Abstract

In this paper the geological survey, based on detailed field work of a significant area of the Sannio-Daunia region which is included between the high valleys of the Fortore and Miscano rivers, allowed us to re-examine such stratigraphic characters as the tectonic arrangements of a mesocenozoic basinal succession known as the Fortore Unit (Dazzaro et alii, 1988), and its relationship to adjacent and coeval basinal units known as the Sannio Unit and the Calcareniti Marne and Argille di Monte Sidone formation of the Daunia Unit. In our opinion the fold-and-thrust belt of the Sannio Mountains belong to the Molise-Sannio Nappe and represent a connecting structural level Between the axial chain (Matese Mountains) and the edge of the chain (Daunia Mountains) with an overall foreland vergence (N and NE). Less penetrative back-thrusts and overturned folds or box-folds are widespread. In this frontal zone of the southern Apennines, the fold-thrust complex is displaced by a basal detachment (about 2000 m deep), probably of Tortonian age, cutting through a group of similar units, including a thin sequence of polychrome (varicolori) clays or argillites, calcirudites and bio-calcarenites, marly limestones, cherty calcilutites, radiolarian cherts and sandstone. In the studied area the aforementioned lithologies form two successions. These are characterised both by a different relative abundance and by the stratigraphic position of pelitic and calciclastic lithofacies: - succession A is predominantly pelitic; - succession B where a greater quantity of marly limestones are present; The successions show both lateral connection (heteropy) and vertical connection. As a rule succession B lies partly over succession A, since both successions are characterized by the same tectono-sedimentary evolution in the early Miocene age; in this period the concordant sedimentation was, at first, arenitic and volcanoclastic (Tufiti di Tusa formation), then exclusively quartz-arenitic (Flysch numidico formation). Moreover, extensive outcrops form important terrigenous and turbiditic deposits; about 1000 m of thinning and fining upward sequence with an important stratigraphic basal unconformity boundary. In our opinion, this silicoclastic formation, called the San Bartolomeo Flysch, corresponds to a wedge top-basin type, which links a palaeo-thrust belt sector to the Tortonian infra-Messinian foreland. In fact, the reconstruction of the field data allows us to propose that the foredeep fill is governed by the pre-existing fold and thrust belt. There is no displacement within the substratum. This depocentre was subsequently fossilized by thin-skinned thrusting which occurred in the early Messinian. The structural interpretation of the geological data is consistent with the stratigraphic results. The imbrications of the Fortore Unit and of the coeval units belong to the same upper level crust. In this level, recurring fault related folding complex structures are present. These structures display both vertical and lateral changes in style. Firstly, upward displacement is very slight, compared to the displacement found in the deepest parts of the stratigraphic sequence. In the upper part of the structures, deformation is expressed by the development of asymmetrical fold pairs (recumbent folds). Secondly, significant along-strike variations in thrust displacement which determine the non-cylindrical shape of the related folds are highlighted; also, tear faults are localized, suggesting a transfer zone in the frontal zone of the chain. This transfer zone has governed the shortening of the sedimentary cover.

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