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1.
We present textural and thickness data on loess from 125 upland sites in west-central Wisconsin, which confirm that most of this loess was derived from the sandy outwash surfaces of the Chippewa River and its tributaries, which drained the Chippewa Lobe of the Laurentide front during the Wisconsin glaciation (MIS 2). On bedrock uplands southeast of the widest outwash surfaces in the Chippewa River valley, this loess attains thicknesses > 5 m. OSL ages on this loess constrain the advance of the Laurentide ice from the Lake Superior basin and into west-central Wisconsin, at which time its meltwater started flowing down the Chippewa drainage. The oldest MAR OSL age, 23.8 ka, from basal loess on bedrock, agrees with the established, but otherwise weakly constrained, regional glacial chronology. Basal ages from four other sites range from 13.2 to 18.5 ka, pointing to the likelihood that these sites remained geomorphically unstable and did not accumulate loess until considerably later in the loess depositional interval. Other OSL ages from this loess, taken higher in the stratigraphic column but below the depth of pedoturbation, range to nearly 13 ka, suggesting that the Chippewa River valley may have remained a loess source for several millennia. 相似文献
2.
毛乌素沙地沉积物粒度特征与土地沙漠化 总被引：5，自引：0，他引：5
为了研究毛乌素沙地沉积物的粒度特征与土地沙漠化的地质成因类型,通过系统研究沙地及相邻地区晚更新世以来在沙地内部、覆沙黄土区、黄土高原和河流谷地与湖泊等不同地质构造地貌条件下所形成的沉积物粒度特征,指出不同地质成因类型的沉积物是这些地区土地沙漠化的物质基础,区分不同类型的沉积物也是构成不同地质成因类型土地沙漠化分类的理论基础,探讨了不同地质作用对土地沙漠化的贡献及其对土地沙漠化防治方面的意义。研究表明,研究区的土地沙漠化是以各种类型的就地起沙为主要特征,地表径流的水力搬运是沙漠化物质搬运的重要途径。现代毛乌素沙地及其邻区的土地沙漠化是对地质历史时期形成的各种沉积物的进一步搬运、分选和再沉积过程,具有相对复杂的粒度特征。 相似文献
3.
NELSON R. HAM JOHN W. ATTIG 《Boreas: An International Journal of Quaternary Research》2001,30(1):31-41
Approximately 35 parallel, discontinuous glacial ridges occur in an area of about 100 km2 in north‐central Wisconsin. The ridges are located between about 6 and 15 km north (formerly up‐ice) of the maximum extent of the Wisconsin Valley Lobe of the Laurentide Ice Sheet. The ridges are between 1 and 4 m high, up to 1 km long, and spaced between 30 and 80 m apart. They are typically asymmetrical with a steep proximal (ice‐contact) slope and gentle distal slope. The ridges are composed primarily of subglacial till on their proximal sides and glacial debris‐flow sediment on the distal sides. In some ridges the till and debris‐flow sediment are underlain by sorted sediment that was deformed in the former direction of ice flow. We interpret the ridges to be recessional moraines that formed as the Wisconsin Valley Lobe wasted back from its maximum extent, with each ridge having formed by a sequence of (1) pushing of sorted ice‐marginal sediment, (2) partial overriding by the glacier and deposition of subglacial till on the proximal side of the ridge, and (3) deposition of debris‐flow sediment on the distal side of the ridge after the frozen till at the crest of the ridge melted. The moraines are similar to annual recessional moraines described at several modern glaciers, especially the northern margin of Myrdalsjokull, Iceland. Thus, we believe the ridges probably formed as a result of minor winter advances of the ice margin during deglaciation. Based on this assumption, we calculate the net rate of ice‐surface lowering of the Wisconsin Valley Lobe during the period when the moraines formed. Various estimates of ice‐surface slope and rates of ice‐margin retreat yield a wide range of values for ice‐surface lowering (1.7–14.5 m/yr). Given that ablation rates must exceed those of ice‐surface lowering, this range of values suggests relatively high summer temperatures along the margin of the Wisconsin Valley Lobe when it began retreating from its maximum extent. In addition, the formation of annual moraines indicates that the glacier toe was thin, the ice surface was clean, and the ice margin experienced relatively cold winters. 相似文献
4.
