The structural geology and basin development of the Romagnan-Umbrian zone (Upper Savio- and Upper Bidente Valleys, N. Italy)

Abstract

Combined stratigraphic and structural geologic studies in the region of the Upper Savio- and Upper Bidente Valleys lead to a re-interpretation of the Romagnan-Umbrian thrust zone, especially of the extent of the Castellaccio thrust unit. The Romagnan-Umbrian thrust zone may be characterized as a generally piggy back propagated, imbricate stack with a complex evolution. The presence of a time equivalent turbidite marker bed (= 'Contessa Bed') within all thrust units enabled the balancing and restoration of the Romagnan-Umbrian thrustzone. Theminimum amount of shortening of the 'Contessa Bed' appears to be about 48% of its original length. Folding accounted for about 49% and thrusting for about 51% of the total amount of shortening. The restoration indicates that, during the 'Contessa' event (dated as the Early Serravallian Globorotolia peripheroronda Zone: I4-14,5 Ma), the Romagnan-Umbrian turbidite basin had a minimum NE-SW width of 50km. After the 'Contessa' event, still during the Early Serravallian, an anticlinal culmination ( = the San Paolo high) formed within the Romagnan-Umbrian turbidite basin, probably generated by blind thrusting during the Serravallian and Early Tortonian. On top of the anticlinal culmination a condensed section of clayey San Paolo Marls accumulated until the Middle Tortonian (9-10 Ma). The more calcareous Veghereto Marls have been deposited on the SW dipping limb of the San Paolo anticlinal culmination. During the Middle Tortonian (9-10Ma) sedimentation in the SW part of the Romagnan-Umbrian turbidite basin was abruptly terminated by the Ligurian overthrust. Simultaneously or shortly thereafter, the Romagnan-Umbrian sequences were also intensively folded and thrusted. The Romagnan-Umbrian thrust zone developed during this main deformation phase of the area. After thrusting, a period of gentle folding, oblique-slip faulting and general uplift completed the structural geologic and sedimentary history of the studied region.