Geological mapping and Tectono-Sedimentary reconstruction of the Messinian-Pleistocene successions in the Ascoli Piceno Area (Southern Marche)

This field-based PhD research project aims to examine a geologically significant complex area of eastern central Italy and is based on some of the results stemming from the development of the Sheet 326 “Ascoli Picenoˮ of the Carta Geologica d’Italia 1:50,000 scale, which covers an area of about 605 km2 in the southern part of the external Marche Region. It portrays the new data stemming from recent sedimentological, structural, and biostratigraphic analysis and provides a sound basis for establishing a first-order tectono-stratigraphic setting of the Upper Miocene– Pleistocene marine succession exposed in the study area. The sedimentary succession exposed inside the study area comprises the pre-orogenic cretaceous- to-lower-miocenic pelagic basin units (Scaglia Rossa, scaglia variegata, Scaglia Cinerea), the pre- orogenic ramp-to-basin Miocene succession (marne con cerrogna, argille a Orbulina), the syn- orogenic foredeep siliciclastic deposits of the Laga formation and the unconformably overlaying late orogenic deposits of the Argille Azzurre developed above deforming thrusts. Furthermore, the interpretation of exploration wells and seismic profiles indicates that, buried beneath the unconformable deposits of the Argille Azzurre, the syn-orogenic turbiditic foredeep succession of the Cellino formation (CEN; Zanclean p.p.) occurs only in the subsurface. The Apennine Messinian to Pleistocene compressional tectonism was mostly coeval with sedimentation and led to the development of fold-and-thrust systems affecting the foreland substratum and the foredeep deposits. In order to outline the tectono-sedimentary evolution of the frontal zone of this sector of the Apennine outer orogenic wedge during the final stages of foredeep infilling and its subsequent inclusion in the wedge-top depozone, surface and subsurface data of the exposed Messinian to Pleistocene deposits have been combined. This approach permitted the recognition of a hierarchy of chronostratigraphic units bounded by regional, tectonically-induced unconformities that can be traced from surface to subsurface. By recording phases of out-of-grade basin margin degradation in response to major phases of structural deformation and depocenter migration towards the foreland, they provide sufficient constraints for defining the relative timing of thrusting and deformation history in the foredeep, which appears to propagate, on the whole, in a forward-breaking sequence. Inside the study area, located in the immediate footwall of the Sibillini Mts. thrust front, the most significant structures are (from west to east): the Acquasanta, Montagna dei Fiori, Appignano- Montalto and Ortezzano-Bellante structures; the Coastal structure is located farther to the east and is beyond the eastern margin of the study area. All these structures comprise N-S-trending, foreland- vergent thrust systems and related anticlines with minor associated back thrusts, and display a complex geometry and tectonic evolution. In the hanging wall of the Acquasanta and Montagna dei Fiori structures the carbonate substratum crops out, whereas the Appignano-Montalto, Ortezzano- Bellante and Coastal Structure are buried and affect the Cellino formation and younger deposits. The syn-orogenic turbidite sediments of the Messinian Laga formation (LAG), crop out extensively in the western half and southeastern portion of the Sheet area. It has been subdivided into three distinct members named, from the oldest to the youngest, Lago di Campotosto member (LAG4), Gessarenitico member (LAG5), and Teramo member (LAG6). The Lago di Campotosto member records thrust-front propagation, bounded by two regional unconformities and showing a north- derived provenance. In the late Messinian, foredeep migration toward the east led to deposition of the sand-prone Gessarenitico member (with resedimented evaporites dated at 5.61 Ma) and the clayey Teramo member, which includes a 5.5 Ma volcaniclastic layer. A further unconformity at 5.42 Ma marks the top of this unit. During the Messinian–Zanclean transition, the basin shifted offshore and was filled by turbidites of the Cellino Formation. Subsequent late Zanclean thrusting fragmented the foredeep into piggy-back basins, progressively filled by syn-tectonic Argille Azzurre deposits (FAA, Zanclean–Calabrian). These comprise the shallow-marine sandstones of the Spungone member and the hemipelagic slope deposits of the Monte dell’Ascensione and Offida members. This research refined the stratigraphic and structural framework of the study area focused on the Messinian-to-Pleistocene deposits through the production of a geological map and two geological sections and contribute to the knowledge of the local tectono-stratigraphic evolution from a physical-stratigraphical and biostratigraphical analysis of the unconformity-bounded units.