Climatic signature of cyclic fluvial architecture from the Quaternary of the central Po Plain, Italy

Sedimentary Geology 209 (2008) 58–68

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Climatic signature of cyclic fluvial architecture from the Quaternary of the central Po Plain, Italy Alessandro Amorosi a,⁎, Marta Pavesi a, Marianna Ricci Lucchi a, Giovanni Sarti b, Andrea Piccin c a b c

Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Via Zamboni 67, 40127, Bologna, Italy Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria 53, 56126 Pisa, Italy Regione Lombardia, D.G. Territorio e Urbanistica, Struttura Sistema Informativo Territoriale, via Sassetti 32, 20124 Milano, Italy

a r t i c l e

i n f o

Article history: Received 20 July 2007 Received in revised form 5 June 2008 Accepted 20 June 2008 Keywords: Fluvial sequence stratigraphy Climate change Pollen Quaternary Po Plain

a b s t r a c t Detailed investigation of middle-upper Quaternary deposits from central Po Plain was carried out on the basis of integrated sedimentological and pollen analyses of a 114 m-long core, and stratigraphic correlations of well data. Stratigraphic architecture of alluvial strata beneath modern Po River reveals distinctive cyclic changes in lithofacies and channel stacking patterns. Each cycle includes basal, silt–clay overbank deposits with thin and lenticular fluvial-channel sands, with upward transition to increasingly amalgamated and more laterally extensive fluvial-channel sand bodies. Lower cycle boundaries, corresponding to the top of laterally extensive fluvial complexes, are locally associated with organic-rich, paludal clays. Pollen records show distinctive cyclic changes that parallel facies architecture. Overbank deposits in the lower parts of cycles are invariably associated with forest expansions. These indicate that major phases of channel abandonment and widespread floodplain aggradation took place at the onset of warm-temperate (interglacial) climatic conditions. The middle portions of cycles record transition to pollen associations diagnostic of glacial periods. Lack of pollen data from sandy facies prevents climatic characterization of laterally extensive fluvial bodies in the upper parts of cycles. On the basis of the overall stratigraphic framework, internal facies architecture and distinctive pollen attributes, the cycles identified in the subsurface of the study area are interpreted to fall in the Milankovitch (100 ka) band, and inferred to be correlative with the transgressive–regressive sequences recently recognized in the coeval successions of the Po coastal plain. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Understanding stratigraphic architecture and sediment-body geometries beneath modern alluvial and coastal plains can help significantly in interpreting spatial distribution of aquifers and aquifer systems. Specifically, the application of sequence-stratigraphic techniques to cyclic fluvial successions of Quaternary age can be used for delineating distribution of aquifer and aquitard sediments, and thus reservoir geometry. Repetitive or cyclic facies patterns within fluvial deposits have been widely described in previous work, and several studies have emphasized a possible allocyclic control on fluvial architecture. However, deciphering the roles played by climate, tectonics and eustacy is generally a very difficult task, owing to objective problems in isolating each factor from the others. Recent studies have shown that climate (Blum, 1993; Blum et al., 1994; Fielding and Webb, 1996; Legarreta and Uliana, 1998; Milana,

⁎ Corresponding author. Fax: +39 051 2094522. E-mail addresses: [email protected] (A. Amorosi), [email protected] (M. Pavesi), [email protected] (M. Ricci Lucchi), [email protected] (G. Sarti). 0037-0738/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2008.06.010

1998; Bridgland, 2000; Aqrawi, 2001; Lewis et al., 2001; Macklin et al., 2002; Antoine et al., 2003; Vandenberghe, 2003; Briant et al., 2005; Kasse et al., 2005) and tectonics (Leeder, 1993; Martinsen et al., 1999; Holbrook and Schumm, 1999; Marzo and Steel, 2000; Vincent, 2001; Adams and Bhattacharya, 2005; Hickson et al., 2005) can be generally regarded as the major controlling factors of fluvial architecture. On the other hand, it has been documented that base-level changes may affect fluvial systems several tens of km, and in some instances few hundreds of km, updip from the shoreline (Schumm, 1993, Leeder and Stewart, 1996; Blum and Törnqvist, 2000; Cattaneo and Steel, 2003; Holbrook et al., 2006). Climate and tectonics are thought to become increasingly important in inland areas (Shanley and Mc Cabe, 1994). Prominent cyclic facies architecture is the dominant feature of the Quaternary alluvial to coastal infill of the Po River Basin (Fig. 1), a rapidly subsiding foreland basin bounded by the Alps to the North and the Apennines to the South (Amorosi and Colalongo, 2005). Subsurface geology of the Po Plain has been widely described on the basis of seismic data (Pieri and Groppi, 1981; Dondi and D'Andrea, 1986; Dalla et al., 1992; Muttoni et al., 2003), and basin geometry depicted through integration of seismic studies with well-log interpretations (Regione Emilia-Romagna and ENI — AGIP, 1998; Regione Lombardia and ENI Divisione AGIP, 2002). This has led to internal subdivision

