Amineralogical-geochronological study has been made of sedimentary levels I2 and I5 in the upper Precambrian of the Adrar, Mauritania, showing that clay minerals can be used for geochronology. This is, however, only possible where early diagenesis has homogenised the 87Sr/86Sr ratio of clay minerals and interstitial waters. Rocks whose clay minerals consist of unaltered mica or illite are not suitable for this approach, as is the case for sediments rich in detrital feldspars: carbonates may however be used.Ages of 964 ± 32 M.Y. (I2) and860± 35 M.Y. (I5) were obtained using a87Rb decay constant of 1.47 × 10?11 yr?1, confirming that the beginning of the late Precambrian sedimentation in the Adrar may be placed at 1000 M .Y. 相似文献
Sub-micron Fe,Mn-oxides in columbite–tantalite minerals are bound to metamict domains in the host. These nano-oxides are secondary
minerals as the metamict zones formed through accumulation of damages from α-recoil, each of which in a small volume destroys
the crystal lattice of the U and Th bearing columbite–tantalite host. Transmission electron microscope investigations demonstrate
that the oxides fall in the compositional range of magnetite–jacobsite–iwakiite (Fe,Mn)3O4 and of hematite-type minerals along the Fe2O3–Mn2O3 join, i.e., minerals that are known to be major carriers of magnetic remanence. Measured magnetic properties of isolated
columbite–tantalite crystals demonstrate in some samples magnetic remanence, which is bound to sub-micron pseudosingle-domain
crystals rather than to the host. Thus, the magnetic remanence is not related to the formation of columbite–tantalite, but
to the crystallization of the nano-oxides, which occur in metamict domains of the host and therefore are secondary. Depending
on the content and distribution of U and Th, the nano-oxides may form diachronously within an individual and among several
host crystals. Magnetic secondary nano-oxides are not restricted to columbite–tantalite minerals, but may occur in other minerals
where high contents of Fe and Mn are associated with high contents of U and Th. Rocks that show the same spatial distribution
for U and Fe, as for instance some red sandstones, may display magnetic properties that are controlled by recoil-induced growth
of secondary Fe-oxides.
Due to the successful operation of dedicated satellite gravity missions, nowadays high-accuracy global gravity field models
have become available. This triggers the challenge to optimally combine this long to medium wavelength gravity field information
derived from space-borne data with high-resolution terrestrial gravity data. In this paper, the least squares collocation
concept is revised with the attempt to consistently unify the combination procedure in such a way that the full information
contained in both data sets is merged. For example, in local or regional geoid determination the remove-restore method is
usually applied only partially taking into account the accuracy of the global model coefficients used for the long-wavelength
reduction. The key advantage of the extended formulation is the fact that it automatically accounts for the error covariance
of all data types involved. The applicability, feasibility and performance of the proposed method is investigated in the frame
of numerical closed-loop simulations. The two main fields of application, i.e., the improvement of a global gravity field
model by terrestrial gravity field data, and, vice versa, the support to a regional geoid solution by the incorporation of
a global gravity field model, have been analyzed and assessed. Although applied under simplified conditions, it could be shown
that the method works and is practically applicable. 相似文献
An evolutionary approach is applied to solve the nonlinear well logging inverse problem. In the framework of the proposed interval inversion method, nuclear, sonic, and laterolog resistivity data measured at an arbitrary depth interval are jointly inverted, where the depth variation of porosity, water saturation, and shale volume is expanded into series using Legendre polynomials as basis functions. In the interval inversion procedure, the series expansion coefficients are estimated by using an adaptive float-encoded genetic algorithm. Since the solution of the inverse problem using traditional linear optimization tools highly depends on the selection of the initial model, a heuristic search is necessary to reduce the initial model dependence of the interval inversion procedure. The genetic inversion strategy used in interval inversion seeks the global extreme of the objective function and provides an estimate of the vertical distribution of petrophysical parameters, even starting the inversion procedure from extremely high distances from the optimum. For a faster computational process, after a couple of thousand generations, the genetic algorithm is replaced by some linear optimization steps. The added advantage of using the Marquardt algorithm is the possibility to characterize the accuracy of the series expansion coefficients and derived petrophysical properties. A Hungarian oil field example demonstrates the feasibility and stability of the improved interval inversion method. As a significance, the genetic inversion method does not require prior knowledge or strong restrictions on the values of petrophysical properties and gives highly reliable estimation results practically independent of the initial model and core information.
