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971.
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
972.
Diffusion of tetravalent cations in zircon 总被引:18,自引:1,他引:18
D. J. Cherniak J. M. Hanchar E. B. Watson 《Contributions to Mineralogy and Petrology》1997,127(4):383-390
Diffusion rates for the three tetravalent cations U, Th and Hf have been measured in synthetic zircon. Diffusant sources
included oxide powders and ground pre-synthesized silicates. Rutherford backscattering spectrometry (RBS) was used to measure
depth profiles. Over the temperature range 1400–1650 °C, the following Arrhenius relations were obtained (diffusion coefficients
in m2sec−1):
log D
Th = (1.936 ± 0.9820) + (− 792 ± 34 kJ mol−1 /2.303 RT)
log D
U = (0.212 ± 2.440) + (− 726 ± 83 kJ mol−1 /2.303 RT)
log D
Hf = (3.206 ± 1.592) + (− 812 ± 54 kJ mol−1 /2.303 RT)
The data show a systematic increase in diffusivity with decreasing ionic radius (i.e., faster diffusion rates for Hf than
for U or Th), a trend also observed in our earlier study of rare earth diffusion in zircon. Diffusive fractionation may be
a factor in the Lu-Hf system given the much slower diffusion rates of tetravalent cations when compared with the trivalent
rare earths. The very slow diffusion rates measured for these tetravalent cations suggest that they are essentially immobile
under most geologic conditions, permitting the preservation of fine-scale chemical zoning and isotopic signatures of inherited
cores.
Received: 12 July 1996 / Accepted: 2 December 1996 相似文献
973.
Effect of tunnel depth on modulus of deformation of rock mass 总被引:1,自引:2,他引:1
M. Verman B. Singh M. N. Viladkar J. L. Jethwa 《Rock Mechanics and Rock Engineering》1997,30(3):121-127
Summary Deformability of rock mass significantly influences its behaviour and is, therefore, an important consideration for the design of underground openings. The modulus of deformation of rock mass is, however, normally obtained from expensive and time-consuming uniaxial jacking tests, whose results often have a large scatter. An empirical correlation has, therefore, been proposed for a quick and inexpensive preliminary estimation of the modulus of deformation of rock mass on the basis of field instrumentation carried out in tunnels in India. 相似文献
974.
Estimation of groundwater evaporation and salt flux from Owens Lake, California, USA 总被引:3,自引:0,他引:3
S. W. Tyler S. Kranz M. B. Parlange J. Albertson G. G. Katul G. F. Cochran B. A. Lyles G. Holder 《Journal of Hydrology》1997,200(1-4):110-135
Groundwater evaporation and subsequent precipitation of soluble salts at Owens Lake in eastern California have created one of the single largest sources of airborne dust in the USA, yet the evaporation and salt flux have not been fully quantified. In this study, we compare eddy correlation, microlysimeters and solute profiling methods to determine their validity and sensitivity in playa environments. These techniques are often used to estimate evaporative losses, yet have not been critically compared at one field site to judge their relative effectiveness and accuracy. Results suggest that eddy correlation methods are the most widely applicable for the variety of conditions found on large playa lakes. Chloride profiling is shown to be highly sensitive to thermal and density-driven fluxes in the near surface and, as a result, appears to underestimate yearly groundwater evaporation. Yearly mean groundwater evaporation from the playa surface estimated from the three study areas was found to range from 88 to 104 mm year−1, whereas mean evaporation from the brine-covered areas was 872 mm year−1. Uncertainties on these mean rates were estimated to be ±25%, based on comparisons between eddy correlation and lysimeter estimates. On a yearly basis, evaporation accounts for approximately 47 × 106 m3 of water loss from the playa surface and open-water areas of the lake. Over the playa area, as much as 7.5 × 108 kg (7.5 × 105 t) of salt are annually concentrated by evaporation at or near the playa surface, much of which appears to be lost during dust storms in area. 相似文献
975.
