共查询到20条相似文献，搜索用时 305 毫秒
1.
Siltstones are unusual rocks. They are mainly made up of quartz. Glacial grinding is considered by some authors to be the main, or even the only, process generating silt. This is supported by: (1) the presence of silt in basal tills from present ice-caps and, above all, (2) by the volumetric importance of Quaternary loess. However, ancient glaciogenic sediments contain only a little silt scattered in the matrix of tillites, or forming thin intercalations in pro- and periglacial deposits. These siltstones represent less than 5% of the total volume of Upper Proterozoic and Upper Ordovician glaciogenic sequences of West Africa. On the other hand, silt is abundant in weathering profiles of tropical, equatorial and mediterranean zones. This silt is generated by splitting of minerals, mainly quartz, inherited from the parent rock. Quartz grains are partly dissolved in situ and their fragments cemented by iron oxi-hydroxides in tropical and equatorial zones and by calcite in mediterranean zones. Silt is particularly abundant in tropical soils, comprising up to 50-75%. Secondary dissolution of the iron or calcite matrix induces disaggregation of these soils and the release of silt which is later reworked and concentrated by wind or running water. Weathering processes, and especially those operating in tropical zones, are the main phenomena generating silt. Glacial grinding would generate only a little silt. A large part of the material of Quaternary loess may be derived from glacially reworked weathering profiles. 相似文献
2.
Baotian Pan Qingyu Guan Hongshan Gao Dongsheng Guan Fenliang Liu Zongmeng Li Huai Su 《Boreas: An International Journal of Quaternary Research》2014,43(1):121-131
The grain size of the loess‐like sediment in the Jinsha River Valley is essentially similar to that of typical loess, and it differs from that of the fluvial sediment in the Jinsha River area. We conclude that the loess‐like sediment is of aeolian origin, as is the loess of the Chinese Loess Plateau. The geochemical characteristics of the loess‐like sediment are similar to those of dammed lake and fluvial sediments in the same area, which is significantly different from those of the typical loess in NW China. The geochemical characteristics show that the loess‐like sediment originated from proximal materials. Combined with the grain‐size distribution pattern of the aeolian deposits, we suggest that the loess‐like sediment of the Jinsha River Valley originated from the dammed lake and fluvial sediments that accumulated in the valley. The local mountain‐valley wind circulation (especially the valley wind in winter and spring) provided the transport force for the loess‐like sediment. The presentation of the loess‐like sediment may indicate the appearance of the grassland and forest steppe and the beginning of the dry‐hot valley phenomenon. 相似文献
3.
对长武和宜川晚第四纪黄土-古土壤序列的高分辨率铁含量分析表明,不同时期的风尘在化学成分上具有较明显的区别。在排除成壤过程中CaCO3淋溶和淀积的相对影响后,研究剖面的铁含量基本反映了原始风尘的铁含量。已有研究表明,西部源区的气溶胶和现代地表流沙中铁的相对含量显著高于东部源区。本文认为,风尘铁含量的变化反映了西部沙漠风尘对黄土堆积的相对贡献。由于西风带北支是西部源区风尘的主要搬运者,风尘中全铁含量可能反映了西风带北支气流的强度,并明显存在着千年尺度的变化。 相似文献
4.
《Journal of South American Earth Sciences》1999,12(3):293-310
A 42 m thick loess–paleosol sequence was studied in the dry pre-Andean valley of Tafi-del-Valle, at the La Mesada site (2280 masl), Tucumán Province in northwest Argentina. The sequence contains 28 paleosols interbedded with 26 loess layers. The latter are coarse loamy and have large hexagonal polyhedrons. The soil layers are restricted to fine loamy Bt horizons, with prismatic structure and organo-argillans on the structural surfaces. Surface (A) and eluviation (E) horizons are systematically absent from the whole sequence. Soil truncation is unlikely as no remains of such horizons or erosional uncomformities were identified. The dated part of the section encompasses a time span of 10,080 years, from 17,580 BP at 5.2 depth to 27,660 BP at 42.3 m depth. A total of 20 Bt/C pairs developed during this time interval, corresponding to a climatic change every 500 years on the average between dry–cool conditions, promoting loess influx, and moist–warm conditions favouring soil development. The depth function of the Bt/C clay ratios reflects the climate improvement during the soil formation interstadials, with an optimum in the upper part of the sequence. This might be related with recurrent northward shifts of the polar front, accompanied by the weakening of the mid-latitude South Pacific anticyclone. Similar short-term periodicity has been identified in the ice cap of Greenland, highlighting the global character of such frequent climatic changes during the Pleistocene. 相似文献
5.
