Geodetic and GIS tools for dam safety: case of Sidi Salem dam (northern Tunisia)

The dams are one of the important engineering structures used for water supply and different other important purposes. In order to provide safety, well-planned and implemented geodetic monitoring is very essential for such engineering structures, which constitute the aim of this study. In fact, micro-geodetic data can be exploited and used to survey the displacement of dam by measuring the horizontal and vertical coordinates at different dates from fixed positions (pillars) to other positions (targets). As a result, the superposition of the geodetic auscultation charts and the factors which can affect the stability of dams such as water level, hydrostatic pressure, and seismic state show a net correlation between these parameters and the auscultation variation at different periods. Geographical information system (GIS) techniques are used for mapping spatial variation of auscultation measures. These synthetic documents can help managers and decision makers in hydraulic management and water system protection.     

Recharge mode and mineralization of groundwater in a semi-arid region: Sidi Bouzid plain (central Tunisia)

Groundwater is the most important source of water supply in Sidi Bouzid plain located in central Tunisia. Proper understanding of the geochemical evolution of groundwater is important for sustainable development of water resources in this region. A hydrogeochemical survey was conducted on the Mio–Plio–Quaternary aquifer system using stable isotopes, radiocarbon, tritium and major elements, in order to evaluate the groundwater chemistry patterns and the main mineralization processes occurring in this system. The chemical data indicate that dissolution of evaporate minerals and evaporation are the main processes controlling groundwater mineralization. The isotopic data show that groundwater in the study area is a mixture of recent shallow waters located upstream and along Wadi Al Fakka bed and paleowaters located towards plain limits and discharge areas. Low ³H and ¹⁴C contents are observed in major part of the plain indicating that recharge of the aquifer occurs mainly through direct infiltration at Wadi Al Fakka while there is no evidence of significant recharge in major part of the plain and mountains piedmonts.     

Sulfur and lead isotopes of Guern Halfaya and Bou Grine deposits (Domes zone,northern Tunisia): Implications for sources of metals and timing of mineralization

The Pb–Zn ore deposits in the Guern Halfaya and Bou Grine areas (northern Tunisia) are hosted mainly by dolostones in the contact zone between Triassic and Upper Cretaceous strata and by Upper Cretaceous limestones. The deposits occur as lenticular, stratiform, vein, disseminations and stockwork ore bodies consisting of sphalerite, galena, pyrite, chalcopyrite and sulfosalt (gray copper). Barite and celestite dominate the gangue, with lesser calcite. The δ³⁴S values of barite and celestite (12.7–15.0‰) at the Oum Edeboua mine are consistent with the reduction of sulfates in Triassic evaporites within the study area (12.8 < δ³⁴S < 14.0‰). The δ³⁴S values in base-metal sulfides from both study areas (2.6–9.5‰) and the presence of bacterial relics suggest involvement of bacterially-mediated sulfate reduction in the mineralization. The present Pb isotope data are homogeneous with ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of 18.723–18.783, 15.667–15.685 and 38.806–38.889, respectively, which suggest a single source reservoir of Pb at depth in the upper crust. The syn-diagenetic mineralization in the Bahloul Formation and the calculated age from the Pb isotopic data suggest an Upper Cretaceous age for the Pb–Zn deposits in the Guern Halfaya and Bou Grine areas. During this period, NE–SW to ENE–WSW trending regional extensional tectonic structures likely favored migration of mineralizing fluids and eventual deposition at Guern Halfaya and Bou Grine.     

Evaluation of Sentinel-1 data for flood mapping in the upstream of Sidi Salem dam (Northern Tunisia)

Flood mapping is a powerful asset that allows drawing better strategies to contain possible economic repercussions and to rescue the affected population. This work is directly unfolded after the rainfall events that occurred in the north of the country, in February 2015, during which certain cities located in the vicinity of the Tunisian basin of Medjerda were flooded by the overflow of the Medjerda river, causing important damage to the towns of Jendouba and Bou Salem. The present research illustrates the potentiality of Sentinel-1 sensor in detecting flood areas in the upstream of Medjerda river. The Medjerda is the most important river in Tunisia, with an annual water potential reaching 0.8 billion m³. We compared the signature of flood water in vertical transmit and horizontal received (VH) and vertical transmit and vertical received (VV) polarizations of radar data. The study proves that the segregation of land/water areas with a threshold technique is better observed in VH polarization rather than VV polarization.     

