Conodontes de la formation de Fromelennes du Givetien de l’Ardenne Franco-Belge

A preliminary detailed list of the conodonts from the Fromelennes Formation is established. This formation is subdivided into three members. At three levels faunal changes are clear enough to characterize important biostratigraphic boundaries. Polygnathus rhenanus, P. dubius and leriodus eslaensis latecarinatus n. subsp. characterize the first level, Spatbognathodus insitus and Schmidtognatbus hermanni the second, and Ancyrodella rotundiloba the third. Only the last two levels are taken into account in defining the boundary between the Givetian and the Frasnian and between the Middle and Upper Devonian. The new subspecies, Icriodus eslaensis latecarinatus, is fully described. Diagrams of other important species and subspecies relevant to the problem are given and they are briefly discussed

Accuracy and reliability of conodont zones: the Polygnathus asymmetricus "Zone" and the Givetian-Frasnain boundary

Limestones from the top of the Bouia Formation, Tafilalt, southeastern Morocco, that traverse the Givetian-Frasnian boundary (base of the Lower P. asymmetricus Zone) were sampled several times for conodonts. In the Bou Tchrafine section near Erfoud a styliolinid coquina bed 6 cm thick was sampled five times, in each case from the whole bed at the same place. The bed immediately below belongs to the Klapperina disparilis Zone and the overlying one to the Middle P. asymmetricus Zone. The conodont fauna from the styliolinid bed varies from one sample to another and has been attributed as follows: once to the upper part of the Lower P. asymmetricus Zone, once to the lower part? of the Lower P. asymmetricus Zone and three times to an interval covering the top of the K. disparilis Zone?, the entire Lowermost and the lower part? of the Lower P. asymmetricus Zone. There is no relationship between the stratigraphic accuracy obtained and the size of the sample. In two nearby outcrops, 300 m and 600 m to the southeast, the microcoquina is thicker (15 cm and 51 cm) and composite; each bed was sampled once or twice and the zonal attribution shows the same variation as in the first outcrop. The varying results from the consecutive samplings are explained not only by the depositional environment (a condensed limestone sequence) but also by the unsatisfactory definition of conodont zones between the K. disparilis Zone and the Middle P. asymmetricus Zone. It would be appropriate to revise that part of the standard conodont zonation on the basis of non-condensed sequences

Conodonts from a potential Eifelian/Givetian Boundary Stratotype at jbel Ou Driss, southern Ma’der, Morocco

Conodonts from a section at Jbel Ou Driss in the Ma'der (pre-Sahara of Morocco) with neritic and pelagic megafaunas, are studied in an attempt to define the Eifelian/Givetian boundary. The conodont succession ranges from the T. kockelianus Zone into the Lower P. varcus Subzone. The ancestry of Polygnathus hemiansatus is established. The section may be considered as a potential global stratotype for the Eifelian/Givetian boundary

Couvinian

In the major part of the Dinant Synclinorium, the Couvinian is characterized by a mixed siliciclasticcarbonate sequence, about 750 m thick in the Couvin area. S e stage is part of a major transgressive cycle that starts in the underlying upper Emsian Hierges Formation. In the southern part of the Namur Synclinorium, its base rests with an angular unconformity on Silurian rocks. In the past, it was internationally used as the lower stage of the Middle Devonian. However, the Eifelian which base is slightly younger was also internationally used in the same sense. In 1980, by a majority vote of the international Subcommission on Devonian Stratigraphy, the base of the Eifelian was chosen as the base of the Middle Devonian Series

Conodontes et Sédimentologie des couches de passage du Givetien au Frasnien dans le Nord du Tafilalt et dans le Ma’der (Maroc Présaharien)

Stratigraphically important conodonts relevant to the Givetian-Frasnian boundary are listed from one section in the Tafilalt area and from threein the Ma'der area. The conodont succession from the Middle Polygnathus varcus Subzone and extending into the Middle Polygnathus asymmetricus Zone is described. The corresponding goniatite sequence ranges from the last occurrence of Maenioceras terebratum into beds containing Manticoceras and Beloceras kayseri. The sedimentology of the beds sampled for conodonts at the Tafilalt section and at two of the Ma'der sections is discussed. The conodont Ancyrodella binodosa is fully described;other relevant taxa are briefly discussed

Conodontes et Acritarches de l’Ordovicien inférieur de la partie Septentrionale de la Cordillère Argentine

