Brian G. Jones

Holocene sea-level change on the southeast coast of Australia: a review

A revised Holocene sea-level curve for the southeast coast of New South Wales, Australia, is presented based on a review of previously published geochronological results for fossil molluscs, organic-rich mud, mangrove roots and fixed biological indicators. It is supplemented by new radiocarbon and amino acid racemization-derived ages on fossil molluscs from transgressive sandsheet facies in back-barrier settings within shallow incised valleys along the southern coast of New South Wales. This data base has been limited to fossils with accurate descriptions of their facies associations and stratigraphic relationships to present mean sea level. Results show that sea level during the Holocene marine transgression rose to between −15 and −11 m at 9400—9000 cal. yr BP. Sea level then rose to approximately −5 m by 8500 cal. yr BP and to approximately −3.5 m between 8300 and 8000 cal. yr BP inundating shallow incised valleys resulting in the deposition of shell-rich transgressive sandsheets within shallow incised bedrock valleys. Present sea level was attained between 7900 and 7700 cal. yr BP, approximately 700—900 years earlier than previously proposed. Sea level continued to rise to between +1 and +1.5 m between 7700 and 7400 cal. yr BP, followed by a sea-level highstand that lasted until about 2000 cal. yr BP followed by a gradual fall to present. A series of minor negative and positive oscillations in relative sea level during the late-Holocene sea-level highstand appear to be superimposed over the general sea-level trend. However, the precise nature of the oscillations are difficult to quantify because of problems associated with accurately determining palaeotidal and wave regimes, climatic conditions and the antecedent morphology of the shallow marine environments during the mid Holocene.

Large-scale washover sedimentation in a freshwater lagoon from the southeast Australian coast: sea-level change, tsunami or exceptionally large storm?

A distinct lens of marine sand, up to 90 cm thick, confined vertically by peat, is found in the upper fill of a closed freshwater back-barrier lagoon on the southeast Australian coast. Coring of the deposit suggests it extends continuously up to 600 m inland and tapers landward rising to ~1.6 m above principle datum. In places the sand is overlain by accumulations of organic-rich silt that contain charophytes, indicating re-establishment of lagoon conditions. Hypotheses considered for the deposition of the sandsheet are higher Holocene sea level, storms and tsunami. Ground-penetrating radar transects of the seaward dune system suggest a penecontemporaneous erosional contact between a series of truncated pre-event dunes and several small overlying post-event dunes. Dating the sandsheet was problematic but it is confined to the last 800 years. The young age combined with a lack of associated beach deposits and evidence of wave scouring suggest that a higher sea-level hypothesis is unlikely. This sand lens is attributed to a large-scale washover event from the southeast. Based on comparisons with modern storm deposits from the same coast and sedimentological diagnostic criteria derived from studies of modern storm- and tsunami-deposited sandsheets, it is concluded that this sand deposit is the product of a short-lived, large-scale overwash event attributed to a late-Holocene tsunami.

Fluvial Architecture of the Hawkesbury Sandstone (Triassic), Near Sydney, Australia

ABSTRACT The Hawkesbury Sandstone has long been assumed to represent the deposits of a large braided river system, comparable in style and magnitude with the modern Brahmaputra River of Bangladesh. Such an interpretation is based mainly on the common occurrence of very large-scale crossbedding, but no architectural studies of the unit have hitherto been carried out. This paper represents a first attempt to estimate the magnitude of Hawkesbury channels and bars on the basis of the preserved architectural evidence. Photomosaics were constructed of two cliff sections south of Sydney, one 5.6 km in length. On the basis of these profiles we estimate that characteristic channel-scale architectural elements are at least 2.7 km wide, and individual macroforms are 5-10 m high, indicating the constructional depth of typical channels. Hollow elements (scoop-shaped units interpreted to have formed at channel confluences) are up to 20 m deep. These magnitudes are large, but measurably smaller that those of channels and bars in the modern Brahmaputra River of Bangladesh.

