Facies and sequence stratigraphic modeling of a Upper Pliocene–Lower Pleistocene fluvial succession (Valdelsa Basin, central Italy)

Sedimentary Geology 294 (2013) 303–314

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Facies and sequence stratigraphic modeling of a Upper Pliocene–Lower Pleistocene fluvial succession (Valdelsa Basin, central Italy) Marco Benvenuti a,⁎, Sara Del Conte b a b

Dipartimento di Scienze della Terra, Università di Firenze, Via G. La Pira 4, 50121 Firenze, Italy TRE Europa, Ripa di Porta Ticinese, 79, 20143 Milano, Italy

a r t i c l e

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Article history: Received 18 March 2013 Received in revised form 17 June 2013 Accepted 21 June 2013 Available online 2 July 2013 Editor: J. Knight Keywords: Fluvial sedimentology Facies analysis Fluvial sequence stratigraphy Piacenzian Valdelsa Basin

a b s t r a c t This paper illustrates the results of sedimentologic and stratigraphic analyses of the upper Piacenzian– Gelasian fluvial succession exposed in the Neogene–Quaternary Valdelsa Basin (central Italy). The succession shows a cyclothemic stacking of gravelly, sandy and muddy lithofacies organized into four monogenic facies associations (A–D). These record depositional environments ranging from braided to low-sinuosity river channels to flood basins. Associations A–D attest to lowstand (A–B), transgressive and high-stand (C–D) depositions in a full cycle of base-level variations. In each association, internal erosional surfaces separate early transgressive association C from the late lowstand association B. The systematic B/C channel scouring is interpreted as the result of a high water/sediment discharge ratio determined by a decrease of coarse-grained sediment supply to the fluvial systems during rise of base level. This erosive surface is conceptually analogous to the ravinement surface sculpted by wave erosion during the transgressive, landward migration of a shoreface. The late transgressive and highstand mud-dominated association D records the flood basin, a depositional environment indicative of a high base level which transformed a former channel belt in a plain dominated by fine-grained sediment settling, bio- and pedoturbation. The studied succession records rhythmic variations of base level and sediment supply to the fluvial systems, in turn regulated by different-rank relative fluctuations of Piacenzian sea level. In this perspective, concepts of sequence stratigraphy and facies analysis are exploited for producing a reliable fluvial sequence stratigraphic model. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Since the early popularization of the Exxon sequence stratigraphy (Wilgus et al., 1988), sedimentary geologists opened a lively discussion on the concept of depositional sequences (see reviews in Emery and Myers, 1996; Catuneanu, 2006; Miall, 2010). The significance and origin of erosional unconformities bounding Exxon-type sequences and the role of alluvial systems in sequence aggradation extended the discussion to the fluvial geomorphological and sedimentological communities (Blum, 1990; Miall, 1991; Blum, 1993; Schumm, 1993; Wescott, 1993; Miall, 1996; Blum and Törnqvist, 2000; Bridge, 2006; Gibling, 2006). The complexity of fluvial dynamics, though not being the main focus of the early Exxon models, appeared to have been largely overlooked for its role in building up the stratigraphic record (Wright and Marriott, 1993; Shanley and McCabe, 1994; Blum and Törnqvist, 2000; Rhee, 2006). In its spatial and temporal development the fluvial system is affected by the interplay of physiography, tectonism, climate and eustasy (Shanley and McCabe, 1994) which results in a complex behavior of rivers in terms of erosion and sedimentation (Schumm, 1977, 2005). This complexity, therefore, makes reference to ⁎ Corresponding author. E-mail address: ma.benvenuti@unifi.it (M. Benvenuti). 0037-0738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sedgeo.2013.06.006

classic “sequential” concepts, such as accommodation, base-level and bounding unconformity, of uncertain value in the stratigraphic interpretation of the fluvial record (Schumm, 1993). This is why the sequence stratigraphy approach to fluvial successions should be based on as many case studies as possible before it may become a well-established tool for stratigraphic analysis in non-marine settings (Blum and Törnqvist, 2000; Postma and Holbrook, 2003; Rhee, 2006). This paper describes the facies analysis of an Upper Pliocene (Piacenzian)–Lower Pleistocene (Gelasian) fluvial succession characterized by a well-developed and laterally continuous rhythmic stacking of conglomerates, sandstones and mudstones. The aim of the study, therefore, is to discuss the cyclic control on the sedimentary facies and key stratigraphic surfaces in producing a reliable fluvial sequence stratigraphic model. 2. General setting and sequence stratigraphy The Valdelsa Basin (central Tuscany, Fig. 1A) is an intermontane depression stretching in a NW–SE direction for about 60 km, confined by two main ridges. To the southwest the Mid Tuscan Ridge consists of limestones, mudstones, sandstones and ophiolites (Ligurid units) thrusted onto Paleozoic metasediments, in turn covered by Triassic evaporites and limestones (Tuscan units). On the opposite Albano–

