首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 343 毫秒
1.
In unglaciated areas of the Mississippi Valley region, the typical full loess–paleosol succession contains the Modern Soil developed in Peoria Silt, weakly developed Farmdale Geosol developed in Roxana Silt, Sangamon Geosol developed in Loveland Silt, and Yarmouth Geosol developed in Crowley's Ridge Silt. Although a fifth loess called the Marianna Silt is reported at one area, the paleosol that separates the Crowley Ridge and Marianna Silts is not well defined. Previous thermoluminescence (TL) and optical stimulated luminescence (OSL) age chronology has suggested multiple phases of Sangamon Geosol developed in Loveland Silt, but clear morphological evidence of polygenetic Sangamon Geosol profiles have not been found. Recently, a thick loess–paleosol sequence has been studied in the middle Mississippi Valley in unglaciated southern Illinois, USA. Soil morphology and analytical results revealed five loesses and associated paleosol units. Two Sangamon Bt horizons were found separated by a thick ACtk horizon, interpreted to indicate two phases of Sangamon Geosol development. This well-preserved loess–paleosol succession provides one of the most complete mid–late Quaternary loess records in the middle Mississippi Valley to date, and is important for studying the stratigraphic framework and paleoclimate and environment changes.  相似文献   

2.
《Quaternary Science Reviews》2007,26(5-6):773-792
New subsurface data reveal a nearly continuous stratigraphic record of Middle to Late Pleistocene loess sedimentation preserved beneath upland summits in eastern Nebraska, USA. Thickness and grain size trends, as well as pedologic evidence, indicate significant changes in loess sources, accumulation rates, and depositional environments. The newly defined Kennard Formation accumulated in the Middle Pleistocene, and may represent multiple thin increments of distal loess from nonglacial sources on the Great Plains. The overlying Loveland Loess, up to 18 m thick and deposited during Oxygen Isotope Stage 6 (OIS 6) (Illinoian glaciation), probably records the emergence of the Missouri River valley as a major glaciogenic loess source. The prominent Sangamon Geosol formed through long-term pedogenic alteration of the upper Loveland Loess during OIS 5 and 4. Thin loess of the Gilman Canyon Formation records slow loess accumulation and pedogenic alteration in OIS 3. The Peoria Loess (OIS 2) is similar in thickness to Loveland Loess, but may have accumulated more rapidly in an environment less favorable to bioturbation. More importantly, comparison of Peoria and Loveland loess thickness trends indicates much greater influx of nonglaciogenic loess from the Great Plains during OIS 2 than in OIS 6, suggesting colder and/or drier conditions in the Midcontinent during OIS 2 than in earlier glacial stages.  相似文献   

3.
美国中西部第四纪冰川与黄土研究的进展和问题   总被引：1，自引：0，他引：1       下载免费PDF全文
冯兆东 《第四纪研究》1994,14(4):362-368
本文对美国中西部和中国黄土高原０．５Ｍａ以来的气候序列进行了对比。０．５—０．１ＭａＢ．Ｐ．，美国中西部间冰期以干暖为特征，１０００００—３５０００ａＢ．Ｐ．，气候从干暖向温湿转化。本次冰进阶段，冰盖达到最大后，风尘堆积占优势，气候湿凉为主。全新世先从温凉转为温湿，继而干暖，后又转向湿凉。总之，０．５Ｍａ以来除末次冰期外，美国中西部与中国黄土高原温度变化的总趋势是一致的，但两地的湿度变化是相反的。  相似文献   

4.
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   

5.
The middle Wisconsinan Gilman Canyon Formation at the Buzzard's Roost type locality in southwestern Nebraska was investigated to document the stratigraphy and to reconstruct the environmental and climate record. The Gilman Canyon Formation was subdivided into three loess units and three soils, with radiocarbon ages constraining it between about 40 ka and 25 ka. Stable carbon isotope ratios, magnetic susceptibility, and carbon content were used to define and characterize soils within both the Gilman Canyon Formation and underlying Illinoian Loveland Loess. At the height of soil development within the Gilman Canyon Formation, climate was supporting C4-dominated grassland, with July temperatures equal to or exceeding those of today. Soil-forming intervals within the Loveland Loess, including the Sangamon Soil, also exhibited relative increases in C4 biomass. Climate, as recorded in the Gilman Canyon Formation, is corroborated by regional proxy data. The formation accumulated during MIS 3, and concurrent soil formation coincided with a summer insolation maximum.  相似文献   

