Discovery of an outcropping Mass Transport Deposit (MTD) in the Palaeogene subalpine synclines of southeastern France: Implications on flexural sub-basin deformation

We document the sedimentary facies of a large Mass Transport Deposit (MTD) within the sand-rich sediment gravity flow-dominated deposits of the Eocene–Oligocene south-western Alpine forelands (Annot Sandstone system). The MTD with an approximated volume of several hundreds of cubic kilometres fills a sub-basin located in the Mont-Tournairet confined sub-basin. Autochthonous facies are very typical sediment gravity-flow deposits (thin-bedded classical turbidites and thick-bedded hyperconcentrated to concentrated flow deposits) that stratigraphically belong to the Annot Sandstone infill. Slumps and internal metre-large fold axes of the deformed stratigraphic intervals indicate a main transport direction towards the northwest. The seafloor instability that led to the mass-flow events within the Mont-Tournairet sub-basin could have been favoured by high sedimentation rates in a small, confined and tectonically active sub-basin, possibly enhanced by local structural deformation associated with the Triassic evaporites on the eastern side of the Mont-Tournairet confined basin. The presence of the MTD suggests that a period of increased flexural subsidence rate and basin deformation occurred in this portion of the subalpine foreland basin. Therefore, the MTD forms a stratigraphic marker of a period of tectonic activity.     

The Jurassic Prehodavci Formation of the Julian Alps: easternmost outcrops of Rosso Ammonitico in the Southern Alps (NW Slovenia)

The Julian Alps are located in NW Slovenia and structurally belong to the Julian Nappe where the Southern Alps intersect with the Dinarides. In the Jurassic, the area was a part of the southern Tethyan continental margin and experienced extensional faulting and differential subsidence during rifting of the future margin. The Mesozoic succession in the Julian Alps is characterized by a thick pile of Upper Triassic to Lower Jurassic platform limestones of the Julian Carbonate Platform, unconformably overlain by Bajocian to Tithonian strongly condensed limestones of the Prehodavci Formation of the Julian High. The Prehodavci Formation is up to 15 m thick, consists of Rosso Ammonitico type limestone and is subdivided into three members. The Lower Member consists of a condensed red, well-bedded bioclastic limestone with Fe–Mn nodules, passing into light-grey, faintly nodular limestone. The Middle Member occurs discontinuously and consists of thin-bedded micritic limestone. The Upper Member unconformably overlies the Lower or Middle Members. It is represented by red nodular limestone, and by red-marly limestone with abundant Saccocoma sp. The Prehodavci Formation unconformably overlies the Upper Triassic to Lower Jurassic platform limestone of the Julian Carbonate Platform; the contact is marked by a very irregular unconformity. It is overlain by the upper Tithonian pelagic Biancone (Maiolica) limestone. The sedimentary evolution of the Julian High is similar to that of Trento Plateau in the west and records: (1) emergence and karstification of part of the Julian Carbonate Platform in the Pliensbachian, or alternatively drowning of the platform and development of the surface by sea-floor dissolution; (2) accelerated subsidence and drowning in the Bajocian, and onset of the condensed pelagic sedimentation (Prehodavci Formation) on the Julian High; (3) beginning of sedimentation of the Biancone limestone in the late Tithonian.     

Facies and basin architecture of the Late Carboniferous Salvan-Dorénaz continental basin (Western Alps, Switzerland/France)

