Geochemische Untersuchungen an Sedimenten des Persischen Golfes

The data obtained can be summed up in 6 points:

1. The sediment consists of varying amounts of quartz (14–23 weight percent, mean value 18%), aragonite, dolomite, Mg-rich calcite, Mg-poor calcite, Na-rich plagioclase (7–10%, m.v. 8%), chlorite (18–32%, m.v. 22%), kaolinite (4–29%, m.v. 13%), illite and mixedlayered illite-montmorillonite (20–42%, m.v. 34%) and organic matter (0.7–2.5%, m.v. 1.1%).
2. The composition of the carbonate fraction suggests a detrital origin for the carbonates deposited along the Persian Coast.
3. The clay mineral distribution follows gradients perpendicular to the long half-axis of the Persian Gulf. This favours detrital origin. The kaolinite distribution suggests an additional detritus from Euphrates and Tigris,
4. Enrichments of trace elements with regard to mean values in clays (Turekian and Wedepohl, 1961) are estimated for Fe, Co, Mo, Ni, V, Zr and Br; impoverishments are recorded for Cu, Mn, K. The contents of Rb, Ti and Zn comply with mean values of clays. The enrichments distribute as follows: chlorite concentrates Mn and Fe, kaolinite Ti and Zr, illite and mixedlayered illite-montmorillonite Rb, V, M, Zn, Mo and Co.
5. The high Br-contents (up to 420 ppm) in the sediment give positive correlations with the concentrations of organic carbon. The X-ray analysis of isolated kerogen confirms an enrichment of Br in this fraction of the organic matter.
6. The distribution of elements in the sediment to a large extent is controlled by a) detritus of Euphrates and Tigris, b) influence of organic matter.

     

Lack of low-grade metamorphism in the Triassic formations of the Argana basin, Morocco: an illite crystallinity re-evaluation

Analysis of mineral assemblages and illite crystallinity of the Arganan Triassic formations show that:

• 1. 

  i) mixed-layers are present across almost all of the section; and

• 2. 

  ii) values of illite crystallinity are quite dispersed and lie mostly in the diagenetic zone, with no gradient related to burial.

It is therefore concluded that, contrary to ideas proposed recently, there is no evidence for very low-grade metamorphism related to crustal thinning during the early rifting of the Central Atlantic. Fluctuations of illite crystallinity values are interpreted as due to complex interactions of detrital heritage and several factors governing illitization and improvement of illite crystallinity. The increase in temperature during burial constitutes only one of them.     

Zur Herkunft des Quarzzements. Abschätzung der Quarzauflösung in Silt- und Sandsteinen

The origin of quartz cement in sandstones can be attributed to supplies (1) from the surrounding shales, and (2) to a lower degree from dissolution of quartz on stylolites within the sandstones. A supply from the surrounding shales, which has been shown by the porosity decrease near the upper and lower surfaces of different sandstones (Füchtbauer, 1974), can be explained by the following observations in Upper Triassic and Middle Jurassic sandstones and siltstones of Northern Germany as well as in concretions of Devonian to Upper Cretaceous age from different localities:

1. Quartz grains in silt layers are flattened by dissolution compared with quartz grains of the same size in the adjacent sandstones, the amount of shrinking being about 35 percent (fig. 1).
2. Concretions prevent the enclosed insoluble residues from diagenesis. The main difference between the concretions and the adjacent shale of 31 occurrences examined is the quartz content, which is by 10–50 percent lower in the adjacent shale, due to diagenetical dissolution (fig. 2).

It is suggested that the dissolved silica was brought to the sandstones by the compaction stream of interstitial water percolating through the rock sequence, and that the sandstones acted as sinks triggering the dissolution. Only a small amount of silica, about 10 percent of the silica from dissolved quartz, is provided by the transition montmorillonite — illite. Both sources together would be able to explain the precipitation of 20 percent quartz cement in a sequence composed of 1/3 sandstones and 2/3 shales. In the sandstones mentioned above stylolites can be observed (fig. 3), the amplitudes of which increase from 0,5–1 mm to 2–5 mm with increasing depth, between 1300 and 2600 metres. The real amount of dissolution on each stylolite — about 4 mm — has been calculated using large mica which were collected by the stylolites from the adjacent sandstone. Using this figure, the decrease of porosity in the sandstones shown in fig. 4 can be quantitatively explained by the frequency of stylolite intercalations. It is suggested that this process, which was due to local diffusion, occurred late in diagenesis, when the compaction stream was already insufficient to move large quantities of silica.     

