Sedimentologia della parte distale della conoide alluvionale del Pliocene superiore (Bacino Tiberino, Umbria)

Abstract

The Tiber Basin is a Plio-Quaternary intermontane basin in Umbria (central Italy). It is made up of 4 lithostratigraphic units: the Fosso Bianco Unit (FBU-Lower Pliocene), Ponte Naja Unit (PNU-Upper Pliocene), S. Maria di Ciciliano Unit (SMCU-Lower Pleistocene), and the Acquasparta Unit (AU-Lower Pleistocene). This paper deals with the sedimentological and pedogenetic features of four facies recognized in ther Ponte Naja Unit. Facies A corresponds to ribbon-shaped, fluvial channel gravel and gravelly sand deposits. Fac ies B is composed of gravel and sandy gravel produced by sheet floods. Facies C is predominant in the sedimentary succession; it consists of clayey sandy silts formed by sheet flood depositions. Facies D corresponds to strata with clues of hydromorphic pedogenesis, identified by the term "paleosols" in this paper. Two main types of "paleosols" have been recognized: one (Type I) having low evolution; and the other (Type II) which is a more evolved "paieosol". Facies characteristics, data on paleocurrents, and composition of gravel channel deposits (facies A) allow PNU to be interpreted as the distal part of a humid muddy alluvial fan probably passing to mud flat. Facies C (sheet flood deposits) and facies D ("paleosols") form 98% of the sedimentary succession and are cyclically alternating. The possible short development time of the "paleosols" makes it possible to identify each couplet (facies C/ facies D) as a high frequency cyclical depositional sequence. Towards the upper part of the succession statistical data shows: a) an increase in channel deposits (facies A); b) a decrease of "paieosol" thickness; c) a decrease of Type II "paieosol" and an increase of the Type I "paieosol" deposits. These characteristics support a variation in the depositional environment, linked to the deposition of a megasequence where the bearing of sedimentologic phenomena was greater than that of pedogenetic processes, and are interpreted as the effect of an alluvial fan progradation.