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Fig. 1. Location of the study area, with position of Core MN1 (see Fig. 3) and section traces of Figs. 2, 5 (AA′) and 6 (BB′).

of the Pliocene–Quaternary succession of the Po Basin into six unconformity-bounded stratigraphic units (UBSU in the sense of Salvador, 1987, 1994). Each unconformity marks a phase of drastic basin reorganization, mainly due to intense tectonic activity. The uppermost UBSU in the Po Basin is termed the “Emilia-Romagna Supersynthem” (Regione Emilia-Romagna and ENI — AGIP, 1998), and includes deposits of middle-late Pleistocene and Holocene age. Stratigraphic correlations between the southern (Emilia-Romagna) and northern (Lombardy) portions of the Po Basin are presently in progress, as part of the Geological Mapping Protocol of Italy at 1:50,000 scale, and the “Emilia-Romagna Supersynthem” has been provisionally referred to as “Po Supersynthem”, in order to encompass the entire Po Basin. This unit exhibits a maximum thickness of 800 m beneath the modern Adriatic coastal plain, and typically wedges out toward the basin margins, i.e. the Apenninic and Alpine foothills (Fig. 2). The lower boundary of Po Supersynthem (red line in Fig. 2) is a major unconformity surface that has been seismically mapped throughout the basin (Regione Emilia-Romagna and ENI — AGIP, 1998; Regione Lombardia and Eni Divisione Agip, 2002). This surface, which is close to the MatuyamaBrunhes reversal on the basis of magnetostratigraphic data, has been recently dated at 0.87 Ma BP (“R” surface of Muttoni et al., 2003). Identification and lateral tracing within the Po Supersynthem of a minor regional unconformity (boundary between Lower and Upper Emilia-Romagna Synthems in Regione Emilia-Romagna and ENI — AGIP, 1998; boundary between Aquifer Groups A and B in Regione Lombardia and ENI Divisione AGIP, 2002) has led to its subdivision into two lower-rank units, namely Lower and Upper Po Synthems (Fig. 2). Each synthem can be thought of as having about four vertically stacked subsynthems, which define geologically and hydraulically distinct aquifers, each a few tens of metres thick (Fig. 2). Tectonics may locally affect this stratigraphic framework, resulting locally in significant thickness variations. Close to the basin margin, stratigraphic architecture is dominated by amalgamated alluvial-fan gravel bodies, passing in distal locations into alternating gravel (fluvial-channel) and predominantly muddy (overbank) sediment bodies (Ori, 1993; Amorosi and Farina, 1995; Amorosi et al., 1996). In this paper we document the cyclic stacking pattern of facies that characterizes subsurface stratigraphy in the basin depocentre, beneath modern Po River, in southern Lombardy (Fig. 1). The highest

