sediment geochemistry
Seabed sediment samples were analysed for their geochemical properties (Radke et al., 2011).
Sediment-geochemical variables include major, minor and trace elements, pigments, carbon and
nitrogen concentrations and isotopes, pore-water constituents, oxygen consumption and carbon
dioxide production rates, as well as a range of derived parameters (e.g. enrichment factors relative to
average upper continental crust).
2.4 Geomorphology
Interpreted local-scale geomorphic maps were produced using ArcGIS 10.1 for each survey area from
multibeam bathymetry grids at 2 m resolution and bathymetric derivatives (e.g. slope; curvature; 1 m
contours). Geomorphic features were identified and mapped using a method adapted from Heap and
Harris (2008).
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
7
2.5 Towed-video
Underwater towed-video was used to ground-truth multibeam bathymetry and backscatter data,
identify seabed habitats, and provide baseline ecological information of the study areas.
Characteristics identified in the towed-video were used to calculate semi-quantitative descriptive
statistics of biota and substrates. Habitat patches were delineated using the Collaborative and
Automative Tools for Marine Imagery (CATAMI) broad-scale scoring method
(http://www.marine.csiro.au/caab/).
2.6 Infauna
Infauna were identified to operational taxonomic units (OTU). Animals that could not be differentiated
to OTU (e.g. damaged or known juvenile specimens) were excluded from analyses. Analysis of
similarities (ANOSIM) was performed to identify differences in infaunal assemblages based on
categorical factors (station, area, rhodolith presence), and the BIO-ENV procedure (Clarke and
Ainsworth, 1993) was performed to fit numerical environmental factors (sediment characteristics,
distance to coast, backscatter, bathymetry, slope) against the species matrix to quantitatively measure
and test their relationships to biological variation.
2.7 Acoustic sub-bottom profiles
Acoustic sub-bottom profiles collected during survey GA0334 were interpreted using the Kingdom
Suite software package. Acoustic facies were identified to provide a semi-quantitative visual method of
assessing shallow sub-surface geology (Kim et al., 2008; Lafferty et al., 2006; Savini et al., 2012).
Acoustic facies descriptions are based on Damuth’s (1975; 1980) echo-type classification of 3.5 and
12 kHz sub-bottom reflections from deep sea surveys, adapted for the range of echo-types observed
in the study area.
8
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
3 Results and interpretation
3.1 Data, samples and limitations
Chirper acoustic sub-bottom profiling was undertaken in tandem with bathymetric swath mapping,
resulting in 2,303 km of profile data. Side-scan sonar was collected in small sections of both areas:
1.26 km2 in Area 1 (one location) and 5.39 km 2 in Area 2 (two locations). Side-scan data did not
provide additional information to that of the bathymetry and backscatter, and are not discussed further.
Gravity coring was attempted, but no penetration of the seabed was possible due to hardness. Ten
surface sediment samples were collected in the core catcher, and utilised for sedimentology.
Sediment samples were collected from 89 grabs at 43 stations (Nicholas et al., 2013), including subsamples for sedimentology (n = 61), environmental geochemistry (n = 15) and biology (n = 52). From
the sedimentology samples, 33 were utilised for grain size and carbonate content, and from the
biology samples, 35 suitable infaunal samples were retained. Sampling was insufficient to accurately
characterise infaunal communities or to examine any but the strongest environmental patterns.
Towed-video was collected along 13 transects (4.25 km of seabed, water depths 39-88 m). More than
six hours of video footage and 20,476 video characterisations were used to calculate descriptive
statistics for biota and substrate types. Towed-video transects were focused on features potentially
relevant to fluid seepage and were not evenly distributed across all geomorphic features.
Because of limited acoustic penetration of the shallow sub-surface (estimated at less than 5 to 30 m
maximum), it was generally not possible to identify tangible relationships between geomorphic
features and faults or other sub-surface structures identified in seismic data. This precluded a detailed
investigation of the shallow sub-surface geology.
3.2 Seabed characteristics
3.2.1 Bathymetry and feature types
The Rottnest Shelf, overlying the Vlaming Sub-basin, is described here in terms of three shelf zones,
two of which are represented in the surveyed areas (Figure 3.1 and Figure 3.2):

The inner shelf (0–30 m water depth), not examined in this study;

The mid shelf (30–100 m water depth), which includes a mid shelf plain (30–65 m water depth)
with ridges, exposed lithified sediments, sand wave fields, potential fault scarps, and rhodolith
beds (Areas 1 and 2); and a mid shelf margin (65–100 m water depth), with mounds, ridges and
rhodolith beds (Area 2); and

