About some widespread Late Pleistocene tephra horizons in Middle Europe
Europe ; Géochimie ; Géographie de l'Europe ; Minéralogie ; Quaternaire ; Téphra ; Téphrochronologie ; Volcanisme
Des analyses par microsonde de minéraux mafiques de niveaux de téphras de Belgique, Allemagne et France ont conduit à la détermination de minéraux selon les classifications recommandées par l'IMA et confirment une corrélation antérieure de niveaux
Two methods that look promising are tephra-hydration dating, and etching of heavy mineral phenocrysts. Basic research into the weathering of tephra will be needed before either method can be used with confidence, but the potential
A dark tephra layer, named a, is found in soils in East and Northeast Iceland. Its volume is approximately 2 km and it covers about 40 % of Iceland. An isopach map and grain-size study suggests that the tephra deposit forms a distinctly two-lobed
The formation of tephra layers in peatlands : an experimental approach
This study takes an experimental approach to investigate the processes of tephra taphonomy. Tephra was applied to peats and movement monitored over periods of up to 6 years. Experiments combined field studies on six British peatlands with rainfall
simulation experiments in the laboratory. The extent of tephra movement most likely depends on the density and porosity of the surface peat; there is no simple relationship with wetness. There is some indication that the extent of tephra movement depends
on the tephra particle size but this will require further work to confirm. The taphonomy of tephra is an important issue which should be considered in all tephrochronology studies in peatlands.
A textural study of basaltic tephras from lower Tertiary diatomites in northern Denmark in Tephra studies.
More than 200 tephra layers deposited in early Tertiary diatomite and clay in northern Denmark record explosive volcanism in the Skagerrak and in the North Sea. The early activity is basaltic, rhyolitic, and mafic and salic alkaline while later
activity is exclusively tholeiitic Fe-Ti-basaltic. Several of the younger basaltic tephras exceed 4 km by volume. The morphology and grain size of the tephras indicate both subaerial and phreatic activity. The Fe-Ti-basaltic tephras are distinctly surtseyan
Old Crow tephra found at the Palisades of the Yukon, Alaska
This paper presents geochemical data that suggest a correlation between this newly discovered tephra and the Old Crow tephra, and provides the first chronologic constraints on the age of Pleistocene sediments at the Palisades of the Yukon.
A Mazama tephra marker bed in the Fraser Lowland, British Columbia
Des carottes de forage provenant du delta et de la plaine d'inondation de la rivière Fraser, Colombie-Britannique, révèlent l'existence d'un horizon marqueur de tephra. Les résultats géochimiques confirment que la couche étudiée appartient
The morphometry of selected tephra samples from Icelandic volcanoes
Analyse statistique de la forme des grains de cendres volcaniques (tephra). Rapports avec la composition chimique du magma, avec les conditions physiques de l'éruption, etc... - (JCB)
Widespread allerod and boreal trachyandesitic to trachytic tephra layers as stratigraphical markers in the Massif Central, France
Etude par microsonde d'une téphra trachyandésitique d'âge Alleröd, découverte dans plusieurs sites de Limagne occidentale et dans les Monts du Forez, appelée Téphra de Ravel. Corrélation avec la téphra de La Taphanel et tentative d'établir les bases
Correlation of the Holocene Jarvis Creek, Tangle Lakes, Cantwell, and Hayes tephras in south-central and central Alaska
The AA. present new data on the glass geochemistry, petrography, and chronology of the Cantwell ash, and test the proposed correlation between proximal tephras at Hayes volcano and the Cantwell ash suggested by Riehle et al. (1990). They also test
for correlations between the Hayes tephras, the well-known Jarvis Creek Ash (Péwé, 1975), and tephra at newly discovered sites at Tangle Lakes.
Geochemical characterization of Late Pleistocene and Holocene tephra layers from the Basin of Mexico, Central Mexico
The aim of this paper is to characterize tephra layers in the southern part of the Basin of Mexico that are potentially useful as stratigraphical markers over the late Pleistocene and Holocene. New geochemical data and radiocarbon dates have been
obtained for tephras collected in sediments of Lake Texcoco and Lake Chalco. The accurate identification and correlation of these tephras will be particularly helpful in the correlation of sediment sequences and the establishment of a volcanic history
The Mount Edgecumbe tephra deposits, a marker horizon in southeastern Alaska near the Pleistocene-Holocene boundary
The AA. correlate tephra samples from several sites, and interpret the radiocarbon data to propose an age range for the sequence of tephra layers. In any case, the Mount Edgecumbe tephra deposits are a widespread, latest Pleistocene stratigraphic
Lake Tapps tephra : an early Pleistocene stratigraphic marker in the Puget Lowland, Washington
Cette étude montre que les tephra du Lac Tapps sont effectivement un marqueur stratigraphique régional pour le sud du Puget Lowland. Les dépôts de la première phase de la glaciation Salmon Springs peuvent être repérés par ces tephra. De plus ces
tephra fournissent un lien entre les sédiments du lobe Puget et ceux déposés par les glaciers alpins Olympic.
Tephrochronology by microprobe glass analysis in Tephra studies.
Basaltic tephra layers form a considerable part of the postglacial tephra deposits in Iceland. They can be related to the volcanic systems that produced them by characteristics based on major and minor element chemistry. Microprobe analysis
of volcanic glass shards have made it possible to relate even very thin (1-2 mm) basaltic tephra layers in distal areas outside the volcanic zones to their source.
Géochronology of Quaternary tephra deposits in Tephra studies.
Three radiometric methods have been used to date Quaternary tephra: fission-track (F-T), K-Ar, and radiocarbon (C-14). The fission-track and K-Ar methods provide direct ages for the tephra because they date phenocrysts from the source magma
that were deposited by the eruption cloud. Radiocarbon dating provides only indirect ages because the carbon used in this method comes from material, such as pre-existing wood, included in the tephra during deposition or collected from underlying