net-to-gross sand ratios are recorded in this part of the basin, thus representing a high potential interest for water research. We first describe fluvial facies characteristics and the pollen record of a 114 mlong Quaternary core (Core MN1 in Fig. 1) that was drilled in 2004 close to the town of Mantua (Southern Lombardy). Then, we outline how these data can be assembled with stratigraphic correlations from water wells to create a sequence-stratigraphic model of fluvial response to climatic and eustatic fluctuations. 2. Methods The data set consists of one continuously-cored borehole (Fig. 1), 114 deep (Core MN1), and poor-quality stratigraphic data from hundreds of water wells. Sediment colour and texture, vertical lithofacies relationships, and the type and concentration of accessory materials, including roots, plant and wood fragments, bioturbation, organic matter, and palaeosols were used as basic tools for facies interpretation. Primary sedimentary structures, especially bedding geometries, are poorly preserved in coarse-grained deposits. Pollen analysis was carried out on fine-grained fractions of the sediment core, in order to reconstruct vegetational and palaeoclimate history of Quaternary stratigraphic units (Moore et al., 1991). A total of 32 samples were analysed for pollen. About 2 g of dry sediment were treated with HCl (20%), HF (40%) and hot NaOH (10%). Pollen concentration was calculated by adding tablets with a known number of Lycopodium spores to a specific weight of sediment. An average of 250 pollen grains per sample was counted. Pollen preservation was generally good in clay samples, while silts and silty sands were barren. Pollen data were assembled in a synthetic percentage diagram, with arboreal pollen (AP) split into five groups, and non-arboreal pollen (NAP) into three (Table 1). Among AP, (i) Quercus-group includes humidity- and warmth-loving components of deciduous broad-leaf forests dominated by oak; (ii) Mountain trees are represented by altitudinal taxa living at present at altitudes N1500 m under cool–wet climate conditions; (iii) Alnus-group is represented by riparian trees, such as Alnus (dominant) and Salix, which grow on frequently flooded soils in proximity to river channels; (iv) Pinus is represented by undifferentiated species of pine; and (v) Pioneer shrubs include taxa (Ephedra, Juniperus and Hippophae) that do not withstand competition

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Fig. 2. Generalized stratigraphy of Po Supersynthem, with its subdivision into Lower and Upper Po Synthems (from Regione Emilia-Romagna and ENI — AGIP, 1998), showing in turn their internal subdivision into subsynthems. Aquifers, corresponding to the coarse-grained portions of individual subsynthems, are coloured in orange and brown. The red line marks the lower boundary of Po Supersynthem. Dashed blue line: freshwater/brackish water boundary. Blue colour: aquifer saturated with brackish waters. For section trace, see Fig. 1.

with other trees, and therefore are considered as important elements of pioneer glacial vegetation. Non-arboreal elements include (i) ubiquitous herbs with various ecological and climate demands; (ii) Poaceae, typically dominating in humid grasslands; and (iii) Artemisiagroup, which includes taxa that withstand dry and cold conditions typical of steppic environments. 3. Stratigraphy of Core MN1 The sedimentological study of Core MN1 shows that the middleupper Quaternary succession of central Po Basin consists entirely of alluvial plain deposits. These include a cyclic alternation of two major facies associations: fluvial-channel and overbank deposits (Fig. 3).

Table 1 Composition of arboreal and non-arboreal pollen groups

AP

Groups

Components

Quercus-group

Acer, Betula, Buxus, Carpinus betulus, Corylus, Fraxinus, Ostrya, Quercus, Quercus ilex, Tilia, Ulmus, Zelkova Abies, Picea, Fagus Alnus, Salix Pinus undiff. Ephedra cf. distachya, Ephedra cf. fragilis, Hippophae cf. rhamnoides, Juniperus type Apiaceae, Asteraceae/Asteroideae, Asteraceae/Cichorioideae, Campanula, Caryophyllaceae, Centaurea, Cruciferae, Ericaceae, Fabaceae, Filipendula, Geranium, Helianthemum, cf. Lamium, Liliaceae, cf. Linaria, Plantago, Primulaceae, Ranunculaceae, Rosaceae, Rubiaceae (Galium), Rumex, Thalictrum, cf. Thymus Poaceae undiff. Artemisia, Chenopodiaceae

