The Monte Santa Maria Tiberina ridge consists of a stack of Late Oligocene-Middle Miocene siliciclastic turbidite successions belonging to the Tuscan and Umbria domains. In this work a new geological map of the area (covering 50 km <sup>2</sup> ) is presented, based on a review and an elaboration of the CARG F 289-Citta di Castello survey. Particular attention is given to developing the lithostratigraphy and the biostratigraphic study of the successions. As regards the Tuscan units, the outstanding results are the division of the Macigno Fm into three members, the recognition of the Marne di Vicchio Fm (in which two key-horizons have been described) and the interpretation of the Arenarie di Celle Fm as a thrust-sheet-top basin deposit. The M. S. Maria Tiberina Fm, whose attribution gave rise in the past to conflicting interpretations, has been formalised as a transitional unit which was deposited, in the west, unconformably upon the Marne di Vicchio Fm of the Falterona Nappe and, in the east, conformably, upon the Marnoso Arenacea Fm. Within the MSMT Fm, three hybrid megaturbidites (Key beds) has been logged, mapped and precisely defined from a biostratigraphical point of view. Furthermore the survey has lead to the description of several contractional and extensional structures and to strict constraints on the timing of the tectonic history which affected, from the Late Aquitanian to present, this part of the Umbria Preapennines.

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