Plinian pumice fall deposit of the Campanian Ignimbrite eruption (Phlegraean Fields, Italy)

A plinian pumice fall deposit associated with the Campanian Ignimbrite eruption (36 ka, Phlegraean Fields caldera, Italy) occurs at the base of the distal grey ignimbrite in 15 localities spread over an area exceeding 1500 km2 between Benevento and the Sorrentina peninsula. In the thickest stratigraphic section at Voscone (130 cm), 45 km east of the Phlegraean caldera centre (Pozzuoli), the deposit consists of two units: the lower fall unit (LFU) is well sorted, exhibits reverse size grading and is composed of equidimensional light-grey pumice clasts with very subordinate accidental lithics; the upper fall unit (UFU) is from well to poorly sorted, crudely stratified, richer in lithics and composed of both equidimensional and prolate pumice clasts. The two fall units show slightly different dispersal axis: N90° for the LFU and N95° for the UFU. Volumes calculated with the method of Pyle (1989) are about 8 km3 for the LFU and 7 km3 for the UFU. The maximum height of the eruptive columns are estimated, using the model of the maximum lithic clasts dispersal, at 44 km for the LFU and 40 km for the UFU, classifying both fall units as ultraplinian in character. Reverse size grading within the LFU suggests an increase of the height of the column and magma discharge rate with time. Moderate sorting and crude stratification of the UFU are consistent with short-period oscillation of the column, possibly associated with repeated partial column collapses. Sharp increases in lithic content at the transition to UFU and within the UFU suggest that changes in the eruptive behaviour was produced by a dramatic increase in conduit/vent erosion. The phase of column instability preceded the emplacement of widely dispersed pyroclastic flow. The ultraplinian nature of the fall fits well with the wide dispersal of the Campanian Ignimbrite with an estimated aspect ratio of 3–4×10−4 (LARI).