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1.
Detailed measurements of the altitudes of the shorelines of former ice-dammed lakes in Glen Roy and vicinity in the Scottish Highlands prove erroneous the conventional view that former shorelines in areas affected by glacio-isostatic uplift are uniformly tilted and/or gently warped. The measurements demonstrate differential uplift of blocks of the earth's crust. The surfaces of some blocks have no detectable tilt whereas others have gradients up to at least 4.6 m/km and are tilted in different directions. In three areas 0.7–2.0 km long, shorelines are distorted by crustal movements; all these areas have landslides attributed to earthquakes that accompanied the release of stress. In the area of greatest local distortion three shorelines rose about 3 m above their average altitudes for the immediately surrounding area, and a fault scarp was produced. Possibly these local distortions accompanied catastrophic lake drainage by jökulhlaup, analogous to crustal movements associated with man-made lakes. A relationship between crustal movement and the limit of a glacial advance is demonstrated. These findings have implications for other parts of the world affected by glacioisostatic uplift.  相似文献   

2.
The sediment profile from Lake Gościąż in central Poland comprises a continuous, seasonally resolved and exceptionally well-preserved archive of the Younger Dryas (YD) climate variation. This provides a unique opportunity for detailed investigation of lake system responses during periods of rapid climate cooling (YD onset) and warming (YD termination). The new varve record of Lake Gościąż presented here spans 1662 years from the late Allerød (AL) to the early Preboreal (PB). Microscopic varve counting provides an independent chronology with a YD duration of 1149+14/–22 years, which confirms previous results of 1140±40 years. We link stable oxygen isotopes and chironomid-based air temperature reconstructions with the response of various geochemical and varve microfacies proxies especially focusing on the onset and termination of the YD. Cooling at the YD onset lasted ~180 years, which is about a century longer than the terminal warming that was completed in ~70 years. During the AL/YD transition, environmental proxy data lagged the onset of cooling by ~90 years and revealed an increase of lake productivity and internal lake re-suspension as well as slightly higher detrital sediment input. In contrast, rapid warming and environmental changes during the YD/PB transition occurred simultaneously. However, initial changes such as declining diatom deposition and detrital input occurred already a few centuries before the rapid warming at the YD/PB transition. These environmental changes likely reflect a gradual increase in summer air temperatures already during the YD. Our data indicate complex and differing environmental responses to the major climate changes related to the YD, which involve different proxy sensitivities and threshold processes.  相似文献   

3.
Two glaciers at Eyjafjallajökull, south Iceland, provide a record of multiple episodes of glacier advance since the Sub-Atlantic period, ca. 2000 yr ago. A combination of tephrochronology and lichenometry was applied to date ice-marginal moraines, tills and meltwater deposits. Two glacier advances occurred before the 3rd century AD, others in the 9th and 12th centuries bracketing the Medieval Warm Period, and five groups of advances occurred between AD 1700 and 1930, within the Little Ice Age. The advances of Eyjafjallajökull before the Norse settlement (ca. AD 870) were synchronous with other glacier advances identified in Iceland. In contrast, medieval glacier advances between the 9th and 13th centuries are firmly identified for the first time in Iceland. This challenges the view of a prolonged Medieval Warm Period and supports fragmentary historical data that indicate significant medieval episodes of cooler and wetter conditions in Iceland. An extended and more detailed glacier chronology of the mid- and late Little Ice Age is established, which demonstrates that some small outlet glaciers achieved their Little Ice Age maxima around AD 1700. While Little Ice Age advances across Iceland appear to synchronous, the timing of the maximum differs between glacier type and region.  相似文献   

