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991.
Hema Achyuthan Nagalakshmi Thirunavukarasu 《Journal of the Geological Society of India》2009,73(5):683-696
In this paper we present Quaternary stratigraphy of the area around Chennai based on archaeological findings on the ferricrete
surface, geomorphological observations supplemented by radiocarbon dating. The coastal landscape around Chennai, Tamil Nadu,
has preserved ferruginised boulder gravel deposits, ferricretes and fluvial deposits of varying thickness. The area studied
is approximately 150 km east to west and 180 km north to south with a broad continental shelf towards the seaward. Several
rivers enter the Bay of Bengal along its shores like the Koratallaiyar, Cooum and the Adyar. Precambrian charnockite and Upper
Gondwana sandstone and shale bedrock rim the shelf margin. For the most part, the Upper Pleistocene-Holocene fluvial sediments
overlie an erosion surface that has cut into older Pleistocene sediments and ferricrete surface. Incised valleys that cut
into this erosion surface are up to 5–6 km wide and have a relief of at least 30 m. The largest valley is that cut by the
Koratallaiyar River. Holocene sediments deposited in the incised valleys include fluvial gravels, early transgressive channel
sands and floodplain silts. Older Pleistocene sediments are deposited before and during the 120-ka high stand (Marine isotope
stage 5). They consist of ferricretes and ferricrete gravel formed in nearshore humid environments. Muddy and sandy clastic
sediments dated to the ca. 5 ka highstand suggest that the climate was semi arid at this time with less fluvial transport.
The coarsening up sequence indicates deposition by high intensity channel processes. Pedogenic mottled, clayey silt unit represents
an important tectonic event when the channel was temporarily drained and sediment were sub aerially exposed. Uplift of the
region has caused the local rivers to incise into the landscape, forming degradation terraces. 相似文献
992.
We estimated the long-term vertical velocity profile across the northeastern Japan forearc by using the height distribution of late Quaternary marine and fluvial terraces, and we correlated the ages of the two marine terraces with marine isotope stages (MIS) 5.5 and 5.3 or 5.1 by cryptotephra stratigraphy. The uplift rate, estimated as 0.11-0.19 m ka− 1 from the relative heights between the terrace surfaces and eustatic sea levels, was nearly equal to, or slightly slower than, the uplift rate farther inland (0.15-0.19 m ka− 1), as determined from the relative heights of fill terrace surfaces. In contrast, the short-term vertical velocity profile, obtained from GPS observations, showed that the forearc is currently subsiding at a maximum rate of 5.4 ± 0.4 mm yr− 1. Thus, the current short-term (geodetic) subsidence does not reflect long-term (geological) tectonic movement. Short-term vertical deformation is probably driven by subduction erosion or elastic deformation caused by interplate coupling, or both. However, long-term uplift is probably due not to moment release on the mega-thrust but to crustal thickening. 相似文献
993.
MARIO MORELLÓN BLAS VALERO-GARCÉS FLAVIO ANSELMETTI† DANIEL ARIZTEGUI‡ MICHAEL SCHNELLMANN§ ANA MORENO¶ PILAR MATA MAYTE RICO JUAN PABLO CORELLA 《Sedimentology》2009,56(5):1505-1534
Lake Estanya is a small (19 ha), freshwater to brackish, monomictic lake formed by the coalescence of two karstic sinkholes with maximum water depths of 12 and 20 m, located in the Pre‐Pyrenean Ranges (North‐eastern Spain). The lake is hydrologically closed and the water balance is controlled mostly by groundwater input and evaporation. Three main modern depositional sub‐environments can be recognized as: (i) a carbonate‐producing ‘littoral platform’; (ii) a steep ‘talus’ dominated by reworking of littoral sediments and mass‐wasting processes; and (iii) an ‘offshore, distal area’, seasonally affected by anoxia with fine‐grained, clastic sediment deposition. A seismic survey identified up to 15 m thick sedimentary infill comprising: (i) a ‘basal unit’, seismically transparent and restricted to the depocentres of both sub‐basins; (ii) an ‘intermediate unit’ characterized by continuous high‐amplitude reflections; and (iii) an ‘upper unit’ with strong parallel reflectors. Several mass‐wasting deposits occur in both sub‐basins. Five sediment cores were analysed using sedimentological, microscopic, geochemical and physical techniques. The chronological model for the sediment sequence is based on 17 accelerator mass spectrometry 14C dates. Five depositional environments were characterized by their respective sedimentary facies associations. The depositional history of Lake Estanya during the last ca 21 kyr comprises five stages: (i) a brackish, shallow, calcite‐producing lake during full glacial times (21 to 17·3 kyr bp ); (ii) a saline, permanent, relatively deep lake during the late glacial (17·3 to 11·6 kyr bp ); (iii) an ephemeral, saline lake and saline mudflat complex during the transition to the Holocene (11·6 to 9·4 kyr bp ); (iv) a saline lake with gypsum‐rich, laminated facies and abundant microbial mats punctuated by periods of more frequent flooding episodes and clastic‐dominated deposition during the Holocene (9·4 to 0·8 kyr bp ); and (v) a deep, freshwater to brackish lake with high clastic input during the last 800 years. Climate‐driven hydrological fluctuations are the main internal control in the evolution of the lake during the last 21 kyr, affecting water salinity, lake‐level changes and water stratification. However, external factors, such as karstic processes, clastic input and the occurrence of mass‐flows, are also significant. The facies model defined for Lake Estanya is an essential tool for deciphering the main factors influencing lake deposition and to evaluate the most suitable proxies for lake level, climate and environmental reconstructions, and it is applicable to modern karstic lakes and to ancient lacustrine formations. 相似文献
994.
