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旨在提高和改善莱茵河的防洪和流域可持续管理水平的“洪水管理行动计划”于1998年开始实施,其内容和措施以及欧洲国家在流域可持续发展管理方面的思路和模式对我国江河流域的防洪和洪泛区管理可资借鉴. 相似文献
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Vesiculated tuffs and associated features 总被引：7，自引：0，他引：7
V. LORENZ 《Sedimentology》1974,21(2):273-291
Vesiculated tuffs are tuffs that contain vesicles between the ash particles. Formation of the vesicles is the result of trapping of steam, the transporting agent of volcanic base surges, in wet, muddy or sticky ash deposited by the base surges. Vesiculated tuffs are described from various maars and tuff-rings in Europe (Iceland, France, Germany) and USA together with associated surface features such as: gravity flowage ripples, mud flow channels, current ripples, and current ridges. Other features described are: plastering of ash against obstacles and vesiculated accretionary lapilli, the latter containing vesicles in the outer layer. Vesiculated base surge deposits probably contained as much as 20–30% of interstitial water and fell out of the base surge clouds en masse owing to non-free flow and consequent accretion. 相似文献
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The aftermath of the Carnian carbonate platform demise: a basinal perspective (Dolomites, Southern Alps) 总被引：1，自引：0，他引：1
In the Dolomites of northernmost Italy the carbonate‐platform growth came to a standstill late in the Early Carnian (Late Triassic). The response to this shutdown of shallow‐water carbonate production in the interplatform basins is largely unknown because erosion has removed most of the soft basinal sediments, giving rise to today's scenic landscape of the Dolomites. Mapping in the central part of the Dolomites and newly available core material has recently revealed a well‐preserved succession of basinal rocks within the Heiligkreuz Hospiz Basin (ital. Ospizio di Santa Croce Basin). In this paper, the regional depositional nature of arrested carbonate platform production is reconstructed by tracing its sedimentological record across the slope and into the basin. The uppermost St. Cassian Formation, the time‐equivalent basinal rocks to the prograding carbonate platforms, is overlain by the Heiligkreuz Formation, whose basal succession was deposited in a restricted and oxygen‐depleted environment immediately post‐dating the platform demise. The succession consists mainly of mudrocks, marlstones, and peloidal packstones, with abundant low‐diversity ostracod and pelecypod fauna and early diagenetic dolomite. C and O isotope values of the basal Heiligkreuz Formation, post‐dating platform demise, average + 2·4 and ? 2·4‰, respectively, and largely overlap the isotopic composition of St. Cassian carbonates. A shift toward slightly lower δ13C values in the Heiligkreuz Formation may reflect incorporation of isotopically depleted C released during bacterial sulphate reduction in the Heiligkreuz sediments. Sedimentological, palaeobiological and geochemical indices suggest that near‐normal marine conditions persisted long after the shutdown of shallow water carbonate‐platform growth, although there are clear indications of severely reduced oxygen levels in the restricted Heiligkreuz Hospiz interplatform basin. The Early Carnian platform demise induced a distinct switch in the locus of carbonate production from the shallow‐water platform and slope to the basin floor and a decrease in the availability of dissolved oxygen in the basinal waters. It is inferred that anoxia extended at least temporarily to the top of the carbonate slope, as indicated by the onlap of normal‐marine mounds by dark marlstones of the basal Heiligkreuz Formation. 相似文献
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Forest vegetation in the southeastern United States extends westward beyond the Ozark and Ouachita plateaus in Arkansas and Missouri into the Central Plains. Along this transect, luxuriant forests give way to mixed forests and grasslands that include smaller trees and progressively fewer tree species and eventually to grassland‐dominated landscapes in central Kansas, Oklahoma, and Texas. This transition is directly related to decreasing precipitation with distance to the west of the Mississippi River valley. Many species, however, have abrupt western range boundaries related to physiography and hydrogeomorphic processes. The western range limits for many trees correspond to Coastal Plain boundaries that at a regional scale impose sharper range boundaries than would be expected on the basis of decreasing precipitation. Also, riparian habitats within stream valleys extending westward from the Coastal Plain provide suitable habitats for trees in the dry regions of the Great Plains. The presence of riparian trees in this region is determined largely by the presence or absence of groundwater conditions necessary for survival. For floodplain trees, then, it is primarily habitat—not climate—that determines the location of range boundaries. 相似文献