郧县前坊村剖面黄土-古土壤序列风化成壤及古气候研究 总被引：2，自引：0，他引：2
秦岭作为我国南方和北方地理、气候、自然环境的天然分界线,并当作是黄土高原的南屏障。汉江上游谷地地区位于秦岭南侧,属于北亚热带气候区域,受季风气候活动影响强烈。为了探讨该地区在亚热带气候背景下,风成黄土成壤改造对气候变化的响应机制,通过对汉江上游谷地前坊村（QFC）剖面磁化率、烧失量、粒度、Rb/Sr等理化性质进行研究。结果表明:①末次冰期以来沙尘暴很有可能越过秦岭在其南侧堆积,黄土-古土壤剖面地层序列从上到下依次为:MS-L0-S0-Lt-L1-AD;这些沉积物完整记录了一级阶地上晚更新世气候变化信息,地层单元受到各个时期不同程度成壤强度的改造。②前坊村剖面中,理化性质在不同地层单元有显著差异。例如磁化率、烧失量、Rb/Sr指标平均含量的高值出现在古土壤层中,低值出现在黄土层;而Zr/Rb含量变化正好相反;这些理化性质表明,在古土壤发育期,水热条件进入最适宜期,沉积物的风化成壤作用显著;在黄土堆积期,气候寒冷干燥,主要以粉尘堆积为主,沉积物的风化成壤作用较弱。③秦岭南侧北亚热带汉江上游前坊村一级阶地剖面化学风化强度变化揭示了黄土-古土壤环境气候变化的规律:末次冰期以来（大约18.0～11.5 ka B.P.）,气候干冷,沙尘暴频繁出现,沉积物以黄土堆积为主,成壤作用微弱,形成马兰黄土（L1）;全新世早期（约11.5～8.5 ka B.P.）,气候由干冷向暖湿方向转变,但主要以干冷为主,形成过渡层（Lt）;在全新世大暖期（8.5～3.1 ka B.P.）,气候条件达到最优阶段,水热配合较好,生物活动活跃,成壤作用十分显著,发育了古土壤S0;到了全新世晚期以来（3.1 ka B.P.）气候又由暖湿向干冷方向转变,成壤作用明显减弱,沙尘暴出现较为频繁,形成了以黄土堆积为主的全新世黄土（L0）。现代表土层（MS）是在（1.5 ka B.P.）以来气候转暖,加之人类长期农业耕作扰动,在全新世黄土L0顶部叠加而形成的。 相似文献
6.
This article highlights the relationship between speleothems growing inside gypsum caves and the particular climate that existed
during their development. Speleothems in gypsum caves normally consist of calcium carbonate (calcite) or calcium sulphate
(gypsum) and the abundance of such deposits greatly differs from zone to zone. Observations carried out over the last 20 years
in gypsum caves subjected to very different climates (Italy, Spain, New Mexico, northern Russia, Cuba, Argentina) highlight
wide variation in their cave deposits. In arid or semi-arid climates, the speleothems are mainly composed of gypsum, whilst
in temperate, humid or tropical regions, carbonate formations are largely predominant. In polar zones no speleothems develop.
These mineralogical details could be useful paleoclimatic indicators of climate change. The interpretation proposed is based
on the fact that in gypsum karst the kind of speleothems deposited is determined by competition between the two principal
mechanisms that cause precipitation of calcite and gypsum. These mechanisms are completely different: calcite speleothem evolution
is mainly controlled by CO2 diffusion, while gypsum deposits develop mostly due to evaporation. Therefore, the prevalence of one kind of speleothem over
the other, and the relationship between the solution–precipitation processes of calcite and gypsum, may provide evidence of
a specific paleoclimate. Additionally, other non-common deposits in gypsum caves like moonmilk, cave rafts and dolomite speleothems
can be used as markers for the prevalence of long, dry periods in humid areas, seasonal changes in climate, or rainfall trends
in some gypsum areas. Moreover, the dating of gypsum speleothems could contribute paleoclimatic data relating to dry periods
when calcite speleothems are not deposited. In contrast, the dating of calcite speleothems in gypsum caves could identify
former wet periods in arid zones. 相似文献
7.