Assessing groundwater vulnerability to nitrate pollution using statistical approaches: a case study of Sidi Bouzid shallow aquifer,Central Tunisia

The study region comprises the Sidi Bouzid shallow aquifer, which is located in the western part of Central Tunisia. It is mainly occupied by agricultural land with intensive use of chemical fertilizers especially nitrates. For this reason, nitrate measurement was performed in 38 water samples to evaluate and calibrate the obtained models. Several environmental parameters were analyzed using groundwater nitrate concentrations, and different statistical approaches were applied to assess and validate the groundwater vulnerability to nitrate pollution in the Sidi Bouzid shallow aquifer. Multiple linear regression (MLR), analyses of covariance (ANCOVA), and logistic regression (LR) were carried out for studying the nitrate effects on groundwater pollution. Statistical analyses were used to identify major environmental factors that control the groundwater nitrate concentration in this region. Correlation and statistical analyses were conducted to examine the relationship between the nitrate (dependent variable) and various environmental variables (independent variables). All methods show that “groundwater depth” and “land use” parameters are statistically significant at 95% level of confidence. Groundwater vulnerability map was obtained by overlaying these two thematic layers which were obtained in the GIS environment. It shows that the high vulnerability area coincides with the likelihood that nitrate concentration exceeds 24.5 mg/l in groundwater. The relationship between the groundwater vulnerability classes and the nitrate concentrations provides satisfactory results; it showed an Eta-squared correlation coefficient of 64%. So, the groundwater vulnerability map can be used as a synthetic document for realistic management of groundwater quality.     

Landscape response to the deposition of airfall pyroclastics from large explosive eruptions: An example from the campanian area (Southern Italy)

We present in this work data about a peculiar type of alluvial fans formed exclusively by volcaniclastic material from large explosive eruptions. Each alluvial fan results formed by the superimposition of several sedimentary bodies, each of them formed by quite homogeneous volcaniclastic material. Lithological analyses allow us to correlate each sedimentary body with the emplacement of pyroclastics from Phlegrean Fields or Somma-Vesuvius. The development of these alluvial fans is controlled by three main factors: area of dispersion of pyroclastics, morphological features of the supplying basin (i.e. mean slopes and area) and climate. Finally, we present an assessment of the minimum time needed for the complete remobilization of pyroclastics of Pomici di Base and Greenish eruptions.     

Cenozoic algal biostromes in the eastern Veneto (northern Italy): a possible example of non-tropical carbonate sedimentation

The rhodolith-bearing biostromes described in this paper form part of an episode of exclusively carbonate sedimentation, restricted in time (Aquitanian) but relatively extensive in space, within a molasse sedimentation basin. The biostromes correspond to an algal biocoenosis on a bar or structural high, are of cyclic character, and make up a minor sequence within the Miocene molasse megasequence.

The foramol skeletal assemblage, paleogeographic conditions of the area, and resemblance of the deposits to other documented mid-latitude limestones suggest that the biostromes are examples of non-tropical carbonate sedimentation.

Trace element contents (Mg, Sr, Mn, Fe) show two distinct diagenetic phases. The first was due to active circulation of oxygenated solutions in a phreatic marine environment, the second to poor circulation of reducing solutions in a fresh-water phreatic environment.     

Geochemical response to varying tectonic settings: an example from southern Sulawesi (Indonesia)

The South arm of Sulawesi was an active continental margin from approximately 60 to 10 Ma, when it collided with the microcontinental fragment of Buton. Pre-collisional samples analyzed for this study are characterized by a geochemical signature typical of arc volcanics: high LILE/HFSE ratios; ⁸⁷Sr/⁸⁶Sr slightly higher than MORB; ¹⁴³Nd/¹⁴⁴Nd ratios similar to MORB. Syn-collisional samples have more enriched isotopic signatures, and are relatively potassium rich. This is interpreted to reflect a larger contribution from subducted sediments, added to the mantle wedge as a silicic melt. Melting of subducted sediments is interpreted to result from a decrease in subduction rate and an increase of temperature in the slab. Magmatism that postdates the collisional event by 10 Ma is characterized by higher Nb/Y ratios than the pre- or syn-collisional samples, and Sr and Nd isotopic signatures intermediate between these two groups. This is likely to reflect melting of a subduction-modified mantle, with a significant contribution from the sub-continental lithospheric mantle. Comparison with post-collisional magmatism from other areas of the world suggests that trace element signatures are similar, but isotopic characteristics are variable. The latter are likely to reflect both the age of the sub-continental lithospheric mantle and the time lag between cessation of subduction and formation of the post-collisional magmas.     