Mountain trees Alnus-group Pinus Pioneer shrubs NAP Ubiquitous herbs

Poaceae Artemisia-group

3.1. Fluvial-channel facies association 3.1.1. Description This facies association is 3 to 20 m thick, and consists predominantly of medium to coarse sand that is moderately well sorted. Gravelly sand was encountered between 5 and 10 m core depth. Individual sand bodies display an erosional lower boundary (Fig. 4A), with a distinctive fining-upward (FU) tendency. Vertically amalgamated, multi-storey bodies, with multiple thinner FU sequences, are recorded between 52 and 73 m core depth. Near the base of sets, the sand is coarser and may contain abundant granules. Sedimentary structures include unidirectional, high-angle cross-bedding (Fig. 4B) and sub-horizontal bedding. Foresets may be planar as well as troughshaped. Silt and clay intercalations are present, but largely subordinate. Wood fragments, up to 30 cm thick (Fig. 4B) are locally dispersed in the lower part of this facies association. The upper boundary of this facies association with the overlying mud is either sharp or gradational. Organic-rich layers commonly cap the FU successions. No fossils were observed within this facies association. 3.1.2. Interpretation The erosional lower boundary of this facies association, combined with its thickness and fining-upward trend, are characteristic features of fluvial-channel deposits. This interpretation is also supported by the presence of unidirectional flow structures and abundance of floated wood within bar deposits. The either sharp or gradational boundary to the overlying mud-prone facies association is interpreted to reflect either abrupt or gradual channel abandonment, respectively. The common presence of organic layers on top of fluvial sands indicates the development of standing bodies of water in paludal environments (swamps), following channel abandonment.

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Fig. 3. Facies interpretation and pollen record of Core MN1 (for location, see Fig. 1). AP: Arboreal pollen; NAP: non-arboreal pollen.

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Fig. 4. Representative photographs of Core MN1. Bottom = lower left angle. (A): Erosional boundary (in red) between floodplain deposits and fluvial-channel sand. Core depth: 68–76 m. (B): Erosional lower boundary of a fluvial-channel body (in red), showing high-angle cross-bedding and wood debris in the basal part of the channel fill. Core depth: 86–93 m. (C): Characteristic features of overbank (floodplain and levee) deposits. Core depth: 95–100 m. (D): Organic-rich, swamp clay. Core depth: 44–47 m.

3.2. Overbank facies association 3.2.1. Description This facies association is dominated by a monotonous succession of rooted and pedogenically modified silts and clays, with subordinate sand intercalations (Fig. 4C). Mud is dark grey and predominantly structureless, with faint horizontal lamination. Clays with fine disseminated plant debris and peat horizons are present at distinct stratigraphic levels (Fig. 4D). Freshwater gastropods are locally encountered. Alternating very fine sand and silt on a cm-scale is also common. Sand-mud contacts can be sharp as well as gradational, and locally wavy. Major sedimentary structures include horizontal lamination and subordinate, small-scale cross lamination. Fine to very fine sand bodies are occasionally present. These are generally less than 1 m thick and display sharp lower boundaries and internal fining-upward trends. Coarsening-upward successions are occasionally encountered. 3.2.2. Interpretation Massive and pedogenically modified silt and clay deposits point to low-energy depositional environments dominated by fallout or weak traction processes, with phases of subaerial exposure. These features suggest deposition in a floodplain and this facies association is believed to predominantly represent overbank deposits. Because of their lithologic characteristics and their stratigraphic position, very close to fluvial-channel deposits (Fig. 3), sand–silt couplets are likely to represent natural levee deposits. Sediment bodies with sharp lower boundary and internal fining-upward trends are interpreted to represent crevasse-channel deposits, while coarsening-upward successions with transition to the underlying mud could be argued to represent crevasse splays. Crevasse-channel sand bodies are tenta-

tively distinguished from their fluvial counterparts by their lesser thickness and grain size. 4. Pollen data The pollen record of MN1 is discontinuous, owing to the presence of four barren intervals that coincide with major fluvial-channel sand bodies (Fig. 3). The lowermost interval suitable for pollen analysis, between about 105 and 91 m core depth, shows high pollen concentration and the dominance of arboreal pollen (AP), mostly represented by the termophilous (warmth-loving) taxa of Quercus-group within alternating floodplain and levee deposits. Alnus-group, which includes riparian trees growing at present on levees or in frequently flooded areas near the river channel, is abundant throughout this interval and partially masks the actual diffusion of other temperate trees (Quercusgroup). Non-arboreal pollen (NAP) is very scarce and represented chiefly by Poaceae, which at present preferably live in humid alluvial plain and swamp environments. Higher up in the stratigraphic column, between 80 and 73 m core depth, the pollen record within swamp/floodplain clays displays remarkably different characteristics. Pollen spectra are dominated by Pinus (about 60% on average), with significant quantities of other mountain conifers (mainly Picea and Abies). Quercus-group is present with very low percentages (b5%), and the total pollen concentration is very low. The highest AP values (90–95%), along with the highest total pollen concentration of the entire record, are from between 46.50 and 45.50 m core depth, coinciding with organic-rich deposits that overlie a 20 m thick fluvial-channel body. Within this stratigraphic interval, the termophilous and humidity-loving trees of Quercus- and Alnusgroup are dominant and show an upward increasing trend, which is

