Quaternary International 288 (2013) 129e138

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Coalescent valley fills from the late Quaternary record of Tuscany (Italy) Alessandro Amorosi a, *, Veronica Rossi a, Giovanni Sarti b, Roberto Mattei b a b

Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Via Zamboni 67, 40126 Bologna, Italy Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria 53, 56126 Pisa, Italy

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 19 October 2011

Three prominent incised-valley fills of post-Last Glacial Maximum age are described from the northern Tuscan coast of Italy. Stratigraphic correlation of core data along a cross-section transversal to the present fluvial-channel axes enables identification of a suite of genetically related valley bodies that fill the incisions made up by Arno, Serchio and CamaioreeStiava rivers during the last glacial/interglacial cycle. The valley fills display different shapes and size, but remarkably similar facies architecture. Valley bodies range between 5 and 10 km in width, and between 30 and 45 m in thickness, with width/thickness ratios of about 100e300. Above a lowstand (and early transgressive?) gravel fluvial deposit, the three incised-valley fills display a distinctive succession of coastal plain to estuarine facies, dated to about 13e8 cal ka BP. Radiocarbon dates document that the three valleys were active simultaneously. Accommodation space was rapidly created during transgression and then filled under conditions of very high sediment accumulation (about 1 cm/y). In contrast to the more common deepening-upward trend recorded by the latest Pleistoceneeearly Holocene valley-fill successions worldwide, sedimentation in the Tuscan valleys equalled, or even exceeded, the rate at which accommodation was created, thus leading to an aggradational, rather than backstepping, stacking pattern of high-frequency (millennial-scale) parasequences. Above the valley fills and on the interfluves, a thin deepening-upward succession of nearshore deposits marks the rapid change from aggradational to retrogradational depositional style. This is invariably overlain by a characteristic shallowing-upward motif of prograding deltaic and coastal facies. The maximum flooding surface, which can be tracked across the whole study area on the basis of subtle palaeontologic indicators, is shown to represent an almost isochronous surface, dated to about 7.8 cal ka BP. Through examination of large-scale geometry and facies attributes of the valley fills, including the relationships between valley bodies and interfluves, this study represents an example of how adjacent river systems with significantly different characteristics may respond simultaneously in a consistent manner to rapid changes in sea-level and climate conditions. Ó 2011 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction Following the interest of early sequence-stratigraphic models in incised-valley fills (Van Wagoner et al., 1990; Wright and Marriott, 1993; Dalrymple et al., 1994; Schumm and Ethridge, 1994; Shanley and McCabe, 1994; Zaitlin et al., 1994) or valley bodies (Gibling, 2006), three-dimensional architecture of valley fills in the rock record has received increasing attention in the past two decades (Greb and Chestnut, 1996; Willis, 1997; Plint, 2002; Feldman et al., 2005). Subsurface analyses have recently focused on buried incised-valley systems of latest Pleistocene to Holocene age. For these examples, valley cutting and filling appear to be controlled primarily by glacial/interglacial sea-level and climate fluctuations

* Corresponding author. E-mail addresses: [email protected] (A. Amorosi), veronica.rossi4@ unibo.it (V. Rossi), [email protected] (G. Sarti). 1040-6182/$ e see front matter Ó 2011 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2011.10.015

(Blum and Törnqvist, 2000; Blum and Aslan, 2006; Anderson and Rodriguez, 2008), with possible superposition of tectonics (Tanabe et al., 2009, 2010). Most studies of late Quaternary incised-valley fills stem primarily from high-resolution seismic data (e.g., Reynaud et al., 1999; Bowen and Weimer, 2003; Nordfjord et al., 2006; Greene et al., 2007; Mattheus et al., 2007; Labaune et al., 2010), and in general very few of these studies have access to deep penetrating cores that enable validation of seismic results. On the other hand, the accurate geometric characterization of Quaternary valley bodies on-land is generally a very difficult task, because studies are generally conducted on the basis of core analysis alone (Gibling, 2006). The coastal plain extending along the Tyrrhenian Sea represents an area where incised-valley fills of late Quaternary age have been characterized in detail, in terms of both geometry and facies architecture. After the early work by Bellotti et al. (1994, 1995), Milli (1997) and Milli et al. (2008) on the Tiber coastal plain, detailed facies documentation of the late Quaternary valley-fill sequences

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from the northern Tyrrhenian coast has been recently provided by Aguzzi et al. (2007) and Amorosi et al. (2008, 2009) for the Arno River system. This paper expands upon previously published material from the Tuscan coast, extending stratigraphic analysis of the post-Last Glacial Maximum (post-LGM) succession to a 30 km-long transect, approximately parallel to the present Tyrrhenian Sea shoreline, north of Arno river mouth (Fig. 1). The aim of this study is to describe the large-scale geometry and internal stratigraphic architecture of three synchronous incised-valley systems (Arno, Serchio and StiavaeCamaiore river systems in Fig. 1) that developed on the Tuscan coast of Italy during the last sea-level cycle. A specific objective is to investigate the response of these three river systems to the post-LGM sea-level rise through comparison of the valley fills in terms of depositional style and sedimentary evolution.