The glacial geomorphology of the Waterville Plateau (ca. 55 km2) provides information on the dynamics of the Okanogan Lobe, southern sector of the Cordilleran Ice Sheet in north‐central Washington. The Okanogan Lobe had a profound influence on the landscape. It diverted meltwater and floodwater along the ice front contributing to the Channeled Scabland features during the late Wisconsin (Fraser Glaciation). The glacial imprint may record surge behaviour of the former Okanogan Lobe based on a comparison with other glacial landsystems. Conditions that may have promoted instability include regional topographic constraints, ice marginal lakes and dynamics of the subglacial hydrological system, which probably included a subglacial reservoir. The ice‐surface morphology and estimated driving stresses (17–26 kPa) implied from ice thickness and surface slope reconstructed in the terminal area also suggest fast basal flow characteristics. This work identifies the location of a fast flowing ice corridor and this probably affected the stability and mass balance of the south‐central portion of the Cordilleran Ice Sheet. Evidence for fast ice flow is lacking in the main Okanogan River Valley, probably because it was destroyed during deglaciation by various glacial and fluvial processes. The only signature of fast ice flow left is the imprint on the Waterville Plateau. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
5.
中国黄土的物质来源及其粉尘的产生机制与搬运过程 总被引：34，自引：10，他引：24
长期以来,黄土高原的物质来源被笼统认为来自包括西北三大内陆盆地(准噶尔、塔里木、柴达木)在内的广阔的沙漠、戈壁地区,而且中国黄土被看作“沙漠”黄土的典型代表。但我们最新的研究揭示出,源自准噶尔和柴达木盆地的粉尘主要由低空气流搬运,受下风方向的山脉阻挡,就近堆积成山麓黄土。源自塔里木盆地的粉尘如果被近地面风搬运,则不可能移出盆地,而是堆积在昆仑山北麓,但如果被飙升到高空,则被西风急流携带,移出盆地而降落在北太平洋地区,无论哪种情况,该盆地的粉尘都不可能对黄土高原有重要贡献。因此,西北三大内陆盆地并非黄土高原的重要物源,而蒙古国南部及与之相邻的包括巴旦吉林、腾格里、乌兰布和、库布其、毛乌素等在内的戈壁、沙漠地区才是黄土高原的主要物质来源区,且主要由近地面风(基本在3000m以下)从上述戈壁、沙漠地区搬运而来。中国黄土并不能简单看作“沙漠”黄土,包括冰川研磨作用、山体剥蚀作用、山前冲洪积作用等在内的“高山过程”,才是产生大量粉土级物质的原因,戈壁、沙漠、黄土的带状分布,只不过是近地面风对山前冲、洪积物质的风力分异而已。 相似文献
6.
Six closely spaced sediment cores taken below the carbonate compensation depth penetrated fine silty muds and entered sandy sediment at 10–12 m below the seafloor. Foraminiferal assemblages and δ8O analyses on planktonic foraminifera indicated that the surface muds down to 2 m are Holocene and derived from local promontories above the CCD. Below these sediments are about 6 m of clays deposited during the late Wisconsin. These are unfossiliferous and have a possible northern source suggested by the higher chlorite content. Sandy sediments below 9 m in the cores contain well preserved benthic foraminifera from the Scotian Shelf. Glacial δ18O values on planktonic tests indicate the sandy sediments are most likely of latest Wisconsin age. Thus during the recent interglacial, the sand fraction of the southern Sohm Abyssal Plain sediments is mostly locally derived, but during glacial periods the sediments have a distant northern source containing quartz sand that was initially deposited on the Scotian Shelf 1,500 km to the north. 相似文献
7.
