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Rend. Online Soc. Geol. It., Vol. 17 (2011) - (DOI 10.3301/ROL.2011.14)

Fluvial response to tectonic deformation: the Ambra Plio-Pleistocene incised-valley deposits (Tuscany, Italy)

V. Bianchi (*), M. Ghinassi (*), M. Aladinucci (**), J. Boaga (*), R. Deiana (*), N. Boscaini (*), G. Moscon (*) & M. Roner (*)


(*) Department of Geosciences, University of Padova, 35137 Padova, Italy.
(**) Weatherford Petroleum Consultants AS, 5147 Bergen, Norway.



Although the relative importance of downstream relative sea/lake-level control versus upstream tectonic and climate forcing in development of valley-fill successions has been emphasized by several authors, only few case studies deal with non-marine valley-fill successions deposited beyond the influence of relative sea-level. Specifically, the erosion of the source area, commonly triggered by the interaction between tectonics and climate, with resulting increase of sediment supply to the valley is the most common process invoked to explain aggradation of valley-fill successions in areas not infl uenced by relative sea-level variations.
On the other hand, the role of epeirogenic movements, whose effects are well known in terms of their control on fluvial style, has been poorly discussed as a possible factor controlling the aggradation of valley-fill successions.
The Plio-Pleistocene deposits of the Ambra River valley, located along the southern margin of the Chianti Mounts (Tuscany, Italy), contain an outstanding example of a fluvial-valley fill not controlled by relative sea-level changes, as attested by the remarkable distance from, and elevation above the coeval paleocoastline. The modern Ambra River drains toward NE, whereas the paleo-Ambra flowed toward SW, as documented by paleocurrent distribution. The valley fill consists of two intervals separated by an erosive surface. The lower interval (40 m thick) is mainly made of gravels and has been the focus of previous studies, which emphasized the role of tectonic and climate in controlling accumulation of the alluvial deposits. The upper interval is 35 m thick and deposited across a sin-sedimentary, NW-SE trending, normal fault.
The present study focuses on the upper interval and is aimed at unraveling the role of a tectonic warping in fluvial sedimentation, with particular emphasis on changes in sedimentological and architectural features of fluvial deposits accumulated across the tectonically disturbed area. Downvalley of the fault zone, the sin-tectonic fluvial deposits are relatively well-exposed and consist of crossto plane-parallel stratified gravels with subordinate sands.
Upvalley of the fault zone, the correlative fluvial succession is poorly exposed and has been mainly analyzed through the integration between ERT (Electrical Resistivity Tomography) investigation and well-core data. Accordingly, it consists of organic-rich mud containing isolated sand bodies interpreted as channel fi lls.
The change in fluvial transport capability recorded by the studied deposits across the fault area is thought to represent the response of the river systems to tectonic movements. Specifically, tectonic upwarping caused a decrease in transport capability in the upstream reaches of the paleovalley, manifested by the aggradation of a poorlydrained floodplain. In contrast, a signifi cant increase in bedload grain-size and fine-sediment bypass is recorded by the gravelly rivers downstream of the upwarping area, where aggradation was promoted by the increase in sediment supply from the erosion of the uplifted area.


KEY WORDS:  non-marine valley-fill,  epeirogenic movements,  Northern Appennines, alluvial sedimentation.

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