Unit bar architecture in a highly-variable fluvial discharge regime: Examples from the Burdekin River,Australia

Unit bars are relatively large bedforms that develop in rivers over a wide range of climatic regimes. Unit bars formed within the highly-variable discharge Burdekin River in Queensland, Australia, were examined over three field campaigns between 2015 and 2017. These bars had complex internal structures, dominated by co-sets of cross-stratified and planar-stratified sets. The cross-stratified sets tended to down-climb. The development of complex internal structures was primarily a result of three processes: (i) superimposed bedforms reworking the unit bar avalanche face; (ii) variable discharge triggering reactivation surfaces; and (iii) changes in bar growth direction induced by stage change. Internal structures varied along the length and across the width of unit bars. For the former, down-climbing cross-stratified sets tended to pass into single planar cross-stratified deposits at the downstream end of emergent bars; such variation related to changes in fluvial conditions whilst bars were active. A hierarchy of six categories of fluvial unsteadiness is proposed, with these discussed in relation to their effects on unit bar (and dune) internal structure. Across-deposit variation was caused by changes in superimposed bedform and bar character along bar crests; such changes related to the three-dimensionality of the channel and bar geometry when bars were active. Variation in internal structure is likely to be more pronounced in unit bar deposits than in smaller bedform (for example, dune) deposits formed in the same river. This is because smaller bedforms are more easily washed out or modified by changing discharge conditions and their smaller dimensions restrict the variation in flow conditions that occur over their width. In regimes where unit bar deposits are well-preserved, their architectural variability is a potential aid to their identification. This complex architecture also allows greater resolution in interpreting the conditions before and during bar initiation and development.     

Controls on channel deposits of highly variable rivers: Comparing hydrology and event deposits in the Burdekin River,Australia

Discharge event frequency, magnitude and duration all control river channel morphology and sedimentary architecture. Uncertainty persists as to whether alluvial deposits in the rock record are a time-averaged amalgam from all discharge events, or a biased record of larger events. This paper investigates the controls on channel deposit character and subsurface stratigraphic architecture in a river with seasonal discharge and very high inter-annual variability, the Burdekin River of north-east Australia. In such rivers, most sediment movement is restricted to a few days each year and at other times little sediment moves. However, the maximum discharge magnitude does not directly correlate with the amount of morphological change and some big events do not produce large deposits. The Burdekin channel deposits consist of five main depositional elements: (i) unit bars; (ii) vegetation-generated bars; (iii) gravel sheets and lags; (iv) antidune trains; and (v) sand sheets. The proportions of each depositional element preserved in the deposits depend on the history of successive large discharge events, their duration and the rate at which they wane. Events with similar peak magnitude but different rate of decline preserve different event deposits. The high intra-annual and inter-annual discharge variability and rapid rate of stage change make it likely that small to moderate-scale bed morphology will be in disequilibrium with flow conditions most of the time. Consequently, dune and unit bar size and cross-bed set thickness are not good indicators of event or channel size. Antidunes may be more useful as indicators of flow conditions at the time they formed. Rivers with very high coefficient of variance of maximum discharge, such as the Burdekin, form distinctive channel sediment bodies. However, the component parts are such that, if they are examined in isolation, they could lead to misleading interpretation of the nature of the depositional environment if conventional interpretations are used.     

Submarine cables: a challenge for ocean management

The international submarine cable industry is a major component of the global telecommunications system, providing important services and requiring increased levels of protection for the maintenance of global economic and maritime security, broadly defined. An overview is given of the submarine cable industry, including its technological developments, legal aspects, security considerations, and implications for integrated ocean planning and management. In the context of multiple ocean use, submarine cables can cause spatial conflicts with other ocean users, particularly the fishing industry. Issues to be examined include compensation for lost or damaged gear, legal liability for cable damage, and regulation and licensing of cables on the seabed of the continental shelf and the high seas. Recent industry-to-industry agreements and programs for compensation to fishers in the USA and Canada are significant advancements in the implementation of integrated approaches to ocean management and planning. The development of transparent and stable interdepartmental processes for the coordinated planning, management and regulation of submarine cables is required for national EEZs. The regulation of international cable industry practices must be harmonized with national management approaches.     