西宁黄土沉积及其物质来源 总被引：10，自引：0，他引：10
西宁黄土包括从Ｓ０至Ｌ１７连续黄土－古土壤序列,黄土开始发育的时间约１．２Ｍａ黄土粒度粗,不稳定重矿物含量高,表明西宁黄土区距黄土物源近,黄土中的石英砂粒多具冰川、冰缘作用的表面机械微结构和形态,结合离石黄土风向主大气环流特征,提出本区黄土主要来地青藏高原第四纪冰川作用和冰缘作用所产生的大量粉砂物质。 相似文献
8.
Accelerator mass spectrometer radiocarbon ages of the Roxana Silt (loess) along the Upper Mississippi Valley of Wisconsin and Minnesota indicate that loess sedimentation of the Roxana Silt occurred between about 55,000 and 27,000 14 C yr B.P. However, due to local environmental controls, the basal age at any given site may range from 55,000 to 35,000 14C yr B.P. The radiocarbon ages presented here are in agreement with previous radiocarbon ages for the Roxana Silt in its type area of west-central Illinois, but indicate that long-term sedimentation rates along the bluffline of the Upper Mississippi Valley were very slow (4-8 cm/1000 yr) compared to long-term sedimentation rates along the bluffline of the type area (40-70 cm/1000 yr). Comparison of radiocarbon ages for midcontinent middle Wisconsinan loess deposits indicates that sedimentation along the Mississippi River valley may have preceded loess sedimentation along the Missouri River valley by as much as 20,000 yr or that basal ages for middle Wisconsinan loess along the Missouri Valley are erroneously young. The bracketing ages for the Upper Mississippi Valley Roxana Silt indicate that the Mississippi River valley was receiving outwash sedimentation between 55,000 and 27,000 14C yr B.P. 相似文献
9.
MICHAEL J. HAMBREY WERNER EHRMANN 《Boreas: An International Journal of Quaternary Research》2004,33(4):300-318
The Mount Cook area in the Southern Alps of New Zealand is heavily glacierized with numerous peaks over 3000 m a.s.l. feeding several large valley glaciers. The region is subject to rapid tectonic uplift and heavy precipitation (up to 15 m per year). This paper describes the clast roundness, clast shape and textural characteristics associated with five glaciers (Fox, Franz Josef, Hooker, Mueller and Tasman) in terms of inputs to the glacier system, transport by the glaciers and reworking following glacial deposition. Inputs include rockfall, alluvial fan and avalanche material delivered to the surface of valley glaciers. Basal debris, where observed at the terminus of two glaciers, consists mainly of incorporated fluvial material. Following deposition, reworking is mainly by subglacial and proglacial streams. The dominant facies are (i) boulder gravel with mainly angular clasts on the steep slopes above the glaciers, (ii) sandy boulder gravel, with mainly angular and subangular clasts, forming lateral and end moraines, and (iii) sandy boulder/cobble gravel with mainly subrounded clasts, and sand, which represent glacially transported sediment reworked by braided rivers. Diamicton is rare in the contemporary glacial environment. Since most sediment associated with glaciers in the Southern Alps lacks unambiguous indications of glacial transport, interpretation of similar sediments in the geological record should not necessarily exclude the involvement of glacial processes. 相似文献
10.
Le-ping Yue Li-rong Yang Zhi-pei Li Min Wang Wei-ji Zhang Hao-gang Nie 《Environmental Geology》2007,51(7):1197-1201
In this paper, nine sediment sequences in the inner desert, desert-loess transitional zone (agro-pastoral transitional zone)
and the northern margin of the Loess Plateau were investigated to study the sediments of Mu Us (Maowusu) and Otindaq (Hunshandake)
sandy land in northern China since the last glaciation. All of these sequences consisted of the upper Holocene dark paleosol
and the lower aeolian sand formed during the last glacial stage. The Mu Us and Otindaq sandy land became the active desert
since the last glaciation and covered a larger sandy area than at present. Mu Us sandy land began to show a landform of sandy
desert and expanded southward towards the northern boundaries of the Loess Plateau. The Holocene was characterized by the
advent of warmer and wetter climatic conditions in northern China, resulting in the rapid growth of surface vegetation and
Mu Us and Otindaq sandy land became a steppe. The thickness of surface soil reached up to as much as 50–80 cm. Thick paleosol
covered the drift sands, and served as a protective layer for the steppe and farmland. Although the last glacial cycle is
far away, paleo-aeolian sands formed during that period are still the material sources of regional desertification. Unreasonable
cultivation destroying the Holocene paleosol, combined with dry climate and strong wind would result in activation of paleo-
aeolian sand leading to the desertification of the study areas. 相似文献
11.
