J Keene

Carbonate sandwaves in Bass Strait

Side‐scan sonar, seabed photography and surficial sediment sampling document a field of sandwaves at 40–46 m water depth in eastern Bass Strait, Australia. The sandwaves are formed of coarse sand and gravel consisting of 50–92% biogenic carbonate, derived largely from the remains of molluscs, bryozoans and echinoderms. Four different scales of bedform are identified: ripples (0.1–0.3 m wavelength, 0.02–0.04 m high), small crest megaripples (4–5 m, 0.2 m), large trough megaripples (3–12 m, 0.1 ‐0.5 m) and sandwaves (55–1730 m, 2–12 m). The ripples are superimposed on both the megaripples and sandwaves. Large megaripples are present in the troughs between the sandwaves, while small megaripples are superimposed on the sandwave crests. Local tidal currents are non‐rectilinear. Sandwaves and their superimposed smaller bedforms are formed transverse to the ebb flow (from 255°) but subparallel to the flood flow, and trough megaripples are aligned transverse to the flood flow (from 135°). Minor deviations of the ...

Middle to early Late Ordovician hydrothermal veining in the Molong Volcanic Belt, northeastern Lachlan Fold Belt: Sedimentological evidence

Detrital volcanic and vein quartz, accompanied by felsic volcanic debris, occur as minor constituents in the Ordovician subduction‐related mafic volcanics of the Molong Volcanic Belt. In the western province of the Molong Volcanic Belt, detrital quartz is present in the three episodes of the mafic Volcanics. Volcanic quartz occurs in allochthonous limestone blocks in the Bendigonian Hensleigh Siltstone overlying the Mitchell Formation. The second volcanic episode (the Fairbridge Volcanics) commenced after a hiatus of approximately 20 million years and lasted around 10 million years from Darriwilian to Gisbornian time. Locally derived vein quartz, volcanic quartz and felsic detritus are concentrated at the bases of autochthonous Wahringa and Yuranigh Limestone Members of the volcanics and are extensive and abundant in basal beds of the regional Eastonian limestone body that transgressed over an eroded volcanic centre at Cargo. This early Eastonian debris, deposited early in an 8 million‐year volcanic hiatus preceding the final Ordovician Bolindian volcanism, establishes a pre‐Eastonian age for mineralisation at Cargo. It is inferred that the pauses in volcanism were preceded by magmatic fractionation, intrusion and hydrothermal activity and followed by erosion, subsidence and deposition of autochthonous limestones. Minor occurrences of vein and volcanic quartz are found in Bolindian volcanogenic sediments of the third volcanic phase. It is concluded that hydrothermal vein formation (and mineralisation by inference) was associated with pauses in volcanic activity throughout the Middle to early Late Ordovician over a wide area in the western province, culminating in the mineralisation at Cargo and Copper Hill near Molong. Volcanism in the eastern province of the Molong Volcanic Belt was continuous from at least Darriwilian to latest Ordovician time. Here, detrital hydrothermal vein quartz and volcanic quartz and felsic detritus are distributed through late Middle and early Late Ordovician turbidites of the Weemalla Formation. The possible existence of cycles in the source area like those of the Fairbridge Volcanics is masked by the distal nature of these deposits. Vein formation occurred in both provinces from late Middle Ordovician to early Late Ordovician, long before the formation of the world‐class mineral deposit at Cadia associated with the latest Ordovician Cadia Monzonite.

Seabed classification of the South Tasman Rise from SIMRAD EM12 backscatter data using artificial neural networks

We have developed an automated method for sea‐floor classification for the South Tasman Rise, based on a SIMRAD EM12‐backscatter (13 kHz) mosaic and 47 sea‐floor samples. The samples have been divided into 3 distinct groups representing: (i) thick blankets of foraminiferal ooze; (ii) mixed sediments comprising sand/silt/mud (turbidites/chalk); and (iii) outcrops of metamorphic basement and volcanic rocks. A total of 515 sub‐areas, each measuring 32 × 32 pixels (∼4 km2) and representing the different seabed types, were extracted from the image from areas 128×128 pixels large, centred on the sample locations. The texture of the sub‐images was analysed by calculating grey‐level run‐length features, spatial grey‐level dependence matrices, and grey‐level difference vectors in four directions. A total of 100 samples for each class and 18 feature statistics were chosen to train an artificial neural network to recognise the textural attributes and their variability for each class. The performance of the network w...

