Deep-water sediment transport patterns and basin floor topography in early rift basins: Plio-Pleistocene syn-rift of the Corinth Rift,Greece

Our current understanding on sedimentary deep-water environments is mainly built of information obtained from tectonic settings such as passive margins and foreland basins. More observations from extensional settings are particularly needed in order to better constrain the role of active tectonics in controlling sediment pathways, depositional style and stratigraphic stacking patterns. This study focuses on the evolution of a Plio-Pleistocene deep-water sedimentary system (Rethi-Dendro Formation) and its relation to structural activity in the Amphithea fault block in the Corinth Rift, Greece. The Corinth Rift is an active extensional basin in the early stages of rift evolution, providing perfect opportunities for the study of early deep-water syn-rift deposits that are usually eroded from the rift shoulders due to erosion in mature basins like the Red Sea, North Sea and the Atlantic rifted margin. The depocentre is located at the exit of a structurally controlled sediment fairway, approximately 15 km from its main sediment source and 12 km basinwards from the basin margin coastline. Fieldwork, augmented by digital outcrop techniques (LiDAR and photogrammetry) and clast-count compositional analysis allowed identification of 16 stratigraphic units that are grouped into six types of depositional elements: A—mudstone-dominated sheets, B—conglomerate-dominated lobes, C—conglomerate channel belts and sandstone sheets, D—sandstone channel belts, E—sandstone-dominated broad shallow lobes, F—sandstone-dominated sheets with broad shallow channels. The formation represents an axial system sourced by a hinterland-fed Mavro delta, with minor contributions from a transverse system of conglomerate-dominated lobes sourced from intrabasinal highs. The results of clast compositional analysis enable precise attribution for the different sediment sources to the deep-water system and their link to other stratigraphic units in the area. Structures in the Amphithea fault block played a major role in controlling the location and orientation of sedimentary systems by modifying basin-floor gradients due to a combination of hangingwall tilt, displacement of faults internal to the depocentre and folding on top of blind growing faults. Fault activity also promoted large-scale subaqueous landslides and eventual uplift of the whole fault block.     

Lithogeochemical localisation of disseminated gold in the White River area, Yukon, Canada

Gold mineralisation in the White River area, 80 km south of the highly productive Klondike alluvial goldfield, is hosted in amphibolite facies gneisses in the same Permian metamorphic pile as the basement for the Klondike goldfield. Hydrothermal fluid which introduced the gold was controlled by fracture systems associated with middle Cretaceous to early Tertiary extensional faults. Gold deposition occurred where highly fractured and chemically reactive rocks allowed intense water–rock interaction and hydrothermal alteration, with only minor development of quartz veins. Felsic gneisses were sericitised with recrystallisation of hematite and minor arsenic mobility, and extensively pyritised zones contain gold and minor arsenic (ca 10 ppm). Graphitic quartzites (up to 5 wt.% carbon) caused chemical reduction of mineralising fluids, with associated recrystallisation of metamorphic minerals (graphite, pyrrhotite, pyrite, chalcopyrite) in host rocks and veins, and introduction of arsenic (up to 1 wt.%) to form arsenopyrite in veins and disseminated through host rock. Veins have little or no hydrothermal quartz, and up to 19 wt.% carbon as graphite. Late-stage oxidation of arsenopyrite in some graphitic veins has formed pharmacosiderite. Gold is closely associated with disseminated and vein sulphides in these two rock types, with grades of up to 3 ppm on the metre scale. Other rock types in the White River basement rocks, including biotite gneiss, hornblende gneiss, pyroxenite, and serpentinite, have not developed through-going fracture systems because of their individual mineralogical and rheological characteristics, and hence have been little hydrothermally altered themselves, have little hydrothermal gold, and have restricted flow of fluids through the rock mass. Some small post-metamorphic quartz veins (metre scale) have been intensely fractured and contain abundant gold on fractures (up to 40 ppm), but these are volumetrically minor. The style of gold mineralisation in the White River area is younger than, and distinctly different from, that of the Klondike area. Some of the mineralised zones in the White River area resemble, mineralogically and geochemically, nearby coeval igneous-hosted gold deposits, but this resemblance is superficial only. The White River mineralisation is an entirely new style of Yukon gold deposit, in which host rocks control the mineralogy and geochemistry of disseminated gold, without quartz veins.     