The outer shelf (100-170 m), beginning approximately at or below mounds on the mid shelf
margin.
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
9
Area 1 is 109 km2 in size, and located on the mid shelf, while Area 2 is 316 km2 in size, extending from
the mid shelf plain to the outer shelf.
Mid shelf plains in areas 1 and 2 (Figure 3.1, Figure 3.2,) are generally flat lying (dipping 0.03°-0.05°
west) and are overlain in places by sandwaves (14% of the plains in Area 1). Sandwaves are
generally sinuous, with crestlines that trend predominately in a NNW–SSE direction, and orthogonal to
the predominant swell regime. The sandwaves in Area 1 (height: 0.4–0.5 m; length: 30–45 m) are
slightly asymmetric, and in many cases have steeper seaward slopes.
Shallow depressions are present within the mid shelf plains of Area 1 (Figure 3.1), with a NW–SE
trending depression that dips southeast (~0.01°, 30% of Area 1; 45-57 m water depths; Table 3.1)
present on the western side. This shallow depression is divided into a northern (8 x 4 km, 2-4 m below
adjacent seabed) and a southern section (4 x 4 km, 2-8 m below adjacent seabed) by a broad,
elevated and shallow ridge oriented sub-orthogonal to the long axis of the depression. This ridge is
additionally truncated by a potential fault scarp oriented parallel to the depression axis.
Sediment sandwaves are present at several locations on the mid shelf, particularly on the
northeastern section of Area 1 (Figure 3.1). In Area 2 much of the mid shelf has a patchy sediment
cover, with localised sediment lobes and sandwaves. Where there is no obvious sediment cover, the
seabed commonly has a rugose texture (Figure 3.2).
Low-lying ridges are present on the mid shelf, the most prominent extending north to south across
Area 2 (Figure 3.2). Ridges are the least abundant (1%) seabed feature in Area 1, occurring at water
depths of 39–52 m. Some low-lying ridge features in Area 1 are locally prominent and rugged, located
in proximity to probable fault scarps. A single north-south aligned, low-lying exposed rocky ridge is
present to the east of the parabolic ridges in Area 2, with its southern-most portion cross-cut by those
ridges.
At least one potential fault scarp is present in Area 1 (Figure 3.1), having an apparent vertical
displacement of 1 m, and is aligned approximately north-northwest.
Parabolic and crescent shaped ridges that stand up to 10 m above the seabed (water depths of
29-54 m) occur in Area 2 (Figure 3.2), and are also present to the north of Area 1 (GA2334
bathymetry; Figure 1.1). In Area 2 these ridges have steep landward-facing slopes, and gentler
sloping seaward flanks. Annular ridges (circles within circles) were located within fields of parabolic
ridges, both in Area 2 and to the north of Area1. Similar circular features also exist within Area 1, but
are not as prominent at the seabed.
More than 2,300 individual mounds are present on the mid shelf margin of Area 2 (Figure 3.2), located
between the outer shelf and the mid shelf plain. The mapped outer shelf and mid shelf margin together
occupy 16 % of Area 2 (Table 3.1). Mounds in this shelf-parallel zone (water depths of 85–100 m)
measure ~ 10–30 m in diameter, have slopes of 9-10°, and stand up to 6 m above the seabed. Some
mounds are aligned parallel or perpendicular to the NNE-SSW trend of low-lying and exposed rocky
ridges. The most prominent mounds are distributed along an east–west alignment where they are
closely spaced these also form ridge-like structures.
The outer shelf in Area 2 (Figure 3.2) is represented by a seabed with soft, possibly muddy sediment,
(indicated by low backscatter values), in water depths of 100-130 m (Area 2 only).
10
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
Figure 3.1 Hill-shaded bathymetry of Area 1 showing location of sampling stations, video transects, side scan
area, and a probable fault scarp; Inset a, location of towed-video transects and sampling stations adjacent to the
probable fault; and Inset b, location of sampling station 37.
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
11
Figure 3.2 Hill-shaded bathymetry of Area 2 overlain with locations of side scan sonar, sampling stations, towed-video transects and principal geomorphic features; Inset a,
location of sampling stations and towed-video transects on the mid-shelf margin in the vicinity of mounds; and Inset b, location of grab sample 17GR37 on a soft sediment
feature.
12
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
Table 3.1 Summary statistics for geomorphic features.
Geomorphic
Feature
Survey
Area
Area
(km2)
Percentage
of Area (%)
Modal Depth
and range (m)
Mean Slope
(° ± 1σ)
Mean Backscatter
(dB ± 1σ)
Mid shelf plain
1
75.1
69
48 (41-52)
1.2 ± 1.1
-11.4 ± 2.0
Low-lying ridge
1
1.1
1
45 (39-52)
4.5 ± 4.1
-10.7 ± 2.5
Depression
1
33.4
30
50 (45-57)
1.1 ± 1.0
-11.1 ± 2.1
Sand wave
1
10.3
9
45 (43-50)
1.0 ± 0.7
-12.0 ± 1.9
Outer shelf
2
25.7
8
121 (96-129)
1.0 ± 0.7
-22.0 ± 3.2
Mid shelf margin
2
25.8
8
71 (63-102)
1.4 ± 1.4
-14.3 ± 2.8
Mounds
2
3.5
1
85 (53-126)
9.5 ± 7.8
-15.7 ± 3.4
Mid shelf plain
2
247.2
78
52 (37-68)
0.9 ± 0.8
-11.5 ± 3.0
Parabolic ridges
2
10.3
3
50 (29-54)
4.6 ± 3.4
-11.6 ± 2.3
Low-lying ridge
2
3.9
1
49 (42-50)
1.5 ± 1.2
-11.8 ± 2.6
Sand wave
2
0.2
<1
41 (40-42)
0.7 ± 0.6
-10.5 ± 2.1
N.B. Sand waves superimpose plains, and as a result, are not used in area calculations.
