I depositi tardo-orogenici della Valle Latina meridionale: stratigrafia e implicazioni cinematiche per l’evoluzione dell’Appennino centrale

Field and laboratory analyses carried out on the sedimentary successions of Monte San Giovanni Campano, Fontana Liri and Rocca d’Arce (FR, southern Latium) allow us to recognize some unconformity-bounded stratigraphic units (UBSU) within the Messinian portion of the local stratigraphical succession. From a regional point of view, the stratigraphical succession of the study area can be divided into four sedimentary cycles separated by regional stratigraphical discontinuities: 1) pre-orogenic deposits; 2) syn-orogenic deposits; 3) late-orogenic deposits; 4) post-orogenic deposits. The aim of this paper is to describe the stratigraphic and structural features of the late-orogenic deposits cropping out in the Monte San Giovanni Campano-Fontana Liri-Rocca d’Arce area. Sediments pertaining to different unconformable cycles have been grouped in the late-orogenic deposits, which according to their structural-stratigraphic features can be referred to thrust-top basins. The younger sedimentary cycle of the late-orogenic deposits, which crops out at Monte San Giovanni Campano and Rocca d’Arce, consists mainly of clays with gypsum and rests unconformably on the underlying formations. Beneath this unconformable sedimentary cycle there are coarsegrained clastic deposits pertaining to an older UBSU, which rests on the pre-orogenic succession of the study area. At Monte San Giovanni Campano and Boville Ernica, a sandy matrix conglomerate with infralittoral marine molluscs, which consists mainly of carbonate pebbles, characterizes the base of this UBSU. These coarsegrained deposits are etheropic with yellowish massive sandstones containing infralittoral marine molluscs and are referable to the lower Messinian. In the localities of Fontana Magna and Paparungi well-stratified bituminous-sandstones characterize this sedimentary cycle, whereas massive sandstones crop out in the Rocca d’Arce and Fontana Liri areas. The recognized unconformable deposits have been split into two different UBSU: 1) Sintema delle Arenarie di Torrice and 2) Sintema delle argille con gessi. These two synthems are part of a Messinian supersynthem: the Supersintema di Monte San Giovanni Campano. This paper deals with the description of the stratigraphical and structural characters of the UBSU recognized in the Monte San Giovanni Campano- Fontana Liri-Rocca d’Arce area. Moreover, the formalization of the Supersintema di Monte San Giovanni Campano together with its related synthems (Sintema delle Arenarie di Torrice and Sintema delle argille con gessi) is proposed in this paper. The structural and stratigraphical features of the recognized UBSU allow us to locate their related sedimentary basins within the frontal portion of the Apennine chain, behind the leading edge of the central Apennines where active thrusts were building the Messinian chain (thrust-top basins). The involvement of these UBSU at the footwall of thrust planes suggests an out-of-sequence re-activation of this sector of the Apennine chain. This evidence allow us to extend more southward the portion of the Apennine chain that was re-activated in out-of-sequence thrusting during the top Messinian/early Pliocene tectono-sedimentary event. Until now, in fact, evidences of out-of-sequence re-activations in the central Apennines during this tectono-sedimentary event were observed just at the hanging wall of the Olevano-Antrodoco-Sibillini Mts overthrust.

Analisi di terreno e di laboratorio effettuate sulle successioni sedimentarie affioranti nei settori di Monte San Giovanni Campano, Fontana Liri e Rocca d’Arce hanno permesso di riconoscere e cartografare alcuni sintemi che interessano la porzione messiniana della successione locale. Tali depositi sono stati accorpati in due diverse unità a limiti inconformi: 1) Sintema delle Arenarie di Torrice e 2) Sintema delle argille con gessi, che fanno parte di un unico supersintema (Supersintema di Monte San Giovanni Campano). In questo lavoro vengono descritte le caratteristiche stratigrafico-strutturali dei cicli discordanti riconosciuti e viene proposta la formalizzazione del Supersintema di Monte San Giovanni Campano e dei sintemi ad esso collegati (Sintema delle Arenarie di Torrice e Sintema delle argille con gessi). Le caratteristiche stratigrafico-strutturali di questi depositi permettono di collocare i relativi bacini di sedimentazione nella porzione frontale della catena appenninica in via di strutturazione (bacini di thrust-top). Il loro successivo coinvolgimento al letto di superfici di sovrascorrimento suggerisce, inoltre, una riattivazione compressiva fuori sequenza, consentendo di estendere molto più a sud la fascia di catena del Messiniano inferiore riattivata durante la fase tettono- sedimentaria del top Messiniano/Pliocene inferiore, fino ad oggi limitata al blocco di tetto del sovrascorrimento Olevano-Antrodoco- M.ti Sibillini.