A new emergent alpine front chain in central Tunisia (Maktar area): filtering gravimetric data contribution

A residual anomaly gravity map of the Maktar area has been processed via several methods including horizontal gradient, upward continuation, and Euler deconvolution to highlight structures hidden by sedimentary cover. These methods reinforce the existence of several faults inferred from geologic studies. This work outlines deep or near-surface faults that had remained hidden until now. The most important set of these faults is NE–SW trending, with depths in excess of 3,000 m. These faults limit the Northwestern limb of Balouta massif and restrain the Northwestern side of Oued Ousafa syncline. This last fault presents a thrust front which constituted the western continuity of a southern Tunisian thetys margin and a western Zaghouan scar prolongation. E–W striking faults refer to an arched beam of faults limiting Kessera plateau and others situated in the northern part of the Maktar plateau. N–S and NW–SE directions correspond to branches of less-apparent importance. Superposition of local maxima specifies dipping vergence of different proven structures. Results from this work show the importance of gravimetric data analysis in Maktar area and provide new insight into Central Atlas Tunisia structure such as new emergent segment front Alpine chain. The proposed map could be a basis for planning future hydrogeological and petroleum research in this region.     

Coastal progradation in response to the recent sea level rise: a case study of El Grine coast, Gulf of Gabes (Southeastern Tunisia)

Over the last 30 years El Grine coast recorded significant morphological changes leading to the shoreline progradation. Diachronic coastal evolution, through a set of satellite images data, supported by sedimentologic analysis of five sand cores have been applied to elucidate tidal flat reaction following the recent sea level rise. We found that an emerging sandy barrier and salt marsh domain were shaped by the local hydro-sedimentary processes. The marine sediments, mobilised by wave and tide currents, have been the sole sedimentary source. Recent sandy accumulations, occurred around 1987, have been able to balance a recent sea level rise. While during a mid-Holocene period, El Grine coast was governed by net erosion tendency inducing cliff recession.     

Genesis of folds in external zones: application of fault propagation fold model. Gafsa Basin example (southern central Tunisia)

The evolution of geological structures is related particularly to reactivation of preexisting fault, thus the importance of tectonic inheritance. Basing on stratigraphic and structural data in external zones leaving the example of Gafsa Basin (southern central Tunisia), we study the evolution of folds during tectonic phases. The structural and stratigraphic data prove that Gafsa Basin is subject for more than one tectonic phase where beginning by Cretaceous extension and reactivated by Atlasic compression. The combination of field results associated to that geomorphology confirms the application of “fault propagation model” as evolution mode of folds. The balanced of cross section, using numerical software Ramp E.M. 1.5.2, shows the importance of tectonic inheritance to interpret evolution of structures reliefs. The deformation increases related to reactivation of old normal fault. The most important deformation is observed in Jbal At Taghli presenting folds in the form of duplex resulted from conjugate activity of tear fault; it is the first interpretation of tear fault activity in surface in the scale of Tunisia. The application of fault propagation fold model to interpret fold genesis confirms the field data and proves the role of tectonic inheritance and reactivation of preexisting faults in the evolution of structures during different tectonic phases.     

北武夷长寿源银铅(锌)矿床地质特征

长寿源银铅(锌)矿区位于武夷隆起北侧.矿体赋存于震旦系洪山组上段的层间破碎带内,呈层状、似层状产出.矿体沿走向具尖灭再现、膨大缩小特征,剖面上呈似层状、透镜状;矿体厚度变化较稳定,有用组份分布较均匀.矿体产状:倾向177~196°,倾角61~75°,与地层产状基本一致.矿石构造以脉状、网脉状、块状构造为主.矿物成分较复杂,主要有方铅矿、深红银矿、辉银矿、自然银、闪锌矿、黄铜矿、金等.围岩蚀变主要有硅化、绢云母化、黄铁矿化、绿泥石化、碳酸盐化、萤石化、高岭土化等.矿床属于层间破碎带热液充填交代型银铅(锌)矿床.     

