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1.
Floodplain deposition is an essential part of the Holocene sediment dynamics of many catchments and a thorough dating control of these floodplain deposits is therefore essential to understand the driving forces of these sediment dynamics. In this paper we date floodplain and colluvial deposition in the Belgian Dijle catchment using accelerator mass spectrometric radiocarbon and optical stimulated luminescence dating. Relative mass accumulation curves for the Holocene were constructed for three colluvial sites and 12 alluvial sites. A database was constructed of all available radiocarbon ages of the catchment and this database was analysed using relative sediment mass accumulation rates and cumulative probability functions of ages and site‐specific sedimentation curves. Cumulative probability functions of ages were split into different depositional environments representing stable phases and phases of accelerated clastic deposition. The results indicate that there is an important variation between the different dated sites. After an initial stable early and middle Holocene phase with mainly peat growth in the floodplains, clastic sedimentation rates increased from 4000 BC on. This first phase was more pronounced and started somewhat earlier for colluvial deposits then for alluvial deposits. The main part of the Holocene deposits, both in colluvial and alluvial valleys, was deposited during the last 1 ka. The sedimentation pattern of the individual dated sites and the catchment‐wide pattern indicate that land use changes are responsible for the main variations in the Holocene sediment dynamics of this catchment, while the field data do not provide indications for a climatological influence on the sediment dynamics. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

2.
M. Fuchs   《Quaternary Research》2007,67(3):349-356
Soil erosion is a natural geomorphological process, which can be triggered by both natural (climate, tectonics, or both) and anthropogenic (e.g., agriculture) perturbation of the ecosystem. Evidence has accrued that the Holocene climate experienced large fluctuations in amplitude and suggestions of human impact on the ecosystem provided by the Neolithic revolution dating back to the early Holocene have been made. The question of whether man or climate was the dominant factor responsible for Holocene soil erosion remains unresolved. To resolve the reasons for Holocene sediment redistribution, high-resolution chronometric data on sediments derived from colluvial and alluvial archives from southern Greece were obtained and combined with available archaeological and paleoclimatic data from the eastern Mediterranean. These data show a significant correlation between sedimentation rates and settlement history. Climatic fluctuations are only weakly correlated with sedimentation history. The results show high sedimentation rates during the Early Neolithic (7th millennium BC) in southern Greece, suggesting that Holocene soil erosion was triggered by human activity and then amplified by enhanced precipitation. This would explain the high sedimentation rates during the Early Neolithic in connection with enhanced precipitation in the eastern Mediterranean, which continued until the mid-Holocene.  相似文献   

3.
Glacial varves can give significant insights into recession and melting rates of decaying ice sheets. Moreover, varve chronologies can provide an independent means of comparison to other annually resolved climatic archives, which ultimately help to assess the timing and response of an ice sheet to changes across rapid climate transitions. Here we report a composite 1257‐year‐long varve chronology from southeastern Sweden spanning the regional late Allerød–late Younger Dryas pollen zone. The chronology was correlated to the Greenland Ice‐Core Chronology 2005 using the time‐synchronous Vedde Ash volcanic marker, which can be found in both successions. For the first time, this enables secure placement of the Lateglacial Swedish varve chronology in absolute time. Geochemical analysis from new varve successions indicate a marked change in sedimentation regime accompanied by an interruption of ice‐rafted debris deposition synchronous with the onset of Greenland Stadial 1 (GS‐1; 12 846 years before AD 1950). With the support of a simple ice‐flow/calving model, we suggest that slowdown of sediment transfer can be explained by ice‐sheet margin stabilization/advance in response to a significant drop of the Baltic Ice Lake level. A reassessment of chronological evidence from central‐western and southern Sweden further supports the hypothesis of synchronicity between the first (penultimate) catastrophic drainage of the Baltic Ice Lake and the start of GS‐1 in Greenland ice‐cores. Our results may therefore provide the first chronologically robust evidence linking continental meltwater forcing to rapid atmosphere–ocean circulation changes in the North Atlantic.  相似文献   

