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Didier Miallier Laurent Michon Jacques Évin Thierry Pilleyre Serge Sanzelle Gérard Vernet 《Comptes Rendus Geoscience》2004,336(15):1345-1353
On the basis of radiocarbon ages obtained on wood burned by base surges and tephrochronological observations, it has been possible to assess that the Puy Chopine, aged ca 9700 yr is older by a few centuries than both the Vasset and Kilian volcanoes. The last two have nearly the same age, ca 9300–9400 yr. Layers of trachytic tephra, most probably originated from Kilian or Vasset, can be observed overlapping those from the Pariou. This last volcano had been previously thought to be the younger of the area on the basis of the observation of trachytic tephra beneath its own tephra. However, new observations allow us to assume that those trachytes originated from the acid phase of the Pariou itself. Therefore, it can be assessed that the four volcanoes erupted according to the following sequence: Chopine/Pariou/?Vasset?/Kilian; the chronological situation of Vasset lies on poor arguments and it should be confirmed. To cite this article: D. Miallier et al., C. R. Geoscience 336 (2004). 相似文献
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The Tedford subfossil locality at Lake Menindee preserves a diverse assemblage of marsupials, monotremes and placental rodents. Of the 38 mammal taxa recorded at the site, almost a third are of extinct megafauna. Some of the bones are articulated or semi-articulated and include almost complete skeletons, indicating that aeolian sediments rapidly buried the animals following death. New optical ages show the site dates to the early part of the last glacial (55,700 ± 1300 yr weighted mean age). This is close to the 51,200-39,800 yr Australia-wide extinction age for megafauna suggested by Roberts et al. [2001, Science 292:1888-1892], but like all previous researchers, we cannot conclusively determine whether humans were implicated in the deaths of the animals. Although an intrusive hearth at the site dating to 45,100 ± 1400 yr ago is the oldest evidence of human occupation of the Darling River, no artifacts were identified in situ within the sub-fossil-bearing unit. Non-anthropogenic causes, such as natural senescence or ecosystem stress due to climatic aridity, probably explain the mortality of the faunal assemblage at Lake Menindee. 相似文献
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The relative weathering of a sequence of five palaesols formed in different parent materials, including bedrock of phonolitic composition, is described and assessed. The three lower palaeosols (units 3–5) are considered to be of interglacial origin. The saprolite (unit 5), in phonolite, formed prior to deposition of the lower Brunhes-age till, is considered to be as old as the Brunhes Matuyama boundary (0.73 myr). The other two palaeosols in this group (units 3 and 4), derived from loess and till, respectively, formed over middle to late Brunhes time under palaeoclimates that were wetter (and presumably warmer), and of sufficient leaching power to move clay, as well as organically complexed extractable Fe and Al. The palaeosols in unit 3 are more complex than the lower ones. Their upper and lower horizons have about the same chemical composition as the lowest two palaeosols, while the middle horizons are similar to the upper two palaeosols (units 1 and 2). Radiocarbon dates for unit 3 are considered to be too young to account for the strong weathering in the upper and lower horizons. This interpretation is supported by high D/L ratios of aspartic acid. Overlying sediments, emplaced by episodic colluvial (unit 2) and slope wash (unit 1) processes, are less weathered; the lower colluvial unit in this group was emplaced during the late stade of the last glaciation. The surface sediments were emplaced more recently during the same interval and weathered mainly during the post-glacial period. Tentative correlations with core 82PCS18 from the Canary Basin show that upper unit 3 is probably equivalent to Isotope Stage 5e, lower unit 3 to Isotope Stage 7, unit 4 to Isotope Stage 8, and unit 5 to Isotope Stage 9. Units 1 and 2 are firmly correlated with Isotope Stage 2. 相似文献
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The upper 40 m of Lake Nyos are retained by a weak natural dam which, if it were to fail, would not only devastate the area hit by the 1986 gas disaster but would also cause a serious flood to surge down the Katsina Ala into Nigeria. The age of the pyroclastic cone, of which the dam is the last remnant, is therefore of great practical importance to the people of the area. If the pyroclastic cone is only a few hundred years old, as some have suggested, then it is eroding away quickly and the dam must surely fail in the very near future. If, on the other hand, it is many thousands of years old, then there is less immediate cause for concern.The age of the pyroclastic cone can be constrained in three ways:(1) Two samples of basalt, one from the dam itself and one from a lava flow which post-dates the pyroclastic cone, have both yielded K–Ar ages in excess of 100,000 years.(2) Photographic evidence indicates that there has been no detectable change (>2 m) to the width of the dam since 1958. This constrains the average erosion rate and suggests that the pyroclastic cone is at least 4000 years old.(3) Cores from sediment deposited after the level of a small lake to the northeast of Lake Nyos was raised by a debris slide from the pyroclastic cone, contain no volcanic ash. A sample from the base of this sequence has yielded a radiocarbon age of 2700 years.The Lake Nyos dam must therefore be, at the very least, a few thousand years old and although its general stability must give serious cause for concern there is no reason to suspect that the rate at which it is currently eroding away is of itself sufficient to pose an immediate threat. 相似文献