In the late Wisconsinan, the South Thompson River valley, British Columbia, was occupied by an ice-dammed lake. After the lake drained, the exposed lacustrine silt became the source material for a Holocene loess. The purpose of this paper is to establish the stratigraphic, depositional and geomorphic framework of loess occurring along the South Thompson River valley immediately east of Kamloops, British Columbia. This montane environment of loess deposition was characterised by active slope and fluvial processes depositing sediments contemporaneously with the accumulation of loess. The loess reaches an average of 4 m in thickness in the central part of the valley and thins towards the valley sides. Two tephras—Mount St Helens Y (3.4 ka) and Mount Mazama (6.8 ka)— occur in the loess and are invaluable stratigraphic markers. Most of the loess was probably deposited between 8.2 ka and 3.4 ka, a period coinciding with mid-Holocene increased summer temperatures and decreased precipitation in south-central British Columbia. Debris flows and small streams, originating on the valley sides, flowed out on to the loess depositing sand and gravel beds. These deposits form a definite proximal—distal relation across valley with the slope-derived sediments decreasing and the loess increasing in thickness towards the centre of the valley. The lactustrine silt particles were mobilised by diurnal mountain and valley, gravity, and canalised winds flowing within the South Thompson valley. An analysis of contemporary wind-flow data was undertaken to provide a possible analogue for valley wind flows in the mid-Holocene. 相似文献
8.
The pedogenic ferrimagnetic minerals have been considered to be the cause of magnetic susceptibility enhancement in loess
deposits distributing in the Chinese Loess Plateau and Central Europe, while “wind intensity” mechanism is proposed to be
responsible for the magnetic susceptibility enhancement of loess in Alaska and Siberia. However, the magnetic enhancement
mechanism is still open for loess in Ili valley, Xinjiang Uygur Autonomous Region, China. To understand this, we conducted
a rock magnetic investigation on Axike section that is located in Ili valley. Results show that transitional stage from magnetic
(χlf) trough to magnetic (χlf) peak corresponds to soil units. The PSD and MD-grained magnetite dominate the magnetic properties of AXK sequences, and
the main factor affecting magnetic properties is the concentration of ferrimagnetic fraction. For the “pedogenesis enhancement”
and “wind intensity” model, it seems hard to explain the magnetic susceptibility enhancement in this area. For the concentration
of fine-grained magnetite in magnetic mineral shows positive relationship with the intensity of pedogenesis, the magnetic
parameters of loess deposit in Ili valley can be used to recover paleoclimatic variations. 相似文献
9.
María Gabriela García Ondra Sracek Diego Sebastián Fernández Margarita del Valle Hidalgo 《Environmental Geology》2007,52(7):1261-1275
The concentration of arsenic measured in groundwater from three aquifers in the study area located in the Eastern Tucuman
province, Argentina, mostly depends on the lithology, but the spatial and temporal variations of concentrations seem to be
also controlled by pH changes, climatic factors, and human perturbations. The highest concentrations of As (more than 1,000 μg
L−1) were found in the shallow aquifer, made of As-rich loess, while the lowest concentrations were measured in the deep confined
aquifer, consisting of alternating layers of alluvial sands/gravels and clays. Intermediate values were measured in the semiconfined
aquifer made of the fluvial sediments deposited in the Salí River valley, that alternate in the upper part of the sedimentary
sequence with layers of loess. Because most of As in the loess is considered to be adsorbed onto Fe-oxyhydroxide coatings,
the increase of pH in the flow direction (west-east) leads to increasing arsenic concentrations towards the eastern border
of the study area. The decomposition of organic wastes poured into the Salí River or associated with local and diffuse sources
of contamination in the eastern part of the study area depletes dissolved oxygen, which leads to the reductive dissolution
of Fe and Mn oxyhydroxides, and to the subsequent release of the adsorbed and co-precipitated As. This process mainly affects
shallow groundwater and the upper part of the semiconfined aquifer. Geochemical and hydrological data also suggest that rising
water table levels at the end of the wet season may also lead to reductive dissolution of As-rich Fe oxyhydroxides in the
shallow aquifer. 相似文献
10.
