Asymmetric,multiple-block collapse at Rotorua Caldera,Taupo Volcanic Zone,New Zealand

Calderas worldwide have been classified according to their dominant collapse styles, although there is a good deal of speculation about the processes involved. Recent laboratory experiments have tried to constrain these processes by modelling magma withdrawal and observing the effects on overlying materials. However, many other factors also contribute to final caldera morphology. Rotorua Caldera formed during the eruption of the Mamaku Ignimbrite. Collapse structure and evolution of Rotorua Caldera is interpreted based its geophysical response, geology and geomorphology, and the stratigraphy of the Mamaku Ignimbrite. Rotorua Caldera is situated at the edge of the extensional Taupo Volcanic Zone, in which major faults strike NE-SW. A second, less dominant fault set strikes NW-SE. These two fault sets have a strong influence on the morphology of Rotorua Caldera. No one style of collapse can be applied to Rotorua Caldera; it was formed during a single eruption, but subsided as many blocks and shows features of trapdoor, piecemeal and downsag types of collapse. Here Rotorua Caldera is described, according to its composition, activity and geometry, as a rhyolitic, single event, asymmetric, multiple-block, single locus collapse structure. The Mamaku Ignimbrite is the only ignimbrite to have erupted from Rotorua Caldera. Extracaldera thickness of the Mamaku Ignimbrite is up to 145 m, whereas inside the caldera it may be greater than 1 km thick. The Mamaku Ignimbrite can be separated into a basal tephra sequence and main ignimbrite sequence. The main ignimbrite sequence contains no observable flow unit boundaries but can be split into lower, middle and upper parts (LMI, mMI, uMI respectively) based on crystal content, welding, jointing, devitrification and vapour phase alteration. Juvenile clasts within the ignimbrite comprise three consanguineous silicic pumice types and andesitic fragments. Only the most evolved pumice type occurs in the basal tephra sequence. All three pumice types occur together throughout the main ignimbrite sequence, whereas the andesitic fragments are only present in uMI. Lithic lag breccias in uMI indicate a late stage of caldera collapse. Concentration of lithic fragments increases towards the middle of the ignimbrite, and may also reflect increased subsidence rate during an earlier stage. Collapse of Rotorua Caldera is thought to have occurred throughout the eruption of the main ignimbrite sequence of the Mamaku Ignimbrite, allowing simultaneous eruption of all the different pumice types and causing the abrupt transition from deposition of the basal tephra sequence to the main ignimbrite sequence.     

A kinematic hardening constitutive model for sands: the multiaxial formulation

This paper explores the possibility of using well-accepted concepts—Mohr-Coulomb-like strength criterion, critical state, existence of a small strain elastic region, hyperbolic relationship for representing global plastic stress–strain behaviour, dependence of strength on state parameter and flow rules derived from the Cam-Clay Model—to represent the general multiaxial stress–strain behaviour of granular materials over the full range of void ratios and stress level (neglecting grain crushing). The result is a simple model based on bounding surface and kinematic hardening plasticity, which is based on a single set of constitutive parameters, namely two for the elastic behaviour plus eight for the plastic behaviour, which all have a clear and easily understandable physical meaning. In order to assist the convenience of the numerical implementation, the model is defined in a ‘normalized’ stress space in which the stress–strain behaviour does not undergo any strain softening and so certain potential numerical difficulties are avoided. In the first part the multiaxial formulation of the model is described in detail, using appropriate mixed invariants, which rationally combine stress history and stress. The model simulations are compared with some experimental results for tests on granular soils along stress paths lying outside the triaxial plane over a wide range of densities and mean stresses, using constitutive parameters calibrated using triaxial tests. Furthermore, the study is extended to the analysis of the effects induced by the different shapes of the yield and bounding surfaces, revealing the different role played by the size and the curvature of the bounding surface on the simulated behaviour of completely stress- and partly strain-driven tests. Copyright © 1999 John Wiley & Sons, Ltd.     

