Detrital modes of recent deep-sea sands and their relation to tectonic setting: a first approximation

Complete petrographic analyses of modern deep-sea sands are surprisingly rare, but are needed if problems such as the influence of tectonics on petrology, or changes in petrology due to diagenesis, are to be understood in ancient sandstones. This paper is a first attempt, using data from our own study of 80 sands plus data from the literature, to establish a baseline for modern deep-sea sands with which ancient sandstones can be compared. We assigned sands to five plate-tectonic categories, each of which has a distinct average petrologic composition: (1) basins associated with trailing-edge continental margins, Q₆₂F₂₆L₁₂; (2) with leading-edge continental margins, either subduction, Q₁₆F₅₃L₃₁, or (3) strike-slip, Q₃₄F₃₉L₂₇; (4) back-arc basins, Q₂₀F₂₉L₅₁; (5) fore-arc basins, Q₈F₁₇L₇₅. The amount and type of lithic fragments and the type of feldspar are particularly important discriminators, the amount of quartz less so. Trailing-edge and fore-arc basins have quite distinctive compositions, but back-arc basins and basins near the two types of leading-edge margins show some overlap, and thus may be hard to differentiate in ancient sandstones.     

Quaternary iron-manganese deposits and associated pelagic sediments (radiolarian clay and chert, gypsiferous mud) from the Tyrrhenian Sea

Various types of pelagic sediments occur on a dolomitic basement located at some 1000 m depth on the eastern slope of the Tyrrhenian Sea, i.e. in the most internal, collapsed and presently submerged portion of southern Apennines. The deposits include laminated limonites, muds with manganosiderite nodules, radiolarian clays, opal chert, gypsiferous muds, lutites with calcareous plankton, and all are of Quaternary age. They are interpreted as products of the interactions between submarine hydrothermal activity and deep-sea sediments, and represent the first documented case of submarine hydrothermal sediments not directly connected to active ridges or volcanic buildings and deposits. The iron-rich sediments are very similar to many other reported examples of submarine thermal activity. The radiolarian clay displays very abundant and almost exclusively radiolarian tests, and no calcareous fossils, but shows numerous dissolution traces of carbonate skeletons. It has been interpreted as deriving from a primary bloom of siliceous plankton followed by an extensive leaching of the calcareous tests on the seafloor. The radiolarian chert shows a very early and yet unknown Opal-A cement. The immediate source of the cement is biogenous silica, whereas the ultimate source is the bloom of siliceous plankton triggered by the hydrothermal and volcanic activity. The gypsum muds are the result of a hydrothermal chemical remobilization of the Messinian sulphate beds which overlie the dolomitic basement.     

翼龙类翼膜的折叠，褶皱以及物质特性

Preservation of wing membranes in pterosaurs is only reported from a few localities in the world, namely from the Karatau Hills, Kazakhstan (Upper Jurassic), from the Solnhofen Lithographic Limestone, Germany (Upper Jurassic), the Daohugou Beds of China (Upper Jurassic or Lower Cretaceous) and the Crato and Santana Formations, Chapada do Araraipe, NE Brazil (Lower Cretaceous).     

A linear approach to the influence of discharge measurement error on flood estimates

ABSTRACT

A linear approach is presented for analysing flood discharge series affected by measurement errors which are random in nature. A general model based upon the conditional probability concept is introduced to represent random errors and to analyse their effect on flood estimates. Flood predictions provided by quantiles are shown to be positively biased when performed from a sample of measured discharge. Though for design purposes such an effect is conservative, this bias cannot be neglected if the peak discharges are determined from stage measurements by means of the extrapolated tail of the rating curve for the gauging station concerned. Monte Carlo experiments, which have been carried out to analyse small sample effects, have finally shown that the use of the method of maximum likelihood is able to reduce the bias due to measurement errors in discharge data.     

EVOLUTION AND INFLEXIONAL INSTABILITY OF TWISTED MAGNETIC FLUX TUBES

This paper presents new results concerning evolution and inflexional instability of twisted magnetic flux tubes in the solar corona. Inflexional configurations, attained when the curvature of the tube axis vanishes, are generally present in coronal magnetic structures and are invariably associated with the early stages of kink formation. New equations for the Lorentz force in orthogonal curvilinear coordinates are applied to study the behaviour of twisted flux tubes in presence of inflexion points. We find that inflexional flux tubes are in disequilibrium and evolve spontaneously to inflexion-free configurations, possibly in braid form. These results have important applications for solar coronal structures. First, they prove that the evolution and relaxation of twisted magnetic fields into braid form is a generic feature, confirming the observational evidence of highly twisted and braided structures present in the solar corona. Secondly, they demonstrate that inflexions can trigger kink instabilities, providing a fundamental mechanism for modeling outbreaks of energy into heat, emitted by flares, microflares and mass ejections.