Abstract The study of Ordovician tephras yields a wealth of valuable information about regional tectonism, sedimentation, stratigraphic correlation, and process rates. As such, these layers are prized by geologists and are the subject of a rich literature. Ordovician tephra studies were pioneering, particularly in the development of chemical fingerprinting to improve precision in tephrochronology. Modern radioisotope geochronology utilizes zircons and other phenocrysts from these layers to generate eruption ages with uncertainty on the order of a hundred thousand years. When integrated with biostratigraphy, chemostratigraphy and astrochronology, tephra ages provide an unparalleled opportunity to constrain process rates. Fifty such Ordovician tephra ages have been published over the last decade from U–Pb analysis of individual zircon phenocrysts, providing geochronological coverage across all stages of the Ordovician. Laurentia dominates this coverage (24) followed by the Baltic Basin (12), North Gondwana (11), Cuyania (four) and the Siberian Tungus Basin (one). Future tephra studies should seek to fill the numerous remaining gaps in the Ordovician time scale.

Abstract The genesis of particles and their transport mechanisms are the two fundamental factors driving the accumulation of sediments associated with volcanism or derived from volcanic sources. These factors are therefore the most important criteria on which to base a useful classification of sediments in such environments. However, the relative significance of the nature of particles v. the transporting mechanism forming a deposit varies in existing terminological schemes: those schemes applied where volcanological contextual information is available tend to give precedence to the transportation process; whereas sedimentological schemes examining ancient deposits tend to focus principally on the nature of particles. Here, we provide an outline of the challenges in classifying volcanically derived sediments and put forward a scheme that aims to bridge current terminological differences and accommodate variable levels of uncertainty. This work defines three endmembers ( primary volcaniclastic , secondary volcaniclastic , volcanic epiclastic ) that correspond to (a) deposits whose particles are produced, transported and emplaced directly by volcanic mechanisms; (b) deposits whose particles are produced directly by volcanic events but transported and accumulated by non-volcanic mechanisms, either in continuum with the events or after interim storage; (c) deposits whose particles are produced by weathering/erosion of volcanic terrains and transportation of derived material by non-volcanic mechanisms. When the complex combination of genetic and transportation processes accumulating volcaniclastic sequences is not clear, but a strong relationship between an eruptive event and the studied volcaniclastic deposit can still be demonstrated, a further category ( volcanogenic ) has been introduced.

Abstract We review here data and information on Antarctic volcanism resulting from recent tephrostratigraphic investigations on marine cores. Records include deep drill cores recovered during oceanographic expeditions: DSDP, ODP and IODP drill cores recovered during ice-based and land-based international cooperative drilling programmes DVDP 15, MSSTS-1, CIROS-1 and CIROS-2, DVDP 15, CRP-1, CRP-2/2A and CRP-3, ANDRILL-MIS and ANDRILL-SMS, and shallow gravity and piston cores recovered in the Antarctic and sub-Antarctic oceans. We report on the identification of visible volcaniclastic horizons and, in particular, of primary tephra within the marine sequences. Where available, the results of analyses carried out on these products are presented. The volcanic material identified differs in its nature, composition and emplacement mechanisms. It was derived from different sources on the Antarctic continent and was emplaced over a wide time span. Marine sediments contain a more complete record of the explosive activity from Antarctic volcanoes and are complementary to those obtained by land-based studies. This record provides important information for volcanological reconstructions including approximate intensities and magnitudes of eruptions, and their duration, age and recurrence, as well as their eruptive dynamics. In addition, characterized tephra layers represent an invaluable chronological tool essential in establishing correlations between different archives and in synchronizing climate records.