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1.
Climate and tectonics play important roles in controlling processes of transport and deposition on alluvial fans, but the bedrock lithology in the fan catchment area is also a significant, independent factor. Adjacent Oligo-Miocene alluvial fan deposits on the northern margin of the Ebro Basin display contrasting depositional characteristics with one dominated by the deposits of debris flows and the other by deposition from flows of water. A difference in clast compositions indicates that the two studied fans (the Nueno and San Julián fans) had contrasting bedrock lithology in their drainage basins. The proximal facies of the Nueno fan body contains matrix-supported conglomerate beds with up to 80% pebble clasts of gypsum in a matrix of gypsiferous sand, interbedded with gypsarenite beds. The drainage basin of this fan was dominated by Triassic bedrock consisting of beds of gypsum, marl and micritic limestone. The San Julián fan body comprises clast-supported, polymict conglomerate beds containing pebbles from Triassic, Cretaceous and Palaeogene limestone units that are exposed in the adjacent part of the basin margin. The interfingering of the deposits of these two fans demonstrates that they were contemporaneous. Given the consistent climate, the differences in fan depositional processes must therefore be attributed to the contrasting bedrock lithology in their drainage basins. A drainage basin consisting mainly of marl and gypsum bedrock provided sufficient fine-grained material to generate debris flows, whereas more dilute, water-lain processes dominated where the drainage basin was largely limestone strata.  相似文献   

2.
The lowest 17-km long reach of the Huerva River valley, down to its confluence with the Ebro River in Zaragoza city, flows across salt-bearing evaporites of the Ebro Tertiary Basin (NE Spain). Upstream, the horizontally lying Miocene evaporites are interfingered with non-soluble distal alluvial fan facies (shales and sandstones). The proportion of soluble facies in the Huerva River valley increases in a downstream direction towards the basin depocenter. On the basis of the type and magnitude of the paleosubsidence features, the valley has been divided into four reaches. Along reach I, undeformed terrace deposits less than 4 m thick rest on insoluble detrital bedrock. In reaches II and III, dissolution at the alluvium–bedrock boundary has generated local thickening, deformation and paleocollapse structures, which only affect the alluvial mantle. In reach IV, terrace deposits thicken to over 60 m resulting from a large-scale synsedimentary subsidence. In this sector, subsidence locally affects to both the alluvium and the underlying bedrock. This indicates that dissolution acts at the rockhead beneath the alluvial cover (alluvial karst) and within the evaporitic substratum (interstratal karst). The development of an intraevaporitic karst in reach IV is attributed to gypsum and salt dissolution. Irregular terrace gravel bodies (gravel pockets) embedded in a fine-grained matrix associated with paleocollapse structures have been interpreted as liquefaction–fluidization structures resulting from ground acceleration and suction induced by catastrophic collapses. Subsidence is currently active in the region affecting areas with a thin alluvial cover in reaches III and IV. The low subsidence activity in most of Zaragoza city is explained by the presence of thickened (around 50 m) and indurated alluvial deposits. In the surrounding area, numerous buildings in Cadrete and Santa Fe villages have been severely damaged by subsidence. Natural and human-induced subsidence favours the development of slope movements in the gypsum scarp overlooking Cadrete village. Several transport routes including the Imperial Canal (irrigation canal) and the recently completed Madrid–Barcelona high-speed railway are affected by human-induced sinkholes. The paleocollapse structures exposed in the trenches of this railway and a ring road under construction point to hazardous locations underlain by cavities and collapse structures where special protection measures should be applied. Rigid structures are recommended beneath the high-speed railway with sufficient strength to span the larger sinkholes with no deformation. Electronic monitoring devices linked to a warning system can detect subtle subsidence-induced deformations in carriageways or railways. This research demonstrates that the study of the paleokarst helps to understand the processes involved in the present-day subsidence phenomena and their general spatial distribution.  相似文献   

