Una deformazione gravitativa profonda di versante all’origine del Piano di Oneto, Val Graveglia (Appennino Ligure)

Viene espressa una nuova ipotesi sulla genesi del Piano di Oneto, ubicato in Alta Val Graveglia (Appennino ligure), sulla base di un riesame degli aspetti deducibili dalle carte geologiche e topografiche e dalle evidenze di terreno.
Il Piano di Oneto costituisce una fra le più ampie superfici pianeggianti nell’area dell’Appennino Ligure e, per tale peculiarità, è oggetto, da oltre un ventennio, di notevole interesse da parte di geologi e geomorfologi. Interpretato come una forma epigea di origine prevalentemente carsica esso è stato definito a più riprese la più grande “dolina” esistente in Liguria Orientale ed attualmente l’area di pertinenza viene considerata e perimetrata quale “monumento naturale” nel Piano del Parco Naturale Regionale dell’Aveto. Tuttavia, la revisione dei dati a disposizione e l’analisi degli elementi morfologici sia alla macroscala che alla mesoscala forniscono, assieme alla presenza di diversi indizi geologici e morfotettonici, nuovi elementi di discussione in parziale contrasto con le interpretazioni che attribuiscono al carsismo un ruolo di primaria importanza nella genesi del Piano di Oneto. Le osservazioni effettuate sulla litologia e struttura del substrato, sulla morfologia e sulla tettonica recente dell’area ci portano a proporre un modello secondo il quale la depressione deriverebbe da un fenomeno gravitativo profondo favorito dalla presenza di un substrato “plastico”, dalle faglie e dal sollevamento plio-quaternario. È stata, inoltre, scartata l’ipotesi del glacialismo quale causa d’origine del Piano di Oneto.

A new hypothesis on the genesis of the Piano di Oneto, located in the Upper Val Graveglia (Ligurian Apennines) (Fig. 1), on the basis of field studies and a re-examination of the features distinguishable in geological and topographical maps and aerial photographs. The Piano di Oneto is one of the largest flat areas of the Ligurian Apennines; interpreted as an epigeal form of prevalently carsic origin, it has been defined more than once as the largest existing “dolina” in eastern Liguria (Fig. 7).
A review of the available data, an analysis of the morphological features at both the macro- and mesoscale and various geological and morphotectonic characteristics provide a new basis for discussion.
From a geological point of view (Fig. 2) the area is characterised by formations associable with the Val di Vara Supergroup (Internal Ligurids of various authors), divided into two main units, the underlying Mt. Porcile Unit and the overlying Mt. Zatta Unit (Gottero Unit of various authors). The former contains notable ophiolitic sequences (basalts, serpentinites), a volcano-sedimentary complex (ophiolitic breccias) and a sedimentary cover (Mt. Alpe Jaspers, Calpionelle Limestones, Palombini Clays). A little to the east of the Val Graveglia – Val di Vara watershed, under the Mt. Porcile Unit, lies the Colli-Tavarone Formation (Upper Cretaceous – Palaeocene), which consists of layered claystones interspersed with thin layers of siltites and calcarenites.
Various authors have studied the structure of this area, which is exceptionally complex. Its complexity is evident in the cross section reported in Fig. 3, the trace of which is visible in Fig. 2. All the structural reconstructions made of the area underline the fact that there is no carbonatic mass under the Piano di Oneto.
Since the Pliocene, the complicated structural situation described above has been subjected to extensional tectonics that have given it its peculiar morphostructural conditions and been the main underlying cause of landslides in this zone as in other parts of Liguria.
An analysis of the orientation of the hydrographic network and the watershed lines indicate their close dependence on the main neotectonic lines, which are oriented ENE – WSW (56° - 66°), NNW – SSE (150°), N – S and E – W. It is possible to say that the majority of the geomorphological alignments correspond to fracture systems or Plio-Quaternary faults but not to the more ancient faults visible in Fig. 2.
In the area under study (Fig. 6) there are morphological features (reverse slopes, bulges and landslides) that are specific signs of gravitational slope deformations.
The studies carried out indicate that the genesis of the Piano di Oneto should be looked for in a “deep-seated gravitational slope deformation” caused by the high energy of the relief associated with the uplift of the zone and the high erosive capacity of the watercourses that have caused the deepening of the hydrographic network.
Given the above scenario, the depression under study could be interpreted as the filling up of a trench running parallel to the ridge (NW– SE) with silty-clayey material washed down from the surrounding slopes (Figs. 6 and 9). We believe that only its extension towards the WSW, where there seem to be Calpionelle Limestones, can be imputed to carsic and erosive phenomena.