Proximal depositional facies from a caldera-forming eruption: the Parata Grande Tuff at Ventotene Island (Italy)

Abstract

Ventotene and S. Stefano islands form part of the southeastern flank of a Pleistocene stratovolcano. The products of 27 eruptions, the majority of which have never been previously identified, comprise the two islands. These products consist chiefly of pyroclastic deposits with only three effusive episodes represented. The youngest and most prominent deposit, hereafter called Parata Grande Tuff, was produced by an eruption that represents the culmination of more than 0.5 Ma of volcanic evolution and that formed a 3-km-wide summit caldera. The Parata Grande Tuff is the focus of this study and consists mostly of pyroclastic flow deposits overlying basal pumice and ash fall beds. The studied sections crop out as far as 2 km from the caldera rim. The flow deposits sequence can be represented by four different lithofacies: (1) massive facies, originated by rapid sedimentation of grains through a boundary layer; (2) coarse-tail graded facies, formed with a relatively lower fallout rate of particles from the suspension toward the boundary layer; (3) inverse-graded facies, considered to have formed by gradual deposition from the base to the top of a traction carpet, when the shear stress dropped below the threshold value to support the weight of the clasts; and (4) sand-wave facies, the product of deposition from an unsteady low-concentration flow. The base of the pyroclastic flow succession is welded and thickens in topographic depressions. The welding of deposits with traction structures and undeformed coarse pumice clasts probably occurred during the final stages of deposition of a formerly paniculate pyroclastic flow. The flow deposits that overlie the welded succession show a drastic increase and change of lithic types that are interpreted to reflect the collapse of the roof of the reservoir triggering magma/water interaction. The uppermost part of the sequence is a succession of layers with dunes and cross stratification alternating with massive beds. The stratified and heterogeneous succession of flow deposits could be due to the unsteady nature of the flow during the final phase of the eruption. A vertical trend of sedimentary facies from massive, to coarse-tail-graded into sand-wave lithofacies is dominant in the PGT sequence and is interpreted as a depositional cycle. Each cycle is suggestive of deposition from a flow that was progressively diluted with time.