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

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Marine Geology 222–2

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, Italyb 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 palaeoen-

vironmental 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 small-

scale, 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.

0025-3227/$ - s

doi:10.1016/j.m

* Correspondi

E-mail addre

23 (2005) 7–18

ee front matter D 2005 Elsevier B.V. All rights reserved.

argeo.2005.06.041

ng author.

ss: [email protected] (A. Amorosi).

A. Amorosi et al. / Marine Geology 222–223 (2005) 7–188

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 evi-

dence 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 sea-

level 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 Tornqvist

(2000) and Cattaneo and Steel (2003), Quaternary

deposits emplaced during the last sea-level cycle con-

stitute 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 Medi-

terranean 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 consti-

tute 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 construc-

tion 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 sur-

faces 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 (Riz-

zini, 1974; Bondesan et al., 1995; Amorosi et al.,

1999, 2003) have shown that the Holocene succes-

sion 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 subdivi-

sion in systems tracts, can be identified in ascend-

ing 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 sea-

level 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

Adriatic S

ea

Ravenna

S9S8S7

S6S5

20 km

R i v e rP o

Ferrara

N

Po Volanodi

Comacchio

MassaFiscaglia

Argenta

S7S6S5

S17

PoPlain

Italy

MediterraneanSea

↕↕↕↕ ↕204 205

11o50lN 12o10lN45o00lE

44o30lE

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.

A. Amorosi et al. / Marine Geology 222–223 (2005) 7–18 9

Holocene T–R sequence, shows increased accom-

modation 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 affi-

nity 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-Milanko-

vitch (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-


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 cyc-

lic 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 strati-

graphic architecture has been observed to include a

distinct stacking pattern of parasequences, i.e., shal-

lowing-upward successions bounded by flooding sur-

faces (Van Wagoner et al., 1990; Kamola and Van

Wagoner, 1995), on a millennial time scale. A simi-

lar 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 strati-

graphic 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 investi-

gate the response of the Po coastal system to ultra-

high-frequency sea-level changes in the Holocene,

through the detailed stratigraphic and sedimentolo-

gical characterization of depositional cycles (para-

sequences) 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 sensi-

tive 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 struc-

tures and accessory components. For detailed facies

description of cores, the reader is referred to pre-

vious work (Amorosi et al., 1999, 2003). Detailed

description of facies associations will not be repeated

here.

2. Microfossil associations

Micropalaeontological analyses of benthic forami-

nifers 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 associa-

tions 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 sub-

environments, which can be distinguished in terms of

depth and minor differences in salinity. The key points

for micropalaeontological interpretation are summar-

ized below.

Associations M (Ma–Me) are indicative of normal

salinity waters and open-marine environments. Parti-

cularly, associations Me (Textularia spp., Miliolidae

spp., Semicytherura spp., and Lepthocythere spp.)

and Md (Miliolidae spp., Elphidium spp., Cribroel-

phidium spp., Pontocythere turbida and Callisto-

cythere spp.), including abundant marine mollusc

shells, display the deepest fauna recorded in the

study units, and are characteristic of offshore–transi-

tion to lower shoreface environments (Fig. 2). Asso-

ciation Mc (Ammonia beccarii, A. papillosa,

Elphidium spp. and P. turbida) forms in high-energy

F

McMc

Ba

Bb

BdBcBb

Me

Md

MbMa

Md

Rm

RbRm

Bd

BcBd

Rm

MeR

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 del-

tas. Microfossil associations that have been subjected

to transport from coeval marine deposits and that may

have been accumulated within nearshore environ-

ments, or brackish-water environments, such as wash-

over lobes, are labelled Rm and Rb, respectively.

Associations B (Ba–Bd) are diagnostic of low-

energy brackish-water, back-barrier environments,

with few foraminifer species. Specific sub-environ-

ments can be defined on the basis of slight differences

in salinity and exchange with marine waters. Associa-

tions 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 depos-

its of modern Adriatic coastal plain near Ravenna

(Amorosi et al., 1999) and Comacchio (Amorosi et

al., 2003), an overall retrogradational and then progra-

dational stacking pattern of facies forms the basic motif

of the Holocene succession in more western areas,

between Argenta and Massa Fiscaglia (Fig. 1), allow-

ing 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 sedimento-

logical 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 shal-

lowing-upward trends, correspond to parasequences in

the sense of Van Wagoner et al. (1990) and are inter-

preted to reflect alternating episodes of rapid relative

204-S7

R

204-S17

205-S5

204-S6204-S5

F

. . . .

. . . .

. . . . .

