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Author(s): C. S. Bristow (corresponding author) [1]; S. D. Bailey [1]; N. Lancaster [2] Linear sand dunes--dunes that extend parallel to each other rather than in star-like or crescentic forms--are [...]

Coastal dune accumulations at locations around the world have formed during sea-level highstands, rising sea levels, falling sea levels, and fluctuating sea levels. Initially, episodes of maximum Pleistocene dune activity in Bermuda, which is the type location for eolianite, were correlated with glacio-eustatic regressions. This model had been abandoned by the 1970s in favor of a rising sea-level model for Bermuda's north shore and autogenic sediment-supply model for the south shore. We report the results of a detailed investigation of the intercalated mid-late Pleistocene beach and coastal dune deposits on the islands of Bermuda, and we test the existing models for these deposits. We contend that the north shore model is invalidated by past misidentification of eoliandeposits as transgressive beach deposits. On the south shore, facies analysis of the marine isotope stage 7 (MIS 7) Belmont Formation reveals that coastal deposition was divided into two phases. S1, which includes foreshore and shoreface progradation, is predominantly marine and coincided with rising relative sea level. S2, which commenced with protosol development on top of the marine section prior to burial by advancing dunes, is nonmarine. We conclude that the two successions S1 and S2 resulted from forcing by glacio-hydro-isostatic relative sea-level change spanning a highstand (rising and then falling), rather than from transgressive or autogenic processes. The finding that, in Bermuda, the construction of protective coastal dunes depends on a falling relative sea level has potential implications for many vulnerable shorelines that face rising relative sea levels associated with climate change. [ABSTRACT FROM AUTHOR]

Ground-penetrating radar and luminescence dating studies of a large, complex, linear dune in the northern part of the Namib Sand Sea provide new information on the age and internal sedimentary structures of these dunes, with important implications for interpretations of paleoclimates and the rock record of eolian sandstones. The dune is a composite feature formed during several episodes of construction, including a hiatus of almost 2000 yr. The oldest sands within the dune are 5700 yr old, indicating complete turnover of sand during the Holocene. The dune has moved laterally by ∼300 m during the past 2500 yr, proving lateral migration of a large linear dune. Dune construction has been affected by climate change, and we attribute the hiatus to increased rainfall and vegetation, which largely halted sand movement and dune building in the Namib Desert during the middle Holocene. [ABSTRACT FROM AUTHOR]

Guichen Bay on the south-east coast of South Australia faces west towards the prevailing westerly winds of the Southern Ocean. The bay is backed by a 4 km wide Holocene beach-ridge plain with more than 100 beach ridges. The morphology of the Guichen Bay strandplain complex shows changes in the width, length, height and orientation of beach ridges. A combination of geomorphological interpretation, shallow geophysics and existing geochronology is used to interpret the Holocene fill of Guichen Bay. Six sets of beach ridges are identified from the interpretation of orthorectified aerial photographs. The ridge sets are distinguished on the basis of beach-ridge orientation and continuity. A 2·25 km ground-penetrating radar (GPR) profile across the beach ridges reveals the sedimentary structures and stratigraphic units. The beach ridges visible in the surface topography are a succession of stabilized foredunes that overlie progradational foreshore and upper shoreface sediments. The beach progrades show multiple truncation surfaces interpreted as storm events. The GPR profile shows that there are many more erosion surfaces in the subsurface than beach ridges on the surface. The width and dip of preserved beach progrades imaged by GPR shows that the shoreface has steepened from around 2·9° to around 7·5°. The changes in beach slope are attributed to increasing wave energy associated with beach progradation into deeper water as Guichen Bay was infilled. At the same time, the thickness of the preserved beach progrades increases slightly as the beach prograded into deeper water. Using the surface area of the ridge sets measured from the orthophotography, and the average thickness of upper shoreface, foreshore and coastal dune sands interpreted from the GPR profile, the volume of Holocene sediments within three of the six sets of beach-ridge accretion has been calculated. Combining optically stimulated luminescence (OSL) ages and volume calculations, rates of sediment accumulation for Ridge Sets 3, 4 and 5 have been estimated. Linear rates of beach-ridge progradation appear to decrease in the mid-Holocene. However, the rates of sediment accumulation calculated from beach volumes have remained remarkably consistent through the mid- to late Holocene. This suggests that sediment supply to the beach has been constant and that the decrease in the rate of progradation is due to increasing accommodation space as the beach progrades into deeper water. Changes in beach-ridge morphology and orientation reflect environmental factors such as changes in wave climate and wind regime. [ABSTRACT FROM AUTHOR]

Abstract The effectiveness of detrital zircon thermochronology as a means of linking hinterland evolution and continental basin sedimentation studies is assessed by using Mesozoic continental sediments from the poorly understood Khorat Plateau Basin in eastern Thailand. New uranium lead (U-Pb) and fission-track (FT) zircon data from the Phu Kradung Formation identify age modes at 141 ± 17 and 210 ± 24 Ma (FT) and 2456 ± 4, 2001 ± 4, 251 ± 3, and 168 ± 2 Ma (U-Pb), which are closely similar to data from the overlying formations. The FT data record post-metamorphic cooling, whereas the U-Pb data record zircon growth events in the hinterland. Comparison is made between detrital zircon U-Pb data from ancient and modern sources across Southeast Asia. The inherent stability of the zircon U-Pb system means that 250 Myr of post-orogenic sedimentary recycling fails to change the regional zircon U-Pb age signature and this precludes use of the U-Pb approach alone for providing unique provenance information. Although the U-Pb zircon results are consistent with (but not uniquely diagnostic of) the Qinling Orogenic Belt as the original source terrane for the Khorat Plateau Basin sediments, the zircon FT cooling data are more useful as they provide the key temporal link between basin and hinterland. The youngest zircon FT modes from the Khorat sequence range between 114 ± 6 (Phra Wihan Formation) and 141 ± 17 Ma (Phu Kradung Formation) that correspond to a Late Jurassic/Early Cretaceous reactivation event, which affected the Qinling Belt and adjacent foreland basins. The mechanism for regional Early Cretaceous erosion is identified as Cretaceous collision between the Lhasa Block and Eurasia. Thus, the Khorat Plateau Basin sediments might have originated from a reactivation event that affected a mature hinterland and not an active orogenic belt as postulated in previous models. [ABSTRACT FROM AUTHOR]

ABSTRACT Zircon is the most widely used mineral in detrital dating studies because it is common to multiple rock types, is chemically and physically resistant, and can endure successive cycles of burial, metamorphism and erosion. Zircon also has the advantage that single grains may be dated by either the fission‐track (FT) or U–Pb method, which, because of their contrasting thermal sensitivities (total resetting occurs at temperatures > 320 °C for FT and > 700–900 °C for U–Pb), can provide unique information about both the age structure and the thermal evolution of a sediment source. However, single method‐based bias and difficulties associated with interpreting measured ages can influence both the quality and the level of useful provenance information. For example, the zircon FT system is sensitive to metamorphic overprinting and hence measured ages alone cannot be interpreted as unambiguously dating formation age of the source rock. In contrast, U–Pb zircon data have high resistance (700–900 °C) to thermal overprinting and therefore recorded formation ages may not relate to an immediate source but may instead reflect a polycyclical history. The focus of this paper is to examine, from a practical standpoint, the provenance potential of detrital zircon fission track data and to investigate the method’s complementary role as an aide to the interpretation of high‐temperature detrital U–Pb zircon data by combining U–Pb and FT methods in a single study. [ABSTRACT FROM AUTHOR]