Australian Paleocyclone Records The Sedimentary Record

The sedimentary record of paleocyclones along the northeast, northern and western Australian coast typically occurs in the form of multiple shore parallel ridges standing up to 5-6 m above Australian Height Datum (AHD) or mean sea level

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Fig. 1 Location sites across northern and Western Australia

Gulf of Carpentaria

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Fig. 1 Location sites across northern and Western Australia

(Fig. 1). The ridges at any one location are composed of either coral fragments, or sand with layers of marine shells, or pure sand, or lithic gravels and in one instance in Western Australia the ridges are entirely composed of one species of shell (F. eragatum). The composition of the ridges ay any one location is a function of the availability of source materials.

Tsunamis are unlikely to be a mechanism responsible for deposition of any of the ridge sequences along the northeast Queensland coast. Eye witness accounts reveal that ridges are commonly formed during tropical cyclone surge and wave events throughout the south Pacific and also along the Queensland coast (Nott, 2003). Postevent surveys of tsunamis over the past decade do not reveal deposition of coral shingle or sand/shell ridges even though these waves impact sections of coast where this sediment is available to be transported landward. Indeed, sediment is deposited onshore but usually as a sheet that tapers in thickness landwards and not as a distinct ridge (Nott, 2006).

Eolian activity is also unlikely to be responsible for deposition of the ridges discussed here. These ridges contain either coral fragments, marine shells or coarsegrained sands derived from the beach which, in the latter case, would have been submerged by storm surge during emplacement of the ridge. As is shown here, and in more detail by (Nott, 2003), only waves and surge generated by intense tropical cyclones can generate marine inundations sufficiently high to reach the crests of these ridges.

At every site examined the ridges get progressively older with distance inland. The most recently deposited ridges occur on the seaward side of the ridge plain. In some locations there can be two distinct sections to the beach ridge plain; an inner (Pleistocene) barrier plain sequence and an outer (more seaward) Holocene sequence. The inner barrier plain sequence was typically deposited during the last interglacial when sea-levels were last at or near their present position. The outer barrier sequence usually begins to form just after 6,000 yrs B.P when present sea-levels stabilized. Hence the oldest, most inland ridge of the outer barrier sequence typically has an age of between 5,500-6,000 yrs B.P. The outer and inner barrier ridge plain sequences are often, but not always, separated by a lagoon or swamp. Many of the sites examined so far throughout northern Australia only have the outer (Holocene) barrier ridge plain preserved.

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