3.
Geological and environmental implications of the evaporite karst in Spain   总被引：2，自引：0，他引：2  
In Spain, evaporite outcrops cover approximately 7% of the total area of the country. Most of the evaporitic formations are made up of Ca-sulfates (gypsum/anhydrite) or Ca-sulfates and halite. Certain Paleogene marine evaporites also contain K-Mg-chlorides, and some Tertiary continental formations bear substantial amounts of Na-sulfates in the subsurface (glauberite and thenardite). Mesozoic evaporitic formations commonly wedge out towards the ground surface, passing into condensed sequences and dissolution-collapse breccias. Some of these highly porous breccias constitute major regional aquifers. In several areas, interstratal karstification of the evaporites has given rise to gravitational deformations such as basin structures, monoclines, and collapse structures covering several square kilometers that record a cumulative subsidence in excess of 200 m (Teruel and Calatayud Grabens). A widespread consequence of evaporite dissolution processes in Spain is the hydrochemical degradation of surface waters. Some of the largest and most outstanding lake systems, from an environmental perspective, occur in karstic depressions developed in evaporitic formations (Fuente de Piedra, Gallocanta, Bujaraloz, and Bañolas lakes). Sinkhole activity is a major geohazard in several evaporite karst areas. The sinkhole risk has a particularly high impact in sectors where Tertiary evaporites are overlain by Quaternary alluvial aquifers (Calatayud, Zaragoza, and Madrid areas). Some of the detrimental effects of subsidence include severe damage to historical monuments (Calatayud), the demolition of a whole village (Puilatos), or the derailment of a freight train (Zaragoza area). The deepest gypsum caves are found in Triassic diapiric structures (El Sumidor Cave, 210 m deep), and the longest ones are developed in horizontally lying Neogene sequences (Sorbas caves, and Estremera maze cave). The Cardona diapir hosts salt caves up to 4,300 m long whose genesis is related to flooding of mine galleries caused by the interception of a phreatic conduit. The main anthropogenic impacts on the endokarstic systems are related to the disposal of wastewaters and the destruction of caves by quarrying. The fluvial valleys that cross Tertiary evaporitic outcrops commonly show peculiar geological characteristics related to dissolution-induced synsedimentary subsidence phenomena: (1) Thickened alluvium filling dissolution basins up to several tens of kilometers long and more than 100 m deep. The largest thickenings are found in areas where the bedrock contains halite and glauberite. (2) Superimposed alluvial units locally bounded by angular unconformities. (3) Abundant deformational structures and paleosinkholes related to the rockhead and/or interstratal karstification of the substratum. These fluvial valleys typically are flanked by a prominent gypsum escarpment. Rock-falls favored by the dissolutional enlargement of joints derived from these scarps are the type of mass movement which has caused the highest number of casualties in Spain.  相似文献   

4.
This paper analyses the causes of the catastrophic failure of an earth dam that took place on 21 January 2001 during the first filling test in the NE of Spain (Altorricón village, Huesca Province). The San Juan reservoir, with a capacity of 850,000 m3, was built in 1999 on gypsiferous mantled pediment deposits overlying Tertiary dispersive clay sediments. The basin of the reservoir was excavated in the alluvial cover and Tertiary bedrock. An earth dam was constructed on the pediment surface along the perimeter of the artificial basin. The dam has a core of compacted clay material derived from the excavation, which is indented 1 m in the Tertiary shales of the bedrock, cutting the highly pervious alluvial mantle. Field observations, analysis of the basin and dam materials and eyewitnesses accounts have helped to infer the processes involved in the failure of the earth dam. These processes include (1) subsidence and ravelling (suffosion) processes induced by the dissolution of the 4-m-thick detrital cover with a gypsum content of around 40%; (2) piping processes affecting the embankments and core of the dam built with dispersive clays that have high exchangeable sodium percentage (ESP) and show active pipes; and (3) water circulation through biogenic burrows in the pediment deposits. This case study demonstrates the frequently hidden limitations that evaporite sediments and dispersive clay materials pose to the construction of dams. It also shows that highly permeable surficial deposits should be stripped before the construction of the dam structure.  相似文献   

