Millennial-scale depositional cycles from the Holocene of the Po Plain, Italy

Marine Geology 222–223 (2005) 7 – 18 www.elsevier.com/locate/margeo

Millennial-scale depositional cycles from the Holocene of the Po Plain, Italy A. Amorosi a,*, M.C. Centineo b, M.L. Colalongo a, F. Fiorini a a

Universita` di Bologna, Dipartimento di Scienze della Terra e Geologico-Ambientali, Via Zamboni 67-40127 Bologna, Italy b Servizio Geologico, Sismico e dei Suoli-Regione Emilia-Romagna, Viale Silvani 4/3, 40122, Bologna, Italy Accepted 15 June 2005

Abstract The Holocene depositional history of southeastern Po Plain on time scales of 103 yr is reconstructed, based upon integrated sedimentological and micropalaeontological analyses of nine continuously-cored boreholes, about 40 m deep. Major palaeoenvironmental changes include the rapid landward migration of a barrier-estuary–lagoon system during the Early–Middle Holocene (transgressive systems tract—TST), followed by extensive delta progradation in the last 6000 yr (highstand systems tract—HST). Detailed facies analysis of cores combined with the identification of 12 microfossils (benthic foraminifer and ostracod) associations allow an ultra-high-resolution sequence–stratigraphic framework to be reconstructed. Particularly, eight smallscale, high-frequency cycles, about 3–5 m thick and spanning intervals of time of about 1000 yr, can be physically traced throughout the study area. Interpretation of these cycles, which are invariably bounded by sharp flooding surfaces and generally show internal shallowing-upward trends (parasequences), indicates that relative sea-level changes during the Holocene were episodic and punctuated by rapid phases of sea-level rise, followed by periods of stillstand (or decreasing sea-level rise). From seaward to landward locations, parasequence boundaries document beach-barrier migration, bay-head delta abandonment and increasing accommodation in the coastal plain. The ensuing phases of sea-level stillstands resulted in the progressive filling of the newly formed accommodation space, through beach progradation, extensive mud deposition in behind-barrier lagoonal (estuarine) and marsh deposits, and aggradation in bay-head delta systems at the head of estuaries. Eustacy appears to be the major controlling factor of the retrogradational stacking pattern of parasequences within the TST. By contrast, a complex interplay of eustacy, sediment supply and subsidence, with an increasing influence of autocyclic mechanisms, such as channel avulsion and delta lobe abandonment, controlled facies architecture within the HST. The maximum flooding surface cannot be assumed to be synchronous, its timing being strongly dependent upon local variations in sediment influx and subsidence.

* Corresponding author. E-mail address: [email protected] (A. Amorosi). 0025-3227/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2005.06.041

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This study shows that the micropalaeontologic characterization of mud-prone (coastal plain and estuarine) successions in terms of water depth and salinity can lead to very accurate sequence–stratigraphic interpretations, allowing identification of parasequence boundaries that may not be detected by conventional stratigraphic approach. D 2005 Elsevier B.V. All rights reserved. Keywords: parasequence; foraminifers; ostracods; Holocene; Po Plain

1. Introduction The impact of climatic changes on coastal systems in the near future must be judged in the perspective of predicting the possible scenarios of environmental changes under rising sea-level conditions. Studies about eustacy and coastal morphology provide evidence for the possible flooding of wide portions of the Italian coasts in the next decades (Colantoni et al., 1997; CENAS, 1998; Marini et al. 2000; Aminti et al., 2001; Silenzi et al., 2002). Detecting the sedimentary response of coastal systems to high-frequency climatic and eustatic variations, thus, is of vital importance for planning protection and management of these highly populated areas. In this respect, the study of past sealevel changes and, specifically, the reconstruction of the palaeogeographic evolution of coastal systems during the Holocene can represent a powerful tool to predict how these coastal environments might alter in the future. As recently observed by Blum and To¨rnqvist (2000) and Cattaneo and Steel (2003), Quaternary deposits emplaced during the last sea-level cycle constitute an accurate archive to this purpose, because of (i) negligible tectonic deformation, if compared with older successions; (ii) high degree of knowledge about climatic and eustatic history, leading in most instances to excellent data sets; (iii) very good chronologic control (see 14C dating of peat horizons and mollusc shells). Modern alluvial plains, but even better coastal plains and deltas, are environments where all these favourable conditions are recorded. The Po River delta is the largest delta in Italy and one of the most important deltas of the entire Mediterranean area. The modern Po delta is very recent in origin and formed only after the bFicarolo avulsion eventQ (1152 A.D.), which caused an abrupt switching of the major distributary channels from southern regions (Comacchio and Ravenna areas — Fig. 1) to the present area.

