Armorican Massif Research Articles

Biostratigraphical and palaeobiogeographical implications of Lower Silurian Radiolaria from black cherts of the Armorican Massif (France)

Abstract. A moderately well-preserved radiolarian assemblage was recovered from organic-rich black chert in a Llandovery (Lower Silurian) sequence that crops out in southern Brittany (Chalonnes-sur-Loire section, Armorican Massif, France). The assemblage is composed of two families (Rotasphaeridae and Haplotaeniatidae), four genera (?Diparvapila, Secuicollacta, Orbiculopylorum, Haplotaeniatum) and 13 species. Some were identified from whole specimens preserved in silica and extracted following dilute hydrofluoric acid processing, while others were recognized in thin-section preparations, as they are beautifully preserved as ‘carbonized’ microfossils. The age range suggested by conodonts and chitinozoans yielded after HF processing from one of the 27 studied samples is in good agreement with the previously published age based on graptolites. The recovered radiolarians are discussed, documented and compared with known Rhuddanian, Aeronian and lower Telychian assemblages in the literature. The stratigraphic ranges are extended for the species Secuicollacta bipola, S. hexactinia, S. parvitesta, Orbiculopylorum granti and O. splendens based on our new data. A significant number of radiolarians found in our samples occur in Llandovery sections from Alaska, Nevada, Arctic Canada and Sweden. These similarities are used to discuss the palaeodistribution of Lower Silurian Radiolaria and our observations support the hypothesis of a wide geographical distribution for these Palaeozoic species.

A U–Pb zircon age (479 ± 5 Ma) from the uppermost layers of the Ollo de Sapo Formation near Viveiro (NW Spain): implications for the duration of rifting-related Cambro-Ordovician volcanism in Iberia

AbstractThe uppermost metavolcanic layer of the Cambro-Ordovician Ollo de Sapo Formation, the largest accumulation of pre-Variscan igneous rocks in the Iberian Peninsula, have been dated in its northernmost part using U–Pb SHRIMP-RG zircon age techniques at 479.0 ± 4.7 Ma. The age obtained is the youngest age found so far in the metavolcanic facies of Ollo de Sapo Formation and represents the cessation of the rifting-related Cambro-Ordovician Ollo de Sapo volcanism at the northernmost tip of the Iberian Peninsula. Our results show that the Cambro-Ordovician volcanism in the NW of the Iberian Peninsula is not as short-lived as previously thought and confirm the correlation between the Cambro-Ordovician volcanic sequences that crop out in the Central Iberian Zone and the French Southern Armorican Massif. Finally, our study suggests that the cessation of the Cambro-Ordovician volcanism along the Ibero-Armorican Arc was synchronic or, less probably, slightly diachronic with younger ages towards the north (in present-day geographical coordinates).

The earliest wood and its hydraulic properties documented inc. 407-million-year-old fossils using synchrotron microtomography

We document xylem structure and hydraulic properties in the earliest woody plant Armoricaphyton chateaupannense gen. nov. & sp. nov. based on c. 407-million-year-old fossils from the Armorican Massif, western France. The plant was small, and the woody axes were narrow and permineralized in pyrite (FeS2). We used standard palaeobotanical methods and employed propagation phase contrast X-ray synchrotron microtomography (PPC-SRμCT) to create three-dimensional images of the wood and to evaluate its properties. The xylem comprised tracheids and rays, which developed from a cambium. Tracheids possessed an early extinct type of scalariform bordered pitting known as P-type. Our observations indicate that wood evolved initially in plants of small stature that were members of Euphyllophytina, a clade that includes living seed plants, horsetails and ferns. Hydraulic properties were calculated from measurements taken from the PPC-SRμCT images. The specific hydraulic conductivity of the xylem area was calculated as 8.7 kg m−1 s−1 and the mean cell thickness-to-span ratio (t/b)2 of tracheids was 0.0372. The results show that the wood was suited to high conductive performance with low mechanical resistance to hydraulic tension. We argue that axis rigidity in the earliest woody plants initially evolved through the development of low-density woods

Variscan orogeny in Corsica: new structural and geochronological insights, and its place in the Variscan geodynamic framework