钙华体次生有机体系的界面结晶作用研究

本文总结了钙华体次生有机体系中生物群落特征和界面结晶行为,根据生物在碳酸盐沉积中的角色,将钙华体中次生有机体系的界面结晶矿化类型分为生物控制矿化沉积和生物诱导矿化沉积。分述了碳酸钙沉积矿化相关的五种生物代谢活动,讨论了生物胞外聚合物（EPS）对碳酸钙矿物晶型和形貌的影响。针对目前钙华研究中存在的问题,今后应深入研究生物体主要构成元素、生物小分子和生物体内部组织在钙华形成或退化中的作用,进一步厘清钙华体与次生有机体系的多界面溶化方式,为钙华保护修复及钙华退化治理措施提供科学依据。      

Equilibrium erosion of soft rock shores with a shallow or absent beach under increased sea level rise

A process-based numerical model was used to explore the response of soft rock shores with low volume beaches to variable rates of sea level rise. Equilibrium recession rates were simulated for ranges of wave height and period, tidal amplitude, rock strength, beach volume and rate of sea level rise. Equilibrium shore profiles were found to be steeper with higher rates of sea level rise. Beaches were represented as protective surfaces yet were found to cause no significant reduction in equilibrium recession rate when their volumes were below a critical threshold. Reduced equilibrium recession rates were found with beaches that extended sufficiently far below low tide level. The model results imply that, given several constraints, a very simple relationship exists between increased rates of sea level rise and the response of eroding composite soft rock/low volume beach shores.     

Neoarchean suprasubduction zone arc magmatism in southern India: Geochemistry,zircon U-Pb geochronology and Hf isotopes of the Sittampundi Anorthosite Complex

The Sittampundi Anorthosite Complex (SAC) in southern India is one of the well exposed Archean layered anorthosite-gabbro-ultramafic rock associations. Here we present high precision geochemical data for the various units of SAC, coupled with zircon U-Pb geochronology and Hf isotopic data for the anorthosite. The zircon ages define two populations, the older yield a concordia age of 2541 ± 13 Ma, which is interpreted as the best estimate of the magmatic crystallization age for the Sittampundi anorthosite. A high-grade metamorphic event at 2461 ± 15 Ma is suggested by the upper intercept age of the younger zircon population. A Neoproterozoic event at 715 ± 180 Ma resulted in Pb loss from some of the metamorphic zircons. The magmatic age of the anorthosite correlates well with the timing of crystallization of the arc-related ~ 2530 Ma magmatic charnockites in the adjacent Salem Block, while the metamorphic age is synchronous with the regional metamorphic event. The geochemical data suggest that the rocks were derived from a depleted mantle source. Sub-arc mantle metasomatism of slab derived fluids and subsequent partial melting produced hydrous, aluminous basalt magma. The magma fractionated at depth to produce a variety of high-alumina basalt compositions, from which the anorthositic complex with its chromite-rich and amphibole-rich layers formed as cumulates within the magma chamber of a supra-subduction zone arc. The coherent initial¹⁷⁶Hf/¹⁷⁷Hf ratios and positive εHf values (1.7 – 4.5) of the magmatic zircons in the anorthosite are consistent with derivation of a rather homogeneous juvenile parent magma from a depleted mantle source. Our study further confirms that the southern part of the Dharwar Craton was an active convergent margin during the Neoarchean with the generation and emplacement of suprasubduction zone arc magmas which played a significant role in continental growth.     

The effect of drawdown on suspended solids and phosphorus export from Columbia Lake,Waterloo, Canada

This study examines the effect of drawdown on the timing and magnitude of suspended solids and associated phosphorus export from a 12 ha reservoir located in an urbanized watershed in southern Ontario, Canada. Water level in Columbia Lake was lowered by 1·15 m over a 2‐week period in November 2001. The total phosphorus (TP) concentrations ranged from 63 to 486 µg L^?1 in Columbia Lake and 71 to 373 µg L^?1 at its outflow. All samples exceeded the Provincial Water Quality Objective of 30 µg TP L^?1. Outflow concentrations of suspended solids and TP increased significantly with decreasing lake level and were attributed to the resuspension of cohesive bottom sediments that occurred at a critical threshold lake level (0·65 m below summer level). Suspended solids at the outflow consisted of flocculated cohesive materials with a median diameter (D₅₀) of c. 5 µm. Particulate organic carbon accounted for 8·5% of the suspended solids export by mass. A total mass of 18·5 t of suspended solids and 62·6 kg TP was exported from Columbia Lake, which represents a significant pulse of sediment‐associated P to downstream environments each autumn during drawdown. The downstream impacts of this release can be minimized if the water level in Columbia Lake is lowered no more than 0·5 m below summer levels. Copyright © 2004 John Wiley & Sons, Ltd.     

