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1.
Landslide deposits dam Lake Oeschinen (Oeschinensee), located above Kandersteg, Switzerland. However, past confusion differentiating deposits of multiple landslide events has confounded efforts to quantify the volume, age, and failure dynamics of the Oeschinensee rock avalanche. Here we combine field and remote mapping, topographic reconstruction, cosmogenic surface exposure dating, and numerical runout modeling to quantify salient parameters of the event. Differences in boulder lithology and deposit morphology reveal that the landslide body damming Oeschinensee consists of debris from both an older rock avalanche, possibly Kandertal, as well as the Oeschinensee rock avalanche. We distinguish a source volume for the Oeschinensee event of 37 Mm3, resulting in an estimated deposit volume of 46 Mm3, smaller than previous estimates that included portions of the Kandertal mass. Runout modeling revealed peak and average rock avalanche velocities of 65 and 45 m/s, respectively, and support a single-event failure scenario. 36Cl surface exposure dating of deposited boulders indicates a mean age for the rock avalanche of 2.3 ± 0.2 kyr. This age coincides with the timing of a paleo-seismic event identified from lacustrine sediments in Swiss lakes, suggesting an earthquake trigger. Our results help clarify the hazard and geomorphic effects of rare, large rock avalanches in alpine settings.  相似文献   

2.
The sandy deposits produced by tsunamis and liquefaction share many sedimentary features, and distinctions between the two are important in seismically active coastal zones. Both types of deposits are present in the wetlands bordering Puget Sound, where one or more earthquakes about 1100 years ago caused both tsunami flooding and sediment venting. This co‐occurrence allows an examination of the resulting deposits and a comparison with tsunami and liquefaction features of modern events. Vented sediments occur at four of five wetland field localities and tsunami deposits at two. In comparison with tsunami deposits, vented sediments in this study and from other studies tend to be thicker (although they can be thin). Vented sediments also have more variable thickness at both outcrop and map scale, are associated with injected dykes and contain clasts derived from underlying deposits. Further, vented sediments tend to contain a greater variety of sedimentary structures, and these structures vary laterally over metres. Tsunami deposits compared with vented sediments are commonly thinner, fine and thin landward more consistently, have more uniform thickness on outcrop and map scales, and have the potential of containing coarser clasts, up to boulders. For both tsunami deposits and vented sediments, the availability and grain size of source material condition the characteristics of the deposit. In the cases presented in this paper, both foraminifera and diatom assemblages within tsunami deposits and vented sediments consisted of brackish and marine species, and no distinction between processes could be made based on microfossils. In summary, this study indicates a need for more careful analysis and mapping of coastal sediments associated with earthquakes to avoid misidentification of processes and misevaluation of hazards.  相似文献   

3.
Onshore tsunami deposits may consist of inflow and backflow deposits. Grain sizes can range from clay to boulders of several metres in diameter. Grain‐size distributions reflect the mode of deposition and may be used to explore the hydrodynamic conditions of transport. The absence of unique sedimentary features identifying tsunami deposits makes it difficult in some cases to distinguish inflow from backflow deposits. On Isla Mocha off central Chile, the 27 February 2010 tsunami left behind inflow and backflow deposits of highly variable character. Tsunami inflow entrained sands, gravels and boulders in the upper shoreface, beach, and along coastal terraces. Boulders of up to 12 t were transported up to 300 m inland and 13 m above sea‐level. Thin veneers of coarse sand were found up to the maximum runup at 600 m inland and 19 m above sea‐level. Backflow re‐mobilized most of the sands and gravels deposited during inflow. The orientation of erosional structures indicates that significant volumes of sediment were entrained also during backflow. A major feature of the backflow deposits are widespread prograding fans of coarse sediment developed downcurrent of terrace steps. Fan sediments are mostly structureless but include cross‐bedding, imbrication and ripples, indicating deposition from bedload traction currents. The sediments are poorly sorted, grain sizes range between medium to coarse sand to gravel and pebbles. An assessment of the backflow transport conditions of this mixed material suggests that bedload transport at Rouse numbers >2·5 was achieved by supercritical flows, whereas deposition occurred when currents had decelerated sufficiently on the low‐gradient lower coastal plain. The sedimentary record of the February 2010 tsunami at Isla Mocha consists of backflow deposits to more than 90%. Due to the lack of sedimentary structures, many previous studies of modern tsunami sediments found that most of the detritus was deposited during inflow. This study demonstrates that an uncritical use of this assumption may lead to erroneous interpretations of palaeotsunami magnitudes and sedimentary processes if unknowingly applied to backflow deposits.  相似文献   

