The Sant'Arcangelo Basin, defined as a piggyback basin (Caldara et alii, 1988a; 1988b), developed during the late Pliocene on the inner front of the last allochthonous Apennine thrust sheet. Four cycles, or depositional sequences, late Pliocene-middle Pleistocene in age, are recognized on the basis of stratigraphic, sedimentologic and structural characters of deposits outcropping in the northern part of the basin. These cycles are separated by unconformities, syntectonic also in origin. Every cycle, several hundred metres thick, represents one or more depositional systems (alluvial, marine-deltaic, lacustrine); within each system some stratigraphic units are distinguished (see geological map). The Caliandro Cycle, late Pliocene in age, is the oldest one; it lies unconformably on the allochthonous units and includes, from west to east, a lower alluvial system evolving to a marine delta, a thick pelitic marine unit corresponding to maximum deepening of the basin, and a regressive fan delta system. The Agri Cycle overlies, with syntectonic unconformity, the alluvial deposits of the Caliandro Cycle, and develops north-eastwards. The facies analysis allows to refer the depositional system to a marine fan delta. The biostratigraphic analysis assigned these sediments to late Pliocene-early Pleistocene. The third cycle (Sauro Cycle), the most eastern one, lies unconformably on the Caliandro Cycle deposits, and with syntectonic unconformity on the conglomerates of the Agri Cycle. The Sauro Cycle, early-middle Pleistocene in age, may be interpreted as a southeastwards prograding marine fan delta system. At the same time, in an innermost area of the basin, owing to a growing anticline, a lacustrine basin develops. The filling of this lacustrine basin, representing the S. Lorenzo Cycle, is made up of alluvial conglomerates and clays with volcaniclastic beds. A K-Ar age of 1.1+ or -0.3 M.y. obtained for the volcanic glasses (Caggianelli et alii, 1992), allows to refer these deposits to the early Pleistocene. Data from facies analysis suggest that the basin was bounded to the West and to the South by emerged Apennine reliefs, while to the ENE it was partially separated from the foredeep by the submarine Stigliano-Montegiordano structural high. The complete uplift and the emersion of this structure took place in middle Pleistocene. It was during this time that alluvial deposits (Conglomerati e Sabbie di Serra Corneta) deposited unconformably on the younger sediments. During the filling of the Sant'Arcangelo Basin the allochthonous sheets moving east-northeastwards, gave rise to syntectonic unconformities between the Caliandro, Agri and Sauro Cycles. These unconformities are more evident in the western proximal areas. Furthermore synsedimentary tectonics is testified by folds which affected the alluvial and deltaic basin-fill sequences. An example of synsedimentary fold is represented, in the middle part of the basin, by an anticline NS directed. This structure separated Sant'Arcangelo Basin into two parts: an inner part toward west where the deposition of lacustrine S. Lorenzo Cycle took place, and an outer one towards east where marine Sauro Cycle developed. During its evolution the Sant'Arcangelo Basin went through modifications, but it kept its features and in particular the connection with the foredeep to the north-east, till the middle Pleistocene. The overall evolution of this piggyback basin was strongly controlled by synsedimentary tectonics; in fact it led to the migration of the basin depocenters from west to east-northeast. The reconstruction of the tectonic and sedimentary phases carried out in the northern part of the basin, well explains the whole evolution of the Sant'Arcangelo Basin. In fact, the oldest deposits (Caliandro Cycle) and the youngest ones (Sauro Cycle) outcrop in the studied area; current investigations suggest that the Sauro Cycle is not represented in the southern part of the basin.

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