Hydro-geomorphic hazards and impact of man-made structures during the catastrophic flood of June 2000 in the Upper Guil catchment (Queyras, Southern French Alps)

The Guil River Valley (Queyras, Southern French Alps) is prone to catastrophic floods, as the long historical archives and Holocene sedimentary records demonstrate. In June 2000, the upper part of this valley was affected by a “30-year” recurrence interval (R.I.) flood. Although of lower magnitude and somewhat different nature from that of 1957 (>100-year R.I. flood), the 2000 event induced serious damage to infrastructure and buildings on the valley floor. Use of methods including high-resolution aerial photography, multi-date mapping, hydraulic calculations and field observations made possible the characterisation of the geomorphic impacts on the Guil River and its tributaries. The total rainfall (260 mm in four days) and maximum hourly intensity (17.3 mm h⁻¹), aggravated by pre-existing saturated soils, explain the immediate response of the fluvial system and the subsequent destabilisation of slopes. Abundant water and sediment supply (landsliding, bank erosion), particularly from small catchment basins cut into slaty, schist bedrock, resulted in destructive pulses of debris flow and hyperconcentrated flows. The specific stream power of the Guil and its tributaries was greater than the critical stream power, thus explaining the abundant sediment transport. The Guil discharge was estimated as 180 m³ s⁻¹ at Aiguilles, compared to the annual mean discharge of 6 m³ s⁻¹ and a June mean discharge of 18 m³ s⁻¹. The impacts on the Guil valley floor (flooding, aggradation, generalised bank erosion and changes in the river pattern) were widespread and locally influenced by variations in the floodplain slope and/or channel geometry. The stream partially reoccupied former channels abandoned or modified in their geometry by various structures built during the last four decades, as exemplified by the Aiguilles case study, where the worst damage took place. A comparative study of the geomorphic consequences of both the 1957 and 2000 floods shows that, despite their poor maintenance, the flood control structures built after the 1957 event were relatively efficient, in contrast to unprotected places. The comparison also demonstrates the role of land-use changes (conversion from traditional agro-pastoral life to a ski/hiking-based economy, construction of various structures) in reducing the Guil channel capacity and, more generally, in increasing the vulnerability of the human installations. The efficiency of the measures taken after the 2000 flood (narrowing and digging out of the channel) is also assessed. Final evaluation suggests that, in such high mountainous environments, there is a need to keep most of the 1957 flooded zone clear of buildings and other structures (aside from the existing villages and structures of particular economic interest), in order to enable the river to migrate freely and to adjust to exceptional hydro-geomorphic conditions without causing major damage.     

Groundwater flow regime under natural conditions as inferred from past evidence and contemporary field observations in a semi-arid basin: Cuenca de la Independencia, Guanajuato, México

This study was carried out in the Cuenca de la Independencia, a semi-arid basin in Central Mexico. The objective is to describe the main features of a groundwater flow regime under natural conditions, based on groundwater discharge manifestations. Information obtained from paleoecological, paleontological, archaeological and historical data suggests that, prior to heavy development (starting in the 1950s), the hydrogeologic regime was characterized by a larger groundwater availability in a more humid and colder climate. Manifestations associated to groundwater discharges are springs, lagoons, wetlands, saline soils, chalcedony deposits, phreatophytes, thermalism, and artesianism. The different types of manifestations and their position in the basin indicate the influence of groundwater flow systems hierarchically nested, forming concentric zones at ground level. The groundwater flow regime corresponds to a classical gravity-induced flow system with generation of local, intermediate and regional patterns. Integrating several types of data to establish the flow geometry and its dynamics has proven a useful tool to increase understanding of the original groundwater regimes. This approach can also be applied in other over-exploited semi-arid basins.     

The lower marine to lower freshwater Molasse transition in the northern Alpine foreland basin (Oligocene; central Switzerland–south Germany): age and geodynamic implications

The Wilhelmine Alpe section near Immenstadt (Allgäu, south Germany), which represents one of the best continuously exposed outcrops within the northern Alpine foreland basin, has been analyzed for magnetostratigraphic and palynostratigraphic signals. The section comprises the marine-to-terrestrial transition from Lower Marine (UMM) to Lower Freshwater Molasse (USM) sediments. Based on the correlation of the local magnetic pattern with the geomagnetic polarity timescale (GPTS) and palynostratigraphic data, an age of about 31 Ma is suggested for the UMM–USM transition in the Wilhelmine Alpe section. A comparison with coeval magnetostratigraphic sections from central and eastern Switzerland indicates that the regression of the UMM sea along the southern margin of the Molasse basin occurred strongly heterochronously between 31.5 and 30 Ma. The heterochroneity is attributed to the deposition of fan-delta and alluvial fan sediments which document that the overall marine conditions during the UMM were accompanied by strong clastic input derived from the rising Alps. This clastic contribution had a much stronger influence on the depositional pattern than previously thought.     

Identifying sources of groundwater recharge in the Merguellil basin (Tunisia) using isotopic methods: implication of dam reservoir water accounting

Thirty-two groundwater samples collected from the Merguellil Wadi basin (central Tunisia) complemented by the Haouareb dam reservoir water samples have been isotopically analysed in order to investigate the implication of the reservoir water to recharging the aquifer, and also to infer the sources, relative ages and mixing processes in the aquifer system. Plots of the stable isotopes data against the local meteoric lines of Tunis-Carthage and Sfax indicate a strong implication of the dam water noticeable up to a distance of 6–7 km. A contribution as much as 80% of the pumped water has been evidenced using isotopic mass balance. In addition, poorly distinguished water clusters in the stable isotope plots, but clearly identified in the diagrams δ¹⁸O versus ³H and ³H versus ¹⁴C, indicate various water types related to sources and timing of recharge. The isotopic signatures of the dam accounting water, the “old” and “native” recharged waters, have been evidenced in relation to their geographical distribution and also to their radiogenic isotopes (³H and ¹⁴C) contents. In the south-western part of the aquifer, mixing process occurs between the dam reservoir water and both the “old” and “native” water components.     