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paralleled by a reduction in pollen of Pinus and mountain trees. A sharp change in the pollen record is observed around 45 m, where total concentration values decrease and pollen spectra are characterized by Pinus and mountain trees, with strongly subordinate Quercusgroup. Alnus-group peaks at 43.50 m core depth, corresponding to levee deposits. The remaining pollen record is characterized by low pollen concentration and high percentages of Pinus and NAP (mainly Poaceae and Artemisia). However, on the basis of minor differences in pollen composition, the upper part of Core MN1 can be subdivided in two intervals: a lower interval (between 31.50 and 27.50 m core depth), with high percentages of Poaceae, indicating proximity to humid environments, and an upper interval (between 27–25 m and 21–17 m core depths), with higher values of Pinus, mountain trees (Picea), pioneer shrubs (Hippophae cf. rhamnoides) and Quercus-group. Quercus-group is here represented almost entirely by Betula, with a significant peak (38%) at 20.90 m core depth, while termophilous taxa like Quercus, Corylus, Tilia and Ulmus are sporadic or absent.

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Hippophae rhamnoides (among pioneer shrubs) is interpreted to represent the onset of interstadial climatic conditions during MIS 3. We do not have definitive data for precise chronologic attribution of the warm-temperate interval between 46.50 and 45.50 m. Pollen data, however, may help to further constrain its age attribution. In particular, the rapid initial development of warm-temperate forests, followed by the expansion of mountain trees indicative of late cooler climate conditions, can be regarded as a peculiar feature of Eemian vegetation dynamics, similarly to what has been reported from several pollen series of Europe (Follieri et al., 1988; De Beaulieu and Reille, 1992; Tzedakis, 1993; Zagwijn, 1996). In this respect, the 46.50–45.50 interval is likely to reflect the Last Interglacial (MIS 5e). This interpretation is consistent with a stratigraphic position below the glacial deposits assigned to the last pleniglacial. As a consequence, cold climate vegetation recorded at 80–73 m and warm-temperate vegetation at 105–91 could reflect the two previous glacial (MIS 6) and interglacial (MIS 7) episodes, respectively (Fig. 3).

5. Palaeoclimatic evolution

6. Cyclic fluvial architecture in the central Po Basin and sequence-stratigraphic interpretation of Core MN1

Pollen spectra from Core MN1 suggest that vegetation dynamics in the central Po Plain fluctuated continuously during middle-late Quaternary, from forest development (documented by repeated AP expansions) to development of shrubby–herbaceous communities, with forest retreat (indicated by Pinus-NAP expansions). Two major warm-temperate phases, marked by forest development, are highlighted in Fig. 3. The older warm-temperate phase, which is recorded within overbank deposits between about 105 and 91 m core depth, shows vegetation features typical of an interglacial period, such as dominance of Quercus-group elements, low Pinus and high total pollen concentration. The younger warm-temperate phase, recorded within swamp/floodplain deposits between 46.50 and 45.50 m core depth, is characterized by well-developed forest vegetation, consisting predominantly of deciduous broad-leaved trees, also distinctive of an interglacial. This second warm phase occurred after the onset of a glacial period, which is documented by the characteristic pollen signature of swamp/overbank deposits at 80–73 m core depth. Glacial conditions are here testified by i) abundance of Pinus, mountain trees and pioneer shrubs, ii) significant occurrence of NAP, and iii) very low total pollen concentration, all representative of a scarce open-vegetation cover, with scattered coniferous forests. Despite the possibility of long-distance transport for coniferous taxa like Pinus, Picea and Abies, living at present at higher altitudes, it is more plausible that these trees survived during glacials in scattered, small populations in lowland areas close to the mountains, where they found favourable moisture conditions (Ravazzi, 2002). The pollen record around 45 m core depth provides indication of a second phase of gradual climate cooling. This is shown by the abrupt decline of the deciduous broad-leaved forests, concurrently with a remarkable decrease in total pollen concentration. The re-expansion of Pinus, mountain trees and NAP point to a cool-temperate, humid climate typical of either the end of an interglacial or the onset of a glacial (Guiot et al., 1989; De Beaulieu and Reille, 1992; Zagwijn, 1996; Tzedakis et al., 2003). The increase in steppic NAP, particularly Poaceae and Artemisia, recorded above 32 m core depth, indicates openvegetation development and transition to cooler and drier, full-glacial climate conditions. The remarkable presence of Picea (see mountain trees between 27 and 25 m, in Fig. 3), the maximum development of which in southern Europe characterizes the middle pleniglacial interstadials (55–35 kyr cal BP — Ravazzi, 2002), concurrently with a radiocarbon date of N45 kyr obtained at this stratigraphic level, suggest attribution of this interval to the last pleniglacial (MIS4). Taking into account the 14C age of 38,600 ± 1050 yr BP obtained around 19 m core depth, the spreading from about 21 m of pioneer vegetation represented mainly by Betula (among Quercus-group) and

