共查询到20条相似文献，搜索用时 31 毫秒
1.
Claire L. Beaney 《Quaternary International》2002,90(1)
Tunnel channels in southeast Alberta are attributed to erosion by channelized, subglacial meltwater flows. An anabranching tunnel channel network dissects the preglacial drainage divide of the ancestral Milk River. Channel morphology and landform associations are used to evaluate competing hypotheses of tunnel channel formation. Mechanisms that invoke subaerial channel incision, direct glacial erosion or steady state, time-transgressive erosion at the ice margin cannot explain convex-up longitudinal channel profiles, anabranching channel networks or confinement to the preglacial drainage divide. Results conclude that the tunnel channel network in southeast Alberta represents late-stage erosion by a channelised subglacial flow of catastrophic dimensions. Interpretations for this tunnel channel network are in agreement with conclusions obtained for the regional subglacial landform assemblage. 相似文献
2.
3.
Three unstable ephemeral-stream channels (arroyos), which drain source areas that have high sediment yields ranging from predominantly sand (Arroyo Calabasas) to a mixture of sand, silt, and clay (Sand Creek) to largely silt and clay (Sage Creek), were resurveyed to provide data on the rates and mechanics of erosion and sedimentation processes during periods ranging from 14 to 22 yr. Channel morphology changed significantly. Erosion occurred through nickpoint recession and bank collapse, but erosional reaches are separated by aggrading or stable-channel reaches. In general, sediment that is eroded, as the nickpoint recedes upstream, is trapped in the widened channel downstream. In this manner sediment is transported episodically out of these basins during a series of cut-and-fill cycles. The manner by which the channels aggrade and the morphology of the aggraded stable channels are controlled by the sediment type. The wide and shallow channel of Arroyo Calabasas is filled by vertical accretion of sand-size sediment. The narrow and deep channels of Sage Creek and Sand Creek are created by the lateral accretion of cohesive fine-grained sediment. The channel modification and the cut-and-fill episodes are dependent on high sediment yields, and therefore they are independent of subtle climatic shifts. Cut-and-fill deposits that have been created in this manner should not be equivalent in age from basin to basin, and therefore channel trenching and filling in the semiarid western United States during the Holocene need not be synchronous. 相似文献
4.
R. Bruce Rains John Shaw Darren B. Sjogren Mandy J. Munro-Stasiuk K. Robert Skoye Robert R. Young Robin T. Thompson 《Quaternary International》2002,90(1)
At some time close to the Last Glacial Maximum (LGM) high-energy, subglacial, Laurentide, meltwater flows eroded a series of discontinuous tunnel channels into the northeastern flanks of the Porcupine Hills and adjacent parts of the high plains near Nanton, Stavely and Claresholm. Discrete channel segments, kilometers long, up to about 1 km wide, and 100 m deep, were carved into Paleocene sandstone and shale of the Porcupine Hills Formation. Floors of Pine, Boneyard, and Crocodile channels all occur at elevations between 1050 and 1175 m a.s.l., and share the characteristic of strongly convex-up long profiles. Intrachannel drainage divides on each channel floor are tens of meters above the water entry and exit points. Formative flows, therefore, must have been pressurized in the subglacial Nye-channels. Prominent scour-holes at some major bends in the channels now host ephemeral ponds or lakes. During the channel erosion, the overlying Laurentide ice surface was probably close to its local LGM maximum elevation of ca. 1400–1500 m a.s.l. Misfit modern streams now drain in opposite directions within the tunnel channels, and there are only minor, local, distal accumulations of sediment derived from the tunnel channel erosion. 相似文献
5.