Sedimentary aspects and paleoenvironmental evolution of a rift basin: Salta Group (Cretaceous–Paleogene), northwestern Argentina

The rift history of the Salta basin is related to the evolution of the Central Andes and to the activity of the Pacific margin, owing to its geographic location. Sedimentation occurred from the Neocomian to the Paleogene, with deposits reaching up to 5,000 m in thickness. Paleoenvironmental analysis reveals an evolutionary history controlled by tectonic and climatic changes. Isolated grabens characterized the early synrift stage; differential subsidence provoked distinct environments in the southern and northern subbasins. In the southern subbasins, alluvial-fan, fluvial-fan and lacustrine deposits prevail, whilst in the northern subbasins eolian and fluvial environments dominate. During the Maastrichtian, two major factors controlled the basin fill: the decrease in tectonic subsidence and a relative sea-level rise as recorded in South America. An extensive and shallow Atlantic marine ingression installed a carbonate system coincident with mainly humid conditions until the Danian. Until the Middle Eocene, the fluvial and lacustrine environmental evolution of the sag basin was controlled especially by the alternation of temperate with dry and humid periods. Paleontological records reflect these climatic changes and show their relationship to the sedimentation regime.     

中国中西部前陆盆地的地质特征及油气聚集

中国中西部前陆盆地具有十分丰富的油气资源,近年来油气勘探也取得了很大的成果,但总体上来说其地质情况复杂、研究程度较低。世界上典型的前陆盆地一般位于造山带和稳定的克拉通之间的狭长槽地,而在中国中西部则主要为与古特提斯造山带在新特提斯阶段再活动有关的陆内会聚形成的“再生”前陆盆地,因此与世界典型前陆盆地相比中国中西部前陆盆地具有一定的“特殊性”。根据两期前陆的叠合程度差异造成的盆地几何形态、挠曲沉降、地层层序、沉积充填、构造变形特征的特点,将中国中西部前陆盆地归纳为改造型、新生型、叠加型和早衰型四种组合类型的前陆盆地。中西部不同组合类型前陆盆地具有不同的结构特征,决定了不同组合类型前陆盆地成藏特征和油气勘探潜力上的差异。     

江西贵溪盆地罗塘群沉积体系特征及其地质意义

早白垩世晚期江西贵溪盆地的控盆断裂活动加强,在盆地边缘的不同地带及盆地中心发育有不同的沉积体系,野外露头岩相特征和剖面的研究表明,贵溪盆地罗塘群主要发育有冲积扇体系、扇三角洲体系、辫状河三角洲体系、湖泊沉积体系及辫状河沉积体系,沉积体系特征主要受控于盆缘断裂活动,不同的沉积体系出现在盆地演化的不同阶段,并由不同的岩相组成.沉积体系的特征、分布及组合型式反映了陆相断陷红盆沉积相带窄、横向及纵向上相变快的特点.     

青藏高原北部温泉活动沉积盆地的沉积特征及其地质意义

晚新生代温泉沉积盆地,是青藏高原腹地在南北向挤压、东西向伸展的构造背景下,沿南北向边界走滑断层,经边界正断层和内部张剪断层的进一步发展而形成的近南北向单断单剪楔形半地堑活动沉积盆地。它可能代表了晚新生代青藏高原第三期强烈挤压隆升事件,是侧向向东剪切挤出的结果。笔者以盆地充填序列和TL、ESR测年资料为主要依据,推测唐古拉山在30 0～2 5 0ka前后全面进入冰冻圈;而以温泉活动沉积盆地为代表的中更新世晚期(2 2 4 .0～1 5 0 .2ka)的冰碛 冰水堆积则对应于青藏高原第三期隆升的断陷盆地发育阶段;中更新世晚期—晚更新世中期(1 4 4 .0～5 6ka)为湖相沉积;晚更新世中期至今(35～0ka)对应于高原缓慢隆升与夷平发育阶段。长江溯源在35ka切割通天河盆地,形成通天河;而在1 6ka侵蚀切穿雁石坪 温泉兵站峡谷,形成布曲河。     

北黄海盆地构造几何学研究新进展

北黄海盆地是发育于隆起背景之上的中、新生代沉积盆地。新一轮资源调查研究表明，北黄海中、新生代沉积盆地的基底由古生界沉积岩层和前寒武纪变质岩系等组成，盆地不同程度地发育下构造层（J3-K1）、中构造层（E2-E3）和上构造层（N）；从油气资源和中、新生代地层发育情况出发，将北黄海海域划分为辽东-海洋岛隆起区、北黄海盆地和胶北．刘公岛隆起区等3个一级构造单元，其中北黄海盆地包括6个二级构造单元和24个三级构造单元；盆地褶皱、断裂构造十分发育，褶皱构造可划分为区域挤压型、局部伴生型和披盖型等三类，断裂构造主要可见近EW—NE向、NW向和NNE向三组，其中近EW—NE向和NNE向断裂比较发育，控制着盆地隆、坳分布格局和沉积特征。