Detrital mode, mineralogy and geochemistry of the Sidi Aïch Formation (Early Cretaceous) in central and southwestern Tunisia: Implications for provenance, tectonic setting and paleoenvironment

The chemical and mineralogical composition of the Sidi Aïch Formation sandstones in central and southwestern Tunisia has been investigated in order to infer the provenance and tectonic setting, as well as to appraise the influence of weathering. The sixteen studied samples are mainly composed of quartz, feldspar, kaolinite and/or illite. Sidi Aïch sandstones are mainly arkosic, potassic feldspar-rich and immature. Much of the feldspar was transformed to kaolinite. Concerning the relation between sandstone detrital composition and their depositional setting, the Sidi Aïch Formation sandstone in the major studied localities, probably accumulated in relatively proximal small basins within the continental interior. However, for the Khanguet El Ouara study site, sandstones may have been deposited in a foreland basin which received recycled sediments from an adjacent orogenic belt.The source area may have included quartzose sedimentary rocks. The dominance of quartz and enrichment in immobile elements suggest that the depositional basins were associated with a passive margin. The petrography and geochemistry reflect a stable continental margin and sediments were derived from granitic and pegmatitic sources located in the southern parts of the Gafsa basin. High values for the chemical index of alteration (CIA) indicate that recycling processes might have been important. Particularly high CIA values in the Garet Hadid locality indicate more intense chemical alteration, either due to weathering processes or tectonic control.     

中侏罗世泥炭地净初级生产力及其对全球碳循环的响应——以吐哈盆地沙尔湖煤田为例

泥炭地具有明显的碳汇能力, 对减缓全球气候变化有着至关重要的作用, 煤作为泥炭地的最终产物, 保存有丰富的泥炭地发育时期的古环境信息, 成煤泥炭地碳聚集速率和净初级生产力(NPP)可以作为恢复古气候的替代指标。吐哈盆地南缘沙尔湖煤田ZK9E02钻孔中侏罗统西山窑组发育视厚度为121.97 m的煤层(C8), 本研究使用频谱分析对其进行米兰科维奇轨道周期的识别及成煤泥炭地发育时限的估算, 然后利用该时限, 结合煤的碳含量以及煤化作用阶段的碳损, 计算出成煤泥炭地的NPP, 进而探究成煤泥炭地的碳汇能力与碳循环关系。研究发现, 所研究煤层的自然伽马测井曲线中蕴含着约405 ka长偏心率、约95 ka短偏心率、约37.6 ka斜率、约18.2 ka岁差的米兰科维奇旋回天文周期, 计算出该成煤泥炭地发育的煤层沉积约2218.54~2347.03 ka。该煤层碳含量74.54%, 考虑到煤化过程中的碳损失约为26.75%, 恢复出成煤期泥炭地碳聚集速率为66.11~69.86 g C/m2·a, NPP为265.2~280.2 g C/m2·a。控制泥炭地NPP的主要因素有CO2含量、古纬度、古大气温度等, 研究区较低的泥炭地NPP水平与当时CO2含量过低或古纬度较低有一定关系。泥炭地NPP在一定程度上反映着大气中CO2的变化, 进一步用来揭示全球碳循环对气候变化的动态响应过程, 这对"深时"高分辨率的古环境研究有重要意义。因此, 预测泥炭地NPP的水平, 研究生态系统中碳元素的最终去向, 亦有利于更好地认识到全球碳循环过程中泥炭地的"碳汇"角色, 助力实现"碳达峰、碳中和"目标。     

Sedimentary response to Late Quaternary sea-level changes in the Romagna coastal plain (northern Italy)