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Fig. 1. A) Geological map of the Valdelsa Basin (after Benvenuti et al., in press) with location of the study area and some stratigraphic sections discussed in the text; B) stratigraphic scheme of the uppermost Miocene–lower Pleistocene succession filling the Valdelsa Basin: the Montespertoli–Tavarnelle buried high subdivided the basin in two adjacent depocenters during the deposition of S1–S3 synthems resulting inactive during the deposition of S4–S7 synthems (see discussion in Benvenuti et al., in press). Due to the occurrence of internal, low-rank unconformities, the S4 and S5 synthems have been further subdivided in sub-synthems; C) detailed geological map of the study area, the boxed area includes the abandoned quarry where the ten logs (see Figs. 3, 4) have been measured.

Chianti mounts, the Ligurid units are thrusted onto upper Oligocene– lower Miocene turbiditic sandstone (Tuscan units). An Upper Miocene– Lower Pleistocene clastic succession up to 2000 m thick (Ghelardoni et al., 1968) has been subdivided into seven unconformity-bounded stratigraphic units (synthems in the sense of ISSC, 1994; Fig. 1B) developed since the latest Messinian (S1 synthem), throughout the Zanclean (S2–S3 synthems), the Piacenzian (S4–S5 synthems) and the Gelasian (lower Pleistocene) (S6–S7 synthems) (Benvenuti and Degli Innocenti, 2001; Benvenuti et al., in press). These synthems reflect major relative sea-level fluctuations representing in this perspective large-scale depositional sequences (Benvenuti et al., in press; Fig. 2) in which lowstand systems tracts (LST) are represented by fluvio-deltaic conglomerates and sandstones, whereas transgressive systems tracts (TST) are typically indicated by prodelta-inner shelf mudstone occurring in the middle-upper part of the synthems. The highstand systems tracts (HTS) are eventually represented by deltaic and/or alluvial sandstone and conglomerate at the top of the synthems. The S5–S6 synthems exposed in the central portion of the basin (Fig. 1B) have been recently discussed for their composite sequential architecture which responded to glacio-eustatic forcing (Benvenuti et al., 2007). Four composite depositional sequences (CDS1–4) stacked within the S5 synthem (Benvenuti et al., 2007; Fig. 2) are in turn composed of elementary depositional sequences (EDS, in the sense of Mutti et al., 1994). From the base, each EDS is characterized by fluvio-deltaic sandstone passing upward into shelfal mudstones bearing shell beds (Benvenuti et al., 2007). CDS4, recording the maximum deepening of the late Piacenzian shallow sea, includes EDSa–d (Fig. 2) which mark four successive steps in the late development of the S5 synthem.

3. Study area and methods The study area is located in the central portion of the Valdelsa Basin at about 10 km SW from the Albano–Chianti mounts which represented the main source for the Pliocene fluvial systems (Benvenuti and Degli Innocenti, 2001; Balestrieri et al., 2013; Benvenuti et al., in press). Inactive quarries on the hilly slopes on the right of the Orme Creek, a left tributary of the Arno River (Figs. 1C, 3A) provide relatively good exposure of alternated yellow-brownish conglomerate, sandstone and grayish mudstone (Fig. 3B). NW–SE trending normal faults interrupt the lateral continuity of these deposits (Fig. 1C). Despite the fault disturbance the unconformable contact between the S5 and S6 synthems, traced out through detailed mapping over a wider area, is well visible in the quarry faces (Fig. 3B). Ten vertical logs (Fig. 4) have been measured and correlated following standardized methods of facies analysis (Dalrymple, 2010 for a review). Different facies have been grouped into two types of associations: monogenic and hybrid facies associations, respectively. Taking as a reference Mutti et al. (1994) (Fig. 5), monogenic facies associations are identified based on the repetition of single or different facies which express the autocyclic behavior of specific depositional systems within a given available accommodation space. Hybrid facies associations are characterized by the stacking of monogenic facies associations which outline changes in accommodation space, hence the role of allocyclic forcing on depositional dynamics (Mutti et al., 1994). S5I–V and S6I–V hybrid associations have been mapped in the S5 and S6 synthems, the latter erosively overlain by the S7 synthem conglomerate and subordinated sandy mudstone, which are not considered in this study (Fig. 1C). Specifically to the S5 synthem, the S5IV association is characterized by the occurrence of