The Conodonts Cordylodus proavus MULLER, K.J., 1959 et Acodus aff. A. deltatus LINDSTROM, M., 1955 are found, respectively, in the lower and the upper parts of the Santa Rosita Formation, the Tremadocian age of which is known from trilobites and dendroid graptolites. Of the acritarchs, Cymatiogalea cuvillieri (DEUNFF, J.) DEUNFF, J., GORKA, H. and RAUSCHER, R., 1974 occurs in the San Jose Shales, as well as in the Caldera Sandstones which contain also Acanthodiacrodium angustum (DOWNIE, C.) COMBAZ, A, 1967. Two conodonts, Bergstroemognathus extensus (GRAVES, R. W. and ELLISON, S., 1941) and Scandodus americanus SERPAGLI, E, 1974, s.f. from the lower part of the San Juan Limestones suggest an Arenigian rather than a Llanvirnian age, as indicated by trilobite evidence. The acritarch Domasia limaciformis (STOCKMANS, F. and WILLIERE, Y.) CRAMER, F.H., 1970, of late Llandoverian to early Wenlockian age in western Europe, is found in the Red Sandstones, in the lower part of the Lipeon Formation

Givetian

The Givetian Stage in the Givet area is represented by the main part of the Hanonet Fm, the Trois-Fontaines Fm, the Terres d'Haurs Fm, the Mont d'Haurs Fm, the Fromelennes Fm and the lowest part of the Nismes Fm. During Eifelian-Givetian transition, the sedimentary systems evolved from a mixed siliciclastic-carbonate ramp to a carbonate platform dominated by cyclic tidal-flat and lagoonal wackestones with local patch reefs or coralgal banks. The shelf had an ESE-WNW trend and extended from the Avesnois basin (northern France) in the west to Aachen (western Germany). The sedimentation in the Ardennes consists of shallow water regressive metric (around 5 m or less) cycles. Stromatopores, corals, brachiopods, algae and cyanobacteria are abundant. Due to this general shallow water setting, the base of the Givetian Stage has been defined by the IUGS Subcommission on Devonian Stratigraphy in a GSSP in southern Morocco referring to the first occurence of the condodont species Polygnathus hemiansatus. The conodont species Icriodus obliquimarginatus appears approximately at the same level in Belgium

Le Silurien supérieur et le devonien inférieur de la Sièrra de Guadarrama (Espagne Centrale). Troisième partie : éléments Icriodiformes, Pelekysgnathiformes et Polygnathiformes

The icriodiform, pelekysgnathiform and polygnathiform conodont elements from the Cercadillo Shales and Limestone are described. The icriodiform elements with one or two postero-lateral processes are assigned to three genera : Caudicriodus n. gen., Praelatericriodus n. gen., and Latericriodus MULLER, K. J . , 1 9 5 6 , herein modified. Caudicriodus forms the main branch from which Praelatericriodus in the Lower Gedinnian and Latericriodus in the Lower Emsian are derived.Caudicriodus? culicellus n. sp. is a newly described Upper Emsian - Lower Couvinian species. A number of species and subspecies described in this part and in the second part by P. BULTYNCK (1971) have been chosen in such a way that they form nine faunastic associations, which have a stratigraphical value, those are from oldest to youngest : I the Praelatericriodus rectangularis — Spathognathodus steinhornensis repetitor — Caudicriodus postwoschmidti — fauna (Lower Gedinnian); II the Praelatericriodus rectangularis — Caudicriodus curvicauda — fauna (Siegenian); III the Praelatericriodus simulator — fauna (Siegenian); IV the Caudicriodus celtibericus — fauna (Lower Emsian); V the Caudicriodus sigmoidalis — Polygnathus dehiscens — Spathognathodus steinhornensis miae — fauna (Lower Emsian); VI the Latericriodus bilatericrescens, Polygnathus dehiscens — Spathognathodus steinhornensis miae — fauna (Lower Emsian); VII the Icriodus fusiformis — 1. aff. I. corniger — I. rectirostratus — Caudicriodus ?culicellus — Polygnathus foveolatus (late forme) — fauna (Upper Emsian); VIII the Icriodus aff. /. fusiformis — I. corniger — 1. rectirostratus — Caudicriodus ? culicellus — Polygnathus foveolatus (late form) — fauna (Upper Emsian — Lower Couvinian); IX the Polygnathus serotinus — fauna (Upper Emsian — Lower Couvinian). Age assignements are based on the Brachiopods and on the association with Conodonts

Back-hopping in Spin-Transfer-Torque switching of perpendicularly magnetized tunnel junctions

We analyse the phenomenon of back-hopping in spin-torque induced switching of the magnetization in perpendicularly magnetized tunnel junctions. The analysis is based on single-shot time-resolved conductance measurements of the pulse-induced back-hopping. Studying several material variants reveals that the back-hopping is a feature of the nominally fixed system of the tunnel junction. The back-hopping is found to proceed by two sequential switching events that lead to a final state P' of conductance close to --but distinct from-- that of the conventional parallel state. The P' state does not exist at remanence. It generally relaxes to the conventional antiparallel state if the current is removed. The P' state involves a switching of the sole spin-polarizing part of the fixed layers. The analysis of literature indicates that back-hopping occurs only when the spin-polarizing layer is too weakly coupled to the rest of the fixed system, which justifies a posteriori the mitigation strategies of back-hopping that were implemented empirically in spin-transfer-torque magnetic random access memories.Comment: submitted to Phys Rev.