Several studies have been conducted in the last decade on the Late Quaternary stratigraphy of the Po Plain (Amorosi et al., 1996, 1999, 2003, 2004; Amorosi and Colalongo, 2005). These studies constitute the landward extension of the studies undertaken during the same period in the adjacent north Adriatic area (Trincardi et al., 1994; Cattaneo and Trincardi, 1999; Trincardi and Correggiari, 2000; Ridente and Trincardi, 2002), providing the basis for the construction of a complete stratigraphic framework at the basin scale. Particularly, Amorosi and Colalongo (2005) have shown that transgressive–regressive (T– R) sequences formed during fourth-order (100 ka) sea-level fluctuations are the dominant feature of Late Quaternary deposits of the Po Plain and that transgressive surfaces, much better than sequence boundaries, are the most readily identifiable key surfaces for sequence–stratigraphic interpretation in this highly subsiding basin. Detailed stratigraphic studies of the uppermost T–R cycle in the subsurface of the Po Plain (Rizzini, 1974; Bondesan et al., 1995; Amorosi et al., 1999, 2003) have shown that the Holocene succession is a few tens of m thick, and separated from the underlying alluvial deposits assigned to the Last Glacial Maximum by a subaerial unconformity marked by an indurated and locally pedogenized horizon. Sediment starvation at this boundary has been inferred to have lasted between 8000 and 15,000 yr (Amorosi et al., 1999, 2003). In terms of sequence–stratigraphic interpretation, three major sedimentary units, reflecting the classical subdivision in systems tracts, can be identified in ascending order: the lower unit, made up of a thick succession of alluvial plain deposits formed during the long period of sea-level fall and subsequent sealevel lowstand, between 125 and 20 kyr BP, includes the falling-stage systems tract (FST) and the lowstand systems tract (LST). The second unit, corresponding to the lower part of the overlying

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11o50lN

N

9

12o10lN 45o00lE

River o P Po di Volano Massa Fiscaglia S7 S8

Ferrara

204

205

S6

Comacchio

Argenta

a

tic Se

↕

S5

S17

S5

S7

Adria

S6

S9

Po Plain

ly

Ita

44o30lE

Mediterranean Sea

20 km

Ravenna

Fig. 1. Sampling sites of sediment cores in the study area and section traces of Figs. 3 and 4. The numbers (204 to the west and 205 to the east) refer to the sheets of the Geological Map of Italy to scale 1 : 50,000.

Holocene T–R sequence, shows increased accommodation and shoreline transgression, which have been interpreted to reflect the landward migration of a barrier-lagoon–estuary system (TST). The upper unit records delta and strandplain progradation, which took place during the following highstand (HST) when riverine sedimentation was enhanced by the decelaration of sea-level rise (Stanley and Warne, 1994). This depositional architecture shows a close affinity with the coeval deltaic and coastal successions described for the last 4th-order cycle from other parts of the world (Oomkens, 1970; Suter et al.,

1987; Demarest and Kraft, 1987; Stanley and Warne, 1994; Gensous and Tesson, 1996; Morton and Suter, 1996; Yoo and Park, 2000; Amorosi and Milli, 2001; Hori et al., 2002; Tanabe et al., 2003). However, although the Holocene stratigraphy beneath the modern delta plains has been largely explored and a worldwide stratigraphic evolution firmly established, the sedimentary response of depositional systems to frequencies at sub-Milankovitch (millennial) scale has been neglected by the majority of traditional sequence–stratigraphic models (Posamentier and Vail, 1988; Hunt and Tucker, 1992; Helland-Hansen and Martinsen, 1996; Posa-