Per Möller Johanna Anjar Andrew S. Murray 《Boreas: An International Journal of Quaternary Research》2013,42(1):25-42
In central and northern Sweden, glacial sediments and landforms, formed during Early and Middle Weichselian stadials and their transition into interstadials, are often preserved in spite of having been overridden by later glacial advances. This study presents an OSL‐dated glacial stratigraphy from Idre in west‐central Sweden, expanding the area in which Middle Weichselian ice‐free conditions have been identified. Three sedimentary units were identified, with the lowermost unit consisting of glaciolacustrine sand, deposited in a stagnant water‐body. Nine OSL samples gave ages ranging from 54 to 41 ka, suggesting deposition during a deglacial phase in MIS 3. Normal faults and silt veins, formed after deposition, indicate that the area was ice‐free for a prolonged period, enabling the melting of buried stagnant ice. Above an erosional unconformity is a sediment unit characterized by gravels and sands deposited in a proximal braided‐river environment. OSL ages range from 180 to 41 ka, indicating poor sediment bleaching during deposition. We thus consider them to give a maximum age of the sedimentation, indicating deposition at or after 41 ka. The uppermost unit consists of a stacked succession of subglacial traction tills and glaciotectonite beds, representing the Late Weichselian glaciation of the area, probably during the inception phase with a wet‐based glacier regime. At the last deglaciation of the area there was extensive meltwater erosion, eroding all sedimentary units and forming a landscape with terraces and channels, and erosional remnants of the uppermost diamict as free‐standing hummocks. 相似文献
6.
Antoni G. Lewkowicz 《Quaternary Research》2011,75(2):334-346
Slope hummocks, a type of nonsorted patterned ground, are composed of stratified, organic, silty sand, and develop through the interaction of niveo-eolian deposition, solifluction, slopewash, and vegetation growth. Fields of hummocks show consistent patterns: forms on convex slopes increase in height downslope until the channel is reached, whereas those on convexo-concave slopes increase on the upper convexity but are buried by niveo-eolian deposition downslope of the snowbank remnant. These trends can be reproduced using a simple numerical model based on measured slope and snow depth profiles, sediment concentrations in the snow and solifluction rates. The model indicates that hummocks transit slopes of 20-40 m in about 2-4 ka, a time-frame that is plausible given site emergence, measured rates of solifluction, and published dates for organic horizons within hummocks on northern Ellesmere Island. Sensitivity analyses show that long-term effect of climate warming on hummock heights may differ depending on whether it is accompanied by precipitation increase or decrease. The required combination of two-sided freezing to promote plug-like movement, incomplete vegetation cover and thin snow that enable eolian erosion during winter and spring, and vegetation growth in snow-bed sites to stabilize niveo-eolian deposits may explain why these forms are important regionally but apparently are not present throughout the Arctic. 相似文献
7.
Tanya C. Forde Mladen R. Nedimović Martin R. Gibling Donald L. Forbes 《Estuaries and Coasts》2016,39(2):363-384
Many coastlines are retreating in response to sea level rise, compounded by glacial–isostatic subsidence in areas marginal to former ice sheets. The resulting barrier and estuarine deposits are dominated by transgressive stratigraphy. Where supplied primarily from relict glacial deposits, this “paraglacial” sediment input may rise and fall, increasing as a new source such as a drumlin headland is exposed to erosion but declining as the source becomes exhausted. Conrads Beach, on the Atlantic coast of Canada, has experienced a succession of barrier growth and reworking as sediment supply from several drumlin sources has varied over the past 3000 years. In the context of long-term regional transgression, there have been intervals of years to centuries characterized by local stability or progradation. Ground-penetrating radar profiles and refraction seismic data were used to image the facies architecture of Conrads Beach to depths of 6–8 and 10–24 m, respectively. Thirteen vibracores provided a record of lithofacies characteristics and geometry. Results show evidence of an estuarine basin at ~2800 years BP. As the outer coast retreated, erosion of drumlins provided multi-century sediment pulses to adjacent beaches and embayments. Locally increased sediment supply fed a prograding beach ridge complex from >600 to ~150 years BP and tidal channels feeding sediment to back-barrier flood delta deposits. This study documents the complexity of coastal adjustment to time- and source-varying sediment supply under long-term rising sea level. It expands and refines previous models, providing guidance required for effective management and hazard mitigation on transgressive paraglacial coasts. 相似文献
8.