Response of coral reefs to climate change: Expansion and demise of the southernmost Pacific coral reef

Received 21 May 2010; revised 20 June 2010; accepted 6 July 2010; published 3 August 2010. [1] Coral reefs track sea level and are particularly sensitive to changes in climate. Reefs are threatened by global warming, with many experiencing increased coral bleaching. Warmer sea surface temperatures might enable reef expansion into mid latitudes. Here we report multibeam sonar and coring that reveal an extensive relict coral reef around Lord Howe Island, which is fringed by the southernmost reef in the Pacific Ocean. The relict reef, in water depths of 25–50 m, flourished in early Holocene and covered an area more than 20 times larger than the modern reef. Radiocarbon and uranium‐series dating indicates that corals grew between 9000 and 7000 years ago. The reef was subsequently drowned, and backstepped to its modern limited extent. This relict reef, with localised re‐establishment of corals in the past three millennia, could become a substrate for reef expansion in response to warmer temperatures, anticipated later this century and beyond, if corals are able to recolonise its surface. Citation: Woodroffe, C. D., B. P. Brooke, M. Linklater, D. M. Kennedy, B. G. Jones, C. Buchanan, R. Mleczko, Q. Hua, and J. Zhao (2010), Response of coral reefs to climate change: Expansion and demise of the southernmost Pacific coral reef, Geophys. Res. Lett., 37, L15602, doi:10.1029/2010GL044067.

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.

The Hawkesbury Sandstone south of Sydney, Australia; Triassic analogue for the deposit of a large, braided river

ABSTRACT The Hawkesbury Sandstone is a Triassic sheet sandstone, extensively exposed in the Sydney Basin, New South Wales, particularly along the coast near Sydney. Unidirectional paleoflow in the sandstone, its freshwater biota, and abundant mudrock intraclasts indicate fluvial deposition. Sheet morphology, low paleocurrent variance, abundant erosion surfaces, and the paucity of in situ mudrocks point to a braided fluvial system. Three facies assemblages have been recognized: stratified sandstone, massive sandstone, and a minor mudrock assemblage. The stratified sandstone assemblage is dominated by stacked sets of planar cross-strata and minor trough cosets in sequences 6-23 m thick, bounded by erosion surfaces. Significant paleocurrent changes between channel sequences indicate that they were initiated by the avulsion of major channel systems. In some cases the channel sequences fine upward, with planar cross-stratal sets overlain by trough sets that decrease in magnitude upwards, fining up into mudrocks. The massive sandstone assemblage occurs principally as massive sandstone in elongate erosional features oriented transverse to paleoflow. The massive sandstone commonly contains large mudrock intraclasts and is attributed to failure of high channel banks and/or large bedforms during falling-water stages. The mudrock assemblage comprises rippled and horizontally laminated fine sandstone, siltstone, and shale, with minor mudstone. It is attributed primarily to floodplain deposition, but abandoned channel fills are also present. The Hawkesbury Sandstone does not conform to existing models for braided-fluvial deposition in that planar cross-strata accumulated in deeper parts of channels, whereas trough cross-strata formed in shallower water. The large scale of planar cross-strata (sets up to 7.5 m thick), mudrock intraclasts (up to 38 m long), bank-collapse scars (up to 11 m deep) and abandoned channel fills (up to 18 m deep) show that the Hawkesbury River was very large, with deep main channels and high but variable discharge.

Continental aridification and the vanishing of Australia's megalakes

The nature of the Australian climate at about the time of rapid megafaunal extinctions and humans arriving in Australia is poorly understood and is an important element in the contentious debate as to whether humans or climate caused the extinctions. Here we present a new paleoshoreline chronology that extends over the past 100 k.y. for Lake Mega-Frome, the coalescence of Lakes Frome, Blanche, Callabonna and Gregory, in the southern latitudes of central Australia. We show that Lake Mega-Frome was connected for the last time to adjacent Lake Eyre at 50–47 ka, forming the largest remaining interconnected system of paleolakes on the Australian continent. The final disconnection and a progressive drop in the level of Lake Mega-Frome represents a major climate shift to aridification that coincided with the arrival of humans and the demise of the megafauna. The supply of moisture to the Australian continent at various times in the Quaternary has commonly been ascribed to an enhanced monsoon. This study, in combination with other paleoclimate data, provides reliable evidence for periods of enhanced tropical and enhanced Southern Ocean sources of water filling these lakes at different times during the last full glacial cycle.