安徽宣城黄土堆积的磁性地层学与古环境意义 总被引：15，自引：0，他引：15
利用磁性地层学方法对安徽宣城剖面的研究表明,B/M界限位于剖面中网纹红土的中下部,结合光释光测年可以外推出长江中下游地区的典型风成沉积开始形成于0.85Ma前后。这一年龄对应于一次全球性的气候转型事件,在海洋沉积与我国北方的黄土-古土壤序列中均记录了这一气候转型事件。同时,0.85Ma对应于中更新世的两大地质事件,一是全球冰量的增加,二是青藏高原的快速隆升,长江中下游地区风成堆积的开始,可能是上述两个因素共同作用的结果。 相似文献
11.
Issanova Gulnura Jilili Abuduwaili Semenov Oleg 《Arabian Journal of Geosciences》2014,7(11):4513-4521
Deflation processes are important in arid environments such as deserts. The deserts of Kazakhstan mostly cover lowlands and extend from the eastern coast of the Caspian Sea to the piedmonts of the Tien-Shan Mountain. Desert areas are also major source areas of dust/sand storm activities. We considered deflation processes in the southern Pre-Balkhash deserts. In Kazakhstan, desertification processes due to wind erosion in the form of dust/sand storms were observed in semi-desert and desert landscapes. During analysis of numerous long-term meteorological data and cartographic materials, we revealed the sand movement directions which allow prediction of future potential sand movement patterns or processes in southern Pre-Balkhash deserts. The Taukum, Moiynkum deserts, Ili river deltas and valleys, and southern coastal of Lake Balkhash are most prone to dust/sand storms. The most frequent storms were observed in the Bakanas weather station (Ile river valley). Sand/dust transport occurs mainly in the east, south-east north-east direction in the southern Pre-Balkhash deserts. The high amount of sand transportation was observed at the Kuigan weather station; low amounts were encountered at the Naimansuiek weather station. The amount of airborne sand/dust varies in accordance with the general and local meteorological features, the complexity of relief forms, soil conditions and properties, lithology, and various contributions of the human activities. Thus, our study on deflation processes in the southern Pre-Balkhash deserts has great importance towards aiding in the prediction and monitoring of dust/sand storms and movement patterns. 相似文献
12.
M. J. OrgeIra A. M. Walther C. A. Vsquez I. Di Tommaso S. Alonso G. Sherwood Hu Yuguan J. F. A. Vilas 《Journal of South American Earth Sciences》1998,11(6):561-570
Environmental magnetic techniques were applied to a loess–paleosol sequence of the Chacopampean plain (Buenos Aires, Argentina). Mineral magnetic carriers and their grain size were identified in order to detect magnetic mineral fluctuations associated with climatic changes. Multidomain magnetite of detrital origin dominates the record. In paleosols, a high coercivity fraction was identified. Horizons with no visual evidence of pedogenesis, but showing magnetic behavior analogous to that of paleosols were observed and are thought to represent environmental conditions similar to those prevailing during paleosol formation.The results suggest that the magnetic signal yielded by paleosols in these South American loess deposits is different from that in the Chinese loess. This may be due to differences in parent materials, diagenetic processes and/or differences in paleoclimatic conditions in both regions. 相似文献
13.