5.
The central Ebro Basin comprises thick evaporite materials whose high solubility produces typically karstic landforms. The sinkhole morphology developed in the overlying alluvium has been studied using gravimetry and ground-penetrating radar (GPR) on stream terraces, as well as analyzing the evolution of sinkhole morphologies observed in aerial photographs taken in 1928, 1957, and 1985. The sinkhole morphologies give some idea of possible subsurface processes as well as an indication of the final mechanisms involve in sinkhole development. On stream terraces and cover pediments the most commonly encountered dolines are bowl-shaped in their morphology with both diffuse and scarped edges. In contrast, dolines developed in the gypsiferous silt infilled valleys have a funnel and well-shaped morphology. The diffuse-edged bowl-shaped dolines are developed through the progressive subsidence of the alluvial cover, due to washing down of alluvial particles through small voids and cracks into deeper subsurface caves, resulting in a decrease alluvial density. Future compaction of the alluvial cover will produce surface subsidences. This type of dolines are associated with negative gravity anomalies. In contrast, the scarped-edge dolines are formed by the sudden collapse of a cavity roof. The cavities and cracks formed in the gypsum karst may migrate to the surface through the alluvial deposits by piping, and they may subsequently collapse. In this instance, the cavities can be detected by both gravity and GPR anomalies where the voids are not deeper than 4–5 m from the surface. These processes forming sinkholes can be enhanced by man-induced changes in the groundwater hydrologic regime by both inflows, due to irrigation, ditch losses, or pipe leakages, and by outflows from pumping activities.  相似文献   

6.
库车坳陷古近系库姆格列木组是盆地内重要的含油气层段。本文应用高精度层序地层学的理论和方法，在库车坳陷古近系库姆格列木组地层中划分出3个三级层序、14个四级层序和数十个五级层序(准层序)。四级层序中发育有冲积扇、辫状河、曲流河、辫状河三角洲和滨浅湖等沉积体系，局部地区发育下切水道、干旱盐湖、泻湖—海湾、半深湖、浊流—重力流等沉积。该组下部四级层序：Psk1、Psk5和中上部四级层序Psk7、Psk12等的砂质沉积形成区内的主力储层，主要分布在盆地北—西北缘和东—东南缘，与其上覆的泥岩、膏泥岩形成有利的储盖组合。  相似文献   

7.
The deposits comprising 'valley-side fans' in a small intermontane basin of the Southern Alps are classified as debris flow, water-laid, intermediate and mixed deposits on the basis of particle size and clast orientation characteristics. Five varieties of debris flow deposit are identified including unimodal and bimodal 'mudflow gravels'. The fans comprise mainly unimodal mudflow gravels which although apparently similar to the mudflow gravels described from montane and periglacial environments are coarser, have less silt and clay and are better sorted than the mudflow deposits described from semiarid alluvial fans. Additions of airborne silt and fine sand to the fan catchments during later stages of fan building gave rise to bimodal debris flow deposits which appear similar to gravels described from cold-climate fans in Tasmania and classed as water-laid deposits. Braided stream deposits were added to the depositional sequence towards the closing stages of fan building indicating that the fan had become 'wetter'. In many places, however, the youngest Pleistocene fan deposits are silt-rich mixtons reflecting a peak in loess deposition.  相似文献   

8.
In the valley of the Ebro River to the southeast of the city of Zaragoza (NE Spain), the dissolution of evaporite sediments (gypsum, halite and Na-sulphates) which underlie alluvial deposits gives rise to numerous sinkholes. These sinkholes are a potential hazard to human safety, particularly where they develop in a catastrophic way. Even slow-developing sinkholes are problematic, as they damage urban and agricultural infrastructure, necessitating costly repairs and vigilant maintenance. To assist in developing avoidance strategies for these hazards, the factors controlling sinkhole occurrence have been assessed using geomorphological maps produced from aerial photographs for 1956 and 1981. Important controls on sinkhole development are found to include underlying geological structure (manifest in preferred orientations of sinkholes on the azimuths N130-150E and N30-40E), and the presence of glauberite in the groundwater flow path, which apparently promotes accelerated gypsum dissolution. Perhaps surprisingly, alluvium thickness does not appear to significantly correlate with the density of sinkholes on the floodplain in this area. The maps for 1956 and 1981 reveal that both human activity and natural processes can serve to obscure the true density of sinkhole development. For instance, a large number of sinkholes which were conspicuous in 1956 have since been back-filled by farmers. In the most fluvially active zone of the Ebro valley (the meander belt), the relatively low density of sinkholes compared with adjoining zones suggests that subsidence is being masked by morpho-sedimentary dynamic processes (aggradation and erosion). Careful geomorphological mapping for different time periods yields a much more accurate impression of the frequency of sinkhole development than would be gained from surveying currently visible sinkholes in the area of interest.  相似文献   

