Note Illustrative della Carta Geologica d'Italia alla scala 1:50.000 foglio 326 Ascoli Piceno

The 326 “Ascoli Picenoˮ Sheet of the Carta Geologica d’Italia 1:50,000 scale, covering an area of about 605 km2 in the southern part of the external Marche Region, has been elaborated through original, 1:10,000 scale field mapping carried out during 2022-2023. It portrays the new data stemming from recent sedimentological, structural, and biostratigraphic analyses and provides a sound basis for establishing a first-order tectono-stratigraphic setting of the Upper Miocene– Pleistocene succession exposed in the study area. The succession exposed inside the Sheet comprises the membro intermedio (SAA2) and the membro superiore (SAA3) of the Scaglia Rossa (Campanian p.p. - Lutetian p.p.) and the marlier scaglia variegata (VAS; Lutetian p.p.- Priabonian p.p.) and Scaglia Cinerea (SCC, Priabonian p.p. - Chattian) formations. The ramp-to-basin Miocene succession is represented by the marne con cerrogna (CRR; Burdigalian p.p.-Tortonian p.p.), whereas the argille a Orbulina (UAM3; Tortonian p.p.-lower Messinian p.p.) of the unità argilloso-marnosa (UAM) marks the transition to syn-orogenic sedimentation and to the foredeep siliciclastic deposits of the formazione della Laga (LAG; Messinian – lowermost Zanclean?). At the surface, these sediments are unconformably overlain by late orogenic deposits of the Argille Azzurre (FAA; upper Zanclean - Gelasian p.p.) 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 formazione di Cellino (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 Sheet 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 Costiera structure is located farther to the east and it is beyond the eastern margin of the Sheet. 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 structures are buried and affect the formazione di Cellino and younger deposits. The Acquasanta anticline is an asymmetric structure with a gently west-dipping western limb, a steep to overturned eastern limb, and a pronounced axial culmination. The anticline, which started to grow during late Messinian, is bounded on its eastern side by the west-dipping, low-displacement Acquasanta thrust fault; a broad syncline, passively transported on top of the Acquasanta thrust, stretches along the western side of the Acquasanta structure. The Montagna dei Fiori anticline, the most prominent structure within the Sheet area, lies to the east of the Acquasanta thrust. The fold profile is asymmetric, with a steep to overturned forelimb and a gently dipping backlimb. The axial culmination of this doubly plunging structure, with the Calcare Massiccio formation exposed in the core, is beyond the investigated area. Within the anticline, surface geological mapping reveals the occurrence of several upward splay thrusts branching off the main thrust zone, giving rise to lower order, thrust-related anticline-syncline-pairs. The thrust fault related to this anticline was mostly active during the late Messinian to earliest Pliocene, but it is buried and can be investigated only by means of subsurface data. Inside the area of the Ascoli Piceno Sheet, seismic reflection data reveal the occurrence of the Appignano-Montedinove and Ortezzano-Bellante structures. The former, active during the Zanclean, caused the overthrusting of the membro di Teramo onto the formazione di Cellino sediments. The latter, which is formed by several east-vergent thrust-related anticlines, has an antiformal geometry (Offida anticline) and it is limited on its eastern side by the west-dipping Offida thrust fault. Owing to the poor seismic images, its internal architecture is not clearly defined but comprises stacked slices of the formazione della Laga and formazione di Cellino sediments. Farther to the east, the Coastal structure is a buried imbricate thrust system extending along the present Adriatic coastline. Stratigraphic evidence suggests that the Ortezzano-Bellante and the Costiera structures were active between the MPl3 and MPl4a and the MPl4a and MPl6 foraminiferal biozones, respectively. The syn-orogenic turbidite sediments of the Messinian formazione della Laga (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, membro del Lago di Campotosto (LAG4), membro gessarenitico (LAG5), and membro di Teramo (LAG6). The membro del Lago di Campotosto accumulated onto the carbonate substratum of the Adria Plate continental margin, dipping westward under the uplifting Apennines and forming an asymmetric basin. It is bounded at the base and at the top by two regional unconformities, which record the main tectono-sedimentary events affecting the Apenninic foreland basin system during the Late Miocene as a result of the thrust-front propagation, that is, the late-Tortonian fragmentation of the Marnoso-arenacea foreland basin system and the intra-Messinian eastward (forelandward) shift of the foredeep depocenter. Sediments of this unit thin