La Formation glaciaire de la Mambéré (République Centrafricaine): Reconstitution paléogéographique et implications à l'échelle du Paléozoique africain

The Mambéré Formation constitutes a horizontal unit located in the western and southwestern part of the Central African Republic. Stratigraphical and sedimentological study provides strong argument to attribute these deposits a glacial origin. A palaeogeographical reconstruction has been outlined in order to precise the age of the formation. Two main categories of glacial deposits have been recognized:

glaciogenic deposits made of basal tills (with facetted pebbles) and flow tills (with flattened blunt pebbles);

reworked glacial deposits formed of sandstones and conglomeratic sandstones, in continuous beds, lenses or isolated blocks, together with siltstones and bedded sandstones.

The southerly provenance of the detrital material is demonstrated by quartz grain surface analysis and heavy-mineral study. This material results essentially from the dismantling of the Precambrian Schistoquartzitic Complex and secondarily from the Granitogneissic Complex. According to the palaeomagnetic polar paths and the migration of the glacial centers over the African continent during the Palaeozoic, the Mambéré Formation may be attributed a Lower Silurian age by reference to similar formations observed in Cameroon or a Lower Carboniferous age by comparison with the glacial formations reported from Niger and Egypt.     

Characteristics and distribution of clay minerals and their effects on reservoir quality: Huagang Formation in the Xihu Sag,East China Sea Basin

Abstract

The characteristics and distribution of clay minerals and their effects on reservoir quality in the Huagang sandstones in the Xihu Sag, East China Sea Basin were studied by using X-ray diffraction, casting thin-sections, scanning electron microscopy, electron microprobe analysis, fluid inclusion analysis, constant-rate mercury injection and nuclear magnetic resonance. Clay minerals consist of kaolinite, chlorite, illite and illite–smectite mixed layer (I/S); kaolinite forms from dissolved feldspars, chlorite occurs as clay coatings that are transformed from clay precursors owing to the flocculation of suspended detrital clays or the crystallisation of pore fluids, and illite forms from the illitisation of detrital smectite, authigenic kaolinite and K-feldspars. Clay distribution is controlled by sedimentary environments, burial history and lithologies. Typical reservoirs in the western sub-sag are thin and developed in braided river facies at relatively shallow burial depths with clays dominated by kaolinite. However, typical reservoirs in the central inversion tectonic zone are thicker and developed in a braided delta front facies at deeper burial depths with clays mainly consisting of chlorite, illite and I/S. High-quality reservoirs are characterised by coarse granularity, high quartz content and low clay content with widespread development of chlorite coatings that inhibit quartz cements at low temperatures. At higher temperatures, the high-quality reservoirs develop more pores providing growth space for quartz cements and result in the coexistence of chlorite coatings and quartz cements. The high-quality reservoirs are controlled by their lithological characteristics rather than chlorite coatings. Illite and I/S clays create severe damage to reservoirs by reducing the size and connectivity of pore-throats.     

Tonminerale in normalsedimenten,hydrothermal beeinflu\ten sedimenten und Erzschlämmen des Roten Meeres

From 14 deeps and other regions of the Red Sea totally 226 samples from 28 cores recovered during the VALDIVIA cruises (1971, 1972) were investigated according to their clay mineral content (<2μm resp. < 6.3 μm) after carbonate dissolution. Three facies groups are to distinguish:

1. normal sediments: dominance of chlorite, kaolinite, illite, small amounts of smectite and sepiolite. Two palygorskite types are present only in a few samples.
2. normal sediments with hydrothermal influence: clay mineral paragenesis similar like that of normal sediments; but increase of smectite and presence of goethite in each sample; partly small contents of talc.
3. heavy metal deposits: dominance of iron-bearing smectite, partly with amorphous components resp. pure ore mineral assemblages with authigenic silicates (talc, quartz, opal, chrysotile, sepiolite, palygorskite, chlorite).