4.
Understanding the impact of past climatic changes on landscape stability is crucial in order to predict and mitigate the effects of future changes. However, in arid and semi‐arid environments, reconstructions are often hampered by a poor understanding of the relationship between sediment deposition and climate. We present here data from central Texas, a region that is sensitive to environmental change but has received relatively little attention. The study integrates a chronology of 29 optically stimulated luminescence (OSL) ages from six sedimentary sites in a range of depositional contexts with a 19 000 a climate record derived from pollen extracted from the adjacent Boriack Bog. By comparing the two records, we aimed to assess the relationship between climate change and geomorphic activity. Data show that extensive aeolian and colluvial deposition occurred during the mid to late Holocene, with sedimentation generally increasing during more arid phases. However, a number of depositional events on slopes were associated with moister episodes, and sediment was also deposited in a summit setting immediately after phases of increased precipitation. Linkages between climate change and geomorphic response were therefore complex, being controlled by both sediment supply and transport energy. The climatic fluctuations identified in the Boriack Bog record highlight the sensitivity of central Texas to environmental change, while disparities in conditions recorded between it and other palaeoenvironmental sites in the southern USA emphasise the need for further work in order to enhance understanding of landscape sedimentary response to climate. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

5.
Tectono-stratigraphic models of foredeep sedimentation have generally presumed a direct link between changing rates of tectonism and concomitant sedimentological response as manifested by change in thickness, composition or depositional environment of sediment accumulating in adjacent basins. Lacustrine limestone units within the early Cretaceous fluvial/lacustrine Gannett Group of western Wyoming exhibit systematic variation in several geochemical proxies of relative rates of precipitation and evaporation, indicating that lakewater chemistry was controlled by variation in regional climate. Change in proportion of allochthonous terrigenous clastic vs. autochthonous carbonate deposition, as well as carbonate Mg/Ca ratio and stable isotopic composition, occurs at two scales. Metre-scale alternation of micritic limestone and argillaceous marl is accompanied by mineralogical and isotopic variation within individual beds, indicating preferential carbonate accumulation during intervals of decreased regional meteoric precipitation relative to lake-surface evaporation. Limestone deposition began during intervals of maximum aridity, and decreased as increased meteoric precipitation-driven flux of terrigenous clastic sediment overwhelmed sites of carbonate accumulation. Similar upsection variation in limestone mineralogy and isotopic composition at a scale of tens of metres reflects the multiple processes of long-term increase in meteoric precipitation and lakewater freshening prior to influx of terrigenous sediment, across-basin fluvial-deltaic progradation, and renewed accumulation of riverine terrigenous units. Such trends suggest that formation-scale alternation between fluvial clastic and lacustrine carbonate deposition was controlled by climate change.  相似文献   

6.
Potshards discovered during excavation of bridge pilasters for a major expressway over the Rio Indio floodplain, a stream incised within the karsts of north‐central Puerto Rico, required large‐scale archaeological excavation. Five‐meter‐deep bridge pilaster excavations in the alluvial valley provide a 4500‐year history of deposition. Stratigraphic analysis of the exposed pilaster walls in combination with textural and organic carbon analyses of sediment cores obtained over a much broader area suggest a fluvial system dominated by overbank deposition. Six sequences of alternating light and dark layers of sediment were identified. The darker layers are largely composed of silts and clays, whereas the lighter layers are rich in sand‐sized sediment. Archaeological evidence indicates the organic‐rich dark layers, believed to be buried A horizons, coincide with pre‐historic occupation by Cedrosan Saladoid, Elenan Ostionoid, and Chican Ostionoid, extending from A.D. 450 to A.D. 1500. Lighter layers below the dark soil horizons are interpreted as overbank deposits from large magnitude flood events. The floodplain aggraded discontinuously with rapid deposition of sand followed by gradual accumulation of silt, clay, and organic material. An approximately 1‐m‐thick layer of coarse sand and gravel halfway up the stratigraphic column represents an episode of more frequent and severe floods. Based on radiocarbon ages, this layer aggraded between A.D. 1000 and A.D. 1100, which is well within the Elenan Ostionoid era (A.D. 900–1200). Rates of sedimentation during this period were approximately 8 mm per year, ten times greater than the estimates of sedimentation rates before and after this flood sequence. The cause for the change in deposition is unknown. Nonetheless the Elenan Ostionoid would have had to endure frequent loss of habitation structures and crops during these events. © 2003 Wiley Periodicals, Inc.  相似文献   