EMMANUELLE DUCASSOU SÉBASTIEN MIGEON THIERRY MULDER ANNE MURAT LUCILLA CAPOTONDI STEFANO M. BERNASCONI JEAN MASCLE 《Sedimentology》2009,56(7):2061-2090
This paper presents an overview of the evolution of the Nile deep‐sea turbidite system during the last 200 kyr, over a series of glacial to interglacial cycles. Six individual deep‐sea fans were identified from an extensive field data set. Each fan comprises a canyon, channel system and terminal lobes. Two of these fan systems were possibly active at the same time, at least during some periods. Large‐scale slope failures destroyed channel segments and caused the formation of new submarine fan systems. These slope failures thus played an important role in the overall evolution of the turbidite system. During the last glacial maximum (ca 25 to 14·8 ka) the central and eastern parts of the Nile deep‐sea turbidite system were relatively inactive. This inactivity corresponds to a lowstand in sea‐level, and a period of arid climate and relatively low sediment discharge from the Nile fluvial system. Rapid accumulation of fluvial flood‐derived deposits occurred across the shallower part of the submarine delta during sea‐level rise between ca 14·8 and 5 ka. The most recent deep‐sea channel–lobe system was very active during this period of rising sea‐level, which is also associated with a wetter continental climate and increased sediment and water discharge from the Nile. Increased sediment deposition in shallower water areas led to occasional large‐scale slope failure. The Nile deep‐sea turbidite system was largely inactive after ca 5 ka. This widespread inactivity is due to retreat of the coastline away from the continental shelf break, and to a more arid continental climate and reduced discharge of sediment from the Nile. The Nile deep‐sea turbidite system may be more active during periods of rising and high sea‐level associated with wetter climates, than during lowstands, and may rapidly become largely inactive during highstands in sea‐level coupled with arid periods. These acute responses to climate change have produced sedimentary/stratigraphic features that diverge from traditional sequence models in their nature and timing. This large‐scale sedimentary system responded to monsoon‐driven climate change and sea‐level change in a system‐wide and contemporaneous manner. 相似文献
995.
996.
Medhat A. El-Bihery 《Environmental Geology》2009,58(5):1095-1105
Recently, Ras Sudr (the delta of Wadi Sudr) area received a great amount of attention due to different development expansion
activities directed towards this area. Although Quaternary aquifer is the most prospective aquifer in Ras Sudr area, it has
not yet completely evaluated. The present work deals with the simulation of the Quaternary groundwater system using a three-dimensional
groundwater flow model. MODFLOW code was applied for designing the model of the Ras Sudr area. This is to recognize the groundwater
potential as well as exploitation plan of the most prospective aquifer in the area. The objectives were to determine the hydraulic
parameters of the Quaternary aquifer, to estimate the recharge amount to the aquifer, and to determine the hydrochemistry
of groundwater in the aquifer. During this work, available data has been collected and some field investigation has been carried
out. Groundwater flow model has been simulated using pilot points’ method. SEAWAT has been also applied to simulate the variable-density
flow and sea water intrusion from the west. It can be concluded that: (1) the direction of groundwater flow is from the east
to the west, (2) the aquifer system attains a small range of log-transformed hydraulic conductivity. It ranges between 3.05
and 3.35 m/day, (3) groundwater would be exploited by about 6.4 × 106 m3/year, (4) the estimated recharge accounts for 3 × 106 m3/year, (5) an estimated subsurface flow from the east accounts for 2.7 × 106 m3/year, (6) the increase of total dissolved solids (TDS) most likely due to dilution of salts along the movement way of groundwater
from recharge area to discharge area in addition to a contribution of sea water intrusion from the west. Moreover, it is worth
to note that a part of TDS increase might be through up coning from underlying more saline Miocene sediments. It is recommended
that: (1) any plan for increasing groundwater abstraction is unaffordable, (2) reliable estimates of groundwater abstraction
should be done and (3) automatic well control system should be made. 相似文献
997.
The sea level rise has its own-bearing on the coastal recession and hydro-environmental degradation of the River Nile Delta.