Crystallinity of the clay and ore minerals is independent from sedimentary overburden. Sepiolite shows in small amounts a wide distribution; palygorskite₂ (d₁₁₀=11.3 Å) yields locally an increased concentration in the range of pteropod layers cemented by aragonite. The environment of ore deposits is characterized by iron-bearing smectite besides the ore minerals.     

U-Pb zircon and Rb-Sr whole-rock dating of low-grade metasediments example: Montagne Noire (Southern France)

Three detrital, Proterozoic zircon suites extracted from siltstones progressively metamorphosed between chlorite- and staurolite-grade independently date the major Caledonian metamorphism within the gneiss dome of the Montagne Noire (Southern France). From this, the following conclusions concerning U-Pb systematics of zircons in low-, medium- and highgrade metamorphic rocks can be drawn:

1. Temperatures of at most 350–400 °C are sufficient to open U-Pb systems of metamict zircons or domains within zircons.
2. The observed open U-Pb system behaviour during metamorphism of the host rocks was found to be due to a low-temperature recrystallisation of highly radiation damaged zircon lattices, probably enhanced by high concentrations of fluid phases in the dehydrating rock volumes.
3. Recrystallisation of metamict zircons under low temperatures causes maximum U-Pb ages for the thermal climax of metamorphism of medium-and high-grade metamorphic rocks, as annealing and accompaning closing of U-Pb systems took place before the maximum temperatures of metamorphism were reached.
4. Low-temperature recrystallisation of old — generally Proterozoic—zircons can readily help to explain the fact that the numerous zircon suites from ancient shield areas yield “lower intercept ages” which are not correlated to any known geological event. Thus, either a weak thermal pulse, not necessarily registered by the mineral assemblage of the host rock, and/or elevated temperatures during burial in the crust might supply enough energy for a structural reordering and simultaneous lead loss of at least the most disordered lattice domains.

In contrast to the U-Pb zircon method, no unambiguous dating of the Caledonian main metamorphism was possible using the Rb-Sr whole-rock technique for phyllites and mica schists sampled in the same metamorphic profile from which the zircon samples were taken. The scatter of data points can best be explained by their rotation around a probable Caledonian isochron. This rotation very probably took place during the later Hercynian orogeny, not significantly affecting the slope of the least square regression line through the scattered data points.     

Geologische und geochemische Untersuchungen im Zentralgneis und in der Greiner Schiefer Serie (Zillertaler Alpen,Tirol)

Geologic, petrological and geochemical investigations have been carried out in the western part of the “Zillertaler Alpen”. Important results are:

1. The premetamorphic material of the Greiner series consists of conglomerates, breccias, arcosic-sandstones or greywackes, bituminous shales, volcanic lavas and tuffs.
2. The southern part of the “Zentralgneis” shows a differentiation trend from alkaline granite to quarzdiorite with predomination of granodiorite.
3. Chemical relationships of granodiorite to its restitic inclusions allow the supposition of a palingenetic origin of the granitic rocks.
4. At least two stages of metamorphism can be differentiated.
5. Parts of the Greiner series, covered by triassic metasediments, are supposed to be of Permian age. A lower age boundary can not yet be given.
6. Some vertical, northeast striking faults with throws of more than 1 or 2 kilometers produced southward verging drag folds.
7. Geodynamic aspects, revealed from regional metamorphism and tectonics, are discussed.

     

Altération supergène des protores à grenats manganésifères dans quelques gisements d'Afrique de l'Ouest

The wet tropical weathering of Precambrian spessartite quartzites leads to the formation of secondary oxidized ores. These ores result from different successions of superimposed elementary processes, among them:

garnet hydrolysis: Mn, Ca, and Fe are leached, and only an amorphous, silicoaluminous phase remains.

lithiophorite neoformation: Added manganese combines with alumina of the amorphous, Si-Al products.

leaching: Voids appear in the central part of the weathered garnets. They may widen, leaving only the quartz matrix with polyhedric voids.

epigenic replacement: The quartz is replaced by cryptomelane, while the garnets may remain unweathered.

