Elsevier

Sedimentary Geology

Volume 209, Issues 1–4, 1 September 2008, Pages 58-68
Sedimentary Geology

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

https://doi.org/10.1016/j.sedgeo.2008.06.010Get rights and content

Abstract

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.

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, 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'Dondi et al., 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, ENI — AGIP, 1998, Regione Lombardia, E, 2002). This has led to internal subdivision of the Pliocene–Quaternary succession of the Po Basin into six unconformity-bounded stratigraphic units (UBSU in the sense of Salvador, 1987, Salvador, 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, ENI — AGIP, 1998, Regione Lombardia, E, 2002). This surface, which is close to the Matuyama-Brunhes 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 m-long 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.

Section snippets

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

Stratigraphy of Core MN1

The sedimentological study of Core MN1 shows that the middle-upper 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).

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

Palaeoclimatic evolution

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

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

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

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

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

Acknowledgements

Bologna University provided funding as part of Progetto Strategico d'Ateneo (Co-ordinator: A. Amorosi). We are grateful to R. Westaway and an anonymous reviewer for their accurate reviews. We also thank Editor C. Fielding for his critical and very helpful comments.

References (72)

  • EmbryA.F.

    Sequence boundaries and sequence hierarchies: problems and proposals

  • HolbrookJ. et al.

    Geomorphic and sedimentary response of rivers to tectonic deformation: a brief review and critique of a tool for recognizing subtle epeirogenic deformation in modern and ancient settings

    Tectonophysics

    (1999)
  • KomatsubaraJ.

    Fluvial architecture and sequence stratigraphy of the Eocene to Oligocene Iwaki Formation, northeast Japan: channel-fills related to the sea-level change

    Sediment. Geol.

    (2004)
  • LewisS.G. et al.

    The fluvial system response to abrupt climate change during the last cold stage: the Upper Pleistocene River Thames fluvial succession at Ashton Keynes, UK

    Glob. Planet. Change

    (2001)
  • López-BlancoM. et al.

    Transgressive–regressive sequence hierarchy of foreland, fan-delta clastic wedges (Montserrat and Sant Llorenç del Munt, Middle Eocene, Ebro Basin, NE Spain)

    Sediment. Geol.

    (2000)
  • López-BlancoM. et al.

    Anatomy of regressive tracts in a regressive sequence set: Vilomara unit, Sant Llorenç del Munt, Ebro Basin, NE Spain

    Sediment. Geol.

    (2000)
  • MacklinM.G. et al.

    Correlation of fluvial sequences in the Mediterranean basin over the last 200 ka and their relationship to climate change

    Quat. Sci. Rev.

    (2002)
  • MarzoM. et al.

    Unusual features of sediment supply-dominated, transgressive–regressive sequences: Paleogene clastic wedges, SE Pyrenean foreland basin, Spain

    Sediment. Geol.

    (2000)
  • OriG.G.

    Continental depositional systems of the Quaternary of the Po Plain (northern Italy)

    Sediment. Geol.

    (1993)
  • RavazziC.

    Late Quaternary history of spruce in southern Europe

    Rev. Palaeobot. Palynol.

    (2002)
  • SteelR.J. et al.

    Anatomy of high-sediment supply, transgressive tracts in the Vilomara composite sequence, Sant Llorenç del Munt, Ebro Basin, NE Spain

    Sediment. Geol.

    (2000)
  • TakanoO. et al.

    Sequence stratigraphic architecture of a differentially subsiding bay to fluvial basin: the Eocene Ishikari Group, Ishikari Coal Field, Hokkaido, Japan

    Sediment. Geol.

    (2003)
  • TzedakisP.C. et al.

    Last Interglacial conditions in southern Europe: evidence from Ioannina, northwest Greece

    Glob. Planet. Change

    (2003)
  • VandenbergheJ.

    Climate forcing of fluvial system development: an evolution of ideas

    Quat. Sci. Rev.

    (2003)
  • ZagwijnW.H.

    An analysis of Eemian climate in western and central Europe

    Quat. Sci. Rev.

    (1996)
  • AdamsM.M. et al.

    No change in fluvial style across a sequence boundary, Cretaceous Blackhawk and Castlegate formations of Central Utah, U.S.A.

    J. Sediment. Res.

    (2005)
  • AguzziM. et al.

    Stratigraphic architecture of Late Quaternary deposits in the lower Arno Plain (Tuscany, Italy)

    Geol. Rom.

    (2005)
  • AmorosiA. et al.

    Large-scale architecture of a thrust-related alluvial complex from subsurface data: the Quaternary succession of the Po Basin in the Bologna area (northern Italy), Gior

    Geol.

    (1995)
  • AmorosiA. et al.

    The linkage between alluvial and coeval nearshore marine succession: evidence from the Late Quaternary record of the Po River Plain, Italy

  • BellottiP. et al.

    Sequence stratigraphy and depositional setting of the Tiber delta: integration of high-resolution seismics, well logs, and archaeological data

    J. Sediment. Res.

    (1994)
  • BlumM.D.

    Genesis and architecture of incised valley fill sequences: a Late Quaternary example from the Colorado River, Gulf Coastal Plain of Texas

  • BlumM.D. et al.

    Fluvial response to climate and sea-level change: a review and look forward

    Sedimentology

    (2000)
  • BriantR.B. et al.

    Climatic control on Quaternary fluvial sedimentology of a Fenland Basin river, England

    Sedimentology

    (2005)
  • BurnsB.A. et al.

    Fluvial response in a sequence stratigraphic framework: example from the Montserrat fan delta, Spain

    J. Sediment. Res.

    (1997)
  • DallaS. et al.

    Late Eocene-Tortonian tectono-sedimentary evolution in the western part of the Padan basin (northern Italy)

    Paleontol. Evol.

    (1992)
  • DondiL. et al.

    La Pianura Padana e Veneta dall'Oligocene superiore al Pleistocene

    G. Geol.

    (1986)
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