rapidly by onlap towards both the north and the east onto an irregular foreland ramp. They are organized in erosionally-based, fining- and thinning-upward bed packages recording the vertical transition from tabular, thick- to very thick-bedded and laterally continuous lobe-axis sandstones to lobe off-axis interbedded sandstones and mudstones that transition across-strike to lobe fringe mud-rich heterolithic successions. Mineralogical-petrographic composition and sole marks converge to indicate a northern provenance of sedimentary flows. In the late Messinian, as a result of the intra-Messinian tectonic phase, the basin depocenter migrated towards the foreland, being placed east to the rising Laga basin and Montagna dei Fiori structure. The turbidite sediments of the membro gessarenitico and membro di Teramo of the formazione della Laga (upper Messinian to lowest Zanclean) accumulated in this new basin, onlapping to the east onto a slightly deforming ramp. The membro gessarenitico is a sandprone unit made up of a basal package of gypsarenitic and gypsum-bearing sandy turbidites, dated at 5.61 Ma, passing upwards to siliciclastic sandstone turbidites. In the Apenninic foredeep, resedimented clastic evaporites postdate the in situ evaporites and their occurrence is the only physical expression of the intra-Messinian unconformity and the corresponding intra-Messinian regional tectonic event. The membro di Teramo is a monotonous succession consisting essentially of thinly-bedded clayey marls intercalated by decametres-scale arenaceous-pelitic bed packages. In the middle portion of the unit, a basinwide key bed represented by a 2 m-thick package of rhyolitic volcaniclastic layers, dated at 5.5 Ma, is present. In the uppermost outcropping stratigraphic level of the member, a series of arenaceous-pelitic bodies represent the possible deep-water counterpart of the latest Messinian brackish- to restricted-marine Formazione a Colombacci. These sediments rest on a minor, regional-scale unconformity placed at 5.42 Ma. During the Messinian – Zanclean transition, the foredeep depocenter gradually shifted eastward, extending the Early Pliocene sedimentation in the present Adriatic offshore, and was progressively filled by the syn-orogenic turbiditic sediments of the formazione di Cellino (CEN). During the late Zanclean, the leading edge of the Apennines migrated eastward and the previous (latest Miocene to Zanclean) foredeep was fragmented by incipient thrusting. As such, its depositional setting evolved into deep-water piggy-back basins bounded on both sides by the crests of active thrust fronts (i.e. the Ortezzano-Bellante structure to the west and Costiera structure to the east) and filled with syn-tectonic, upper Zanclean to Calabrian sediments of the Argille Azzurre (FAA), a thick, mainly pelitic, clastic wedge composed of shallow- water, shelf and slope deposits. Three members have been distinguished within the Argille Azzurre, one at the base (membro dello Spungone, FAA3) and two in its upper portion (membro del Monte dell’Ascensione, FAA4, and membro di Offida, FAA5). The membro dello Spungone is a shallow-marine, mixed siliciclastic- carbonate sandstone unit showing a progressive deepening-upward trend. At the surface, it rests unconformably onto folded, Upper Miocene sediments of the formazione della Laga and exhibits a high-frequency cyclicity consisting of top-truncated, meter- to decimeter-scale high-frequency sequences. The membro del Monte dell’Ascensione (Gelasian p.p.) and the membro di Offida (upper Gelasian to Calabrian p.p.) are bounded by basin-scale, tectonically enhanced unconformities, and are mostly composed of hemipelagic mudstones. At least three coarse-grained, north-south-oriented submarine canyon-fills, including conglomerate- and sand-rich facies associations, occur encased in the hemipelagic slope mudstones of the Argille Azzurre at different stratigraphic levels; one intercalated in the Piacenzian hemipelagic mudstones (FAA) and two in the Gelasian mudstones of the membro del Monte dell’Ascensione. Palaeocurrents show a longitudinal dispersal pattern from the south and indicate that the large-volume turbidity currents entering these slope canyons from the west were diverted northward by the structural relief generated by the thrust-related Ortezzano - Bellante structure. The UBSU mapped within the post-orogenic continental succession coincide with those distinguished in the neighbouring Sheets of the Marche and Abruzzo regions and are, from the oldest to the youngest: i) sintema di Urbisaglia (URB), comprising alluvial and travertine deposits; ii) supersintema Colle Ulivo – Colonia Montani (AC), comprising alluvial and travertine deposits; iii) sintema di Matelica (MTI), comprising alluvial, travertine, and glacis deposits; iv) sintema di Valle Majalama (AVM), comprising the alluvial deposits of the subsintema di Castelnuovo al Vomano (AVM6b).