Cation ordering in BaAl2Ge2O8-feldspar: implications for the phase transition in anorthite

BaAl₂Ge₂O₈-Feldspar undergoes an order-disorder phase transition I2/c↔C2/m at T _tr ≈1690 K. The thermodynamics of the Al,Ge cation ordering process is described in terms of the compressible Ising model in mean field approximation. The mean field potential predicts a first order character of the phase transition. This is compared to antiferromagnetic ordering in a two-dimensional square Ising model with NN-pair interactions and four-spin interactions on alternating squares. Calculated order parameters and short range ordering are in good agreement with the corresponding properties observed in BaAl₂Ge₂O₈-feldspar by means of X-ray diffraction, hard mode infrared spectroscopy and TEM. Using known calorimetric data a similar model is postulated for Al,Si ordering in anorthite, CaAl₂Si₂O₈, for which the derived potential describes a transition with slightly stronger first order character at T _tr ≈1928 K. Received: 30 January 1998 / Revised, accepted: 29 August 1998     

Analytical two-dimensional model of a quadrupole magnetosphere

A model of a quadrupole magnetosphere is developed with the aid of conformal mappings. Such a type of magnetosphere might have occurred at the Earth during times of polarity reversals of the dipole field, when the residual field essentially may have had a quadrupole structure. The magnetospheric field-line configuration is calculated for an axial quadrupole directed perpendicular to the solar-wind flow. The model is two-dimensional and leads to analytical formulae.     

A laboratory simulation of pyroclastic flows down slopes

Laboratory experiments are described which explore the dynamical consequences of buoyant convective upflow observed above hot pyroclastic flows. In nature, the convection is produced by the hot ash particles exchanging heat with air mixed into the front and top of the pyroclastic flow. This effect on the buoyancy due to the mixing of air and ash has been modelled in the laboratory using mixtures of methanol and ethylene glycol (MEG), which have a nonlinear density behaviour when mixed with water. Intermediate mixtures of these fluids can be denser than either initial component, and so the laboratory experiments were inverted models of the natural situation. We studied MEG flowing up under a sloping roof in a tank filled with water. The experiments were performed both in a narrow channel and on a laterally unconfined slope. The flow patterns were also compared with those of conventional gravity currents formed using fresh and salt water. The presence of the region of reversed buoyancy outside the layer flowing along the slope had two significant effects. First, it periodically protected the flow from direct mixing with the environment, resulting in pulses of relatively undiluted fluid moving out intermittently ahead of the main flow. Second, it produced a lateral inflow towards the axis of the current which kept the current confined to a narrow tongue, even on a wide slope.In pyroclastic flows the basal avalanche portion has a much larger density contrast with its surroundings than the laboratory flows. Calculations show that mixing of air into the dense part of a pyroclastic flow cannot generate a mixture that is buoyant in the atmosphere. However, the overlying dilute ash cloud can behave as a gravity current comparable in density contrast to the laboratory flows and can become buoyant, depending on the temperature and ash content. In the August 7th pyroclastic flow of Mount St. Helens, Hoblitt (1986) describes pulsations in the flow front, which are reminiscent of those observed in the experiments. As proposed by Hoblitt, the pulsations are caused by the ash cloud accelerating away from the front of the dense avalanche as a density current. The ash cloud then mixes with more air, becomes buoyant and lifts off the ground, allowing the avalanche to catch up with and move ahead of the cloud. The pulsing behaviour at the fronts of pyroclastic flows could account for the occurrence of cross-bedded layer 1 deposits which occur beneath layer 2 deposits in many sequences.     

Pogo and Pangaea

Long wavelength magnetic anomalies of crustal origin derived from the POGO and MAGSAT satellite data often display a strong continuity across the now-rifted continental margins when the continents are reassembled into Pangaea. These anomalies predate the breakup of the supercontinent and represent major blocks whose crustal properties are broadly similar even though those blocks are no longer contiguous.