4.
Under the WCoE/IPL project, Landsat Enhanced Thematic Mapper plus (ETM+) and ASTER GDEM data were used to analyze factors associated with frequent debris flow and rock-debris avalanche on the Nigeria-Cameroon mountain range. Detailed processing of the ETM+ imageries of the vulnerable portions on the western part of Adamawa plateau was carried out to identify structural trends relatable to the slope failures. The processing identified the structural characteristics of the study area and clearly distinguished the lineaments associated with the debris flow and rock-debris avalanches. The strongly weathered and fragmented gneissic rocks, shallow groundwater table, as well as the distribution of faults and joint sets were significant factors predisposing the slopes to rapid movements. Recent landslides occurred at the locations with high lineament density and in areas close to major faults and long lineaments. The interaction among the moving mass, slope geometry, local geology, topography, and drainage then resulted in a long runout and a high degree of spreading. Analysis of the ASTER–GDEM delineated landforms, slope morphology, and drainage patterns, which was not possible with conventional mapping techniques. Integration of these results yielded reasonable interpretation of the predisposing factors for the long runout failures and provided logical basis for future landslide susceptibility analysis in the area. Accurate investigation of the predisposing factors and characteristics of landslides in environments that have experienced past failures is therefore important in understanding areas that may be susceptible to landsliding in future.  相似文献   

5.
Coastal boulder fields provide clues to long-term frequency-magnitude patterns of coastal flooding events and have the potential to play an important role in coastal hazard assessment. Mapping boulders in the field is time and labour-intensive, and work on intertidal reef platforms, as in the present study, is physically challenging. By addressing coastal scientists who are not specialists in remote sensing, this contribution reports on the possibilities and limitations of digital applications in boulder mapping in Eastern Samar, Philippines, where recent supertyphoons Haiyan and Hagupit induced high waves, coastal flooding and boulder transport. It is demonstrated how satellite imagery of sub-metre resolution (from Pléiades and WorldView-3 imagery) enables efficient analysis of transport vectors and distances of larger boulders, reflecting variation in latitudes of both typhoon tracks and approaching angles of typhoon-generated waves. During the investigated events, boulders with a-axes of up to 8 m were clearly identified to have been shifted for up to 32 m, mostly along the seaward margin of the boulder field. It is, however, hard to keep track of smaller boulders, and the length of a-axes and b-axes including their orientation is often impossible to map with sufficient accuracy. Orthophotographs and digital surface models created through the application of an unmanned aerial vehicle and the ‘Structure from Motion’ technique provide ultra-high-resolution data, and have the potential to not only improve the results of satellite image analysis, but also those from field mapping and may significantly reduce overall time in the field. Orthophotographs permit unequivocal mapping of a-axes and b-axes including their orientation, while precise values for c-axes can be derived from the respective digital surface models. Volume of boulders is best inferred from boulder-specific Structure from Motion-based three-dimensional models. Battery power, flight speed and altitude determine the limits of the area covered, while patches shielded by the boulders are difficult to resolve. For some tasks, field mapping remains mandatory and cannot be replaced by currently available remote sensing tools: for example, sampling for rock type, density and age dating, recording of lithological separation of boulders from the underlying geological unit and of geomorphic features on a millimetre to decimetre-scale, or documentation of fine-grained sediment transport in between the boulders in supratidal settings. In terms of future events, the digital products presented here will provide a valuable reference to track boulder transport on a centimetre to decimetre-scale and to better understand the hydrodynamics of extreme-wave events on a fringing reef coastline.  相似文献   

6.
A history and conception of glacial events for the central Karakoram Himalaya, proposed some 80 years ago by Giotto Dainelli, are largely accepted today. However, certain deposits identified as terminal moraine complexes marking glacial episodes were actually emplaced by rock avalanches. In the Skardu and Shigar intermontane basins of Baltistan, at least 15 rock avalanche events were previously mapped as moraine or till. Criteria used for distinguishing these catastrophic landslide deposits emphasize homogeneous lithology of rubble and matrix, clast shape, facies characteristics, the large scale unity of emplacement, and morphological relations to valley topography. The deposits of three events, at Katzarah, Satpura, and the north end of Shigar Valley, have been reconstructed in detail. Thick supraglacial debris does not result in similar deposits. Extensive valley fills, river terraces, large sediment fans, and lacustrine sediments formerly attributed to late-glacial conditions are reinterpreted as postglacial events involving rock avalanches that interrupted fluvial development. Existing reconstructions of glaciations are left in doubt, especially late-glacial events in the central Karakoram, as are the roles assigned to Karakoram, main Indus Valley, and western Himalayan ice.  相似文献   