Although stratigraphic information from water wells typically is imprecise and in places inaccurate, the large number of available records and their calibration with Core MN1 allow derivation of the major facies associations that characterize stratigraphic architecture in the study area. Despite difficulties in extrapolating results from a single site to the basin scale, generalizations seem possible based on overall sedimentological characteristics of the alluvial system, which do not display significant changes throughout the study area. Two regional, volumetrically significant facies associations form the basic motif of Quaternary cyclicity in the Po Basin (Figs. 5 and 6): floodplain muds with subordinate sand bodies, typically representing an overbank association, and medium to coarse sand bodies, interpreted as a fluvial-channel association. Levee and crevasse deposits are not detected in the data from the water wells. Correlation of numerous water well records documents that fluvial-channel bodies have considerable continuity and an overall sheet-like geometry, their horizontal dimensions greatly exceeding the vertical dimension (Figs. 5 and 6). The thickness of these sedimentary bodies displays remarkably homogeneous values. The surfaces chosen for our stratigraphic subdivisions are placed at the generally abrupt transition from laterally extensive coarse-grained bodies to overlying, organic-rich mudstones (Figs. 5 and 6). This subdivision, which leads to identification of stratigraphic units with a lower, mudstone-dominated portion, and an upper interval characterized by increasing clustering of sand bodies, is here preferred from an operational point of view to depositional cycles bounded by channelfill sand facies. Specifically, the tops of amalgamated fluvial-channel bodies can be physically traced throughout the basin and represent comparably smoother (and more easily mappable) surfaces than lower bounding surfaces. The latter show evidence of incision of up to several metres into the underlying substrate, and thus appear considerably more irregular (Figs. 5 and 6). Organic-rich layers are commonly encountered in lower parts of cycles (see peat distribution in Figs. 5 and 6). Calibration of fluvial-channel/overbank cyclicity (Figs. 5 and 6) with pollen data from Core MN1 (Fig. 3) provides the basis for climatic characterization of middle-late Quaternary sedimentation in the central Po Basin. Specifically, the laterally extensive mud-prone intervals identified in the lower parts of cycles at 30–40 m and 90– 100 m below sea level (Figs. 5 and 6), can readily be correlated in Core MN1 to the major two expansions of temperate forests (e.g. peaks in AP within overbank deposits around 45 and 100 m core depth, respectively — Fig. 3). This peculiar pollen signature suggests that generalized channel abandonment and widespread floodplain aggradation in the study area took place concurrently with the development

64 A. Amorosi et al. / Sedimentary Geology 209 (2008) 58–68 Fig. 5. Geologic cross-section, showing cyclic facies architecture in the study area (see Fig. 1, for location). Heavy stippled lines represent regionally extensive bounding surfaces of transgressive–regressive (T–R) sequences. The boundary between Lower Po Synthem and Upper Po Synthem is taken from Regione Lombardia and ENI Divisione Agip (2002).