Heat flow anomalies provide critical information in active tectonic environments. The Gulf of Cadiz and adjacent areas are affected by the plate convergence between Africa and Europe, causing widespread deformation and faulting. Active thrust faults cause lateral movement and advection of heat that produces systematic variations in surface heat flow. In December 2003 new heat flow data were collected during the research vessel Sonne cruise SO175 in the Gulf of Cadiz over two sites of recent focused research activity: (i) the Gulf of Cadiz sedimentary prism and (ii) the Marques de Pombal escarpment. Both features have also been discussed as potential source areas of the Great Lisbon earthquake and tsunami of 1755. Background heat flow at the eastern terminus of the Horseshoe abyssal plain is about 52–59 mW/m2. Over the Gulf of Cadiz prism, heat flow decreases from ∼57 mW/m2 to unusually low values of 45 mW/m2 roughly 120 km eastward. Such low values and the heat flow trend are typical for active thrusting, supporting the idea of an east-dipping thrust fault. Slip rates are 10 ± 5 mm per year, assuming that the fault dips at 2°. A fault dipping at 5°, however, would result into slip rates of 1.5–5 mm per year, suggesting that subduction has largely ceased. Based on seismic data, the Marques de Pombal fault is interpreted as part of an active fault system located ∼100 km westward of Cape San Vincente. Heat flow over the fault is affected by refraction of heat caused by the 1 km high escarpment. Thermal models suggest that the slip rate along the fault must either be small or shear stresses acting on the fault are rather high. With respect to other fault zones, however, it is reasonable to assume that the fault's slip rate is small. 相似文献
The <2 μm fraction of 45 sediment samples from the Elbe and Weser rivers and from the southeastern North Sea (German Bight) was analyzed for its contents of clay minerals and selected chemical elements. This was done to provide new information to the controversial question about the origin and distribution of clays in this near-shore marine region. The proportions of smectite, chlorite, illite and kaolinite were determined from intensities of the (001) basal reflection of the XRD pattern using weighted integrated factors and IR-mineral spectra, which were fitted to the values of the chemical analyses. Illite – the major clay mineral – was characterized by its K/Rb ratios and K-Ar ages. Anthropogenic metal contamination was demonstrated by the Cu, Pb and Zn contents. 相似文献
Marine Isotope Stage 11 has been proposed as an analogue for the present interglacial interval; yet, terrestrial climate records from both this region and time interval are rare. The sediments deposited at Lake El'gygytgyn (67°30′N, 172°5′E) in Far East Russia contain a 3·56 Ma record of climate variability. This study presents a high‐resolution record of sediment colour change from Marine Isotope Stage 8 to 12 (ca 275 to 475 ka) and demonstrates the link between lake catchment processes and climate variability. The hue colour parameter, calculated from data collected via colour reflectance spectroscopy in the visible spectrum (380 to 720 nm), exhibits correspondence with global climate records. Determining the source of sediment colour changes was achieved through detailed mineralogical and sedimentological methods, and linked to colour changes through a series of colour sensitivity tests. Mineralogical data, measured by X‐ray diffraction, reveal fluctuations in concentrations of clay minerals corresponding to colour changes. Further analyses of the clay mineral assemblages show no change in relative clay mineral abundances, yet demonstrate a lake catchment dominated by physical weathering processes. Using measured mineral abundances, reconstructions of sediment colour based on colour reflectance mineral standards link mineral and clay mineral content to overall sediment colour. Colour sensitivity tests demonstrate the ability of iron oxide minerals to stain sediments red. Additionally, colour sensitivity to organic matter content was tested, suggesting that organic content drives variability in the red portion of the spectrum and darkens the overall colour signal. Sediment colour is then ultimately linked to physical weathering of bedrock minerals, with small amounts of chemical weathering producing iron oxides during wet intervals. Fluctuations in the sediment colour reveal a high‐resolution record of wet/dry cycles, and provide new information about wet periods for the Russian Arctic region not yet understood from other lake proxy records. 相似文献