The Salvan‐Dorénaz Basin formed during the Late Palaeozoic within the Aiguilles‐Rouges crystalline basement (Western Alps) as an asymmetric, intramontane graben elongated in a NE–SW direction and bounded by active faults. At least 1700 m of fluvial, alluvial fan and volcanic deposits provide evidence for a strong tectonic influence on deposition with long‐term, average subsidence rates of > 0·2 mm yr^?1. The early basin fill was associated with coarse‐grained alluvial fans that were dominated by braided channels (unit I). These issued from the south‐western margin of the basin. The fans then retreated to a marginal position and were overlain by muddy floodplain deposits of an anastomosed fluvial system (unit II) that drained towards the NE. Deposition of thick muds resulted from a reduction in the axial fluvial gradient caused by accelerated tectonic subsidence. Overlying sand‐rich meandering river deposits (unit III) document a reversal in the drainage direction from the NE to the SW caused by synsedimentary tectonism, reflecting large‐scale topographic reorganization in this part of the Variscides with subsidence now preferentially in the W and SW and uplift in the E and NE. Coarse‐grained alluvial fan deposits (unit IV) repeatedly prograded into, and retreated from, the basin as documented by coarsening‐upward cycles tens of metres thick reflecting smaller scale tectonic cycles. Volcanism was active throughout the evolution of the basin, and U/Pb isotopic dating of the volcanic deposits restricts the time of basin development to the Late Carboniferous (308–295 Ma). ⁴⁰Ar/³⁹Ar ages of detrital white mica indicate rapid tectonic movements and exhumation of the nearby basement. In unit I, youngest ages are close to that of the host sediment, but the age spectrum is wide. In unit II, high subsidence and/or sedimentation rates coincide with very narrow age spectra, indicating small, homogeneous catchment areas. In unit III, age spectra became wider again and indicate growing catchment areas.     

Identification of Quaternary thrusts, folds and faults in a low seismicity area: examples in the Southern Alps (France)

In low seismicity areas, folds, faults and striated pebbles in recent alluvial deposits can demonstrate the Quaternary activity of tectonic structures and can reveal their kinematics. In the Digne nappe (Southern Alps), an out-of-sequence thrust occurred in the late Quaternary in response to WSW-trending compression. The presence of late Quaternary compressional deformation in the Valavoire thrust could have resulted from the activity of the underlying Durance flexure with a maximum Pliocene–Quaternary uplift rate of about 0.1 mm yr⁻¹. The Quaternary top surface of the Valensole basin, that truncates SW-vergent thrust propagation folds, is folded above the Lambruissier anticline. Exceptional conditions resulted in the local preservation of this Quaternary fold morphology created with a minimum uplift-rate of 0.05 mm yr⁻¹ under a NE-trending compression. At the front of the Digne nappe the deformation is characterized by WSW to WNW trends of compression and low strain rates during the Quaternary period.     

Remobilisation of deep Na-Cl waters by a regional flow system in the Alps: Case study of Saint-Gervais-les-Bains (France)

The hydrothermal system of Saint-Gervais-les-Bains, France is located in a south western low-elevation point of the Aiguilles Rouges crystalline Massif. The crystalline rocks are not directly outcropping in the studied area but certainly exist beyond 300 m depth. Uprising waters are pumped from two different aquifers below the Quaternary deposits of the Bon Nant Valley. In the Lower Trias-Permian aquifer crossed by De Mey boreholes (27–36 °C), the ascending Na-SO₄ and high-Cl thermal water from the basement (4.8 g/L) is mostly mixed by a Ca-SO₄ and low-Cl cold water circulating in the autochthonous cover of the Aiguilles Rouges Basement. The origin of the saline thermal water probably results from infiltration and circulation in the basement until it reaches deep thrust faults with leaching of residual brines or fluid inclusions at depth (Cl/Br molar ratio lower than 655). The dissolution of Triassic halite (Cl/Br > 1000) is not possible at Saint-Gervais-les-Bains because the Triassic cold waters have a low-Cl concentration (< 20 mg/L). Water–rock interactions occur during the upflow via north–south strike-slip faults in the basement and later on in the autochthonous cover. For the De Mey Est borehole, gypsum dissolution is occurring with cationic exchanges involving Na, as well as low-temperature Mg dissolution from dolomite in the Triassic formations. The aquifer of imbricated structures (Upper-Middle Trias) crossed by the Lépinay well (39 °C) contains thermal waters, which are strongly mixed with a low-Cl water, where gypsum dissolution also occurs. The infiltration area for the thermal end-member is in the range 1700–2100 m, close to the Lavey-les-Bains hydrothermal system corresponding to the Aiguilles Rouges Massif. For the Ca-SO₄ and low-Cl end-member, the infiltration area is lower (1100–1300 m) showing circulation from the Mont Joly Massif. The geothermometry method indicates a reservoir temperature of probably up to 65 °C but not exceeding 100 °C.     

