The Côte d’Ivoire–Ghana transform margin is bounded to the south by a prominent marginal ridge. ODP Leg 159 shipboard analyses on sediments from four sites document three distinct transform margin sedimentary and tectonic stages of evolution: (1)?an intracontinental stage of transform faulting recorded in deformed lacustrine to marine siliciclastic sequences; (2)?a marginal ridge uplift stage, recorded by shallow water limestones, appears coeval with the passing of a hot, oceanic spreading center just south of the sediment wedge; and (3)?cool ing subsidence of the transform margin recorded in bathyal to abyssal sediments emphasizes a passive margin stage. These results are consistent with previously published models of evolution. 相似文献
Abstract. Fucus serratus provides a habitable site For much epifauna if conditions are right. Epifaunal colonies in their turn as well Provide habitats in which meiofauna can dwell. Thus micro-environment is provided which is Dependent on colony-species, -size and silt for its riches. Analysis of population data shows the facts are Turbellarian abundance rarely relates to other taxa, However the flatworms' population count Closely relates to total meiofaunal amount. Experiments on bryozoan-choice by Monocelis lineata show This worm prefers Flustrellidra as a place to go. This may in part be due to thigmotactic deference But mainly seems related to a feeding preference. In addition to behaviour towards food and concavity, Responses are shown to light, temperature, current and gravity. In the discussion thus it is partly explained How meiofauna finds epifauna and there is maintained. 相似文献
Using the IRAM interferometer we have observed four carbon stars (U Cam, CIT6, Y CVn, IRC+40540) in the HCN(J=1 0) and CN(N=1 0) lines. Here we present some results for CIT6 and U Cam. 相似文献
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
The 1900–1700 Ma Waterberg Group in the main Waterberg fault-bounded basin consists of dominantly coarse siliciclastic red
beds with minor volcanic rocks. The sedimentary rocks were deposited mainly by alluvial fans, fluvial braidplains and transgressive
shallow marine environments, with lesser lacustrine and aeolian settings. Uplifted, largely granitic source areas were located
along the Thabazimbi-Murchison lineament (TML) fault system in the south, and along the Palala shear zone in the northeast.
Palaeoplacer titanomagnetite-ilmenite-zircon heavy mineral deposits, best developed in the Cleremont Formation in the centre
of the basin, reflect initial fluvial reworking and subsequent littoral marine concentration. Coarse alluvial cassiterite
placer deposits are found in the Gatkop area in the southwest of the basin, and appear to have been derived from stanniferous
Bushveld Complex lithologies south of the TML. Hydrothermal zinc and U-Cu mineralisation in the Alma lithologies in the same
area appears to be related to the TML fault system. Small manganese deposits and anomalous tungsten values occur in the south
of the basin, where they are again closely spatially associated with the TML. Copper-barium mineralisation is found associated
with dolerite dykes, and in stratigraphically controlled, inferred syngenetic settings. The most interesting of these apparently
syngenetic occurrences is found within green coloured reduced mudrocks and inferred volcanic rocks, at an unconformity developed
within the overall red bed sequence of the Waterberg Group, adjacent to the TML in the southwest of the basin. The most important
potential mineralisation in the main Waterberg basin thus encompasses shoreline placer Ti and the possibility of substantial
sediment-hosted copper deposits.
Received: 31 May 1996 / Accepted: 17 February 1997 相似文献
In my paper I will try to give some geographical and economical aspects on the recent development of airfreight.Some models for forecasting will be presented, especially three made by (i) DOUGLAS Aircraft's RD; (ii) IATA Freight Traffic Forecasts and (iii) A trend model modified in my department. These three models will be applied and compared with special respect to Finnish airfreights during the 80's. 相似文献