7.
Laminated sediments in Lake Ohau, Mackenzie Basin, New Zealand, offer a potential high‐resolution climate record for the past 17 kyr. Such records are particularly important due to the relative paucity of detailed palaeoclimate data from the Southern Hemisphere mid‐latitudes. This paper presents outcomes of a study of the sedimentation processes of this temperate lake setting. Hydrometeorological, limnological and sedimentological data were collected over a 14 month period between 2011 and 2013. These data indicate that seasonality in the hydrometeorological system in combination with internal lake dynamics drives a distinct seasonal pattern of sediment dispersal and deposition on a basin‐wide scale. Sedimentary layers that accumulate proximal to the lake inflow at the northern end of the lake form in response to discrete inflow events throughout the year and display an event stratigraphy. In contrast, seasonal change in the lake system controls accumulation of light (winter) and dark (summer) laminations at the distal end of the lake, resulting in the preservation of varves. This study documents the key processes influencing sediment deposition throughout Lake Ohau and provides fundamental data for generating a high‐resolution palaeoclimate record from this temperate lake.  相似文献   

8.
Research on colluvial depositional systems has recently emphasized periglacial and high‐altitude settings, and the relations between Quaternary slope stratigraphy and climate change. This article examines the role of variable slope morphology, surface hydrology and microclimate in controlling colluvial sedimentation along a coastal tract of the hyperarid Atacama Desert in northern Chile. Direct accessibility of active surfaces is accompanied by uninterrupted stratigraphic exposures along the base of slopes, allowing direct comparisons between surface processes and the resulting sedimentary record. Four slope sectors are identified, based on differences in morphology and processes over active surfaces. Colluvial sedimentation is controlled by complex interactions of slope gradients and profiles, exposure to dominant winds, and potential runoff pathways, which vary considerably between different sectors. Major differences are evident between these hyperarid deposits and slope sedimentation in periglacial and temperate settings, including the complete absence of pedogenic activity and clay minerals; the volume of aeolian deposits and their role in controlling processes which redistribute sediment downslope, extending colluvial aprons; and the occurrence of runoff processes only where favoured by particular topographic configurations. Depositional surfaces range from steep talus cones, to debris‐flow‐dominated and aeolian‐dominated colluvial aprons, to an aeolian ramp subject to reworking by mass flows and flash floods. Consequently, facies associations and architectures at outcrop are highly variable and highlight the importance of spatial variations in slope morphology and processes in producing distinct, coeval colluvial stratigraphies within a single environmental context. Discrepancies between active processes and the corresponding stratigraphic signatures are also evident in some sectors; for example, preservation of alluvial and aeolian facies in stratigraphic sections does not always reflect the dominant processes over active slopes. Together with the spatial variability in processes and deposits along these slopes, this suggests that caution is required when extracting palaeoenvironmental information from analyses of colluvial successions.  相似文献   