E.M. Rutledge M.J. Guccione H.W. Markewich D.A. Wysocki L.B. Ward 《Engineering Geology》1996,45(1-4):167-183
Loesses of the Lower Mississippi Valley (LMV) are world-famous. Sir Charles Lyell (1847), Hilgard (1860), Stafford (1869), Call (1891) and Mabry (1898), thought the LMV loess was a single water deposit although “double submergence” was noted by Call (1891) and Salisbury (1891). Shimek (1902) and Emerson (1918) recognized LMV loess as a wind deposit which came from the valley. Although wind-deposited loess gained wide acceptance, Russell (1944a) published his controversial theory of “loessification” which entailed weathering of backswamp deposits, downslope movement and recharge by carbonates to form loess. Wascher et al. (1947) identified three LMV loesses, mapped distributions and strongly supported eolian deposition. Leighton and Willman (1950), identified four loesses and supported eolian deposition as did Krinitzsky and Turnbull (1967) and Snowden and Priddy (1968), but Krinitzky and Turnbull questioned the deepest loess. Daniels and Young (1968) and Touchet and Daniels (1970) studied the distribution of loesses in south-central Louisiana. West et al. (1980) and Rutledge et al. (1985) studied the source areas and wind directions which deposited the loesses on and adjoining Crowley's Ridge. B.J. Miller and co-workers (Miller et al., 1985, 1986, Miller and Alford, 1985) proposed that the Loveland Silt was Early Wisconsin rather than Illinoian age and advanced the name Sicily Island loess. They proposed the underlying loess was Illinoian and advanced the name Crowley's Ridge. We termed the loesses, from the surface downward, Peoria Loess, Roxana Silt, Loveland/Sicily Island loess, Crowley's Ridge Loess and Marianna loess. Researchers agree that the surfical Peoria Loess is Late Wisconsin and the Roxana Silt is Late to Middle Wisconsin, but little agreement exists on the age of the older loesses. Pye and Johnson (1988) proposed Early Wisconsin for the Loveland/Sicily Island. McKay and Follmer (1985) suggested this loess correlated with a loess under Illinoian till. Clark et al. (1989) agreed on Crowley's Ridge, but suggested the Loveland/Sicily Island loess on Sicily Island was older. Mirecki and Miller (1994) and Millard and Maat (1994) suggested an Illinoian age for the Loveland/Sicily Island loess. Miller and co-workers suggested, as did Pye and Johnson (1988), an Illinoian age for the Crowley's Ridge loess. McKay and Follmer (1985) suggested it correlated with a loess under “Kansan” till. Stratigraphy indicates the Marianna is the older of the five loesses.
Researchers identified loess on both the east and west side of the LMV as well as on higher terraces within the valley. Many researchers assumed unaltered loesses were commonly yellowish brown, and silts or silt loams (West et al., 1980; Miller et al., 1986). The nonclay fraction of unweathered LMV loesses was dominated by quartz followed * Corresponding author. by carbonates, mainly dolomites, followed by feldspars, and micas. Clays were dominated by montmorillonite followed by micaceous minerals, kaolinite and vermiculite (Miller et al., 1986). Soils in the Crowley's Ridge loess are most developed, followed by the soils in the Loveland/Sicily Island which are more developed than the modern soils in the Peoria Loess. Soils in the Roxana and Marianna loesses are least developed and the Farmdale Soil of the Roxana is the weaker of the two (Miller et al., 1986). There is certainly overlapping range in the degree of soil development in the various loesses. 相似文献
14.
J.C. Bidegain Y. Rico A. Bartel M.A.E. Chaparro S. Jurado 《Quaternary International》2009,209(1-2):175
Quaternary loess sequences of Argentina, with interbedded loess and buried soils (palaeosols), provide terrestrial records of past climates and environmental conditions. Study of rock magnetic parameters measured over a large area of the Pampean loess seems to indicate that the existing magnetoclimatological models cannot adequately account for the complexities of the Pampean loess.The Chinese loess has been considered as typical, where magnetic properties are largely controlled by pedogenesis. On the other hand, the Siberian loess is an alternative magnetoclimatological model in which palaeosols appear as magnetic lows and the intercalated loess as magnetic highs. Argentine loess is apparently closer to the Siberian model. However, considering the data obtained in Argentina, the situation seems to be more complicated. The higest magnetic values (SIRM and susceptibiliy values) in silty and sandy loess indicate a more efficient entrainment of dense iron oxides particles during stormy dry (glacial) intervals.The parent material shows the highest susceptibility values (>100 × 10−8 m3/kg) while the waterlogged horizons show the lowest ones (below 20 × 10−8 m3/kg). Pedogenesis resulting in the development of BC and B soil horizons of palaeosols in the parent loess produced decreases in susceptibility values and increases in the F factor. This phenomenon occurs at some degree of humidity in which the process of gleying caused the total depletion of both susceptibility and frequency factor. The magnentic data allows consideration of the relevance of major cycles (arid/humid) separated by discontinuities as the main factor favoring one particular behavior of the magnetic parameters. The B horizons of palaeosols developed during an arid cycle will not show a notable difference in the magnetic records from the parent material. In contrast, during humid climate condition the pristine loess can be progresively obliterated by pedogensis through to the extreme situation of gleying. 相似文献
15.