12.
Aeolian Facies Belts in the Taklimakan Desert 总被引：4，自引：0，他引：4
Li Baosheng Dong Guangrong Zhang Jiasheng Li Sen Jin Heling Chen Huizhong Wen Xiangle Wang Yao Lanzhou Desert Research Institute Chinese Academy of Sciences Lamhou Gansuand Zhu YizhiXi''an Open Laboratory of Loess Quaternary Geology Chinese Academy of Sciences Xi''an Gansu Fei Zhenbi 《《地质学报》英文版》1995,69(3)
At least at the beginning of the last glacial epoch, the facies belts of dune sand, sandy loam and loess formed by winds had existed in the Taklimakan desert and areas south of it. There were no appreciable changes in the NE and NW wind systems and their wind fqrces that deposited dune sand, sandy loam and loess in the global cold stage since the last glacial epoch (accordingly no marked shifts of the boundaries of these aeolian facies belts took place. In the global warm stage since then, the climate in the Taklimakan desert and areas south of it became warm and dry, resulting in ablation of substantial volumes of ice and snow in their surrounding mountains and thus forming alluvial and diluvial deposits in the region. The alluvial-diluvial actions, however, failed to change the general framework of aeolian facies belts. 相似文献
13.
Li Baosheng Dong Guangrong Zhang Jiasheng Li Sen Jin Heling Chen Huizhong Wen Xiangle Wang Yao Zhu Yizhi 《《地质学报》英文版》1995,8(3):317-328
At least at the beginning of the last glacial epoch, the facies belts of dune sand, sandy loam and loess formed by winds had existed in the Taklimakan desert and areas south of it. There were no appreciable changes in the NE and NW wind systems and their wind forces that deposited dune sand, sandy loam and loess in the global cold stage since the last glacial epoch; accordingly no marked shifts of the boundaries of these aeolian facies belts took place. In the global warm stage since then, the climate in the Taklimakan desert and areas south of it became warm and dry, resulting in ablation of substantial volumes of ice and snow in their surrounding mountains and thus forming alluvial and diluvial deposits in the region. The alluvial-diluvial actions, however, failed to change the general framework of aeolian facies belts. 相似文献
14.
The total rare-earth element values(ΣREE)of loess in the Xinjiang region vary over a range of 128-200 ppm ,with an average of 153ppm .The average REE content of loess lies between the earth‘s crust (155ppm) and sedimentary rocks(151ppm).The Xinjiang loess,with the REE distribu-tion patterns characterized by negative slopes ,is rich in the Ce-family elements, and has a distribu-tion pattern characteristic of sedimentary rocks.The North Xinjiang loess is relatively depleted in Tb,but rich in Yb and Lu.The South Xinjiang loess is relatively rich in light rare-earth elements.This is full proof that the Xinjiang loess comes partly from weathered materials(clay rock,sandstone)in the region studied.The REE distribution patterns in the Xinjiang loess are similar to those in the precipitated dust and Aeolian sand,indicating the same material source.The REE distribution pat-terns in the Xinjiang loess are also similar to those in loess from the middle Yellow River Valley,China and Taskent,the former USSR.This implies that loesses of the three locations(Xinjiang,the mid-dle Yellow River Valley and Taskent) come from a common material source.But the REE patterns in the Xinjiang loess are different from those in wall rocks (volcanic rock,K-bearing volcanic rock).Generally ,LREE/HREE,Eu/Eu* and Ce/Ce* ratios reflect the features of parent materials of loess,indicating that the parent rocks were probably in the early stage of alkaline weathering and the weathered materials existed in an oxidation environment with basic mediums under arid-climatic conditions before transport.As a result,the migration ability of the REE is weak. 相似文献
15.
