New evidence of the Littorina transgressions in the Kattegat: Optically Stimulated Luminescence dating of a beach ridge system on Anholt, Denmark

Recent geomorphological and sedimentological investigation of the raised beach ridge plain on Anholt in the Kattegat Sea, Denmark, has demonstrated how these deposits, when combined with Optically Stimulated Luminescence (OSL) dating, can provide information about past changes in relative sea level. The southwest–northeast trending plain is divided into three topographic levels, one of which has been surveyed using Ground-Penetrating Radar (GPR). Samples retrieved from the survey line indicate that the initial part of the beach ridge plain formed about 7700 years ago in response to a mid-Holocene Littorina transgression. The raised marine deposits document a subsequent rise in relative sea level of about 9 m, and OSL dates indicate that this transgression took place about 6650 years ago. The flooding of the Kattegat Sea 7700 years ago might possibly be related to a rapid global sea-level rise peaking around 7500 years ago.     

Petrogenesis of Tertiary Continental Intra-plate Lavas from the Westerwald Region, Germany

Tertiary volcanic rocks from the Westerwald region range frombasanites and alkali basalts to trachytes, whereas lavas fromthe margin of the Vogelsberg volcanic field consist of morealkaline basanites and alkali basalts. Heavy rare earth elementfractionation indicates that the primitive Westerwald magmasprobably represent melts of garnet peridotite. The Vogelsbergmelts formed in the spinel–garnet peridotite transitionregion with residual amphibole for some magmas suggesting meltingof relatively cold mantle. Assimilation of lower-crustal rocksand fractional crystallization altered the composition of lavasfrom the Westerwald and Vogelsberg region significantly. Thecontaminating lower crust beneath the Rhenish Massif has a differentisotopic composition from the lower continental crust beneaththe Hessian Depression and Vogelsberg, implying a compositionalboundary between the two crustal domains. The mantle sourceof the lavas from the Rhenish Massif has higher ²⁰⁶Pb/²⁰⁴Pband ⁸⁷Sr/⁸⁶Sr than the mantle source beneath the Vogelsbergand Hessian Depression. The 30–20 Ma volcanism of theWesterwald apparently had the same mantle source as the QuaternaryEifel lavas, suggesting that the magmas probably formed in apulsing mantle plume with a maximum excess temperature of 100°Cbeneath the Rhenish Massif. The relatively shallow melting ofamphibole-bearing peridotite beneath the Vogelsberg and HessianDepression may indicate an origin from a metasomatized portionof the thermal boundary layer. KEY WORDS: continental rift volcanism; basanites; trachytes; assimilation; fractional crystallization; partial melting     

Quaternary forminiferal stratigraphy in boring 81/29 from the central North Sea

A 140.2 m deep boring (BH 81/29) from the central North Sea (British sector) has been investigated for its foraminiferal content. Fourteen assemblage zones are identified, and these are correlated with other records from the North Sea region. The stratigraphical interpretation of BH 81/29 is supported by palaeomagnetic data and by amino acid dates and thermoluminescence dates from the same boring. Foraminiferal zones 14 to 8, from the bottom of the core, have been referred to the Early Pleistocene. Zones 7 to 4, which occur above the Bruhnes/Matuyama boundary, seem to belonged in the Middle Pleistocene, and zones 3 to 1 are referred to the Late Pleistocene. A characteristic feature of the present sequence is that a major part of the Quaternary record seems to be missing. As is also known from other areas of the North Sea, interglacial deposits are especially badly represented.     

Quaternary erosional surfaces in the Danish North Sea

Studies of a deep high-resolution reflection seismic profile through the eastern North Sea basin show that at least four erosional phases have affected the area during the Saalian, Weichselian and Holocene. Foraminiferal investigations of five boreholes make it possible to date the erosional events. When looking at the restricted area of this study, the deep incised valleys appear to have developed during sea-level fall and lowstand as the Quaternary ice sheets were established. Further erosion took place during the deglaciation of the area and the valleys were further deepened when used as drainage paths. The oldest erosional phase recognized from the seismic profiles is interpreted to be of Saalian age. Two later erosive phases were associated with intra-Weichselian glacial advances. The uppermost erosive surface represents river valleys at the transition from the Weichselian glacial to the Holocene.     

