The lateral facies of ignimbrites at Laacher See volcano

Abstract

Valley-fill ignimbrites of the Middle Laacher See Tephra (MLST) in the proximal depositional fan south of Laacher See volcano are laterally continuous with fine-grained overbank-facies deposits, 0.5–1.5 m thick, covering higher elevations and interfluves between the paleovalleys. The overbank deposits consist of up to 12 ash layers, each 4–10 cm thick, which show internal structures typical of ash-flow transport, such as poor sorting, reverse size-grading of pumice, local normal grading of coarse ash-sized lithics above a fine-grained basal layer, cross-stratification behind obstacles, and erosional unconformities. Thickness, median grain-size, and number of individual layers decrease systematically with distance from the vent. Overbank ash layers thicken at the valley slopes and form discrete valley-fill ignimbrite flow units in the paleochannels with median grain size increasing from ≤63 μm to ≤350 μm. Toward the center of paleochannels, however, the well-defined overbank facies is obscured by mutual erosion of individual flow units. Overlapping data fields in ternary grain-size variation diagrams indicate the overbank facies to have evolved from ash flows chiefly through depletion of lapilli and coarse ash. Overbank-facies ash layers do not represent dust layers resulting from elutriation clouds of ash flows (co-ignimbrite ash) or surge deposits developed on higher ground due to low concentration of solids. They are similar in some parameters to Taupo-type ignimbrite veneer deposits, but are interpreted differently. The thin, fine-grained, Laacher See ash layers are thought to have been deposited from diluted portions of the flow proceeding directly from the eruption column while the main pyroclastic flows were confined to the paleovalleys radiating away from the eruptive center. The wide distribution of the thin ash layers is attributed to the balance of deposition from various flow parts and turbulent entraining and heating of ambient air that sustained sufficient mobility of the diluted flows to spread across hills and level ground.