In Western Corsica, remnants of pre-batholitic lithological and metamorphic assemblages are preserved as km-scale septa enclosed within Lower Carboniferous to Early Permian plutons. Two groups of septa were recognized: (1) the Argentella and Agriates-Tenda fragments correspond to Neoproterozoic rocks deformed and metamorphosed during the Cadomian–Panafrican orogeny, and (2) the Zicavo, Porto-Vecchio, Solenzara–Fautea, Belgodère, Topiti, and Vignola fragments consist of Variscan metamorphic rocks. The lithological content and the main ductile deformation events for each septum are presented. In the Zicavo, Porto-Vecchio, and Topiti septa, a top-to-the-SW ductile shearing (D1 event) coeval with an amphibolite facies metamorphism is responsible for crustal thickening at ca 360 Ma. This main event was preceded by eclogite and granulite facies metamorphic events preserved as restites within migmatites dated at ca 345–330 Ma. A top-to-the-SE ductile shearing (D2 event) coeval with the crustal melting accommodated the exhumation of the D1 event. In contrast, the Belgodère segment is peculiar as it exhibits a top-to-the-E vergence, although retrogressed high-pressure rocks are also recognized. The pre-Permian fragments are arranged in four NW–SE-striking stripes that define a SW–NE zoning with (1) a Western domain in Topiti, Vignola, Zicavo, Porto-Vecchio, and Solenzara–Fautea; (2) a Neoproterozoic basement with its unconformable Early Paleozoic sedimentary cover in Argentella; (3) an Eastern metamorphic domain in Belgodère; (4) another Neoproterozoic basement with its Upper Paleozoic sedimentary cover in Agriates-Tenda. The Argentella basement is separated from the Western and Eastern domains by two sutures: S1 and S2. The Variscan Corsica represents the Eastern part of the Sardinia–Corsica–Maures segment. The comparison of this segment with other Variscan domains allows us to propose some possible correlations. We argue that the Western domain, Argentella, Belgodère, and Agriates-Tenda domains can be compared with the Southern Variscan belt exposed in French Massif Central–Southern Massif Armoricain, Armorica microblock, Léon block, respectively.

Retrogressed lawsonite blueschists from the NW Iberian Massif: P–T–t constraints from thermodynamic modelling and 40Ar/39Ar geochronology

Blueschist facies terranes in the Variscan Ibero-Armorican Arc are restricted to scarce and relatively small areas. One of these examples is the Ceán Unit, which is the westernmost exposure of the middle allochthonous sheet of the Variscan belt in the Malpica–Tui Complex (NW Iberian Massif). The Ceán Unit is a highly condensed metamorphic succession with a lower part in the blueschist facies and an upper part without HP relicts. It comprises variable proportions of glaucophane–chloritoid-bearing metapelites and mafic rocks with abundant well-preserved pseudomorphs after euhedral lawsonite. Both lithologies show systematic changes in texture and mineral composition that are spatially related depending on deformation. The metamorphic evolution of the metabasic rocks has been constrained in the P–T space through pseudosection approach and is characterised by H2O-undersaturated prograde evolution induced by the crystallisation of lawsonite. Peak conditions in the blueschist/LT-eclogite facies have been constrained at ca. 2.2 GPa and 560 °C. Exhumation-related metamorphism is characterised by a nearly isothermal decompression from the lawsonite-bearing fields to fields with stable albite at P ≈ 1 GPa. This lead to the pseudomorphism of lawsonite in the early-decompression stages, and a subsequent amphibolite–greenschist facies overprint at P < 0.8 GPa and T ≈ 440–480 °C. The preservation of the lawsonite crystal shape despite complete retrogression indicates that pseudomorphism occurred as a static process and that particular levels of the blueschist host rock were not affected by penetrative deformation during exhumation. 40Ar/39Ar step heating of phengitic muscovite from the pelitic schists interbedded with the lawsonite pseudomorph-bearing metabasic rocks yield plateau ages of ca. 363 ± 2 and 354 ± 1 Ma. The older age is interpreted as the age of the peak blueschist facies metamorphism. The age of 355 Ma is interpreted as a cooling age and is inferred to represent some point relatively close to peak conditions at the onset of the isothermal decompression. 40Ar/39Ar dating of muscovite from the quartzo-feldspathic mylonites of the Bembibre–Ceán detachment, at the base of the Ceán Unit, yields an age of ca. 337 ± 3 Ma, interpreted as the age of the post-nappe extensional tectonics. Similar data obtained from the blueschists of Ile de Groix (Armorican Massif; Bosse et al. in Chem Geol 220:21–45, 2005) support the equivalence of the Ceán Unit and the Upper Unit of Ile de Groix along the Ibero-Armorican Arc and suggest that these units share a blueschist facies event constrained at ca. 360–370 Ma, that is inferred to represent the Late Devonian–Early Carboniferous subduction of the northern margin of Gonwana beneath Laurussia.

A Lower Silurian (Aeronian) radiolarian assemblage from black cherts of the Armorican Massif (France)

The Chalonnes-sur-Loire outcrop is the most complete Llandovery section in the Ligerian domain (Armorican Massif, NW France); it displays a ca. 10 m-thick sequence of organic-rich black bedded cherts alternating with black graptolitic shales, which were deposited above glaciomarine diamictites of the Hirnantian glaciation. It is likely the black cherts accumulated under eutrophic waters, on the outer shelf part (distal offshore) of a Gondwanan margin, situated at intermediate to high latitudes of the Southern hemisphere. Hydrofluoric acid processing allowed the extraction of radiolarians; seven species are identified in the best preserved sample discovered so far. All seven are common species in tropical assemblages known from Alaska and Nevada, which are characteristic of the Orbiculopylorum assemblage, of Aeronian to early Telychian age. This age is in good agreement with independent age control from graptolites in the Chalonnes section, suggestive of an Aeronian age for the radiolarian-yielding level.

Fingerprinting the provenance of building stones: a case study on the Louvigné and Lanhélin granitic rocks (Armorican massif, France)

Abstract Two examples of granitic stones from Brittany (western France) commercialized under the names of “gris-bleu de Louvigné” and “bleu de Lanhélin” were characterized in order to explore how the provenance of a building stone can be traced back with a maximum of confidence. For this purpose, petrographical, geochemical and magnetic characteristics, representing more than 70 quantitative and qualitative variables, were compiled for a total of 32 samples. We have defined two reference populations for these building stones and have extracted their discriminative characteristics. We have then compared four randomly selected samples and two foreign commercial counterparts of these stones to the reference populations. Discriminative variables differ from one case of comparison to the other, which indicates that a combination of various tools and variables will be generally required to unequivocally fingerprint the origin of a given granitic stone. Where several quarries are mining a single geological unit within a composite intrusion, the provenance of a granitic rock can be defined at the scale of the intrusion. In addition, stones coming from two different intrusions from the same batholith can be distinguished. We conclude that the provenance of any granitic building stone is identifiable, especially if the intrinsic variability of a population of samples representative of that stone has been previously circumscribed. This study underlines that the compilation of databases for building stone identity cards is an essential first step toward the creation of official labels guaranteeing stone provenances.

Relief variation and erosion of the Variscan belt: detrital geochronology of the Palaeozoic sediments from the Mauges Unit (Armorican Massif, France)

Abstract The sediments of the Mauges Unit located in the internal zone provide an opportunity of studying the evolution of relief during Palaeozoic time. U–Pb dating on zircon and 39 Ar/ 40 Ar on white mica are used to constrain the age and nature of the sources. The first relief identified is marked by an Early Devonian unconformity interpreted as the opening of a northern back-arc basin. Detrital minerals are first reworked from underlying layers indicating a local supply. Magmatic zircons at c. 400 Ma then record the emergence of a magmatic arc. During the Middle Devonian, the gap in the sedimentary record is attributed to an emersion followed by the disappearance of the relief during the Late Devonian. At the Devonian–Carboniferous boundary, the main collision is followed by the onset of a relief. The continental sedimentation in the Ancenis Basin (late Tournaisian–Viséan) is a coarsening-upwards megasequence indicating an increasing and/or approaching relief. The detrital minerals record the progressive exhumation of Variscan metamorphic (mica at c. 350 Ma) and magmatic rocks (zircons at c. 390–340 Ma). The Serpukhovian–Bashkirian sedimentation records the erosion of a proximal metamorphic source (Champtoceaux with micas at c. 350–340 Ma) showing a much shorter drainage system. Supplementary material: Sample coordinates and U–Pb on zircon and 39 Ar/ 40 Ar on white mica analyses are available at http://www.geolsoc.org.uk/SUP18730 .

Composition and significance of the Katian (Upper Ordovician) conodont fauna of the Vaux Limestone (‘Calcaire des Vaux’) in Normandy, France; pp. 214–219

Study of new conodont collections from the Vaux Limestone exposed at its classical locality at Saint-Hilaire-la-Gerard in the Sees syncline, the conodont fauna of which was previously described in a preliminary way by Weyant et al. (1977, Un episode calcaire ashgillien dans l'est du Massif armoricain; incidence sur l'âge des depots glacio-marins fini-ordoviciens. Comptes Rendus de l'Academie des Sciences, Paris, 284, Serie D, 1147-1149), has provided significant new information about Late Ordovician conodonts from Normandy. A more precise age of this formation has been established based on an unexpectedly abundant conodont fauna of low diversity. Representatives of Amorphognathus, Hamarodus, Sagittodontina, Scabbardella and Eocarniodus are present and the fauna is referable to the middle Katian-lower Hirnantian A. ordovicicus Zone. Our study, the first illustrating Ordovician conodonts from Normandy, shows that the fauna is closely similar to middle Katian faunas from other parts of continental Europe and represents the Sagittodontina robusta-Scabbardella altipes biofacies of the Mediterranean Province.

Correlation of the nappe stack in the Ibero-Armorican arc across the Bay of Biscay: a joint French–Spanish project

Abstract A correlation between allochthonous units exposed in the NW Iberian Massif and the southern Armorican Massif is carried out based on lithological associations, structural position, age and geochemistry of protoliths and tectonometamorphic evolution. The units on both sides of the Bay of Biscay are grouped into Upper, Middle and Lower allochthons, whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the Parautochthon. The Lower Allochthon represents a fragment of the outermost edge of Gondwana that underwent continental subduction shortly after the closure of a Palaeozoic ocean which, in turn, is represented by the Middle Allochthon. The latter consists of supra-subduction ophiolites and metasedimentary sequences alternating with basic, mid-ocean ridge basalt (MORB)-type volcanics, with inheritances suggesting the proximity of a continental domain. Seafloor spreading began at the Cambro-Ordovician boundary and oceanic crust was still formed during the Late Devonian, covering the lifetime of the Rheic Ocean, which is possibly represented by the Middle Allochthon. The opening of the oceanic domain was related to pulling apart the peri-Gondwanan continental magmatic arc, which is represented by the Upper Allochthon.

The Cadomian Orogen: Neoproterozoic to Early Cambrian crustal growth and orogenic zoning along the periphery of the West African Craton—Constraints from U–Pb zircon ages and Hf isotopes (Schwarzburg Antiform, Germany)

The Cadomian Orogen in the NE Bohemian and the northern Armorican Massifs shows a distinct orogenic zoning from recent NW to SE consisting of (i) an outboard sitting continental crustal unit comprising Neoproterozoic rocks associated with c. 2.0Ga old Icartian Basement, (ii) a magmatic arc and a back-arc basin, (iii) a foreland or retro-arc basin, and (iv) the passive margin of the back-arc basin. New U–Pb zircon ages of detrital zircon of Neoproterozoic to Fortunian siliciclastics from the Schwarzburg Antiform in the Saxo-Thuringian Zone (NE Bohemian Massif) identify the West African Craton as the hinterland for the Cadomian Orogen as demonstrated by zircon populations dated at 1.8–2.2, 2.5–2.7, 3.0–3.1, and 3.4–3.5Ga. The dominant zircon population (c. 50–70% in each sample) is derived from a Cadomian magmatic arc in a time slice of c. 570–750Ma. The magmatic activity of the Cadomian arc stopped at c. 570Ma. Closure of the back-arc basin by arc-continent collision occurred between c. 570 and 542Ma under the formation of a foreland (retro-arc) basin. A short-living remnant basin existed between c.542 and 540Ma. Granitoid plutonism at 539–540Ma documents the final pulse of the Cadomian Orogeny. Hf isotope compositions, calculated ɛHfi values and TDM model ages for detrital and magmatic zircon show that during the c. 180Ma long Cadomian magmatic arc activity juvenile arc magmas were contaminated by recycling of Eburnian and Archaean crust. Mixing with an evolved continental crust is always present. The inferred geotectonic setting is a continental magmatic arc during the Neoproterozoic developed on a stretched Archaean and Palaeoproterozoic (Eburnian) crust. In the West African crustal evolution it can be demonstrated that during Eburnian orogenic processes (c. 1.8–2.2Ga) in most cases a 2.5–3.4Ga old basement became reworked. Archaean 2.5–2.9Ga magmas remelted a 3.0–3.4.Ga crust. Zircon grains with an age of 3.0–3.1 and 3.4Ga are derived from juvenile magmas. Two zircon grains dated at 2779±22 and 3542±28Ma imply reworking of pre-existing Eoarchean to Hadean crust and show TDM model ages of 3.98 and 4.29Ga, respectively.

Monazite EMP-Th-U-Pb age pattern in Variscan metamorphic units in the Armorican Massif (Brittany, France)

Monazite EMP-Th-U-Pb age pattern in Variscan metamorphic units in the Armorican Massif (Brittany, France)

Contribution of Morphometrics to the Systematics of the Ordovician GenusNeseuretus(Calymenidae, Trilobita) from the Armorican Massif, France

The genusNeseuretusHicks, 1873 is the most abundant trilobite of the Ordovician siltite succession of the Andouillé and Traveusot Formations in the French Armorican massif. The systematics of some species ofNeseuretusis still unclear. Armorican and Iberian domains formed part of a distinctive paleobiogeographical province in the Ordovician and, while fiveNeseuretusspecies were defined in Iberia that follow each other through time, from the Middle to the Upper Ordovician, only oneNeseuretusspecies,N. tristani, has been identified in the Armorican massif so far. The discovery of new fossil deposits in the Ménez-Belair syncline has led to the identification and re-description of threeNeseuretustaxa:Neseuretus avusHamman, 1977 from the early-middle Darriwilian,Neseuretus tristani(Brongniart in Desmarest, 1817) from the late–middle Darriwilian to late Darriwilian andNeseuretus tardus(Hammann, 1983) from the Darriwilian–Sandbian boundary and early Sandbian. Morphological characters of the cephalon and pygidium were determined that can definitively distinguish the three identified taxa. These results stem from both traditional and geometric (outline and landmark-based) morphometric analyses. The three taxa follow each other through time and constitute a morphological sequence that contributes to improving the stratigraphy of the Middle and Upper Ordovician.

Multiscale full waveform inversion

We develop and apply a full waveform inversion method that incorporates seismic data on a wide range of spatio-temporal scales, thereby constraining the details of both crustal and upper-mantle structure. This is intended to further our understanding of crust–mantle interactions that shape the nature of plate tectonics, and to be a step towards improved tomographic models of strongly scale-dependent earth properties, such as attenuation and anisotropy. The inversion for detailed regional earth structure consistently embedded within a large-scale model requires locally refined numerical meshes that allow us to (1) model regional wave propagation at high frequencies, and (2) capture the inferred fine-scale heterogeneities. The smallest local grid spacing sets the upper bound of the largest possible time step used to iteratively advance the seismic wave field. This limitation leads to extreme computational costs in the presence of fine-scale structure, and it inhibits the construction of full waveform tomographic models that describe earth structure on multiple scales. To reduce computational requirements to a feasible level, we design a multigrid approach based on the decomposition of a multiscale earth model with widely varying grid spacings into a family of single-scale models where the grid spacing is approximately uniform. Each of the single-scale models contains a tractable number of grid points, which ensures computational efficiency. The multi-to-single-scale decomposition is the foundation of iterative, gradient-based optimization schemes that simultaneously and consistently invert data on all scales for one multi-scale model. We demonstrate the applicability of our method in a full waveform inversion for Eurasia, with a special focus on Anatolia where coverage is particularly dense. Continental-scale structure is constrained by complete seismic waveforms in the 30–200 s period range. In addition to the well-known structural elements of the Eurasian mantle, our model reveals a variety of subtle features, such as the Armorican Massif, the Rhine Graben and the Massif Central. Anatolia is covered by waveforms with 8–200 s period, meaning that the details of both crustal and mantle structure are resolved consistently. The final model contains numerous previously undiscovered structures, including the extension-related updoming of lower-crustal material beneath the Menderes Massif in western Anatolia. Furthermore, the final model for the Anatolian region confirms estimates of crustal depth from receiver function analysis, and it accurately explains cross-correlations of ambient seismic noise at 10 s period that have not been used in the tomographic inversion. This provides strong independent evidence that detailed 3-D structure is well resolved.

Le Néolithique ancien à l’ouest de la Bretagne : nouvelles découvertes à Pen Hoat Salaün (Pleuven, Finistère)

Excavated in 2008 and covering an area of 3700m2 in preparation for the development of a large supermarket, the site of Pen Hoat Salaün yields evidence for two occupations, one attributable to the Bertheaume group (Early Mesolithic, 8300–8200 years BP), and the other to the end of the Early Neolithic (4800–4700years BP). Located at the top of a slope at the edge of a plateau and overlooking a graben, the site commands an extensive panoramic view towards the north-east. The granitic substrate is capped by a cover of Quaternary surface formations, no more than 1m in thickness and consisting of three distinct sandy-loam levels (US 1–3). The two lowermost levels are made up of reworked and colluvial materials, derived from the granitic sands and residual lœssal formations of the plateau, whereas the uppermost level corresponds to the present-day soil horizon developed on recent agricultural colluvium. Moreover, excavation of this cover has led to the discovery of lithic objects attributed primarily to the Mesolithic in a lower archaeological level (US 3), while a mixture of fragments attributed to the Mesolithic and Neolithic is found in a higher archaeological level (US 2). This latter lithic material is associated with ceramic shards, but to a lesser degree (than in US 3). A total of 763 shards are recorded, showing bowl or bottle-like forms; the decorations are dominated by panels of embossed buttons, cordons – including one small part with finger-printed ornament – and ribs in relief, as well as impressions made with a blunt point or spatula. Although some characters can be distinguished with different proportions in the two lithic assemblages, there is also a clear lack of tools characteristic of the early or middle Neolithic period (arrowheads, borers and burins). Moreover, we note the disappearance in this area of the laminar debitage sequences so characteristic of the Villeneuve Saint-Germain culture, in favour of the production of flakes. This sparse lithic assemblage is accompanied by a muscovite-sillimanite gneiss axehead and a fragment of a schist/wackestone ring. The former item belongs to a context – also known as carnacean – in which axeheads have high social value, with a possible alpine origin in view of its type (probably repolished Bégude), even though such rocks are also present in the Armorican Massif. A serpentinite ring-disc discovered in 2007 in a trial trench 100m south-west of the excavation area appears to represent another exceptional item, possibly originating in the Alps, though a local provenance is discussed. Although not strictly associated with the shards, four radiocarbon dates obtained on burned brushwood indicate an age range of 4800–4700years BP. While it is out of the question to speak in terms of an association, the Pen Hoat Salaün assemblage offers a good outline of stylistic and technonolgical features at the end of the early Neolithic, as a prelude to the Castellic.

Transgressive systems tract of a ria-type estuary: The Late Holocene Vilaine River drowned valley (France)

The understanding of control parameters of estuarine-infilling at different scales in time and space is important for near-future projections and better management of these areas facing sea-level change. During the last 20 years, the stratigraphy of late Quaternary estuaries has been extensively investigated leading to the development of depositional models which describe the sedimentary facies distribution according to changes in sea level, tidal range and wave activity but few of them describe in detail the influence of the estuarine geomorphology. On the Atlantic coast, recent studies provide a detailed understanding of the sedimentary infill succession of incised valleys during the late Holocene transgression with recent emphasis on the macrotidal bay of the Vilaine River in South Brittany. The latter has shown the importance of the inherited morphology in controlling the preservation of the sedimentary deposits. However, until now, there has been little information available on the geometry of the underlying bedrock of the estuary and its immediate approaches including the Holocene history of the most proximal part of the Vilaine River estuary. The recent acquisition of very-high resolution seismic reflection (Chirp) and sediment core data in the Vilaine Estuary provides the opportunity to address several of these issues leading to the identification of the controlling factors that played a role in the sedimentary filling of the estuary. The estuary of the Vilaine River is one of the most sheltered estuaries of the Atlantic coast and thus shows an excellent record of the successive transgressive phases since the last glacial period. The valley of this ria-type estuary is incised in the Hercynian magmatic and metamorphic rocks of the Armorican Massif along the striking branches of the South Armorican Shear Zone and filled with well-stratified sediments. The study of the morphology of the substratum in the estuary reveals narrow paleovalleys (1 km wide) with a maximum depth of 20 m. The shapes and dimensions of these paleovalleys are comparable with those observed upstream in the present-day Vilaine River valley. Facies analysis of seismic and sedimentary data, complemented by 21 AMS radiocarbon age dating, enables the reconstruction of the infilling of the estuarine valley. The valley is filled by Pleistocene to early Holocene alluvial, coarse-grained, pebbly sands overlain by mid- to late Holocene marine sands, silts and muds. From 10,000 to 5000 cal. BP, the marine transgression shows a rapid initial phase of transgression followed by a progressive slowing down which can be recognised over more than 90 km in an upstream direction. The rapid transgression developed homogeneously over the entire length of the estuary with a widespread expression of tidal currents (tide-dominated estuary). The slow phase of the transgression gives rise to a differentiated expression of the hydrodynamic factors according to the geomorphological setting. The sedimentary deposits of the innermost and narrow part of the estuary reflect a strong control of tidal and river currents when the influence of tidal currents increases at the expense of fluvial control in a downstream direction. The sedimentary filling of the outer reaches of estuary shows a strong influence of tidal currents modulated by changes in sea level where, from 5700 to 3000 cal. BP, the progressive decrease in accommodation space promotes the expression of wave action (wave-dominated estuary). Finally, from 5000 years BP to present day, the stabilisation of sea level gives birth to scour and lag sedimentary morphologies (scour and lag estuary). The sedimentary record appears thus strongly marked by the Holocene transgression and the geomorphological setting where hydrodynamic parameters superimposed on the geomorphological and eustatic contexts expressed in different ways in time and space govern the evolution of the estuarine system.

Heavy-mineral, mineral-chemical and zircon-age constraints on the provenance of Triassic sandstones from the Devon coast, southern Britain

An integrated heavy-mineral, mineral-chemical and zircon-dating study of the Triassic succession exposed on the south Devon coast, in the western part of the Wessex Basin, indicates derivation from a combination of granitic and metasedimentary lithologies of ages of mostly over 550 Ma. These sources were probably located at a relatively proximal location near the southern margin of the basin. Derivation from more distal sources in the Armorican Massif or local Variscan sources to the west appears unlikely in view of the scarcity of Permo-Carboniferous (Variscan-age) zircons. The Budleigh Salterton Pebble Bed Formation was derived from a different combination of source lithologies than the Otter Sandstone Formation, the former including staurolite-grade metasediments that were absent in the catchment area of the Otter Sandstone. The Devon coast succession has provenance characteristics that differ from equivalent sandstones further east in the Wessex Basin, and from sandstones in the East Irish Sea Basin to the north. These differences indicate that sediment supply patterns to the linked Triassic basin systems in southern Britain are complex, involving multiple distinct sub-catchment areas, and that heavy-mineral studies have considerable potential for unravelling these sub-catchment area sources.

La fin du Tardiglaciaire dans le Grand-Ouest de la France

In recent years, the Late Glacial period of Western France has been the scene of exciting new research. Prior to this research, the human occupation of Western France at the very end of the Pleistocene was seldom considered in discussions of European Prehistory. The presence of few stratified sites and the acidic soils of the Armorican Massif, which preserve few organic remains, are probably to blame for this treatment. A systematic study of assemblages collected in the 1970s-80s and the excavation of new sites allow us to re-examine the Final Palaeolithic of Western France. This new research has allowed us to propose a new chrono-cultural organization of the lithic industries. Because few sites are well-dated or stratified in Western France, this new chronology is based on lithic technology and a comparison of assemblages from the surrounding areas. In this paper, I specifically deal with assemblages long thought to be Magdalenian. However, some are in fact younger and represent a different technology that was replaced at the very end of the Late Glacial, during the Younger Dryas-Preboreal transition. These so-called ’ Post-Azilian’ industries mark a clear technical departure from the Azilian that preceded and the early Mesolithic that succeeded them. Contrary to the Azilian, where non-uniform elongated flakes were the main objective, blades and bladelettes are the central focus of reduction in post-Azilian societies. Post-Azilian blanks are regular and standardized. They are produced from methods using high-quality raw materials. The bladelettes are used to make different kinds of projectile points. The design of projectiles varies across Europe and these different types are seen as the cultural markers of separate traditions : Laborian/ Epilaborian, Ahrensburgian/ Epiahrensburgian, Swiderian,… The assemblages of Western France show an interesting diversity of projectile points. Straight backed points are most common. These projectile points testify to southern influences and are characteristic of the Epilaborian in Southern France. The presence of these projectile points in Western France is not surprising considering the fact they have been found in surrounding areas ; it is however unexpected to find elements similar to the Ahrensburgian and Epiahrensburgian. The presence of Ahrensburgian tanged points and oblique truncated points in Western France show the extension of these Northern European traditions farther West. While these axial points are the most common, our work also suggests the existence of trapezoidal elements probably used as cutting points. Well known during the second half of the Mesolithic and the Neolithic, this kind of projectile point seems to appear much earlier than usually considered. We are currently working on the origin of this concept (changes in hunting strategies, diversification of targets) and its link with the environmental changes of the end of the Pleistocene. While projectile point types vary throughout Europe, study of the lithic sub-system shows the diffusion of strong technical concepts over a wide area. The assemblages of these different European traditions testify to the presence of homogeneous objectives and methods of production. This homogeneity suggests that there may be a large European techno-complex composed of several traditions that are not necessarily contemporaneous or geographically restricted. This new view of the very end of the Late Glacial period allows us to develop new hypotheses about the socio-economic organization of these human societies. From this perspective it is interesting to consider the impact climatic instability may have had on populations during this period.

Meduanarostrum, a New Middle Pragian Rhynchonellid (Brachiopod) Genus from the Armorican Massif, NW France, and Meduanarostridae Nov. Fam.

A new rhynchonellid genus, Meduanarostrum, with type species M. letissieri (Oehlert, 1877), is described from the Laval Synclinorium, Armorican Massif, where it is present in a restricted area along the Mayenne river valley. A middle Pragian age is attributed to the new genus after evaluation of the scanty stratigraphic information found in the literature. Meduanarostrum nov. gen. is designated as the type genus for the new family Meduanarostridae.

An Atypical Silurian Myodocope Ostracod from the Armorican Massif, France

A new myodocope ostracod Sineruga insolita gen. et sp. nov. is herein described from the Armorican Massif (France). Sineruga resembles Silurian myodocopes (i.e., bolbozoids and cypridinids) in having anterior features (thinner carapace) possibly related to vision. On the other hand, it resembles entomozoid ostracods in having a bean-shaped outline along with a deep adductorial sulcus and a simple muscle spot, but lacks their characteristic ribbed ornament. The data available suggest that Sineruga insolita was probably an atypical, non-ribbed member of the entomozoids thus indicating that early entomozoids may have had smooth representatives. Comparisons with other Recent and fossils ostracods show that the presence of a rostrum and/or symmetrical vision related carapace features (i.e., indicating lateral eyes) can be used as diagnostic characters for myodocope ostracods. The position and shape of the dorsal connection and to a lesser extent that of the sulcus and the adductorial muscle scar can be used to...