The Petrology and Geochemistry of Vesteris Seamount, Greenland Basin--an Intraplate Alkaline Volcano of Non-Plume Origin

Vesteris Seamount is a solitary alkaline volcano in the GreenlandBasin some 280 km NW of Jan Mayen. Topographic and geophysicalstudies have shown no sign of an associated plume trace. Evidencefrom ash layers in sediment cores around the volcano and datingof dredged samples show that it has been active in Quaternarytimes. The lavas from Vesteris studied here consist of basanites,tephrites, mugearite, and alkali basalts. Crystal fractionationmodels are consistent with the generation of the tephrites andmugearite from a basanitic parent. Extensive kaersutite fractionationis required late in the fractionation sequence to produce theextreme mugearite composition. Na-Al-Fe-rich green cores tomany clinopyroxene phenocrysts at Vesteris suggest a fractionationhistory beginning at high pressure in the mantle. Differencesbetween Vesteris and Jan Mayen in the ratios of highly incompatibletrace elements such as Ce/Pb and Rb/Cs, which will not normallybe fractionated from one another during mantle melting, suggestthat the two are not derived from the same source. Relativelyunradiogenic Sr isotope ratios (compared with Bulk Earth), andhighly incompatible trace element patterns similar to thosefor St. Helena, suggest that Vesteris magmas are derived froma depleted, asthenospheric source. We propose that the Vesterisbasanites are very low degree partial melts ({small tilde}1%)of this source, most probably those which give rise to the seismiclow-velocity zone (LVZ). Such small-degree melts may preferentiallytap small-scale heterogeneities in the asthenosphere. Vesterislies at the intersection of two major structural trends in theGreenland Basin—(1) a zone of major reorientation of spreadingdirection on the Mohns Ridge north of Jan Mayen and (2) theextension of the Kolbeinsey Ridge axis. We propose that a combinationof the extensional stress fields related to these two lineamentsproduces sufficient dilation of the lithosphere at Vesteristo allow magmas from the LVZ to reach the surface.     

Magma Evolution of the Sete Cidades Volcano, Sao Miguel, Azores

The Sete Cidades volcano (São Miguel, Azores) is situatedat the eastern end of the ultraslow spreading Terceira riftaxis. The volcano comprises several dominantly basaltic pre-calderaeruptions, a trachytic caldera-forming stage and a post-calderastage consisting of alternating trachytic and basaltic eruptions.The post-caldera flank lavas are more primitive (>5 wt %MgO) than the pre-caldera lavas, implying extended fractionalcrystallization and longer crustal residence times for the pre-caldera,shield-building lavas. Thermobarometric estimates show thatthe ascending alkali basaltic magmas stagnated and crystallizedat the crust–mantle boundary (15 km depth), whereas themore evolved magmas mainly fractionated in the upper crust (3km depth). The caldera-forming eruption was triggered by a basalticinjection into a shallow trachytic magma chamber. Lavas fromall stages follow a single, continuous liquid line of descentfrom alkali basalt to trachyte, although slight differencesin incompatible element (e.g. Ba/Nb, La/Nb) and Sr isotope ratiosimply some heterogeneity of the mantle source. Major and traceelement data suggest similar partial melting processes throughoutthe evolution of the volcano. Slight geochemical differencesbetween post- and pre-caldera stage lavas from the Sete Cidadesvolcanic system indicate a variation in the mantle source compositionwith time. The oxygen fugacity increased from the pre-calderato the post-caldera stage lavas, probably as a result of theassimilation of crustal rocks; this is supported by the presenceof crustal xenoliths in the lavas of the flank vents. The lavasfrom the Sete Cidades volcano generally have low Sr isotoperatios; however, rocks from one post-caldera vent on the westernflank indicate mixing with magmas resembling the lavas fromthe neighbouring Agua de Pau volcano, having higher Sr isotoperatios. The different magma sources at Sete Cidades and theadjacent Agua de Pau volcano imply that, despite their closeproximity, there is only limited interaction between them. KEY WORDS: crystallization depth; fractionation; stratigraphy; Terceira rift; volcanic stages