2. Geological setting The coastal plain of northern Tuscany between Pisa and Viareggio, facing the Tyrrhenian Sea to the west, is ca. 550 km2 wide and has a characteristic triangle shape wedging out northwards, toward Versilia (Fig. 1). The coastal plain is bordered by the Leghorn and Pisa Hills to the south, and by the Apuane Alps and Pisani Mountains to the east. The fluvial network in this area includes three major river basins. Arno and Serchio rivers are the main fluvial courses, with catchment areas of 8228 km2 and 1400 km2, respectively. Arno River is 241 km long, with water discharge of about 100 m3/s (Autorità di Bacino del Fiume Arno; http://www. arno.autoritadibacino.it), while Serchio River is 111 km in length, with water discharge of 46 m3/s (Cortecci et al., 2008). North of Massaciuccoli Lake (see Fig. 1) a network of smaller fluvial channels

Fig. 1. Location of the study area, showing approximate geometry of the three post-Last Glacial Maximum paleovalleys and related interfluves. The bold line indicates the section trace of Fig. 6 and its components (Figs. 3e5, indicated by arrows). The fence diagram of Fig. 2 (Arno River paleovalley) is evidenced by dotted lines. The dashed line shows the maximum landward position of the shoreline during the Holocene, as reconstructed by subsurface data.

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and ditches flow from the Apuane Alps towards the Versilia coastline. These form the CamaioreeStiava river basin. Due to proximity of the Apuane Alps to the Tyrrhenian Sea, Camaiore River is just 19 km long, while Stiava River is even shorter, its course having been modified by human intervention. The north Tuscan coastal plain represents the inshore portion of the subsiding half-graben Viareggio Basin (Argnani et al., 1997; Martini et al., 2001; Pascucci, 2005). This basin was formed since the late Tortonian due to the opening of the Tyrrhenian Sea and the counter-clockwise migration of the Apenninic foredeepeforeland system (Malinverno and Ryan, 1986; Patacca et al., 1990; Martini and Sagri, 1993). Seismic investigations have shown that the Viareggio Basin is up to 2500 m thick (Mariani and Prato, 1988) and includes five unconformity-bounded units of upper Miocene to Holocene age (Pascucci, 2005). Detailed stratigraphic studies of the late Quaternary succession, which consists predominantly of continental to shallow-marine alternations, have been concentrated during the last decade around the city of Pisa. These studies led to identification of two transgressiveeregressive sequences formed in response to the last two interglacialeglacial cycles (base of MIS 1 and 5e, respectively e Aguzzi et al., 2005). Within the youngest sequence, a palaeovalley fill consisting primarily of estuarine deposits and related to the Arno River was identified on the basis of remarkable (15e50 m) thickness variations of the post-LGM succession (Aguzzi et al., 2007; Amorosi et al., 2008; see Figs. 1 and 2). This valley fill accumulated during the dramatic Lateglacialeearly Holocene eustatic sea-level rise that occurred worldwide in response to the generalized phase of climate amelioration and the consequent

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disappearance of large portions of the glacial ice sheets (Fairbanks, 1989; Bard et al., 1996). In contrast, no detailed subsurface studies are available for the wide portion of the Tuscan coastal plain extending north of Arno River, between Pisa and Viareggio (Fig. 1).

3. Dataset A dense grid of borings and sediment cores (ca. 3500 stratigraphic data) were used for this study. A wealth of subsurface information exists for the Pisa area and the Arno River in general, whereas lower density stratigraphic data are available for Serchio and Camaioree Stiava river systems (Fig. 1). Where data include sufficiently detailed material (23 sediment cores), facies interpretation was derived, along with identification of diagnostic stratigraphic markers, such as indurated horizons and palaeosols. Facies characterization of the post-LGM succession also benefited from the analysis of the meiofauna (benthic foraminifers and ostracods) found within six reference cores (Bergamin et al., 2006; Amorosi et al., 2009; Carboni et al., 2010). A total of twenty-seven radiocarbon dates previously published in Bergamin et al. (2006), Amorosi et al. (2009) and Carboni et al. (2010) allowed tying facies interpretation from cored wells into a sequence-stratigraphic framework in the sense of Posamentier et al. (1988) and Plint and Nummedal (2000). Conventional ages from Bergamin et al. (2006) were calibrated using CALIB 5.1 calibration program (referenced as Stuiver and Reimer, 1993) along with the Marine04 marine dataset (Hughen et al., 2004). No local reservoir correction was applied. Radiocarbon ages younger than 15,000 years BP are reported in text as cal BP.