6.
Geochemical and isotopic studies of aeolian sediments in China   总被引：5，自引：0，他引：5  
The Sr and Nd isotopic, rare earth element (REE) and major element compositions, together with mineral and grain‐size proportions, are reported for aeolian loess deposits and desert sands from several Chinese localities. The study was carried out in order to examine regional variations in the isotopic and geochemical features of these aeolian sediments, and to constrain the provenance of Chinese loess. Samples include loesses from the Tarim and Junggar basins and desert sands from the Taklimakan desert in north‐west China, loess from the Ordos area and desert sands from the Tengger and Mu‐us deserts in north‐central China, as well as loess and desert sands from the Naiman area, north‐east China. REE distributions show minimal variation among the Chinese loess deposits, whereas those for the desert sands show regional variations. New isotopic data document a latitudinal variation in Sr and Nd isotopic features for the loesses and desert sands. The Naiman and Junggar loesses have distinctly lower 87Sr/86Sr ratios and higher εNd(0) values than the loesses from the Tarim Basin, the Ordos area and the Loess Plateau. Among the desert sands, the Naiman samples have higher εNd(0) values than the Taklimakan, Tengger and Mu‐us samples. Isotopic data suggest that loesses of the Loess Plateau were supplied from the Tarim Basin loesses and Taklimakan Desert sand, and that the Naiman loesses were supplied from the Junggar Basin loesses. The latitudinal variation in the loesses and desert sands may be partly explained by isotopic variations reported previously for moraines from the Tianshan and west Kunlun Mountains, which are possible sources for the loesses and desert sands. These inferences on the provenance of the loesses and desert sands are consistent with the dust transport pattern over East Asia.  相似文献   

7.
The Rocks loess section, in unglaciated western Kentucky, provides a high-resolution environmental record during the last glacial maximum onset. The Peoria Silt (9 m thick) contains 26 terrestrial gastropod species, with up to 15 species within a single 5 cm interval. Thirteen radiocarbon ages, using shells or charcoal, range between 30 and 24.5 cal ka; younger loess has been leached or eroded. Stratigraphic shifts in gastropod assemblages imply significant cooling, particularly ~27 cal ka, as solar insolation was decreasing and the southern Laurentide Ice Sheet rapidly advancing. Midwestern to southern species (e.g. Anguispira kochi, Gastrocopta pentodon, Hawaii miniscula, Helicodiscus parallelus, Vallonia perspectiva) occur only in the lowermost Peoria Silt (~30–27 cal ka). In contrast, cold-tolerant species (Columella alticola, Vertigo modesta, Vallonia gracilicosta) occur only in full glacial Peoria Silt (27–24.5 cal ka). Inferred mean July temperatures, from mutual climatic range methods, range from ~23 °C at 30 cal ka, cooling to ~18 °C by 26 cal ka; about 3–8 °C cooler than today (~26 °C). Superimposed on this cooling trend are multi-centennial variations in detrital carbonate, fossil shell concentrations, palaeotemperature estimates, and oxygen isotope values (Vertigo, Discus, Helicodiscus). The finer-scale variations imply relatively synchronous fluctuations in glacial sediment supply, loess sedimentation, and climate.  相似文献   

8.
黄土的构度指标及其试验确定方法   总被引：4，自引：4，他引：0  
黄土的结构性是一个重要的物理性质指标,它与土的粒度、密度和湿度一起影响土的力学性质。依据加荷、扰动、浸水释放土结构势的方法,由土的无侧限抗压强度定义了黄土结构性的构度指标。通过多个场地不同黄土的无侧限单轴压缩试验,测试分析了不同含水率原状土、重塑土和饱和原状土的单轴压缩应力-应变曲线及无侧限抗压强度变化规律,由此得出土的构度指标的变化规律。并且,在粒度、密度、湿度相似条件下,比较分析了土的构度指标与其压缩性之间的关系。土的构度指标与粒度、密度和湿度指标一起能够合理描述土的物理力学基本性质。  相似文献   

9.
《International Geology Review》2012,54(12):1143-1149
Loess and similar rocks of China occur in five distinct environments: 1) on the slopes of high and moderately high mountains, 2) in belts of weathered talus and coarse residual deposits in foothills, 3) on floors of intermontane basins, 4) in thick deposits on high plateaus, and 5) on valley terraces. Loess rocks are found in the following types of deposits: primary loess, weathered slump and talus, stream alluvium, and coarse eluvial deposits. Engineering characteristics of the loess differ depending on environment and type of deposit. Specific gravity ranges from 1. 0 to 1. 5, porosity is generally high but decreases at depth, permeability ranges widely, compaction capacity is greater in loesses of lower specific gravity. — M. Russell  相似文献   

10.
Buraczynski, J., 1988. Lithological, mineralogical and geochemical characteristics of loess in the Rhinegraben. Eng. Geol., 25: 201–208.