A. Amorosi et al. / Sedimentary Geology 209 (2008) 58–68 Fig. 6. Geologic cross-section, showing cyclic facies architecture in the study area (see Fig. 1, for location). Keys as in Fig. 6. The boundary between Lower Po Synthem and Upper Po Synthem is taken from Regione Lombardia and ENI Divisione Agip (2002).

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of warm-temperate climatic phases, i.e. the onset of interglacial conditions. The middle portions of cycles, instead, display pollen associations diagnostic of glacial conditions. Lack of pollen data from the laterally extensive fluvial-channel bodies in the upper parts of cycles prevents climatic characterization of coarse-grained fluvial sedimentation. The fluvial cycles described in this paper closely resemble the transgressive–regressive facies tracts identified by Burns et al. (1997), López-Blanco et al. (2000a,b) and Marzo and Steel (2000) within the Eocene fan-delta complexes of Spain. In terms of sequence stratigraphy these cycles are consistent with the conceptual model of transgressive–regressive (T–R) sequences of Embry (1993, 1995). Transgressive overbank intervals passing upwards to amalgamated fluvial-channel bodies have been described from the Eocene Escanilla Fm. (Kjemperud et al., 2004) and the Cretaceous Blackhawk–Lower Castlegate Fm. (Van Wagoner, 1995; Adams and Bhattacharya, 2005). In the Middle Castlegate Fm., fourth-order T–R cycles are revealed by the repeated vertical stacking of retrogradational estuarine, centralbasin, estuarine-fill and fluvial-channel belt deposits (McLaurin and Steel, 2000). A similar facies architecture of Quaternary deposits has also been documented from the Tyrrhenian coast of Italy, beneath Arno (Aguzzi et al., 2005, 2007) and Tiber (Bellotti et al., 1994) coastal plains. Recent sequence-stratigraphic work on fluvial successions has shown that in the landward reaches of the coastal plain the transgressive surface generally separates amalgamated channels below from more aggradational channels above (Wright and Marriott, 1993; Zaitlin et al., 1994; Olsen et al., 1995; Takano and Waseda, 2003; Komatsubara, 2004; Plink-Björklund, 2005). Following these models, we are inclined to interpret the thick overbank units with ribbon-shaped sand bodies that overlie the sheet-like fluvial bodies as undifferentiated transgressive (TST) and highstand (HST) deposits. Their lower bounding surfaces, at 108 m and 48 m core depths, respectively, could thus represent an approximation of the transgressive surfaces (TS). The above interpretation is supported by the interglacial signature of the basal mud-prone intervals. Specifically, the characteristic pollen spectra observed few metres above the fluvial-channel/overbank transition, recording the rapid development of warm-temperate forests with maximum expansion of Quercus-group, display strong similarities with pollen associations reported from transgressive backbarrier and coastal deposits beneath the present Po coastal plain (Amorosi et al., 2004), which directly overlie the TS. This correlation further suggests that the overbank, organic-rich deposits at the base of our fluvial cycles may represent the landward equivalent of the TST. We do not have definitive data for the precise sequencestratigraphic interpretation of the laterally extensive sand bodies in the upper parts of cycles. The development of fluvial sedimentary packages during sea-level fall (FST) has been recently documented from highly subsiding basins (Blum and Törnqvist, 2000; Törnqvist et al., 2003; Amorosi et al., 2004; Amorosi and Colalongo, 2005; Blum and Aslan, 2006), and also based on experiments (Milana and Tietze, 2007; Swenson and Muto, 2007). In the case of central Po Plain, given the stratigraphic position of fluvial-channel bodies just below “transgressive” overbank deposits, and following the existing stratigraphic practice (e.g. Shanley and Mc Cabe, 1993; Olsen et al., 1995; Plint et al., 2001; Plink-Björklund, 2005), we are inclined to assign the thick fluvial bodies, showing the highest net-to-gross sand ratios, to the lowstand systems tract. It cannot be ruled out, however, that a significant portion of these fluvial-channel bodies could have formed during the early phases of transgression. 7. Factors controlling alluvial architecture in the Po Basin The striking regularity shown by middle-late Quaternary depositional cycles in the central Po Plain constitutes an intrinsic limitation