The various ore types are characterized by the sequence of transformations and by the volumes that each transformation affects. These transformations are old because they are observed not only in situ ores but also in removed detrital ores. In the same way the major part of the accumulated stocks of oxidized ores must be old.     

Continental and marine paleoenvironments reflected by west Pacific clay sedimentation

Various stratigraphic and paleoenvironmental results arise from mineralogical, geochemical and micromorpho logical investigations on Late Cretaceous and Cenozoic sediments recovered by the Deep Sea Drilling Project in the West Pacific:

Stratigraphic correlations. The deep pelagic red clays deposited on either side of the Shatsky Rise contain a variety of clay minerals, which point to the importance of eolian detrital supply and allow rough stratigraphic correlations.

Climate and paleocirculation. Neogene sediments of the Lord Howe Rise area reflect closely the relationship between the evolution of the Antarctic ice sheet and related aridification stages, and the atmospheric and oceanic circulation pattern since 14 my.

Regional and global tectonics. The post-Eocene deposits around New Zealand express a sequence of tectonic events and of relaxation stages, associated with changes in the circulation regime. Along the inner wall of the Japan Trench, clay mineral successions document the existence of an outer continental landmass until the late Oligocene, its collapse associated with volcanism close to the Paleogene-Neogene boundary, and an increased detrital influence from Japan since the early-middle Miocene.

Volcanism, diagenesis and detrital supply. The Aptian to Maastrichtian sediments in the Mariana Basin document a complex sequence of geochemical and geodynamical events: proximal volcanism and local supply from subaerial archipelagoes; early marine diagenesis during lithospheric subsidence and plate migration; and finally the occurrence and evolution of terrigenous input from distant sources distributed by marine currents.

     

Age and origin of detrital zircons from the pre-Permian basements of the Bohemian Massif and the Alps

U-Pb isotopic analyses were made on detrital zircon populations from sandstones and quartzites of the pre-Permian basement in an attempt to shed light on the presedimentary history of the zircons and the age of their primary source rocks. Eight rock samples were collected from the Saxothuringian and Moldanubian parts of the Bohemian Massif, the western part of the Upper Austroalpine Nappes, and the Southern Alps. The heterogeneous populations were separated into fractions of different size, magnetic susceptibility, color, and shape. Because of their typically pitted surface all zircon grains from the sandstones and quartzites appear to be detrital. Only in three samples from the Alps—one from a contact metamorphic aureole—the zircons show surface recrystallization and minor new growth. With the exception of some euhedral crystals in the Saxothuringian quartzites all zircon fractions have highly discordant U-Pb ages. On a concordia diagram their data points scatter slightly around best-fit lines with upper intersections between 2000 and 2300 m.y. From this pattern the following conclusions are reached:

1. A large proportion of the material of the metasedimentary basement rocks in the Bohemian Massif as well as in the Alps derives from one or more sources, about 2000 to 2300 m.y. old.
2. The estimated proportion of detrital zircons with primary ages of 700 to 1500 m.y. is less than 10%.
3. The existence of a regional high-grade metamorphism in the Bohemian Massif as well as in the Alps during 700 to 1500 m.y. can be excluded. From Rb-Sr isotopic data, a metamorphism for the time prior to 1500 m.y. is very unlikely.

The lower intersections of the best-fit lines with the concordia curve cannot be clearly correlated with an episodic disturbance of the U-Pb systems during weathering and sedimentation and/or during regional metamorphism. For the zircons of the Bohemian Massif a disturbing event, about 550 to 600 m.y. ago, is likely. Clear, euhedral, but nevertheless detrital zircons found among the zircon populations of two Saxothuringian quartzites (“Plattenquarzit” of the pre-Ordovician “Arzberger Serie” and Lower Ordovician “FrauenbachQuarzit”) crystallized most probably during the Upper Proterozoic and/or the Assyntian petrogenesis. The highly discordant age pattern of the detrital zircons from the Alps is likely to be the result of the Caledonian and/or Hercynian (=Variscan) metamorphism. Differences in concentration levels of common lead in detrital zircons and the problem of red zircons as indicators of Precambrian origin are discussed.     

