I depositi Plio-Pleistocenici di Valle Incisa del torrente Ambra (Toscana, Italia) : interazione tra tettonica e sedimentazione.

Abstract

The Pleistocene fluvial succession of the Ambra River, located along the Southern margin of the Chianti Ridge (Tuscany, Italy). The modern Ambra River drains toward NE, whereas, during the Early Pleistocene, the paleo-Ambra drained toward SW, as documented by a well-developed, 60-70 m thick valley-fill succession. Valley-fill aggradation was promoted by an increase in sediment supplied from the source area, and was not forced by any relative sea-level rise (Aldinucci et al., 2007), as attested by the distance from and elevation on the Pleistocene coastline, along with the presence of several rocky thresholds. The paleovalley trends almost in a NS direction and develops across a NE dipping, sin-sedimentary normal fault. Downstream of the fault zone, the valley fill consists of two main gravel-dominated intervals separated by an unconformity surface. The present study focuses on the uppermost interval of the valley-fill succession. These deposits have been examined through a detailed geological and geomorphological mapping (1:10.000 scale), facies analyses (bed-by-bed logging), paleocurrents measurement, geophysics investigations (ERT: Electric Resistivity Tomography) and structural analyses. Downstream of the fault zone, the upper valley fill consists of well-stratified, gravel-bed river deposits lacking any floodplain mud. Upstream of the fault zone, the upper valley fill is made of floodplain organic-rich mud hosting channelized sandy bodies. The intense CO2 emission affecting the fault area indicates a recent tectonic activity, whereas deformations of the gravels in the lower part of the valley-fill succession, point to a post-depositional fault activity, which reasonably occurred during accumulation of the upper valley-fill deposits. Such hypothesis is consistent with the significant changes affecting the upper valley-fill deposits across the fault zone (i.e. sandy to muddy upstream of the fault and gravelly downstream of the fault). These modifications in fluvial depositional styles fit with those described by Holbrook and Schumm (1999) in the framework of interaction between fluvial sedimentation and longitudinal modifications of riverine profile induced by epeirogenic movements. In particular, tectonic warping caused a decrease in transport capability, with consequent aggradation of fine-grained deposits in the upstream reaches of the paleovalley. On the contrary, downstream of the upwarped area aggradation of gravely facies occurred as consequence of increase transport capability due to steepening of the riverine profile. In order to determine the age of tectonic deformation, a paleomagnetic study has been carried out on the fine-grained deposits. This study allowed to ascribe the study deposits to the Olduvai subchron