Sediment flux distribution in the Southern Brazil Basin during the late Quaternary: the role of deep-sea currents

Detailed sedimentological and stratigraphic analyses were carried out on seven Kullenberg cores collected across the Brazilian continental margin during the French cruises Byblos and Apsara III, in order to highlight the factors controlling the sediment flux distribution in the Southern Brazil Basin during the late Quaternary. On the continental slope and upper continental rise above 3000 m depth, sediment fluxes are important and highly variable (4·2–14·2 g cm^?2 10^?3 yr). The sediments show a pelagic or turbiditic character, depending on the width of the shelf and proximity of canyons. The material is characterized by high kaolinite contents, and originates from the coastal rivers draining the South American continent north of Rio de Janeiro. On the middle continental rise between 3000 and 4000 m depth, sediment fluxes are the lowest observed in the area (0·9 g cm^?2 10^?3 yr), because terrigenous input is trapped at shallower depths on the São Paulo Plateau. Pelagic settling is the dominant process. In the deep domains, below 4000 m depth, contouritic accumulations are developed on the path of the northwards moving Antarctic bottom water (AABW) currents. The deposits consist of fine-grained silty-clayey muds with very low carbonate contents. The sediment fluxes (1·45 g cm^?2 10^?3 yr) are higher than on the middle continental rise, as a consequence of fine-grained terrigenous supply derived from higher latitudes (Argentine Basin and Southern Ocean), and transported in the basin through the Vema Channel by the AABW currents. This material is characterized by high smectite and chlorite contents. These data reveal large sediment flux variations which are linked to distinct depth-related domains. Such a distribution is the consequence of the presence of two available sources of terrigenous sediments: (1) the Brazilian continental areas with a downslope material transport and a sediment distribution controlled by the morphology of the margin, and (2) the Argentine Basin with an alongslope material transport by deep-sea currents which dominate the sedimentation in the abyssal domains.     

Experimental modelling of forearc basin development during accretionary wedge growth

We present results of three sand-box experiments that model the association between tectonic accretion and sedimentation in a forearc basin. Experimental sedimentation occurs step by step in the forearc basin during shortening of the sand wedge. In each experiment, the development of the accretionary wedge leads to the formation of a major backthrust zone. This major deformation zone accounts for the thickening in the rear part of the wedge. In natural settings this tectonic bulge dams sediments that are transported toward the trench from mountainous terrain behind the forearc. We test the variation of friction along the déollement and note the following: (1) shortening of a low-friction wedge involves a mechanical balance between forethrusts and backthrust propagation and this balance is recorded by the sedimentary sequence trapped in the forearc basin. Indeed, if most of the movement occurs along the backthrust, the deepening of the basin will be larger and consequently the thickness of the sedimentary sequence will be greater. (2) Such balance does not exist in the case of a high-friction wedge. (3) Variation of friction along the décollement during shortening of the sand wedge leads to modification in the forearc basin filling. Thus, for similar increments of convergence, the sequence deposited in the forearc basin shows relatively larger thickness when the wedge is shortened above a high-friction décollement. We suggest that contraction and thickening in the rear part of the wedge is an efficient mechanism to, initiate and develop a forearc basin. Thus, this kind of basin occurs in convergent settings, without collapse related to local extension or tectonic erosion. They represent a sedimentary trap on a passive basement, bounded by a tectonic bulge. The Quaternary Hikurangi forearc basin, southeast of the North Island of New Zealand, is bounded by two actively uplifting ridges. Thus, this basin is considered to be a possible example of the basins modelled in our experiments, and we suggest that the limit between the basin and the wedge could be a complex backthrust zone.     

Garnet re‐equilibration by coupled dissolution–reprecipitation: evidence from textural,major element and oxygen isotope zoning of ‘cloudy’ garnet

The analysis of texture, major element and oxygen isotope compositions of cloudy garnet crystals from a metapelite sampled on Ikaria Island (Greece) is used to assess the model of growth and re‐equilibration of these garnet crystals and to reconstruct the pressure–temperature–fluid history of the sample. Garnet crystals show complex textural and chemical zoning. Garnet cores (100–200 μm) are devoid of fluid inclusions. They are characterized by growth zoning demonstrated by a bell‐shaped profile of spessartine component (7–3 mol.%), an increase in grossular from 14 to 22 mol.% and δ¹⁸O values between 9.5 ± 0.3‰ and 10.4 ± 0.2‰. Garnet inner rims (90–130 μm) are fluid inclusion‐rich and show a decreasing grossular component from 22 to 5 mol.%. The trend of the spessartine component observed in the inner rim allows two domains to be distinguished. In contrast to domain I, where the spessartine content shows the same trend as in the core, the spessartine content of domain II increases outwards from 2 to 14 mol.%. The δ¹⁸O values decrease towards the margins of the crystals to a lowest value of 7.4 ± 0.2‰. The outer rims (<10 μm) are devoid of fluid inclusions and have the same chemical composition as the outermost part of domain II of the inner rim. Garnet crystals underwent a four‐stage history. Stage 1: garnet growth during the prograde path in a closed system for oxygen. Garnet cores are remnants of this growth stage. Stage 2: garnet re‐equilibration by coupled dissolution–reprecipitation at the temperature peak (630 < T < 650 °C). This causes the creation of porosity as the coupled dissolution–reprecipitation process allows chemical (Ca) and isotopic (O) exchange between garnet inner rims and the matrix. The formation of the outer rim is related to the closure of porosity. Stage 3: garnet mode decreases during the early retrograde path, but garnet is still a stable phase. The resulting garnet composition is characterized by an increasing Mn content in the inner rim’s domain II caused by intracrystalline diffusion. Stage 4: dissolution of garnet during the late retrograde path as garnet is not a stable phase anymore. This last stage forms corroded garnet. This study shows that coupled dissolution–reprecipitation is a possible re‐equilibration process for garnet in metamorphic rocks and that intra‐mineral porosity is an efficient pathway for chemical and isotopic exchange between garnet and the matrix, even for otherwise slow diffusing elements.     