The coast of Ceará State in NE Brazil is covered by vast fields of active and stabilized coastal sand dunes. Its tropical climate is characterized by two seasons, wet and dry, with wind intensity determined by the meridional shift of the Intertropical Convergence Zone. The wind power is negatively correlated with precipitation, and precipitation is negatively correlated with the difference between sea surface temperatures of the tropical Atlantic north and south of the equator. We present a model suggesting that during the Late Pleistocene wind power determined the mobility and stability of the dunes. Sand dunes accumulated during periods of high wind power (as it is today) and stabilized when wind power was low. Once the dunes were stabilized by vegetation they could not be activated even by increased wind power. Samples that were taken for luminescence dating from 25 stabilized dunes along the coasts of Ceará gave ages ranging from135 ka to < 100 yr. We postulate that these luminescence ages fall at the beginning of wet periods in NE Brazil characterized by low wind power. These paleoclimatic wet periods correlate well with the cold periods of stades in Greenland ice-core records. 相似文献
16.
黄河中游岩土侵蚀问题及防治对策探讨 总被引：1，自引：0，他引：1
黄河中游出露地层岩性主要有古生界奥陶系碳酸盐岩、石炭系和二叠系碎屑岩;中生界三叠系砂泥岩、侏罗系砂泥岩和白垩系砂泥岩;新生界古近系砂泥岩、新近系(原为第三系)红土及第四系风积黄土和现代风积沙。第四系黄土厚度大,结构松散,遇水易解体,具湿陷性,是黄河中游粗泥砂的主要物源;第四系风积沙分布于黄河中游北部,亦是黄河中游粗泥砂的主要物源。岩土侵蚀类型有五:一是风力侵蚀—风沙侵蚀;二是降雨对谷坡面的面状冲刷—坡面侵蚀;三是地表径流对谷底的侧蚀—沟谷侵蚀;四是滑塌对谷坡坡体的破坏—重力侵蚀;五是潜蚀。并对不同侵蚀类型提出了不同的防治对策。 相似文献
17.
丰宁滦河上游的河谷地带有多处黄土堆积,由于该地区黄土分布零星、厚度较小,在以往的研究中却很少引起人们的关注,然而该地区黄土堆积是河北坝上地区环境演化和气候变化的重要地质记录.为了探讨该地区黄土的成因、物源以及化学风化强度,选取代表性的黄土-古土壤剖面77个样品进行了常量元素分析,并与已知典型风成堆积物的地球化学元素特征进行对比.结果表明:(1) 丰宁剖面的主要常量元素(SiO2、Al2O3和Fe2O3)之和以及UCC(Upper Continental Crust)标准化曲线均与典型风成堆积物具有较好的相似性, 表明丰宁黄土和古土壤为风成成因;(2) 丰宁黄土处于初等化学风化阶段,古土壤则进入了中等化学风化阶段.与典型风成堆积物相比,化学风化强度序列为:宣城风成红土＞＞西峰红黏土、镇江下蜀土＞丰宁古土壤、洛川古土壤＞洛川黄土＞丰宁黄土;(3) 常量元素迁移特征表明丰宁黄土和古土壤的化学风化已完成了早期去Na、Ca阶段,并进入到了中期去K阶段;(4) 丰宁黄土、古土壤的K2O/Al2O3和Fe2O3/Al2O3比值与洛川黄土、洛川古土壤、镇江下蜀土、西峰红黏土较为接近,表明这些风成堆积物可能具有相似的源区.而TiO2/Al2O3比值小于其他风成堆积物, 指示丰宁剖面具有其他物源. 相似文献
18.
A. B. Klimchouk 《Environmental Geology》1997,31(1-2):1-20
Most of exogeneous epigenetic sulfur deposits are clearly associated with intensely karstified carbonate and sulfate rocks.
This paper demonstrates, using the Pre-Carpathian region as an example, that karstification is one of the most important processes
controlling the formation of sulfur deposits. This is determined by a coincidence of some major prerequisites of these two
processes.
