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Record 2014/49 | GeoCat 78846

Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth BasinSummary results from marine survey GA0334

Nicholas, W. A., Howard, F., Carroll, A., Siwabessy, J., Tran, M., Radke, L., Picard, K. and Przeslawski, R.

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Seabed environments and shallow sub-surface 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 sub-surface 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

http://www.creativecommons.org/licenses/by/3.0/au/deed.en

mailto:[email protected]

http://dx.doi.org/10.11636/Record.2014.049

Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin iii

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

iv Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin

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

Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin v

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

vi Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin

Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin 1

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.

2 Seabed environments and shallow sub-surface geology of the Vlaming Sub-basin, offshore Perth Basin

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 sub-aerially 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 sub-bottom 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.


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 CO2 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).


1.2 Recent Geology of the Rottnest Shelf The recent geology (mid to late Quaternary) of the Rottnest Shelf, underlain by the Vlaming Sub-basin, 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).


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).


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.


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).


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.

http://www.marine.csiro.au/caab/


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 km2 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 sub-samples 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.


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).


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.


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.


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 local-scale variability in hydrodynamic conditions created by the interaction of geomorphic features with oceanographic conditions and sediment supply.


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.


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);


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.


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).


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.


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


Table 3.2 Parameters on habitats and associated bedforms recorded in towed-video characterisations.

Characterisation types Characterisations (% of total)

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 (dombio3 >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

Sponge areas (subdombio4) 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 1Domsub = dominant substrate type; 2subsub = sub dominant substrate type; 3dombio = dominant biota type; 4subdombio = sub-dominant 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.).


Table 3.3 Geomorphic feature types in the survey areas, their average characteristics, and number of associated towed-video characterisations.

Geomorphic feature Survey Areas

Depth (m) average (min; max)

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)%

Mid shelf plains 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

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


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.


Table 3.5 Descriptions of acoustic facies recognised and interpretations.

Type Example Description Distribution Interpretation

Dis

tinct

ech

oes

(Cla

ss I)

IA-1

Distinct (~1 ms), continuous, and sharp water bottom reflection with no apparent sub-bottom reflectors.

Outer shelf and mid shelf plains, Area 2.

Lithified or dense sediment directly underlying hard seabed.

IA-2

Distinct, continuous, and sharp water bottom reflection with one distinct, continuous sub-bottom reflector. Unit between reflectors uniform and transparent.

Distinct patches throughout the mid shelf plains.

Loose sediment covering lithified or dense sediment. Occurs as sheets or in the lee of more elevated features.

IB-1

Distinct, continuous, and sharp water bottom reflection with numerous distinct, continuous, wavy, and sometimes contorted sub-bottom reflectors.

Mid shelf plains, Area 2.

Antiformal lithified rock or dense sediment immediately underlying seabed.

IB-2

Distinct, continuous, and sharp water bottom reflection with numerous indistinct, westerly-dipping, sub-parallel sub-bottom reflectors.

Mid shelf plains, Area 2.

Westerly dipping strata, mid shelf plains.

Indi

stin

ct e

choe

s: C

ontin

uous

, pro

long

ed (C

lass

IIA

) 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.

Throughout Area 1.

Common over seabed apparently lacking unconsolidated sediment.

IIA-2

Indistinct, continuous, wavy (20 m wavelength), and fuzzy water bottom reflection with one apparent sub-bottom reflector wedging out in both directions.

Specific patches, Area 1.

Sand-wave field with up to 3 ms of sediment over lithified or dense sediments.

IIA-3

Indistinct, continuous, and fuzzy water bottom reflection with two or more converging, indistinct, discontinuous, and hummocky sub-bottom reflectors, wedging out over 100’s of metres.

Southeastern corner, Area 1.

Mid shelf plain, exposed and partially covered seabed with sub-aerially exposed surfaces beneath.


Type Example Description Distribution Interpretation In

dist

inct

ech

oes:

Hyp

erbo

lae

(Cla

ss II

B)

IIB-2a

Irregular, indistinct, and wavy (20m wavelength) water bottom reflection consisting of overlapping hyperbolae, with no apparent sub-bottom reflection.

Outer mid shelf (Area 2).

Parabolic ridges, Area 2. Potentially more than one generation of sub-surface features.

IIB-2b

Indistinct, irregular, transparent, and overlapping hyperbolic reflections, often overlapping with a distinct water bottom reflection.

Mid shelf margin, Area 2, in water depths of 70– 85 m.

Mounds. The transparency is an acoustic artefact caused by the size and location of mounds in relation to sonar beam footprint and path.

Com

posi

te

IB-2/ 1A-2

Distinct, continuous, and sharp water bottom reflection with numerous indistinct, westerly-dipping, and sub-parallel sub-bottom reflectors, downlapping onto an indistinct and continuous sub-bottom reflector.

Mid shelf plains, Area 2

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 sub-bottom 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.


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, sub-parallel 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 crescent-shaped 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.


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


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


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.


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).


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 side-scan 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:

• 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.


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.


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.


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