Data from 17 continuously cored boreholes, 40–170 m deep, reveal the subsurface stratigraphy of the Romagna coastal plain. Sedimentological and microfaunal data allow the distinction of eight facies associations of Late Pleistocene–Holocene age, including 18 lithofacies and 16 faunal associations. Ten ¹⁴C dates provide the basis to establish a sequence stratigraphic framework for the succession corresponding to the upper part 35 ky BP of the last glacio-eustatic cycle. The eight facies associations can be grouped into lowstand, transgressive and highstand systems tracts. The upper part of the lowstand systems tract consists of alluvial plain deposits. These accumulated during the Late Pleistocene when the shoreline was ≈250 km south of its present-day position. A pronounced stratigraphic hiatus (between 25 and 8·8 ky BP) is invariably recorded at the upper boundary (transgressive surface) of these Pleistocene, indurated and locally pedogenized alluvial deposits. The succeeding postglacial history is represented by a well developed transgressive–regressive cycle. Transgressive deposits, interpreted to reflect the rapid landward migration of a barrier–lagoon system, include two wedge-shaped, paralic and marine units. These thicken in opposite directions and are separated by a ravinement surface. Above the transgressive deposits, the maximum flooding surface (MFS) marks the change from a transgressive barrier–lagoon complex to a prograding, wave-dominated delta system (early Po delta). The MFS can be traced landwards, where it constitutes the base of lagoonal deposits. An aggradational to progradational stacking pattern of upper delta plain (marsh), lower delta plain (lagoon/bay), and delta front (beach ridge) deposits reflects the progressive increase in the sediment supply/accommodation ratio during the following highstand. The alluvial deposits capping the sequence accumulated by the 13th century AD, in response to an avulsion event that caused abandonment of the former Po delta lobe and the northward migration of the Po River towards its present position.     

Use of GIS for optimalization of human activity in a catchment area: An example of the Beauce region (France)

The quality of groundwater often represents a limiting factor for its exploitation. The chemical composition of groundwater is a very complicated process involving geochemical, hydrochemical, and biochemical interaction between the water and the rock with which it is in contact. In addition, as a result of human activity, the deterioration of groundwater quality is a serious problem. The use of a geographic information system (GIS) combining various data may help solve this problem. The article describes multicriterion analysis applied to the Beauce basin in France, which is a part of the SAGE project of the BRGM Orléans.     

Detecting a sequence boundary across different tectonic domains: an example from the middle Miocene of the northern Apennines (Italy)

Sequence stratigraphic concepts can provide a powerful tool for understanding the tectono-sedimentary evolution of areas extending across different tectonic domains. An example is provided by the upper Serravallian strata of the northern Apennines, where a sedimentological and biostratigraphic study allows a sequence boundary to be traced across the foredeep and piggy-back basin successions. Turbidite sedimentation of predominantly alpine and subordinate apenninic provenance occurred in the apenninic foreland basin throughout the middle Miocene. Deep-water sedimentation in the foredeep was laterally associated with deposition in shelf to slope environments in the piggy-back basins. In the piggy-back basin succession, the upper Serravallian sequence boundary is a laterally extensive unconformity within homogeneous marly deposits. This unconfonnity is laterally correlative with the base of lenticular turbidite bodies. A stratigraphic lacuna affecting Zone N14 characterizes the marginal areas, where glaucony-rich deposits assigned to Zone N15 unconformably overlie marls displaying association of Zone N13. In the depocentres, where no significant stratigraphic gap has been detected, the sequence boundary is narrowly constrained to lowermost Zone N14. The upper Serravallian unconformity of the piggy-back basins succession is correlative with time-equivalent features in two distinct parts (inner basin and outer basin) of the foredeep. In the inner basin the sequence boundary separates basin margin turbidites from overlying slope hemipelagites. In a more external position (outer basin) the sequence boundary is the base of a characteristic mega turbidite of apenninic provenance (Turrito layer). In other sectors of the outer basin, where turbidite sedimentation was entirely of alpine provenance, the sequence boundary has no clear physical expression. The observed facies distribution in the study area suggests that an important thrusting event affected the northern Apennines in the late Serravallian, resulting in submarine channel incision and nondeposition in the piggy-back basins. Compressional activity in the foredeep was responsible for the closure of the inner basin and subsequent shifting of turbidite sedimentation in the outer basin. Slope instability led to widespread remobilization of previously deposited turbidites, triggering turbidite events of huge volume. The different characteristics of the sequence boundary in the various parts of the foredeep constitute an example of differential response of a multisourced supply system to tectonic deformation.     