9.
The GIS approach to evaporite-karst geohazards in Great Britain   总被引：2，自引：0，他引：2  
Evaporite karst in Great Britain has formed in Permian and Triassic gypsum, and in Triassic salt. Active dissolution of these deposits can occur on a human rather than a geological timescale causing subsidence and building damage. The British Geological Survey has taken two approaches towards understanding and advising on hazards caused by dissolution of these soluble rocks. At a detailed level, a national database and GIS of karstic features is being populated. Information gathered includes dolines, springs, stream sinks, caves and building damage. At a national level, the soluble rocks in Great Britain have been identified and digital-map polygon information relating to them was extracted from the British 1:50,000-scale digital geological map. These areas have been assessed, and in places their margins extended to include some overlying rocks where subsidence features are known to penetrate upwards through the overlying sequence. The national areas have then been assessed using detailed local information to assign a susceptibility rating from A (extremely low) to E (high), depending on the nature and regularity of the subsidence events that occur. This national zonation of the soluble rocks can be used for planning, construction and in the insurance businesses. This has proved useful for assessing the potential stability of linear routes, such as roads and pipelines or for other important structures such as bridges and buildings. The information can also be used to delineate zones of karstic groundwater flow.  相似文献   

10.
This paper concerns the evaporite units, depositional systems, cyclicity, diagenetic products and anhydritization patterns of the Calatayud Basin (nonmarine, Miocene, central Spain). In outcrop, the sulphate minerals of these shallow lacustrine evaporites consist of primary and secondary gypsum, the latter originating from the replacement of anhydrite and glauberite. In the evaporative systems of this basin, gypsiferous marshes of low salinity can be distinguished from central, saline lakes of higher salinity. In the gypsiferous marsh facies, the dominant, massive, bioturbated gypsum was partly replaced by synsedimentary chert nodules and siliceous crusts. In the saline lake facies, either cycles of gypsiferous lutite‐laminated gypsarenite or irregular alternations of laminated gypsum, nodular and banded glauberite, thenardite and nodular anhydrite precipitated. Early replacement of part of the glauberite by anhydrite also occurred. Episodes of subaerial exposure are represented by: (1) pedogenic carbonates (with nodular magnesite) and gypsiferous crusts composed of poikilitic crystals; and (2) nodular anhydrite, which formed in a sabkha. Additionally, meganodular anhydrite occurs, which presumably precipitated from ascending, highly saline solutions. The timing of anhydritization was mainly controlled by the salinity of the pore solutions, and occurred from the onset of deposition to moderate burial. Locally, a thick (>200 m) sequence of gypsum cycles developed, which was probably controlled by climatic variation. A trend of upward‐decreasing salinity is deduced from the base to the top of the evaporite succession.  相似文献   

11.
The Lower Murrumbidgee alluvial fan at the eastern edge of the Murray Basin is comprised of high‐yielding coarse‐grained aquifers and interlayered fine‐grained deposits that exert an important control on recharge and vertical leakage of contaminants such as salt. Concerns over increasingly saline shallow groundwater, particularly in irrigation areas, has focused investigations on the depositional origin and spatial distribution of these fine‐grained deposits (aquitard units), which may constitute both a source of leachable salt and a barrier to leakage. Detailed laboratory analysis of a minimally disturbed core to 83m depth, obtained from a drillhole adjacent to an irrigation bore, was augmented with geophysical investigations from this and other boreholes near the apex of the alluvial fan. Previously mapped clay units (aquitards) are redefined as clayey silts based on clay content variation between 10% and 30%. Mineralogical and lithostratigraphic evidence for three clayey silt units is presented: a lower unit (75–83m), a middle unit (45–64m) and an upper unit (0–16m). Electrical image surveys indicate that the upper unit is discontinuous, interrupted by large palaeodrainage features probably containing sands and gravels. These palaeodrainage channels are buried beneath a veneer of clay and significantly increase recharge and leakage. Some evidence suggests an aeolian component near the surface and within the middle clayey silt unit. However, mixing with fluvial deposits and subsequent weathering has also occurred. The clayey silt units are extensively weathered and oxidised, with the degree of oxidation increasing towards the surface and adjacent to aquifers saturated with oxygenated groundwater. Post‐depositional weathering of the middle and lower units may also have been associated with leaching of salts. No salt remains in the middle and lower units, but 10.2kg/m2 is stored within 15m of the surface at the Tubbo site. The upper clayey silt unit is a significant source of leachable salt, which is associated with increasing shallow groundwater salinity at some sites.  相似文献   

