Laacher See Tephra: A widespread isochronous late Quaternary tephra layer in central and northern Europe

Abstract

A late Quaternary tephra layer, widespread in central and northern Europe, resulted from explosive Plinian and phreatomagmatic eruptions of the Laacher See Volcano 11,000 yr B.P. The tephra is distinguished from other late Quaternary andesitic-rhyolitic airfall tuff layers in northern Europe and from basaltic or trachytic tuff deposits in southern Europe by its phonolitic composition and abundance of sanidine, plagioclase, clinopyroxene, amphibole, and sphene. The proximal tephia sequence at Laacher See is divided into three main deposits: the predominantly Plinian deposits of Lower and Middle Laacher See Tephra (LLST and MLST) and phreatomagmatic deposits of the Upper Laacher See Tephra (ULST). The MLST member is further subdivided into beds A, B, and C1, C2, and C3. The chemical composition of the magma is highly differentiated phonolite in the LLST to MLST B sections but mafic phonolite in the MLST C1 to ULST sections. All deposits are considered to be isochronous, the frequency maximum of 16 radiocarbon datings indicating an eruption about 11,000 ±50 yr B.P. Distal ash was deposited in three main fans directed to the northeast (LST traced up to 1,100 km distance), south (LST traced up to 600 km), and southwest (LST traced up to 100 km). Tephrostratigraphic correlation of the distal ash deposits is based on (a) the major-element composition of glass shards, (b) lithology, and (c) heavy-mineral analyses. The northeastern fan consists of deposits from LLST, MLST B, and MLST C1 eruptive phases, the southern fan comprises MLST A, MLST C2, and ULST deposits, and the southwestern fan consists exclusively of ash from the ULST eruptive phase. Northeastern transport of ash during eruptive phases, with high Plinian eruption columns, but southern and southwestern transport of ash along phases of relatively low eruption columns, are interpreted in terms of prevailing southwesterly paleowinds at high altitudes (tropopause level?) but northerly winds dominating in the lower atmosphere. The Laacher See eruption columns were emplaced into an atmosphere vertically zoned with respect to paleowind directions, which also explains the near-vent shifting of LLST, MLST B, and MLST C1 iso-pach axes from east-southeast to northeast within the first 20 km of transport.