Fig. 2. Three-dimensional architecture of the post-LGM Arno valley fill, showing distinctive along-dip variability in facies architecture. SB: Sequence Boundary, TS: Transgressive Surface, BRS: Bay Ravinement Surface, TRS: Tidal Ravinement Surface, WRS: Wave Ravinement Surface, MFS: Maximum Flooding Surface (modified from Amorosi et al., 2008). Reference cores are shown in red.

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4. Late Quaternary valley-fill architecture Anatomy and depositional architecture of Arno, Serchio and CamaioreeStiava incised-valley systems are described below. Owing to inhomogeneous data distribution in the study area (see Fig. 1), the transversal cross-sections of Figs. 3e5 were constructed at variable distance from the present shoreline, following the paths of the highest data density (see oblique section traces relative to shoreline position at peak transgression e Fig. 1). As a consequence, the Arno section (Fig. 3) depicts facies architecture of the valley fill at relatively proximal (behind beach-barrier) locations relative to the other two sections (Figs. 4 and 5), which instead include nearshore sands. Fig. 2 summarizes the characteristic longitudinal facies variations from proximal to distal locations along the Arno valley fill, showing stratigraphic relationships between littoral, estuarine and continental deposits. For detailed lithofacies documentation, the reader is referred to previously published papers (Aguzzi et al., 2007; Amorosi et al., 2008). The diagnostic sedimentological and micropalaeontological features of the major facies associations shown in Figs. 3e5 are summarized in Table 1. 4.1. Arno valley fill Preliminary description of the anatomy and stratigraphy of the Arno valley fill has been reported by Amorosi et al. (2008), and is implemented here through additional data and re-examination of the subsurface database (Figs. 2 and 3). The valley body, which roughly coincides with the present Arno River course, shows a total thickness of ca. 40e45 m and is 5e8 km wide. The interfluve plateaus, which can be easily differentiated from the post-LGM succession by their overconsolidated state (see core c1690 in Fig. 3), are located 15e20 m below sea-level on both sides of the valley axis. In correspondence with the depocentre (core D283 in Fig. 3), the valley fill is floored by a thin fluvial gravel deposit. This is

abruptly overlain by a thick succession of LateglacialeHolocene (ca. 13,000e7800 cal BP) coastal sediments onlapping onto the valley flanks. High-frequency, millennial-scale cyclicity has recently been documented from reference core S1 (Amorosi et al., 2009), where subtle changes in palaeosalinity evidenced by palaeontologic (benthic meiofauna) indicators have enabled reconstruction of alternating freshwater and brackish sub-environments during valley filling. Stratigraphic correlations transversal to the Arno River show that individual small-scale cycles, about 10 m thick, are bounded by flooding surfaces (parasequences sensu Van Wagoner et al., 1990) and display similar facies architecture, with sharp-based central/ outer-estuarine clays grading upwards into inner-estuary and coastal-plain deposits (Fig. 3). Based upon shallowing/deepeningupward trends and lateral stratigraphic relationships, facies architecture shows an aggradational, rather than retrogradational stacking pattern of parasequences within the valley body. A laterally extensive stratigraphic interval, made up of soft clays with abundant brackish fossils, can be correlated across the interfluves, documenting generalized flooding after valley filling (Fig. 3). This stratigraphic interval, which represents the landward equivalent of the nearshore and shallow-marine deposits recorded at more distal locations (see maximum flooding deposits in Fig. 2), is overlain by a 5e10 m thick, middleelate Holocene “regressive” succession of prograding deltaic and alluvial plain deposits. 4.2. Serchio valley fill The Serchio valley fill is 8e10 km wide, with maximum thickness of about 30e35 m (Fig. 4). The palaeovalley displays broad interfluves located around 20e25 m below sea-level. Two fluvial terraces along the southern flank of the valley are revealed at ca. 30 and 35 m depth by the diagnostic gravel-based, finingupward successions, capped by well developed palaeosols.

Fig. 3. Cross stratigraphic section depicting facies architecture of post-LGM Arno valley fill (see Fig. 1, for location). Core S1 is projected onto the section line. Reference cores are box-bordered. Radiocarbon dates after Amorosi et al. (2009).

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Fig. 4. Cross stratigraphic section depicting facies architecture of post-LGM Serchio valley fill (see Fig. 1, for location). Cores sgiul-39 and DN1 are projected onto the section line. Reference cores are box-bordered. Radiocarbon dates after Carboni et al. (2010).