Integrated 3D geophysical and geological modelling of the Hercynian Suture Zone in the Champtoceaux area (south Brittany, France)

This paper combines geological knowledge and geophysical imagery at the crustal scale to model the 3D geometry of a segment of the Hercynian suture zone of western Europe in the Champtoceaux area (Brittany, France). The Champtoceaux complex consists of a stack of metamorphic nappes of gneisses and micaschists, with eclogite-bearing units. The exhumation of the complex, during early Carboniferous times, was accompanied by deformation during regional dextral strike–slip associated with a major Hercynian shear zone (the South Armorican Shear Zone, SASZ). Dextral shearing produced a km-scale antiformal structure with a steeply dipping axial plane and a steeply eastward plunging axis. Armor 2 deep seismic profile shows that the regional structure was cut by a set of faults with northward thrusting components. Based on the seismic constraint, direct 2D crustal-scale modelling was performed throughout the Champtoceaux fold on seven radial gravity profiles, also using geological data, and density measurements from field and drill-hole samples. The 3D integration of the cross-sections, the digitised geological map, and the structural information (foliation dips) insure the geometrical and topological consistency of all sources of data. The 2D information is interpolated to the whole 3D space using a geostatistical analysis. Finally, the 3D gravity contribution of the resulting model is computed taking into account densities for each modelled geological body and compared to the Bouguer anomaly. The final 3D model is thus compatible with the seismic and gravity data, as well as with geological data. Main geological results derived from the modelling are (i) the overall 3D geometry of the south dipping thrust system interpreted on the seismic profile emphasises northward thrusting and folding of the Champtoceaux complex which was coeval with strike–slip along the South Armorican Shear Zone; (ii) the gravity modelling suggests the presence of a relatively dense body below the Champtoceaux complex that could be interpreted as a result of relative uplift of midcrustal material during thrusting along the E–W trending wrench–thrust system; (iii) the northern limb of the Champtoceaux anticline is a relatively shallow feature; and (iv) Vigneux synkinematic granitic body is a laccolith sheared and rooted along the southern branch of the SASZ and spreads away from the strike–slip zone within weak country-rocks.     

High-resolution (2–7 kHz) acoustic and geometric characters of submarine creep deposits in the South Korea Plateau, East Sea

A synthesis of high-resolution (Chirp, 2–7 kHz) seismic profiles in the South Korea Plateau reveals that large masses of wavy stratified sediment (≈60–90 m thick) cover broad, gently sloping (<0·5°) ridges in water depths of 1000–2000 m. The wavy stratified sediment (WSS) is characterized by wavy (0·2–5 km in wavelength and <15 m in relief), continuous reflective layers with a basal deformed zone that overlies undeformed, strong reflectors. The WSS exhibits systematic variation in wave dimensions and thickness of internal reflective layers with changes in slope gradient. The troughs of the waves are commonly associated with internal growth faults, and wave amplitude generally increases with subbottom depth. On steep slopes around the ridges, the WSS masses are bounded downslope by slide and slump deposits including slightly translated or rotated WSS blocks. The acoustic and geometric characters, and association with downslope slides and slumps on the steeper slopes, suggest that the WSS masses were most probably formed by slow creep movement before slope failure. In the absence of significant sediment input to the South Korea Plateau, the deep (1000–2000 m in water depth) mass movements were probably triggered by earthquakes that have occurred frequently in this region. Some slightly displaced, intact WSS blocks in the associated slides and slumps downslope reflect a progressive evolution from submarine creep into slide and slump.     