Detailed fieldwork and new extensive 14C dating of residual channel infillings provide data for the reconstruction of the Late‐glacial channel downcutting and incision history of the Venlo–Boxmeer lower reach of the River Meuse (= Maas) in the southern Netherlands. Within a period of 500–1300 yr after Late‐glacial climatic amelioration, the Meuse responded to increased discharges and decreased sediment supply by adjusting the width/depth ratio of its channels. Two main phases of channel downcutting are followed by two main phases of floodplain lowering and narrowing, indicating net floodplain degradation by the fluvial system as a non‐linear response to Late‐glacial and Early Holocene climate change. Some 1300 yr after initial late‐glacial warming, channels downcut rapidly during the Early Bølling (13.3–12.5 kyr BP) and adopted a high‐sinuosity meandering style. Channel downcutting paused around 11.9 kyr BP, possibly in response to rising groundwater levels and/or the Older Dryas cooling event. Between 11.9 and 11.3 kyr BP a new floodplain was formed. Then, lateral erosion took place and initiated a first phase of 2.6 m floodplain lowering during the Late Allerød. Gradual climate deterioration during the Allerød progressively broke up soils and vegetation cover, from 11.3 to 10.9 kyr BP. The Meuse gradually adjusted to an increased ratio of sediment supply over transport capacity through higher width/depth ratios. Main channels became shallower and adopted a low‐sinuosity pattern, finally culminating in a braided river system during the Younger Dryas. The final Holocene warming resulted, within 500 yr, in renewed rapid channel downcutting by a single low‐sinuosity channel during the Early Preboreal, followed by a second phase of 1.8–2.8 m floodplain lowering. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
9.
Stefan Wenau Tiago M. Alves 《Boreas: An International Journal of Quaternary Research》2020,49(4):799-812
Tunnel valleys are major features of glaciated margins and they enable meltwater expulsion from underneath a thick ice cover. Their formation is related to the erosion of subglacial sediments by overpressured meltwater and direct glacial erosion. Yet, the impact of pre-existing structures on their formation and morphology remains poorly known. High-quality 3D seismic data allowed the mapping of a large tunnel valley that eroded underlying preglacial delta deposits in the southern North Sea. The valley follows the N–S strike of crestal faults related to a Zechstein salt wall. A change in downstream tunnel valley orientation towards the SE accompanies a change in the strike direction of salt-induced faults. Fault offsets indicate important activity of crestal faults during the deposition of preglacial deltaic sediments. We propose that crestal faults facilitated tunnel valley erosion by acting as high-permeability pathways and allowing subglacial meltwater to reach low-permeability sediments in the underlying Neogene deltaic sequences, ultimately resulting in meltwater overpressure build-up and tunnel valley excavation. Active faults probably also weakened the near-surface sediment to allow a more efficient erosion of the glacial substrate. This control of substrate structures on tunnel valley morphology is considered as a primary factor in subglacial drainage pattern development in the study area. 相似文献
10.
James A. Smith Claus-Dieter Hillenbrand Alastair G.C. Graham 《Quaternary Research》2009,71(2):190-200
Subglacial meltwater plays a significant yet poorly understood role in the dynamics of the Antarctic ice sheets. Here we present new swath bathymetry from the western Amundsen Sea Embayment, West Antarctica, showing meltwater channels eroded into acoustic basement. Their morphological characteristics and size are consistent with incision by subglacial meltwater. To understand how and when these channels formed we have investigated the infill of three channels. Diamictons deposited beneath or proximal to an expanded grounded West Antarctic Ice Sheet are present in two of the channels and these are overlain by glaciomarine sediments deposited after deglaciation. The sediment core from the third channel recovered a turbidite sequence also deposited after the last deglaciation. The presence of deformation till at one core site and the absence of typical meltwater deposits (e.g., sorted sands and gravels) in all three cores suggest that channel incision pre-dates overriding by fast flowing grounded ice during the last glacial period. Given the overall scale of the channels and their incision into bedrock, it is likely that the channels formed over multiple glaciations, possibly since the Miocene, and have been reoccupied on several occasions. This also implies that the channels have survived numerous advances and retreats of grounded ice. 相似文献
11.