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mentier and Allen, 1999; Plint and Nummedal, 2000). Modern stratigraphy from coastal areas is able to investigate sequences reflecting shorter time periods than fourth-order cycles. For instance, recurring cyclic patterns at the scale of 5th- and 6th-order cycles have been described from the Mississippi delta (Lowrie and Hamiter, 1995) and the Ebro delta in Spain (Somoza et al., 1998), where Holocene stratigraphic architecture has been observed to include a distinct stacking pattern of parasequences, i.e., shallowing-upward successions bounded by flooding surfaces (Van Wagoner et al., 1990; Kamola and Van Wagoner, 1995), on a millennial time scale. A similar organization in parasequences has been identified by Thomas and Anderson (1994) and Nichol et al. (1996) for the infilling of incised-valley systems. A comparable facies architecture, with backstepping estuarine facies in response to step-like sea-level rise events, followed by active aggradation during decelerated sea-level rise, has been described by Hori et al. (2002) from the Changjiang (Yangtze) River mouth, in East China. Analogies with this stratigraphic framework have been illustrated by Amorosi and Milli (2001), who identified a characteristic pattern of parasequences in the Holocene of Po and Tevere delta systems in Italy, although no detailed analyses of these millennial-scale cycles were undertaken in terms of geometry, composition and internal architecture. The main focus of this paper, which expands upon previous work by the authors, is to investigate the response of the Po coastal system to ultrahigh-frequency sea-level changes in the Holocene, through the detailed stratigraphic and sedimentological characterization of depositional cycles (parasequences) on time scales of 103 yr. Parasequence analysis was carried out on the basis of integrated facies and palaeoecological observations, the latter based upon analysis of benthic foraminifers and ostracods. We chose as test area a relatively inner portion of the coastal system, located almost entirely behind the line of maximum shoreline transgression, between Argenta and Massa Fiscaglia (Fig. 1). This part of the system, which consists of a complex pattern of freshwater, brackish and shallow-marine environments is particularly sensitive to subtle changes in relative sea-level and

salinity, providing thus a key contribution to our purpose. Nine continuous cores, approximately 40 m in length, were drilled by Geological Survey of Regione Emilia–Romagna in the study area, as part of the Geological Mapping Protocol of Italy to 1 : 50,000 scale. Core recovery was 100%. Eleven accelerator mass spectrometry (AMS) radiocarbon dates were obtained on wood fragments, peats, and mollusc shells. Datings are reported as conventional (uncalibrated) 14C ages. Facies analysis was carried out based on lithology, grain size, sedimentary structures and accessory components. For detailed facies description of cores, the reader is referred to previous work (Amorosi et al., 1999, 2003). Detailed description of facies associations will not be repeated here.

2. Microfossil associations Micropalaeontological analyses of benthic foraminifers and ostracods were carried out on 228 samples, leading to a precise palaeoecological characterization of facies associations. The internal composition of the 12 mixed benthic foraminifers and ostracods associations illustrated in Fig. 2 has been recently described in detail by Amorosi et al. (2004) and Fiorini, 2004, and for this reason will not repeated here in detail. The labels M (Marine), R (Reworked), B (Brackish) and F (Freshwater) reflect a specific palaeoenvironmental significance. Lower case letters define specific subenvironments, which can be distinguished in terms of depth and minor differences in salinity. The key points for micropalaeontological interpretation are summarized below. Associations M (Ma–Me) are indicative of normal salinity waters and open-marine environments. Particularly, associations Me (Textularia spp., Miliolidae spp., Semicytherura spp., and Lepthocythere spp.) and Md (Miliolidae spp., Elphidium spp., Cribroelphidium spp., Pontocythere turbida and Callistocythere spp.), including abundant marine mollusc shells, display the deepest fauna recorded in the study units, and are characteristic of offshore–transition to lower shoreface environments (Fig. 2). Association Mc (Ammonia beccarii, A. papillosa, Elphidium spp. and P. turbida) forms in high-energy

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Bd F R

11

Ma Mb Me Md

Ba Rb Rm

Bb Mc

Bc

Mc

Bd Rm Bb Bc

Bd

Rm

Md Me

Fig. 2. Environmental zonation of the 12 microfossil associations identified in cores (see Fig. 3). See text for description.