Kinetic Modeling of Diagenesis of Eogene Lacustrine Sandstone Reservoirs in the Jianghan Basin,Southeastern China

In the Tuoshi oilfield,located in the Cenozoic Jianghan Basin of southeastern China ,there have been found hydrocarbon reservoirs hosted in lacustrine sandstones of the Eogene Xingouzui Formation.The main diagenetic features identified in these sandstones include the dissolution of detrital K-feldspar and albite grains,the precipitation of quartz as overgrowths and /or cements ,and the precipitation and /or transformation of clay minerals.These diagenetic features were interpreted to have occurred in early,intermediate and late stages,based on the burial depth.The kinetics of fluid-mineral reactions and the concentrations of aqueous species au each stage of diagenesis were simulated numerically for these lacustrine sandstones,using a quasi-sta-tionary state approximation that incorporates simultaneous chemical reactions in a time-space continuum.During the early diagenetic stage,pore fluid was weakly acidic,which resulted in dissolution of K-feldspar and albite and,therefore,led to the release of K^ ,Na^ ,Al^3 and SiO2(aq) into the diagenetic fluid.The increased K^ ,Na^ ,Al^3 and SiO2(aq) concentrations in the diagenetic fluid caused the precipitation of quartz,kaolinite and illite.At the beginning of the intermediate diagenetic stage the concentration of H^ was built up due to the decomposition of organic matter,which was responsible for further dissolution of K-feldspar and albite and pre-cipitation of quartz,kaolinite,and illite.During the late diagenetic stage,the pore fluid was weakly alkaline,K-feldspar became stable and was precipitated with quartz and clay minerals.When the burial depth was greater than 3000 m,the pore fluids became supersaturated with respect to allbite,but undersaturated with respect to quartz,resulting in the precipitation of albite and the dissolution of quartz.The diagenetic reactions forecasted in the numerical modeling closely matched the diagenet-ic features identified by petrographic examination, and therefore,can help us to gain a better understanding of the diagenetic processes and associated porosity evolution in sandstone reservoirs.     

Clay-lead sorption relations

Bays, lagoons, and estuaries are sites where normal physicochemical processes result in accumulations of sediment and certain chemicals. Changes in water velocity and chemistry, and chemical interactions of sediment, biota, and water are factors that contribute to concentrating trace metals in coastal and lake sediments. To evaluate whether lead concentration is affected by mineralogy, kaolinite, illite, montmorillonite, and a zeolitic tuff were suspended in 10 and 20 mg/l concentrations of lead solutions [Pb(NO₃)₂] which were pH-adjusted incrementally through a range of 2.5 to 11.0. Samples were centrifuged after 24 hours to separate liquid from suspended sediment. Sediment-free solutions were run as controls. Lead concentrations were determined by atomic adsorption spectrophotometry. Results indicate that montmorillonite (Wyoming Bentonite) particles serve as lead adsorption nuclei over a broad pH range. Maximum sorption occurs as the solution reaches a pH of about 7.5. The kaolinite clay from Georgia strongly adsorbs trace amounts of lead at pH ranging from 3.0 to 4.5, where up to 95 percent of the lead is adsorbed by the clay. Little adsorption difference was found between the Fithian illite clay and zeolitic tuff from the Nevada Test Site in comparison to sediment-free solutions which were pH-adjusted. In concentrations of 10 to 20 mg/l montmorillonite and kaolinite clays serve as nucleation sites capable of adsorbing up to 95 percent of trace concentrations of lead within 24 hours. It appears that accumulations of lead in coastal lake and estuarine sediments are significantly influenced by:

1. pH changes which occur as river and coastal waters mix resulting in precipitation of lead, and
2. sorption of lead by suspended clays.