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Fig. 2. The Valdelsa synthems architecture expressed in terms of large-scale depositional sequences further subdivided into low-rank sequences for S5 synthem. The question marks refer to an uncertain chronostratigraphic position of S2 synthem (see Benvenuti et al., in press, for details). Legend for codes: EDS: elementary depositional sequence CDS: composite depositional sequence; FRST: forced regression systems tract; LST: lowstand systems tract; TST: transgressive systems tract; CS: condensed section; MFS: maximum flooding surface; HST: highstand systems tract.

brackish gastropods and bivalves in the mudstones which overlie the conglomerates. This fossil record is the only evidence of some marine influence in a portion of the basin dominated by terrestrial settings since the Piacenzian (Del Conte, 2007). The erosive bounding surface of the S6 deepens to the NW justifying the lack of associations S5IV–V in the studied sections.

to well rounded, characterized by an overall clast-supported texture in places marked by zones with abundant sandy matrix. These deposits are crudely bedded (Fig. 6A). Decimeter thick horizontal bedding is outlined by the alignments of larger clasts among which crude normal grading is observed. Few beds show sharp grading from a framework of pebble–cobble passing upward into massive coarse sand. Clasts are imbricated pointing to palaeoflows running to the SW.

4. Facies description and interpretation The following is a synthetic description and process interpretation of gravelly (G), gravelly–sandy (Gs), sandy (S) and muddy (M) facies (Figs. 4, 5). 4.1. Gravelly facies G This consists of calcareous–arenaceous pebble to cobble-sized conglomerate and subordinate coarse sandstone. Clasts are sub-rounded

4.1.1. Process interpretation The overall tabular geometry of single beds and bedsets hints to gravel deposition from poorly confined flows. The thickness of the single beds and the textural features suggest that parent flows were shallow, quite flashy in their character and dominated by bedload. Crude grading and abrupt grain-size partitions indicate modes of transport and deposition described in both subaerial (Todd, 1989; Benvenuti and Martini, 2002; Benvenuti, 2003) and subaqueous settings (Sohn, 1997), in which sediment-laden tractional flows undergo

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Fig. 3. A) Aerial view of the abandoned quarry and location of the ten logs; B) panoramic view of the E–W trending quarry face, persons for scale. S5III–S6I: hybrid facies associations; other codes refer to the facies described in the text.

lateral expansion due to rapid loss of flow confinement. This determines an abrupt collapse of the suspended load that rapidly increases the concentration of the bedload giving rise to bipartite flows. The lower flow region is a hyperconcentrated, non-Newtonian flow similar to debris flows, whereas the upper region may still behave as a turbulent flow (Todd, 1989; Sohn et al., 1999; Benvenuti and Martini, 2002). The sediment transport was directed to the SW towards the contemporary coastline.

4.2. Gravelly-sandy facies Gs This consists of pebbly, subordinately cobbly, conglomerate and medium-coarse grained sandstone arranged in concave-plane lens-shaped beds from decimeters to meters thick and up to several meters wide (Fig. 6A, C). Internally, these beds may be characterized

by massive texture or crude normal grading, mm–dm thick planar and concave inclined lamination. The latter is characterized by a variable degree of sediment sorting and fabric. Thick gravelly layers may show textures ranging from clast-supported massive to openwork to clast dispersed in abundant sandy matrix (bimodal) fabric (Fig. 6B, F). The laminasets are normally oriented transverse to the bed geometry and dipping in a NW–SE direction (Fig. 6D–E).

