Stratigraphic constraints on the CROP-04 seismic line interpretation: San Fele 1, Monte Foi 1 and San Gregorio Magno 1 wells (Southern Apennines, Italy)

This paper illustrates the results of stratigraphic investigations on commercial wells establishing important constraints on the interpretation of the structural architecture of the Southern Apennines. The San Fele 1 and Monte Foi 1 wells, located not far from the outer (north-eastern) margin of the mountain chain, penetrated a thick antiformal stack of Lagonegro imbricates that overlies buried Apulia carbonates seated at depths exceeding 6000 meters below sea level and not reached by the boreholes. These wells document a considerable telescopic shortening and consequent thickening of the Apennine nappes, namely of the Lagonegro Units. The thickness of the drilled antiformal stack is at least four times greater than the thickness of the involved stratigraphic sequence that does not exceed 1500-1600 metres from the Monte Facito Formation to the Galestri Formation. The San Gregorio Magno 1 well is located in correspondence to an axial culmination of the buried Apulia-carbonate duplex system where the total thickness of the Apennine nappes averages 2500 metres. Before reaching the buried duplex system, the well crossed shallow-water carbonates referable to the Apenninic Platform and deep-water deposits referable to the Lagonegro Basin. A system of SW-dipping normal faults, well expressed on the surface, is responsible for the lack of discrete intervals of the Apenninic-Platform sequence (absence in the well of Liassic shallow-water limestones that crop out not far from the wellsite in the footwall of the fault) and of the Lagonegro basinal sequence (absence in the well of the upper Triassic cherty limestones systematically present, in complete sections, between the Scisti Silicei Formation and the Galestri Formation). The shallow-water Apulia carbonates reached by San Gregorio Magno 1 are stratigraphically overlain by siliciclastic deposits displaying close similarities with the lower Pliocene siliciclastic deposits covering the Apulia carbonates in the High Agri Valley. These deposits, interpreted as a wildflysch stuffed with olistostromes derived from the Apennine nappes, are characterized by medium to coarse-grained sandstones and subordinate granule conglomerates containing well-rounded shallow-water-derived carbonate lithoclasts (mostly Jurassic-lower Cretaceous shallow-water ooidal grainstones and Paleogene shelf-derived bioclastic packstones with Alveolina and Nummulites) together with well-rounded quartz grains associated with K-feldspars. Additional constraints on the interpretation of the internal structural architecture of the Apennine chain derive from the well Acerno 1, located about 30 kilometres W of San Gregorio Magno 1. The stratigraphic data of this borehole have provided new tools for the final interpretation of the controversial Campagna tectonic window where an important breach has cut across and displaced the primary low-angle thrust surface separating the Apenninic-Platform carbonates from the underlying Lagonegro-Basin deposits. Another remarkable piece of information coming from Acerno 1 is represented by the considerable thickness of the Lagonegro deposits that are organized into several tectonic slices the total thickness of which exceeds 3000 metres. The existence of a thick pile of Lagonegro deposits in quite internal areas of the mountain chain, not far from the Tyrrhenian coast, supports the attribution to the Lagonegro Units of a characteristic package of reflectors recognized at the base of the Apenninic Platform in the south-western portion of the CROP-04 seismic line