Southeast Australia: A Cenozoic Continental Margin Dominated by Mass Transport

The Southeast Australian continental margin extends for 1,500 km northward from Bass Strait to the Great Barrier Reef. Mass transport dominates the continental slope, which stretches from the shelf break around 150 m depth to the abyssal plain around 4,500 m depth. The continental slope has average slopes of 2.8–8.5° and extends seaward from the shelf break an average distance of 50 km. Margin structure results from Late Cretaceous rifting, producing exposed fault blocks and igneous complexes on the lower slope, and an overlying sediment wedge around 0.5 km thick, centered at the shelf break. Recent collection of multibeam echosounding and high-resolution seismic data provide a detailed view of mass-transport features over a 900 km length of the margin. The features are mostly slab slides, box canyons, and linear canyons. They are ubiquitous along the steep rifted margin, but absent in regions of gentler slopes such as submarine plateaus and failed rift arms. Submarine landslides range in scale from hundreds of small slides of <0.5 km3 volume, up to the largest documented slide of 20 km3. However, potential future slide masses of basement blocks up to 105 km3 have been identified. Cores that penetrated the basal-slide surface show variable sediment accumulation, since the mass-movement event, but four penetrations show accumulations of <2 m, and one of <0.6 m. At current accumulation rates, these data indicate that many landslides occurred less than 25 ka, with some as recent as 6 ka. Mass movements appear to follow a pattern of box canyon development exploiting structural trends in pre-rift and syn-rift strata, until the canyon head intersects the toe of the Tertiary sediment wedge. Once this occurs, sediment creep, faulting and failure of the wedge toe migrates up slope, finally reaching the upper slope and Quaternary deltaic depocenters.

Submarine Landslides on the Upper Southeast Australian Passive Continental Margin – Preliminary Findings

The southeast Australian passive continental margin is narrow, steep and sediment-deficient, and characterized by relatively low rates of modern sedimentation. Upper slope (<1,200 m) sediments comprise mixtures of calcareous and terrigenous sand and mud. Three of twelve sediment cores recovered from geologically-recent, submarine landslides located offshore New South Wales/Queensland (NSW/QLD) are interpreted to have sampled failure surfaces at depths of between 85 and 220 cm below the present-day seabed. Differences in sediment physical properties are recorded above and below the three slide-plane boundaries. Sediment taken directly above the inferred submarine landslide failure surfaces and presumed to be post-landslide, returned radiocarbon ages of 15.8, 20.7 and 20.1 ka. The last two ages correspond to adjacent slide features, which are inferred to be consistent with their being triggered by a single event such as an earthquake. Slope stability models based on classical soil mechanics and measured sediment shear-strengths indicate that the upper slope sediments should be stable. However, multibeam sonar data reveal that many upper slope landslides occur across the margin and that submarine landsliding is a common process. We infer from these results that: (a) an unidentified mechanism regularly acts to reduce the shear resistance of these sediments to the very low values required to enable slope failure, and/or (b) the margin experiences seismic events that act to destabilise the slope sediments.

Physical Properties and Age of Continental Slope Sediments Dredged from the Eastern Australian Continental Margin – Implications for Timing of Slope Failure