钱塘江河口为强冲积性河口,在洪潮水流共同作用下河床冲淤剧烈,极端洪水条件下河床的冲刷深度是过江隧道工程的关键问题之一。基于河床演变分析、动床数值模拟和动床物理模型等研究手段,建立了钱塘江河口过江隧道河段洪水冲刷深度的预测模型,分别经钱塘江河口的典型实测地形、水流泥沙及河床冲淤等实测资料进行验证。在此基础上预测了某过江隧道河段在极端洪水作用下河床最大冲刷深度,三种研究方法所得的结果定性定量基本合理,且与后来地质详勘的沉积分析成果基本一致,进一步表明了预测模型的可靠性,预测的最大冲刷深度可为过江隧道的合理埋设提供科学依据。 相似文献
12.
Timothy P. Lane David H. Roberts Colm Ó Cofaigh Brice R. Rea Andreas Vieli 《Boreas: An International Journal of Quaternary Research》2016,45(2):220-234
The Uummannaq region is a mosaic of glacial landsystems, consistent with hypothesized landscape distribution resulting from variations in subglacial thermal regime. The region is dominated by selective linear erosion that has spatially and altitudinally partitioned the landscape. Low altitude areas are dominated by glacial scour and higher elevations are dominated by plateaux or mountain valley and cirque glaciers. The appearance and nature of each landscape type varies locally with altitude and latitude, as a function of bedrock geology and average glacial conditions. Selective linear erosion has been a primary control on landscape distribution throughout Uummannaq, leading to plateau formation and the growth of a coalescent fjord system in the Uummannaq region. This has allowed the development of the Uummannaq ice stream's (UIS) onset zone during glacial periods. Fjord development has been enhanced by a downstream change in geology to less‐resistant lithologies, increasing erosional efficiency and allowing a single glacial channel to develop, encouraging glacier convergence and the initiation of ice streaming. The landscape has been affected by several periods of regional uplift from 33 Ma to present, and has been subject to subsequent fluvial and glacial erosion. Uplift has removed surfaces from the impact of widespread warm‐based glaciation, leaving them as relict landsurfaces. The result of this is a regional altitude‐dependent continuum of glacial modification, with extreme differences in erosion between high and low elevation surfaces. This study indicates that processes of long‐term uplift, glacial erosion/protection and spatial variability in erosion intensity have produced a highly partitioned landscape. 相似文献
13.
High resolution shallow seismic data was acquired from inner continental shelf of Goa,west coast of India to map underlying stratigraphic and buried geomorphic features of shelf strata.Seismic data revealed characteristic channel incisions beneath 4-15 m thick sediment layer and corresponds to multi cycle incisions.Stratigraphic analysis of these incision signatures reveals three prominent subaerial unconformities S6,S7 and S9.These unconformities were exposed during the last glacial,penultimate glacial(MIS-6)and prior to penultimate glacial(MIS-8)periods.On the basis of interpreted age of subaerial unconformities and differences in their morphological features,observed channel incisions have been divided grossly into three phases of incision.Phase-1 incisions are older than ～330 kyr BP,whereas,incisions of Phase-2 and Phase-3 correspond to ～320-125 kyr BP and ～115-10 kyr BP respectively.Plan form of these incisions varied from a straight channel type to ingrown meander and then to anastomosing channel types.These channels meet at the confluence of present-day Mandovi and Zuari rivers.The confluence point has varied in due course of time because of cyclic incision and burial with repeated sea level fluctuations.The preserved main channel width varies from ～100 m to 1000 m.and maximum channel depth reaches up to ～35 m.Comparison of quantitative and qualitative morphologic results of different phases of incisions suggest that Phase-2 channels had ～33% more mean bank full discharge than that of the Phase-3 channels.Phase-2 incisions had been carved in higher hydraulic energy condition as compared to Phase-3 incisions implying that the Indian summer monsoon was better during formative stages of Phase-2 incisions. 相似文献
14.
