Precambrian fluvial systems, lacking the influence of rooted vegetation, probably were characterised by flashy surface runoff, low bank stability, broad channels with abundant bedload, and faster rates of channel migration; consequently, a braided fluvial style is generally accepted. Pre-vegetational braided river systems, active under highly variable palaeoclimatic conditions, may have been more widespread than are modern, ephemeral dry-land braided systems. Aeolian deflation of fine fluvial detritus does not appear to have been prevalent. With the onset of large cratons by the Neoarchaean–Palaeoproterozoic, very large, perennial braided river systems became typical. The c. 2.06–1.88 Ga Waterberg Group, preserved within a Main and a smaller Middelburg basin on the Kaapvaal craton, was deposited largely by alluvial/braided-fluvial and subordinate palaeo-desert environments, within fault-bounded, possibly pull-apart type depositories.
Palaeohydrological data obtained from earlier work in the Middelburg basin (Wilgerivier Formation) are compared to such data derived from the correlated Blouberg Formation, situated along the NE margin of the Main basin. Within the preserved Blouberg depository, palaeohydrological parameters estimated from clast size and cross-bed set thickness data, exhibit rational changes in their values, either in a down-palaeocurrent direction, or from inferred basin margin to palaeo-basin centre. In both the Wilgerivier and Blouberg Formations, calculated palaeoslope values (derived from two separate formulae) plot within the gap separating typical alluvial fan gradients from those which characterise rivers (cf. [Blair, T.C., McPherson, J.G., 1994. Alluvial fans and their natural distinction from rivers based on morphology, hydraulic processes, sedimentary processes, and facies assemblages. J. Sediment. Res. A64, 450–489.]). Although it may be argued that such data support possibly unique fluvial styles within the Precambrian, perhaps related to a combination of major global-scale tectono-thermal and atmospheric–palaeoclimatic events, a simpler explanation of these apparently enigmatic palaeoslope values may be pertinent. Of the two possible palaeohydrological formulae for calculating palaeoslope, one provides results close to typical fluvial gradients; the other formula relies on preserved channel-width data. We suggest that the latter will not be reliable due to problematic preservation of original channel-widths within an active braided fluvial system. We thus find no unequivocal support for a unique fluvial style for the Precambrian, beyond that generally accepted for that period and discussed briefly in the first paragraph. 相似文献
This article discusses the involvement of older women and men in voluntary work in rural communities in the Netherlands. Gender,
ageing and civil society form the core concepts in the article. Politicians and policy makers in the Netherlands are worried
about the perceived decline of civil society and about the consequences of this decline for specific categories of older women
in terms of well-being and social exclusion. The paper presents the results of two surveys of involvement of older women and
men in civic organisations, voluntary work and local actions. The surveys indicate high levels of involvement of both women
and men. No gender differences are found in activity level. However, the data demonstrate the gendered nature of civil society.
Women are more engaged in informal voluntary work and informal local actions, while men are more active in formal organisations
and functions. This gender segmentation is not absolute, however. Many older women and men transgress the traditional feminine–masculine
boundaries. The surveys highlight the involvement of women and men in rural communities, but indicate lower activity levels
in urban areas. 相似文献
This study explains why a number of El Nino properties (period, amplitude, structure, and propagation) have changed in a coherent manner since the late 1970s and why these changes had almost concurred with the Pacific decadal climate shift. Evidence is presented to show that from the pre-shift (1961-1975) to the post-shift (1981-1995) epoch, significant changes in the tropical Pacific are found in the surface winds and temperature, whereas changes in the thermocline are uncertain. Numerical experiments with the Cane and Zebiak model demonstrate that the decadal changes in the surface winds qualitatively reproduce the observed coherent changes in El Nino properties. The fundamental factor that altered the model's El Nino is the decadal changes of the background equatorial winds and associated upwelling. The annual cycle is also necessary for the mean state to modulate El Nino. From the pre- to post-shift epoch, the changes in the background winds and upwelling modify the structure of the coupled mode (eastward displacement of the equatorial westerly anomalies) by reallocating anomalous atmospheric heating and SST gradient along the equator. This structural change amplifies the ENSO cycle and prolongs the oscillation period by enhancing the coupled instability and delaying transitions from a warm to a cold state or vice versa. The changes in the mean currents and upwelling reduce the effect of the zonal temperature advection while enhance that of the vertical advection; thus, the prevailing westward propagation is replaced by eastward propagation or standing oscillation. Our results suggest a critical role of the atmospheric bridge that rapidly conveys the influences of extratropical decadal variations to the tropics, and the possibility that the Pacific climate shift might have affected El Nino properties in the late 1970s by changing the background tropical winds and the associated equatorial upwelling. 相似文献
One of the first operations in a seismic signal processing system applied to earthquake data is to distinguish between valid and invalid records. Since valid signals are characterized by a combination of their time and frequency properties, wavelets are natural candidates for describing seismic features in a compact way. This paper develops a seismic buffer pattern recognition technique, comprising wavelet-based feature extraction, feature selection based on the mutual information criterion, and neural classification based on feedforward networks. The ability of the wavelet transform to capture discriminating information from seismic data in a small number of features is compared with alternative feature reduction techniques, including statistical moments. Three different variations of the wavelet transform are used to extract features: the discrete wavelet transform, the single wavelet transform and the continuous wavelet transform. The mutual information criterion is employed to select a relatively small set of wavelets from the time–frequency grid. Firstly, it is determined whether wavelets can capture more informative data in an equal number of features compared with other features derived from raw data. Secondly, wavelet-based features are compared with features selected based on prior knowledge of class differences. Thirdly, a technique is developed to optimize wavelet features as part of the neural network training process, by using the wavelet neural network architecture. The automated classification techniques developed in this paper are shown to perform similarly to human operators trained for this function. Wavelet-based techniques are found to be useful, both for preprocessing of the raw data and for extracting features from the data. It is demonstrated that the definition of wavelet features can be optimized using the classification wavelet network architecture. 相似文献