     

Environmental impacts by input of substances in landscapes of the District (Bezirk) of Leipzig,Germany

Problems of landscape and resource protection resulting from the intensification of land-use can be mastered only by intersectoral planning and a land management considering (landscape-)ecological principles right from the beginning. In the district of Leipzig ecological studies in the '80ies have focussed on:

1. Determination of the regional pattern of atmospheric immissions;
2. Registration of heavy metals in soil and vegetation;
3. Soil compaction, soil erosion;
4. Study of stress indicators in the aeration zone and in the top-most aquifer in order to examine barrier effects in the percolation process.

First results are discussed.     

Kyanite eclogites

Prior experimental work has shown that in the laboratory the mineralogy of eclogites is sensitive to the ratio of CaO ∶ MgO ∶ FeO and that the reaction pyroxene + kyanite?garnet + quartz proceeds to the right at high pressures in rocks rich in magnesium and to the left in rocks rich in calcium and iron. Typical basalts crystallized at high pressure never contain kyanite. The chemistry and mineralogy of a large number of naturally occurring eclogites show they belong to three classes.

1. Kyanite-free magmatic eclogites, rich in magnesium, from:
2. kimberlites
3. dunites and serpentinites.
4. Kyanite-bearing eclogites and grosspydites rich in CaO and low in FeO with intermediate MgO from:
5. kimberlites
6. gneisses.
7. Kyanite-free eclogites of metamorphic origin rich in iron with low magnesium and intermediate amounts of calcium from:
8. glaucophane schists
9. gneisses.

     

The Araguainha astrobleme/Central Brazil

The following facts have supported the origin of the Araguainha circular structure in Central Brazil by a meteoritic impact:

1. the almost circular contour
2. the impact-morphologic sequence including a central uplift, ring walls and a basin rim of escarpments
3. outcrops of suevites and mixed breccias
4. the evidence of shock metamorphism
5. the presence of shatter cones, and
6. negative anomalies of the total intensity of the magnetic field at the center of the ring structure.

     

Zur Geologie und Mineralogie der Kalk- und Gipskrusten Algeriens

Caliches: Large areas of the northern Sahara and the Algerian High Planes are covered by mostly 1–5 m thick caliches. Their age (Pliocene in the Sahara) decreases to the north and their precipitation is generally independent of groundwater. Their profile is composed (from top to base) as follows:

Upper soil, loose and mostly of eolian origin.

Upper part of caliche, with very characteristic, dense, partly layered-knobby texture, formed slowly by solutional and reprecipitational processes of ± freely outcropping caliches under addition of eolian material.

Under part of caliche, highly porous, somewhat chalky and greyish-white; precipitated mainly by capillar rise of solutions in permeable and calcareous rocks.

Substratum, preferentially calcareous sandstones, alluvial deposits and marls.

The mineralogy of the caliches (whose main components are represented in fig. 4 A-C) is rather monotonous: in addition to relicts of the substratum (partly dissolved or pushed aside by precipitation of calcite), there are only newly formed low-Mg-calcite and some quarzine (length-slow quartz). Sr-contents of calcite rise clearly from substratum to upper part of caliche. Gypsiferous Crusts (or Cementations): They are found mainly in the surroundings of Chotts (flat, ± saline lakes) and in oases of the NE-Algerian Sahara. Their formation began — mostly caused climatically — after the period of caliche formation and is still continuing in some places. Most of these gypsum-crusts are formed by evaporation of near-surface groundwaters in sandy soils. Water saturated in gypsum precipitates large crystals of gypsum (relatively low in Sr), partly filled by sand, at groundwater-surface. Fine crystalline crusts (relatively high in Sr) are formed by ascendent waters with lower gypsum content ± directly under the landsurface.     

The sedimentation of copper-bearing shales (Kupferschiefer) in the sudetic foreland

The profile of the copper-bearing shales (Kupferschiefer) displays 3 types of lithological developments of the Sudetic Foreland:

in reduction facies: with the notably diminishing of participation of clay minerals and organic substance from bottom to top.

Many horizons of concentration of copper, lead and zinc are present.

in oxidation facies: where marley shales are forming, the metals decrease to trace contents.

in intermediate zones, from reduction facies to oxidation facies shales, the lithological formation is changing and the greatest variability is encountered in the intensity of metal contents.