LA METEORITE DE TILLABERI

The Tillaberi stone fell in April 1970 in Republic of Niger. In the 157 g piece sent to Paris, a centimeter wide lithic inclusion is found. Contrary to previous assumptions, the stone is an L₆ chondrite in which few chondrules can be distinguished; olivine is Fa 25 ± 0.4 percent with minor amounts of Ca, Cr, Ti; orthopyroxene is Fs 21.6 ± 1.2 percent with a rather large scatter of the amounts of minor elements. The feldspar, well developed, contains 10 to 11 percent An, 84 to 85 percent Ab, five percent Or, and 0.77 ± 0.09 percent FeO. The lithic inclusion contains much feldspar which corresponds to 14 to 17 percent An, 75 to 79 percent Ab, four to five percent Or without almost any FeO. This inclusion contains also rounded grains of olivine and minute crystals of chromite. It has a frothy microdoleritic texture with a frozen border against the chondritic material. As the nickel rich grains are either martensite or acicular plessite and the silicates are undeformed, a quick cooling after a short but intense heating is postulated to account for the mineralogical characteristics.     

DÉTERMINATION DU DÉBIT DES PUITS INCOMPLETS DE NAPPES LIBRES FONCÉS EN TERRAIN ISOTROPE ET ANISOTROPE

ABSTRACT

In this paper, the author relies on Darcy's law and on an hypothesis formulated from experimental considerations to establish a semi-empirical formula for the calculation of the discharge of partially penetrating wells of free aquifer sunk in isotropic soils.

This relatively simple relation applied to 27 cases studied under an experimental design or numerical computations, reveals itself to have an unexpected accuracy.

Starting from previous formula, the author establishes another one for partially penetrating wells bored in anisotropic soils.     

Transformation of Irradiance Measurements into Spectral Actinic Flux for Photolysis Rates Determination

The actinic flux is the only radiometric quantity suitable for photolysisfrequency determination. It is derived from solar spectral irradiancemeasurements performed by a portable spectroradiometer in the 300–850nm wavelength range. The spectral irradiance is first divided into a directpart and a diffuse part, according to the atmospheric conditions, and thenconverted into the corresponding actinic flux quantity. As an intermediary,the ratio of diffuse actinic flux to diffuseirradiance is calculated by the spherical harmonics radiative code withrespect to wavelength, solar zenith angle, surface albedo, and aerosolproperties.The results of extensive sensitivity studies of asa function of the main atmospheric parameters are discussed and lead to theconclusion that aerosol optical depth is the major critical value for aprecise determination. The global algorithm totransform spectral irradiance into actinic flux is finally applied forphotodissociation rate calculations by convolution of the obtained actinicflux spectra with the absorption cross-sections and quantum yields of themolecule studied. Photolysis rates of different atmospheric photooxidants havebeen measured with this spectroradiometric method during the summers of 1993and 1994 in Brittany and in Portugal. The ozone and nitrogen dioxidephotodissociation rates obtained present a good agreement with thej(O₃) and j(NO₂) actinometerresults, for the same experimental conditions in Brittany.     

Structure and evolution of a Messinian mixed carbonate‐siliciclastic platform: the role of evaporites (Sorbas Basin,South‐east Spain)

The Sorbas Member is a late Messinian complex sedimentary system that formed immediately following deposition of the Messinian evaporites in the Sorbas Basin (South‐east Spain). This work describes the sequence architecture and facies organization of a continuous kilometre long, alluvial fan to open platform transect near the village of Cariatiz in the north‐east of the basin. The post‐evaporitic Cariatiz platform was a mixed carbonate‐siliciclastic system composed of four intermediate‐frequency, fifth‐order depositional sequences (Depositional Sequences 1 to 4) arranged in an overall prograding trend. The intense fracturing and brecciation of these deposits is attributed to the deformation and dissolution of an evaporite body measuring several tens of metres in thickness. The four sequences display significant spatial–temporal variability in both architecture and facies distribution, with two main phases: (i) Depositional Sequences 1 and 2 are ooid and oobioclastic dominated, and show normal marine faunas; and (ii) Depositional Sequences 3 and 4 show a higher siliciclastic contribution and are microbialite dominated. These important changes are interpreted as modifications of the primary controlling factors. Following an initial 70 m drowning, possibly linked to increased oceanic input, Depositional Sequences 1 to 3 were controlled mainly by eustatic variations and inherited topography; their progradation destabilized the evaporite body near the end of the Depositional Sequence 2 period. During the second phase, Depositional Sequences 3 and 4 recorded a progressive restriction of the Sorbas Basin related to a 30 to 40 m fall in water level that was driven mainly by regional factors. These regional factors were dissolution and gravity‐induced deformation of the evaporites and correlative evaporative fluid circulation associated with the contrasted arid/humid regional climate that, respectively, controlled sequence geometry and fluctuating water salinity which caused a microbialite bloom.