3.2.2 Sedimentology
Sediments in the study area are dominantly gravelly-sand in texture (slightly gravelly sand (n = 21),
gravelly sand (n = 7), sand (n = 3), sandy gravel (n = 3), slightly gravelly mud (n = 1) and muddy
gravel (n = 1)), and carbonate in composition (mean = 82 %, range = 33-98% CaCO3; Figure 3.3).
Sand content ranged from 6 to 100% (mean = 88 ± 20%). Mud content was generally < 1 %. Mean
grain size and bulk carbonate content was influenced strongly by the presence of large carbonate
clasts (generally rhodoliths) in all grab samples. On average, sediment samples from Area 1 have
lower proportions of carbonate, a similar range of grain sizes and are better sorted (Figure 3.3).
Sediment texture appears to be influenced by the presence of geomorphic features such as ridges.
For example, samples collected either side of the probable fault in Area 1 (Figure 3.1), have
contrasting textures of muddy gravel (poorly sorted) and fine sand (well-sorted). These suggest localscale variability in hydrodynamic conditions created by the interaction of geomorphic features with
oceanographic conditions and sediment supply.
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
13
Figure 3.3 Sediment characteristics for Areas 1 and 2 with a) carbonate content in the bulk samples plotted
against water depth; b) mean sediment grain size plotted against water depth as determined by the laser method,
and c) sediment sorting against water depth. Not shown in c) is a muddy gravel sample with phi = 101.4 from
sample 12GR25.
14
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
3.2.3 Sediment (environmental) geochemistry
Anomalous light rare earth element (LREE) and silver (Ag) concentrations, relative to aluminium (Al),
were evident in grab sample 17GR037 at station 17, Area 2 (Figure 3.4). Sediments in this sample
consisted of moderately well-sorted gravelly sand, and were better sorted and lower in carbonate than
sample 17GC10 collected nearby. The location of 17GR037 (but not 17GC10) coincides with one of
many soft sediment accumulations (sand wave) evident between low-lying ridges which trend in a
north to north-east direction and the parabolic ridge complex, which also traps sediment on its western
margins. In contrast, most of the samples from Area 2 consisted of sediments sampled from within the
parabolic ridge complex, and were geochemically similar to sediments collected in Area 1, although
closer to the low Al end-member.
Figure 3.4 Plots of a) light rare elements and b) silver against aluminium concentrations indicating higher
concentrations of LREE and Ag at station 17.
3.2.4 Observations from towed-video
Characterisations of towed-video (Table 3.2) indicate the seabed is dominated by rock (51% of all
locations observed), mostly in areas of low to moderate relief (57%), and provides important habitat for
a range of benthic species. Sandy sediment occurs in 25% of all locations, and is associated with flat
relief (38%), and 2-dimensional (19%) and 3-dimensional (6%) ripples/sand waves.
Benthic habitats (Figure 3.5, Figure 3.6, Figure 3.7, Figure 3.8) were broadly classified into five main
categories (modified from Przeslawski et al. 2011 and Carroll et al. 2012):
1.
Barren sediments: Sandy sediments with little evidence of infaunal (e.g. bioturbation) or
epifaunal activity, with sand waves or sand ripples (Figure 3.6 g,h);
2.
Rhodolith beds: unconsolidated and relatively flat to low relief areas of rhodoliths with sparse
epibenthic algae (limited to red algal species), (Figure 3.6 c);
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
15
3.
Mixed patches (octocorals and sponges): Rocky outcrops supporting locally abundant patches
of macroalgae (kelp and red algae), sponges and octocorals, interspersed with areas of sandy
sediment and low epifaunal cover. Macroalgae and sponges commonly occupy distinct and
localised areas with rare overlap. Rocky outcrops may be covered with a thin veneer of
sediment; however, epibenthic growth of sessile organisms indicates that a hard substratum is
present (Figure 3.6 d);
4.
Mixed gardens (sponges and octocorals): Continuous bedrock supporting diverse and
abundant areas of macroalgae (kelp and red algae), sponges and octocorals. Additionally,
bedrock features may be interspersed with areas of sandy sediment, but the majority of the
transect is dominated by mixed gardens (Figure 3.6 a,b); and
5.
Mixed gardens (sponges and octocorals with hard coral): Continuous bedrock supporting
diverse and abundant areas of macroalgae (kelp and red algae), sponges and octocorals with
instances of reef-forming hard coral (Figure 3.6 e,f).
Figure 3.5 Representative towed-video images from a transect of the mid shelf margin across mounds in Area 2
(12cam02), with: a) the edge of a mound with a veneer of sand covering hard substratum with epifaunal growth
(sponges); b) hard consolidated rock with sponge growth; c) hard consolidated rock supporting massive barrel
sponge; d) edge of mound supporting laminar sponge growth; and; e) sand ripples.
16
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
Figure 3.6 Representative images of habitats showing: a) mixed sponge and octocoral gardens at 22CAM06
(~43 m); b) massive barrel sponges on consolidated hard grounds (CHG) at 23CAM04 (~41 m); c) unconsolidated