Impact of synsedimentary metre-scale normal fault scarps on sediment gravity flow dynamics: An example from the Grès d’Annot Formation, SE France

Synsedimentary faults result in the direct interaction between tectonic and sedimentary processes at similar spatio-temporal scales. Sedimentological analysis of sediment-laden gravity flows in the northern part of the Grès d'Annot Formation (Sanguinière sub-basin, Col de la Moutière/Tête Ronde) has revealed the presence of fault scarps of metre-scale height. These synsedimentary fault scarps were sufficient to disturb the sediment gravity flow dynamics resulting in (i) a strong variation of the erosive behaviour of a concentrated flow and (ii) the transformation of a strongly stratified, laminar hyperconcentrated flow into a turbulent flow, in short distance (less than 500 m). These disturbances develop without the flows being deviated by the fault scarps but produce great facies heterogeneity, the least homogeneous facies (mixing sand and clay) being localized on the upstream obstacle side, the most homogeneous (massive sand) downstream.     

Hydromechanical properties and alteration of natural fracture surfaces in the Soultz granite (Bas-Rhin, France)

Natural fractures are characterized by rough surfaces and complex fluid flows. A large distribution of apertures (residual voids) within their walls and the presence of contact points (in situ normal loads) produce heterogeneous flows (channeling). The resulting permeabilities, porosities or fluid–rock exchange surfaces cannot be realistically modeled by parallel and smooth plate models. Four natural fractures are sampled at different depths and degrees of alteration in the Soultz sandstone and granite (EPS1 drillhole, Soultz-sous-Forêts, Bas-Rhin, France). The fracture surfaces are measured with mechanical profilometry and maps of asperity heights (XYZ). Resulting local apertures (XYe) are then calculated. A statistical study of the surface profiles (XZ) show that the fractures are more or less rough and tortuous according to the types of alteration. Altered samples are characterized by smoother surfaces of fractures. Such differences imply that (i) the average fracture aperture is not representative for the whole fracture and that (ii) the different local apertures should be integrated in hydraulic and mechanical models. A hydraulic model (finite difference calculations) of fluid flow, taking into account the elastic closure (Hertz contact theory) of fractures with depth, is used. Maps of contact points and relative local loads within the fracture planes are compared to flow maps. They show different channeling of fluid flows. Strongly altered fractures are characterized by homogeneous fluxes despite the presence of numerous contact zones during the closure of fracture. By contrast, fresh fractures develop, increasing fluid flow channels with depth.Fracture closure (increasing normal stress) does not systematically increase the channeling of fluid flow. There is evidence for a general smoothing out of the irregularities of the fracture walls due to precipitation of secondary minerals, indicating that the cubic law can be commonly valid, also at great crustal depth but this validity depends on the degree of fracture alteration. Mineralogical and geochemical observations, thus, should be taken into account to perform more accurate permeability calculations and models of fluid circulation in fracture networks.     

Anatomy of ancient passive margin slope systems: Aptian gravity-driven deposition on the Vocontian palaeomargin, western Alps, south-east France

The Aptian succession on the Vocontian palaeomargin (south-east France) consists of marl and marly calcareous pelagic slope facies together with a range of gravity-driven deposits (slumps, debris-flow deposits, turbidite packages and massive sandstones). The massive sandstones were emplaced by high-density turbidity currents and are associated with extensive clastic sills and dykes. The sedimentology is constrained by a high-resolution bio- and lithostratigraphic framework and permits a detailed analysis of the slope succession including: (1) a sequence stratigraphical analysis of the slope deposits; and (2) lateral tracing of individual sedimentary packages downslope. The resulting model for the Vocontian slope represents an alternative to the ‘classic’ Exxon delta-fed, mud-rich model. Key elements of the Vocontian model are: (1) an emphasis on lowstand slope erosion and complex slope morphology controlled by contemporary tectonism and salt diapirism; (2) slope deposition in confined erosional and structurally controlled conduits rather than the buildout of slope fans/channel-levee complexes; (3) a dominance of large-volume muddy slump and transitional debris-flow deposits, with subordinate sandy turbidites, including significant massive sandstone facies; (4) common sand injections (sills and dykes) associated with the massive sandstone facies; and (5) minimal downslope evolution of the flows, with the nature of the source sediment being the over-riding factor determining flow behaviour and deposit character. The Vocontian system is a rare instance in which large sections of a ‘fossil’ passive margin slope are preserved in the geological record. The slope deposits differ from the classic models of turbidite systems that have mainly been built from peripheral foreland basins, and the new insight makes it possible to compare ancient and present-day passive margin slope systems.     

Coupling between hydrogeology and deformation of mountainous rock slopes: Insights from La Clapière area (southern Alps,France)

Meteoric infiltration influence on large mountainous rock slopes stability is investigated by comparing hydrogeologic and gravitational structures from detailed mapping of the ‘La Clapière’ slope. The slope infiltrated waters are trapped in a perched aquifer that is contained in deposits inside tensile cracks of the upper part of the slope. Flow rates of 0.4 to 0.8 l?s^?1 from the perched aquifer to the landslide cause landslide accelerations. Numerical modeling shows that a 0.75 l?s^?1 infiltration yield increases conditions for toppling with failure through tilting of large rock volumes from the perched aquifer bottom down to the foot of the slope. To cite this article: Y. Guglielmi et al., C. R. Geoscience 337 (2005).     

The Valais units in Savoy (France): a key area for understanding the palaeogeography and the tectonic evolution of the Western Alps

The Valais units in Savoy (Zone des Brèches de Tarentaise) have been re-mapped in great detail and are subject of combined stratigraphic, structural and petrological investigations summarized in this contribution. The sediments and rare relics of basement, together with Cretaceous age mafic and ultramafic rocks of the Valais palaeogeographical domain, represent the heavily deformed relics of the former distal European margin (External Valais units) and an ocean–continent transition (Internal Valais unit or Versoyen unit) that formed during rifting. This rifting led to the opening of the Valais ocean, a northern branch of the Alpine Tethys. Post-rift sediments referred to as “Valais trilogy” stratigraphically overlie both External and Internal Valais successions above an angular unconformity formed in Barremian to Aptian times, providing robust evidence for the timing of the opening of the Valais ocean. The Valais units in Savoy are part of a second and more external mid-Eocene high-pressure belt in the Alps that sutured the Briançonnais microcontinent to Europe. Top-N D1-deformation led to the formation of a nappe stack that emplaced the largely eclogite-facies Internal Valais unit (Versoyen) onto blueschist-facies External Valais units. The latter originally consisted of, from internal to external, the Petit St. Bernard unit, the Roc de l’Enfer unit, the Moûtiers unit and the Quermoz unit. Ongoing top-N D2-thrusting and folding substantially modified this nappe stack. Post 35 Ma D3 folding led to relatively minor modifications of the nappe stack within the Valais units but was associated with substantial top-WNW thrusting of the Valais units over the Dauphinois units along the Roselend thrust during W-directed indentation of the Adria block contributing to the formation of the arc of the Western Alps.     

Repeated cycles of submarine channel incision, infill and transition to sheet sandstone development in the Alpine Foreland Basin, SE France

The Grès de Champsaur turbidite system, deposited in a distal setting in the Alpine Foreland Basin of south‐eastern France, exhibits a repeated upsection alternation in sand body geometry between incised channels and sheet sands. The channels form symmetric lenticular erosional features, of width 900–1000 m (measured between the lateral limits of incision) and depth 65–115 m, and can be traced axially for up to 5 km. In each case, the channel fill is capped by a laterally persistent sandy sheet‐form interval, which lies upon a fine‐grained substrate beyond the channel margins. No intrachannel elements have been traced into the substrate sequence, suggesting that, before infill, the channels acted as open sea‐floor conduits of essentially the same dimensions as the preserved channel deposits. The channels are vertically stacked, although axial erosion juxtaposes younger channel axis deposits against the fill of older channels and their channel‐capping sheet sandstones to produce an apparently well‐connected composite sandstone body geometry. The predominant channel‐fill facies comprises coarse‐grained, amalgamated sandstones, which are commonly parallel‐ or cross‐stratified. Subsidiary facies of finer grained sandstone–mudstone couplets and clast‐bearing muddy debrites are commonly preserved as erosional remnants, suggesting a complex channel history of aggradation and erosion. The repeated cycles of channel incision, infill and transition to sheet sandstone development indicate repetitive incision and healing of the palaeo‐sea floor. A model is proposed that links incision to the development of relatively steep axial gradients (parallel to the mean dispersal direction) and the return to sheet‐form deposition to the re‐establishment of lower axial gradients, with the repetitive switch between incisional channels and sheet sandstones driven by changes in sediment input rate against a background of ongoing sea‐floor tilting.     

Carbonate platform-to-basin correlation by means of grain-composition logs: an example from the Vercors (Cretaceous, SE France)

Quantitative logs of grain composition for several sections of platform top and flank of the Vercors carbonate platform (Early Cretaceous, SE France) provide platform-to-basin correlation with a resolution of a few metres over an area of 70 km². Grain composition was determined by point-counting thin sections. Point-count groups that characterize palaeoenvironmental realms (i.e. open sea, platform margin) were defined for the platform–basin trajectory. Grain-composition logs revealed marked peaks in the number of open-sea biota and peaks in ooid abundance. The peaks in open-sea biota correspond to back-stepping intervals and deepening upward facies successions at the platform margin. These peaks probably relate to incipient drowning of the platform and may be used to delineate marine-flooding surface-bounded sequences. Peaks in ooid occurrence show no relationship with the progradation, aggradation or retreat of the platform. Apparently, the oolitic sands were not part of a facies tract that shifted up and down the platform. Instead, they represent a depositional mode that was either on or off. Times of prolific ooid production and shedding probably occurred during wide but shallow submergence of the platform, accompanied by suitable water chemistry. Peaks in both ooids and open-sea biota are excellent markers for platform-to-basin correlation, as they are recorded in successions on the platform top as well as on the flank. Altogether, the grain-composition logs show that each of the lithologically rather similar platform tongues of the Vercors has a unique signature or compositional fingerprint. These compositional fingerprints are most helpful in evaluating the lateral extent of different stratigraphic units. In outcrops of the Vercors platform, the physical tracing of bedding surfaces delineate wedges of toe-of-slope sediments that show a conspicuous thinning towards the platform. However, our correlation shows that these sediment bodies are not truly basin-restricted wedges but have a platform top equivalent. This implies that these units were, at least partly, deposited during high stands of sea level that flooded the platform.     

Perched lobe formation in the Gulf of Cadiz: Interactions between gravity processes and contour currents (Algarve Margin, Southern Portugal)

The Gulf of Cadiz is swept by the strong saline Mediterranean Outflow Water (MOW). On the Algarve Margin (South Portugal), this current has constructed fine-grained contourite drifts. This margin is dissected by the Portimao Canyon and three short channels that only incise the upper slope, and are absent on a terrace located at mid-slope depths along the Algarve Margin. High-resolution seismic profiles and sediment cores highlight the original architecture of the sedimentary deposits on this terrace. Coarse-grained lenticular chaotic bodies formed during major relative sea-level lowstands are intercalated within the drift. The lobate shape and sandy nature of the lenticular chaotic bodies and their location at the mouths of the three short channels suggest they are gravity-generated deposits that are perched on the middle continental slope.In the Gulf of Cadiz, the interaction between contour current and gravity processes is strongly controlled by climatic variations and relative sea-level changes during the late Quaternary. During cold periods when sea-level was low, erosion intensified on the continental shelf and the deepest part of MOW was active. Sediment was transported downslope through the channels and deposited on sedimentary lobes perched on the mid-slope terrace. During warm periods when relative sea-level was high, the supply of sediment from the shelf was shut off and the shallowest part of MOW was more active. Contourite drifts fill the channels and bury the sandy lobes.     

新疆库车盆地康村地区砂岩型铜矿成矿模式研究

新疆库车盆地分布着一系列砂岩型铜矿床和矿点,文章在野外调查的基础上开展康村地区铜矿沉积环境和成矿模式研究,总结了区内沉积环境、沉积相与铜矿化体的配置关系,进而探讨了矿化成因和成矿模式。结果表明：康村地区新近系中可以识别出主要的沉积相类型有冲积扇相、扇三角洲相和湖泊相,以及5种亚相和7种微相。砂岩型铜矿化与沉积环境相关,矿（化）体主要分布在扇三角洲平原相和扇三角洲前缘相。2件样品LA-ICP-MS测年数据分别为427.4±6.6 Ma、387.3±3.3 Ma和424.6±2.3 Ma、279.9±6.8 Ma,表明康村地区中新统吉迪克组（N1j）含铜砂砾岩的物源主要来自于库车盆地北西南天山造山带古生代晚志留世至早二叠世地质演化所形成的地层和岩浆岩产物。康村地区的矿化类型为砂岩型铜矿化,深部铜富集为硫酸盐热化学还原作用（TSR）型矿化成因,近地表矿化为后期盆地地上和地下卤水改造成因。库车盆地康村地区的铜矿化形成于新近系砂岩成岩之后,为后生富集,主要受新近纪后期断裂构造及后生卤水控制。     

Fault array evolution in extensional basins: insights from statistical analysis of gravel deposits in the Cecina River (Tuscany,Italy)

Two statistical analyses of gravel clasts from the Lower Pleistocene deposits in the Lower Cecina Valley (Tuscany, Italy) have been combined to unravel changes in the palaeo‐drainage system. Data from 16 outcrops were collected and 6400 clasts described. Facies analysis, micro‐palaeontology and macro‐palaeontology and petrographic characteristics of the gravel deposits have highlighted the presence of three allostratigraphic units. Clast lithology is the main discriminator among these units. Cluster and principal component analyses of the 6400 clasts have improved understanding of the stratigraphy of the Lower Pleistocene deposits and constrain the re‐routing of the lower palaeo‐Cecina River from a supposedly south‐east to north‐west direction to the present east to west direction. Short rivers feeding small fan deltas represented by the oldest allostratigraphic units were abandoned in the Lower Pleistocene, when the re‐routing of the Cecina River caused the capture of these streams. This evolution suggests a change in the tectonic regime of the area. The fan deltas developed on the hanging wall of normal faults sub‐parallel to the coast; a change to a transtensile tectonic regime caused the deviation of the main river channel toward the present coast and the formation of a pull‐apart basin, which is now exploited by the Cecina River. This study illustrates the value of lithological analyses of gravel deposits for understanding the tectonic evolution of an area.     

鄂尔多斯盆地店头地区砂岩型铀矿成因初步探讨

通过电子显微镜及电子探针分析,对鄂尔多斯盆地东南部店头地区砂岩型铀矿物的赋存状态、主要伴生元素及其沉淀机制进行了初步探讨。研究区主要的铀矿物类型为沥青质铀矿,以显微浸染状充填于砂岩孔隙及胶结物中;与U密切伴生的元素有Fe、Se、Pb、Si、Ti、Cr、S等,与铀矿物伴生的矿物组合有黄铁矿、方铅矿、钛铀矿以及与其关系密切的硒矿化;主要含铀层位富含有机质及黄铁矿,夹有透镜状、条带状煤线,具备丰富的还原物质,有利于铀元素的还原富集,因此认为影响研究区铀沉淀的主要地球化学障是还原障和吸附障。     

On the tectonics of the Neocomian Rio do Peixe Rift Basin, NE Brazil: Lessons from gravity, magnetics, and radiometric data

A geophysical perspective based on well-acquired gravity, magnetic, and radiometric data provides good insights into the basin architectural elements and tectonic evolution of the Rio do Peixe Basin (RPB), an Early Cretaceous intracontinental basin in the northeast Brazilian rift system, which developed during the opening of the South Atlantic. NW–SE-trending extensional forces acting over an intensively deformed Precambrian basement yielded a composite basin architecture strongly controlled by preexisting, mechanically weak fault zones in the upper crust. Reactivated NE–SW and E–W ductile shear zones of Brasiliano age (0.6 Ga) divided the RPB into three asymmetrical half-grabens (Brejo das Freiras, Sousa, and Pombal subbasins), separated by basement highs of granite bodies that seem to anchor and distinguish the mechanical subsidence of the subbasins. Radiometric and geopotential field data highlight the relationship between the tectonic stress field and the role of a preexisting structural framework inserted in the final rift geometry. The up-to-2000 m thick half-grabens are sequentially located at the inflexion of sigmoidal-shaped shear zones and acquire a typical NE–SW-oriented elliptic shape. The Sousa Subbasin is the single exception. Because of its uncommon E–W elongated form, three-dimensional gravity modeling reveals an E–W axis of depocenters within the Sousa Subbasin framework, in which the eastern shoulders are controlled by NE–SW-trending faults. These faults belong to the Precambrian structural fabric, as is well illustrated by the gamma ray and magnetic signatures of the basement grain. Release faults were identified nearly perpendicular or oblique to master faults, forming marginal strike ramps and horst structures in all subbasins. The emplacement mechanism of Brasiliano granites around the RPB was partially oriented by the same structural framework, as is indicated by the gravity signature of the granitic bodies after removal of the gravity effect of the basin-filling deposits. The RPB major-fault occurrence along the releasing bend of a strong discontinuity – the so-called Portalegre Shear Zone – in addition to the configuration of a gentle crustal thinning, according to gravity field studies, suggests that a crustal discontinuity governs the nucleation of the RPB, followed probably by small displacement in deep crustal levels accommodating low-rate stretching during basin subsidence.     

The Zn-Pb deposits of Casario (Ligurian Alps,NW Italy): Late Palaeozoic sedimentary-exhalative bodies affected by the alpine metamorphism

In the Ligurian Alps (South-Western Italian Alps), Zn-Pb deposits occur within late Palaeozoic meta-sedimentary units belonging to the Briançonnais Zone near Casario (Tanaro valley). Different types of sulphide-rich, lens-shaped mineralizations are recognized: sphalerite-galena massive sulphide bodies, pyrite-rich lenses and sulphide-rich quartz–carbonate-chloritoid granofels. Sulphide lenses and host rocks are affected by at least three ductile deformation phases and by a polyphase alpine metamorphism, whose climax conditions are estimated, based on P-T pseudosection calculations, at T = 300-325 °C and P = 0.55-0.60 GPa. In all the mineralized lenses the ore minerals are represented, in variable amount, by Fe-poor sphalerite, galena, pyrite and arsenopyrite (± tetrahedrite, chalcopyrite and pyrrhotite); the gangue consists of quartz, carbonate (sideritemagnesite ± rhodochrosite s.s.), Fe-chloritoid, muscovite-phengite and chlorite. The mineralizations are associated with chloritoid – carbonate micaschists displaying a finely bedded texture, with sharp between-bed compositional contrast, which suggests their exhalative origin.

In spite of the tectono-metamorphic overprint, some pre-metamorphic features of the hydrothermal system are still recognized, like relics of the hydrothermal feeding system, primary growth textures and sulphide-rich microbreccias. These massive sulphide lenses, which share many characters with the SEDEX deposits, testify to the occurrence of an exhalative event of Upper Carboniferous age previously unrecognized in the Ligurian Briançonnais Unit.