In the depocentre, intercepted by reference core ENEA, the valley fill includes a 7 m-thick basal succession of fluvial gravels with subordinate sands. Above this gravel body the lower valley-fill succession is characterized by an alternation of clays and silty clays containing vegetal remains, wood fragments and non-marine molluscs (freshwater and terrestrial molluscs within core ENEA e Nisi et al., 2003; Carboni et al., 2010). This facies association,

assigned to the Lateglacial period (around 13,200e12,400 cal BP), is consistent with a freshwater humid environment, such as a coastal plain or inner estuary (Fig. 4). The upper portion of the valley-fill succession, which is dated to the early Holocene period (Carboni et al., 2010), consists of central- and outer-estuarine deposits with brackish bivalves (Cardium glaucum). These are overlain by clays and sands containing freshwater molluscs, with rare brackish

Fig. 5. Cross stratigraphic section depicting facies architecture of post-LGM CamaioreeStiava valley fill (see Fig. 1, for location). Reference core is box-bordered. Cores R3A12, DB-Salov and sira241 are projected onto the section line. Radiocarbon dates after Bergamin et al. (2006).

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Table 1 Diagnostic features of the major facies associations depicted in Figs. 3e5, as revealed by core analysis. Facies association

Lithology, sedimentary structures and accessory components

Micropaleontological content

Fluvial channel/Fluvial terrace

Erosional-based, poorly sorted gravel body with fining-upward trend. Typically indurated and pedogenized at top (fluvial terrace) Organic-rich, dark grey to black clay and peat alternating with silt and silty sand. Wood fragments very abundant. Local presence of sand bodies with both fining- and coarsening-upward trends

Barren

Coastal plain and Inner estuary

Central and outer estuary/Lagoon

Homogeneous succession of grey silty clay, with local concentration of brackish molluscs. Rare sand layers with wave ripples at their tops. Wood and shell fragments scattered

Beach Barrier

Fine to very fine sand and silty sand, with typical fining-upward trend. Mollusc shells very abundant (transgressive sand below MFS). Well sorted, medium to coarse sand with coarsening-upward trend. Mollusc shells, wood fragments and plant debris abundant (delta front above MFS) Bioturbated silty clay and clay, with thin sand intercalations, wood fragments

Prodelta

foraminifers (Carboni et al., 2010). This latter unit indicates a “regressive” tendency atop the valley fill, with return to innerestuarine/coastal-plain conditions (Fig. 4). The overlying beach-barrier sands record the onset of marine transgression in the study area, while maximum marine ingression is observed at the transition with overlying prodelta clayesand alternations, dated to 7800 cal BP. Upwards, the vertical succession of beach-barrier (delta front) and lagoonal (delta plain) deposits documents middleelate Holocene delta progradation (Nisi et al., 2003; Carboni et al., 2010). 4.3. CamaioreeStiava valley fill The CamaioreeStiava valley fill is about 33 m thick and ranges between 6 and 8 km in width (Fig. 5). The interfluve plateaus, identified around 25 m depth, appear to be connected throughout steep valley flanks with the depocentre area, intercepted at ca. 60 m depth (cores R47 and DB-Salov). Low data density prevents identification of fluvial terraces deposits in this valley fill, although a typical flat terrace morphology is clearly depicted by stratigraphic correlations between cores ICRAM and DBCAV (Fig. 5). A basal fluvial gravel body, approximately 3 m thick, is recorded at the base of the valley-fill succession, in correspondence of the valley axis. Above these fluvial deposits, the valley fill is mainly composed of a thick succession of estuarine clays, with lateral transition to coastal plain sediments. The estuarine deposits, containing a peculiar marine-brackish meiofauna, accumulated during the early phase of sea-level rise dated to between ca. 11,300 and 7800 cal BP (Bergamin et al., 2006). A thick succession of beachbarrier sands overlies the valley fill and the adjacent interfluves. Shoreline transgression is dated to about 7800 cal BP (Bergamin et al., 2006). 4.4. Summary characteristics The three late Quaternary incised-valley bodies related to the Arno, Serchio and CamaioreeStiava palaeovalleys rest on a succession of older alluvial deposits, and record a short period of channel entrenchment followed by valley filling associated to the last glacioeustatic cycle. Having formed completely during one sea-level cycle,

Rare freshwater-oligohaline ostracods, mainly belonging to genera Candona and Pseudocandona (coastal plain). Slightly brackish water assemblage dominated by the eurihaline ostracod species Cyprideis torosa. Among foraminifers Ammonia tepida and A. parkinsoniana locally occur (inner estuary) Moderately to highly marine-influenced brackish water meiofauna, with very abundant Ammonia tepida and A. parkinsoniana, and subordinate Criboelphidium spp., Haynesina germanica and Quinqueloculina seminula. Cyprideis torosa is the dominant ostracod Rare and poorly preserved shallow-marine foraminifers (Ammonia beccarii and A. inflata, Elphidium spp.) and ostracods (Pontocythere turbida)