4.2.1. Process interpretation The geometry of the gravelly-sandy beds hints to sediment transport by channelized tractional flows. Sediment texture and structure suggest that parent flows were variably charged with sediment. Lens-shaped beds characterized by massive texture or crude normal grading are derived from concentrated flows in which massive settling of the sediment load prevented the development of bedforms. The latter developed in less

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Fig. 4. The logged quarry faces as schematically indicated in Fig. 3A. Codes for facies as in the text; sb: sequence boundary for hybrid facies associations; frs: fluvial ravinement surface (see text).

concentrated flow as indicated by inclined laminasets recording the migration of dune to bar-scale bedforms. The rhythmic variations of textures observed in the cross bedding have been described in natural and experimental gravelly bedforms (Smith, 1972; Iseya and Ikeda, 1987; Anketell and Rust, 1990; Carling, 1990; Bridge and Lunt, 2006). The lateral grading of these laminae, expressed by alternation of clast-supported, matrix-rich, openwork textures, has been related to a dynamic sorting of bedload during single depositional events. In some cases (see below) occurrence of massive texture within thick laminae may reflect particularly high-magnitude events. The orientation of the laminasets orthogonal to the geometry of the encasing beds attests to a dominant lateral or oblique bedform migration in the channels. The direction of the cross lamination trending to NW–SE is consistent with channels oriented from NE to SW.

4.3. Sandy facies S Yellowish pebbly to silty sandstone arranged in dm–m thick lenses are characterized by a concave base and planar to convex top (Fig. 6D–E). These lens-shaped beds may be several meters wide. Internal structure ranges from massive-normal graded at the base of

the beds, where fine pebbles often occur, to laminated at the top. Lamination shows an inclined planar, less frequently concave, geometry. Similarly to facies Gs, inclined laminasets are often transverse to the bed geometry. The overall fining of the beds is outlined in the topmost part by massive silty sandstone locally rich in oxidized vegetal remains. 4.3.1. Process interpretation The bed geometry, sediment textures and sedimentary structures point to deposition from channelized turbulent flows carrying a sandy bedload. The latter fed the development of dune- to bar-scale bedforms characterized by both frontal and lateral accretion in respect to palaeoflows directed to the SW. Convex-up bed terminations point to sediment lobes. Lobate beds in a lateral position in respect to the channelized beds are interpreted as crevasse splays. Convex tops of the channelized beds (Fig. 6D–E) suggest channel overfilling. 4.4. Muddy facies M Yellowish sandy silt (Ma) and grayish clayey silt (Mb) are arranged in dm-scale tabular beds (Fig. 7A). These beds, generally massive and

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Fig. 5. Conceptual models for monogenic and hybrid facies associations: 1–4: single facies; A–D: monogenic facies association. After Mutti et al. (1994).

occasionally thinly laminated, may contain dispersed, carbonized and oxidized vegetal matter, root traces, scattered shells of land snails and rare vertebrate bones, calcareous nodules and diffuse mottling. Large vegetal remains such as carbonized tree stumps in life position (Mc), may locally occur in these deposits (Fig. 7B). 4.4.1. Process interpretation The sedimentary features of this facies suggest primary depositional processes related to the settlement of fine-grained sediments carried in suspension by waterflows and deposited in a setting characterized by contrasting redox conditions. Diffuse bioturbation by terrestrial organisms and the evidence of soil formation attest to a subaerial setting. Ma and Mb deposits indicate conditions of contrasting drainage with Ma pointing to well-drained condition whereas Mb is indicative of waterlogging. Vegetal remains in life position in Mc attest to soil development, a process also expressed by diffuse mottling and horizons rich in calcareous nodules.

4.5. Monogenic facies associations: depositional environments 4.5.1. Association A This association is made up of repetition of facies Gs arranged in bedsets up to 5 m thick (Fig. 4) which rest erosively over facies association D. This erosive contact points to the incision of relatively wide alluvial valleys followed by deposition of gravels and sands in multi-story channels. The classic channel-bar sedimentological models (Miall, 1996, for a review) provide a suitable reference for the paleoenvironmental interpretation of this association. Cross lamination is indicative of the development of low-flow regime bedforms which in the majority migrated transversally or laterally to palaeoflow direction. This means that channels were characterized by a low sinuosity pattern (cf., Billi et al., 1987). 4.5.2. Association B This facies association is made exclusively of facies G, arranged into bedsets up to 8 m thick, which conformably rests on facies