shallow environments, such as tidal inlets and flood/ ebb tidal deltas, whereas Mb (Ammonia tepida, A. parkinsoniana, Cribroelphidium spp., Semicitherura spp., P. turbida and Loxoconcha spp.) and Ma (A. tepida and A. parkinsoniana, Loxoconcha stellifera and P. turbida) show an increasing influence of riverine waters in low-energy environments, such as bays or prodeltas. Association R includes a microfauna that has been reworked from older formations, transported by rivers, and emplaced within crevasse splays or bay-head deltas. Microfossil associations that have been subjected to transport from coeval marine deposits and that may have been accumulated within nearshore environments, or brackish-water environments, such as washover lobes, are labelled Rm and Rb, respectively. Associations B (Ba–Bd) are diagnostic of lowenergy brackish-water, back-barrier environments, with few foraminifer species. Specific sub-environments can be defined on the basis of slight differences in salinity and exchange with marine waters. Associations Bd (A. tepida and A. parkinsoniana – dominant –, Cyprideis torosa) and Bc (A. tepida, A. parkinsoniana and C. torosa) include an intermixture of marine and brackish-water species, and are diagnostic of outer- and central lagoon/estuary environments, respectively, whereas Bb (C. torosa) is characteristic of an inner lagoon/estuary, with no significant marine influence. Finally, Association Ba (Trochammina inflata) is recorded at the landward margin of the lagoonal or estuarine complex.

Association F (Candona spp.), which lacks any foraminifer, is characteristic of freshwater settings, such as swamps and shallow lakes.

3. Anatomy of parasequences in the Holocene of the Po Plain Similarly to what observed in the subsurface deposits of modern Adriatic coastal plain near Ravenna (Amorosi et al., 1999) and Comacchio (Amorosi et al., 2003), an overall retrogradational and then progradational stacking pattern of facies forms the basic motif of the Holocene succession in more western areas, between Argenta and Massa Fiscaglia (Fig. 1), allowing identification of the TST and overlying HST (Fig. 3). Within this T–R sequence, eight lower-rank cycles, approximately 3–5 m thick and with a time duration of about 1000 yr, can be identified and physically traced throughout the study area, on the basis of sedimentological and micropalaeontological data. The bounding surfaces of these comparatively thin packages mark abrupt landward shifts of facies, and thus represent bflooding surfacesQ, although in some instances they are surfaces across which there is simply evidence of bdeepeningQ, rather than bfloodingQ (Bhattacharya, 1993). The resulting cycles, which are bounded by isochronous flooding surfaces and show internal shallowing-upward trends, correspond to parasequences in the sense of Van Wagoner et al. (1990) and are interpreted to reflect alternating episodes of rapid relative

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204-S17

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flooding surface

alluvial sands and clays

brackish-water (lagoonal, central estuary) clays

freshwater (swamp, inner estuary) clays

beach-ridge sands

bay-head delta sands

marine (bay, outer estuary) clays

Last Glacial Maximum unconformity Bc foraminifer and ostracod association sample for micropaleontological analysis R

. . . . .

LITHOLOGY

. . . .

sand

peat

silt and clay

organic-rich layer

14

7,735 uncalibrated C date (yr BP)

Fig. 3. Detailed stratigraphic cross-section (location is shown in Fig. 1), showing facies architecture, attribution of the study samples to the twelve microfossil associations, and subdivision in eight parasequences. TST: transgressive systems tract, HST: highstand systems tract.

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205-S5

Bb

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sea-level rise and subsequent stillstand (or decelerating sea-level rise). The eight parasequences identified in the study area (Fig. 3) are numbered 1–8 in ascending order. Parasequence 1 is recorded uniquely at the seaward end of the correlation panel (see core 205-S5), as a thin peat horizon at the very base of the Holocene succession, but shows a greater thickness at relatively seaward locations (Fig. 4). Parasequences 1 to 3 belong to the TST, whereas parasequences 5 to 8 correspond to the HST. The turnaround between transgressive and regressive strata, which defines the maximum flooding surface, i.e., the boundary between TST and HST (Posamentier and Vail, 1988; Galloway, 1989), is located in the lower part of Parasequence 4 (Fig. 3). At any single core, the number of readily visible parasequences varies due to the thickness of the Holocene T–R cycle overlying the Last Glacial Maximum unconformity (LGMU) and to the extension of the flooding surfaces. Owing to the onlap relationships of transgressive deposits onto LGMU, a decreasing number of parasequences is recorded from downdip to

