Le Bassin lusitanien (Portugal) à l'Aptien supérieur–Albien : organisation séquentielle,proposition de corrélations, évolutionThe Lusitanian Basin (Portugal) during the late Aptian–Albian: sequential arrangement,proposal of correlations,evolution

. Three transgressive–regressive 2nd-order cycles were identified in the Upper Aptian–Albian fluvial and marine deposits of the Lusitanian Basin. Its widespread nature, probably including eustatic origin, allows correlation between the southern package, with precise stratigraphic positioning, and the northern series with a poorly constrained age. The main unconformities can be related to the onset of an oceanic crust, in the western margin of Galicia during the Late Aptian, in the bay of Biscay during the Early Albian, and, to the northwest of the Galicia triple point, during the Middle to Late Albian transition, and, at the Albian–Cenomanian boundary, to a probable compressive event with Africa due to the rotation of Iberia. To cite this article: J. Dinis et al., C. R. Geoscience 334 (2002) 757–764.     

Sedimentary petrology and geochemistry of siliciclastic rocks from the upper Jurassic Tordillo Formation (Neuquén Basin, western Argentina): Implications for provenance and tectonic setting

The Upper Jurassic Tordillo Formation is exposed along the western edge of the Neuquén Basin (west central Argentina) and consists of fluvial strata deposited under arid/semiarid conditions. The pebble composition of conglomerates, mineralogical composition of sandstones and pelitic rocks, and major- and trace-element geochemistry of sandstones, mudstones, and primary pyroclastic deposits are evaluated to determine the provenance and tectonic setting of the sedimentary basin. Conglomerates and sandstones derived almost exclusively from volcanic sources. The stratigraphic sections to the south show a clast population of conglomerates dominated by silicic volcanic fragments and a predominance of feldspathic litharenites. This framework composition records erosion of Triassic–Jurassic synrift volcaniclastic rocks and basement rocks from the Huincul arch, which was exhumed as a result of Late Jurassic inversion. In the northwestern part of the study area, conglomerates show a large proportion of mafic and acidic volcanic rock fragments, and sandstones are characterised by a high content of mafic volcanic rock fragments and plagioclase. These data suggest that the source of the sandstones and conglomerates was primarily the Andean magmatic arc, located west of the Neuquén Basin. The clay mineral assemblage is interpreted as the result of a complex set of factors, including source rock, climate, transport, and diagenesis. Postdepositional processes produced significant variations in the original compositions, especially the fine-grained deposits. The Tordillo sediments are characterised by moderate SiO₂ contents, variable abundances of K₂O and Na₂O, and a relatively high proportion of ferromagnesian elements. The degree of chemical weathering in the source area, expressed as the chemical index of alteration, is low to moderate. The major element geochemistry and Th/Sc, K/Rb, Co/Th, La/Sc, and Cr/Th values point to a significant input of detrital volcanic material of calcalkaline felsic and intermediate composition. However, major element geochemistry is not useful for interpreting the tectonic setting. Discrimination plots based on immobile trace elements, such as Ti, Zr, La, Sc, and Th, show that most data lie in the active continental margin field. Geochemical information is not sufficiently sensitive to differentiate the two different source areas recognized by petrographic and modal analyses of conglomerates and sandstones.     

Chemical and isotopic assessment of surface water–shallow groundwater interaction in the arid Grande river basin,North-Central Chile

ABSTRACT

This paper analyses the composition of surface water and shallow groundwater in the Grande River basin, North-Central Chile, using this information to characterize water interactions. Chemical and isotopic data for surface water and groundwater (7 and 6 sampling locations, respectively) were obtained from three sampling campaigns performed in March–April (autumn), August–September (late winter) and December (early summer) 2012. Precipitation samples were also collected. Data was processed using spatial distribution charts, Piper and Stiff diagrams, and multivariate analysis. In general, the results for each method converge on a high degree of connectivity between surface water and shallow groundwater in the study area. Furthermore, approximately a 10% of groundwater contribution to the surface flow discharge was estimated for a particular reach. This multi-method approach was useful for the characterization of surface water–groundwater interactions in the Grande River basin, and may become a suitable and replicable scheme for studies in arid and semi-arid basins facing similar water management challenges.

Editor D. Koutsoyiannis; Associate editor B. Dewals     

A Criterion for Brittle Failure of Rocks Using the Theory of Critical Distances

This paper presents a new analytical criterion for brittle failure of rocks and heavily over-consolidated soils. Griffith’s model of a randomly oriented defect under a biaxial stress state is used to keep the criterion simple. The Griffith’s criterion is improved because the maximum tensile strength is not evaluated at the boundary of the defect but at a certain distance from the boundary, known as half of the critical distance. This fracture criterion is known as the point method, and is part of the theory of critical distances, which is utilised in fracture mechanics. The proposed failure criterion has two parameters: the inherent tensile strength, σ ₀, and the ratio of the half-length of the initial crack/flaw to the critical distance, a/L. These parameters are difficult to measure but they may be correlated with the uniaxial compressive and tensile strengths, σ _c and σ _t. The proposed criterion is able to reproduce the common range of strength ratios for rocks and heavily overconsolidated soils (σ _c/σ _t = 3–50) and the influence of several microstructural rock properties, such as texture and porosity. Good agreement with laboratory tests reported in the literature is found for tensile and low-confining stresses.     