9.
Tunnel valleys are common throughout the terrain of the Saginaw Lobe of the Laurentide Ice Sheet in southern Michigan. The set of valleys described in this paper is regularly spaced in a radial pattern behind the Kalamazoo Moraine, an ice‐marginal position formed during retreat from the Last Glacial Maximum. These valleys are divided into proximal and distal groups lying north and south, respectively, of a major river valley that cross‐cuts the tunnel valleys at right angles. Based on a series of rotasonic borings and core analysis, the proximal valleys are shallow, contain minimal sediment fill, and overlie fine‐grained diamicton and glaciolacustrine sediment, whereas the distal valleys are deeply incised into the substrate and are partially filled with coarse sediment. The distal valleys terminate within a broad zone of high‐relief, hummocky topography representing stagnation and collapse behind the Kalamazoo ice margin. The proximal valleys occur within a more subdued landscape located farther from the ice margin. Although some elements of existing genetic models are consistent with these valleys, none appears to be completely compatible with their stratigraphy and morphology. Initial incision of the valleys could have involved short‐lived moderate‐ to high‐discharge flows, followed by deposition during or after the events. The deep incision and thick, coarse sediment in distal valleys in the stagnant marginal zone probably involved supraglacial meltwater draining to the bed as the margin downwasted. Fining‐upward eskers inset into the valleys were formed by flows of declining energy in small late‐stage conduits.  相似文献   

10.
全新世长江泥沙堆积的时空分布及通量估算   总被引：1，自引：1，他引：0  
利用长江中下游、河口及口外、浙-闽沿岸陆架6个主要沉积盆地的40个晚第四纪钻孔及其年代学数据和长江口外、陆架的浅地层剖面,计算了全新世不同阶段各沉积盆地的沉积速率,并进行了近7 000年来泥沙堆积通量的估算。研究发现全新世早期距今10 000年至8 000年间长江口下切古河谷是长江泥沙的主要堆积中心,沉积速率可高达15m/ka。随着海平面上升,全新世中期长江中下游也成为长江泥沙的重要沉积盆地,其中江汉盆地的沉积速率可达10m/ka。近2 000年来,口外、陆架的堆积呈明显增加趋势,反映长江中下游盆地和河口可容空间日益减小。根据沉积速率估算,距今7 000年来长江中下游堆积泥沙约13 074×108 t,同期水下三角洲和陆架的泥沙堆积量约为9 470×108 t。研究还发现全新世以来有两个异常低沉积速率时期：距今8000-7 000年期间上述各沉积盆地沉积速率均显著低,未见长江泥沙的沉积中心; 距今4 000-2 000 年期间长江口呈现低沉积速率。 这两次异常的原因推测与海平面、气候波动事件密切相关。  相似文献   

11.
Coastal outcrops of Upper Pleistocene deposits in north-eastern Mallorca (Balearic Islands, Spain) have been examined and a chronological framework established using Optically Stimulated Luminescence (OSL) dating. The outcrops record a complex interaction between aeolian, colluvial and alluvial fan deposition that result in a variable stratigraphical architecture. Dating and facies analysis make it possible to identify four aeolian activity episodes during MIS 5c/b, 4 and 3, and also add to the understanding of the environmental history of north-eastern Mallorca over the last 100 ka. The new data suggest enhanced aeolian transport and dune formation during cold climatic intervals when sea level was low, sediment supply (exposed marine carbonate sediment) was high, winter windiness was high, and vegetation on coastal plains at a minimum.  相似文献   

12.
《Quaternary Science Reviews》2007,26(22-24):2801-2822
Late Cenozoic fluvial successions are widespread in India. They include the deposits of the Siwalik basin which represent the accumulations of the ancient river systems of the Himalayan foreland basin. Palaeomagnetic studies reveal that fluvial architecture and styles of deposition were controlled by Himalayan tectonics as well as by major climatic fluctuations during the long (∼13 Ma) span of formation. The Indo-Gangetic plains form the world's most extensive Quaternary alluvial plains, and display spatially variable controls on sedimentation: Himalayan tectonics in the frontal parts, climate in the middle reaches, and eustasy in the lower reaches close to the Ganga–Brahmaputra delta. Climatic effects were mediated by strong fluctuations in the SW Indian Monsoon, and Himalayan rivers occupy deep valleys in the western Ganga plains where stream power is high, cut in part during early Holocene monsoon intensification; the broad interfluves record the simultaneous aggradation of plains-fed rivers since ∼100 ka. The eastward increase in precipitation across the Ganga Plains results in rivers with low stream power and a very high sediment flux, resulting in an aggradational mode and little incision. The river deposits of semi-arid to arid western India form important archives of Quaternary climate change through their intercalation with the eolian deposits of the Thar Desert. Although the synthesis documents strong variability—both spatial and temporal—in fluvial stratigraphy, climatic events such as the decline in precipitation during the Last Glacial Maximum and monsoon intensification in the early Holocene have influenced fluvial dynamics throughout the region.  相似文献   