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updip locations, causing a dramatic landward-thinning of the TST. The high density of micropalaeontological data ensures an accurate positioning of the basal flooding surfaces of parasequences (Fig. 3). The flooding surface at the base of Parasequence 1, which merges with LGMU and corresponds to the base of TST (transgressive surface or binitial transgressive surfaceQ of Nummedal et al., 1993), documents the onset of a coastal plain in response to rising sea level, at about 9,400 yr BP, with abundant development of freshwater (swamp) environments replacing the former alluvial plain. The overlying Parasequence 2 is bounded by a flooding surface marking the onset of a brackish (association Bc), wave-dominated estuarine environment over the previously exposed area (Amorosi et al., 2003), with development of a bay-head delta complex at the head of the estuary (core 204-S7). The lower boundary of Parasequence 3 records the rapid landward shift of the bay-head delta sands (core 204-S6). At downdip locations (core 205-S5), the flooding surface is marked by rapid transition from a brackish (microfossil association Bc) to a marine

W

E

204S-17 204-S5

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4,015 5,340 5,680

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7,735

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29,030 18,830

alluvial plain sands and clays

coastal plain clays, sands and peats

transgressive barrier and beach-ridge sands

bay-head delta sands

lagoonal, bay and estuarine clays

offshore-transition and prodelta clays

FST

LGM unconformity

4 9,500

parasequence boundary parasequence 14

C date

Fig. 4. Simplified stratigraphic cross-section (location is shown in Fig. 1), showing parasequence architecture of Holocene deposits beneath the modern Po coastal plain. FST: falling-stage systems tract, LST: lowstand systems tract, TST: transgressive systems tract, HST: highstand systems tract, TS: transgressive surface, MFS: maximum flooding surface The two dates reported for core 204-S7 are projected from Bondesan et al. (1999).

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(association Ma) environment, with upward return to brackish-water conditions (association Bb). The lower part of Parasequence 4 marks a further updip migration of the river mouths (bay-head delta sands in core 204-S5), in response to continuing transgression. Subsurface mapping based on about 200 piezocone penetration tests (Centineo, 2001) shows that the bay-head delta complex emplaced at peak transgression is larger than the older bay-head deltas, extending along strike through coalescence of sand bodies formed at adjacent river mouths. At more seaward locations (cores 204-S6 and 204-S7), centralestuary, brackish conditions (associations Bd/Bc and Bb) developed on top of continental environments, whereas beach-ridge sands (corresponding to the maximum transgressive limit of the shoreline — see Fig. 4) are recorded in core 205-S5. At this site, a truncated coarsening-upward sequence including a brackish microfauna (association Bc) is recorded below the nearshore sands (association Rm), and is interpreted to reflect migration (early transgressive phase) of a flood-delta or a washover lobe into the estuary, predating the establishment of a transgressive barrier, through a marine ravinement surface. Local superposition of microfossil association Rb onto sediments bearing a brackish microfauna in the upper part of parasequence 4 (cores 204-S6 and 204-S7) is interpreted to reflect sand input into the lagoon by normal storm and tidal processes (see Fig. 2). Because of a major decrease in sea-level rise, aggradation and progradation became dominant at about 6000 yr BP and generalized highstand deposition took place in southeastern Po Plain, with subsequent outbuilding of a wave-dominated delta system (Amorosi and Milli, 2001). In this period, several delta lobes were constructed and then abandoned, as a result of distributary–channel avulsion and migration. Between 5500 and 4000 yr BP the delta plain area experienced alternating development of terrestrial conditions and re-flooding, leading to renewed parasequence development. Two phases of localized transgression took place in response to delta lobe switching processes, resulting in formation of wide bays (lower part of Parasequences 5 and 6). In both instances, the basal flooding surfaces are marked by a tongue of very shallow-marine deposits (microfossil association Ma) interfingering with sediments formed in a brackish-water environment.