4.
Multiproxy climate records from Iceland document complex changes in terrestrial climate and glacier fluctuations through the Holocene, revealing some coherent patterns of change as well as significant spatial variability. Most studies on the Last Glacial Maximum and subsequent deglaciation reveal a dynamic Iceland Ice Sheet (IIS) that responded abruptly to changes in ocean currents and sea level. The IIS broke up catastrophically around 15 ka as the Polar Front migrated northward and sea level rose. Indications of regional advance or halt of the glaciers are seen in late Alleröd/early Younger Dryas time and again in PreBoreal time. Due to the apparent rise of relative sea level in Iceland during this time, most sites contain evidence for fluctuating, tidewater glacier termini occupying paleo fjords and bays. The time between the end of the Younger Dryas and the Preboreal was characterized by repeated jökulhlaups that eroded glacial deposits. By 10.3 ka, the main ice sheet was in rapid retreat across the highlands of Iceland. The Holocene thermal maximum (HTM) was reached after 8 ka with land temperatures estimated to be 3 °C higher than the 1961–1990 reference, and net precipitation similar to modern. Such temperatures imply largely ice-free conditions across Iceland in the early to mid-Holocene. Several marine and lacustrine sediment climate proxies record substantial summer temperature depression between 8.5 and 8 ka, but no moraines have been detected from that time. Termination of the HTM and onset of Neoglacial cooling took place sometime after 6 ka with increased glacier activity between 4.5 and 4.0 ka, intensifying between 3.0 and 2.5 ka. Although a distinct warming during the Medieval Warm Period is not dramatically apparent in Icelandic records, the interval from ca AD 0 to 1200 is commonly characterized by relative stability with slow rates of change. The literature most commonly describes Little Ice Age moraines (ca AD 1250–1900) as representing the most extensive ice margins since early Holocene deglaciation, with temperature depressions of 1–2 °C compared to the AD 1961–1990 average. Steep north–south and west–east temperature gradients are reconstructed in the Holocene records of Iceland, suggesting a strong maritime influence on the terrestrial climate of Iceland.  相似文献   

5.
In the summer of 1999, the Maritimes and Northeast Pipeline Company excavated a 3-m-deep trench across northern Nova Scotia exposing a continuous transect of surficial deposits along a 237-km corridor. A Lateglacial palaeosol with preserved A horizon (peat and wood) buried under 2-10 m of surface till consisted mainly of herbaceous plant material with few large wood fragments. Large pieces of wood from two sites yielded conventional radiocarbon ages of 10.9 14C kyr BP (GSC-6435) and 10.8 14C kyr BP (GSC-6419). Previous to these finds, only a few localities were known to reveal till overlying peat, so the extent of Younger Dryas (YD) glaciers could not be clearly established. Glacial flow lines indicated by fabric and fluting of the YD surface till sheet in northern Nova Scotia and ice-marginal deposits imply an ice cap centred over eastern P.E.I. and the southern Gulf of St. Lawrence. This glacier also dammed a series of glacial lakes against the highland-rimmed west coast of Cape Breton Island. Glaciers developed and advanced during the YD in the uplands and offshore shelf areas from small remnants of Late Wisconsinan ice. Renewed ice growth was enabled by increased precipitation and local cooling in the Gulf of St. Lawrence due to deflection of the jet stream and expanded sea-ice cover in the North Atlantic. The YD may provide an analogy to glacier development in Maritime Canada during the interglacial/glacial transition.  相似文献   

6.
This study provides an insight into the impact of probably the largest flood ever to have been identified in mainland Britain by examining new sedimentary evidence from the Auchteraw terrace, Fort Augustus, Scotland. Study of three sections reveals a succession consisting of: (1) sheet gravels; (2) large trough-shaped depressions infilled with cross-stratified sands and gravels; (3) smaller-scale, finer-grained cross-strata; and (4) sheet-like, occasionally channelized, bimodal sand and gravel beds. This study shows that both the sedimentology and morphology of the Auchteraw terrace are consistent with jökulhlaup deposition and reveal a greater variety of lithofacies types than identified in previous studies of jökulhlaups from ice-dammed lakes. The fine-grained nature of the sediment discussed in this study emphasizes the importance of sediment supply for the formation of distinctive jökulhlaup successions. The sedimentary evidence recorded here provides a valuable tool for the interpretation of the magnitude and frequency of proglacial meltwater flows associated with Pleistocene ice sheets worldwide.  相似文献   