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The area of the Dolomites in the Southern Alps exhibits some of the best outcrops of carbonate platforms and platform-to-basin transitions in the world. The region has attracted geologists since the early 19th Century and has been a centre of stratigraphic and sedimentological research ever since. The interpretation of the platforms in the Dolomites as coral reefs dates back to the 1850s and was inspired by the work on modern reefs in the Pacific and Indian Oceans at that time. Very soon, however, studies in the Dolomites triggered developments of their own. Fieldwork on these Triassic atolls led to a first understanding of the dramatic facies changes at the periphery of reefs and atolls, including the recognition of slope facies and basinward transport of platform material by gravity. In addition, the platforms were used to prove long-term syndepositional subsidence on the scale of kilometres as early as 1860. After World War II, the study of modern reefs and platforms led to process-based facies models and this, in turn, guided the sedimentological analysis of the Alpine–Mediterranean belt. Repeatedly, however, the roles became reversed and the Alpine–Mediterranean platforms contributed to the fundamental underpinnings of sedimentology. In the 1980s, mountain-size outcrops in the Alps and the Apennines were recognized as models for the interpretation of shoal-water carbonates in seismic data. Seismic models of outcrops in the Dolomites, for instance, gave rise to the concept of pseudo-unconformities, i.e. rapid lateral facies changes that appear as unconformities in seismic images. In the 1990s, the Alpine–Mediterranean Mesozoic along with the North American and European Palaeozoic revealed the significance of microbially induced carbonate precipitation in constructing large limestone bodies. Studies of ancient rocks led this development because this particular carbonate factory is far less prominent now than it was at certain times in the past. 相似文献
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Ekaterina V. KOROCHANTSEVA Mario TRIELOFF Cyrill A. LORENZ Alexey I. BUYKIN Marina A. IVANOVA Winfried H. SCHWARZ Jens HOPP Elmar K. JESSBERGER 《Meteoritics & planetary science》2007,42(1):113-130
Abstract— Radiochronometry of L chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 Myr ago. Fossil meteorites from Sweden indicate a highly increased influx of extraterrestrial matter in the Middle Ordovician ～480 Myr ago. An association with the L‐chondrite parent body event was suggested, but a definite link is precluded by the lack of more precise radiometric ages. Suggested ages range between 450 ± 30 Myr and 520 ± 60 Myr, and can neither convincingly prove a single breakup event, nor constrain the delivery times of meteorites from the asteroid belt to Earth. Here we report the discovery of multiple 40Ar‐39Ar isochrons in shocked L chondrites, particularly the regolith breccia Ghubara, that allow the separation of radiogenic argon from multiple excess argon components. This approach, applied to several L chondrites, yields an improved age value that indicates a single asteroid breakup event at 470 ± 6 Myr, fully consistent with a refined age estimate of the Middle Ordovician meteorite shower at 467.3 ± 1.6 Myr (according to A Geologic Time Scale 2004). Our results link these fossil meteorites directly to the L‐chondrite asteroid destruction, rapidly transferred from the asteroid belt. The increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate, which implies severe environmental stress. 相似文献
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Marina A. IVANOVA Cyrill A. LORENZ Mikhail A. NAZAROV Franz BRANDSTAETTER Ian A. FRANCHI Lyuba V. MOROZ Robert N. CLAYTON Andrew Yu. BYCHKOV 《Meteoritics & planetary science》2010,45(7):1108-1123
Abstract– Dhofar (Dho) 225 and Dho 735 are carbonaceous chondrites found in a hot desert and having affinities to Belgica‐like Antarctic chondrites (Belgica [B‐] 7904 and Yamato [Y‐] 86720). Texturally they resemble CM2 chondrites, but differ in mineralogy, bulk chemistry and oxygen isotopic compositions. The texture and main mineralogy of Dho 225 and Dho 735 are similar to the CM2 chondrites, but unlike CM2 chondrites they do not contain any (P, Cr)‐sulfides, nor tochilinite 6Fe0.9S*5(Fe,Mg)(OH)2. H2O‐contents of Dho 225 and Dho 735 (1.76 and 1.06 wt%) are lower than those of CM2 chondrites (2–18 wt%), but similar to those in the metamorphosed carbonaceous chondrites of the Belgica‐like group. Bulk compositions of Dho 225 and Dho 735, as well as their matrices, have low Fe and S and low Fe/Si ratios relative to CM2 chondrites. X‐ray powder diffraction patterns of the Dho 225 and Dho 735 matrices showed similarities to laboratory‐heated Murchison CM2 chondrite and the transformation of serpentine to olivine. Dho 225 and 735’s oxygen isotopic compositions are in the high 18O range on the oxygen diagram, close to the Belgica‐like meteorites. This differs from the oxygen isotopic compositions of typical CM2 chondrites. Experimental results showed that the oxygen isotopic compositions of Dho 225 and Dhofar 725, could not be derived from those of typical CM2 chondrites via dehydration caused by thermal metamorphism. Dho 225 and Dho 735 may represent a group of chondrites whose primary material was different from typical CM2 chondrites and the Belgica‐like meteorites, but they formed in an oxygen reservoir similar to that of the Belgica‐like meteorites. 相似文献
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