Sedimentary facies,production rates and facies models for recent coralline algal gravels,Co. Galway,Ireland

Five sedimentary facies are described from SCUBA diving examination and sampling of Mannin Bay, Ireland. A Bank facies is built up by the unattached coralline algae Lithothamnium corallioides and Phymatolithon calcareum. This autochthonous facies occurs in shallow sheltered environments. In exposed areas a rippled Clean Algal Gravel facies is found composed of coralline and molluscan debris. In intermediate energy areas a Muddy Algal Gravel facies is found with small amounts of live corallines. Sheltered creeks have a Mud facies which is partly carbonate and partly terrigenous. The shallow water coralline algal sediments are overlapped by a Fine Sand facies of mixed biogenic composition. Each facies is characterized by particular phenotypic growth forms of the unattached corallines. Rates of organic calcium carbonate production are obtained which are found to be similar to rates from shallow tropical non-reef environments. The carbonate sediments of Mannin Bay are compared with similar sediments from Kilkerrin Bay. Ireland, from Brittany and from Falmouth Harbour. From these comparisons, facies models are proposed for these carbonate sediments. The major factor controlling facies distribution is coastal morphology. The present day shelf is considered to be too exposed to preserve complete sequences of the shallow water sediments.     

Surface lowering of limestone pavement as determined by cosmogenic (36Cl) analysis

Determination of the rate and total amount of limestone pavement surface lowering is a critical issue in developing models of regional landscape change in limestone terrain. Erratic‐capped pedestals have frequently been used for this purpose but problems concerning definition and measurement of pedestal height, and the absence of a secure timeframe for erratic emplacement, have resulted in conflicting interpretations. We have used cosmogenic (³⁶Cl) to establish the emplacement age of erratic boulders and the total amount of pavement surface lowering at sites in northwest England. Since erratic emplacement at 17.9 ka the limestone pavement has been lowered by 22–45 cm (average: 33 ± 10 cm), assuming lowering was continuous. Although indicating some spatial heterogeneity, the results contrast with earlier reported values based on the measurement of pedestal heights and inferred age for deglaciation. We consider that changes in climate and the character and duration of regolith covers to have been important influences in promoting surface lowering. It is argued that nivation (chemical and mechanical snow‐related processes) associated with several cool/cold periods is likely to have played an important role in surface lowering. Complicating factors associated with surface lowering (thickness and longevity of snow and regolith covers) are identified but as yet cannot be quantified. Copyright © 2012 John Wiley & Sons, Ltd.     

Paraglacial sedimentation and denudation processes in an Alpine valley of Switzerland. An approach to the quantification of sediment budgets

Abstract

As a result of recent drillings in the Walensee Valley (eastern Switzerland) a new facies model for the Quaternary filling of Alpine valleys has been developed. A detailed lithological model and some new radiocarbon dating allowed the calculation of regional sedimentation and denudation rates and their change during the Late- and Postglacial period. It is shown that these changes follow the paraglacial sedimentation model by Church and Ryder [1]. The absolute quantification of the sediment budgets between the Last Glaciation and today points to denudation rates in the order of 1.5 mm y^?1 for the catchment of the Lake of Walenstadt. This is 50 % higher than suggested from current tectonic and isostatic estimates up to now. In that case present day uplift of the Alps would not be in balance with denudation. © Elsevier, Paris     

Magma fragmentation dynamics: experiments with analogue porous low-strength material

 Experiments were conducted on the fragmentation of analogue low-strength porous material (plastiprin) by rapid decompression in a shock-tube-type apparatus. The porous samples (length=365 mm, cross-section dimensions 40×40 mm) pressurized by air to pressures up to 0.9 MPa, were rapidly decompressed to 0.1 MPa. Rapid decompression of samples caused fragmentation and ejection of the fragmentation products into a large volume tank. The process of analogue material fragmentation was documented using high-speed cinematography and dynamic pressure measurements. The duration of the fragmentation event is significantly shorter than that of the ejection event. The fragmentation of material precedes the acceleration of fragments. As a result of fragmentation, sub-parallel fractures are generated. The characteristic fragment size decreases as the initial pressure differential increases. The ejected fragments obtain velocities of 60 m/s. The mechanisms of material fragmentation during unloading and fragmentation wave propagation are discussed. The experimental results provide insight into the fragmentation dynamics of highly viscous magmas in which brittle failure at high strain rate is possible. Received: 23 July 1997 / Accepted: 23 November 1997     

实践中发展的最大有效力矩准则

最大有效力矩准则发表以来, 不断得到新的野外观察和实验结果的验证, 正在得到国内外同行的支持。岩石的变形行为取决于自身力学性质、构造层次(包括温度、压力、流体压力等因素)和应变速率。浅构造层次中平面菱网状构造和剖面共轭膝褶带, 钝角面对缩短方向, 受最大有效力矩准则控制, 是对慢应变速率的构造响应。证明应变速率对变形行为的控制程度不亚于构造层次。韧、脆性构造的共存意味着构造演化过程为快、慢应变速率的交替。      

Trace metal composition of and accumulation rates of sediments in the Upper Gulf of Thailand

Sediment cores and grab samples were collected in the Upper Gulf of Thailand to determine sedimentation rates and to determine if metal concentrations reflect anthropogenic inputs.Accumulation rates of sediments in the Upper Gulf measured using the ²¹⁰Pb method, appear to vary from ca. 4 to 11 mm yr^?1. Sediment budgets suggest that little of the sediment delivered to the Upper Gulf by the major rivers is ultimately transported to the Lower Gulf. Metal concentrations in Upper Gulf sediments appear to be dominantly controlled by natural inputs.     