7.
The 1918 eruption of the glacially capped Katla volcano, southern Iceland, generated a violent jökulhlaup, or glacial outburst flood, inundating a large area of Mýrdalssandur, the proglacial outwash plain, where it deposited ca 1 km3 of volcaniclastic sediment. The character of the 1918 jökulhlaup is contentious, having been variously categorized as a turbulent water flow, a hyperconcentrated flow or as a debris flow, based on localized outcrop analysis. In this study, outcrop‐based architectural analyses of the 1918 deposits reveal the presence of lenticular and tabular bedsets associated with deposition from quasi‐stationary antidunes and down‐current migrating antidunes, and from regular based bedsets, associated with transient chute‐and‐pool bedforms, all of which are associated with turbulent, transcritical to supercritical water flow conditions. Antidune wavelengths range from 24 to 96 m, corresponding to flow velocities of 6 to 12 m sec?1 and average flow depths of 5 to 19 m. This range of calculated flow velocities is in good agreement with estimates made from eyewitness accounts. Architectural analysis of the 1918 jökulhlaup deposits has led to an improved estimation of flow parameters and flow hydraulics associated with the 1918 jökulhlaup that could not have been achieved through localized outcrop analysis. The observations presented here provide additional sedimentological and architectural criteria for the recognition of deposits associated with transcritical and supercritical water flow conditions. The physical scale of sedimentary architectures associated with the migration of bedforms is largely dependent on the magnitude of the formative flow events or processes; sedimentary analyses must therefore be undertaken at the appropriate physical scale if reliable interpretations, regarding modes of deposition and formative flow hydraulics, are to be made.  相似文献   

8.
This paper describes the internal architecture of a push moraine formed by a winter-spring surge of Hagafellsjökull-Eystri (Iceland) in 1998/99. The sedimentary architecture of this push moraine consists of a multilayered slab of glaciofluvial sediments with a monoclinal structure that has been displaced laterally by the advancing ice margin. The crest and ice-distal face of the moraine consist of subhorizontal sediment sheets, while the ice-proximal face dips steeply (45° to 90°) towards the ice margin. The core of the moraine consists of frozen sediment and thin slabs of glacier ice are embedded in its proximal face. The sediment slabs are characterized by both brittle and ductile styles of deformation. We argue that the observed variation in deformation style is dependent on whether the glacial foreland was frozen or unfrozen at the time of displacement. Frozen foreland would behave in a brittle fashion, while unfrozen foreland is likely to have deformed in a more ductile manner. The associated spatial variations in the degree of foreland freezing could be explained by variation in ice-marginal snow cover. We conclude that the thermal regime of the foreland, and the timing of the ice advance, is of importance to the style of internal deformation found within ice-marginal push moraines.  相似文献   

9.
《Geodinamica Acta》2013,26(1):81-100
The North Volcanic Zone of Iceland was unglaciated during most interglacials. Subsequently, the region was covered by the Weichselian ice cap. A widespread interglacial complex, the Sy?ra Formation, has been mapped in this zone. It covers probably O.I.S.5e, 5d and 5c. Its formation and preservation are discussed in terms of rift and volcanism activity, in interrelations with the former deglaciation. A topographic bulge, presumed of glacio-isostatic origin, limited the downstream drainage of the Jökulsa a Fjolum river enabling the interglacial sedimentation and the excavation of one of the canyons of Dettifoss. Effusive volcanic activity in the rift is important prior to the Sy?ra 4 unit in association with an early abrupt event (SY2: Sy?ra ash), related to a phreato-magmatic eruption at the eastern hyaloclastite ridge or from the Askja volcano and to jökulhlaup events. It corresponds probably to ash Zone B as defined by Sejrup et al., (1989) on the Northern Iceland shelf. The previous activity of hyaloclastite ridge is recorded during the Marine Isotope Stage 6 (MIS 6 = Saalian) and its deglaciation, a younger effusive event is dated at 80 ka. The Interglacial paleo-seismic region is similar to the present one; during deglaciation, the seismic zone is widened, up to 60 km to the East. Continuous micro-seismicity related to dyke intrusion and effusive or phreato-magmatic eruptions develop at the onset of deglaciation. It is discrete during the full interglacials, and most intense during pyroclastic eruptions. A comparison with the Late Glacial/Holocene deglaciation is provided in the same region.  相似文献   