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Fig. 6. A) Facies G–Gs exposed on the N–S trending quarry faces, within S5III hybrid facies association: the apparent massiveness of facies G reveals at a closer view amalgamated plane and crudely graded decimeter-thick beds, hammer for scale; frs: fluvial ravinement surface; B) close view of the rhythmic layering which characterize low-angle inclined gravelly beds of facies Gs (see D–E), hammer for scale: cs: clast-supported texture; bim: bimodal texture made of pebbles floating in a sandy matrix; C) erosive contact between facies G and Gs within S6II hybrid facies association (see also Fig. 4), the rod is 1 meter long; D) the mid-upper portion of the S6II hybrid facies association showing the stacking of facies Gs, S and Ma–b: the low-angle bedding of facies Gs is related to a bar migrating laterally within a ENE–WSW trending channel incised on facies G (not visible), person for scale; E) line drawing of the outcrop in D; F) the unconformable transition between S5II and S5III hybrid facies associations; codes for textures of Gs as in Fig. 5B, op: openwork texture (photograph by Stefano Dominici). Trowel on the right corner for scale.

association A and in turn is erosively overlain by facies association C (Fig. 4). Association B testifies to the deposition from relatively shallow and poorly confined flows within alluvial systems sharing depositional features with alluvial fans and gravel-bed rivers. Recently, the geomorphologic and sedimentologic differences between alluvial fans and rivers (see discussion in Blair and McPherson, 1994), have been further reduced by the concept of distributive fluvial system (DFS, Hartley et al., 2010). This term refers to rivers entering an actively subsiding plain, which lose their confinement and create the fan-shaped distributive planforms. Association B, therefore, may record the development of DFS. Nevertheless, the relevance of unconfined flows is a feature shared also by braided rivers. Despite the classic reference to channel-bar complexes as diagnostic of ancient braided rivers (Eynon and Walker, 1974; Smith, 1974; Miall, 1978; Bridge and Lunt, 2006), the highly variable water discharge and sediment supply made the braidplains prone to rapid channel avulsion and overbank deposition (Carson, 1984; Ashworth et al., 2007). Association B, therefore, may record a highly-supplied braided river. In the Southern Apennines of Italy, similar rivers are presently represented by the so-called fiumare, seasonal, high-gradient gravelbed streams characterized by morphologic and hydrologic features intermediate between rivers and alluvial fans (Biamonte, 1993; Sorriso-Valvo and Terranova, 2006). Though low-discharge planforms of the fiumare are normally characterized by a pattern of shallow braiding channels (Biamonte, 1993; Billi and Biamonte, 1995; Sabato and Tropeano, 2004), the related gravelly-sandy deposits are dominated by plane bedding, hinting to flashy flows expanding over the fiumara valley bottom. Summing up, association B shares features with a range of alluvial systems in which large amount of sediment is supplied by poorly-confined flows supply to a coastal–alluvial plain. A further discussion in the next section (hybrid facies associations) will bring arguments to discriminate the more appropriate depositional system.

4.5.3. Association C This facies association is composed of facies Gs and S forming bedsets up to 5 m thick. This association rests on association B by a high-relief erosive surface, in turn passing upwards into association D. Association C records the multi-story filling of single stream channels incised into association B. As for facies association A, cross bedding is indicative of the development of low-flow regime bedforms which migrated transversally or laterally to the palaeoflow direction within sinuous channels. In this association channels range from few meters to some tens of meters in width and up to 3 m deep. Wider channels are filled by large-scale laterally-accreted bars which show an internal architecture made of low-angle (b10°) inclined beds (Fig. 6D–E). Sandstone occurs in the topmost channels, which in some cases developed as sinuous chute channels on the larger bars. On the whole this association is referred to low-sinuosity gravel-sand bed rivers (cf., Billi et al., 1987).

4.5.4. Association D This association is made of a lateral and vertical alternation of facies Ma–c, Gs and S forming bedsets up to 10 m thick. Beds of facies Gs and S are generally thinner, on the order of few dm–1 m thick, and finer-grained than those characterizing associations A and C. These thinner beds occur as isolated lens-shaped, both concave-down and convex-up, bodies within the predominant muddy deposits and they increase in frequency in the upper portion of this association. Association D records the sedimentary development of a low-energy subaerial setting comparable with a floodplain characterized by overbank flood-flows, temporary water stagnation, pedo- and bioturbation (Bridge, 2003). Facies Ma–c attests to the vertical accretion of the plain from sediment settling, punctuated by pedogenic modification under a variable hydrologic regime. Gravelly-sandy lensshaped bodies record overbank flows which fed crevasse channels and splays.