to the hypothesis of autocyclic processes (channel switching or avulsion) as the driving mechanism for cyclic facies architecture. The cycles identified in the subsurface of central Po Plain should thus be ascribed to an allocyclic control. The long-cored pollen series described in this paper, albeit discontinuous, shows that repeated alternation of overbank and fluvial facies is paralleled by a distinctive pollen signature, suggesting that T–R sequence development was intimately related to interglacial/glacial cycles. The laterally amalgamated fluvial-channel bodies (mobile channel-belts of Gibling, 2006), which may be several tens of km in width, are interpreted as complex systems of laterally migrating, braided- and low-sinuosity rivers that developed under conditions of increased sediment supply. Low accommodation during lowstand (and early transgressive?) phases favoured lateral migration of river channels, with widespread development of scour-and-fill episodes (Komatsubara, 2004). However, the multi-storey character of channelfill deposits of central Po Basin, which may exceed 30 m in composite thickness, suggests continuous creation of accommodation by tectonic subsidence, which thus played a fundamental role in shaping T–R sequences. The sharp transitions at cycle boundaries from sheet-like fluvialchannel bodies to organic-rich clays and their diagnostic pollen signal (sharp increase in arboreal pollen) may suggest that generalized development of paludal areas and poorly drained floodplains took place in the study area in response to rapid sea-level rise close to the onset of interglacial periods (TST). During these phases, river channels were probably essentially non-migrating, as suggested by the strongly lenticular, ribbon-shaped channel geometries (single ribbon-shaped or fixed channels of Gibling, 2006). Increased accommodation due to the combined effect of subsidence and sea-level rise led to widespread aggradation, and great volumes of sediments were stored in the floodplains (Steel et al., 2000). The progressive decrease up-section in the relative proportion of overbank fines, which is accompanied by an increase in the thickness of fluvial-channel units, could thus indicate decreasing accommodation during highstand conditions (HST). It is likely that tectonic subsidence was locally able to compensate for subsequent sea-level fall, allowing accommodation creation and accumulation of alluvial plain deposits even during the falling-stage or forced-regressive interval (see preservation of falling-stage deposits in Amorosi and Colalongo, 2005). In this period, tectonic subsidence was the key factor in controlling preservation of alluvial deposits. Although we have no direct means for accurately determining how much time is represented by each depositional cycle, the presence of eight distinct aquifers of approximately constant thickness (Fig. 2), bounded by an unconformity surface dated at 870 kyr BP and marking the onset of the first major Pleistocene glaciation in the Alps (Muttoni et al., 2003), suggests an average duration of about 100 kyr for each cycle. Evidence from pollen and sequence-stratigraphic interpretation, although restricted to a single core (MN1), is fully consistent with this framework, suggesting that these repetitive facies patterns could be linked to Milankovitch-band (100 ky) climatic cycles. Additional pollen series from coeval fluvial succession, however, should be investigated elsewhere in the Po Basin, in order to test this interpretation. 8. Conclusions A well-developed cyclic pattern of facies within middle-upper Quaternary deposits of central Po Plain reveals a characteristic climatic signature of alluvial architecture. Fluvial-channel deposits, 10 to 30 m thick, form regionally extensive sheet-like bodies showing a high degree of channel clustering. Laterally continuous overbank deposits, with a locally high organic content, cap the amalgamated sand bodies, which thus may serve as regionally significant aquifers. The abrupt transitions from laterally extensive coarse-grained bodies to overlying, organic-rich mudstones represent the most readily identifiable and easily mappable surfaces, and for this reason

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are taken as bounding surfaces of fluvial depositional cycles. The basal portions of these cycles display expansions of warm-temperate vegetation, which are diagnostic of interglacial conditions. On the basis of the overall stratigraphic architecture and diagnostic pollen signature, the laterally extensive mud-prone intervals in the basal portions of cycles are interpreted to represent the landward equivalent of transgressive deposits. This allows subdivision of alluvial deposits into vertically stacked, transgressive–regressive (T–R) sequences. Rapid sea-level rise at the onset of the interglacials resulted in fluvial-channel abandonment and widespread floodplain aggradation. On the other hand, increased channel clustering in the upper part of T–R sequences indicates decreasing accommodation and increasing sediment supply, which probably occurred close to the glacial/interglacial transition. 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