渤海古沙漠沉积特征分析 总被引：4，自引：0，他引：4
根据世界沙漠的分布特点和沙漠的发育条件,作者认为晚更新世末期渤海气候干冷,雨量稀少,西北风强盛,松散沉积物丰富,具备发育沙漠的条件.文章从以下方面证明渤海古沙漠的存在:(1)渤海约有四分之一区域为残留砂沉积;(2)在全新世盖层下的更新世砂质沉积具有高倾角的前积纹层;(3)渤海东南缘陆地和岛屿有古渤海沙漠衍生黄土分布.综合分析认为末次冰期时,渤海中部和辽东湾古沙漠发育程度好,渤海湾次之,莱州湾未发育沙漠. 相似文献
16.
SEAN J. FITZSIMONS 《Boreas: An International Journal of Quaternary Research》1989,18(2):127-134
Pebble counts of the lithology of glacial sediments in the King Valley show that the content of distantly derived erratics of many sections decreases upwards in near surface sediments. Two factors that contribute to this lithological stratification are dilution of the erratic content of surface sediments by locally derived rocks and lithological stratification of debris within the Pleistocene King Glacier. The common diluting mechanism appears to have been slope detritus derived from the valley sides and small hills that crop out on the valley floor. Lithological stratification of debris in the King Glacier resulted from the altitude of the equilibrium line of the King Glacier relative to the position and altitude of the rock source areas and the thermal regime at the ice-bed interface. The Jurassic dolerite and Permian sediments that crop out above the equilibrium line altitude were transported in subglacial and englacial positions. In contrast, below the equilibrium line sediments that accumulated and were transported in a supraglacial position contained no erratic lithologies. When deposited, the supraglacial sediments formed a siliceous, non-erratic cover over sediments that were transported in subglacial and englacial positions. The model of the mode of sediment transport in the King Valley may have application to areas of alpine glaciation where the distribution of some rock types is restricted to areas above the equilibrium lines of glaciers. 相似文献
17.
Li Peiying Cheng Zhenbo Lu Houyuan Liu Guohai First Institute of Oceanography SOA Qingdao Shandong Liaoning Teachers University Dalian Liaoning Liu Linqun 《《地质学报》英文版》1992,66(3):311-325
This Paper discusses the origin of the Coastal Zone Loess on the Liaodong Peninsula, Liaoning Province.The loess is composed mainly of proximal materials transported by low-altitude air flow and less commonly ofinland distal materials carried by high-altitude air flow. The main provenance is the Liaodong Bay. When thesea level declined during glacial period, the bottoms of the Liaodong Bay and Bohai Sea emerged, wheredesertization occurred. As a result, submarine sediments were transported by strong northwesterly winds to theeastern coastal zone of the peninsula and deposited there. These materials mixed with the windblown dust car-ried by atmospheric circulation from the interior of the continent, forming the loess. 相似文献
18.
黄河源风沙沉积及意义 总被引：8，自引：0，他引：8
考察发现黄河源发育有广泛的现代、古代风沙沉积和沙漠化草地。地貌、沉积和年代学的分析表明,这些沙丘都是就地形成的半成熟沙丘,粒度粗,分选中等,形成于四个主要时期,即倒数第二次冰期末、末次冰期最盛期,全新世寒冷时期和现代,为青藏高原及邻区黄土提供了丰富的粉尘。 相似文献
19.