Lithological properties of loesses have considerable practical significance. Together with the origin of the material and the genetic–facial variability they are characterised by granulometry, chemical and mineral composition and textural features. In particular, the clay fraction and carbonate content point to the fraction and carbonate content point to the geotecnical properties of loesses.  相似文献   


11.
Loess on the high mountains of northwestern China is largely controlled by the topographic features, distributed mainly on the northern slopes of the Tianshan and Kunlun Mountains as well as in the Ili Valley (between the northern and central Tianshan ranges). Loess distributed in different regions has different ages, with the oldest (early Pleistocene) loess occurring in the Ili Valley. Geochemical, mineralogical and granulometric analyses demonstrate that the loess in the three different regions has different provenance. The loess on the northern Tianshan and Kunlun Mountains is mainly derived from the Junggar and Taklimakan Deserts, respectively, whereas the loess in the Ili Valley is derived mainly from the Sary–Ishikotrau Desert located in the Republic of Kazakhstan. However, these deserts serve mostly as holding areas for the silts rather than original source regions. The production of the silt-sized particles is mainly associated with glacial abrasion and tectonic-induced rock denudation. Thus, the loess in the studied regions can be described as “mountainous” loess rather than simply “glacial” loess or “desert” loess. It is unlikely that the mechanisms of loess formation are the same in different regions, given their dependence on specific geomorphological conditions and associated processes.  相似文献   

12.
Previous work has demonstrated that magnetic properties and sediment particle size of Chinese loess deposits provide information on past behaviour of Chinese summer and winter monsoons, respectively (Heller and Evans, 1995; Derbyshire et al, 1997). It has been suggested that the East Asian winter monsoon system in particular is teleconnected on earth orbital and sub-orbital timescales to climatic states and events in high northern latitudes, especially northern hemisphere ice volume and Heinrich events, (Porter and An, 1995; Chen et al., 1997). However, the majority of this research has been performed around the central part of the Chinese Loess Plateau where the rate of dust accumulation is relatively low, thus limiting the potential resolution of palaeoclimatic records. Here we present a high resolution (5mm / ~20 yr interval) magnetic susceptibility record from an thick loess deposit located at Caoxian in the north-western part of the Loess Plateau. The record spans the “Lateglacial” (last glacial / interglacial transition) and when placed on a palaeomagnetic chronology shows a relationship with the GISP2 proxy air temperature record from Greenland (Grootes et al., 1993; Meese et al., 1994; Stuiver et al 1995). The results demonstrate that several Lateglacial climatic fluctuations previously reported in the North Atlantic region and in Europe are also recorded in China. In addition, the apparent absence of a signal corresponding to the Bølling Interstadial in China during the early deglaciation suggests that, at certain times, the apparent North Atlantic — Asian monsoon teleconnection may have collapsed. The demonstrated ability of the loess deposits to resolve sub-millennial scale climate variations points to their potential as a previously unexplored archive for very high-resolution studies of terrestrial climate.  相似文献   