The separation of the influence of nutrients and climate on the varve time-series of baldeggersee, Switzerland 总被引:7,自引:2,他引:7
The varve data-set from a freeze-core taken in the deepest part of Baldeggersee was subjected to different multivariate statistical
analyses in order to estimate the amount of variance in the varve thickness measures explicable by past climate and by the
trophic state of the lake. A comparison of two different time-periods (1902 to1992 versus 1920 to 1980) revealed that the
lake restoration programme since 1982 has had a significant impact on the formation of the seasonal layers. Results of the
partitioning of the variance in the varve thickness measures showed that about two thirds of the variance are unexplained
by a climate and trophic state model and that trophic state explains 6%, whereas climate accounts for about 28% of the variance
before the effect of lake restoration had a strong impact on the varves. Among the climate parameters the amount of annual
precipitation is a strong predictor for explaining the thickness of both dark layer and total couplet thickness, whereas summer
precipitation is important for the thickness of the light layer. 相似文献
976.
977.
978.
P. V. S. Rama Rao P. T. Jayachandran P. Sri Ram B. V. Ramana Rao D. S. V. V. D. Prasad K. K. Rose 《Annales Geophysicae》1997,15(6):729-733
The characteristics of VHF radiowave scintillations at 244 MHz (FLEETSAT) during a complete solar cycle (1983–93) at a low-latitude station, Waltair (17.7°N, 83.3°E), are presented. The occurrence of night-time scintillations shows equinoctial maxima and summer minima in all the epochs of solar activity, and follows the solar activity. The daytime scintillation occurrence is negatively correlated with the solar activity and shows maximum occurrence during the summer months in a period of low solar activity. The occurrence of night-time scintillations is inhibited during disturbed days of high solar activity and enhanced during low solar activity. 相似文献
979.
B. Soukharev 《Annales Geophysicae》1997,15(12):1595-1603
The interaction between the factors of the quasi-biennial oscillation (QBO) and the 11-year solar cycle is considered as an separate factor influencing the interannual January–March variations of total ozone over Northeastern Europe. Linear correlation analysis and the running correlation method are used to examine possible connections between ozone and solar activity at simultaneous moment the QBO phase. Statistically significant correlations between the variations of total ozone in February and, partially, in March, and the sunspot numbers during the different phases of QBO are found. The running correlation method between the ozone and the equatorial zonal wind demonstrates a clear modulation of 11-y solar signal for February and March. Modulation is clearer if the QBO phases are defined at the level of 50 hPa rather than at 30 hPa. The same statistical analyses are conducted also for possible connections between the index of stratospheric circulation C1 and sunspot numbers considering the QBO phase. Statistically significant connections are found for February. The running correlations between the index C1 and the equatorial zonal wind show the clear modulation of 11-y solar signal for February and March. Based on the obtained correlations between the interannual variations of ozone and index C1, it may be concluded that a connection between solar cycle – QBO – ozone occurs through the dynamics of stratospheric circulation. 相似文献
980.
S. Klimov S. Romanov E. Amata J. Blecki J. Büchner J. Juchniewicz J. Rustenbach P. Triska L. J. C. Woolliscroft S. Savin Yu. Afanas’yev U. de Angelis U. Auster G. Bellucci A. Best F. Farnik V. Formisano P. Gough R. Grard V. Grushin G. Haerendel V. Ivchenko V. Korepanov H. Lehmann B. Nikutowski M. Nozdrachev S. Orsini M. Parrot A. Petrukovich J. L. Rauch K. Sauer A. Skalsky J. Slominski J. G. Trotignon J. Vojta R. Wronowski 《Annales Geophysicae》1997,15(5):514-527
The plasma-wave experiment ASPI (analysis of spectra of plasma waves and instabilities) on board the INTERBALL spacecraft is a combined wave diagnostics experiment. It performs measurements of the DC and AC magnetic field vector by flux-gate and searchcoil sensors, the DC and AC electric field vector by Langmuir double probes and the plasma current by Langmuir split probe. Preliminary data analysis shows the low noise levels of the sensors and the compatibility of new data with the results of previous missions. During several months of in-orbit operation a rich collection of data was acquired, examples of which at the magnetopause and plasma sheet are presented in second part of the paper. 相似文献