rhodolith beds over a sand base at 14CAM03 (~43 m); d) bryozoan and algal communities on CHG at 26CAM08
(~45 m); e) reef-building scleractinian corals on CHG at 23CAM04 (~41 m); f) reef-building scleractinian corals on
CHG at 22CAM06 (~43 m); g) unconsolidated sand waves with small coralline encrusted rubble in troughs at
25CAM07 (~53 m); and h) unconsolidated sand ripples at 13CAM01 (~50 m).
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
17
Figure 3.7 False colour hill-shaded bathymetry of Area 1 showing dominant substrates and biota derived from
towed-video characterisations parallel (Inset a) and perpendicular (Inset b) to the probable fault.
18
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
Figure 3.8 False colour hill-shaded bathymetry with locations of towed-video transects showing dominant substrates and biota (Inset a) in the vicinity of mounds on the mid
shelf margin, and (Inset b) across an annular ridge, located within the parabolic ridge complex
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
19
Table 3.2 Parameters on habitats and associated bedforms recorded in towed-video characterisations.
Characterisations
(% of total)
Characterisation types
Characterisations
(total number)
Dominant substratum of Rock ( >61% only)
50.7
10390
Dominant substratum of rock (>81%)
29.8
6098
Dominant substratum of sand/mud
24.8
5069
Rhodoliths (subsub2)
9.5
1952
Rhodoliths domsub1 >40%
14.5
2975
Areas of rhodoliths (all)
24.3
4980
Areas without rhodoliths
74.4
15232
Areas of kelp (dombio >1%)
78.3
16038
Kelp/algal beds (>20% )
46.0
9422
Kelp/algal beds (>60%)
0.8
154
Sponge areas (dombio) (any sponge areas)
6.8
1384
Sponge areas (dombio >20% cover)
1.5
298
10.0
2056
Hard coral areas (dombio)
0.1
11
Hard coral areas (subdombio)
0.5
107
Octocoral (only subdombio)
3.5
723
Bryozoa (only subdombio)
0.2
45
Flat relief
37.5
7672
Low/mod relief
57.4
11753
2D waves/ripples
18.5
3793
3D waves/ripples
5.8
1191
3
(subdombio4)
Sponge areas
1
Domsub = dominant substrate type; 2subsub = sub dominant substrate type; 3dombio = dominant biota type; 4subdombio = subdominant biota type.
3.2.5 Biophysical characterisation of geomorphic features
Biological assemblages varied with geomorphic setting (Table 3.3, Table 3.4). In general, the various
geomorphic features contained common morphologies of sponges (laminar, branching, barrel, and
cup). Consolidated substrates supported hard scleractinian corals, sponges, extensive kelp (Eklonia
sp.) and red algae beds, and were also colonised by calcareous coralline algae. Reef-building hard
corals (e.g. Turbinaria sp.) were observed on hard substrates and outcrops at some stations
(Figure 3.6 e). Rhodoliths (free-living coralline red algae) were found in large aggregations over areas
of consolidated rock, adjacent to rocky outcrops, and in large fields of flat relief (Figure 3.6 c).
Rhodolith beds had relatively little visible growth of other biota apart from the presence of red algae
and sparse patches of bryozoans (Adeona sp.).
20
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
Table 3.3 Geomorphic feature types in the survey areas, their average characteristics, and number of associated
towed-video characterisations.
Depth (m)
average (min; max)
Survey
Areas
Geomorphic feature
Characterisations
(% of total; number)
No.
of stations
Mid shelf plains
1&2
-48.9 (-44.6; -54.2)
57.1% (11689)
8
Depression
1
-46.1 (-45.6; -46.6)
2.1% (431)
2
Parabolic ridges
2
-43.3 (-39.2; -51.3)
31.4% (6430)
5
Mounds
2
-85.9 (-82.2; -91.7)
4.7% (953)
1
Mid shelf margin
2
-87.7 (-86.0; -91.9)
3.5% (709)
1
On mid shelf plains (Table 3.4), macroalgae was the dominant biota type (48.5%) and local
occurrences of reef-building hard coral were sub-dominant (0.2% of observations). Unconsolidated
rhodolith beds were observed across large fields of flat relief plains (16%) and in areas of flat relief
adjacent to parabolic ridges. Low-relief depressions supported localised areas of flat rhodolith beds
(2.1% of observations), macroalgae (2.1% occurrence) and localised areas of sponges (0.2%),
bryozoans (0.02%) and octocorals (0.01%).
Parabolic ridges supported high densities of biota, with reef-forming hard corals dominant (0.1%
occurrence) and sub-dominant biota types (0.3% occurrence). Rocky outcrops supported macroalgae
(Eklonia sp. and red algae as an understory species; 29% of video observations), massive sponges
(2%) and bryozoans (Adeona sp.; 0.2%).
Mounds (high-relief) supported localised habitat types dominated by sponges (4.3%), with octocoral
growth subdominant (3.2%). Sponges and octocorals were rare in the inter-mound environment
(0.3%), suggesting perhaps an unsuitable substrate for their colonisation and growth.
Table 3.4 Average parameters for ecological habitats by geomorphic feature type.
No. rhodolith
occurrence
in geomorph
Sponge
occurrence
Dominant
(subdom)%
Hard coral
occurrence
Dominant
(subdom)%
Macroalgae
occurrence
Dominant
(subdom)%
Octocoral
occurrence
Dominant
(subdom)%
Bryozoan
occurrence
Dominant
(subdom)%
3332 (16.3%)
0.2% (7.5%)
0 (0.2%)
48.5% (0.2%)
0 (0.01%)
0 (0.03%)
Depressions
431 (2.1%)
0 (0.2%)
0
2.1%
0 (0.005%)
0 (0.02%)
Parabolic
ridges
1648 (8.0%)
2% (2.5%)
0.1% (0.3%)
28.8% (1.0%)
0
0 (0.2%)
Mounds
0
4.3%
0
0
0 (3.2%)
0
Mid shelf
margin
0
0.3%
0
0
0 (0.3%)
0
Mid shelf
plains
3.2.6 Infaunal assemblages
Nine hundred and sixty three individuals representing approximately 246 OTU’s (e.g. sponge species
