Lake‐level fluctuations at Lake Bourget (eastern France) around 4500–3500 cal. a BP and their palaeoclimatic and archaeological implications

On the basis of sedimentological analysis of two cores taken at Chatillon, Lake Le Bourget (northern French Pre‐Alps), and well dated by radiocarbon dates in addition to tree ring dates obtained from an archaeological layer, this paper presents a high‐resolution lake‐level record for the period 4500–3500 cal. a BP. The collected data provide evidence of a complex palaeohydrological (climatic) oscillation spanning the ca. 4300–3850 cal. BP time interval, with major lake‐level maxima at ca. 4200 and 4050–3850 cal. a BP separated by a lowering episode around 4100 cal. a BP. The lake‐level highstands observed at Chatillon between 4300 and 3850 cal. BP appear to be synchronous with (i) a major flooding period recorded in deep cores from the large lakes Le Bourget and Bodensee, and (ii) glacier advance and tree line decline in the Alps. Such wetter and cooler climatic conditions in west‐central Europe around 4000 cal. a BP may have been a nonlinear response to decrease and seasonal changes in insolation. They may also provide a possible explanation for the general abandonment of prehistoric lake dwellings north of the Alps between 4360 and 3750 cal. a BP. Copyright © 2012 John Wiley & Sons, Ltd.     

A Markov Chain Model for Subsurface Characterization: Theory and Applications

This paper proposes an extension of a single coupled Markov chain model to characterize heterogeneity of geological formations, and to make conditioning on any number of well data possible. The methodology is based on the concept of conditioning a Markov chain on the future states. Because the conditioning is performed in an explicit way, the methodology is efficient in terms of computer time and storage. Applications to synthetic and field data show good results.     

Mapping Coastal Aquifers by Joint Inversion of DC and TEM Soundings-Three Case Histories

Electrical and electromagnetic methods are well suited for coastal aquifer studies because of the large contrast in resistivity between fresh water-bearing and salt water-bearing formations. Interpretation models for these aquifers typically contain four layers: a highly resistive unsaturated zone; a surficial fresh water aquifer of intermediate resistivity; an underlying conductive, salt water saturated aquifer; and resistive substratum. Additional layers may be added to allow for variations in lithology within the fresh water and salt water layers. Two methods are evaluated: direct current resistivity and time domain electromagnetic soundings. Use of each method alone produces nonunique solutions for resistivities and/or thicknesses of the different layers. We show that joint inversion of vertical electric and time domain electromagnetic soundings produces a more tightly constrained interpretation model at three test sites than is produced by inversion methods applied to each data set independently.     

Monitoring alluvial sand suspension by eddy correlation

Eddy correlation techniques are standard tools in micrometeorology and oceanography to measure momentum and contaminant transport across turbulent boundary layers. They can, in theory, be used to estimate the net vertical suspended sediment flux directly over different areas of an alluvial channel boundary, and thus disclose ongoing erosion/deposition patterns. The basic principles and main problems in applying the technique to alluvial suspension are first introduced. Results from a trial application of the method in a large sand bed river are then presented; the focus of the analysis is on the substantial (and surprising) contributions of multi-minute flow fluctuations to suspension work in the study environment. The data were collected in a 10 m deep channel of the Fraser River near Mission, British Columbia, Canada. Turbulent fluctuations of flow components streamwise and normal to the bed, along with the output of an optical suspended sediment sensor, were monitored over 7 h, 1 m above the bed. Flow velocities averaged 0·9 ms^?1 and mean suspended sediment concentrations 500 mgl^?1, at sensor level above 1–5 dm high dunes. Spectral analysis of the records reveals that approximately 30 per cent of the vertical suspended sand mixing across the sensor level (and roughly as much of the momentum exchange) was linked to gradual flow oscillations with periods between 1 and 13·6 min (underlying briefer, turbulent fluctuations). Extended periods of sediment-rich, slightly upward directed but slower mean flow alternated with periods of sediment-poor, slightly downward and faster mean flow; these slow fluctuations involved 10–20 cms^?1 changes in 5 min average flow speed, 2–4· changes in vertical flow angle and 100 mgl^?1 changes in mean sand concentration. To obtain accurate eddy-correlation estimates of the vertical suspension flux in the study conditions, hour-scale flow and turbidity records that include many of these multi-minute cycles appear to be necessary. The spectra of the Fraser River near-bed signals do not conspicuously differ in overall shape (in terms of low-frequency content and location of peak) from turbulent spectra encountered in some atmospheric boundary layers. Nonetheless, the long period fluctuations observed on the Fraser River may not be turbulent; rather they may reflect slowly evolving perturbations in the near-bed streamlines, caused by bedform translation or gradual fluctuation within the large-scale streamwise cells of the secondary flow.     