The aqueous geochemistry of Zr and the solubility of some Zr-bearing minerals

Literature data on the thermodynamics of complexation of Zr with inorganic species, at 25°C, have been critically reviewed. The preponderance of published complexation constants deal with F⁻ and OH⁻ ions. Stability constants for the complexation reactions are relatively independent of ionic strength and thus recomended values for each ligand type are averages of the most reliable data. Complexation constants under elevated conditions (T 250°C andP_v = P_H2O) have been predicted for various Zr complexes (F⁻, Cl⁻, SO₄²⁻ and OH⁻) using Helgeson's electrostatic approach. Predominance diagrams (calculated for simple systems with these constants) suggest that, over a wide range of pH conditions, Zr(OH)_4(aq) will dominate the aqueous geochemistry of Zr except under very high activities of competing ligands (e.g., F⁻, SO₄²⁻).The solubilities of vlasovite [Na₂ZrSi₄O₁₁] and weloganite [Sr₃Na₂Zr(CO₃)₆·3H₂O have been measured in KCI solutions (0.5–1.0 M) at 50°C. Weloganite dissolution is complicated by the predictable precipitation of strontianite (SrCO₃) whereas vlasovite dissolves incongruently. Solubility products for the dissolution of welonganite and vlasovite are determined to be −28.96±0.14 and −20.40±1.18, respectively. Concentrations of Zr up to 10⁻³ m were present in the experimental solutions; the presence of large amounts of Zr in aqueous solutions support the possibility of extensive remobilization of Zr during hydrothermal mineralization.     

Density‐Driven Free‐Convection Model for Isotopically Fractionated Geogenic Nitrate in Sabkha Brine

Subsurface brines with high nitrate (NO₃^?) concentration are common in desert environments as atmospheric nitrogen is concentrated by the evaporation of precipitation and little nitrogen uptake. However, in addition to having an elevated mean concentration of ～525 mg/L (as N), NO₃^? in the coastal sabkhas of Abu Dhabi is enriched in ¹⁵N (mean δ¹⁵N ～17‰), which is an enigma. A NO₃^? solute mass balance analysis of the sabkha aquifer system suggests that more than 90% of the nitrogen is from local atmospheric deposition and the remainder from ascending brine. In contrast, isotopic mass balances based on Δ¹⁷O, δ¹⁵N, and δ¹⁸O data suggest approximately 80 to 90% of the NO₃^? could be from ascending brine. As the sabkha has essentially no soil, no vegetation, and no anthropogenic land or water use, we propose to resolve this apparent contradiction with a density‐driven free‐convection transport model. In this conceptual model, the density of rain is increased by solution of surface salts, transporting near‐surface oxygenated NO₃^? bearing water downward where it encounters reducing conditions and mixes with oxygen‐free ascending geologic brines. In this environment, NO₃^? is partially reduced to nitrogen gas (N₂), thus enriching the remaining NO₃^? in heavy isotopes. The isotopically fractionated NO₃^? and nitrogen gas return to the near‐surface oxidizing environment on the upward displacement leg of the free‐convection cycle, where the nitrogen gas is released to the atmosphere and new NO₃^? is added to the system from atmospheric deposition. This recharge/recycling process has operated over many cycles in the 8000‐year history of the shallow aquifer, progressively concentrating and isotopically fractionating the NO₃^?.     

Estimating sand concentrations using ADCP-based acoustic inversion in a large fluvial system characterized by bi-modal suspended-sediment distributions