Strongly fluvial-influenced marine meiofauna, tolerant to salinity stressed conditions and remarkable organic matter content (Ammonia tepida and A. parkinsoniana, Cribroelphidium granosum, Haynesina germanica, and Palmoconcha turbida)

they correspond to “simple” incised-valley fills in the sense of Zaitlin et al. (1994). The valley bodies show remarkably similar facies organization. This includes basal fluvial-channel deposits overlain by a variety of transgressive, mud-dominated coastal plain and estuarine facies. Transgressive coastal deposits overlie the valley fills and the adjacent interfluves and are capped, in turn, by a progradational succession of deltaic and coastal sediments. The coastalplain valley fills documented in this paper display different shapes and size. Widths range between 5 and 10 km, while the valley bodies have thickness of 30e45 m, with resulting width/thickness ratios of about 100e300. All these values fall within the field of “valley fills associated with alluvial and marine strata” (see classification of Gibling, 2006). The three fluvial bodies described from the subsurface of the Tuscan coastal plain match perfectly the three diagnostic criteria set out by Fielding and Gibling (2005) for valley fills, i.e. i) wide traceability of the basal erosion surfaces and of the correlative surfaces in extra-channel (interfluve) position (three distinct valley systems separated by two interfluves), ii) dimension of the overall valley body an order of magnitude larger than those of other channel forms in the system (the latter are a few meter in thickness and tens of meter in width); and iii) scale of erosional relief on the basal surface several times the depth of scour evident from component channel fills. 5. The post-LGM succession of the Tuscan coastal plain 5.1. Sequence-stratigraphic interpretation and palaeoenvironmental evolution The most striking feature of the post-LGM succession of Tuscany along the 30 km-long transect that runs oblique to the modern shoreline, between Pisa and Viareggio, is its remarkable thickness variation, from about 15 to 60 m. Its lower boundary traces out the shape of the three palaeovalleys described in the previous sections (from south to north, Arno, Serchio and CamaioreeStiava e Fig. 6). The post-LGM succession overlies alluvial deposits that accumulated during the last phase of base-level fall and subsequent lowstand, between about 125 ka BP (onset of forced regression, according to terminology by Catuneanu et al., 2009) and 19 ka BP

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Fig. 6. Representative cross section of post-LGM coalescent valley fills buried beneath the Tuscan coastal plain and its sequence-stratigraphic interpretation (for section trace, see Fig. 1). Subdivision into systems tracts (FST, LST, TST and HST) and stratigraphic position of the key surfaces for sequence-stratigraphic interpretation (SB, TS, MFS) are shown. Two particularly salient features are illustrated: (i) the three valley fills display fairly consistent geometry and facies architecture, being invariably separated by pedogenized interfluves onto which the post-LGM sequence has minimum thickness; (ii) the MFS is observed to represent an almost isochronous surface, its seeming topographic relief representing instead the result of an oblique section trace. For facies description, see Figs. 3e5.

(end of regression e see Lambeck et al., 2002). In sequencestratigraphic terms, these alluvial deposits include the fallingstage systems tract (FST) of Plint and Nummedal (2000) and the lowstand systems tract (LST), which are separated by the sequencebounding unconformity (SB) of Hunt and Tucker (1992). Stratigraphic positioning of SB is a highly debated issue in sequence stratigraphy (Catuneanu et al., 2009), and several models place SB at the onset of sea-level fall, i.e. below, and not above, FST (Posamentier et al., 1992; Kolla et al., 1995; Morton and Suter, 1996). Given the shallow depths of investigation of the Tuscan logs, the deeper candidate for SB cannot be tracked in the cross-sections. For this reason, SB was placed in coincidence of the maximum regressive surface of Helland-Hansen and Martinsen (1996), i.e. the top of FST. Lack of control on the geometry of SB on the basis of borehole data alone makes precise separation of FST from LST very difficult. The authors are inclined to place SB in coincidence of the erosional contact between the gravel bodies observed in the three valley fills around 50e60 m depth, and the underlying finer-grained alluvial deposits (Figs. 3e6). This implies that the 3e7 m thick fluvialchannel gravels that overlie SB represent the preserved lowstand deposits on the three valley floors (see also Foyle and Oertel, 1997). Following the last phase of stream rejuvenation, a stable river pattern was established in the three narrow valleys, most fluvial sediment being conveyed to the lowstand delta systems through the incised valleys. It cannot be ruled out, however, that part of the gravel bodies accumulated under early transgressive conditions. There is local evidence (Serchio and Arno valleys) for active terrace formation during channel entrenchment, suggesting a complex history of alternating incision and aggradation in the fluvial drainage systems (Foyle and Oertel, 1997; Muto and Steel, 2004; van der Schriek et al., 2007; Strong and Paola, 2008; Vis and Kasse, 2009). Specifically, a major erosion surface is observed to deeply incise and crosscut at different stratigraphic levels fluvialterrace deposits made up of sand and gravel-prone facies grading upwards into typically pedogenized silts and clays (see Fig. 4). Fluvial terraces are widely represented within modern valleys (Blum et al., 1994), but are rarely identified within the dataset of incised-valley fills (Gibling, 2006; van der Schriek et al., 2007). Along the study sections, the observed patchy distribution of terrace deposits (FST) is more likely to reflect low density and quality of borehole descriptions rather than poor terrace development. On the interfluves separating the major valley bodies the three deeply incised erosional surfaces pass into correlative subaerial exposure surfaces, evidenced by characteristic indurated and weathered horizons associated with significant stratigraphic hiatuses (“interfluve sequence boundary” of Van Wagoner et al.,

