La formazione Plio-Pleistocenica di Santopadre (Lazio, Italia centrale)

Abstract

The Santopadre Formation outcrops on a vast area - about 150 km²-of south-central Latium (central Italy) between the Emico-Simbruini Mountains, Monte Cairo, and the middle Liri river valley. The sedimentation basin lies on a Meso-Cainozoic carbonate and terrigenous substratum, dislocated by tectonic structures oriented mainly NW-SE, E-W and N-S. The Formation, having a thickness up to about some hundred metres, consists of two members: the lower one is composed of coarse- to fine-grained silty sandstones and clayey siltstones, deposited in a fiuvio-Iacustrine and palustrine environment; the upper one is composed mainly of alluvial sediments, with facies from psephitic to psammitic and pelitic. The two members' lithological composition is strictly connected with the nature of the substratum, but scarse allogenic lithotypes also occur. Facies analyses of the upper member sediments allowed to distinguish four main litho facies. i) Lithofacies A): heterometric matrix-supported breccias and conglomerates referred to high viscosity debris flows; ii) lithofacies B1): stratified alluvial clasts-supported conglomerates referred to sheet flood deposits; iii) lithofacies B2): cross-bedded conglomerates showing an erosional contact with stream channel deposits, iv) lithofacies C: alternating sandstones and siltstones of backswamp overbank environment. On the whole, the upper member of the Formation represents a very large alluvial fan or rather several anastomosing alluvial fans fed by streams flowing mainly from NW and N as the paleocurrent pattern shows. In its distal part the fan grades into an alluvial braided streams plain. Grain-size analyses indicate that the deposits become finer and finer passing from the apical part - made up of mainly psephitic sediments - to the distal psammitic one. Morphometric analyses on pebbles indicate higher and higher roundness, sphericity and prolate indexes in the same direction. A reconstruction of the Formation depositions evolution suggests that the lower member was deposited in a fluvio-lacustrine environment at the valley outlet, characterized by moderate transport energy with flood episodes and formation of ponds and swamps, adjacent to mountains not yet affected by uplift movements. A strong and rapid uplifting of the mountain range, associated with extensional tectonic movements, caused the production of a huge quantity of detrital material, which would have been transported and deposited downstream, under favorable climatic conditions. Subsequently, the Formation was dislocated by extensional faults that often re-utilized older structures, mainly trending NW-SE to E-W and N-S and showing displacements of at least several tens of metres. The Formation was thus subdivided into tilted, uplifted and/or depressed blocks. The age of the Formation has not been determined; morphologic observations and the relationships with the underlying and overlying Plio-Pleistocene formations suggest that it could have been deposited under temperate climatic conditions during a long time interval in the Upper Pliocene-Lower Pleistocene period.