12.
非海相盆地准层序级别的有机相变化及其地质意义   总被引：17，自引：0，他引：17  
柴达木盆地北缘侏罗系由非海相冲积-三角洲及湖泊沉积体系构成。托阿尔阶的沉积序列为下部的冲积体系向上过渡为滨浅湖、中深湖沉积体系,后者发育一系列在垂向上向上变粗-变浅的旋回层并构成准层序。在准层序级别,有机相参数发生规律性变化。岩石的有机质含量向上增加,显示与粉砂质含量呈正相关。其原因可能是冲积物源的植物碎屑和粉砂颗粒具有相似的水动力均衡条件。有机母质具有高等陆生植物和低等水生生物输入的双重旋回特征。成熟度居于临界状态 (镜质体反射率平均 0.57% )。自生的黄铁矿 /菱铁矿比与沉积物堆积速率之间存在一定的联系。湖泛面位于准层序的下部且直接上覆高有机质的暗色泥岩。  相似文献   

13.
Mitigation works are very essential for mitigation of debris-flow hazards in mountainous areas. Usually, it is difficult to assess the effectiveness of existing mitigation works in a catchment. This paper presented a method for quantitative assessment of debris flow mitigation measures by using Kanako system, a user-friendly GUI-equipped debris flow simulator that allows good visualization and easy interpretation. Kanako 2D (Ver. 2.04) was applied to a case study at Caijia Gully, Sichuan Province, China. Mitigation works including check dams, drainage channel, and deposition basin were constructed in the gully in 2001 and 2006. Kanako 2D can simulate debris flow from steep area to alluvial fan. 1D simulation was applied for assessing the effect of the check dams at the lower part of the gully, and 2D simulation was applied for the effect of the drainage channel and deposition basin on the alluvial fan. The simulation results indicate that debris flow will cause great damage to residential area on the alluvial fan if mitigation measures were not implemented in the gully. For old dams which have been filled up with deposits of previous debris flows, the results show that they still have the function for controlling debris flow due to the gradient reduction of the channel bed in front of the dams by the trapped debris flow deposition. After the comprehensive control of debris flow including trapping, drainage, and deposition in the gully, the simulation results indicate that the risk on the alluvial fan can be reduced to an acceptable level.  相似文献   

14.
The Salvan‐Dorénaz Basin formed during the Late Palaeozoic within the Aiguilles‐Rouges crystalline basement (Western Alps) as an asymmetric, intramontane graben elongated in a NE–SW direction and bounded by active faults. At least 1700 m of fluvial, alluvial fan and volcanic deposits provide evidence for a strong tectonic influence on deposition with long‐term, average subsidence rates of > 0·2 mm yr?1. The early basin fill was associated with coarse‐grained alluvial fans that were dominated by braided channels (unit I). These issued from the south‐western margin of the basin. The fans then retreated to a marginal position and were overlain by muddy floodplain deposits of an anastomosed fluvial system (unit II) that drained towards the NE. Deposition of thick muds resulted from a reduction in the axial fluvial gradient caused by accelerated tectonic subsidence. Overlying sand‐rich meandering river deposits (unit III) document a reversal in the drainage direction from the NE to the SW caused by synsedimentary tectonism, reflecting large‐scale topographic reorganization in this part of the Variscides with subsidence now preferentially in the W and SW and uplift in the E and NE. Coarse‐grained alluvial fan deposits (unit IV) repeatedly prograded into, and retreated from, the basin as documented by coarsening‐upward cycles tens of metres thick reflecting smaller scale tectonic cycles. Volcanism was active throughout the evolution of the basin, and U/Pb isotopic dating of the volcanic deposits restricts the time of basin development to the Late Carboniferous (308–295 Ma). 40Ar/39Ar ages of detrital white mica indicate rapid tectonic movements and exhumation of the nearby basement. In unit I, youngest ages are close to that of the host sediment, but the age spectrum is wide. In unit II, high subsidence and/or sedimentation rates coincide with very narrow age spectra, indicating small, homogeneous catchment areas. In unit III, age spectra became wider again and indicate growing catchment areas.  相似文献   