1990; Gibling and Bird, 1994; Gibling and Wightman, 1994; McCarthy and Plint, 1998). At these locations SB merges with the transgressive surface of erosion (TS), with no intervening lowstand and early transgressive deposits. Across the study area, TS (corresponding to the maximum regressive surface of Catuneanu et al., 2009) is the most readily identifiable stratigraphic surface, marking the sharp facies change from purely fluvial deposits to organic-rich clay deposits. This locally erosional surface, which represents the base of the transgressive systems tract (TST), is strongly diachronous and reflects the complex, inherited pre-transgressive topography of the three valley systems (Rossi et al., 2011). Early (latest Pleistocene) transgressive deposits are recorded uniquely in the depocentres (Figs. 3e6), while there is widespread evidence of generalized flooding of the valleys during the early Holocene. The transgressive surface marks the evolution of the incised-valley systems into wave-dominated estuaries, as documented by reconstruction of elongate beach ridges at the mouth of the estuaries during transgression. The TST, which accumulated between ca. 13,000e7800 cal BP, records the progressive infilling and drowning of the Tuscan palaeovalleys in response to the rapid post-glacial sea-level rise. The three valley fills exhibit the traditional vertical succession of estuarine facies both predicted by conceptual models (Nichols, 1991; Dalrymple et al., 1992; Zaitlin et al., 1994) and observed within late Quaternary successions (Allen and Posamentier, 1993; Nichol et al., 1996; Hori et al., 2002; Li et al., 2002; Tanabe et al., 2006). Specifically, a composite succession of organic-rich muds formed in a coastalplain or inner-estuarine environment, with upper transition to central- and outer-estuarine clays, invariably forms the lower TST, which exhibits a characteristic onlapping geometry onto the valley walls (Figs. 3e5). An aggradational stacking pattern of highfrequency depositional cycles is recorded within lower TSTs (e.g., valley fills), suggesting that in the early stages of transgression sediment accumulation within the estuaries kept pace to increasing accommodation due to rapid sea-level rise. At comparatively distal locations (see Figs. 4 and 5), the upper TST is characterized by a marked deepening-upward tendency (transition to thin nearshore and shallow-marine deposits), with a typical erosional lower boundary (wave ravinement surface of Swift (1968) and Nummedal and Swift (1987)). The upper TST marks the change from an aggradational to retrogradational depositional style (Fig. 4). Separating the TST from the overlying highstand systems tract (HST) is the maximum flooding surface (MFS in Fig. 6), the age of which is well constrained in all valley fills around 7800 years BP on the basis of radiocarbon dates (Figs. 3e6). This almost isochronous surface, which represents the Holocene peak of transgression in the

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study area, has no obvious physical expression, but has been identified from core data at the turnaround from upward-deepening to upward-shallowing facies successions, in coincidence of samples containing the most marine-influenced microfossil assemblage (Amorosi et al., 2009; Rossi et al., 2011). Within lithologically homogeneous beach-barrier deposits, the upward transition from fine-grained to coarse-grained littoral sands has been taken as possible indication of MFS (see Table 1). The gentle NW-dipping of MFS observed in Fig. 6 is due to the oblique character of the section trace relative to the palaeoshoreline (see Fig. 1). Finally, a single shoaling succession, 15e30 m thick, composes the HST. This is interpreted to reflect deltaic and coastal plain progradation during the late Holocene phase of deceleration in sea-level rise. 5.2. Timing of valley incision and filling In early sequence-stratigraphic models, glacio-eustatic changes of high magnitude and frequency, such as those characterizing the Quaternary period, have been inferred to generate large valleys, especially in coastal areas (Posamentier, 2001), and maximum rates of incision have been related to maximum rates of sea-level fall (Van Wagoner, 1995). Subsequent studies, however, have shown that climate may represent an important controlling factor in shaping incised valleys (Blum and Price, 1998). These studies have addressed the relative importance of upstream climatic vs. downstream sea-level control on fluvial system evolution (Blum and Törnqvist, 2000). Chronologic control of incised-valley formation in the Tuscan coastal plain unfortunately relies upon scattered data only, due to the fact that “bedrock” fluvial deposits and basal fluvial gravels in most instances are not constrained by radiocarbon dates, nor by pollen data, due to lack of suitable material. The lack of stronger radiocarbon age constraints limits precise definition of the timing of valley incision (see also Posamentier and Vail, 1988; Li et al., 2002; Blum and Aslan, 2006). However, given the overall age attribution to MIS 3 of the deposits underlying SB (see core ICRAM in Figs. 5 and 6) and the calibrated age of 27.5 ka (late MIS 3) obtained from a buried fluvial terrace in the Arno valley system (Aguzzi et al., 2007), it can be assumed that valley incision presumably culminated at the MIS 3/2 transition (Late Pleniglacial), as documented by literature examples from coeval coastal plain successions (Dabrio et al., 2000; Wellner and Bartek, 2003; Blum et al., 2008; Kasse et al., 2010). It is likely that the initial MIS2 sea-level fall promoted valley entrenchment, although changes in discharge regimes and sediment supply (Blum and Valastro, 1994; Blum et al., 2000; Simms et al., 2007) cannot be ruled out as possible additional controlling factors. Unlike the timing of valley incision, for which high-resolution chronologic framework is not available, the time required for valley filling is instead reasonably well constrained. Based upon the available radiocarbon dataset, transgressive sedimentation under coastal-plain or inner-estuarine conditions started at about 13,000 cal BP (Fig. 6). On the other hand, initial deposition across the interfluves, associated with sea-level rise, took place around 8000 cal BP, thus implying that the three valley systems were filled in about 5000 years. This assumption is further supported by the age (7.8 ka cal BP) and stratigraphic position of the MFS, an almost isochronous stratigraphic marker which is generally recorded 2e3 m above the interfluves (see Fig. 6). A very high rate of sediment accumulation, on the order of about 6e9 mm/y, was induced by rapidly created accommodation into the valley, due to the combination of rapid eustatic sea-level rise and subsidence-related processes. The narrow, funnel-shaped geometry of the north Tuscan valleys and their proximity to the Apenninic chain favoured a continuous high supply of sediment to the fluvial mouths. This led

to an aggradational (and locally even progradational) stacking pattern of facies even under transgressive conditions. This aggradational trend is clearly detected within the two major valley fills (Arno and Serchio valley systems in Figs. 3 and 4), while it was not identified in the CamaioreeStiava valley-fill succession, probably due to paucity of high-resolution core data. This peculiar facies architecture contrasts markedly with the more common retrogradational (deepening-upward) trend observed within several coeval valley systems from the Gulf of Mexico and other regions (see Greene et al., 2007; Simms et al., 2007; Anderson and Rodriguez, 2008). The transgressive phase was punctuated by episodes of rapid sea-level rise separated by distinct short-term phases of bay-head delta progradation (Fig. 2). A distinctive climate signature of millennial-scale parasequence development (Fig. 3) has been documented from the Arno incised-valley by Amorosi et al. (2009), who showed that the basal flooding surfaces of individual parasequences are invariably associated with abrupt shifts to warmer climate conditions. This stepwise trend of sea-level rise is consistent with the most recent sea-level curves from the Tuscan area (Lambeck et al., 2011), which show a discontinuous post-LGM sealevel rise, with major transgressive pulsations separated by periods of slow transgression or stillstand. However, more detailed facies and pollen characterization from Serchio and CamaioreeStiava valley fills is needed before a similar parasequence facies architecture can be generalized to the entire Tuscan coastal plain. It is very likely that the characteristic ups and downs shown by the sequence-bounding unconformity across the Arno, Serchio and CamaioreeStiava valley systems (Fig. 6) can be traced further south, along the Tyrrhenian Sea coast, across other Apenninic river systems. In particular, a retrogradational stacking pattern of bayhead deltas has been reported from late Quaternary deposits beneath Ombrone River (Bellotti et al., 2004), suggesting a similar parasequence architecture related to a step-like, sea-level rising trend. In a more southern position, close to the city of Rome, a cyclic pattern of swamp/estuarine clays and bay-head delta sands composes the post-glacial Tiber incised-valley fill (Milli, 1997; Amorosi and Milli, 2001). Finally, 30 km north of Naples, supporting evidence for another incised-valley system is given by a thick late Quaternary succession made up of alternating coastal plain and estuarine clays beneath the present Volturno coastal plain (D. Ruberti, pers. comm. 2009). This suggests that a stratigraphic pattern of coalescing valley fills can presumably tracked along the Tyrrhenian coast for a total length of at least 500 km.

6. Conclusions Several previous papers have documented the occurrence of incised-valley fills within the sedimentary record, providing detailed documentation of their internal architecture. As for Quaternary examples, very high-resolution seismic profiling has provided new insights into the geometry of buried valley bodies. Few studies, however, have dealt with synchronous valley incision during the last glacial/interglacial cycle as deduced from borehole data. Close examination of a valuable stratigraphic dataset beneath the Tuscan coastal plain penetrating the post-LGM succession shows remarkable thickness variations and abrupt lateral facies changes, which indicate the presence of three coexisting, simple incised-valley systems of latest Pleistocene to early Holocene age, oriented roughly perpendicular to the present shoreline. Application of sequence-stratigraphic analysis through identification of significant stratigraphic surfaces (unconformities and flooding surfaces) enables interpretation of depositional-facies origin and distribution of the post-LGM succession in the study area.