13.
Various lines of evidence support conflicting interpretations of the timing, abruptness, and nature of climate change in the Great Plains during the Pleistocene–Holocene transition. Loess deposits and paleosols on both the central and northern Great Plains provide a valuable record that can help address these issues. A synthesis of new and previously reported optical and radiocarbon ages indicates that the Brady Soil, which marks the boundary between late Pleistocene Peoria Loess and Holocene Bignell Loess, began forming after a reduction in the rate of Peoria Loess accumulation that most likely occurred between 13.5 and 15 cal ka. Brady Soil formation spanned all or part of the Bølling-Allerød episode (approximately 14.7–12.9 cal ka) and all of the Younger Dryas episode (12.9–11.5 cal ka) and extended at least 1000 years beyond the end of the Younger Dryas. The Brady Soil was buried by Bignell Loess sedimentation beginning around 10.5–9 cal ka, and continuing episodically through the Holocene. Evidence for a brief increase in loess influx during the Younger Dryas is noteworthy but very limited. Most late Quaternary loess accumulation in the central Great Plains was nonglacigenic and was under relatively direct climatic control. Thus, Brady Soil formation records climatic conditions that minimized eolian activity and allowed effective pedogenesis, probably through relatively high effective moisture.Optical dating of loess in North Dakota supports correlation of the Leonard Paleosol on the northern Great Plains with the Brady Soil. Thick loess in North Dakota was primarily derived from the Missouri River floodplain; thus, its stratigraphy may in part reflect glacial influence on the Missouri River. Nonetheless, the persistence of minimal loess accumulation and soil formation until 10 cal ka at our North Dakota study site is best explained by a prolonged interval of high effective moisture correlative with the conditions that favored Brady Soil formation. Burial of both the Brady Soil and the Leonard Paleosol by renewed loess influx probably represents eolian system response that occurred when gradual change toward a drier climate eventually crossed the threshold for eolian activity. Overall, the loess–paleosol sequences of the central and northern Great Plains record a broad peak of high effective moisture across the late Pleistocene to Holocene boundary, rather than well-defined climatic episodes corresponding to the Bølling-Allerød and Younger Dryas episodes in the North Atlantic region.  相似文献   

14.
黄土是美国内布拉斯加州地表分布范围最广的沉积物之一,其中末次冰期形成的皮奥瑞亚黄土（Peoria loess,PL） 质量累积速率（Mass Accumulation Rate, MAR） 在比格内尔山（Bignell Hill,BH） 达到世界最高。探究此地区黄土快速沉积 的原因一直是北美中部大平原黄土研究的热点。本文通过对内布拉斯加州中部BH 样品的Nd、Sr 同位素、主微量和稀土元 素的分析,发现内布拉斯加黄土的化学成分相对均一,与上部大陆地壳（UCC） 和中国洛川黄土相比,Fe、Mg 值较低, K、Na 含量相对较高。BH 黄土的化学风化指数（CIA） 值平均为65.7,小于洛川黄土,显示出较弱的化学风化强度。Sr-Nd 同位素结果显示,除了普遍认为的怀特组沉积为黄土的主要源区外,还存在另一个87Sr/86Sr 比值更高, εNd更负的源区,推测 这个源区为北美劳伦冰盖冰川沉积,其对黄土的贡献达30%~45%。BH 黄土剖面的CIA、Na/K 和碳酸盐含量变化表明,在PL 形成时期（约25~18 ka）,化学风化较弱,气候干冷,有助于风尘产生与堆积。  相似文献   

15.
Although barite nodules are extremely rare in the World Ocean, they are rather widespread in the Japan Sea. They were first discovered by Japanese scientists in the Honshu Island borderland (Okada et al., 1971; Sakai, 1971). They were later reported from the North Yamato Rise (Lipkina and Tsoi, 1980) and Okushiri Ridge (Astakhova et al., 1990) (Fig. 1, Table 1). All barite nodules in the Japan Sea are confined to Miocene marine sediments.  相似文献   

16.
A thin mantle of cover-loam over much of North-East Essex has been recognised as consisting largely of loess. The cover-loam represents the intermixing, to a varying degree, of a layer of loessial silt with a thin layer of underlying sand which is also of likely aeolian origin. The heavy mineral content of the coarse silt fraction of North-East Essex loess is generally similar to that of last glaciation age loesses elsewhere in Eastern England, Belgium and the Netherlands. This suggests the North-East Essex loess is part of a single loess sheet deposited over Eastern England and parts of Western Europe. Nevertheless, detailed examination of the heavy minerals content from all of these areas reveals slight areal variations especially in the proportion of hornblende. These differences show the coarse silt from North-East Essex loess to have closest affinities with that from Norfolk. A distant source for the loess within the present North Sea Basin is proposed on textural evidence. Loess accumulation in North-East Essex probably commenced in the few thousand years leading up to the maximum extent of Devensian ice (about 18,000 years B.P.) and may have continued to about 14,000 years B.P.  相似文献   

17.
Lost loesses     
《Earth》2006,74(3-4):309-316
Loesses form wide belts in front of previously glaciated areas. Their thicknesses may be considerable, changing in Eurasia from maximally a few metres in the west to a hundred metres or more in the east. The Eastern (particularly Chinese) loesses are mostly unrelated to glaciations. The periglacial loesses from China and elsewhere predominantly date from the last Pleistocene glaciation: relatively few comparable occurrences are known from earlier Quaternary glaciations. As it is difficult to imagine that the conditions in front of the land-ice masses during the earlier glaciations differed fundamentally from those of the last one, considerable quantities of loess must have disappeared. This disappearance, which is commonly ascribed to fluvial and eolian erosion, is not easily explained as equivalent deposits that may have the older loesses as a source, are practically absent. A possible explanation might be that loess is recycled during successive glaciations. Some loess disappears during interglacials by erosion, but this quantity is more than compensated by the formation of new silt particles. The implication would be that the loess deposits increase in volume for each new glaciation.  相似文献   