Quantifying sediment flux within rivers is a challenge for many disciplines due, mainly, to difficulties inherent to traditional sediment sampling methods. These methods are operationally complex, high cost, and high risk. Additionally, the resulting data provide a low spatial and temporal resolution estimate of the total sediment flux, which has impeded advances in the understanding of the hydro-geomorphic characteristics of rivers. Acoustic technologies have been recognized as a leading tool for increasing the resolution of sediment data by relating their echo intensity level measurements to suspended sediment. Further effort is required to robustly test and develop these techniques across a wide range of conditions found in natural river systems. This article aims to evaluate the application of acoustic inversion techniques using commercially available, down-looking acoustic Doppler current profilers (ADCPs) in quantifying suspended sediment in a large sand bed river with varying bi-modal particle size distributions, wash load and suspended-sand ratios, and water stages. To achieve this objective, suspended sediment was physically sampled along the Paraná River, Argentina, under various hydro-sedimentological regimes. Two ADCPs emitting different sound frequencies were used to simultaneously profile echo intensity level within the water column. Using the sonar equation, calibrations were determined between suspended-sand concentrations and acoustic backscatter to solve the inverse problem. The study also analyzed the roles played by each term of the sonar equation, such as ADCP frequency, power supply, instrument constants, and particle size distributions typically found in sand bed rivers, on sediment attenuation and backscatter. Calibrations were successfully developed between corrected backscatter and suspended-sand concentrations for all sites and ADCP frequencies, resulting in mean suspended-sand concentration estimates within about 40% of the mean sampled concentrations. Noise values, calculated using the sonar equation and sediment sample characteristics, were fairly constant across evaluations, suggesting that they could be applied to other sand bed rivers. © 2018 John Wiley & Sons, Ltd.     

The zoogeomorphology of case-building caddisfly: Quantifying sediment use

Caddisfly (Trichoptera) larvae are an abundant and widespread aquatic insect group characterized by the construction of silk structures, including nets and cases. Case-building caddisfly have the potential to modify the sorting and mobility of sand and fine gravel via: (1) case construction, resulting in altered sediment properties; (2) transporting sediment incorporated into cases over the river bed; and (3) changing the structure of river beds via burrowing activity. To investigate these mechanisms, it is necessary to understand the mass, size distribution and spatial variability of sediment use by case-building caddisfly larvae. We quantified the mineral sediment used by individuals and communities of case-building caddisfly in 27 samples, from three sites on a gravel-bed stream. The mass and size distribution of sediment in individual cases varied between taxa (mass = 0.001–0.83 g, D₅₀ = 0.17–4 mm). The mean mass of sediment used by the caddisfly community was 38 g m⁻² and varied locally. Sediment use was predominantly coarse sand (D₅₀ = 1 mm). 64% of sediment use was attributable to Agapetus fuscipes (Glossosomatidae). Due to within-species variability in case mass, the abundance of most taxa, including A. fuscipes, was only weakly associated with the mass of sediment used by this species, at the river scale. Whilst the caddisfly community used a small percentage of the total sediment available (average 2.99% of the 1–1.4 mm size fraction), A. fuscipes used more fine sediment in their cases at sites where it was more available. Despite variability in local habitat, all sites supported diverse case-building caddisfly communities utilizing mineral sediment. Consequently, geomorphological effects of case-building caddisfly are potentially widespread. The results provide novel insights into the specific grain sizes and quantities of fine sediment used by caddisfly larvae, which represents an important step towards understanding their zoogeomorphic activities. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.     

Order of diagenetic events controls evolution of porosity and permeability in carbonates

The properties of carbonate rocks are often the result of multiple, diagenetic events that involve phases of cementation (porosity occlusion) and dissolution (porosity enhancement). This study tests the hypothesis that the order of these events is a major control on final porosity and permeability. A three-dimensional synthetic model of grainstone is used to quantify trends that show the effect of early cementation, non-fabric selective dissolution, and then a second-generation of (post-dissolution) cement. Models are 3 mm³ with a resolution of 10 μm. Six simple paragenetic sequences are modelled from an identical starting sediment (without accounting for compaction) where the same diagenetic events are placed in different sequences, allowing for quantification of relative changes in the resultant porosity and permeability for each diagenetic event, the trajectory through time, as well as for each final rock. All modelled paragenetic sequences result in reductions in porosity and permeability, but the order of diagenetic events controls the trajectory and final rock properties. Differences in the order of early cement precipitation alone produce variable final values, but all follow the porosity–permeability relationship as expressed by the Kozeny-Carman equation. However, final values for the sequences which include a phase of dissolution fall on a new curve, which departs from that predicted by the Kozeny-Carman relationship. This allows an alternative form of porosity–permeability relationship to be proposed: κ = 2280ϕ–30,400, where ϕ is porosity (%) and κ is permeability (mD). Hence while the Kozeny-Carman relationship predicts porosity–permeability changes that occur with cementation, it is unable to capture accurately changes within the pore network as a result of dissolution. Although the results may be dependent on the properties of the initial carbonate sediment and simplified diagenetic scenarios, it is suggested that this new porosity–permeability relationship may capture some generalized behaviour, which can be tested by modelling further sediment types and diagenetic scenarios.     