Spatial and temporal changes in dry-snow line altitude on the Antarctic Peninsula

Drastic changes were detected in glacial systems of the Antarctic Peninsula in the last decades. The observed phenomena comprise the disintegration of ice shelves, acceleration and thinning of glaciers, and retreat of glacier fronts. However, due to the lack of consistent systematic observations in particular of the higher parts of the glacial systems, it is difficult to predict further responses of the Antarctic Peninsula glaciers to climatic change. The present paper analyses spatial and temporal variations of changes in the dry-snow line altitude on the Antarctic Peninsula as extracted from a time series (1992–2005) of ERS-1/2 SAR and Envisat ASAR data. Upward changes in dry-snow line altitude were observed in general, and are attributed to extreme high-temperature events impacting the central plateaus of the Antarctic Peninsula and the increasing duration of warming periods. A mean decrease in dry-snow line altitude was detected on the west side of the peninsula and is identified as a response to recorded increase in precipitation and accumulation. These results validate the capability of SAR data for deriving superficial parameters of glaciers to be used as indicators of climatic changes in high-latitude regions where operational restrictions limit conventional meteorological observations.     

Sixty thousand years of magmatic volatile history before the caldera-forming eruption of Mount Mazama, Crater Lake, Oregon

The well-documented eruptive history of Mount Mazama, Oregon, provides an excellent opportunity to use pre-eruptive volatile concentrations to study the growth of an explosive silicic magmatic system. Melt inclusions (MI) hosted in pyroxene and plagioclase crystals from eight dacitic–rhyodacitic eruptive deposits (71–7.7?ka) were analyzed to determine variations in volatile-element concentrations and changes in magma storage conditions leading up to and including the climactic eruption of Crater Lake caldera. Temperatures (Fe–Ti oxides) increased through the series of dacites, then decreased, and increased again through the rhyodacites (918–968 to ~950 to 845–895?°C). Oxygen fugacity began at nickel–nickel-oxide buffer (NNO) +0.8 (71?ka), dropped slightly to NNO +0.3, and then climbed to its highest value with the climactic eruption (7.7?ka) at NNO +1.1 log units. In parallel with oxidation state, maximum MI sulfur concentrations were high early in the eruptive sequence (~500?ppm), decreased (to ~200?ppm), and then increased again with the climactic eruption (~500?ppm). Maximum MI sulfur correlates with the Sr content (as a proxy for LREE, Ba, Rb, P₂O₅) of recharge magmas, represented by basaltic andesitic to andesitic enclaves and similar-aged lavas. These results suggest that oxidized Sr-rich recharge magmas dominated early and late in the development of the pre-climactic dacite–rhyodacite system. Dissolved H₂O concentrations in MI do not, however, correlate with these changes in dominant recharge magma, instead recording vapor solubility relations in the developing shallow magma storage and conduit region. Dissolved H₂O concentrations form two populations through time: the first at 3–4.6 wt% (with a few extreme values up to 6.1 wt%) and the second at ≤2.4 wt%. CO₂ concentrations measured in a subset of these inclusions reach up to 240?ppm in early-erupted deposits (71?ka) and are below detection in climactic deposits (7.7?ka). Combined H₂O and CO₂ concentrations and solubility models indicate a dominant storage region at 4–7?km (up to 12?km), with drier inclusions that diffusively re-equilibrated and/or were trapped at shallower depths. Boron and Cl (except in the climactic deposit) largely remained in the melt, suggesting vapor–melt partition coefficients and gas fractions were low. Modeled Li, F, and S vapor–melt partition coefficients are higher than those of B and Cl. The decrease in maximum MI CO₂ concentration following the earliest dacitic eruptions is interpreted to result from a broadening of the shallow storage region to greater than the diameter of subjacent feeders, so that greater proportions of reservoir magma were to the side of CO₂-bearing vapor bubbles ascending vertically from the locus of recharge magma injection, thereby escaping recarbonation by streaming vapor bubbles. The Mazama melt inclusions provide a picture of a growing magma storage region, where chemical variations in melt and magma occur due to changes in the nature and supply rate of magma recharge, the timing of degassing, and the possible degree of equilibration with gases from below.     