10.
Fossil pollen, plant macrofossils, gastropods, and elemental and stable-isotope geochemistry in a sediment core from Twiss Marl Pond, southern Ontario, Canada, were used to document climate oscillations during the Last Glacial–Interglacial transition (13,000–8500 14C BP) and understand their ecological effects. Chronology was provided by AMS 14C dating and regional pollen correlation. Oxygen isotope (δ18O) results from mollusc shells, Chara-encrustations and bulk carbonates show a classic climate sequence of a warm Bølling–Allerød (BOA) at 12,500–10,920 14C BP, a cold Younger Dryas (YD) at 10,920–10,000 14C BP, the Holocene warming at 10,000 14C BP, a brief Preboreal Oscillation (PB) at 9650 14C BP, and a possible Gerzensee/Killarney (G/K) cooling shortly before 11,000 14C BP.Clay sediments at the base of the core contain high herb and shrub pollen and abundant arctic/alpine plant macrofossils, indicating a treeless tundra with severe soil erosion in watershed. During the BOA warm period, authigenic marl began to be deposited, and Picea woodland became established. The establishment of Picea woodland after peaks of δ18O and of carbonate accumulation suggests a lagged response of upland vegetation to BOA warming. In contrast, the occurrence of warmth-loving aquatics Najas flexilis and Typha latifolia at that time indicates sensitive responses of aquatic plants. The YD cooling is indicated by a 1.5‰ negative excursion in δ18O, an increase in minerogenic matter and higher concentrations of erosion-derived elements (Al, Na, K, Ti and V). Pollen data show no forest transformation in response to YD cooling, which is attributed to the insensitive nonecotonal vegetation at that time. However, more openings in the forests and increased erosion in the watershed are indicated by a slight increase of herb pollen, high concentrations of erosion elements and a Pediastrum peak. The onset of the Holocene was marked by an abrupt increase of 2‰ in δ18O and the replacement of Picea woodland by Pinus-dominated forest. The Picea recurrence at 9650 14C BP demonstrates sensitive response of ecotonal vegetation to the PB climate oscillation, which is also indicated by 0.4‰ negative excursion of δ18O. These new results suggest the importance of multiproxy records for reliable paleoclimate reconstruction.Reevaluation and revised chronologies of previously published sites (Gage Street, and Nichols Brook) in the eastern Great Lakes region show their major δ18O shifts correlative to the YD and PB oscillations as documented from Twiss Marl Pond and nearby Crawford Lake. The sequence and magnitude of climatic oscillations from these sites match in detail with records from the Atlantic Seaboard, suggesting that these oscillations are an expression of broad-scale, probably global, climate change rather than local meltwater-induced climate cooling.  相似文献   

11.
The timing and magnitude of sea-surface temperature (SST) changes in the tropical southern South China Sea (SCS) during the last 16,500 years have been reconstructed on a high-resolution, 14C-dated sediment core using three different foraminiferal transfer functions (SIMMAX28, RAM, FP-12E) and geochemical (Uk′37) SST estimates. In agreement with CLIMAP reconstructions, both the FP-12E and the Uk′37 SST estimates show an average late glacial–interglacial SST difference of 2.0°C, whereas the RAM and SIMMAX28 foraminiferal transfer functions show only a minor (0.6°C) or no consistent late glacial–interglacial SST change, respectively. Both the Uk′37 and the FP-12E SST estimates, as well as the planktonic foraminiferal δ18O values, indicate an abrupt warming (ca. 1°C in <200 yr) at the end of the last glaciation, synchronous (within dating uncertainties) with the Bølling transition as recorded in the Greenland Ice Sheet Project 2 (GISP2) ice core, whereas the RAM-derived deglacial SST increase appears to lag during this event by ca. 500 yr. The similarity in abruptness and timing of the warming associated with the Bølling transition in Greenland and the southern SCS suggest a true synchrony of the Northern Hemisphere warming at the end of the last glaciation. In contrast to the foraminiferal transfer function estimates that do not indicate any consistent cooling associated with the Younger Dryas (YD) climate event in the tropical SCS, the Uk′37 SST estimates show a cooling of ca. 0.2–0.6°C compared to the Bølling–Allerød period. These Uk′37 SST estimates from the southern SCS argue in favor of a Northern Hemisphere-wide, synchronous cooling during the YD period.  相似文献   

12.
The headlands between the fjords Arnarfjördur and Patreksfjördur in northwest Iceland, consisting mostly of Upper Tertiary plateau basalt lavas, have been geologically mapped in detail. Magnetic properties of samples from some 50 lava flows and dykes in the region have been measured; the mean palaeomagnetic pole position obtained from forty normally magnetized lavas is at 88°N113°E. The application of the present work to stratigraphic correlation and magnetic anomaly interpretation in northwest Iceland is discussed.  相似文献   

