Fluvial Aggradation vs. Tectonic Deformation: the Ambra Plio-Pleistocene Incised-valley Deposits (Tuscany, Italy)

Valeria Bianchi, Massimiliano Ghinassi, and Mauro Aldinucci

Few authors highlight the importance of tectonic and climatic upstream control in development of valley-fill aggradation, rather than downstream relative sea/lake-level changes. Furthermore, the presence of sinsedimentary tectonic warping can cause a modification of fluvial patterns, with consequent development of heterogeneous valley-fill architectures. The effects of these modifications are well-known in a modern setting, but no fossil cases have been documented in detail.

An outstanding example of a fluvial valley-fill uninfluenced by relative sea-level changes is represented by Plio- Pleistocene deposits of the Ambra River valley, located across the Chianti Ridge (Central Tuscany, Italy), represent. The paleo-valley was separated from the coeval paleo-coastline by a large distance and several rocky shoulders. Study deposits were mapped at 1:10.000 scale over an area of about 40 km², and represent the fluvial infill of a SSE flowing paleovalley, which crosses the modern watershed between the Siena and the Valdarno Basin.

The valley fill succession consists of two main intervals separated by an erosive surface. The lower one (40 m thick) is gravel-dominated and has been the focus of previous studies, which emphasized the role of climate in the accumulation phase of the alluvial deposits. The upper interval is 25 m thick and it is deposited across a sinsedimentary normal fault NW-SE trending (i.e. almost transverse to the valley axis).The present study focuses on the upper interval and is aimed to unravel the role of the tectonic disturb in fluvial sedimentation, with particular weight 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 cross- to 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 of ERT (Electrical Resistivity Tomography) investigation and well-core data. Accordingly, it consists of organic-rich mud containing isolated sand bodies interpreted as channel fills.

The analysis of studied deposits across the fault area provides a high-resolution response of the river systems to tectonic movements recorded by the change in fluvial transport capability. Specifically, the decrease in transport capability is caused by tectonic upwarping in the upstream reaches of the paleovalley, manifested by the aggradation of a poorly-drained floodplain. In contrast, a significant increase in bedload grain-size and fine-sediment bypass is recorded by the gravelly rivers downstream of the uplifted area, where aggradation was promoted by the increase in sediment supply from the erosion of the uplifted area.

Because of the sharp porosity change which characterizes the upper interval deposits and because of the confined architectures of valley-fill deposits, the present case study may represent an helpful model for understanding how these kind of reservoirs form and for predicting their architectures.

AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013