Abstract
The Lower Val d’Orcia Basin (40 km southeast of Siena) is filled with Upper Miocene continental deposits overlain by Lower to Middle Pliocene shallow-marine and transitional deposits with minor alluvial clastics. The Montenero Gilbert delta (Early Pliocene) is located on the southern margin of the basin, and evolved through several main depositional stages. The first progradational stage was controlled by the development of an elongated fault-generated depression, acting as a log-line and deflecting sediments laterally with respect to the main track. Such a progradational track switched laterally of about 35 degrees in consequence of fault activity, characterised by a localised transpression linked to strike-slip movements. The location of the delta on the Grosseto-Pienza Line, an Apenninic transversal alignment where strike-slip movements have been presumed, could be significant, even if the limited fault extension prevents the definition of its regional significance.
During the second stage, the Montenero delta was characterised by an abrupt increase in sediment discharge, pointing to a forced regression. Such a forced regression has been interpreted as due to the uplift of the Seggiano-Montalcino Ridge related to emplacement of the M. Amiata magmatic complex.
Keywords: Gilbert delta, Lateral switching, Tectonic control, Forced regression, Northern Apennines.