Contamination of port zone sediments by metals from Large Marine Ecosystems of Brazil

Sediment contamination by metals poses risks to coastal ecosystems and is considered to be problematic to dredging operations. In Brazil, there are differences in sedimentology along the Large Marine Ecosystems in relation to the metal distributions. We aimed to assess the extent of Al, Fe, Hg, Cd, Cr, Cu, Ni, Pb and Zn contamination in sediments from port zones in northeast (Mucuripe and Pecém) and southeast (Santos) Brazil through geochemical analyses and sediment quality ratings. The metal concentrations found in these port zones were higher than those observed in the continental shelf or the background values in both regions. In the northeast, metals were associated with carbonate, while in Santos, they were associated with mud. Geochemical analyses showed enrichments in Hg, Cd, Cu, Ni and Zn, and a simple application of international sediment quality guidelines failed to predict their impacts, whereas the use of site-specific values that were derived by geochemical and ecotoxicological approaches seemed to be more appropriate in the management of the dredged sediments.     

Chemical and carbon isotopic evolution of hydrocarbons during prograde metamorphism from 100°C to 550°C: Case study in the Liassic black shale formation of Central Swiss Alps

Hydrocarbon distributions and stable isotope ratios of carbonates (δ¹³C_car, δ¹⁸O_car), kerogen (δ¹³C_ker), extractable organic matter (δ¹³C_EOM) and individual hydrocarbons of Liassic black shale samples from a prograde metamorphic sequence in the Swiss Alps were used to identify the major organic reactions with increasing metamorphic grade. The studied samples range from the diagenetic zone (<100°C) to amphibolite facies (∼550°C). The samples within the diagenetic zones (<100 and 150°C) are characterized by the dominance of C_<20n-alkanes, suggesting an origin related with marine and/or bacterial inputs. The metamorphic samples (200 to 550°C) have distributions significantly dominated by C₁₂ and C₁₃n-alkanes, C₁₄, C₁₆ and C₁₈n-alkylcyclopentanes and to a lesser extend C₁₅, C₁₇ and C₂₁n-alkylcyclohexanes. The progressive ¹³C-enrichment (up to 3.9‰) with metamorphism of the C_>17n-alkanes suggests the occurrence of cracking reactions of high molecular weight compounds. The isotopically heavier (up to 5.6‰) C_<17n-alkanes in metamorphic samples are likely originated by thermal degradation of long-chain homologous with preferential release of isotopically light C₁ and C₂ radicals. The dominance of specific even C-number n-alkylcyclopentanes suggests an origin related to direct cyclization mechanism (without decarboxylation step) of algal or bacterial fatty acids occurring in reducing aqueous metamorphic fluid conditions. The regular increase of the concentrations of n-alkylcycloalkanes vs. C_>13n-alkanes with metamorphism suggests progressive thermal release of kerogen-linked fatty acid precursors and degradation of n-alkanes. Changes of the steroid and terpenoid distributions are clearly related to increasing metamorphic temperatures. The absence of 18α(H)-22,29,30-trisnorneohopane (Ts), the occurrence of 17β(H)-trisnorhopane, 17β(H), 21α(H)-hopanes in the C₂₉ to C₃₁ range and 5α(H),14α(H),17α(H)-20R C₂₇, C₂₉ steranes in the low diagenetic samples (<100°C) are characteristic of immature bitumens. The higher thermal stress within the upper diagenetic zone (150°C) is marked by the presence of Ts, the disappearance of 17β(H)-trisnorhopane and thermodynamic equilibrium of the 22S/(22S + 22R) homohopane ratios. The increase of the ααα-sterane 20S/(20S + 20R) and 20R ββ/(ββ + αα) ratios (from 0.0 to 0.55 and from 0.0 to 0.40, respectively) in the upper diagenetic zone indicates the occurrence of isomerization reactions already at <150°C. However, the isomerization at C-20 (R → S) reaches thermodynamic equilibrium values already at the upper diagenesis (∼150°C) whereas the epimerisation at C-14 and C-17 (αα → ββ) arrives to constant values in the lower anchizone (∼200°C). The ratios Ts vs. 17α(H)-22,29,30-trisnorneohopane [(Ts/(Ts + Tm)] and 18α(H)-30-norneohopane (C₂₉Ts) vs. 17α(H),21β(H)-30-norhopane [C₂₉Ts/(C₂₉Ts + C₂₉)] increase until the medium anchizone (200 to 250°C) from 0.0 to 0.96 and from 0.0 to 0.44, respectively. An opposite trend towards lower values is observed in the higher metamorphic samples.The occurrence of specific hydrocarbons (e.g., n-alkylcyclopentanes, cadalene, hydrogenated aromatic compounds) in metamorphic samples points to kerogen degradation reactions most probably occurring in the presence of water and under reducing conditions. The changes of hydrocarbon distributions and carbon isotopic compositions of n-alkanes related to metamorphism suggest that the organic geochemistry may help to evaluate the lowest grades of prograde metamorphism.