Upper Oligocene-Lower Miocene shallowing-upwardlacustrine sequences controlled by periodic and non-periodic processes(Ebro Basin, northeastern Spain)

The lacustrine facies from two sections (Candasnos and Fraga) ofthe Oligocene-Miocene Torrente de Cinca lithostratigraphic Unit in thecentral part of the Ebro Basin (Spain) have been analysed to determine theinfluence of orbital parameters in lacustrine sedimentation. The unit ispredominantly composed of limestones and marls, and represents a shallowlacustrine freshwater system. The sedimentological features of the faciesstudied demonstrate that the lower part of the Candasnos section representsoffshore lacustrine subenvironments whereas the upper part, and the whole ofthe Fraga section, characterise marginal lacustrine areas. Series of stratalthickness variations of limestone, marl, and limestone/marl couplets fromboth sections have been analysed using spectral analysis. This shows thatinformation corresponding to periodic cycles only appears in the offshorefacies, that is to say, in the lower part of Candasnos section, and disappearsin the marginal facies where non-periodic cycles exist. Furthermore, thespectral analysis of the offshore facies highlights the existence of a peak inthe power spectrum with a period of around 7 (6.8 to 7.8) that can berecognised in the field as shallowing-upward lacustrine sequences.Magnetostratigraphic data from the Candasnos section allow us to establish atime span of 2,808 years for the limestone/marl couplet from the lower partof this section, and between 19,000 and 22,000 years for the periodic cycleidentified, thus representing the climatic precession cycle. Shallowingsequences from marginal areas do not correspond with any periodiccycle.     

Alpujarride attribution of the supposed ‘Almagride Complex’ (Betic Internal Zone,Almerı́a Province,Spain)Appartenance Alpujarride du prétendu « Complexe almagride » (Zones internes bétiques,province d'Almeria,Espagne)

The differentiation of units in the Sierra de Almagro has been a source of controversy. There were defined the Almagride and Ballabona–Cucharón complexes, the former considered by several authors as part of a Subbetic metamorphosed and outcropping in a tectonic window. In this study, the units of Ballabona, Almagro and Cucharón are integrated into a single one, that of Tres Pacos, because they correspond to different parts of the same stratigraphic series. This unit is tectonically over the Nevado–Filabride Complex. The existence of the Almagride and Ballabona–Cucharón complexes is discarded and their units form part of the Alpujarride Complex. To cite this article: C. Sanz de Galdeano, F.J. Garc??a Tortosa, C. R. Geoscience 334 (2002) 355–362.     

Tectonique transpressive en terre Adélie au Paléoprotérozoı̈que (Est Antarctique)Palaeoproterozoic transpression in Terre Adélie (East Antarctica)

The Palaeoproterozoic units of Terre Adélie show two types of structural domains associated with HT–LP metamorphic conditions: domes and NS–N340° striking vertical shear zones. Shear zones reflect dextral transpressive motions. Domes reflect sub-vertical shortening and principal stretching subparallel to shear zones. They could partly result from longitudinal flow coeval with transpression. Deformations are comparable to those described along the eastern and western boundaries of the Archean Gawler Craton (South-East Australia), which underlines the continuity between these two areas before opening of the Austral Ocean. To cite this article: A. Pelletier et al., C. R. Geoscience 334 (2002) 505–511.     

Erosion and sediment transport in High Arctic rivers, Svalbard

This paper discusses sediment yield, sediment delivery and processes of erosion in rivers subject to High Arctic conditions in Svalbard. Long-term measurements reveal large variations between rivers and from year to year in each individual river. In the unglacierized catchment of Londonelva, annual sediment transport varied between 28 and 93 t/yr, with a mean sediment yield of 82.5 t/km²/yr. In the glacier-fed rivers Bayelva and Endalselva, the suspended sediment transport varied in the range of 5126 t/yr to 22797 t/yr during a 12-year period. A mean of 11 104 t/yr gave rise to a mean sediment yield of 359 t/km²/yr for the whole Bayelva catchment area. The sediment yield of the glacier and the moraine area was estimated at 586 t/km²/yr. A conceptual model used to interpret the long- and short-term patterns of sediment concentration in the meltwater from the glacier and erosion of the neoglacial moraines is proposed. Evidence is found that a proportion of the sediments are delivered by a network of englacial and subglacial channels that exist even in cold ice. Regression analyses of water discharge versus suspended sediment concentration gave significant correlations found to be associated with the stability of ice tunnels in cold ice. Large floods have been found to flush the waterways and exhaust the sediment sources. A long-term change in the exponent of regression lines is attributed to changes in sediment availability caused by flushing and expansion of tunnels and waterways by large floods and a subsequent slow deformation of them caused by the ice overburden and the glacier movement. A comparison of sediment yields from a number of polythermal and temperate glaciers in various areas showed large differences that were attributed primarily to bedrock susceptibility to erosion and, secondarily, to glaciological parameters.     

Habitat, occurrence and conservation of Saharo-Arabian-Turanian element Forsskaolea tenacissima L. in the Iberian Peninsula

The aim of this study is to assess the Iberian populations of Forsskaolea tenacissima L. according to its biogeographical interest, habitat, geographical range and conservation status. Results point out that they are restricted to gravel wadis of Tabernas Desert (SE Spain), are scarcely included in protected areas and represent historically isolated populations with relict behaviour. We also describe a new association, Senecioni-Forsskaoleetum tenacissimae. Conservation status of species is cause for concern and two conservation actions must be carried out. Firstly, protected areas should house Forsskaolea populations and secondly, phytosociological characterization of a community allows inventorying its habitat and directing conservation efforts to community level.     

Permeability-porosity Relationships in Rocks Subjected to Various Evolution Processes

— It is well known that there is no “universal” permeability-porosity relationship valid in all porous media. However, the evolution of permeability and porosity in rocks can be constrained provided that the processes changing the pore space are known. In this paper, we review observations of the relationship between permeability and porosity during rock evolution and interpret them in terms of creation/destruction of effectively and non-effectively conducting pore space. We focus on laboratory processes, namely, plastic compaction of aggregates, elastic-brittle deformation of granular rocks, dilatant and thermal microcracking of dense rocks, chemically driven processes, as a way to approach naturally occurring geological processes. In particular, the chemically driven processes and their corresponding evolution permeability-porosity relationships are discussed in relation to sedimentary rocks diagenesis.     

Analytical Model for Permeability Evolution in Microcracking Rock

— Analytical expressions to predict the enhancement of permeability due to stress-induced microcracking in initially low porosity rock are presented. A fracture mechanical microcrack model is employed to derive integrated effective hydraulic variables as a function of stress, which are then used to calculate the evolution of permeability using the statistically-based Dienes model. The model enables determination of permeability enhancement as a function of two loading parameters and three material parameters. Results are in reasonable agreement with experimental measurements and indicate that appreciable increases in permeability can be anticipated during brittle failure. The analytical nature of the model makes it easily incorporatable into numerical models that require quantification of the permeability evolution as a function of stress, for which there is currently no law.     

Extracting flow structures from tracer data

A method of visualizing structures in closed chaotic flows out of homogenous particle distributions is presented in the example of models of a meandering jet. To this end, the system will be leaked or opened up by defining a region of the flow, so that a particle is considered to be escaped if it leaves this region. By applying this method to an ensemble of nonescaped tracers, we are able to characterize mixing processes by visualizing the converging and stretching filamentations (stable and unstable manifolds) in the flow without using additional mathematical tools. The possibility of applying the algorithm to analyze buoy data, and a comparison with the finite time manifolds are discussed.