Within these two parasequences, the depositional environments shoal upwards from resumed marine conditions to brackish (Parasequence 5) or continental (Parasequence 6) conditions. A laterally extensive peat horizon, dated at about 4000 yr BP and traceable throughout the entire study area, is recorded at top of Parasequence 6. Peat layers have been observed to be particularly abundant within highstand deposits, where they commonly occur at top of shallowing-upward cycles (Breyer, 1997). It seems likely that progradation and lateral switching of delta lobes in combination with subsidence and sediment compaction created in this period interdistributary swamps and shallow embayments favourable for the generation of peat. Peat is thought to represent the latest stage of filling of these interdistributary areas (Milli, 1997). The re-establishment of freshwater swamp environments at top of the peat horizon is interpreted as a new flooding surface at base of Parasequence 7, followed by upward transition to alluvial plain facies. The development of Parasequence 8, dated to XII– XVI century A.D. on the basis of historical data (Bondesan et al., 1999), is related to the abandonment of a former delta lobe fed by the Po di Volano distributary channel, owing to a catastrophic avulsion event in 1152 (the bRotta di FicaroloQ in Ciabatti, 1967). The abrupt shifting of the Po River toward a northern position, which led to the construction of modern Po Delta, caused a dramatic lowering of sediment supply to the previously active delta lobe, which was submerged due to continuing subsidence and replaced by a brackish environment (see microfossil association Bb above the parasequence-bounding surface in Fig. 3).

4. Sequence stratigraphic architecture and parasequence development Prolongation of the stratigraphic cross-section of Fig. 3 basinwards, into the coastal area studied recently by Amorosi et al. (2003), allows the construction of a general stratigraphic scheme for the Holocene T–R cycle of the Po Plain, showing the linkage between continental and coeval nearshore deposits (Fig. 4). A stratigraphic framework as refined as the one shown for the landward area is

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not available for the coastal zone, due to i) poor recovery of loose sands during drilling, ii) lower density of palaeoecological data, and iii) poor chronologic control. The overall internal architecture of the systems tracts, however, appears to be controlled by the stacking pattern of parasequences developed on a millennial time scale, and provides the basis to reconstruct coastal evolution in the study area during the last 10 kyr. Lowstand deposition (LST) was restricted to the deepest part of a broad and shallow incised valley formed during the previous phase of sea-level fall (Amorosi et al., 2003), and consists of alluvial plain sediments, the deposition of which occurred after the Last Glacial Maximum (Fig. 4). Early transgression (TST), documented by Parasequences 1, took place between 10,500 and 9400 yr BP and was characterized by the generalized development of a coastal plain within the incised valley. At that time, most of the area comprised between Argenta and Massa Fiscaglia (Fig. 3) was subaerially exposed and subjected to soil formation in the interfluves (see examples in Aitken and Flint, 1996; McCarthy and Plint, 1998). The transgressive coastal plain deposits, which constitute the upper part of the incised-valley fill, differ from the underlying lowstand alluvial-plain deposits for the abundance of organic clays and peats, lack of paleosols, and lack of brownish and yellowish alteration colours, suggesting frequently submerged environments with very short phases of subaerial exposure (Amorosi et al., 2003). The transgressive nature of Parasequences 1 is supported by its stratigraphic correlation with the coeval sand-ridge deposits documented further east by Colantoni et al. (1990) and Correggiari et al. (1996) in the present Adriatic Sea offshore. These relict, transgressive sand bodies have been interpreted to reflect the drowning and reworking of pre-existing coastal barriers, according to the transgressive submergence model of Penland et al. (1988). On land, the development of Parasequence 2 was accompanied by flooding in a more southern position (Comacchio area), with local establishment of brackish conditions (Amorosi et al., 2003). Late transgression was characterized by extensive flooding by brackish and then marine waters. Rapid transit of a wave-dominated estuary over the coastal plain occurred between 9400 and 7000 yr BP,

