The Ilopango caldera complex, El Salvador: Stratigraphic revision of the complete eruptive sequence and recurrence of large explosive eruptions

Abstract

Ilopango caldera erupted episodically at least 13 tuff-forming eruptions with a minimum estimate volume of 1–5km3 DRE per eruption, reaching up to 150km3 DRE for the first caldera-forming eruption. All tuffs are of dacitic-rhyolitic composition. The complete pyroclastic sequence spans a range in time from 1.785 to 0.0015 Ma, and based on stratigraphy and geochronology constraints can be divided into three formations: the Comalapa, Altavista and Tierras Blancas formations. In this work, we focus on the members of the newly described Altavista Formation (middle part of Ilopango caldera volcanic sequence), which consist of six con- solidated pyroclastic deposits or tuffs. Each tuff corresponds to a specific eruption followed by a period of quiescence during which soil beds were developed on the deposits. The ages of the Altavista Formation ranges from 918 to 257ka, based on new 40Ar/39Ar, U/Pb-zircon, and U/Th-zircon analyses. The tuffs of this for- mation show similar characteristics in mineralogy and composition. They are calcalkaline, rhyodacitic tuffs, with plagioclase, clinopyroxene, and hornblende. From field mapping and descriptions of the deposits, we have inferred the eruptive styles that include pumice fallouts, pyroclastic density currents and also hydromag- matic explosions. The common vent in all tuffs was the Ilopango caldera and each member of the Altavista Formation could correspond to a caldera collapse event, except for one of the six eruptions. The volume of each member was estimated to be >30km3 DRE, which is the same order of magnitude than that estimated for the Tierra Blanca Joven (TBJ) eruption at about 1500 B. P, and smaller than those of the ignimbrites of the Comalapa Formation, the first three members of the Ilopango caldera reported previously. The tuffs of the Altavista Formation are visible up to 15–20 km away from the caldera's topographic margin. The recurrence interval of large explosive events at the Ilopango caldera was established by integrating the stratigraphic and geochronologic data of all 13 ignimbrites and pumice fallouts erupted from Ilopango caldera since the first one at 1.78 Ma to the last explosive event (TBJ).