Fault-sourced alluvial fans and their interaction with axial fluvial drainage: An example from the Plio-Pleistocene Upper Valdarno Basin (Tuscany, Italy) Francesco Fidolini a, ⁎, Massimiliano Ghinassi b , Mauro Aldinucci c , Paolo Billi d , Jacopo Boaga b , Rita Deiana b , Lara Brivio b a Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, 50121 Firenze, Italy b Dipartimento di Geoscienze, Università di Padova, Via Gradenigo 6, 35131 Padova, Italy c Weatherford Petroleum Consultants AS, 5147 Bergen, Norway d Dipartimento di Scienze della Terra, Università di Ferrara, Via Saragat 1, 44100 Ferrara, Italy abstract article info Article history: Received 9 July 2012 Received in revised form 5 February 2013 Accepted 8 February 2013 Available online 19 February 2013 Editor: B. Jones Keywords: Alluvial fan Axial fluvial drainage Base level Palaeohydrology Northern Apennines The present study deals with the fault-sourced, alluvial-fan deposits of the Plio-Pleistocene Upper Valdarno Basin (Northern Apennines, Italy). Different phases of alluvial fan aggradation, progradation and backstep are discussed as possible effects of the interaction among fault-generated accommodation space, sediment supply and discharge variations affecting the axial fluvial drainage. The Upper Valdarno Basin, located about 35 km SE of Florence, is filled with 550 m palustrine, lacustrine and alluvial deposits forming four main unconformity-bounded units (i.e. synthems). The study alluvial-fan deposits belong to the two uppermost synthems (Montevarchi and Torrente Ciuffenna synthems) and are Early to Middle Pleistocene in age. These deposits are sourced from the fault- bounded, NE margin of the basin and interfinger with axial fluvial deposits. Alluvial fan deposits of the Montevarchi Synthem consist of three main intervals: i) a lower interval, which lacks any evidence of a depositional trend and testify balance between the subsidence rate (i.e. fault activity) and the amount of sediment provided from the mar- gin; ii) a coarsening-upward middle interval, pointing to a decrease in subsidence rate associated with an augment in sediment supply; iii) a fining-upward, upper interval (locally preserved), documenting a phase of tectonic quiescence associated with a progressive re-equilibration of the tectonically-induced morphological profile. The basin-scale unconformity, which separates the Montevarchi and Torrente Ciuffenna synthems was due to the en- trance of the Arno River into the basin as consequence of a piracy. This event caused a dramatic increase in water discharge of the axial fluvial system, and its consequent embanking. Such an erosional surface started to develop in the axial areas, and propagated along the main tributaries, triggering erosion of the alluvial fan deposits. Alluvial-fan deposits of the Torrente Ciuffenna Synthem accumulated above the unconformity during a phase of tectonic quiescence, and show a fining-upward depositional trend. This trend was generated by a progressive decrease in sediment supply stemming out from upstream migration of the knickpoints developed during the embanking of the axial system. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Alluvial-fan sedimentary successions have been reported from dif- ferent geological settings (Heward, 1978a,b; Gloppen and Steel, 1981; Blair, 1987; DeCelles et al., 1991; Nemec and Postma, 1993; Ravnas and Steel, 1998; Mack and Leeder, 1999; Benvenuti, 2003) in which sediment supply, accommodation space and their changes are driven by the interplay among eustasy, climate and tectonics (Catuneanu, 2002). When deposition is not affected by shoreline variations (Shanley and McCabe, 1994), the development of alluvial-fan succes- sions and the variability of their internal architecture are controlled by tectonics of the basin margin relief, climate influence on inland denudation and flow discharge (Blum and Price, 1998; Gibling et al., 2005). Fault-sourced alluvial fans caught the attention of geomorpholo- gists and sedimentologists for the wide variability of the sedimentary succession characteristics, the involved sedimentary processes and the fan morphodynamic evolution and forcings. The geomorphologi- cal approach is based on the study of modern systems and deals with alluvial fan geometry and its relationship with the geomorphic processes acting in the headwater (Bull, 1962, 1964; Viseras et al., 2003). The sedimentological and stratigraphic approach focuses on fossil successions and gives information on temporal variability of the systems in terms of changes in accommodation space and sedi- ment supply (Steel et al., 1977; Heward, 1978a,b; Paola et al., 1992). Nevertheless, important issues concerning sedimentary successions associated with fault-sourced alluvial fans are still a matter of discus- sion. In fact, the development of coarsening-upward successions in Sedimentary Geology 289 (2013) 19–39 ⁎ Corresponding author. E-mail address: francesco.fidolini@unifi.it (F. Fidolini). 0037-0738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sedgeo.2013.02.004 Contents lists available at SciVerse ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo