Stephan Roger

40Ar/39Ar dating of a tephra layer in the Pliocene Senèze maar lacustrine sequence (French Massif Central): constraint on the age of the Réunion–Matuyama transition and implications on paleoenvironmental archives

Abstract The sedimentary sequence cored in the Seneze maar (Velay, France) is very favourable for magnetostratigraphy of the Late Pliocene and for studying the paleoenvironmental changes that occurred during that period. The magnetostratigraphic studies revealed that the sediments from this lacustrine sequence were deposited during the Matuyama reverse period, except for the upper part where a short normal polarity episode was recorded. A tephra layer interbedded in the sediments contemporaneous with this normal polarity event is dated at 2.10±0.01 Ma by the 40 Ar/ 39 Ar method. Using this chronological marker, it can be inferred that the normal polarity episode recorded in the Seneze sequence corresponds to the Reunion event and that the age of the normal–reverse transition closing this event is ca. 2.09 Ma. These chronological constraints also permit to (1) demonstrate that the ‘Villafranchian’ mammal fauna found in the Seneze maar is younger than 2.09 Ma and (2) correlate the climatic events recorded in the Seneze sequence to the marine δ 18 O records. This suggests that the Seneze pollen sequence (5–120 m depth) ranges from isotopic stage 85 to 76.

Messinian events: new constraints from sedimentological investigations and new 40Ar/39Ar ages in the Melilla–Nador Basin (Morocco)

Abstract New 40Ar/39Ar ages obtained both from within a platform and from the adjacent basin allow, through the extension of a previous argon dataset, the establishment of a chronological framework for the Messinian carbonate complex of Melilla (Morocco). In the platform, prograding bioclastic deposits began around 6.87±0.02 Ma, whereas the youngest preserved deposits are 5.77±0.04 Ma old. The southern part of the basin was uplifted and emergent prior to 6.23±0.03 Ma. The Tortonian/Messinian and the Messinian/Pliocene boundaries have not yet been identified in the Melilla region. Durations of the prograding bioclastic deposits (ca. 410 ka), the prograding Porites reefs (ca. 360 ka) and the preserved part of the terminal carbonate complex (TCC) (ca. 330 ka) can be estimated. Sedimentological and geochronological investigations do not indicate any major subaerial unconformity nor any major time gap. If a major sea level fall did exist during Messinian times, it developed after 5.77±0.04 Ma. Chronological correlations show that the onset of the Sorbas gypsum deposition (Southern Spain), considered as the marker of the Messinian Salinity Crisis onset, is contemporaneous with aggrading Porites reef building of the terminal carbonate complex of Melilla. A major paleoceanographic change in the marginal areas of the Mediterranean, with the cessation of upwelling systems, could be partly coeval with the onset of the Messinian Salinity Crisis.

40Ar/39Ar dating of the pre-evaporitic Messinian marine sequences of the Melilla basin (Morocco): a proposal for some biosedimentary events as isochrons around the Alboran Sea

In the Mediterranean region, the Melilla basin (NE Morocco) represents a key area that recorded biosedimentary events and environmental changes relative to the pre-evaporitic Messinian times. 40Ar/39Ar dating of volcanic tuffs interbedded all along the Melilla Messinian shelf carbonates and coeval basin deposits has been performed in order to date accurately three main pre-evaporitic biosedimentary events: the prograding bioclastic deposition and the oligospecific prograding Porites coral reef buildings with coeval Halimeda blooms, both within the platform, and the diatomite deposition basinward. The new age data allow a precise chronological framework to be established for the pre-evaporitic sequence of Melilla basin demonstrating that diatomitic deposits are coeval with both prograding bioclastic and reefal units. The prograding bioclastic carbonate unit related to boreal influences in coeval basinal diatomites, began at least at 6.73±0.02 Ma and ended at 6.46±0.03 Ma. The oligospecific prograding Porites coral-reefs and Halimeda beds and coeval warm-water diatomites began at least at 6.46±0.03 Ma. Previous 40Ar/39Ar ages indicate that they ended prior to 6.0±0.1 Ma. Both biosedimentological similarities and chronological accordance within several platforms and adjacent basins all around the Alboran Sea show that these main bioevents, dated in Melilla, are synchronous over the Alboran realm. This accurate time scale for these pre-evaporitic biosedimentary events (6.9–6.0 Ma) is in accordance with the most recent work on the latter period corresponding to the Messinian Salinity Crisis.

40Ar/39Ar dating on tephra of the Velay maars (France): implications for the Late Pleistocene proxy-climatic record

Abstract Pollen contents and magnetic records of the Pleistocene sedimentary sequences cored in the Praclaux and Lac du Bouchet maars (Velay region, French Massif Central) demonstrated that they have high potential as detailed climatic and environmental archives. Tephra layers interbedded in these sequences have been studied in order to perform isotopic dating. First, petrographic studies and 40 Ar/ 39 Ar ages have shown that these tephra layers are the witness of the same eruption from the Sancy volcano centre, located 100 km to the north. According to fourteen 40 Ar/ 39 Ar ages, carried out on sanidine and biotite grains, this eruption occurred 275±5 ka ago. This result allows to construct a time scale for a composite high resolution record of the climate covering the last 300,000 years and to establish an absolute chronology for the palaeoclimatic events recorded in these sequences. These tephra layers are interbedded in organic layers, deposited during an interstadial stage called the Amargiers interstadial which, according to its age, can be considered as equivalent to isotopic stage 9a. Moreover, the geographical extension of the tephra and its well-constrained age give to this tephra the interest of a chronostratigraphic marker for upper Quaternary sequences in southeastern France.

Rifting actif ou passif en Éthiopie ? éléments de réponse apportés par l'étude des xénolites péridotitiques de la région du lac Tana

The comparative study of peridotitic xenoliths from Oligocene and Quaternary volcanic formations of north-western Ethiopia shows that the textural, mineralogical, thermal and geochemical characteristics of the lithospheric mantle of this region were acquired in Oligocene times. These data suggest that the impinging of the Afar plume beneath the Ethiopian lithosphere occurred prior to the traps emplacement and the Plio-Quaternary rifting. This favours a model of active rifting for the Afar triple junction.