Fossil surfaces of erosion and non-deposition are common in limestone sequences from the Mesozoic in western Europe and in the Tethys and have been described under the name ‘hardgrounds’. They are of shallow water as well as of oceanic origin. A modern example in the Pacific is described in this paper. The Carnegie Ridge, an east-west trending shallow ridge between South America and the Galapagos Islands, has a central, deeper saddle where erosion has removed most of the sediment cover down to a hard chalk and chert bed (acoustic basement), and has cut intricate channel patterns on the south flank and two deep canyons on the north. The erosion has produced a karst-like relief of steep-walled channels, cliffs, and corroded chalk remnants. The floors of the channels are covered with ferromanganese oxide crusts or crust fragments over which loose sediment is being transported. In the two canyons on the north flank, this sediment consists of foraminiferal sand travelling downslope in the form of barchan dunes. All sediment down to acoustic basement has been stripped from the Carnegie Ridge crest except where it is protected behind basement ridges and pinnacles. Surface features of the eroded chalk are strongly reminiscent of features observed in Mesozoic hardgrounds. Current measurements over several days indicate a net northward movement, slow but possibly adequate to keep the sea floor free of fresh deposits. The rates, however, seem inadequate to explain the formation of the deep channels, and there is no evidence for the southward flow which is implied by the southern channel system. A process of combined carbonate dissolution and removal by the current of fresh sediment and dissolution residues can account for the required erosion in about two million years. Regional unconformities identifiable in seismic reflection profiles and dated in cores are of middle to late Pliocene age, suggesting that the formation of the erosion surface began 2-3 million years ago. Buried Miocene unconformities of local extent show that the present erosion period had minor precursors possibly related to short-lived increases in current action and carbonate dissolution. 相似文献
15.
The most conspicuous erosional features associated with constricted bedrock channel reaches and knickpoints are potholes.
The presence and morphology of potholes have been attributed to a number of factors by earlier researchers. Amongst these
factors, the role of substrate rock properties has received very little quantitative attention. The main objective of this
study is to quantitatively evaluate the physical properties of bedrock in order to test the possible influences of rock properties
on the occurrence and morphology of potholes. The area selected for this study is a large scabland area developed by the Indrayani
river at Shelarwadi near Pune. This site is ideally suited for the study since it is featured by wide straths, multiple inner
channels and several hundred potholes. A transect-based quadrat method was used in this study. Within each quadrat, the pothole
dimensions, the joint length, joint direction, the rock mass strength and the distance from active channel were measured.
The analysis reveals a weak correlation between pothole size and rock properties. The distance from the active channel emerges
as the most significant factor, suggesting that the hydraulics of flows is the key factor and substrate characteristics play
only a secondary role. 相似文献
16.
深水水道沉积构型及其演化一直是沉积学界研究的热点。基于Rio Muni盆地深水区470 km2高分辨率三维地震数据,在精细地震解释的基础上,对研究区发育的深水水道的沉积构型、演化进行探讨。主要取得4点认识：(1)深水水道的弯曲度主要受控于物源供给和海底坡度,研究区发育弯曲水道、顺直水道2类深水水道;(2)起源于陆架边缘的深水水道,物源供给相对充分,弯曲度高,其剖面往往不对称,水道壁发育滑塌或阶地,垂向演化具有侧向迁移特征,发育废弃水道;(3)而起源于中上陆坡的深水水道,其弯曲度低,剖面具有U形特征,水道壁光滑无滑塌或阶地, 主要以垂向加积为主。由于物源供给不足,顺直水道逐渐被深海泥质披覆沉积充填;(4)同一条深水水道,由上陆坡向下陆坡,随着海底坡度的降低,其弯曲度呈增大趋势。 相似文献
17.
3D seismic data from the Dogger Bank, North Sea, allow the mapping of Late Pleistocene and Holocene depositional systems in unprecedented detail. The data demonstrate that glacial processes resulted in the development of incised tunnel valley systems during the Weichselian and that these were subsequently modified by fluvial processes in a pro-glacial setting. Subsequently, the Dogger Bank formed an emergent plain during the Holocene with a complex meandering river system, associated tributary or distributary channels and lakes, dominating the region. Prior to the sea level rising sufficiently to submerge the Dogger Bank around 7500 yr ago, the meandering river system was replaced by a dendritic channel network of potential fluvial, estuarine or intertidal origin. As the Holocene depositional features bear no systematic relationship to the bathymetry this study demonstrates that previously published bathymetry-based models for the Holocene palaeogeographic development of the North Sea require modification. 相似文献
18.