7.
The Sumatra–Andaman Tsunami left distinctive sedimentological and geomorphological signatures in the area of Khao Lak. Fine-grained sediments, predominantly layers of cohesive, carbonate-rich, fine-sandy silt with thicknesses of 1–10 cm, erosionally overlying pre-tsunami sandy soils and sediments, represent the most common tsunami deposits in the study area. Petrographically, they differ significantly from other coastal sediments and affiliated soils. Due to their grain size and corresponding clay mineral content, muddy shelf sediments (sub-wave base) are indicated as a main source. The present results suggest that indications of shelf influence, although varying regionally, might contribute to the identification of fine-grained tsunami sediments and their differentiation from storm sediments. However, the observed differences of tsunami sediments to soils and other coastal sediments, especially with respect to carbonate mineralogy, might disappear in short geological time under conditions of intensive weathering and bioturbation. At Cape Pakarang, hundreds of boulders with up to 24 tons were deposited on the foreshore and upper shoreface. Applying Nott’s (Earth Planet Sci Lett 210:269–276, 2003) formulas, minimum flow velocities of 3.9 m/s are required to transport the largest boulders. The devastating tsunami effect of both, onshore flow and backflow, is documented by damaged human constructions. Geomorphological effects include intensive widening of estuary mouths and the development of erosional channels. Now, estuary mouths are reduced, and erosional channels cut off from the sea due to the formation of a post-tsunami beach ridge.  相似文献   

8.
Catastrophic volcanic debris avalanches reshape volcanic edifices with up to half of pre-collapse cone volumes being removed. Deposition from this debris avalanche deposit often fills and inundates the surrounding landscape and may permanently change the distribution of drainage networks. On the weakly-incised Mt. Taranaki ring-plain, volcanic debris avalanche deposits typically form a large, wedge shape (in plan view), over all flat-lying fans. Following volcanic debris avalanches a period of intense re-sedimentation commonly begins on ring-plain areas, particularly in wet or temperate climates. This is exacerbated by large areas of denuded landscape, ongoing instability in the scarp/source region, damming of river/stream systems, and in some cases inherent instability of the volcanic debris avalanche deposits. In addition, on Mt. Taranaki, the collapse of a segment of the cone by volcanic debris avalanche often generates long periods of renewed volcanism, generating large volumes of juvenile tephra onto unstable and unvegetated slopes, or construction of new domes with associated rock falls and block-and-ash flows. The distal ring-plain impact from these post-debris avalanche conditions and processes is primarily accumulation of long run-out debris flow and hyperconcentrated flow deposits with a variety of lithologies and sedimentary character. Common to these post-debris avalanche units is evidence for high-water-content flows that are typically non-cohesive. Hence sedimentary variations in these units are high in lateral and longitudinal exposure in relation to local topography. The post-collapse deposits flank large-scale fans and hence similar lithological and chronological sequences can form on widely disparate sectors of the ring plain. These deposits on Mt. Taranaki provide a record of landscape response and ring-plain evolution in three stages that divide the currently identified Warea Formation: 1) the deposition of broad fans of material adjacent to the debris avalanche unit; 2) channel formation and erosion of Stage 1 deposits, primarily at the contact between debris avalanche deposits and the Stage 1 deposits and the refilling of these channels; and 3) the development of broad tabular sheet flows on top of the debris avalanche, leaving sediments between debris avalanche mounds. After a volcanic debris avalanche, these processes represent an ever changing and evolving hazard-scape with hazard maps needing to be regularly updated to take account of which stage the sedimentary system is in.  相似文献   

9.
Six years of intensive study of the shallow water sediments of the northwestern Gulf of Mexico have indicated that there are many criteria by which ancient sediments deposited under similar conditions can be recognized. Thus the presence of echinoid fragments and of glauconite-filled Foraminifera tests favors marine shelf deposition over bay deposits; calcareous aggregates and grains of gypsum suggest high salinity bay deposits; abundance of wood fibers, high mica content, ferruginous aggregates, and well laminated sediments all are suggestive of deposition near river mouths; greater roundness distinguishes dunes from adjacent beach sands; and coarser grain size distinguishes beach sands from shallow shelf sands in the vicinity.Faunal assemblages of Foraminifera, Ostracoda, and Mollusca all serve to distinguish between bay and shelf deposits. Within the bays the faunas show a close relationship to salinity conditions whereas on the shelf the faunas are arranged in bands parallel to the shore and to the depth contours. Certain generalizations concerning the relation of faunas to environment can be made without detailed knowledge of the species.Contribution from the Scripps Institution of Oceanography, New Series No. 997. Investigation supported by a grant from American Petroleum Institute, Project 51.  相似文献   

10.
Sedimentary indicators of catastrophic glacial megafloods—plane-bedded angular gravel, cobbles, and boulders—are described in several sections of the high terraces of the Chuya River valley. The principal difference of these sediments from typical alluvium of this area is demonstrated. The clast roundness, grain size of clasts, and sedimentary structures of the high-terrace deposits of the Chuya and Katun’ valleys indicate the same facies originated through megaflood deposition. These results are at odds with ideas of alluvial, glacial, or glaciofluvial genesis of the high-terrace deposits of the Chuya River.  相似文献   

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