Forestepping-backstepping stacking pattern of volcaniclastic successions: Roccamonfina volcano, Italy

Abstract

We use Unconformity Bounded Stratigraphie Units (UBSU) to reconstruct the chronostratigraphy of volcaniclastic deposits of Roccamonfina Volcano (southern Italy). Significant discontinuities of different hierarchical order in the stratigraphic succession of the volcano have allowed us to identify the UBSU supersynthems, synthems and subsynthems. The hierarchical order has been determined by the importance and duration of the discontinuities bounding such units and by their geographic distribution. Supersynthems are bounded by regional-scale unconformities that may correspond to important tectonic events, changes in the style of eruptive activity or volcano-tectonic collapses. Synthems are bounded by unconformities marked by paleosols that are representative of considerable hiatus and extended all over the synthem. They are composed of complex sequences of depositional units. At Roccamonfina the synthems reveal cyclical eruptive activity. Unconformities bounding the subsynthems have been recognized through a detailed facies analysis. More than 100 stratigraphic sections have been analyzed and correlated, restoring them to single successive datum planes. It has been possible to reconstruct the original stratigraphic relationships, the geometry of the units, and the effects of syn-eruptive tectonic activity. This procedure revealed that the unconformity surfaces bounding depositional units show different physical expression in the proximal, medial and distal areas: (a) erosive surfaces may become amalgamation surfaces; (b) amalgamation of several depositional units can locally produce a single lithostratigraphic unit. In this case the unconformity surfaces bounding the units can be identified by the presence of discontinuous pumice and lithic concentration zones; (c) non-depositional surfaces in proximal areas (bypassing surfaces) or in interfluve areas may correspond to downslope deposits; (d) unconformity surfaces can bound different lithostratigraphic units that are the result of down-current facies variation. The pyroclastic units at Roccamonfina are organized as forestepping-backstepping sequences. We interpret this organization as resulting from waxing-waning processes associated with eruption and emplacement. In fact, the internal geometry of each synthem indicates a forestepping phase until the maximum eruption rate is achieved; as it later declines deposition occurs upslope (backstepping). This mechanism is also reproduced at smaller scale within each subsynthem. The forestepping-backstepping organization of the deposits provides an explanation for the variability of physical, chemical and sedimentological parameters observed in the Roccamonfina volcaniclastic deposits.