Marine-planation terraces on the shelf around Grand Cayman: a result of stepped Holocene sea-level rise

The shelf around Grand Cayman consists of two seaward-sloping terraces separated by a mid-shelf scarp . Except along the exposed windward margin where coral growth is dominant, the upper terrace (0-10 m bal) largely consists of a barren rocky pavement traversed by erosional furrows. Exposure related trends in the morphology and distribution of these erosional features, and the lack of coral growth , demonstrates that the terrace is the result of contemporary erosion during seasonal storms . The upper terrace is terminated by a mid-shelf scarp (10-20 m bal) that, in most areas, is partially to completely buried by modern carbonate deposits . Along narrow sections of the leeward shelf, the scarp is commonly exposed and displays an erosional intertidal notch at - 18.5 m. The lower terrace (12-40 mbsl) extends from the mid-shelf scarp to the shelf edge. Its surface is a modern reef-and- sediment wedge that thickens toward the shelf edge, reaching up to 40 m in thickness. These deposits are underlain by a seaward-sloping bedrock terrace (20-40 m bal). This buried terrace and the mid-shelf scarp, which are geomorphic equivalents of the upper terrace and coastal cliff, represent an earlier episode of marine planation when sea level was stabilized at a lower position. The contemporary erosional features of the upper -shelf terrace and the presence of identical terraces around recently uplifted islands demonstrates that the terraces on Grand Cayman were sculptured by marine erosion during the last deglacial sea-level rise. The lower terrace and the mid-shelf scarp were eroded during a slow-rise episode from 11- 7 ka and were subsequently drowned by an extremely rapid, 5 m rise-event at - 7 ka. Following this catastrophic event, which drowned fast -growing Acropora reefs in other areas of the Caribbean , sea-level stabilized and rose slowly to its present position , producing the upper terrace. This pronounced stepped pat tern in Holocene sea-level rise remains to be confirmed from outside the Caribbean-Atlantic reef province but is consistent with the stepped nature of pre-Holocene sea-level curves. The presence of seaward sloping terraces on many shelves around the world suggests that erosional terrace cutting is a common phenomenon during sea-level rise. In contrast, terraces in areas that have undergone relative sea-level fall are constructional in origin, produced entirely by reef accretion. This suggests that there is a genetic relationship between the sea-level cycle and terrace type: erosional terraces form during rise and constructional terraces during fall.

Massive sandstone facies in the Hawkesbury Sandstone, a Triassic fluvial deposit near Sydney, Australia

ABSTRACT Massive sandstone is a common facies in the Hawkesbury Sandstone, a Triassic formation attributed to deposition by a large braided river. The massive sandstone occurs as sheets and in elongate depressions trending perpendicular to the paleoslope, as indicated by the unidirectional mean orientation of abundant cross-strata. Some of the depressions filled with massive sandstone also contain angular mudstone intraclasts, up to several meters in length. Massive sandstone is also associated with deformed cross-strata, formed by progressive loss of lamination during mass movement down foreset slopes. Foreset failure is attributed to liquefaction due to falling water level, or to collapse of adjacent mud banks. The latter mechanism resulted in rapid loading of bedform foresets, and introduce large mudclasts into the massive sand. Sand and mudclasts accumulated in transverse scours at the foot of the foreset slopes, but also travelled along the scours, or spilled out to form massive sheet sands. The abundance of massive sandstone in the Hawkesbury Sandstone is attributed to deposition in a large river, in which flood stage bedforms were probably up to 15 m high. Because of the size of the bedforms, large volumes of liquefied sand were generated when foreset slopes failed.

Holocene Sea Level Fluctuations and the Sedimentary Evolution of a Barrier Estuary: Lake Illawarra, New South Wales, Australia

Abstract Lithostratigraphy of the Holocene Lake Illawarra barrier estuary on the New South Wales coast, Australia, adds details to previous models of barrier estuary evolution. Establishment of a detailed chronology, with the use of 121 aspartic acid–derived ages and six radiocarbon ages, has allowed the definition of a five-stage geomorphic model for the infill of the barrier estuary. A broad incised valley formed during the sea level lowstand represents the initial stage. Stage two is represented by a basal transgressive marine sand sheet deposited in response to rising sea levels associated with the last postglacial marine transgression, which inundated the shallow incised valley ca. 8000–7500 years ago. This feature is not present in the deeper and narrower incised valleys used to establish previous barrier estuary models. The more open marine conditions, with a diverse assemblage of estuarine and marine mollusc species, persisted until ca. 5000 years ago when the barrier started to become emergent and resulted in the development of a low-energy back-barrier lagoonal environment (stage 3). A late Holocene regression (1–2 m) of sea level between 3200–2500 years ago (stage 4) further restricted oceanic circulation and increased the rate of fluvial bay-head delta progradation. The final stage has seen a rapid extension of the fluvial deltas and increased rates of lagoonal sedimentation during the past 200 years as a result of land clearing for agriculture and urban and industrial development. This five-stage evolutionary model of barrier estuary evolution developed for Lake Illawarra can be applied to other shallow estuaries on tectonically stable, wave-dominated coastlines.