Attempts are made here to use remote sensing to detect the coastal recession in some selected parts and delineating the chemistry
of groundwater aquifers and surface water, which lie along south-mid-northern and coastal zone of the Nile Delta. Eight water
samples from groundwater monitoring wells and 13 water samples from surface water were collected and analyzed for various
hydrochemical parameters. The groundwater samples are classified into five hydrochemical facies on Hill-Piper trilinear diagram
based on the dominance of different cations and anions: facies 1: Ca–Mg–Na–HCO3–Cl–SO4 type I; facies 2: Na–Cl–HCO3 type II; facies 3: Na–Ca–Mg–Cl type III, facies 4: Ca–Na–Mg–Cl–HCO3 type IV and facies 5: Na–Mg–Cl type V. The hydrochemical facies showed that the majority of samples were enriched in sodium,
bicarbonate and chloride types and, which reflected that the sea water and tidal channel play a major role in controlling
the groundwater chemical composition in the Quaternary shallow aquifers, with a severe degradation going north of Nile Delta.
Also, the relationship between the dissolved chloride (Cl, mmol/l), as a variable, and other major ion combinations (in mmol/l)
were considered as another criterion for chemical classification system. The low and medium chloride groundwater occurs in
southern and mid Nile Delta (Classes A and B), whereas the high and very high chloride (classes D and C) almost covers the
northern parts of the Nile Delta indicating the severe effect of sea water intrusion. Other facets of hydro-environmental
degradation are reflected through monitoring the soil degradation process within the last two decades in the northern part
of Nile Delta. Land degradation was assessed by adopting new approach through the integration of GLASOD/FAO approach and Remote
Sensing/GIS techniques. The main types of human induced soil degradation observed in the studied area are salinity, alkalinity
(sodicity), compaction and water logging. On the other hand, water erosion because of sea rise is assessed. Multi-dates satellite
data from Landsat TM and ETM+ images dated 1983 and 2003 were used to detect the changes of shoreline during the last two
decades. The obtained results showed that, the eroded areas were determined as 568.20 acre; meanwhile the accreted areas were
detected as 494.61 acre during the 20-year period. 相似文献
998.
WANG An WANG Guocan LI Dewei XIE Defan LIU Demin Faculty of Earth Sciences China University of Geosciences Wuhan Hubei China State Key Laboratory of Geological Processes Mineral Resources China 《《地质学报》英文版》2009,83(1)
The Hohxil region in the northern Qinghai-Tibet Plateau is occupied by numerous plateau lakes,which have long been inferred as being tectonic products.However,so far little evidence has been found to support this tentative inference.Field survey and morphotectonic analysis of TM satellite images in the eastern segment of the Hohxil region revealed that Kusai Lake and Yelusu Lake are S- shaped pull-apart basins,which were dominated by left strike-slip master faults trending WNW-ESE. The pull-apart distanc... 相似文献
999.
粒度和磁化率是两个研究黄土古气候最常用的古环境变化指示参数,它们随着黄土古土壤地层变化而出现峰和谷的对应已经被证明是反映了天文轨道要素的周期变化。文章试图忽略这些受控于轨道要素的气候周期变化,而主要侧重考察黄土地层这两个参数的平均值(或背景值)所反映的长期变化趋势。对兰州九州台黄土进行了系统采样和测量,发现兰州九州台黄土剖面粒度和磁化率曲线显示出两个明显的趋势,粒度从剖面底部向上有明显逐渐变粗趋势,指示着冬季风增强,与此同时,磁化率自下而上却逐渐增大,指示着夏季风增强的趋势。与黄土高原其他黄土剖面磁化率和粒度曲线对比发现,这是两个普遍存在的趋势。地理位置靠近青藏高原的剖面,这两个增大的趋势更明显。冬、夏季风同时逐渐增强是海陆热力差异增大所引起,反映了青藏高原第四纪时期的逐渐不断的隆升过程。因此,根据粒度和磁化率曲线变化趋势线的变化特点可以帮助分析和反推第四纪以来青藏高原隆升的过程。兰州九州台以及黄土高原各剖面粒度和磁化率曲线的线性变化趋势则可能指示着第四纪以来青藏高原是逐渐均匀缓慢的变化过程。我们对22Ma以来风积地层记录的变化趋势也做了分析。前人过去普遍认识的第四纪以来跳跃式或间歇式剧烈隆升在我们的数据中没有得到反映。黄土高原西部西宁、兰州、靖远等剖面磁化率显著的增长趋势可能与青藏高原隆升到一定高度后高原季风加强所致。 相似文献
1000.
编辑按 杨怀仁先生是我国著名的地貌学与第四纪地质学家,在地貌与第四纪科学研究、教学和人才培养方面做出了杰出贡献。前不久,杨先生因病逝世。《第四纪研究》特刊载南京大学地理与海洋科学学院杨达源教授等撰写的纪念短文,以表达学界对先生的缅怀之情。 相似文献