Edward A Bryant

Evidence of Tsunami Sedimentation on the Southeastern Coast of Australia

In coastal regions, the highest magnitude storms cannot always be invoked to account for large-scale, anomalous sediment features. Any coastline in the Pacific Ocean region can be affected by tsunamis, including Australia, which historically lacks evidence of such events. Geologically, tsunamis along the New South Wales coast have deposited a suite of Holocene features that consist of anomalous boulder masses, either chaotically tossed onto rock platforms and backshores or jammed into crevices; highly bimodal mixtures of sand and boulders; and dump deposits consisting of well-sorted coarse debris. In addition, many coastal aboriginal middens were distributed by such events. Within estuaries, tsunamis have left a record of stranded run-up ridges that have been interpreted mistakenly as cheniers. Dating of such deposits indicates that several events have affected this coastline since 3000 BP. In contrast to storm waves, tsunamis can leave a depositional imprint of their passage characterized by chaotic sorting and mixing of sediments either from different coastal environments or of different sediment sizes. The preservation potential of these deposits is high where sediments have been deposited above present sea level or stranded inland.

Geological Indicators of Large Tsunami in Australia

Tsunami waves can produce four general categories of depositional and erosional signatures that differentiate them from storm waves. Combinations of items from these categories uniquely define the impact of palaeo-tsunami on the coastal landscape. The largest palaeo-tsunami waves in Australia swept sediment across the continental shelf and obtained flow depths of 15–20 m at the coastline with velocities in excess of 10 m -1. In New South Wales, along the cliffs of Jervis Bay, waves reachedelevations of more than 80 m above sea-level with evidence of flow depths in excess of 10 m. These waves swept 10 km inland over the Shoalhaven delta. In northern Queensland, boulders more than 6 m in diameter and weighing 286 tonnes were tossed alongshore above cyclone storm wave limits inside the Great Barrier Reef. In Western Australia waves overrode and breached 60 m high hills up to 5 km inland. Shell debris and cobbles can be found within deposits mapped as dunes, 30 km inland. The array of signatures provide directional information about the origin of the tsunami and, when combined with radiocarbon dating, indicate thatat least one and maybe two catastrophic events have occurred during the last 1000 years along these three coasts. Only the West Australian coast hashistorically been affected by notable tsunami with maximum run-up elevations of 4–6 m. Palaeo-tsunami have been an order of magnitude greater than this. These palaeo-tsunami are produced most likely by large submarine slides on the continental slope or the impactof meteorites with the adjacent ocean.

Catastrophic wave erosion on the southeastern coast of Australia: Impact of the Lanai tsunamis ca. 105 ka?

Sand barriers along the coast of southern New South Wales, dating from the last interglacial, have been almost completely destroyed, most probably by a catastrophic tsunami. Evidence for catastrophic wave erosion can also be traced to heights of at least 15 m above present sea level on coastal abrasion ramps. These erosional features lie above the range of effective erosion by contemporary storm waves, and cannot be attributed to either eustatic fluctuations or local uplift. Chronological evidence for the timing of the destruction of the last interglacial barriers suggests that tsunamis generated by the submarine slide off Lanai in the Hawaiian Islands 105 ka traveled across the Pacific and eroded this coast.

Sea Level, Storm, or Tsunami: Enigmatic Sand Sheet Deposits in a Sheltered Coastal Embayment from Southeastern New South Wales, Australia

Abstract An extended period of marine-dominated back-barrier sedimentation, truncated by the deposition of two large-volume, laterally extensive sand sheets that extend up to 3 km inland from the modern coastline, is present at Minnamurra in southeastern Australia. The sand sheets are anomalous as they drape the entire back-barrier succession and infill the estuary. The lower sand sheet contains mud clasts and significant amounts of organic matter including coastal dune grasses indicating that significant overwash and erosion incorporated material from both the barrier and estuary into the lower sand sheet deposit. The first sand sheet is overlain by a poorly developed soil that records a time break before the now infilled system was inundated once more. The second sand sheet covers the poorly developed soil and incorporates a number of erosional features, such as ripped-up soil clasts, cobbles, and organic-rich sand. Along this part of the coast, large laterally extensive sand sheets are spatially and sedimentologically unique, occurring only in embayments that face southeast. This, along with a lack of evidence of wave action or beach structures, suggests that the deposits are not evidence for higher Holocene sea level but are the result of overwash sedimentation from the southeast. The sand sheets share many of the characteristics of those emplaced by tsunami waves, although the possibility of exceptionally large storm events cannot be entirely discarded. However, such a storm would have to be several orders of magnitude higher than those in recorded history.

Bedrock-Sculpturing by Tsunami, South Coast New South Wales, Australia

Bedrock-sculpturing resulting in s-forms is associated with catastrophic flooding in near- and subglacial environments produced by flow velocities approximating

Theoretical constraints and chronological evidence of Holocene coastal development in central and southern New South Wales, Australia

10 m S^{-1}

The impact of tsunami on the coastline of Jervis Bay, Southeastern Australia

. These velocities can also be produced by extreme tsunami generated by submarine landslides or comet impacts with oceans. Repetitive tsunami events during the late Holocene have overwashed headlands along the New South Wales south coast and produced two suites of bedrock-sculptured terrain. At the smaller scale, s-forms similar to muschelbrüche, v-shaped grooves, and sichelwannen have developed on upslopes while broad potholes, flutes, and transverse troughs have formed on headland crests. Cavitation features consisting of sinuous grooves, impact marks, drill holes, and cavettos appear more ubiquitously. At the larger scale stripped ramps, large potholes, cascade channels and canyon-like features have been generated. Six flow phenomena: Mach-stem waves, jetting, vortex impingement, horseshoe vortices, helical flow, and multiple vortex formation are all involved, either singly or in combination with each other, in the creation of bedrock-sculptured features and terrain. Tsunami-sculptured terrain undoubtedly has a global distribution whose extent requires further investigation.

Extreme Marine Inundations (Tsunamis?) of Coastal Western Australia

Abstract Lord Howe Island, situated 600 km east of Australia, provides a unique opportunity to evaluate Late Quaternary highstands of sea level in the Tasman Sea. The mid-ocean island, which is the site of the southernmost coral reef, is composed of basalts of late Tertiary age, and calcarenites derived from bioclastic reefal carbonates. Both erosional and depositional evidence of Late Quaternary highstands of sea level is preserved. Uranium-series disequilibrium dating of coral clasts from a calcarenite beach facies at Neds Beach on the northeast of the island yielded a mean age of 136,000 yr B.P. Thermoluminescence dating of the quartz sand fraction from the same deposit, using fine-grained and coarse-grained methods, yielded ages of 138,000 and 116,000 yr B.P., respectively. These ages are interpreted to indicate that this beach unit, within which fossil bones and eggs of the extinct horned turtle, Meiolania , are found, formed during the Last Interglacial when the sea was 2–4 m above present. Benches and platforms developed on Tertiary basalt and on Late Pleistocene calcarenite on the more sheltered lagoonal shore on the west of the island indicate a sea level up to 1.5 m higher than present during the Holocene. Cemented boulder conglomerates (ca. 3000 yr B.P.) at North Head, and emergent mollusc-rich carbonate muds (ca. 900 yr B.P.) within an embayment fill at Old Settlement Beach, further support this interpretation. These palaeo-sea-level data from the Tasman Sea support previous estimates of the height of the Last Interglacial sea surface relative to eastern Australia, and supplement a growing body of evidence for a higher sea level in the region during the mid to late Holocene.