A large number of submarine landslides were identified on the continental slope of the southeastern Australian margin during voyages aboard the RV Southern Surveyor in 2008. Preliminary sedimentological, geotechnical and biostratigraphic data are reported for dredge samples of Neogene compacted, calcareous sandy-muds recovered from submarine scarps located on the mid-continental slope. The scarps are interpreted to represent submarine landslide failure surfaces. Slope stability modeling using classical soil mechanics techniques and measured sediment shear-strengths indicate that the slopes should be stable; however, the ubiquity of evidence for mid-slope landslides on this margin indicates that their occurrence is a relatively commonplace event and that submarine-landsliding can probably be considered a normal characteristic of the margin. This presents an apparent contradiction that is probably resolved by one or both of the following: an as yet unidentified mechanism acts to reduce the shear resistance of these sediments to values low enough to enable slope failure; or geologically frequent seismic shaking events large enough to mobilise slides. It is hypothesised that the expansion of the Antarctic Icesheet in Mid-Miocene time and the consequent large-scale production of cold, equator-ward migrating, bottom water has caused significant erosion and removal of material from the mid and lower slope of the Australian continental margin in the Tasman Sea since the Mid-Miocene. It is also hypothesised that erosion due to equator-ward moving bottom water effectively and progressively removed material from the toe of the continental slope sediment wedge. This rendered the slope sediments that were deposited throughout the Tertiary more susceptible to mass failure than would have otherwise been the case.

Petrology and diagenesis of Narrabeen group sandstones, Sydney Basin, New South Wales∗

The petrology of fluvial sandstones from the Narrabeen Group was investigated to document sandstone composition and to assess factors controlling sandstone diagenesis. The sandstones are dominated by detrital quartz and lithic grains. The average composition varies in ascending stratigraphic order from R (rock fragments) > Q (quartz) to Q > R. Major diagenetic events in order of occurrence were: (i) formation of hematite, chlorite and mixed‐layer illite/smectite, and first generation of kaolin (I); (ii) precipitation of calcite, siderite and ankerite; (iii) dissolution of labile detrital grains and carbonate cement, and formation of second generation of kaolin (II); (iv) quartz overgrowth and precipitation of illite; and (v) precipitation of calcite and ankerite. These events were controlled both by detrital mineralogy and pore‐water chemistry. Kaolin (I) and chlorite are mutually exclusive. At shallow burial depth different pore‐water chemistry prevailed in different parts of the basin through meteoric w...

Morphology of Australia’s Eastern Continental Slope and Related Tsunami Hazard

Morphologic characterisation of five distinct, eastern Australian upper continental slope submarine landslides enabled modelling of their tsunami hazard. Flow depth, run-up and inundation distance has been calculated for each of the five landslides. Future submarine landslides with similar characteristics to these could generate tsunami with maximum flow depths ranging 5–10 m at the coastline, maximum run-up of 5 m and maximum inundation distances of 1 km.

Sedimentology, structure and age estimate of five continental slope submarine landslides, eastern Australia

ABSTRACT Sedimentological and accelerator mass spectrometry (AMS) 14C data provide estimates of the structure and age of five submarine landslides (∼0.4–3 km3) present on eastern Australias continental slope between Noosa Heads and Yamba. Dating of the post-slide conformably deposited sediment indicates sediment accumulation rates between 0.017 m ka–1 and 0.2 m ka–1, which is consistent with previous estimates reported for this area. Boundary surfaces were identified in five continental slope cores at depths of 0.8 to 2.2 m below the present-day seafloor. Boundary surfaces present as a sharp colour-change across the surface, discernible but small increases in sediment stiffness, a slight increase in sediment bulk density of 0.1 g cm–3, and distinct gaps in AMS 14C ages of at least 25 ka. Boundary surfaces are interpreted to represent a slide plane detachment surface but are not necessarily the only ones or even the major ones. Sub-bottom profiler records indicate that: (1) the youngest identifiable sediment reflectors upslope from three submarine landslides terminate on and are truncated by slide rupture surfaces; (2) there is no obvious evidence for a post-slide sediment layer draped over, or burying, slide ruptures or exposed slide detachment surfaces; and (3) the boundary surfaces identified within the cores are unlikely to be near-surface slide surfaces within an overall larger en masse dislocation. These findings suggest that these submarine landslides are geologically recent (<25 ka), and that the boundary surfaces are either: (a) an erosional features that developed after the landslide, in which case the boundary surface age provides a minimum age for the landslide; or (b) detachment surfaces from which slabs of near-surface sediment were removed during landsliding, in which case the age of the sediment above the boundary surface indicates the approximate age of landsliding. While an earthquake-triggering mechanism is favoured for the initiation of submarine landslides on the eastern Australian margin, further evidence is required to confirm this interpretation.