1), were collected from 35 grab samples. Infaunal assemblages sampled from these seabed
sediments were dominated by crustaceans (75.4% of individuals, 66.7% of OTU's; Figure 3.9), with
polychaetes a minor component (13.7% of individuals, 16.7% of OTU's). The remaining taxa included
nematodes, echinoderms and molluscs, and epifaunal organisms including cnidarians, sponges, and
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
21
bryozoans. Some of the observed infauna are similar to assemblages described elsewhere in
Australia, including from Joseph Bonaparte Gulf (surveys SOL4934, SOL5117, SOL5463; Heap et al.,
2010; Anderson et al., 2011; Przeslawski et al., 2011). No known species indicative of gas or fluid
seeps were recorded in this collection.
Infaunal assemblages were more similar between replicate grabs, than among different stations
(ANOSIM: R = 0.455, p < 0.001). There was no difference in infaunal assemblages between samples
collected from areas with rhodoliths and those without (ANOSIM: R = -0.003, p = 0.53). The only
combination of environmental factors to significantly explain variation in infaunal assemblages were
depth, carbonate content and distance from the coast (BIO-ENV: ρ = 0.218, p = 0.04).
Figure 3.9 Representative infauna from grab samples dominated by crustaceans: (a) gammarid sp. 55 (30GR61);
(b) tanaid sp. 6 (30GR61); (c) gammarid sp. 20 (20GR43); (d) isopod sp. 2 (20GR43); (e) gammarid sp. 25
(29GR56); and (f) decapod sp. 10 (30GR61).
3.3 Shallow sub-surface geology
3.3.1 Acoustic facies overview
Penetration of the shallow sub-surface by acoustic sub-bottom profiling was generally up to ~ 5 ms
TWT (rarely to ~ 10 ms TWT) in Area 1, and ~10ms TWT (rarely to ~ 15 ms TWT) in Area 2. As such,
only the upper few tens of metres (potentially 20–40 m) of the sub-surface geology could be imaged.
In Area 1, few sub-surface features were identifiable. In Area 2 westerly-dipping reflectors were visible
beneath the mid shelf plain. Ten acoustic facies (Table 3.5) were identified, principally from Area 2,
characterised by more distinct reflections with low to moderate amplitude.
22
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
Table 3.5 Descriptions of acoustic facies recognised and interpretations.
Indistinct echoes: Continuous, prolonged (Class IIA)
Distinct echoes (Class I)
Type
Example
Description
Distribution
IA-1
Distinct (~1 ms), continuous,
and sharp water bottom
reflection with no apparent
sub-bottom reflectors.
Outer shelf and Lithified or dense
mid shelf plains, sediment directly
Area 2.
underlying hard
seabed.
IA-2
Distinct, continuous, and
Distinct patches Loose sediment
sharp water bottom reflection throughout the covering lithified or
with one distinct, continuous mid shelf plains. dense sediment.
sub-bottom reflector. Unit
Occurs as sheets or
between reflectors uniform
in the lee of more
and transparent.
elevated features.
IB-1
Distinct, continuous, and
Mid shelf plains,
sharp water bottom reflection Area 2.
with numerous distinct,
continuous, wavy, and
sometimes contorted subbottom reflectors.
IB-2
Distinct, continuous, and
Mid shelf plains, Westerly dipping
sharp water bottom reflection Area 2.
strata, mid shelf
with numerous indistinct,
plains.
westerly-dipping, sub-parallel
sub-bottom reflectors.
IIA-1
Indistinct (~2 ms),
continuous, and fuzzy water
bottom reflection with no
apparent sub-bottom
reflectors or two converging
sub-bottom reflectors
wedging out over 100s of
metres.
IIA-2
Indistinct, continuous, wavy
Specific
(20 m wavelength), and
patches, Area
fuzzy water bottom reflection 1.
with one apparent subbottom reflector wedging out
in both directions.
Sand-wave field with
up to 3 ms of
sediment over
lithified or dense
sediments.
IIA-3
Indistinct, continuous, and
Southeastern
fuzzy water bottom reflection corner, Area 1.
with two or more converging,
indistinct, discontinuous, and
hummocky sub-bottom
reflectors, wedging out over
100’s of metres.
Mid shelf plain,
exposed and partially
covered seabed with
sub-aerially exposed
surfaces beneath.
Throughout
Area 1.
Interpretation
Antiformal lithified
rock or dense
sediment
immediately
underlying seabed.
Common over
seabed apparently
lacking
unconsolidated
sediment.
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
23
Composite
Indistinct echoes: Hyperbolae
(Class IIB)
Type
Example
Description
Distribution
Interpretation
IIB-2a
Irregular, indistinct, and wavy Outer mid shelf
(20m wavelength) water
(Area 2).
bottom reflection consisting
of overlapping hyperbolae,
with no apparent sub-bottom
reflection.
Parabolic ridges,
Area 2. Potentially
more than one
generation of subsurface features.
IIB-2b
Indistinct, irregular,
transparent, and overlapping
hyperbolic reflections, often
overlapping with a distinct
water bottom reflection.
Mounds. The
transparency is an
acoustic artefact
caused by the size
and location of
mounds in relation to
sonar beam footprint
and path.
IB-2/
1A-2
Distinct, continuous, and
Mid shelf plains,
sharp water bottom reflection Area 2
with numerous indistinct,
westerly-dipping, and subparallel sub-bottom
reflectors, downlapping onto
an indistinct and continuous
sub-bottom reflector.
Mid shelf
margin, Area 2,
in water depths
of 70– 85 m.
Generally westerly
dipping sedimentary
strata, potentially
lithified.
3.3.2 Area 1
Three principal acoustic facies were present in Area 1:

primarily indistinct, continuous, and fuzzy water bottom reflections;

indistinct, continuous seabed reflectors with high-frequency waves (20 m wavelength), overlying
a fuzzy water bottom reflector; and

an indistinct (2 ms TWT), continuous and fuzzy bottom echo, sometimes with two or more
converging sub-bottom reflectors.
Ridges and probable fault scarps (linear) observed at the seabed were also visible in the acoustic subbottom profiles as seabed features (Figure 3.10). Annular ridges were not distinct in profile. Because
of limited sub-surface penetration, it was not possible to relate the ridges, probable fault scarps, and
annular ridges to faults or other sub-surface structures.
24
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
Figure 3.10 Representative acoustic sub-bottom line GA334_229 from Area 1 located across the fault (centre of
image) visible in Figure 3.1.
3.3.3 Area 2
The principal acoustic facies in Area 2 were:

Distinct, continuous and sharp water bottom reflectors with either no sub-surface reflectors, or
numerous distinct wavy and contorted sub-bottom reflectors;

Distinct continuous and sharp water bottom reflectors with numerous westerly dipping, subparallel sub-bottom reflectors;

Irregular, indistinct in places, and wavy water bottom reflectors; and

Indistinct, irregular, transparent and overlapping hyperbolic reflections, commonly overlapping a
sistinct water-bottom reflector.
Mounds on the outer shelf were commonly characterised by indistinct, transparent and overlapping
hyperbolic reflectors that commonly overlapped the seabed reflector. Nested parabolic to crescentshaped ridges were commonly characterised by irregular, indistinct and overlapping hyperbolic echoes
without apparent sub-bottom reflections. At the locations of some parabolic ridges, flat to very gently
dipping reflectors were present beneath the seabed reflector.
Mid shelf plain subsurface strata generally dipped to the west at 1° to 3°, occasionally with curved to
sinuous cross-sectional form (Figure 3.11a). An antiformal structure was observed at one location
(Figure 3.11b), possibly related to primary bedding. Low-lying, shelf-parallel ridges observed landward
and seaward of the parabolic ridges could not be associated with faults or other structures in their
shallow sub-surface. There were, however, enhanced seaward-dipping sub-surface reflectors, directly
beneath some of these ridges. In general, exposures of rugose seabed were associated with stronger,
gently dipping reflectors in the shallow sub-surface.
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
25
Figure 3.11 a) Sub-bottom profile GA334_039 characterised by echo-type 1B-2, which represent bedding planes
dipping from the seafloor at different angles. b) Sub-bottom profile GA334_055 characterised by echo-type 1B-1
and 2, which potentially may represent bedding planes in shallow marine limestones. Vertical exaggeration = 20x
26
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
4 Interpretation and implications for CO2 storage
4.1 Seabed characteristics of the Rottnest Shelf, Vlaming Sub-basin
4.1.1 Mounds on the mid shelf margin
The origin of the mounds present between 75 and 100 m water depth (Area 2) was not established
here, but based on morphology and comparison with similar features on the Carnarvon Shelf, it is
tentatively suggested that these features are bioherms or similar and related to shoreline processes
(Nichol and Brooke, 2011; Nichol et al., 2012). This, however, requires further investigation. Similar
mounds are present directly north of Area1 (GA2434 survey), at water depths of 100–105 m, in what
seems to be a northward continuation of those visible in Area 2.
The low-lying linear ridges trending north-northeast between 75 and 60 m water depth are likely to be
related to rock outcropping directly at the seabed.
4.1.2 Features of the mid shelf plain
The parabolic ridges on the outer mid shelf are interpreted as palaeo-dunes. Similar features are
visible on the seabed directly north of Area 1, observed in bathymetry collected during a previous
survey, GA2434. Sub-aerial dunes, with a range of ages, exist over wide expanses of Western
Australia’s coast (Nichol and Brooke, 2011), and are present up to 10 km distant from the shoreline.
The morphological similarity of the ridges on the outer mid shelf with aeolian dunes is interpreted as
evidence that the ridges were formed in a sub-aerial environment, close to a palaeo shoreline during a
time of lowered sea-level. But their location does not indicate a precise position of a shoreline.
Additionally, several generations of dunes may form at approximately similar locations.
Annular ridges commonly located in association with palaeo-dunes, and observed within Areas 1 and
2, are also present at water depths of 40–45 m directly north of Area 1. These do not have well
defined analogues onshore. Additional detailed data is required to determine their origin. There is no
evidence at present to suggest that they are directly related to focused fluid seepage from depth.
However, this cannot be ruled out.
Faults appear to be present at the seabed in Area 1 (cf. Langhi et al., 2013), but were not indicated at
those surface locations in earlier studies (e.g. Bale, 2004). Faulting at the seabed in Area 1 implies
neo-tectonic activity to the immediate north of Rottnest Island. If, as is likely, the faults in Area 1
intersect Tamala Limestone, then they must have been active at sometime in the recent past (~ 0.5
Ma). In contrast, faults evident in deeper seismic profiles have not been observed on the seabed in
Area 2, nor in the shallow sub-surface.
The reason for the anomalous geochemistry in sample 17GC037 in Area 2 is not understood.
4.1.3 Shallow Sub-surface Geology
Though the limited depth of acoustic sub-bottom profiling largely precluded an assessment of
lithology, stratigraphy and structure beneath the seabed (Area 1), westerly-dipping reflectors were
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
27
clearly visible in the shallow sub-surface of the mid shelf (dominantly from Area 2). Previous drilling, in
addition to seabed samples collected in the current study, may suggest that the dipping strata are
consolidated carbonates. For example, in Warnbro 1 well, 264 m of coastal limestone was noted to
directly overlie the Paleocene–Eocene Kings Park Formation. On Rottnest Island, the Quaternary