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Fig. 7. A) Panoramic view of the N–S trending quarry face showing the upper portion of S5III hybrid facies association dominated by monogenic association D; B) typical aspect of the monogenic facies association D with Mb mudstone colonized by trees (Mc) and incised by small sandy channels, the outcrop is 1 m high; C) the unconformable transition between S5III and S6I hybrid facies associations; the sand in the upper part of the monogenic facies association D hints to a coarsening-upward trend which anticipates the arrival of facies Gs of the S6I association; D) panoramic view of S5IV hybrid facies association exposed during the excavation of a municipal dump located few kilometers SE of the study area (see Fig. 1C); the banded mudstone bears shells of brackish gastropods and bivalves hinting to marine influence in a portion of the basin dominated by continental deposition (see discussion in the text).

4.6. Hybrid facies associations and sequence stratigraphy Each repetition of A–D associations represents a single hybrid facies association which records cyclic variations of depositional and palaeoenvironmental conditions. In the studied succession, four hybrid associations, S5II–III and S6I–II, respectively, reflect the dynamics of the accommodation space recorded by the physical surfaces and the intervening deposits within each association (Fig. 8). From the base, the bounding erosive surface separating monogenic association D from association A represents a first order unconformity (sb in Fig. 8), that marks a significant decrease in accommodation during a relative fall of base level that, given the vicinity of the coeval coastline (Benvenuti and Degli Innocenti, 2001; Del Conte, 2007), was connected to relative fluctuations of sea level. Association A records the early valley-fill related to low-sinuosity multi-story channels, whereas the overlying association B attests to progressive valley aggradation under a slow increase in accommodation space. Associations A and B may record respectively the early and late lowstand of sea level. In this interpretation the depositional system which best fits association B is a gravel-bed river, characterized by shallow braided channels showing some sedimentological analogy with the modern apenninic fiumara streams. The latter develop in geomorphic, hydro-climatic, structural conditions which may be somewhat different from those existing in the Piacenzian Valdelsa Basin. Nevertheless, the concurrent uplifting relief on the NE margin of

the basin (Balestrieri et al., 2013; Benvenuti et al., in press) and a regional sub-tropical climate prone to high-magnitude floods (Benvenuti et al., 1999), made available huge sediment and water discharges to the late Pliocene Valdelsa fluvial systems. In this perspective the latter may be considered convergent with the fiumara streams. The low-rate creation of accommodation space expected in the late lowstand (Wright and Marriott, 1993) justifies the filling of the incised valley by laterally mobile multiple, wide and shallow channels. In the hypothesis of a DFS fan, association B would testify to a vertical transition from a fluvial valley represented by association A, to a related fan that originally should have been located in a more distal position. In this case the fan deposits over the valley fill should point to a backstepping attesting to the creation of accommodation under a rising base-level. The latter process seems more reasonably indicated by the overlying associations C–D. Association C marks a change of fluvial style, from poorly confined coarse deposition to finer-grained deposition in single sinuous channels, that may be ascribed to an early transgressive stage. The systematic channel incision recording the transition from association B to C may represent a feature apparently not consistent with a rising base level expected during a transgression (discussed below). The progressive creation of accommodation space is confirmed by association D. The presence of association D at the top of each sequence shows a recurring vertical facies development characterized by waterlogging and poorly-drained soil development at the base followed by crevasse deposition to the top. These

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Fig. 8. Synthesis of the facies analysis and sequence stratigraphy of the studied succession: codes for facies and monogenic facies associations as in the text; codes for systems tract as in Fig. 2; eLST–eTST: early lowstand/transgressive systems tract; lLST–lTST: late lowstand/transgressive systems tract; sb: sequence boundary; frs: fluvial ravinement surface.

features suggest a transition from a maximum increase to progressive reduction of accommodation space as expected between transgressive and highstand system tracts (Catuneanu, 2006). Organic (associations S5II–III) and calcareous (associations S6I–II) paleosols hint to periods of low sediment supply which, in the frame of hybrid associations, resulted in the formation of key stratigraphic surfaces (cf., Kraus, 1999) which may be a terrestrial equivalent of the marine maximum flooding surface (Posamentier and Allen, 1999). The latter attests to sediment starvation of the coastal areas and inner shelf in the acme of a transgression.