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Despite contrasting facies characteristics, due almost uniquely to comparison of different valley segments, the three valley fills share a common depositional style, including a transgressive succession of inner- to outer-estuary facies above lowstand/early transgressive fluvial deposits. Where high-resolution core descriptions are available, including micropalaeontological and pollen characterization, a hierarchy of bounding-surfaces can be used to delineate depositional cycles of lower rank (parasequences) within the valley bodies. The vertical succession of facies suggests that the three valleys experienced a common depositional history, from time of formation to their ultimate filling. Fluvial incisions were produced near or during the LGM, while wave-dominated estuaries developed into the valley systems with the ensuing, post-LGM phase of sea-level rise. The three valley fills keep record of very high sedimentation rates, of about 1 cm/y. Contrary to the most common models of incised-valley filling, sediment accumulation in the Tuscan valleys kept pace with sea-level rise during the early stages of transgression, leading to a distinctive aggradational (rather than backstepping) facies architecture within the valley bodies. Acknowledgements This work was supported by the Municipality of Pisa in the form of a grant to Giovanni Sarti. The authors thank two anonymous reviewers for insightful critiques that improved the manuscript. References Aguzzi, M., Amorosi, A., Sarti, G., 2005. Stratigraphic architecture of late Quaternary deposits in the lower Arno Plain (Tuscany, Italy). Geologica Romana 38, 1e10. Aguzzi, M., Amorosi, A., Colalongo, M.L., Ricci Lucchi, M., Rossi, V., Sarti, G., Vaiani, S.C., 2007. Late Quaternary climatic evolution of the Arno coastal plain (Western Tuscany, Italy) from subsurface data. Sedimentary Geology 211, 211e229. Allen, G.P., Posamentier, H.W., 1993. Sequence stratigraphy and facies model of an incised valley fill: the Gironde Estuary, France. Journal of Sedimentary Petrology 63, 378e392. Amorosi, A., Milli, S., 2001. Late Quaternary depositional architecture of Po and Tevere river deltas (Italy) and worldwide comparison with coeval deltaic successions. Sedimentary Geology 144, 357e375. Amorosi, A., Sarti, G., Rossi, V., Fontana, V., 2008. Anatomy and sequence stratigraphy of the late Quaternary Arno valley fill (Tuscany, Italy). In: Amorosi, A., Haq, B.U., Sabato, L. (Eds.), Advances in Application of Sequence Stratigraphy in Italy. GeoActa, Special Publication, vol. 1, pp. 55e66. Amorosi, A., Ricci Lucchi, M., Rossi, V., Sarti, G., 2009. Climate change signature of small-scale parasequences from Lateglacial-Holocene transgressive deposits of the Arno valley fill. Palaeogeography, Palaeoclimatology, Palaeoecology 273, 142e152. Anderson, J.B., Rodriguez, A.B., 2008. Response of Gulf Coast Estuaries to Sea-Level Rise and Climate Change. Geological Society of America. Special Paper 443. Argnani, A., Bernini, M., Di Dio, G.M., Papani, G., Rogledi, S., 1997. Stratigraphic record of crustal scale tectonics in the Quaternary of the Northern Apennines (Italy). Il Quaternario 10, 595e602. Bard, E., Hamelin, B., Arnold, M., Montaggioni, L., Cabioch, G., Faure, G., Rougerie, F., 1996. Deglacial sea-level record from Tahiti corals and the timing of global meltwater discharge. Nature 382, 241e244. Bellotti, P., Chiocci, F.L., Milli, S., Tortora, P., Valeri, P., 1994. Sequence stratigraphy and depositional setting of the Tiber delta: integration of high-resolution seismics, well logs, and archeological data. Journal of Sedimentary Research 64, 416e432. Bellotti, P., Milli, S., Tortora, P., Valeri, P., 1995. Physical stratigraphy and sedimentology of the Late Pleistocene-Holocene Tiber Delta depositional sequence. Sedimentology 42 (4), 617e634. Bellotti, P., Caputo, C., Davoli, L., Evangelista, S., Garzanti, E., Pugliese, F., Valeri, P., 2004. Morpho-sedimentary characteristics and Holocene evolution of the emergent part of the Ombrone River delta (southern Tuscany). Geomorphology 61, 71e90. Bergamin, L., Di Bella, L., Frezza, V., Devoti, S., Nisi, M.F., Silenzi, S., Carboni, M.G., 2006. Late Quaternary palaeoenvironmental evolution at the Versilian coast (Tuscany, Italy): micropalaentological proxies. In: Coccioni, R., Marsili, A. (Eds.), Proceedings of the Second and Third Italian Meetings on Environmental Micropaleontology. Grzybowski Foundation, Special Publications, vol. 11, pp. 1e11.

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