13.
This study presents the first multi-proxy palaeoenvironmental and palaeoclimatic history for northern South America based on the palaeolimnological reconstruction of a pond located in a dry paramo at 3570 masl. During the Last Glacial Maximum (LGM), the study area was under glacial conditions, then during global events Heinrich Stadial 1 (HS1), Bølling–Allerød (BA), and the Younger Dryas (YD), the pond expanded, accumulation rates and proxies for erosion reached the highest values, indicative of humid conditions, with maxima in humidity during the BA and YD. Dry conditions and pond desiccation occurred in the Greenlandian–Northgrippian and by 6010 cal a bp the area was transformed into the mire of today. Comparisons with records from other sites in South America indicate that changes in humidity are most likely controlled by the Intertropical Convergence Zone, mainly during the glacial and postglacial, and by changes in the Pacific Ocean, more pronounced after the YD.  相似文献   

14.
千年尺度气候变率的研究   总被引：5，自引：0，他引：5  
2 0世纪后期 ,千年 (ka)尺度气候变率的研究取得了重要的进展 ,这表现在以下几个方面 :(1)格陵兰冰芯及深海沉积证明 ,在末次冰期中普遍存在平均周期为 1 5ka的循环 ,有人认为全新世也存在这种循环 ,小冰期就是最近一个循环的冷期。 (2 )每个循环由 1个相对暖期 (间冰阶 )及 1个冷期(冰阶 )组成 ,称为Dansgaard/Oeshger循环 (D/O循环 )。 (3)连续几个D/O循环的冰阶气温愈来愈低 ,海因里希事件 (H事件 )就发生在最冷的冰阶之后。 (4 )自 15kaBP到 6 8kaBP共确定出 6次H事件 ,分别称为H1…H6,有的作者认为新仙女木事件 (YD)与H事件形成机制近似 ,可以称为H0 。 (5 )D/O循环与H事件的成因 ,目前尚无定论。但热盐环流 (THC)变化的学说得到了较多作者的承认。这个学说认为 :北大西洋北部的大量融冰使海面为冷的淡水控制 ,影响了大传送带中海水的下沉 ,从而削弱了深水的形成。北大西洋THC减弱 ,使向北输送的热量减少 ,使北大西洋气候更寒冷。一旦深水形成再次增加 ,完成一个D/O循环。H事件形成的机制与之类似 ,不过过程变化更为激烈。 (6 )这样 ,THC有 3种模态 :现代模 (北大西洋有两个泵 )、冰阶模 (一个泵 ) ,及H事件 (无泵 )。 (7)海洋环流模式已经对THC的变化及模态之间的转换进行了模拟 ,至少在一定程度?  相似文献   

15.
《Sedimentary Geology》2007,193(1-4):131-148
This paper characterises the sedimentary impact of a glacial outburst flood or ‘jökulhlaup’ on an ice-contact delta topset at Russell Glacier, Kangerlussuaq, west Greenland. Rapid drainage of an ice-dammed lake in July 1987 generated a jökulhlaup with a peak discharge of ∼ 1300 m3 s− 1, which drained across a 500-m-wide, 200-m-long, delta top into a proglacial lake. The delta topset comprises boulder clusters, ice block obstacle marks with relief of up to 4 m, and is graded to lake levels up to 6 m higher than those during typical non-jökulhlaup conditions. The delta top was dissected by the 1987 jökulhlaup causing a fan-shaped extension of the delta front by 30 m. Surface grain size on the delta decreases rapidly away from the main flood flow direction, reflecting rapid downstream reduction in sediment transport capacity. The 1987 jökulhlaup was predominantly fluidal and turbulent and had peak stream powers of 2846 W m− 2 proximally and < 400 W m− 2 distally. Delta topset sedimentation can be characterised by four lithofacies associations in order of decreasing flow energy: (A) coarse-grained deposits related to a flow expansion; (B) finer-grained peripheral deposits located at the margins of the main flow; (C) lobate bars and delta fronts deposited within distal locations and (D) fine-grained deposits at distance from the delta front associated with slackwater conditions. Jökulhlaup-dominated delta topsets are controlled by the geometry of the channel expansion into the proglacial lake, jökulhlaup hydrograph form, the sediment availability and character, proglacial lake basin depth and surface area, lake outflow spillway erodibility and cross-sectional area, and history of previous jökulhlaups.  相似文献   