库车坳陷北部白垩系沉积速率分析

库车坳陷白垩纪经历了一个相对独立的一级构造旋回,结合坳陷北部库车河白垩系剖面详细的磁性地层学结果,应用地层回剥分析方法恢复了库车坳陷白垩系时期的沉降历史,计算得到亚格列木组、舒善河组、巴西盖组、巴什基奇克组沉积速率,分别为11.5 cm/ka、2.5 cm/ka、2.9 cm/ka、1.7 cm/ka。结合裂变径迹资料和岩相古地理特征,将库车坳陷白垩系沉降历史分为四个演化阶段:（1）Berriasian中期（141.9141.2 M a）:极快速沉降;（2）Berriasian中晚期至A lb ian中晚期（141.2101 M a）,持续缓慢沉降;（3）A lb ian末期至Campan ian早期（10179.1 M a）:构造隆升,沉积剥蚀;（4）Campan ian中晚期—M aastrichtian期（79.165.6 M a）:极慢速沉降,接受沉积。早白垩世早中期沉降曲线呈“上凹”特征,反映了构造活动由早期剧烈沉降到后期减弱,最后趋于稳定的过程。该沉降过程与典型的前陆盆地挠曲沉降过程并不一致。      

Late Miocene–Pliocene eclogite facies metamorphism, D'Entrecasteaux Islands, SE Papua New Guinea

The D'Entrecasteaux Islands of south‐eastern Papua New Guinea are active metamorphic core complexes that formed within a region where the plate tectonic regime has transitioned from subduction to rifting. While rapid, post 4 Myr exhumation and cooling of amphibolite and greenschist facies rocks that constitute the footwall of the crustal scale detachment fault system have been previously documented on Fergusson and Goodenough Islands of the D'Entrecasteaux chain, the timing of eclogite facies metamorphism in rocks of the footwall was unknown. Recent work revealed that at least one of the eclogite bodies formed during the Pliocene. We present combined in situ ion microprobe U–Pb age analyses of zircon from five variably retrogressed eclogite samples from Fergusson and Goodenough Islands that document Late Miocene–Pliocene (8–2 Ma) eclogite formation on these islands. Textural relationships and zircon–garnet rare earth element partition coefficients indicate that U–Pb ages constrain zircon crystallization under eclogite facies conditions in all samples. Results suggest westward younging of eclogite facies metamorphism from Fergusson to Goodenough Island. Present‐day exposure of Late Miocene–Pliocene eclogites requires exhumation rates > 2.5 cm yr^?1.     

Relations entre production organique et apports terrigènes dans les sédiments fluviatiles holocènes : observations et conclusions hétérodoxes

Accumulation of organic matter in fens of fluvial valleys is often related to a low terrigenous matter delivery and to palaeoenvironmental conditions inducing low mechanical erosion. These assumptions come from the interpretation of contents in organic (MO) and mineral (MM) matters in sediments, expressed in percents, and then exactly anticorrelated. Calculation of mass accumulation rates of MO $(T_{{\mathrm{a}}_{\mathrm{MO}}})$ and MM $(T_{{\mathrm{a}}_{\mathrm{MM}}})$, expressed in g?m^?2?yr^?1, shows that $T_{{\mathrm{a}}_{\mathrm{MO}}}$ and $T_{{\mathrm{a}}_{\mathrm{MM}}}$ generally are not anticorrelated and that high values of $T_{{\mathrm{a}}_{\mathrm{MO}}}$ and $T_{{\mathrm{a}}_{\mathrm{MM}}}$ could appear simultaneously. That expression of MO and MM accumulation makes it possible to precise the climatic and human impact on sedimentation. To cite this article: J.-J. Macaire et al., C. R. Geoscience 337 (2005).     

Investigation of a large collapse sinkhole affecting a multi-storey building by means of geophysics and the trenching technique (Zaragoza city,NE Spain)

An active sinkhole around 100 m long has been investigated in the city of Zaragoza (NE Spain). Subsidence activity on this depression, including the sudden occurrence of a collapse sinkhole 5 m across, led to the abandonment of a factory in the 1990s. At the present time, a building with 100 flats and shallow pad foundations partially built on the sinkhole, is affected by rapid differential settlement. The development of the sinkhole results from the karstification of the halite- and glauberite- bearing bedrock and the sagging and collapse of the overlying bedrock and alluvium, more than 30 m thick. GPR and electrical resistivity profiles have provided information on the distribution and geometry of the subsidence structure. The application of the trenching technique and geochronological methods (AMS and OSL dating) has allowed us to infer objective and practical data on the sinkhole including (1) Limits of the subsidence structure, (2) subsidence mechanisms, (3) cumulative subsidence (>408 cm), (4) subsidence rates on specific failure planes (>1.8 cm/year), (5) episodic displacement regime of some fault planes. The available information indicates that the progressive deformation recorded in the building will continue and might be punctuated by events of more rapid displacement. This work illustrates the practicality of the trenching technique for the study of sinkholes in mantled karst areas.