Testing solar forcing of pervasive Holocene climate cycles

The temporal and spatial extent of Holocene climate change is an area of considerable uncertainty, with solar forcing recently proposed to be the origin of cycles identified in the North Atlantic region. To address these issues we have developed an annually resolved record of changes in Irish bog tree populations over the last 7468 years which, together with radiocarbon‐dated bog and lake‐edge populations, extend the dataset back to ～9000 yr ago. The Irish trees underpin the internationally accepted radiocarbon calibration curve, used to derive a proxy of solar activity, and allow us to test solar forcing of Holocene climate change. Tree populations and age structures provide unambiguous evidence of major shifts in Holocene surface moisture, with a dominant cyclicity of 800 yr, similar to marine cycles in the North Atlantic, indicating significant changes in the latitude and intensity of zonal atmospheric circulation across the region. The cycles, however, are not coherent with changes in solar activity (both being on the same absolute timescale), indicating that Holocene North Atlantic climate variability at the millennial and centennial scale is not driven by a linear response to changes in solar activity. Copyright © 2005 John Wiley & Sons, Ltd.     

Source mechanism of volcanic tremor: fluid-driven crack models and their application to the 1963 kilauea eruption

We propose a model for the mechanism of magma transport based on a fluid-filled tensile crack driven by the excess pressure of fluid. Such a transport mechanism can generate seismic waves by a succession of jerky crack extensions, if the fracture strength of rock varies in space, or if there is a difference between the dynamic and static values of the critical stress intensity factor. We also find that the opening and closing of a narrow channel connecting two fluid-filled cracks may be a source of seismic waves. Using the finite-difference method, we calculated the vibration of dry and fluid-filled cracks generated by: (1) a jerky extension at one end or at both ends and (2) a jerky opening of a narrow channel connecting two cracks. We then calculated the far-field and near-field radiation from these vibrating cracks. The spectra show peaked structures, but interestingly, most high-frequency peaks are only present in the near-field and cannot be transmitted to the far-field. The spectral features described above are often observed for volcanic tremors and in some cases for seismic signals associated with hydraulic fracturing experiments.We first consider as a model of volcanic tremor randomly occurring jerky crack extensions, and derive a formula relating the tremor amplitude to the excess pressure in the magma, the incremental area in each extension, and the frequency of extensions. These parameters are also constrained by other observations, such as the rate of magma flow.Our model was tested quantitatively against observations made in one of the best-described case histories of volcanic tremor: the October 5–6, 1963 Kilauea flank eruption. We found that a single, long crack extending from the summit to the eruptive site cannot explain the observations. The model of a steadily expanding crack ran into difficulties when quantitative comparisons were made with observations. The extension of crack area needed to explain the amplitude of volcanic tremor should accompany a large increase in tremor period which was not observed.Our second model is a chain of cracks connected by narrow channels which open and close. The length of each crack is around 1 km, the channel area connecting neighboring cracks is about 10³m², and the channel opens jerkily with the magmatic excess pressure of about 20 bars. The frequency of jerky opening of each channel is about once in 15 seconds. The channel is closed after each jerky opening, as soon as magma is moved through the channel.     

Water Table Uncertainties due to Uncertainties in Structure and Properties of an Unconfined Aquifer

We consider two sources of geology‐related uncertainty in making predictions of the steady‐state water table elevation for an unconfined aquifer. That is the uncertainty in the depth to base of the aquifer and in the hydraulic conductivity distribution within the aquifer. Stochastic approaches to hydrological modeling commonly use geostatistical techniques to account for hydraulic conductivity uncertainty within the aquifer. In the absence of well data allowing derivation of a relationship between geophysical and hydrological parameters, the use of geophysical data is often limited to constraining the structural boundaries. If we recover the base of an unconfined aquifer from an analysis of geophysical data, then the associated uncertainties are a consequence of the geophysical inversion process. In this study, we illustrate this by quantifying water table uncertainties for the unconfined aquifer formed by the paleochannel network around the Kintyre Uranium deposit in Western Australia. The focus of the Bayesian parametric bootstrap approach employed for the inversion of the available airborne electromagnetic data is the recovery of the base of the paleochannel network and the associated uncertainties. This allows us to then quantify the associated influences on the water table in a conceptualized groundwater usage scenario and compare the resulting uncertainties with uncertainties due to an uncertain hydraulic conductivity distribution within the aquifer. Our modeling shows that neither uncertainties in the depth to the base of the aquifer nor hydraulic conductivity uncertainties alone can capture the patterns of uncertainty in the water table that emerge when the two are combined.     

A new record of Colpocoryphe (Trilobita) from the Skiddaw Group of the Cross Fell inlier,northern England

The trilobite Colpocoryphe sp. is described for the first time from the Kirkland Formation, Llanvirn, of the Skiddaw Group in the Cross Fell inlier. It is considered to be closely related to a form previously recorded from the upper Arenig of the Lake District. This new record supports the previously suggested Gondwanan aspects of the northern England Avalonian fauna during the early Ordovician. Copyright © 2000 John Wiley & Sons, Ltd.