16.
《Quaternary Science Reviews》2007,26(19-21):2375-2405
Late Devensian glacigenic sediments and landforms along the north-west coast of Wales document the advance and subsequent retreat of the eastern margin of an Irish Sea Ice Stream that met, coalesced and ultimately uncoupled from ice radiating outwards from the adjacent Welsh Ice Cap centred over Snowdonia. Across the boundary between the two former ice masses is a set of sediment–landform assemblages that reflect rapidly changing erosional and depositional conditions during ice interaction. From the inner part of the ice-stream the assemblages range outwards, from a subglacial depositional assemblage, characterised by drumlin swarms; through a subglacial erosional assemblage, marked by prominent bedrock scours and large subglacial rock channels; through an ice-marginal assemblage, identified by closely spaced, glaciotectonised push moraines and intervening marginal sandur troughs; into a freely expanding proglacial sandur and lacustrine delta assemblage. The ice-marginal assemblage provides evidence for numerous oscillatory episodes during retreat and at least 20 ice-marginal limits can be identified. At least 11 of these display multiple criteria for identifying readvance and, in the ideal case, is characterised by a moraine form built by localised tectonic stacking of diamict to the rear, fronted by a clastic wedge of ice-front alluvial fan gravel and intercalated flow till. The distribution of sediment–landform assemblages suggests a highly dynamic, convergent ice-stream flow pattern, with high ice velocity, a sharply delineated lateral shear margin, pervasive ice-marginal glaciotectonic deformation and a tightly focused ice-marginal sediment delivery system; all signature characteristics of contemporary ice streams.  相似文献   

17.
The general subject of this paper is subglacial deformation beneath Breiðamerkurjökull, a surging Icelandic glacier. More specifically it discusses the evolution and the role of fluid pressure on the behaviour of subglacial sediments during deformation. During Little Ice Age maximum, the two outcrops studied, North Jökulsarlon (N-Jk) and Brennhola-Alda (BA), were located at 2550 m and 550 m respectively from the front of the Breiðamerkurjökull. Sedimentological analysis at the forefield of the glacier shows thick, coarse glaciofluvial deposits interbedded with thin, fine-grained shallow lacustrine/swamp deposits, overlain by a deformed till unit at N-Jk. BA outcrop shows fine-grained shallow lacustrine/swamp deposits overlain by a deformed till unit. The sequence of deformation events from one outcrop to the other is similar. First, major thrust planes, which were rooted in shallow lacustrine/swamp deposits developed by glacially induced simple shear. Next, the thrusts were folded, indicating the deformation of hydroplastic sediment assisted by moderate fluid pressure. Then clastic dyke swarms crosscut the sedimentary succession, proving that fluid overpressure caused hydrofracturing associated with fluidisation. Finally, as water escaped from the glacier bed, fluid pressure dropped, and normal faulting occurred in brittle-state subglacial sediments. Fluid-pressure variations are related to glacier dynamics. They control the deformation sequence by modifying subglacial rheological behaviour and the nature of the subglacial tectonism.  相似文献   

18.
Jökulhlaups are the consequence of a sudden and significant release of meltwater from the edge of a glacier. Such floods are sourced commonly from ice-dammed lakes, but occasional volcanic eruptions beneath ice can produce intense jökulhlaups due to prodigious rates of meltwater release. Globally, volcanogenic jökulhlaups have caused fatalities and damage to infrastructure within effected catchments. Here, we present the results of one-dimensional hydraulic modelling of the inundation area of a massive, hypothetical jökulhlaup on the Jökulsá á Fjöllum River in northeast Iceland; the floodwater source for this simulation is an eruption within the ice-filled caldera of Bárðarbunga: an active volcano beneath the Vatnajökull ice cap. Remotely sensed data were used to derive a digital elevation model and to assign surface-roughness parameters. We used a HEC-RAS/HEC-GeoRAS system to host the hydraulic model; to calculate the steady water-surface elevation; to visualise the flooded area; and to assess flood hazards. Maximum discharge was set notionally at 180,000 m3 s?1 and the duration and volume of the jökulhlaup were placed at 39 h and 14 km3, respectively. During the simulated rise to maximum discharge, the mean velocity of the jökulhlaup was 2.8 m s?1 over a distance of 120 km. At the height of the jökulhlaup an area of 460 km2 was inundated. Modelling results showed that, along short reaches, stream-power values exceeded 11,000 W m?2; such energy conditions would have allowed boulders up to 10-m in diameter to be mobilised by the jökulhlaup. Unsteady flow was simulated along a 22-km reach of the flood tract and it revealed strong spatial and temporal variations in flood power. Besides providing insight into the erosional and depositional effects of a volcanogenic jökulhlaup, the modelling results enable estimates of the relative timing and location of likely flooding hazards.  相似文献   

