Doctor of Philosophy
School of Earth and Environmental Sciences - Faculty of Science
Switzer, Adam D, Depositional characteristics of recent and late Holocene overwash sandsheets in coastal embayments from southeast Australia, PhD thesis, School of Earth and Environmental Sciences, University of Wollongong, 2005. http://ro.uow.edu.au/theses/436
Sedimentary evidence for large-scale washover deposition along the Australian southeast coast is found as sandsheets in estuaries and anomalous boulder accumulations on rocky ramps and headlands. Large imbricated allochtonous boulders found elevated up to 33m above present sea-level indicate high energy deposition on sheltered rock ramps and coastal headlands and attest to very high wave energy in the past. Analysis of the sedimentary structures within the boulders can in places be related to ramp lithology and identify significant transport distances where the original position of the boulder is identified. The most striking boulder deposits are found in the ambient location of a large bay and have been transported more than 30 m horizontally along a boulder ramp and elevated to a height of more than 7 m above present sea-level. Although this evidence is striking, it is impossible to identify if the boulders were moved in one movement or have been moved by several events over time.
This study presents sedimentological evidence for a tsunami event(s) found in the upper fill of several embayments along the coast. Laterally extensive marine sandsheets are identified in back-barrier lagoons and elevated shell-rich sands are found on the margin of a large drowned river valley. These deposits exist up to 3km from the influence of modern coastal processes. The back-barrier sandsheets contrast with the finer confining sediments of the coastal lagoon and barrier estuary.
Interpretation of the depositional history of the deposits is conducted with reference to a global review of overwash deposition and a modern analog from the southeast Australian coast. In the absence of any geological evidence for known tsunami events on this coast, the deposits are compared to two modern sandsheets that were deposited during two large storms in 2001. This study therefore allows direct comparison of the deposits of unknown source to the storm deposited sediments found Abrahams Bosom Beach.
A large sandsheet is identified at Killalea Lagoon is composed of mixed marine sediment dominated by dune and beach sand but also containing platy heavy mineral assemblages indicative of nearshore to inner shelf sediments. This marine sediment is mixed with clumps of coastal vegetation and rip-up clasts of soil and lagoonal muds and clays.
The marginal drowned river valley material in a small embayment at Batemans Bay includes a coarse shell-unit containing large, often articulated, bivalve shells and oysters, along with cobbles of mixed lithology in a matrix of marine sand suggesting deposition of predominantly seaward tidal channel material.
The advantage of the sandy deposits lies in the analysis of their internal sedimentology and the presence of datable peat and shelly sands that confine the sand sheets and coarse shell-rich deposit respectively. The internal sedimentology of the deposit yields little structure and sediments consist of a series of massive, laminated and graded beds that often incorporate organic debris. These deposits contrast with storm washover deposits investigated at Abrahams Bosom Beach from the same coast that consist of thin graded beds of beach face and dune sediment only. These latter beds are a few centimeters thick and can be traced throughout the deposit. The storm deposits are the result of numerous wave-generated landward pulses while the larger more chaotic deposit is the result of several very large pulses capable of eroding and transporting shelf, nearshore, dune and terrestrial sediments landward followed by partial reworking by back flow. These characteristics are indicative of the chaotic reworking of such deposits by shortlived high-energy events attributed to tsunami.
Dating of the Batemans Bay sequence was problematic and the shell-rich unit can only be confined to an age of around 1000 yrs At Killalea Lagoon optically stimulated luminescence (OSL) dating of quartz sediments from the sandsheets supplemented dating of the confining peat deposits by AMS radiocarbon.. Although affected by groundwater contamination, dates from the Killalea Lagoon site suggest that the depositional event is attributed to a large-scale inundation event around 1500AD adding to a considerable bank of published dates that cluster around this period. Possible tsunami sources include sediment slides off the continental slope of Australia or New Zealand, seismic events in New Zealand and the Macquarie Ridge and bollide impacts in the Tasman Sea or southwest Pacific.
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