5. Discussion 5.1. Early transgressive fluvial erosion: a conceptual analog of the marine ravinement surface The occurrence of an erosive contact between the late lowstand deposits of association B and those of association C (frs in Fig. 8), can be used to help sequence stratigraphic interpretation. The systematic presence of this erosive surface in each sequence raises a

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question about the apparent loss of accommodation in a period of net rising base level. In general terms, channel incision is not only regulated by base-level variation, being also a primary autocyclic process in fluvial systems, dependent on the ratio between water and sediment discharge (Best and Ashworth, 1997; Blum et al., 2000). The B–C erosive contact may reflect a variation of water and sediment discharge rather than a fall of base level. A decrease of sediment grain size and volume, determined by the rising base level, resulted in a relative increase of water discharge with consequent higher erosive power of flowing water (cf., Blum et al., 2000) and channel incision on the deposition of association B. If this process happened in an early transgressive stage, then a conceptual analogy exists between this channel incision and wave erosion in nearshore settings recorded by the ravinement surface (Davis and Clifton, 1987; Posamentier and Allen, 1999). The latter is an erosional transgressive surface (Cattaneo and Steel, 2003; Catuneanu, 2006), generated during the landward migration of the shoreline. Similarly, the B–C contact indicates erosion under a rising base level which reduced sediment supply to the fluvial systems. A hypothetic coupled development of fluvial (frs) and marine (mrs) ravinement surfaces is proposed for the studied succession (Fig. 9). In the model the frs developed as the consequence of base-level rise during the early transgression after sea level lowstand. With the progression of sea-level rise, the landward migration of the shoreline created the mrs on the backshore deposits, meanwhile the fluvial valleys were filled with association C. The progressively reduced fluvial sediment supply to the shoreline justifies the shoreface erosion and retreat characterizing this stage. The maximum landward retreat of the shoreline would have resulted in the sandy overfilling of the fluvial valleys, followed by the starving of fluvial coarse-grained sediment with the establishment of late transgressive mud-dominated subaerial and submarine environments. 5.2. The flood basin: a late transgressive-highstand depositional system in fluvial sequences The predominance of facies Ma–c in association D, compared to the facies Gs and S and the limited dimension of the related channels and crevasse splays (Figs. 4, 7B, 8), outline the apparent paradox of huge vertical aggradation of mud in relation to small fluvial channels. By this fact, the progressive rise of base level switched off the large fluvial systems recorded by associations A–B, giving rise to fluvial channels which reached the smallest sizes in association D. Despite a suitable reference to a floodplain, more properly association D may represent a specific alluvial environment sharing depositional features with the so-called flood basin. In fluvial geomorphology flood basin indicates 1) the tract of land actually submerged during the highest known flood in a specific region, or 2) the flat, wide area lying between a low, sloping plain and the natural levee of a river (McGraw-Hill, 2003). Similarly, in the sedimentologic literature the flood basin (frequently floodbasin) indicates small lakes and ponds in a floodplain, dominated by organic-rich mudstone and associated channelized and overbank sandstone (Ethridge et al., 1981; Jorgensen and Fielding, 1996; Pole, 2001; Cohen

et al., 2005; Jinnah and Roberts, 2011). In analogy with the estuarine and the lagoon deposits, diagnostic of the transgressive systems tract (Dalrymple et al., 1992; Allen and Posamentier, 1993), facies association D records the flood basin as a specific depositional system related to a stage of high base level, rather than a sub-environment of the alluvial plain. This depositional environment, dominated by fine-grained deposition mostly supplied from overland flows and only in part from the overbank of small channels, shows also convergence with the muddy playa lakes of endorheic basins (Tunbridge, 1984). Furthermore, the flood basin may be a depositional reference also for the mudstones rhythmically alternating with channelized conglomerate reported from alluvial successions of tectonically-active basins, and referred to rising base level during subsidence pulses (Paola et al., 1992; Bentham et al., 1993; Crews and Ethridge, 1993).