13.
Abstract

Quaternary alluvial and colluvial sediments infill major river valleys and form alluvial fans and colluvium-filled bedrock depressions on the range fronts and within the Mount Lofty Ranges of southern Australia. A complex association of alluvial successions occurs in the Sellicks Creek drainage basin, as revealed from lithostratigraphy, physical landscape setting and optically stimulated luminescence (OSL) ages. Correlation of OSL ages with the Marine Oxygen Isotope record reveals that the alluvial successions represent multiple episodes of alluvial sedimentation since the penultimate glaciation (Marine Isotope Stage 6; MIS 6). The successions include a penultimate glacial maximum alluvium (Taringa Formation; 160?±?15?ka; MIS 6), an unnamed alluvial succession (42?±?3.2?ka; MIS 3), a late last glacial colluvial succession within bedrock depressions (ca 15?ka; MIS 2) and a late last glacial alluvium (ca 15?ka; MIS 2) in the lowest, distal portion of Sellicks Creek. In addition, the Waldeila Formation, a Holocene alluvium (3.5?±?0.3?ka; MIS 1), and sediments deposited during a phase of Post-European Settlement Aggradation (PESA) are also identified. The age and spatial distribution of the red/brown successions, mapped as the Upper Pleistocene Pooraka Formation, directly relate to different topographic and tectonic settings. Neotectonic uplift locally enhanced erosion and sedimentation, while differences in drainage basin sizes along the margin of the ranges have influenced the timing and delivery of sediment in downstream locations. Close to the Willunga Fault Scarp at Sellicks Creek, sediments resembling the Pooraka Formation have yielded a pooled mean OSL age of 83.9?±?7?ka (MIS 5a) corroborating the previously identified extended time range for deposition of the formation. Elsewhere, within major river valleys, the Pooraka Formation was deposited during the last interglacial maximum (128–118?ka; MIS 5e). In general, alluviation occurred during interglacial and interstadial pluvial events, while erosion predominated during drier glacial episodes. In both cases, contemporaneous erosion and sedimentation continued to affect the landscape. For example, in the Sellicks Creek drainage basin, which lies across an actively uplifting fault zone, late glacial age sediments (MIS 2) occur within the ranges and near the distal margin of the alluvial fan complex. OSL dating of the alluvial successions reported in this paper highlights linkages between the terrestrial and marine environments in association with sea-level (base-level) and climatic perturbations. While the alluvial successions relate largely to climatically driven changes, especially in major river valleys, tectonics, eustasy, geomorphic setting and topography have influenced erosion and sedimentation, especially on steep-sloped alluvial fan environments.
  1. KEY POINTS

  2. Luminescence dating of the Sellicks Creek alluvial fan complex reveals that sedimentation occurred predominantly during the later stages of glacial cycles accompanying lower sea-levels than present.

  3. Luminescence dating confirms that the stratigraphically lower portions of the Pooraka Formation are beyond the range of radiocarbon dating.

  4. Upper Pleistocene alluvial fan sedimentation at Sellicks Creek correlates with pluvial events in southeastern Australia.

  相似文献   

14.
《Geodinamica Acta》1999,12(5):291-301
As a result of recent drillings in the Walensee Valley (eastern Switzerland) a new fades model for the Quaternary filling of Alpine valleys has been developed. A detailed lithological model and some new radiocarbon dating allowed the calculation of regional sedimentation and denudation rates and their change during the Late- and Postglacial period. It is shown that these changes follow the paraglacial sedimentation model by Church and Ryder [1]. The absolute quantification of the sediment budgets between the Last Glaciation and today points to denudation rates in the order of 1.5 mm y−1 for the catchment of the Lake of Walenstadt. This is 50 % higher than suggested from current tectonic and isostatic estimates up to now. In that case present day uplift of the Alps would not be in balance with denudation.  相似文献   

15.
Lynchets are ridges formed by erosion and sediment accumulation downstream of agricultural plots and offer valuable insights into past agricultural activity. These microtopographies cover vast areas and serve as indicators of historical changes in land use. As a result, their ubiquity across Europe makes them particularly interesting. In this study, we propose a geoarchaeological approach to analyze six lynchets, four in the Vosges Mountains and two on the Lorraine Plateau (France). The lynchets can be considered soil archives with no stratigraphic organization or chronological sequence from bottom to top, making it difficult to determine the age of the lynchets and identify changes in land use over time. To this end, we propose the analysis of historical and geo-historical archives combined with the “pedosedimentary” archives of lynchets through charcoal identification and dating combined with near-infrared spectroscopy to determine the age, vegetation, and past land use changes associated with lynchet landscapes. By combining these multiple data sources, we are better able to show the chronological development of these ancient agricultural systems and uncover valuable information on landscape history. Charcoal dating suggests a higher frequency of fires from the Middle Ages. The dating aligns with the regional dynamics of anthropogenic fires, indicating a potential use of fire for cultural purposes. We also demonstrate the difficulty of extrapolating the dating of a lynchet to the entire lynchet system. Our results highlight the difficulties of interpreting the formation and dating of lynchets and the lynchet system on the sole basis of charcoal analysis. However, we highlight the value of applying pedoanthracology to lynchets to determine the dynamics of land use change in former fields.  相似文献   

16.
The Maastrichtian chalk of the southern Central Graben, Danish North Sea, is a homogeneous pure white coccolithic chalk mudstone deposited in a deep epeiric shelf sea, which covered large parts of northern Europe. The sediment displays a pronounced cyclicity marked by decimetre‐thick bioturbated beds alternating with slightly thinner non‐bioturbated, mainly laminated beds. The laminated half‐cycles consist of alternating millimetre‐thick, graded, high‐porosity laminae and non‐graded, low‐porosity laminae. The cyclicity has been interpreted previously as caused by periods of slow background sedimentation and bioturbation interrupted by periods of rapid deposition of laminated beds, with the latter reflecting random and local resedimentation processes. Based on textural and structural analysis, the millimetre‐scale, non‐graded laminae are interpreted as having been deposited directly from pelagic rain of pelleted coccoliths representing the primary production. The graded laminae were deposited from small‐volume, low‐density turbidity currents and suspension clouds. The sedimentation rates of the cyclical chalk are similar to those known elsewhere, and the lamination is interpreted as having been preserved from destruction through bioturbation by anoxic conditions at the seafloor. Bioturbated–laminated cycles are thus formed by slow sedimentation during alternating seafloor redox conditions probably on a Milankovitch scale. A direct implication of this interpretation is that the cycles are areally widespread, probably extending throughout the southern Central Graben area and may be useful for correlation and high‐resolution cyclostratigraphy in the chalk fields of the Danish North sea. If the laminated half‐cycles represent a few rapid resedimentation events, with a high sedimentation rate as suggested by most workers, then the sediment would not be truly cyclic, but would represent event sedimentation within a pelagic background represented by the bioturbated beds. In this case, the cycles would have very limited potential for correlation.  相似文献   

