Interpreting flash flood palaeoflow parameters from antidunes and gravel lenses: An example from Montserrat,West Indies

The wavelength of stationary water‐surface waves and their associated antidune bedforms are related to the mean velocity and depth of formative flow. In past published sand‐bed flume experiments, it was found that lens structures were preserved during antidune growth and change, and the dimension of the lenses was empirically related to antidune wavelength, and thus could be used to estimate flow velocity and depth. This study is the first to compare observations of formative flow conditions and resulting sedimentary structures in a natural setting, testing the previously published relationship at a field‐scale. Trains of stationary and upstream migrating water‐surface waves were prevalent during the flash flood in October 2012 in the Belham Valley, Montserrat, West Indies. Wave positions and wavelengths were assessed at 900 sec intervals through the daylight hours of the event within a monitored reach. The wave data indicate flow depths up to 1·3 m and velocity up to 3·6 m sec^?1. Sedimentary structures formed by antidune growth and change were preserved in the event deposit. These structures include lenses of clast‐supported gravel and massive sand, with varying internal architecture. The lenses and associated low‐angle strata are comparable to sand‐bed structures formed from stationary and upstream migrating waves in flume experiments, confirming the diagnostic value of these structures. Using mean lens length in the event deposit underestimated peak flow conditions during the flood and implied that the lenses were preserved during waning flow.     

512—1铀矿床矿化特征及其成因探讨

512-1铀矿床位于白依背斜北翼志留系下统中,含矿岩层为碳质泥质板岩所夹的硅灰岩透镜体。区内岩浆岩不发育。铀矿体受地层、岩性、东西向走向断层和北东向切层断裂破碎带控制,呈透镜状,以边幕式斜列产出。合矿岩石主要为破碎或碎裂的硅质灰岩、硅岩。铀赋存形式以吸附状态为主,有少量的沥青铀矿、铀黑和铀的次生矿物。铀通过同生富集,后生再造等多期地质作用富集成工业矿体。     

The influence of lee sediment behind large bed elements on bedload transport rates in supply-limited channels

A coarse surface layer can help to limit bedload transport rates in channels with cobble and gravel beds. In these systems, periodic boulder-sized clasts often exist with small deposits of fine material in the lee of these large bed elements. A combined field and flume study was conducted to investigate the potential impact of lee deposits with distinctly finer sediment-sizes behind boulders on bedload transport rates. Detailed sediment characterizations were performed on surface, subsurface, and lee sediments in two coarse-bedded Connecticut channels. Bedload measurements also were conducted in a series of flows that approached the bankfull level in these two systems to determine transport rates and the size distribution of bedload material. A 6-m long, 0.5-m wide flume was used to model these systems with fine sediment passing over a fixed bed of sediment particles with uniform-sized, large bed elements. Sediment distributions of the lee deposits in the two Connecticut channels indicate that lee deposits may be produced from winnowing of sediments from the surface layer. Lee deposits also exhibit sediment distributions similar to bedload sediment distributions from low to near-bankfull flow in one of the two channels. Bedload sediments in the second channel were finer than lee deposits, presumably from selective entrainment of fines. Flume experiments demonstrate that bedload transport rates are lower for periods of steady flow relative to periods that include either an increase or decrease in discharge. The results show that lee sediments establish a metastable deposit behind each obstruction for a given discharge. Either increases or decreases in discharge disrupt this temporary stability and increase sediment delivery to the main flow. The study suggests that the influence of the rate of change in discharge may be as important as the absolute magnitude of discharge on sediment transport rates at moderate and low discharges in sediment-limited systems with large bed elements.     

大桥坞火山岩型铀矿床地质特征及勘查方法评述

通过新一轮找矿工作（2006-2008年）的勘查实践及研究认为：（1）大桥坞火山岩型铀矿床赋矿主岩为火山角砾岩,具两种类型的角砾岩,一种为火山通道相隐爆角砾岩．另一种为火山口相受层间构造控制的透镜状角砾岩;NW向组断裂构造是主控矿构造,矿床各矿带铀矿体的总体走向主要为NW向。（2）普查阶段勘查工程的布设应注重灵活机动原则,随地质认识的加深和变化及时调整工程布置方案。     

Formation and growth of multiple,distinct ice lenses in frost heave

This paper presents a fully coupled thermo-hydro-mechanical (THM) model which simulates frost heave in fully saturated soils. The model is able to simulate the formation and growth of multiple distinct ice lenses. The basic equations of the system were derived using the continuum theory of mixtures, nonequilibrium thermodynamics, and fracture mechanics, considering skeleton deformation, water flow and heat transport. Central to this model is the coupled transport of mass due to the temperature gradient across the frozen fringe, which acts as the main driving force of the phenomenon. The model is formulated in terms of measurable physical properties and thus no ad hoc parametrization is required. In an ice-lens-free state, the system is solved as a continuum using the finite element method (FEM). It is then locally treated as a discontinuous system upon the formation of ice lens, by enriching the elements carrying the embedded ice lens(es) using the extended finite element method (X-FEM). The accuracy and efficiency of the proposed model has been verified using several laboratory tests on Devon silt samples at different overburden pressures and thermal boundary conditions. Shut-off pressures have been also estimated and compared with the experimental results.     