The concentrations of copper sulphides occur in lagoonal areas in which an oxigen deficiency was associated with a great amount of organic matter. The beds formed in the oxidation facies represent the coastal zones of an open sea basin where the influence of inflow and outflow were considerable.     

The roles of pedogenesis and diagenesis in clay mineral assemblages: Lower Cretaceous fluvial mudrocks,Nova Scotia,Canada

Clay mineral assemblages in alluvial mudrocks are important for paleoclimatic interpretation and for understanding burial diagenetic cementation in sandstones, but it is commonly difficult to unravel the relative importance of source weathering, pedogenesis and diagenesis in their origin. The clay mineral assemblages in fluvial overbank mudrocks from the Lower Cretaceous Chaswood Formation in central Nova Scotia, investigated by X-ray diffraction analysis of the < 2 µm fraction of 45 samples, include kaolinite, illite, vermiculite, and mixed layer kaolinite/expandable clay and mica/vermiculite. The assemblages vary with depositional facies. Wetland organic-rich mudrocks have large amounts of amorphous material and kaolinite is the dominant clay mineral. In the eastern part of the basin, where overbank mudrocks were episodically uplifted by syn-sedimentary strike-slip faulting, cumulate ultisol and alfisol paleosols are common. In the ultisols, hematite is enriched and kaolinite increases at the expense of illite in the B horizon. Alfisols contain more illite and vermiculite and the B horizon is enriched in goethite. In the western part of the basin, where thin sandstones with abundant diagenetic kaolinite cement are interbedded with the mudrocks, the distinctive clay mineral assemblage of mica/vermiculite mixed layer, vermiculite with 15.5 Å peak, and kaolinite/expandable mixed layer clay with a 17.7 Å peak is interpreted to result from bacterially-mediated oxidation of organic matter below the paleo-water table during early burial diagenesis. Deeper burial diagenesis may lead to slightly higher kaolinite crystallinity. Volcanic ash appears to alter to kaolinite/expandable mixed layer clay with a 7.9 Å peak. Comparison with the continuously subsiding and rapidly accumulated Wessex Formation of southern England, formed at a similar paleolatitude, shows the strong role of pedogenic processes and early diagenesis by meteoric water in development of clay mineral assemblages in the locally tectonically uplifted Chaswood Formation.     

Charriage crustal,anatexie et decrochements ductiles dans les Maures orientales (Var,France) au cours de l'orogenese varisque

The kinematics of the deformational events recorded in the catazonal gneisses within the eastern part of the Maures massif (Variscan basement of Provence, southeastern France) has been established. These events can be correlated with both the metamorphic and the magmatic evolution, and the orogenic history of the eastern Maures then consists of the following stages:

1. -HP-metamorphism preserved only in relict eclogites and quartzites rich in calc-silicates,
2. -Catazonal metamorphism involving anatectic melting, broadly contemporaneous with large-scale horizontal transport towards the NNE. During this event, the lithologic units were disrupted and intensively mylonitized.
3. -Intrusion of a first generation of anatectic granitoids.
4. -A second tectonic event under epi-to mesozonal metamorphism conditions is responsible for the northward displacement of the eastern Maures relative to the western part along a sinistral strike-slip fault 4 km wide (Ramatuelle — Plan de la Tour fault) in which the early anatectic granitoids have been mylonitized. Outside the fault zone, this event is marked by upright to W-vergent open folds trending N-S, i.e. parallel to the transport direction.
5. -A moderate cataclastic reactivation of the Ramatuelle — Plan de la Tour fault with a dextral sense of shear, locally accompanied in the eastern part of the area by minor lowtemperature thrusting towards the south.
6. -Intrusion of a second generation of anatectic granites about 320 Ma ago.

The metamorphic, magmatic and tectonic evolution of the eastern Maures suggests a continuous orogenic history in an area of rapid crustal thickening by large-scale thrusting within the continental crust. This evolution may be related to the development of continental subduction during the continent/continent collision responsible for the Variscan orogeny in southern Europe.