15.
Kerman city has a semiarid-arid climate with an average annual precipitation of about 158 mm. The area is underlain by soluble subsoil and alluvial deposits, overlying highly fractured Cretaceous limestones. Geo-environmental studies indicate that both paleokarst and active karst features are developed in the area. The paleokarsts were developed in the Upper Cretaceous limestone during the cold, humid periods of Post Cretaceous and probably Early Quaternary time and include honeycombs, solution flutes, rillenkarren, caverns, and solution collapse dolines. Active karst landforms occur by combined piping-induced and limestone solution at depth in subsoils, and alluvial deposits and bajada that overly potent karstic limestones and cover subsidence sinkholes and subjacent alluvial karst collapse dolines. Many factors, such as soluble compounds (salt and gypsum), desiccation cracks, and Qanat (dug water wells), could contribute to the development of karstic landforms. The most immediate cause for active karst landforms is considered to be the drawdown of the water table in the area. There is an increasing demand for groundwater consumption to irrigate pistachio fields. Excessive pumping of the groundwater lowers the water table about 80 cm per year. This rate of drawdown accelerates land subsidence (about 6 cm per year), creates circular patterns of fractures in the ground and in buildings, disrupts agricultural work and urbanization projects, and tilts foundations. These geohazards indicate that ground sinking and karstification are in progress in the alluvial deposits and underlying limestones. The disturbance and expense caused by the geohazards could be mitigated by the application of overhead sprinkler irrigation for pistachio fields or by planting less thirsty plants.  相似文献   

16.
人类活动影响下娘子关岩溶水系统地球化学演化   总被引：17，自引：5，他引：12  
王焰新  高旭波 《中国岩溶》2009,28(2):103-102
娘子关泉是我国北方最大的岩溶泉之一,也是阳泉市工农业生产和人民生活的重要供水水源。地下水地球化学演化分析表明,在地下水由补给区向排泄区运移过程中,除固有的水岩相互作用外,由于受采矿活动和地表水入渗补给的影响,岩溶水由低离子含量的HCO3-SO4或HCO3型水逐渐成为SO4型、SO4-HCO3型和SO4-HCO3-Cl型水。在泉群集中排泄区,区域流动系统与局部流动系统的地下水发生混合作用,最终形成了水质相对良好的HCO3-SO4型或SO4-HCO3型岩溶泉水。在此过程中,地下水对方解石和白云石也由最初的溶解作用演变为沉淀再结晶。尽管石膏呈持续溶解现象,但在采煤活动严重影响区域,石膏的沉淀也可能出现。地球化学模拟表明,在岩溶含水层中,地下水首先以方解石（白云石）的溶解为主;随着石膏溶解数量的增加,方解石（白云石）的溶解开始受到抑制,进而发生沉淀,石膏的溶解成为控制地下水水化学的主导过程。当矿坑水混入时,地下水相对石膏过饱和,地下水对碳酸盐岩含水介质的溶蚀能力得到增强。随着水岩反应的演进,铁氢氧化物大量沉淀,通过共沉淀和吸附作用去除了地下水中的重金属类污染物。   相似文献   

17.
The study consists of the identification of landforms subject to alluvial fan flooding in active sectors of the Zonda range piedmont. In the Department of Pocito, located about 5 km southwest of San Juan City, a series of alluvial fans have been identified. These alluvial fans are located downstream of the natural drainage basins covering an area of approximately 130 km^2 towards the eastern slopes of the Zonda range at a median elevation of 2,000 m a.s.l.  相似文献   