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favoured by the low coastal gradient. During this period, a transgressive barrier complex migrated rapidly toward more western positions, because of the fast Early Holocene sea-level rise. Three different shorelines, related to Parasequences 2, 3 and 4, have been reconstructed on the basis of stratigraphic position of transgressive barrier sands in cores. Landward of the shoreline, previously exposed areas were rapidly covered by brackish waters, as a result of the dramatic backstepping of the estuarine system. At the upstream portion of the estuary, the obvious retrogradational stacking pattern of the three bay-head deltas described at length in the previous section (see Fig. 3) is time equivalent with the pattern of backstepping shorelines identified downdip, showing strong similarities with the theoretical models of Dalrymple et al. (1992) and Nichol et al. (1994). At peak transgression, during emplacement of Parasequence 4, the shoreline was located 30 km W of its present position. With the ensuing phase of sealevel highstand (HST), sediment supply overwhelmed the rate of relative sea-level rise and coastal progradation took place, with rapid basinward shift of sedimentary facies and outbuilding of a wave-influenced, arcuate Po delta, with its adjacent system of beachridge strandplains (Parasequences 5 to 8). Lateral tracing of parasequence boundaries becomes increasingly difficult within the highstand deposits, owing to the development of different patterns (shallowing vs. deepening) at the same time in different parts of the basin (see Wehr, 1993; Martinsen and Helland-Hansen, 1994). This is most obvious when trying to locate the maximum flooding surface (MFS) at the basin scale. On the basis of the shoreline trajectory (Fig. 4), the MFS should be placed within Parasequence 4, i.e., at the turnaround between landward stepping and basinward stepping nearshore sand bodies. However, at relatively inland locations (cores 204-S7, 204-S5 and 204-S17) the greatest degree of marine influence is recorded higher up in the stratigraphic column (Fig. 3), within Parasequence 5 (see microfossil association Ma in core 204-S7, and association Ba in core 204-S5) or Parasequence 6 (see association Ma in core 204-S7, and association Bc in core 204-S17). This implies that during deposition of parasequences 5 and 6 one part of the basin (the delta plain) was experiencing bmaximum floodingQ with lagoonal/bay deposits, while at seaward (delta front)

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locations regression of the shoreline due to delta progradation was taking place (Fig. 4). This anomaly is interpreted to reflect a local drop in sediment supply in the delta plain due to episodes of delta-lobe switching that, combined with subsidence, locally increased the rate of relative sea-level rise, leading to localized marine incursion and transgression. In summary, correlation of individual parasequences based upon closely-spaced cores in the Holocene deposits of the Po Plain allows to document a significant diachroneity of the MFS on the scale of 103 yr at the basin scale. The stacking pattern of parasequences within the TST exhibits a consistent pattern throughout the study area and appears to have been controlled mostly by acceleration and deceleration of sea-level rise. By contrast, parasequence development in the HST seems to reflect fluctuations in sediment supply rather than changes in relative sea level. During this period, local (autocyclic) processes, such as distributary channel avulsion and delta lobe abandonment, prevailed on external (allocyclic) controlling factors.

5. Conclusions The construction of an ultra-high-resolution sequence–stratigraphic framework for the Holocene transgressive-regressive cycle of the Po Plain, through integrated sedimentological and micropalaeontological characterization of short (103-yr) time-scale cyclicity, represents a key to define a generalized predictive model of sedimentary response of the Adriatic coastal system to high-frequency climatic and eustatic variations. Detailed observations of cores from nine boreholes in the present Po coastal plain enables recognition of eight small-scale cycles, 3–5 m thick, bounded by flooding surfaces and generally displaying internal shallowing-upward trends (parasequences). Facies architecture in the TST is punctuated by characteristic landward-stepping geometries within coastal-plain and then estuarine deposits, reflecting a transgressive evolution controlled primarily by millennial-scale changes in the rate of sea-level rise during the Early Holocene. Lateral tracing of parasequence boundaries in the TST is straightforward. By contrast, the influence of changes in sea-level may be overprinted in the

HST by fluctuations in sediment supply and subsidence, and cyclic facies pattern within deltaic deposits are likely to be related to autocyclic variations in sediment flux, with no significant change in sea level. Local lobe switching produced parasequences limited in areal extent that are virtually indistinguishable from successions of broad regional significance. As a consequence, the maximum flooding surface is markedly diachronous on the scale of 103 yr and does not have any chronostratigraphic significance on this scale of observation. Characterization and correlation of small-scale depositional cycles bounded by flooding surfaces (parasequences) look very promising to understand the sedimentary response of coastal systems to alternating phases of rapid sea-level rise and stillstand.

Acknowledgements Thanks are due to Raffaele Pignone (Geological Survey of Regione Emilia-Romagna) for providing access to cores. We express our gratitude to Salvatore Milli and Yoshiki Saito for their helpful review of the manuscript.

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