R. W. Young

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.

Comparative uranium-thorium and thermoluminescence dating of weathered quaternary alluvium in the tropics of Northern Australia

Abstract Thermoluminescence (TL) age determinations of alluvial sediments in the tropics are evaluated by comparison with U/Th age determinations of pedogenic accumulations in the alluvium of the lower Gilbert River, a large fan delta in the wet-dry tropics of northern Queensland, Australia. This study extends U/Th dating by applying it not only to calcretes, but also to Fe/Mn oxyhydroxide/oxide accumulations. While a direct correlation cannot be made between U/Th dates from secondary minerals and TL dates from the host sediments, both sets of data show broad consistency. In addition to providing a minima for acceptable TL ages, U/Th dates are useful for determining the chronology of pedogenesis/diagenesis. They show that calcretes and ferricretes have formed under similar climatic conditions in the wet-dry tropics of northern Australia during the late pleistocene. Beneath about 5–12 m the Gilbert fan delta consists of an extensive sand body older than 85,000 yr and probably about 120,000 yr in age, representative of a period of major fluvial activity not repeated since this time. Above are muds and fine sandy muds that extend uninterrupted to the present surface except in the downstream fan where they are bisected by a thin unit of medium sand that TL dates at 40,000–50,000 yr B.P. A system of sandy distributary channels over the fan surface represents an early Holocene fluvial phase probably more active than at present.

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.

Wearing Down, Wearing Back, and Gorge Extension in the Long-Term Denudation of a Highland Mass: Quantitative Evidence from the Shoalhaven Catchment, Southeast Australia

The long standing issue of the dominance of scarp retreat versus summit lowering in the denudation of a highland mass is investigated with supporting evidence provided by Tertiary basalts throughout the Shoalhaven catchment in southeast Australia. Both of these forms of denudation are found to be insignificant compared to the role of fluvial gorge extension over the last 30 m.y. Headward advancement of the Shoalhaven Gorge has been occurring at approximately 15 times the rate of major escarpment retreat, 250 times the average rate of summit lowering, and over 500 times the rate of interfluve consumption. These estimates show that the headward erosion of gorges is the most important process denuding the highlands in the Shoalhaven region and possibly elsewhere in the highlands of eastern Australia. Over the long term, the highlands in this region will become considerably more dissected well before they decrease substantially in height or are narrowed. This conclusion has important implications for models predicting isostatic rebound from assumed character and rates of denudation.

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 age, extent and geomorphological significance of the Sassafras basalt, south‐eastern New South Wales

. 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.

Character and origin of deep arenaceous weathering mantles on the bega batholith, southeastern Australia

Abstract Silcretes on the N.S.W. coast near Ulladulla have long been attributed to. a sub‐basaltic origin, but field evidence is at odds with all variations of the sub‐basaltic hypothesis, and one site shows good evidence that the basalt post‐dates the silcrete. K‐Ar ages averaging 29.7 ± 0.5 Ma from the basalts provide a minimum age for silcrete development in this area. Furthermore, the K‐Ar dates, together with evidence for an erosional rather than tectonic origin of the coastal lowland, demonstrate that the adjacent tablelands reached their present elevation prior to the mid‐Oligocene.