. . . .

sea level

alluvial sands and clays

freshwater (swamp, inner estuary) clays

brackish-water (lagoonal, central estuary) clays

bay-head delta sands

beach-ridge sands

marine (bay, outer estuary) clays

flooding surface

Last Glacial Maximum unconformity

5m

0 3km

HST

TST

uncalibrated C date (yr BP)14

Bc foraminifer and ostracod association

7,735

FACIES ASSOCIATION

LITHOLOGY

sand

silt and clay

peat

organic-rich layer sample for micropaleontological analysis

1

2

3

45

6

78 Bc

Ma

Bb

F

F

BcBd

F

Bc

Bd

RmBc

BbMa

Ma

Bc

Bc

Bb

RBcMaBdMaRb

Bb

Bb

R

R

R

Rb

Rb

Bd/Bc

R

F

F

F

R

Ba

F

F

F

F

F

Bc

F

F

7,735

6,895

5,340

4,015

9,455

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.

A.Amorosiet

al./Marin

eGeology222–223(2005)7–18

12


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 trans-

gressive and regressive strata, which defines the max-

imum 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 Holo-

cene 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 num-

ber of parasequences is recorded from downdip to

204S-17204-S5 204-S6 204-S7 205-S

W

0 3km

5m

5,680

7,735

23,320

4,015

6,895

29,030

5,340

6,200

5,070

lagoonal, bay andestuarine clays

coastal plain clays,sands and peats

bay-headdelta sands

alluvial plainsands and clays

Fig. 4. Simplified stratigraphic cross-section (location is shown in Fig. 1), s

modern Po coastal plain. FST: falling-stage systems tract, LST: lowstan

systems tract, TS: transgressive surface, MFS: maximum flooding surface T

et al. (1999).

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 sur-

face at the base of Parasequence 1, which merges with

LGMU and corresponds to the base of TST (trans-

gressive 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 fresh-

water (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 (Amor-

osi 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

12

3

45

67

8

LST

FST

HST

MFS

TS

TST

5205-S6

205-S7205S8 205-S9

E

18,830

10,450 9,500

9,4558,740

4offshore-transitionand prodelta clays

transgressive barrierand beach-ridge sands parasequence

boundaryparasequence

9,500 C date14

LGM unconformity

howing parasequence architecture of Holocene deposits beneath the

d systems tract, TST: transgressive systems tract, HST: highstand

he two dates reported for core 204-S7 are projected from Bondesan


(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), central-

estuary, brackish conditions (associations Bd/Bc and

Bb) developed on top of continental environments,

whereas beach-ridge sands (corresponding to the max-

imum 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, pre-

dating the establishment of a transgressive barrier,

through a marine ravinement surface. Local super-

position of microfossil association Rb onto sediments

bearing a brackish microfauna in the upper part of

parasequence 4 (cores 204-S6 and 204-S7) is inter-

preted 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 deposi-

tion took place in southeastern Po Plain, with sub-

sequent outbuilding of a wave-dominated delta

system (Amorosi and Milli, 2001). In this period,

several delta lobes were constructed and then aban-

doned, 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 sedi-

ments formed in a brackish-water environment.

Within these two parasequences, the depositional

environments shoal upwards from resumed marine

conditions to brackish (Parasequence 5) or continen-

tal (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 Para-

sequence 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 micro-

fossil association Bb above the parasequence-bound-

ing 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 con-

struction 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


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 chron-

ologic 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 character-

ized 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 for-

mation 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 trans-

gressive 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, accord-

ing to the transgressive submergence model of Pen-

land 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,

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 posi-

tion 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 back-

stepping shorelines identified downdip, showing

strong similarities with the theoretical models of Dal-

rymple 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 sea-

level highstand (HST), sediment supply overwhelmed

the rate of relative sea-level rise and coastal prograda-

tion took place, with rapid basinward shift of sedi-

mentary facies and outbuilding of a wave-influenced,

arcuate Po delta, with its adjacent system of beach-

ridge strandplains (Parasequences 5 to 8).

Lateral tracing of parasequence boundaries

becomes increasingly difficult within the highstand

deposits, owing to the development of different pat-

terns (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 land-

ward 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 strati-

graphic column (Fig. 3), within Parasequence 5 (see

microfossil association Ma in core 204-S7, and asso-

ciation 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)


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 switch-

ing 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 parase-

quences based upon closely-spaced cores in the Holo-

cene 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 decelera-

tion of sea-level rise. By contrast, parasequence deve-

lopment 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) con-

trolling 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 micropalaeontologi-

cal characterization of short (103-yr) time-scale cycli-

city, 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 influ-

ence of changes in sea-level may be overprinted in the

HST by fluctuations in sediment supply and subsi-

dence, 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 indistinguish-

able 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 alter-

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

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