Figure 4 - uploaded by Giada Bufarale
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Profile A-B: sandbars, for location refer to Figure 3. Length: ~1755 m. Top: orthophoto from SLIP Enabler portal (Landgate Imagery). Darker colours represent areas covered with seagrass meadows. Pink dash line depicts the crest of the sandbars. Bottom: uninterpreted and interpreted seismic profile showing the buried architecture below the sandbars. The reflector TP is almost flat; conversely, reflector TP1 is irregular with a possible palaeochannel, marked with dash line. The vertical axis corresponds to the depth below sea level (BSL) and the scale is in metres. The sound velocity in the sediments is equivalent to ~2000 m/s.
Source publication
High-resolution shallow seismic profiles collected along the inner shelf in Geographe Bay (south-west Australia) illustrate a highly-variable buried architecture. Three main acoustic units, separated by unconformities, correspond to different geological facies, deposited under various sea-level conditions. The acoustic basement (Unit B) belongs to...
Context in source publication
Context 1
... 1, 3). The distance between successive sandbar crests increases to the north-east, ranging from about 250 m to more than 1.5 km. These sandbars vary greatly in size from about 100-300 m wide with the dune crest up to ~3 m above the seabed, and thin seaward. They are generally asymmetrical, with an almost bare stoss flank and a vegetated lee side (Fig. ...
Citations
... While it is becoming well documented that nonreefal accumulations, such as stacked aeolianites and beachrocks, also have the ability to form bathymetric highs on the modern seafloor (e.g., Brooke et al., 2017;Bufarale et al., 2019;Green et al., 2020;Lebrec et al., 2022aLebrec et al., , 2022bO'Leary et al., 2020;Passos et al., 2019) and can misleadingly exhibit reefal morphologies in seismic-reflection data (Bubb & Hatlelid, 1977;Salzmann et al., 2013), pre-Quaternary carbonate aeolianites and other relict coastal features are rarely documented in the geologic literature (e.g., Abegg & Handford, 2001;Dodd et al., 2001;Kindler & Davaud, 2001;McKee & Ward, 1983;Smith et al., 2001), and non-reefal carbonate buildups are seldom described by seismic interpreters. This is particularly puzzling given the ability of drowned coastal features to exhibit buildup morphologies and to form both carbonate and siliciclastic barrier complexes-composed of beachrocks, aeolianites and other coastal sedimentary deposits preserved through early cementation-forming seafloor ridges enclosing lagoons, bays or estuaries (e.g., Alcántara-Carrió et al., 2013;Brooke et al., 2010;De Falco et al., 2015;Gardner et al., 2007;Lebrec et al., 2022a;Locker et al., 1996;Mellett et al., 2012;Passos et al., 2019;Sade et al., 2006;Wenau et al., 2020). ...
Linear buildups formed in tropical carbonate environments are often interpreted as bioconstructed reefs. Nevertheless, coastal processes can also form extensive sedimentary ridges exhibiting buildup morphologies. This study investigates two Miocene ridges developed along the Australian North West Shelf using 3D seismic and well data. Ridge 1 is ca. 30 m thick and >60 km long, and it is made of foraminiferal pack‐grainstones. It protects a lagoon with pinnacle morphologies. Ridge 2 is ca. 150 m thick and >80 km long. It is composed of quartz sand forming lobes. Both ridges have a continuous curvilinear front and are in a mid‐shelf setting. They mimic the modern Australian coastline. It is then proposed that Ridge 1 is either: (1) a barrier reef developed on a drowned shoreline, or (2) stacked carbonate aeolianites and beachrocks acting as a barrier. Ridge 2 is interpreted as stacked deltaic sands. This study demonstrates that lithified and buried coastal features of carbonate and siliciclastic nature can form extensive ridges exhibiting buildup morphologies. It is proposed that ridges formed by stacked coastal features are overall continuous with a curvilinear front, while reefal ridges are more discontinuous and exhibit deeper and more stable passes.
The Busselton area in southwestern Australia is characterised by three distinct coastal plains along the foot of Whicher Range that formed mainly by marine attrition during progressive sea-level falls through the Cenozoic and, to a lesser extent, erosion by long-lived rivers. None of the geological and geomorphic units, all of which overlie the Lower Cretaceous Leederville Formation, show evidence of tectonic tilting so, in the absence of masking dune systems, this area offers a reference for Cenozoic global sea levels.
The oldest landform, a marine erosional surface at 112–166 m ASL on Whicher Range, is a remnant of the Blackwood Plateau capped by in situ laterite of likely Eocene age. The Whicher Scarp, with relief of about 120 m, formed by marine erosion removing much of the Leederville Formation during a progressive Eocene–Miocene sea-level fall (~43–13 Ma). At 72–83 m ASL, the Yelverton Bench represents a probable Miocene stillstand during this fall. The scarp below this bench is characterised by a piedmont laterite lithologically and spatially distinct from the older plateau laterite on Whicher Range. The toe-line at 41 m ASL marks the base of the Whicher Scarp and the beginning of the coastal plains—it represents the geomorphic expression of a buried Pliocene (~2.8 Ma) erosional surface at 29 m ASL.
At 21–41 m ASL, the Ambergate Plain is a terrestrially re-sedimented marine erosion surface covered by continuous strand facies of the upper Pliocene Yoganup Formation, which in turn is overlain by lateritized clay that may correlate with the Pliocene–Pleistocene Guildford Formation. The main heavy-mineral strands formed as ancestral shorelines and are embedded within the Yoganup Formation across the entire Ambergate Plain. The Cemetery Scarp, with a relief of 11 m and associated erosion surface at 5 m ASL, cuts into the Ambergate Plain and probably formed during an early Pleistocene interglacial highstand, possibly MIS 11 (~400 ka).
The Ludlow Plain at 3–5 m ASL is covered by low eolian swales and ridges of shelly calcareous sand up to 6 m thick containing coral fragments of possible MIS 5e (~124 ka) age attributed to the Tamala Limestone, which marks the beginning of marine platform carbonate production. The Busselton Wetland Plain was formed during a gentle recession after the Holocene highstand at 7.5 ka following the last glacial maximum and is recognised from a low scarp on the seaward edge of the Ludlow Plain. Although the Capel River has a history spanning the last 30–40 Ma, most rivers draining the scarp postdate the Pliocene. The build-up of barrier beach dunes during the last 7500 years next to the present coast has diverted rivers on the Wetland Plain and forced outlets to Geographe Bay to migrate laterally. Lateritization was episodic, principally in the Eocene, Miocene and Pliocene, but after the deposition of the Guildford Formation, did not extend through the Pleistocene or Holocene.