18.
冲积扇砂砾岩储层是准噶尔盆地一类重要的油气储层类型,由于其具岩相类型多、连续性差等特点,对冲积扇内部岩相成因解释一直是冲积扇相带认知的基础和难点。以准噶尔盆地西北缘现代白杨河冲积扇为例,在大量的野外露头资料和粒度分析数据的基础上,结合冲积扇源区母岩类型、水文资料以及冲积扇文献资料,对现代白杨河冲积扇岩相的类型、成因及分布规律进行探讨。按沉积机制,白杨河冲积扇属于辫状河型冲积扇,具有规模大(扇体总面积约327.6 km^2),坡度平缓(约1‰～7‰),沉积粒度粗等特征。在白杨河冲积扇内共可识别出16种岩相类型,并根据岩相形成的流体动力差异划归为5类成因,即重力流成因、高流态牵引流成因、低流态牵引流成因、静水沉积成因以及风成沉积成因。重力流以洪流沉积为主;高流态牵引流主要包括片流沉积和湍流沉积;低流态牵引流以砂(砾)质河道沉积为主;静水沉积以蓄水细粒沉积为主;风成沉积以风携细粒沉积为主。根据各岩相沉积构造、粒度特征及展布规模,可将岩相划分为四类:Ⅰ类岩相沉积构造特征明显并具有较大展布规模;Ⅱ类岩相沉积构造特征明显但展布规模局限;Ⅲ类岩相为不具层理构造但具有较大展布规模的岩相;Ⅳ类岩相不具层理构造并且展布规模局限。其中Ⅰ类和Ⅱ类岩相多为牵引流成因,多发育于洪水期扇体扇中、扇缘区域以及间洪期扇体的扇中区域,并可在地下继承性发育为较好的储集相带。  相似文献   

19.
Subsidence hazards due to evaporite dissolution in the United States   总被引：1，自引：4，他引：1  
Evaporites, including gypsum (or anhydrite) and salt, are the most soluble of common rocks; they are dissolved readily to form the same type of karst features that typically are found in limestones and dolomites, and their dissolution can locally result in major subsidence structures. The four basic requirements for evaporite dissolution to occur are: (1) a deposit of gypsum or salt; (2) water, unsaturated with CaSO4 or NaCl; (3) an outlet for escape of dissolving water; and (4) energy to cause water to flow through the system. Evaporites are present in 32 of the 48 contiguous states of the United States, and they underlie about 35–40% of the land area. Karst is known at least locally (and sometimes quite extensively) in almost all areas underlain by evaporites, and some of these karst features involve significant subsidence. The most widespread and pronounced examples of both gypsum and salt karst and subsidence are in the Permian basin of the southwestern United States, but many other areas also are significant. Human activities have caused some evaporite–subsidence development, primarily in salt deposits. Boreholes may enable (either intentionally or inadvertently) unsaturated water to flow through or against salt deposits, thus allowing development of small to large dissolution cavities. If the dissolution cavity is large enough and shallow enough, successive roof failures above the cavity can cause land subsidence or catastrophic collapse.  相似文献   

20.
There are two different dune systems in central Australia; regional quartz dunefields and transverse gypsiferous dunes associated with playa lakes. These two systems, especially gypsiferous dunes at Lake Amadeus, the largest playa in central Australia, provide a sedimentary, geomorphological and environmental history of the region during the late Quaternary. The gypsifierous dunes consist of a surficial gypcrete overlying an aeolian sediment sequence below, a mixture of gypsum sand and quartz sand. No clay pellets have been found in the dune sequence, in significant contrast to the gypsiferous clay dunes in other parts of Australia. Three possible models of the environmental controls of gypsiferous dune formation are discussed. The most plausible one suggests simultaneous gypsum precipitation and deflation. Sandsized gypsum was precipitated in a groundwater-seepage zone around the playa margin during seasonally high water-tables and these crystals were deflated onto land during dry intervals, forming the marginal gypsiferous dunes. These processes require conditions of high regional water-table, strong climatic seasonality and probably a windier and overall wetter climate. At least two separate gypsiferous-duneforming episodes can be recognized. The age of formation of the younger one has been dated by thermoluminescence at 44–54 ka. The gypcrete crust capping the dunes is characterized by intergrown microcrystalline gypsum crystals, showing evidence of leaching, dissolution and recrystallization. It is interpreted as a pedogenic product formed during a stable period after accumulation of the gypsiferous dune. After the construction of the younger gypsiferous dune, there was a major episode of activation of regional quartz dunefields which formed thick quartz sand mantles overlying gypsiferous dunes on both playa margins and the dune islands within the playa. An equivalent aeolian sand layer was deposited within the playa. Soil structures in this unit indicate that the sand sheet over the playa was later colonized by vegetation. Activation of the regional dunefields suggests a major period of dry climate, which, although not dated, may correlate with the last glacial maximum identified as a period of maximum aridity from 25 to 18 ka at other sites in Australia.  相似文献   

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