Complex variations during a caldera-forming Plinian eruption, including precursor deposits, thick pumice fallout, co-ignimbrite breccias and climactic lag breccias: The 184 ka Lower Pumice 1 eruption sequence, Santorini, Greece

Abstract

The 184ka Lower Pumice 1 eruption sequence records a complex history of eruption behaviours denoted by two significant eruptive phases: (1) a minor precursor (LP1-Pc) and (2) a major Plinian phase (LP1-A, B, C). The precursor phase produced 13 small-volume pyroclastic fallout, surge and flow deposits, which record the transition from a dominantly magmatic to a phreatomagmatic eruptive style, and exhibit a normal (dacite to andesitic-dacite) to reverse (andesitic-dacite to dacite) compositional zonation of juvenile pyroclasts in the stratigraphy. Incipient bioturbation and variability in unit thickness and lithology reflect multiple time breaks and highlight the episodic nature of volcanism prior to the main Plinian eruption phase. The Plinian magmatic eruption phase is defined by three major stratigraphic divisions, including a basal pumice fallout deposit (LP1-A), an overlying valley-confined ignimbrite (LP1-B) and a compositionally zoned (rhyodacite to basaltic andesite) lithic-rich lag breccia (LP1-C), which caps the sequence. This sequence records the initial development of a buoyant convective eruption column and the transition to eruption column and catastrophic late-stage caldera collapse events. Similarities in pyroclast properties (i.e., chemistry, density), between the Plinian fallout (LP1-A) and pyroclastic flow (LP1-B) deposits, indicate that changes in magma properties exerted no influence on the dynamics and temporal evolution of the LP1 eruption. Conversely, lithic breccias at the base of the LP1-B ignimbrite suggest that the transition from a buoyant convective column to column collapse was facilitated by mechanical erosion of the conduit system and/or the initiation of caldera collapse, leading to vent widening, an increase in magma discharge rate and the increased incorporation of lithics into the eruption column, causing mass overload. Lithic-rich lag breccia deposits (LP1-C), which cap the eruption sequence, record incremental, high-energy caldera collapse events, whereby downfaulting occurred in discrete jumps, resulting in variable magma discharge rates and the development of a fissure vent system.