NEIL F. GLASSER JAMES L. ETIENNE MICHAEL J. HAMBREY JEREMY R. DAVIES RICHARD A. WATERS PHILIP R. WILBY 《Boreas: An International Journal of Quaternary Research》2004,33(3):224-237
This article presents the results of a geomorphological and sedimentological investigation of former glacial meltwater drainage in the region of the lower Afon Teifi, one of the major rivers of southwest Wales. Former drainage characteristics in the region are reconstructed concentrating on palaeo-drainage routes associated with successive Pleistocene glaciations and their role in the Quaternary evolution of the lower Teifi. Mapping of these features throughout a c. 100 km 2 area reveals a complex evolution in the establishment of the present-day drainage system, with evidence for the following surface channel types: (i) type 1 channels of primary subglacial origin cut during the late Devensian (late Wisconsinan/late Weichselian) glaciation; (ii) type 2 channels representing either pre-late Devensian subaerial fluvial run-off, unconnected to the course of the preglacial Afon Teifi, or originating as subglacial chute channels; (iii) type 3 channels developed as subglacially modified pre-late Devensian tributaries of the Afon Teifi. Two further features are also described: (iv) type 4 channels are drift-plugged abandoned preglacial courses of the Afon Teifi, and (v) type 5 channels formed as lateglacial and post-late Devensian gorges which bypass type 4 channels. A relative chronostratigraphy based on channel geomorphology and sedimentology reveals an evolutionary sequence considerably more complicated than identified in previous studies, with extensive modification of the lower Afon Teifi region by glacial meltwater during at least two periods of Pleistocene glaciation. 相似文献
19.
Site assessment and monitoring data were analyzed for 26 stream restoration projects in North Carolina where the channel was
reconfigured. Post-project changes in channel capacity were highly variable from site to site, but more than 60% of the projects
underwent, on average at a given site, at least a 20% change in channel capacity. An analysis of site and basin geomorphology
revealed that large post-construction adjustments were associated with highly dynamic stream channels characterized by a combination
of high sediment transport capacity, large sediment supply, and/or easily eroded bank materials. In-stream structures along
dynamic, reconfigured channels also exhibited high incidences of damage. Thus, the design and construction of channels in
a state of equilibrium, which do not exhibit excessive erosion or deposition along highly dynamic rivers is currently problematic.
In light of these findings, a conceptual framework based on geomorphic parameters is put forth to assess the likelihood of
project success early in the design process to (1) eliminate high risk sites from consideration of channel reconfiguration
and (2) improve upon the implemented management strategies that are ultimately used. It is also argued that where space permits
an enhanced natural channel, adjustment approach is likely to be more effective than projects based on natural channel design. 相似文献
20.
In Scandinavia, most fluvial erosion takes place in the Quaternary glacial overburden at a restricted number of small source areas along individual drainage channels. As a consequence, a sample of active stream sediment is representative of only a very limited portion of the drainage area. This restriction makes stream sediment less reliable for regional exploration than generally expected. Overbank (levee or river-plain) sediment produced during large floods is an alternate more representative sampling medium. The sediment suspended during a flood has a much more widespread origin, and when the load is deposited upon the flood plain, nearly horizontal strata are formed and preserved at levels above the ordinary stream channel. A composite sample through a vertical section of such strata represents a great number of sediment sources that have been active at different times and forms an integrated sample of the entire catchment area. Because young sediments overlay older, the uppermost layers will be contaminated by pollutants in industrialized regions, but those at depth may remain pristine and will to a greater extent reflect the natural pre-industrial environment. In regional geochemical mapping, overbank sediment can be sampled at widely spaced sites, keeping costs per unit area low. Examples from Norway (1 sample station per 500 km2) show that overbank sediment produces broad geochemical patterns with high contrasts reflecting the bedrock geochemistry. Some patterns agree with known geological units and metallogenic provinces, but hitherto unknown major structures have also been indicated. A large Mo-deposit missed by a traditional stream survey is readily detected in the overbank sediment. It is concluded that overbank sediment is a promising alternate sample medium that should be tested in other physiographic regions. 相似文献