The application of confocal microscopy to the study of stone weathering

Confocal scanning light microscopy is a valuable new method for examining the nature and progress of stone weathering at a level of resolution between optical microscopy and scanning electron microscopy. Not only is it non-destructive, but it can also create three-dimensional images.     

Inversion of reflection seismograms by differential semblance analysis: algorithm structure and synthetic examples1

Seismograms predicted from acoustic or elastic earth models depend very non-linearly on the long wavelength components of velocity. This sensitive dependence demands the use of special variational principles in waveform-based inversion algorithms. The differential semblance variational principle is well-suited to velocity inversion by gradient methods, since its objective function is smooth and convex over a large range of velocity models. An extension of the adjoint state technique yields an accurate estimate of the differential semblance gradient. Non-linear conjugate gradient iteration is quite successful in locating the global differential semblance minimum, which is near the ordinary least-squares global minimum when coherent data noise is small. Several examples, based on the 2D primaries-only acoustic model, illustrate features of the method and its performance.     

Intra-arc opening and closure of a marginal sea: The case of the Guerrero Terrane (SW Mexico)

Abstract The pre-Oligocene structure of southwest Mexico, south of the trans-Mexico volcanic axis, is investigated from Taxco (Guerrero state, abbreviation: Gro) to the Pacific coast. Three volcano-sedimentary units are recognized; from east to west the calc-alkaline Teloloapan, tholeiitic Arcelia and calc-alkaline Zihuatanejo suites. Structural and stratigraphic data show that the Teloloapan volcanic arc, active during ?Late Jurassic and early Cretaceous, was built upon continental basement. The Teloloapan lavas are overlain by the Albian–Cenomanian Morelos platform carbonates and followed by the Upper Cretaceous Mexcala flysch. In contrast, the Arcelia pillow lavas are associated with sandstones and cherts of Albian-?Cenomanian age. The Zihuatanejo arc was also installed upon continental basement and its magmatic activity was in part coeval with Arcelia magmatism. Unlike the almost undeformed Zihuatanejo volcanic rocks, all the other volcanic units are involved in east-vergent thrusting and recumbent folding associated with ductile tectonics, as well as the Late Cretaceous Mexcala flysch overlying the Morelos platform carbonates. Contrasting with previous views, the present results do not support a major mid-Cretaceous thrusting event in the study area. The new geodynamic interpretation proposed here considers that the Arcelia rocks were formed in a marginal basin situated east of the Zihuatanejo arc. Closure of this basin in Paleocene times is responsible for the east vergent thrust tectonics in SW Mexico.     

The Santa Rita gold deposit in the Proterozoic Paranoá Group, Goiás, Brazil: An example of fluid mixing during ore deposition