Evolution of drainage,sediment-flux and fluvio-deltaic sedimentary systems response in hanging wall depocentres in evolving non-marine rift basins: Paleogene of Raoyang Sag,Bohai Bay Basin,China

The dynamics of sediment feeding into rift basins and the geomorphologic nature of source areas are critical elements in understanding the evolution of rifted basins. This study integrates seismic, well and geochronologic data on the western dipslope of the Raoyang Sag, a rift associated sub-basin to the larger Bohai Bay Basin of China to define the history of drainage development for the basin and to assess the sedimentologic response to drainage evolution events. In the Paleogene-age Lixian Slope, as indicated by paleo-drainage configuration, progradational seismic geometries, compositional maturity and zircon-tourmaline-rutile maturity index trends, three drainages; the paleo-Daqing River, paleo-Tang River and paleo-Dasha River drainages were feeding three closely spaced hanging wall deltaic depositional systems; Delta A fed from the northwest, Delta B fed from the west and Delta C fed from the southwest, respectively. From the late Eocene to early Oligocene, a decrease in sediment-flux into the hanging wall is documented and petrographic analysis is used to link these changes to stream-capture in the upstream catchment of the Daqing River. This change is coupled with morphologic changes in the geometries of Deltas A and C, both of which show decreasing deltaic areas, changes in lobe geometry and changes in distributary channel sizes. In addition, the progradational direction of Delta C changes from perpendicular-to-the-rift axis to prograding oblique-to-the-rift axis. It is apparent that the progradation and retrogradational changes in rift margin deltas do not happen in isolation, but such changes can affect growth and progradation direction in adjacent deltas. This work shows that the decrease in sediment-flux, caused by a drainage capture, will result in a decrease in distributary channel size and delta size and may result in upstream deltas taking advantage of such decreasing confinement to prograde more obliquely to the rift axis.     

Ontogenetic patterns of concentration indicate lagoon nurseries are essential to common grunts stocks in a Puerto Rican bay

Estimates of abundance and size of three commercially exploited grunt species indicate ontogenetic changes in habitat utilization concentrate their juveniles within the lagoon of the Bay of La Parguera, Puerto Rico. Eleven biotopes, defined by four benthic structures (reef, mangrove, vegetation beds and unconsolidated sediments) and three geographic zones (inner lagoon, outer lagoon and bank shelf) were sampled randomly by visual surveys. French, bluestriped and white grunt (Haemulon flavolineatum, Haemulon sciurus and Haemulon plumeri) were common in the bay and appeared to exhibit similar life history patterns of cross-shelf migration and habitat selection. Recently settled grunts were dispersed over vegetated and unconsolidated soft-bottom sediments of the bay. The juvenile stage occurred in highest densities in shallow lagoon biotopes among the submerged prop-roots of mangrove stands and on inshore reefs. Length data indicates that grunts migrate offshore to adult habitat via increasingly deep reefs. Indices of biotope nursery function based on standing stock estimates of juveniles identified three biotopes, all within the inner lagoon as essential habitat for juveniles of 5–10 cm length interval. This concentration of juveniles within biotopes of the lagoon could represent a bottleneck to recruitment for grunt stocks. Evidence that quantity and quality of lagoon nurseries may limit recruitment indicates that these areas represent a key component of a marine protected area designed to restore fisheries within the bay.     

Prevention of beach erosion at Aberystwyth: A success story

In a previous paper in this journal (Wood 1978) the geological and human factors involved in coastal erosion at the north end of the Promenade at Aberystwyth were evaluated. It was concluded that only the addition of suitable material to the beach (replenishment) would halt erosion and protect this portion of the promenade. Even while the paper was passing through the press replenishment was in progress (Autumn 1977), and the purpose of this note is to evaluate its success and to look forward to the future.