5.3. Land–sea correlation of small-scale sequences in the S5 synthem A physical correlation between the fluvial deposits of the upper portion of the S5 synthem and the deltaic–marine equivalent deposits 12 km to the W (Fig. 10A), illustrates the facies gradient determined by high frequency sea/base-level fluctuations. Taking into account the uppermost composite sequence of the S5 synthem (CDS4, Fig. 2) in the central portion of the basin, a correlation between EDSc and S5IV is proposed based on the following arguments. The mudstones resting over the deltaic sandstone within EDSc, bear a benthic foraminifer association that records the maximum deepening of a shelf (up to − 120 m; Benvenuti et al., 2007; Dominici et al., 2008) reached during development of the S5 synthem. EDSc resulted from a high frequency sea-level fluctuation representing at the same time the maximum flooding stage of the medium- (CDS4) and large-scale sequences coinciding with the S5 synthem. The relative maximum level reached by the sea in the Valdelsa Basin during the late Piacenzian transformed the coastal alluvial plain to the NE into the lagoon recorded by the brackish mudstones of association S5IV (Fig. 6D). Association S5III is therefore the landward equivalent of EDSb whereas association S5III is correlated with the topmost EDS of CDS3. The continental-marine facies tract resulting from the correlation suggests the following generalizations (Fig. 10B): 1) the relative fall of sea/base-level which determined the fluvial incision in the alluvial plains was followed by the valley aggradation and related delta progradation during the lowstand stage; 2) the early transgressive stage was characterized on the alluvial plain by channel incision and aggradation (association C). The contemporary delta fronts underwent backstepping and were starved with sediments, consequently the sandy bottoms was affected by pervasive bioturbation. The hypothetic shoreline migrated landward generating a marine ravinement surface younger than the fluvial one (Fig. 9); 3) in the late transgressive stage, due to high base level, the former channel belts became flood basins (association D, lower part)

Fig. 9. Hypothesis on the formation of fluvial (frs) and marine (mrs) ravinement surfaces. Transgression starts at time T1 when the shoreline is still supplied by channels bounded by the frs resting on the lowstand portion of the fluvial valley fill. The continuing rise of sea level (T1–T3) results in a storage of progressively finer-grained sediments in the alluvial plain and starving of the nearshore with consequent wave erosion and shoreline retreat recorded by the mrs. The maximum shoreline retreat coincides with the overfilling of the fluvial valleys in the alluvial plain. The depositional record of this early transgressive stage is represented offshore by a transgressive lag (Posamentier and Allen, 1999; Cattaneo and Steel, 2003; Catuneanu, 2006). Drawing not to scale.

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Fig. 10. A) Land–sea correlation in the uppermost portion of S5 synthem (see discussion in the text). The location of the sections is reported in Fig. 1A. Symbols for the San Maiano section as in Fig. 2. B) Hypothetical depositional gradient during a cycle of relative sea-level fluctuations inspired to the lateral facies development recorded in the uppermost synthem S5. SB: sequence boundary; fsr: fluvial ravinement surface; mrs: marine ravinement surface. Arrows indicate the trajectory of the shoreline (black dots).

and similar to the related inner shelf were dominated by mud deposition. 4) highstand of sea/base-level is recorded in the upper portion of the correlated sequences by an overall coarsening-upward of grain size. In the flood basin this is related to the arrival of sand as overbank flows (association D, upper part) which anticipate renewed valley incision starting a further small-scale depositional sequence.

6. Conclusions The upper Piacenzian–Gelasian cyclothemic fluvial succession exposed in the Neogene–Quaternary Valdelsa Basin has been interpreted as the response of fluvial systems to high-frequency variation of the base level resulting in two types of facies associations. Monogenic associations A, B, C and D, record specific alluvial environments whereas hybrid facies associations, including associations A–D, record a full cycle of base-level variations discussed in sequence-stratigraphic terms. Together with an overall stacking of lowstand, transgressive and highstand deposits, each hybrid association shows specific features such as erosional surfaces delimiting the early transgressive (association C) from the late lowstand (association B) fluvial valley fill, and the flood basin (association D) as a transgressive depositional system. The B/C channel scouring conceptually converges with the marine ravinement surface generated by wave erosion during the transgressive landward migration of a shoreface. Specifically, channel scouring resulted from a high water/sediment discharge ratio during the early rise of the base level. The channel incision may have occurred before the development of the marine ravinement surface. The flood basin is considered indicative of a rising base level sharing a sequential significance with the transgressive estuaries and lagoons of nearshore settings. The correlation between the fluvial hybrid associations and small-scale shallow marine depositional sequences along the Piacenzian land-sea gradient, attests that the fluvial systems responded to the sea-level fluctuations through valley incision, recurring variation of river channel

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