Tamala Limestone was found at depths of up to 70 m below present sea-level (Playford, 1988).
Dipping reflectors present within acoustic sub-bottom profiles from Area 2 may represent low-angle
bedding commonly observed within aeolian limestone. Alternatively, the dipping strata may relate
more closely to a shallow marine limestone, perhaps equivalent to the Minim Cove and the
Peppermint Grove members of the Tamala Limestone (Murray-Wallace and Kimber, 1989), or the
Rottnest Limestone. In particular, the dip of the strata are very gentle, compared to that of typical
dunes (20° to over 30°), and the seabed exposures of the strata seem more similar to beach ridges on
the Rockingham and Becher banks (Searle1984). Thus, the seabed in Area 2 may be underlain by
lithified beach ridge or sub-littoral deposits to the east of the palaeo-dunes, rather than aeolian
sediments (Bastian, 1996). Sub-aerial surfaces, exposed during lower periods of sea level appear to
exist underneath the palaeo-dunes.
4.2 Implications for fluid migration
Bale (2004) previously identified major faults in deep seismic profiles underlying Area 2, together with
potential gas chimneys and hydrocarbon-related diagenetic zones (HRDZs) that may intersect the
Tamala Limestone. In preparation for this marine survey Geoscience Australia identified indicators of
potential seepage and fault reactivation in seismic data below the study areas (Nicholas et al., 2013;
Borissova et al., 2013). The current study has, however, been unable to confirm or deny the
connectivity of seabed features to potential fluid migration pathways in the sub-surface, in part due to
the limited penetration of acoustic sub-bottom profiles. No active fluid seepage was observed on the
seabed within the survey areas, no tangible geochemical evidence for hydrocarbon seepage was
detected, and no acoustic anomalies potentially indicative of fluid migration or accumulation were
detected in the sub-bottom profiles.
Playford (1988) noted that solution pipes extend below sea level at several localities on Rottnest
Island, and must have formed when sea-level was lower than present. Sub-aerial exposure of the
Tamala Limestone under these conditions would have resulted in carbonate dissolution by meteoric
waters and the formation of cavities and pits (Smith et al., 2012). The presence of dissolution-related
porosity within the Tamala Limestone suggests that potential horizontal and vertical conduits for fluid
migration are abundant in the shallow sub-surface of the study area (Smith et al., 2011). It has been
suggested that groundwater discharge is occurring along the coast and offshore from springs via
solution channels within the Tamala Limestone (Allen, 1981; Davidson, 1995). On the shelf, there is
evidence for submarine groundwater discharge (SGD) presently occurring on an offshore barrier reef
several kilometres from the coast, just inshore of Area 1 (Greenwood et al., 2013). Furthermore,
measured Radon activities outside of Cockburn Sound indicate that SGD occurs in shelf waters
adjacent to Cockburn Sound, within strata equivalent to those on Rottnest Island and shelf, and
potentially throughout the regional coastal waters offshore Perth (Burnett et al., 2006; Loveless et al.,
2008). In Geographe Bay, forming the southernmost portion of the Southern Rottnest Shelf, the
Dunsborough Fault was identified as a probable zone of active and focused SGD (Varma et al., 2010),
and offshore seabed depressions and faults elsewhere on this shelf are likely to be preferred locations
for SGD. Thus, there is the potential for groundwater discharge within the survey areas, some of which
may be associated with faulting.
28
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
Although evidence for groundwater seepage has been documented from the region, the nature of
Quaternary carbonates may potentially have discouraged focused fluid seepage to the seabed. Within
the Tamala Limestone hydraulic conductivity is comparatively high, even in the absence of dissolution
features (e.g. solution pipes and pits), due to secondary porosity provided by bedding planes and
fractures (Smith et al., 2011). This implies that fluid seepage may be diffuse, rather than focused, and
may not form identifiable features at the seafloor.
Episodic sub-aerial exposure of the shelf during periods of lower sea level can result in the
development of hard, calcretised crusts within Quaternary carbonates, and can act as barriers to
vertical fluid migration (Semeniuk, 1986; Murray-Wallace and Kimber, 1989; Dravis, 1996; Perry et al.,
2003). Potentially, fluids migrating upward from deeper strata may have been forced to migrate
laterally beneath weathered carbonate crusts, preventing focused seepage to the seabed.
Periods of lower sea level are likely to have induced cementation of carbonate sediment
accumulations on the shelf, such as dunes, as in the nearby coastal regions (Semeniuk, 1986). While
it is possible that seepage of fluids sourced from depth may have occurred in places, the extensive
nature of carbonate cementation within the study area suggests that the major influence has been
oscillations in sea-level, and the associated sub-aerial exposure and water-table fluctuations. Sub-sea
cementation is also possible (Keene and Harris, 1995). Additionally, it is possible that groundwater
discharge occurred within the study areas during periods of lowered sea level (Smith et al., 2011).
In terms of potential fluid seepage from the underlying sedimentary basin, additional data is required
to determine whether seabed features are in related to seepage (see Recommendations below).
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
29
5 Summary
Two areas of the Rottnest Shelf, southwestern Australia, were surveyed to characterise the seabed
environments and shallow sub-surface geology of the Vlaming Sub-Basin, offshore Perth Basin, under
the Australian Government’s National CO2 Infrastructure Plan. Seabed geomorphic features and
associated habitats were investigated to identify possible evidence for fluid seepage from the
underlying sedimentary basin. Methods used include: multibeam bathymetry, backscatter and sidescan sonar for seabed mapping; acoustic sub-bottom profiling for the imaging of shallow sub-surface
geology; and grab sampling for biological, sedimentological, and geochemical characterisation.
Key observations from the survey are:
30

Seabed geomorphic features identified include plains, ridges, palaeo-dunes, mounds, shallow
depressions, sediment waves, and probable fault scarps. The seabed comprised areas of hard,
lithified carbonate sediments, and areas thinly veneered with unconsolidated carbonate
sediments. Sediments were dominantly gravelly carbonate sands, with generally low mud
content.

Sponges, kelp, bryozoans, calcareous algae, octocorals (soft corals), hard corals and rhodoliths
occurred on hard substrates. Unconsolidated sediments (e.g. sand) occupied 25% of all
locations and were associated with flat relief (91%), 2-dimensional (60%) and 3-dimensional
(17%) ripples/waves and relatively low cover of sponges, bryozoans, octocorals and
macroalgae.

Though probable faults are present at the seabed in Area 1, it was not possible to recognise
these in the acoustic sub-bottom profiles, in part due to poor acoustic signal penetration, and
perhaps due to scale differences. Faults and potential seepage indicators within the deeper
sedimentary basin, visible in seismic profiles, had no obvious linkages to seabed features in the
acoustic sub-bottom profiles.

Indicators of fluid flow (chemical signatures, salinity anomalies or carbonate chimneys) at the
seabed or features formed by sudden fluid release (pockmarks or plumes of anomalous water)
were not identified.

The apparent lack of fluid-related features at the seabed within the study area, despite previous
studies indicating the existence of hydrocarbon and groundwater seeps in the region, may be
due to hard layers (e.g. calcretised crusts) and high hydraulic conductivity within the Quaternary
limestones of the Rottnest Shelf. These would potentially impede focused vertical fluid flow to
the seabed. Furthermore, there are no soft sediment indicators of fluid release (i.e. pockmarks)
in the study areas because the sediment type precludes their formation at present on this
carbonate shelf.

It is possible that significant fluid migration processes may not have been active in most recent
times, during which the shelf carbonates were deposited. Additionally, if seepage is present it
may be below current detectable limits.
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
6 Recommendations
To provide further evidence on the distribution of fluid seepage at the seabed, and within the shallow
sub-surface geology of the Vlaming Sub-basin, it is recommended that drilling of mounds, parabolic
ridges, annular ridges and faults be conducted. Drilling may provide information on the chemical
composition, mineralogy, the type of cementation and lithological composition of the shallow geology.
Additionally, an acoustic sub-bottom study of the shallow geology capable of penetration to ~ 100 m,
linked to shallow drilling, may clarify the relationship among ridges, mounds and faults at the seabed
and features present in deeper seismic profiles. Furthermore, developing analytical methods that have
greater seepage detection capability (dependant on detection limits) than those currently available is a
necessity to further develop our understanding of seepage on the seabed.
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
31
7 Acknowledgements
This document is the summary product for the marine survey GA0334, and would not have been
possible without the input, help and guidance of a large number of people. We are particularly grateful
to Dr Irina Borissova and Tanya Whiteway for their involvement in leading the organising of the marine
survey. Grateful thanks go to the laboratory staff at Geoscience Australia, including Christian Thun,
Jessica Byass, and colleagues. Early versions of this document were commented on by Dr Diane
Jorgensen. Dr Riko Hashimoto provided guidance and comments which particularly improved this
document. Reviews by Dr Huang Zhi and George Bernardell further improved this and made the
document easier to read.
32
Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin
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