Zoned Zircon from Eclogite Lenses in Marbles from the Dabie-Sulu UHP Terrane, China： A Clear Record of Ultra-deep Subduction and Fast Exhumation

Eclogite lenses in marbles from the Dabie-Sulu ultrahigh-pressure (UHP) terrane are deeply subducted meta-sedimentary rocks. Zircons in these rocks have been used to constrain the ages of prograde and UHP metamorphism during subduction, and later retrograde metamorphism during exhumation. Inherited (detrital) and metamorphic zircons were distinguished on the basis of transmitted light microscopy, cathodoluminescence (CL) imaging, trace element contents and mineral inclusions. The distribution of mineral inclusions combined with CL imaging of the metamorphic zircon make it possible to relate zircon zones (domains) to different metamorphic stages. Domain 1 consists of rounded, oblong and spindly cores with dark-luminescent images, and contains quartz eclogite facies mineral inclusion assemblages, indicating formation under high-pressure (HP) metamorphic conditions of T = 571-668℃and P = 1.7-2.02 GPa. Domain 2 always surrounds domain 1 or occurs as rounded and spindly cores with white-luminescent images. It contains coesite edogite facies mineral inclusion assemblages, indicating formation under UHP metamorphic conditions of T = 782-849℃and P > 5.5 GPa. Domain 3, with gray-luminescent images, always surrounds domain 2 and occurs as the outermost zircon rim. It is characterized by low-pressure mineral inclusion assemblages, which are related to regional amphibolite facies retrograde metamorphism of T = 600-710℃and P = 0.7-1.2 GPa. The three metamorphic zircon domains have distinct ages; sample H1 from the Dabie terrane yielded SHRIMP ages of 245±4 Ma for domain 1, 235±3 Ma for domain 2 and 215±6 Ma for domain 3, whereas sample H2 from the Sulu terrane yielded similar ages of 244±4 Ma, 233±4 Ma and 214±5 Ma for Domains 1, 2 and 3, respectively. The mean ages of these zones suggest that subduction to UHP depths took place over 10-11 Ma and exhumation of the rocks occurred over a period of 19-20 Ma. Thus, subduction from～55 km to > 160 km deep mantle depth took place at rates of approximately 9.5-10.5 km/Ma and exhumation from depths >160 km to the base of the crust at～30 km occurred at approximately 6.5 km/Ma. We propose a model for these rocks involving deep subduction of continental margin lithosphere followed by ultrafast exhumation driven by buoyancy forces after break-off of the UHP slab deep within the mantle.     

Finite element analysis of flow patterns near geological lenses in hydrodynamic and hydrothermal systems

We use theoretical and numerical methods to investigate the general pore-fluid flow patterns near geological lenses in hydrodynamic and hydrothermal systems respectively. Analytical solutions have been rigorously derived for the pore-fluid velocity, stream function and excess pore-fluid pressure near a circular lens in a hydrodynamic system. These analytical solutions provide not only a better understanding of the physics behind the problem, but also a valuable benchmark solution for validating any numerical method.
  Since a geological lens is surrounded by a medium of large extent in nature and the finite element method is efficient at modelling only media of finite size, the determination of the size of the computational domain of a finite element model, which is often overlooked by numerical analysts, is very important in order to ensure both the efficiency of the method and the accuracy of the numerical solution obtained. To highlight this issue, we use the derived analytical solutions to deduce a rigorous mathematical formula for designing the computational domain size of a finite element model. The proposed mathematical formula has indicated that, no matter how fine the mesh or how high the order of elements, the desired accuracy of a finite element solution for pore-fluid flow near a geological lens cannot be achieved unless the size of the finite element model is determined appropriately.
  Once the finite element computational model has been appropriately designed and validated in a hydrodynamic system, it is used to examine general pore-fluid flow patterns near geological lenses in hydrothermal systems. Some interesting conclusions on the behaviour of geological lenses in hydrodynamic and hydrothermal systems have been reached through the analytical and numerical analyses carried out in this paper.     

On deposits controlled by tectonic lenses

Tectonic lenses (e.g., boudinage structures,pinch-and-swell structures) are common geologicalstructures observed in various scales ranging fromseveral hundreds of kilometers (e.g., lenticular terranesor massifs) in orogenic belts to a few millimeters (e.g.,core-mantle structures and porphyroclasts) in petro-graphic thin-sections. The most frequently observedtectonic lenses are those developed in metamorphicrocks where compositional layers with distinctrheological strengths coexist. No matter h…     

Hydrate bearing clayey sediments: Formation and gas production concepts

Hydro-thermo-chemo and mechanically coupled processes determine hydrate morphology and control gas production from hydrate-bearing sediments. Force balance, together with mass and energy conservation analyses anchored in published data provide robust asymptotic solutions that reflect governing processes in hydrate systems. Results demonstrate that hydrate segregation in clayey sediments results in a two-material system whereby hydrate lenses are surrounded by hydrate-free water-saturated clay. Hydrate saturation can reach ≈2% by concentrating the excess dissolved gas in the pore water and ≈20% from metabolizable carbon. Higher hydrate saturations are often found in natural sediments and imply methane transport by advection or diffusion processes. Hydrate dissociation is a strongly endothermic event; the available latent heat in a reservoir can sustain significant hydrate dissociation without triggering ice formation during depressurization. The volume of hydrate expands 2-to-4 times upon dissociation or CO₂CH₄ replacement. Volume expansion can be controlled to maintain lenses open and to create new open mode discontinuities that favor gas recovery. Pore size is the most critical sediment parameter for hydrate formation and gas recovery and is controlled by the smallest grains in a sediment. Therefore any characterization must carefully consider the amount of fines and their associated mineralogy.