RICHARD H. FILLON GIFFORD H. MILLER JOHN T. ANDREWS 《Boreas: An International Journal of Quaternary Research》1981,10(1):107-124
Eight Labrador Sea piston cores with faunal and ash-zone stratigraphies correlated to deep-sea oxygen isotope stages were used to compute Labrador Sea terrigenous sand input rates (mg/cm2/1000 years) during the last 100,000 years. Sources of the sand in Labrador Sea cores are likely to be ice-rafting, turbid glacial meltwater inflow or deflation and wind erosion of unvegetated landscapes in the wake of retreating continental ice sheets. High levels of sand input to the Labrador Sea are therefore undoubtedly glacier-related while low levels of sand input are not. Comparison of the history of Labrador Sea sand input with the chronology of glacial and non-glacial events on Baffin Island reveals that the era of highest sand input rates, the isotopic stage 5a/4 transition, closely coincided with an episode of early Foxe glacier advance to tidewater (Ayr Lake Stade) along the outer coast of Baffin Island ca. 80,000 B.P. to 60,000 B.P. The period of lowest Labrador Sea sand input rates, late isotopic stage 3 to the present, largely corresponds to a major disconformity in the raised marine and glacigenic sediments on Baffin Island, but includes also the late Foxe/early Holocene Cockburn glacial advance (which did not reach the outer coast of the island) and the modern glacial minimum. Labrador Sea and central-subpolar North Atlantic sand input histories are reciprocally related over the last 80,000 years. Accelerated sand input in the Labrador Sea during times of reduced sand input in the North Atlantic implies: (1) major early Wisconsin glacier expansion in the circum Labrador Sea/Baffin Bay region and/or; (2) a surface circulation pattern in the North Atlantic which inhibited iceberg melting there while delivering icebergs and relatively warm surface water into the Labrador Sea. Conversely, reduced sand input in the Labrador Sea during times of accelerated sand input in the North Atlantic implies: (1) late Wisconsin glacier recession in the circum Labrador Sea/Baffin Bay region and/or; (2) a circulation pattern which carries icebergs southward and eastward away from the Labrador Sea. These implications are discussed in the light of paleoceanographic evidence for three periods - 80,000 B.P. to 57,000 B.P.; 25,000 B.P. to 13,000 B.P.; and 13,000 B.P. to 9800 B.P 相似文献
20.
Dust transport and the question of desert loess formation 总被引：24，自引：0，他引：24
Although contemporary dust storms are frequent in arid and semi-arid areas, desert loess deposits are poorly developed. Much of the World's loess occurs in mid-latitude areas which experienced glaciation during the Pleistocene. Ocean core evidence indicates that dust transport from sub-tropical deserts increased during cold stages of the Pleistocene, but loess formed only on certain desert margins, for reasons which have not been fully explained. This paper re-examines the mechanisms of dust transport and deposition, and the circumstances leading to the accumulation of thick loess. Typical loess is composed mainly of medium silt grains which are transported in short-term suspension a few metres above the ground. Significant thicknesses of loess form only when dust is trapped within a limited area, often relatively close to the source. Dust particles finer than 20 μm are transported mainly in long-term suspension over a greater height range and may be widely dispersed. The availability of silt and the frequency, magnitude and direction of dust-transporting winds are important factors governing the potential for loess formation, but the existence of a suitable dust trap is particularly important. Traps may be formed by topographic obstacles, areas of moist ground, or vegetated surfaces. Vegetation adjacent to glacial outwash plains and braided meltwater streams trapped dust in mid-latitudes during the Pleistocene. Dust blown during glacial periods from certain deserts, notably in Sinai, Soviet Central Asia and China, accumulated as loess in neighbouring semi-arid regions. On the margins of other deserts loess formation was inhibited partly by the absence of vegetation traps. During most of the Holocene net dust deposition rates in all desert-marginal areas have been too low for significant loess accumulation. This is mainly due to a reduction in silt availability and a tendency towards landscape stability. Reported dust storm frequencies during the past 50 years over-estimate the longer-term Holocene dust flux due to the effects of human activities. Much modern dust owes its origin to erosion of cultivated soils in semi-arid areas and is finer than typical loess. 相似文献