18.
J. Šajgalik 《GeoJournal》1991,24(2):165-174
The Slovak Carpathians in Pleistocene presented a very articulate territory, so that loesses, as well as loess-like deposits could occur here with various lithofacial types.Based on a complex evaluation of a great number of loess samples of the Danubian lowland and their analysis by methods of mathematical statistics, it was possible to create a realistic model of the base soil. For the statistical processing we picked 4 selective sets of the Danubian lowland hilly country, where each set can be considered as a homogeneous engineering geological type.By comparing the loess properties of the Danubian lowland with the loesses of other European countries (mainly USSR), it can be stated that the studied loesses are not so dangerous from the sagging standpoint. The increase of the oedometric modulus with the depth was shown. Similar relationships were derived also for the parametres of shear strength.The regional characteristics of the loess mechanical properties will serve well in the initial phases of investigation, planning and designing, but also in more exact geoo technical calculations.  相似文献   

19.
Valleys tributary to the Mississippi River contain fossiliferous slackwater lake sediment (Equality Formation) deposited in response to aggradation of the Mississippi River valley during the last glaciation. In the St. Louis Metro East area, the lower part of the Equality Formation is primarily laminated, fossiliferous silt and clay deposited from about 44,150 to 24,310 14C yr B.P. The upper Equality Formation is primarily very fine sand to silt deposited from about 21,200 to 17,000 14C yr B.P. Among the four cores that sample this succession in the St. Louis Metro East area, core MNK-3 (38.64EN, 90.01EW) was selected for detailed study. Three sources are distinguished by the following characteristics: (1) gray smectite-quartz-Se-rich, feldspar-poor material of the Des Moines, Wadena, and James lobes; (2) reddish brown kaolinite-Cu-Fe-rich sediment of the Superior and Rainy lobes; and (3) brown illite-dolomite-Sr-rich sediment of the Lake Michigan and Green Bay lobes. The earliest sediments (44,150 to 41,700 14C yr B.P.) were derived from the central and western provenances and are chronocorrelative with the lower Roxana Silt. A hiatus occurred from about 41,700 to 29,030 14C yr B.P. when much of the middle Roxana Silt (Meadow Member) was deposited on adjacent uplands. The youngest sediment includes evidence of heightened activity of the Superior Lobe at about 29,000 14C yr B.P., the Lake Michigan and Green Bay lobes from about 25,000 to 24,000 14C yr B.P., and the Wadena-Des Moines-James lobes at about 21,000 14C yr B.P.  相似文献   

20.
Although loess–paleosol sequences are among the most important records of Quaternary climate change and past dust deposition cycles, few modern examples of such sedimentation systems have been studied. Stratigraphic studies and 22 new accelerator mass spectrometry radiocarbon ages from the Matanuska Valley in southern Alaska show that loess deposition there began sometime after 6500 14C yr B.P. and has continued to the present. The silts are produced through grinding by the Matanuska and Knik glaciers, deposited as outwash, entrained by strong winds, and redeposited as loess. Over a downwind distance of 40 km, loess thickness, sand content, and sand-plus-coarse-silt content decrease, whereas fine-silt content increases. Loess deposition was episodic, as shown by the presence of paleosols, at distances >10 km from the outwash plain loess source. Stratigraphic complexity is at a maximum (i.e., the greatest number of loesses and paleosols) at intermediate (10–25 km) distances from the loess source. Surface soils increase in degree of development with distance downwind from the source, where sedimentation rates are lower. Proximal soils are Entisols or Inceptisols, whereas distal soils are Spodosols. Ratios of mobile CaO, K2O, and Fe2O3 to immobile TiO2 show decreases in surface horizons with distance from the source. Thus, as in China, where loess deposition also takes place today, eolian sedimentation and soil formation are competing processes. Study of loess and paleosols in southern Alaska shows that particle size can vary over short distances, loess deposition can be episodic over limited time intervals, and soils developed in stabilized loess can show considerable variability under the same vegetation.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号