17.
长江口沉积物210Pb分布及沉积环境解释   总被引：19，自引：0，他引：19  
在长江河口潮滩、分流河道和水下三角洲共获得18个柱样,进行沉积学分析和210Pb测定,并对其中6根柱样进行137Cs测定。经研究发现,长江口外在水深25～30m,122°30′N,31°00′E附近存在一个泥质沉积中心,沉积速率达2.0～6.3cm/yr。另外,在潮滩和涨潮槽也获得较高沉积速率,其中南汇和横沙岛潮滩沉积速率(1.03～1.94cm/yr)高于崇明东滩(0.51～0.76cm/yr),涨潮槽沉积速率也达0.86cm/yr。此外,在石洞口、南汇、九段沙潮滩和三角洲前缘有部分柱样未获沉积速率,推测为沉积环境不稳定或沉积速率过快所致。  相似文献   

18.
在长江河口潮滩、分流河道和水下三角洲共获得18个柱样,进行沉积学分析和210Pb测定,并对其中6根柱样进行137Cs测定。经研究发现,长江口外在水深25～30m,122°30′N,31°00′E附近存在一个泥质沉积中心,沉积速率达2.0～6.3cm/yr。另外,在潮滩和涨潮槽也获得较高沉积速率,其中南汇和横沙岛潮滩沉积速率(1.03～1.94cm/yr)高于崇明东滩(0.51～0.76cm/yr),涨潮槽沉积速率也达0.86cm/yr。此外,在石洞口、南汇、九段沙潮滩和三角洲前缘有部分柱样未获沉积速率,推测为沉积环境不稳定或沉积速率过快所致。  相似文献   

19.
The deeply dissected Southwest Grand Banks Slope offshore the Grand Banks of Newfoundland was investigated using multiple data sets in order to determine how canyons and intercanyon ridges developed and what sedimentary processes acted on glacially influenced slopes. The canyons are a product of Quaternary ice‐related processes that operated along the margin, such as ice stream outwash and proglacial plume fallout. Three types of canyon are defined based on their dimensions, axial sedimentary processes and the location of the canyon head. There are canyons formed by glacial outwash with aggradational and erosional floors, and canyons formed on the slope by retrogressive failure. The steep, narrow intercanyon ridges that separate the canyons are composite morphological features formed by a complex history of sediment aggradation and degradation. Ridge aggradation occurred as a result of mid to late Quaternary background sedimentation (proglacial plume fallout and hemipelagic settling) and turbidite deposition. Intercanyon ridge degradation was caused mainly by sediment removal due to local slump failures and erosive sediment gravity flows. Levée‐like deposits are present as little as 15 km from the shelf break. At 30 km from the shelf, turbidity currents spilled over a 400 m high ridge and reconfined in a canyon formed by retrogressive failure, where a thalweg channel was developed. These observations imply that turbidity currents evolved rapidly in this slope‐proximal environment and attained flow depths of hundreds of metres over distances of a few tens of kilometres, suggesting turbulent subglacial outwash from tunnel valleys as the principal turbidity current‐generating mechanism.  相似文献   

20.
The present paper refers to research conducted in the tectonic-karst depression of Campo Felice in the central Apennines (Italy), in which glacial, alluvial and lacustrine sediments have been preserved. Stratigraphic interpretations of sediments underlying the Campo Felice Plain are based on evidence obtained from nine continuous-core boreholes. The boreholes reach a depth of 120 m and provide evidence of five sedimentation cycles separated by erosion surfaces. Each cycle is interpreted as an initial response to a mainly warm stage, characterized by low-energy alluvial and colluvial deposition, pedogenesis, and limited episodes of marsh formation. In turn, a mainly cold stage follows during which a lake formed, and glaciers developed and expanded, leading to deposition of glacial and fluvioglacial deposits. The chronological framework is established by eleven accelerator mass spectrometer 14C ages and three 39Ar-40Ar ages on leucites from interbedded tephra layers. These age determinations indicate five glacial advances that respectively occurred during marine oxygen isotope stages 2, 3-4, 6, 10 and 14.  相似文献   

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