Gold and platinum group elements in cobaltarsenide ores: Hydrothermal concentration from a serpentinite source-rock (Bou Azzer,Morocco)

Summary The cobalt-arsenide ores of Bou Azzer are located along the borders of serpentinite massifs (Upper Proterozoic ophiolite complex) in carbonate-quartz lenses resulting from hydrothermal carbonate alteration of serpentinite. The cobalt ores contain an average gold content of 5–20 ppm; gold is mainly located in skutterudite (120 ppm av.), whereas the Fe-arsenide (loellingite) contains < 1 ppm Au. Similarly the highest PGE contents are found in skutterudite (up to 2 ppm total PGE). All the arsenide ores of Bou Azzer exhibit the same chondrite normalized PGE pattern displaying positive Rh and negative Pt anomalies, and a slight positive slope (Pd/Ir = 1 to 2). This uncommon PGE pattern closely resembles to that of sulphides of komatiites.In serpentinite, the PGE patterns are typical of slightly depleted mantle rocks, and the associated podiform chromitites are within the range of ophiolitic chromitites, except for Pd and Au enrichment.Horizons of sulphide-bearing serpentinites show relatively high contents of noble metals and display PGE patterns which closely resemble those of the Co-arsenide ores, although an order of magnitude lower. These sulphides probably correspond to the remobilization during serpentinization of primary magmatic sulphides. The sulphiderich horizons are a possible source-rock for the noble metals of the Bou Azzer cobaltarsenide ores.

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Gold und Platingruppen-Elemente in Kobalt-Arsenid Erzen: Hydrothermale Anreicherung aus einem Serpentinit (Bou Azzer, Marokko)

Zusammenfassung Die Kobalt-Arsenid Erze von Bou Azzer kommen entlang den Grenzen eines Serpentinit-Massifs (Oberproterozoischer Ophiolit-Komplex) in Karbonat-Quarz-Linsen vor, die auf hydrothermale Umwandlung des Serpentinits zurückgehen.Die Kobalt-Erze enthalten 5–20 ppm Gold; dieses kommt hauptsächlich in Skutterudit (120 ppm) vor, während die Fe-Arsenide (Loellingit) weniger als 1 ppm Gold enthalten. Die höchsten PGE Gehalte kommen ebenso in Skutterudit vor (bis zu 2 ppm PGE). Alle Arsenid-Erze zeigen das gleiche Verteilungsbild mit positiven Rh und negativen Pt Anomalien, und eine leicht positive Neigung (Pd/Ir = 1 bis 2). Diese ungewöhnlichen PGE Verteilungsbilder erinnern an die von Sulfiden aus Komatiiten.Die PGE Verteilung in Serpentiniten ist typisch für leicht verarmte Mantelgesteine, und die assoziierten podiformen Chromitite liegen innerhalb des Bereiches für ophiolitische Chromitite, mit Ausnahme der Anreicherung in Pd und Au.Lagen von Sulfid-führenden Serpentiniten zeigen relativ hohe Gehalte an Edelmetallen, und PGE-Verteilungsmuster die denen von Co-Arseniderzen sehr ähnlich sind, obwohl sie um eine Größenordnung niedriger liegen. Diese Sulfide dürften Produkte der Remobilisierung primärer magmatischer Sulfide während der Serpentinisierung sein. Die Sulfid-reichen Lagen sind als ein mögliches Ursprungsgestein für die Edelmetalle der Kobalt-Arsenid-Erze von Bou Azzer zu sehen.

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With 8 Figures     

Two stages of high-pressure metamorphism in the Main Uralian Fault area (northern Urals)

Metamorphism in the northern sector of the Main Uralian Fault (MUF) area, northern Urals, is considered by the example of the Salatim glaucophane-schist and Belokamenka kyanite-staurolite complexes. New isotope-geochronological dates for metamorphic rocks of the MUF area are presented. The obtained data evidence the existence of two metamorphic events, of Early and Late Devonian ages, which apparently correspond to the wedging-up of subduction paleozones.