The Holocene Secche di Lazzaro phreatomagmatic succession (Stromboli, Italy): evidence of pyroclastic density current origin deduced by facies analysis and AMS flow directions

Abstract

The edifice of Stromboli volcano gravitationally collapsed several times during its volcanic history (> 100 ka-present). The largest Holocene event occurred during the final stage of the Neostromboli activity (similar to 13-5 ka), and was accompanied by the emplacement of phreatomagmatic and lahar deposits, known as the Secche di Lazzaro succession. A stratigraphic and paleomagnetic study of the Secche di Lazzaro deposits allows the interpretation of the emplacement and the eruptive processes. We identify three main units within the succession that correspond to changing eruption conditions. The lower unit (UA) consists of accretionary lapilli-rich, thinly bedded, parallel- to cross-stratified ash deposits, interpreted to indicate the early stages of the eruption and emplacement of dilute pyroclastic density currents. Upward, the second unit (UB) of the deposit is more massive and the beds thicker, indicating an increase in the sedimentation rate from pyroclastic density currents. The upper unit (UC) caps the succession with thick, immediately post-eruptive lahars, which reworked ash deposited on the volcano's slope. Flow directions obtained by Anisotropy of Magnetic Susceptibility (AMS) analysis of the basal bed of UA at the type locality suggest a provenance of pyroclastic currents from the sea. This is interpreted to be related to the initial base-surges associated with water-magma interaction that occurred immediately after the lateral collapse, which wrapped around the shoulder of the sector collapse scar. Upward in the stratigraphy (upper beds of UA and UB) paleoflow directions change and show a provenance from the summit vent, probably related to the multiple collapses of a vertical, pulsatory eruptive column

Stratigraphy and Tectonics of the Sengkang Basin, South Sulawesi

“…One of the factors that strongly control the transport and emplacement of a pyroclastic flow is the flow's temperature (Porreca et al, 2007;Giordano et al, 2008). Consequently, the temperature may control the distance the flow travels.…”

Mapping and Assessment of Volcanic Hazards Related to the Ignimbritic Eruption by AMS in Bambouto Volcano (Cameroon Volcanic Line)

“…the magnetic foliation is subvertical). Normal fabrics may correspond to two different situations [27]: in some cases, K 1 is consistent with the flow direction inferred from the foliation imbrication, whereas in other cases it is normal to the flow direction (Fig. 8a).…”

Paleoflow directions of a subaqueous lahar deposit around the Miocene Keserűs Hill lava dome complex (North Hungary) as constrained by photo-statistics and anisotropy of magnetic susceptibility (AMS)

“…Comparing the shape anisotropies with fabrics from PDC deposits that are emplaced under high particle concentration and laminar shearing conditions (e.g. Macdonald and Palmer, 1990;Ort et al, 1999Ort et al, , 2003Porreca et al, 2003;Giordano et al, 2008), we can conclude that P values in our data set are lower than those in PDC deposits where P values are generally ≥3 % (e.g. Baer et al, 1997;Piper et al, 2002;Petronis and Geissman, 2009).…”