19.
Gondwana coals of the Rio Bonito Formation (Paraná Basin) in Southern Brazil have generally large ash yields, so they could be better called coaly siltstones than coal. In addition, hummocky cross stratification (HCS) was found in several coal beds of the Rio Bonito Formation throughout the basin. In this formation, the frequent and close relationship between facies involving rocks generated by subaqueous gravity flows (diamictites) and coal itself provides an excellent depositional model based on resedimentary processes acting during deposition, as well as a stratigraphic rearrangement of the present units.In the State of Rio Grande do Sul (southern part of Paraná Basin), coals are actually prodelta deposits related to delta-front diamictite and conglomeratic sandstone with sigmoidal bedding. Coal-forming organic sediments would come from trees plucked by the floods, as indicated by the wood logs floating in the diamictite, and reworking of previous peat accumulations. Every coal layer is covered generally by paleosoil siltstones, which represent colonization at the top of the catastrophic flood deposit, ending a sedimentary cycle.In case of Brazilian coal settings, several authors recognized deltas (fan deltas or braid deltas). Here is particularly considered the general environment as a salted interior sea (lago mare, Hsü et al. sense).The present study will refer to three important lithostratigraphic units in the Carboniferous–Early Triassic cycle: the Itararé Group, the Rio Bonito Formation, and the Palermo Formation.Although the preferential mode of occurrence of HCS in shallow marine environments indicates a genesis attributed to storm action, other causes, such as catastrophic flooding, have been advanced. Mutti et al. [Mem. Sci. Geol. 48 (1996) 233] described flood-dominated deltaic systems with thick conglomerate, sandstone, and pelitic deposits, derived from small- to medium-scale fluvial systems and mountain-bordered drainage basins adjacent to the sea. In such settings, seaward sediment flow can increase dramatically when weather conditions can supply water in such amounts to produce catastrophic floods. Thick and laterally extensive sandstone lobes with HCS are the fundamental depositional elements of fan deltas and other river-dominated delta systems.Diamictites and coal together could be a result from Jökullhlaups—an Icelandic term for glacial outburst flood—in case of catastrophic floods coming from a melting mountain glacier, similar to the Columbia River Valley Scablands (15,000 BP) and in modern Iceland examples.  相似文献   

20.
Sediment records from two lakes in the east-central Sierra Nevada, California, provide evidence of cooling and hydrological shifts during the Younger Dryas stade (YD; ~ 12,900-11,500 cal yr BP). A chironomid transfer function suggests that lake-water temperatures were depressed by 2°C to 4°C relative to maximum temperatures during the preceding Bølling-Allerød interstade (BA; ~ 14,500-12,900 cal yr BP). Diatom and stable isotope records suggest dry conditions during the latter part of the BA interstade and development of relatively moist conditions during the initiation of the YD stade, with a reversion to drier conditions later in the YD. These paleohydrological inferences correlate with similar timed changes detected in the adjacent Great Basin. Vegetation response during the YD stade includes the development of more open and xeric vegetation toward the end of the YD. The new records support linkages between the North Atlantic, the North Pacific, and widespread YD cooling in western North America, but they also suggest complex hydrological influences. Shifting hydrological conditions and relatively muted vegetation changes may explain the previous lack of evidence for the YD stade in the Sierra Nevada and the discordance in some paleohydrological and glacial records of the YD stade from the western United States.  相似文献   

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