Geological map of Pliocene-Pleistocene deposits of the Ambra and Ombrone valleys (Northern Siena Basin, Tuscany, Italy)

The study area is located across the Chianti Ridge (Tuscany, Italy), between the Upper Valdarno Basin and the Siena Basin. This area covers about 25 km(2), forming a narrow belt oriented N-S and drained by the Ambra and Ombrone creeks, which flow northward and southward, respectively. Field mapping was carried out at 1:10,000 scale through an allostratigraphic-sedimentological approach. The study area deposits represent the infill of a SW-draining paleovalley, cut both in pre-Neogene bedrock and marine Pliocene deposits of the Siena Basin. The valley-fill succession consists of two main allounits (V1 and V2), which are conformably stacked in the northern sector of the study area, and unconformably offset in the southern sector. V1 comprises gravelly to sandy fluvial deposits, whereas V2 deposits show noticeable downvalley variability. V2 consists of poorly drained floodplain deposits in the northern sector of the paleovalley, whereas gravel and sand-bed river deposits fill its southern part. Alluvial-fan and palustrine deposits are also associated with V2 fluvial facies. A normal fault trending NW-SE is the main structural feature of the area. This fault cuts the V2 unit lowering the upstream reach and is thought to have promoted the marked facies changes observed in the fluvial deposits of unit V2.