In the Podol'sky and Bukovinsky regions the Miocene aquifer system is well drained by erosion valleys; the giant network caves
known here in gypsum formed under past artesian conditions. In the region of sulfur deposits, associated with the same karstified
gypsum strata, true artesian conditions still prevail. Hydrogeologic data show that abundant cavities detected in the vicinity
of sulfur deposits can be interpreted as having the same origin as the fossil caves of the Podol'sky and Bukovinsky regions.
The current widespread belief that the gypsum/anhydrite stratum in the region is aquifuge separating the Miocene aguifers
is inadequate. This belief caused much controversy with regard to the genetic interpretations of sulfur deposits in the region.
Caves formed by the upward water flow through the gypsum/anhydrite stratum govern the water exchange between the aquifers
within the aquifer system.
A new karst model for the formation of sulfur deposits is suggested. It agrees well with the hydrogeological features of the
Miocene sequence and with biogeochemical mechanisms of sulfur origin in low-temperature diagenetic environments.
Received: 16 February 1995 · Accepted: 4 June 1996 相似文献
19.
Lateral stresses existing in soil deposits are important clues to recent stress history because they tend to remain “locked in” following adjustments in the overburden load: Thus a high lateral stress is partly retained following a reduction in vertical stress, as by glacial melting or erosion. Conversely, during loess deposition a low lateral stress should be retained if the soil structure is preserved under increasing vertical stress. This has not previously been verified in the field because of the difficulty of measuring lateral stresses in situ in an hydrocollapsible material.
Tests were conducted at two sites in Missouri river valley loess using theKo Stepped Blade, which uses data extrapolation to give stress on a zero thickness blade. In basal loess where the moisture content was at or above the liquid limit,Ko = 1.0 ± 0.02. In the upper to middle loess zoneKo is as low as 0.2 to 0.4, lower than the values of 0.4 to 0.5 that would be expected if the soil were normally consolidated. Near the ground surface, lateral stresses are higher soKo > 1, attributed to post-depositional weathering and the presence of smectite clay minerals. The vertical sequence of lateral stress from high to low to high again should contribute a tension-induced cleavage if lateral confinement is removed by excavation or erosion. This stress condition is most pronounced in loess close to its source, where the potential tension zone was found to extend to a depth of about 7 m compared to 4 m at the other site. Loess cleavage appears to result from stress relief and is not a unique directional property of this material. 相似文献
20.
Tropical wetlands in Central Brazil are located in savanna areas and are made up of more terrestrial-type wetlands – campo limpo úmido (grassland-type savanna), campo sujo úmido (shrub-type savanna), mata galeria (riverine forest); as well as more aquatic-type wetlands – vereda (valley-side marsh with palm groves of Mauritia flexuosa), várzea (backswamp), lake, and river. They are regulated by a seasonal climatic regime characterized by a wet-rainy season from October to March and then followed by a dry season. Underground water is abundant and rivers frequently overflow during the rainy season. Many of these wetlands which are protected by law are significant regulators of water quality. In order to predict tropical wetland function and draw up environmental management policies, it is important to understand how abiotic and biotic factors influence tropical wetland origin and spatial distribution. In this large-scale study we set out to investigate the influence that geological and geormorphological settings, i.e. geological substrates and geomorphological units, have on the spatial distribution of tropical wetlands in Central Brazil. Two watersheds, the Caiapó and Piracanjuba, were selected in order to carry out the study. They present different types of rock and unconsolidated surface materials. Planation surface, escarpment and steep slope, gentle slope, and aggradation are the dominant geomorphological units in the watersheds. Principal component analysis was conducted in order to determine the influence of the selected abiotic variables on the spatial distribution of tropical wetlands. The study showed that the presence of sedimentary and low-grade metamorphic rocks and planation surface strongly influence the spatial distribution of the more terrestrial-type wetlands. Geological and geomorphological settings explain about 60% of the variability in the spatial distribution of these wetland types. No clear effect of the abiotic variables selected was observed on the more aquatic-type wetlands. An association between stratified layers or planar geological surfaces, groundwater discharge zones, and slope breaks is suggested to explain the influence of the geological and geomorphological settings on the wetlands under review. The study demonstrated the importance of considering abiotic factors, not usually included in classification schemes, to further understand the spatial distribution of tropical wetlands. 相似文献