The Santa Rita gold deposit (Central Goiás, Brazil) is hosted by Middle to Upper Proterozoic carbonate-pelite sequences of the Paranoá Group that have been metamorphosed in the greenschist facies. The ore is contained in pyrite-bearing quartz-carbonate veins. The mineralization is structurally controlled by WNW-ESE high-angle faults and fractures resulting from the reactivation of older NE-SW lineaments. Pyrite is the sole sulphide and it shows growth zones enriched in Co, Ni and As (up to 4 wt%). Hydrothermal alteration zones are enriched in Co, Ni and As and are characterized by diffuse albitization, carbonatization, silicification and pyritization.A fluid inclusion study on quartz from pyrite-bearing quartz-carbonate veins led to the identification of two fluids: (1) a highly saline CO₂---N₂-rich aqueous fluid with halite and ± sylvite daughter minerals, and (2) a CO₂---N₂ rich aqueous fluid with moderate salinity. The two fluid types occur in the same quartz domain and display great variation in the degree of filling and notable dispersion of the microthermometric data. On heating, all the inclusions decrepitate between 200° and 300°C. Raman spectrometry detected high concentrations of N₂ in the gas phase, with a molar ratio between 1 and 19 and a small proportion of CH₄ (up to 2 mole %).The simultaneous entrapment of compositionally variable fluids in the system H₂O---CO₂---N₂---NaCl---KCl allow us to propose a mechanism of heterogeneous trapping. The entrapment may result from the mixing of a high-salinity fluid (H₂O---NaCl---KCl system) with a carbonic fluid (H₂O---CO₂---N₂ system) produced by the devolatilization process of carbonate and phyllitic host rocks. Considering the absence of spatially and temporally related igneous activity and the low P-T regional metamorphism in the Paranoá Group, the brines are inferred to result from leaching of evaporites occurring in the lower part of the Paranoá lithostratigraphic column.Gold was probably initially transported as an AuCl₂-complex (T>300°C, low pH, moderate ƒ_O2-pyrite field stability). As temperature decreased below 290°C, the “switch-over” process would lead to the predominance of Au(HS)₂⁻ in the fluid. Pyrite precipitated in this temperature interval. The oscillatory zoning of the As---Co---Ni-bearing pyrites indicates episodic fluctuation of the fluid composition. Such changes in fluid composition are favoured by a mechanism of fluid mixing by intermittent supplies in the hydrothermal system. The proposed mechanism of heterogeneous trapping of two separate fluids in the system H₂O---CO₂---N₂---NaCl---KCl and the resulting changes in the physicochemical conditions caused by the fluid mixing appears as a conspicuous process for the Santa Rita hydrothermal fluid evolution.A model based on the existence of a Proterozoic geothermal system involving the regional thermal gradient is proposed.     

Regional heavy mineral survey in the exploration for gold using regression: Grenville Province, southwestern Quebec

A geochemical reconnaissance heavy mineral survey has been carried out in the area of the Gatineau River Valley, over a part of the Grenville Province of Québec. The majority of samples show gold concentrations below the detection limit of 5 ppb, and anomalies are erratic. To circumvent these problems and to present an accurate image of the gold-bearing potential of the area, a regression analysis has been applied to the samples whose gold concentrations were above the detection limit. The regression of gold on chalcophile pathfinder elements (As, Se and Pb) explains 11% of the gold variance.Bedrock lithologies, overburden cover and physico-chemical characteristics are important factors influencing the chalcophile element dispersion pattern. Principal component analysis was used to characterize the principal inter-element relationships and to identify the factors influencing the dispersion of chalcophile elements. A filtering procedure was used to remove the background effects influencing the pathfinder elements, and to enhance the influence of mineralization. This procedure uses standard formulas for partial reconstruction of the data matrix from results of principal component analysis. The regression model constructed with these corrected data explains 20% of gold variance. It constitutes an improvement over the model which uses non-filtered data.The equation inferred from regression was applied to the whole data set and provided an estimation of gold at each sampling point. Exploration targets are defined from highest gold concentrations estimated by regression. The regression model aided in the definition of some well defined anomalies associated with geological structures favourable to gold mineralization.     

K-Ar Radiometric Age Determinations of White Micas from the Piemont Zone,French-Italian Western Alps

The olivine-clinopyroxene (Fe-Mg partition) geothermometer of Powell and Powell (1974) was derived on the basis of clinopyroxene mixing parameters which imply a large miscibility gap in CaFeSi₂O₆-CaMgSi₂O₆-CaAl₂SiO₆ clinopyroxenes at 950° C. Application of the Powell and Powell thermometer in an empirical way (ignoring likely errors in mixing parameters) is limited by the fact that almost all natural clinopyroxene-olivine pairs are constrained by the form of the equation to have 1-bar temperatures within the narrow (915–1,060° C) limits of their calibration points.Temperatures obtained from the Powell and Powell thermometer have been compared with those obtained from clinopyroxene-orthopyroxene (miscibility gap) thermometers; agreement with the latter is poor. There are discrepancies between calculated and observed (experimental) temperatures for iron-rich clinopyroxene-olivine pairs. It is concluded that application of the thermometer in its present form is unlikely to produce reliable results.