The modern Po Delta system: Lobe switching and asymmetric prodelta growth

www.elsevier.com/locate/margeo

Marine Geology 222–2

The modern Po Delta system: Lobe switching

and asymmetric prodelta growth

Annamaria Correggiari *, Antonio Cattaneo 1, Fabio Trincardi

ISMAR (CNR), v. Gobetti 101, 40129 Bologna, Italia

Accepted 15 June 2005

Abstract

The modern Po Delta system, comprising five main delta lobes, has been investigated by integrating VHR seismic surveys,

recorded offshore from water depths as shallow as 5 m to the toe of the prodelta in about 30 m, with accurate historical

cartography extending back several centuries. Previous studies give sedimentological and geochronological information from

precisely positioned sediment cores. This combined historical and stratigraphic reconstruction of the modern Po prodelta allows

volumetric reconstructions indicating an average sediment load of 9.4 106 t yr�1 for Po di Pila and Po di Goro-Gnocca lobes.

This estimate is remarkably consistent with the total sediment load of 11.5 106 t yr�1 available for parts of the last century from

a gauge station at the apex of the delta plain (in Pontelagoscuro).

These integrated stratigraphic studies allow to explain the key characters of the Po delta system: a) the marked asymmetry of

the whole delta-prodelta system reflecting prevailing sediment dispersal to the south of each individual delta outlet; b) the shore-

parallel overlapping of successive prodelta lobes fed by distinct river outlets of ever changing relative importance; c) the delta

outlets being artificially forced in a fixed position so that natural avulsion is prevented and delta lobes undergo headland retreat

leaving a marked erosion on the prodelta; d) the presence of prodelta lobes showing widespread bcut-and-fillQ features (rangingfrom 100 to 300 m and depths up to 4–5 m filled with massive silt to very fine sand) offshore of short-lived very active

distributary channels (e.g.: Po di Tolle lobe) and suggesting that, in some particular interval, short-lived episodes of submarine

erosion are induced by catastrophic increases in river discharge (of natural origin or induced by human maintenance). The

seismic stratigraphy of the modern Po Delta documents that markedly distinct prodelta architectures form when a newly

activated lobe is located updrift (north, in this case) or downdrift (south) of the one that is retreating: in the first case the

abandoned lobe becomes sheltered by the new, rapidly advancing, one; in the opposite case the retreating lobe is updrift and a

substantial portion of the sediment is cannibalized and transported to the new lobe, downdrift.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Po Delta; prodelta lobes; subaqueous channels; seismic stratigraphy; Adriatic; Little Ice Age

0025-3227/$ - s

doi:10.1016/j.m

* Correspondi

E-mail addre1 Present addr

23 (2005) 49–74

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

argeo.2005.06.039

ng author. Tel.: +39 051 6398869; fax: +39 051 6398940.

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

ess: Ifremer-Brest-GM, BP70 29280 Plouzane, France.

A. Correggiari et al. / Marine Geology 222–223 (2005) 49–7450

1. Introduction

Increasing concern about the fate of river-borne

pollutants is fueling detailed studies on sediment

distribution in prodelta deposits, their geochemical

composition and their faunal content. Improved

understanding of the evolution of prodelta deposits

is necessary to direct environmental studies and may

help identify the impact of human activities on river

catchments, delta plains and adjacent coastal systems

(Stanley and Warne, 1994; Stanley, 2001). In parti-

cular, late Holocene prodelta systems are potential

archives for reconstructing these short-term environ-

mental changes over the last few millennia. Exploit-

ing prodelta systems as stratigraphic archives for

short-term environmental change requires a detailed

analysis of their internal geometry and a precise

chronological assessment of each elementary deposi-

tional unit. A significant improvement in our under-

standing of delta and prodelta systems may derive

from an integration of geophysical records and pre-

cisely positioned cores, offshore, with historical

information, on land, including old maps (in some

deltas spanning the last few centuries), and written

accounts (Trincardi et al., 2004). In the Mediterra-

nean, fine-grained prodelta deposits are typically 20–

30 m thick and can be used to complement recon-

structions of the evolution of their parent delta

systems (Bellotti et al., 1994; Cattaneo et al.,

2003; Correggiari et al., 2005).

In shore-normal sections, late Holocene prodelta

deposits show a relatively simple and laterally uniform,

wedge-shaped geometry. Offshore major delta systems,

however, a much more complex picture can be detected,

particularly if shore-parallel profiles are taken into

account and the spatial resolution between shore-normal

profiles is increased. The modern Po Delta system,

deposited during the last ca. 500 yr, includes an exten-

sive prodelta composed of multiple prodelta lobes each

extending to the south and coalescing into an undiffer-

entiated distal prodelta system (Correggiari et al.,

2005). These prodelta units evolved under the impact

of short-term climate change, river avulsion and delta

lobe switching as well as increasing anthropogenic

forcing (Ciabatti, 1967; Nelson, 1970; Cencini, 1998;

Castiglioni et al., 1999; Roveri et al., 2001).

The overall evolution of the Po Delta system during

the last few thousands of years was presented by

Correggiari et al. (2005) defining the major phases of

delta and prodelta outbuilding since the Bronze Age

(3500 yr BP) and emphasizing the shore-parallel com-

ponent of prodelta growth based on reconstructions of

sedimentary facies and geochronological information

(mainly AMS 14C dates) from sediment cores in the

prodelta and one borehole taken from a coastal lagoon.

This paper focuses on the growth of the modern Po

Delta during the last 500 yr by combining ancient

cartographic data, on land, with seismic-stratigraphic

and core data, offshore. Based on this data integration,

a detailed reconstruction of the evolutionary phases of

the main elementary delta lobes is proposed and the

following aspects are discussed: 1) the geometric rela-

tionships among prodelta lobes to reconstruct the

major phases of delta and prodelta outbuilding during

the last 500 yr; 2) a semi-quantitative estimate of the

volumes of individual delta and prodelta lobes and

their offshore distribution; 3) the impact of human

activities on the delta and their consequences for pro-

delta construction; and 4) the possible differences in

style of sediment transport during distinctive phases of

delta growth. We anticipate that thickness reconstruc-

tions from seismic profiles, offshore, only record the

net accumulation that is preserved after possible epi-

sodes of lobe retreat and offshore erosion; they are

therefore minimum estimates for each interval of delta

growth and prodelta lobe activity.

2. Background

2.1. Po River basin and delta

The Po River basin is elongated East–West,

bounded by the Alps to the North and West and by

the Apennines to the South, and opens to the Adriatic

Sea to the East. The total area of the Po basin’s

watershed is 74,500 km2, of which 30,790 km2 are

above 200 m in elevation and the remainder is an

aggrading alluvial plain (Nelson, 1970). About half

of the Po plain is below 50 metres of elevation. The

Po River is 691 km long and has a relatively steep

gradient upstream of Pontelagoscuro (located 100 km

landward of the present delta mouth of Po di Pila),

where the river enters the coastal region and flattens

considerably. The Po Delta is located at the eastern end

of the Po plain. The area of the modern Po Delta is 380

A. Correggiari et al. / Marine Geology 222–223 (2005) 49–74 51

km2. At the time of maximummarine transgression (ca.

5–6 kyr BP; Trincardi et al., 1996; Cattaneo et al.,

2003), the shoreline was on the order of 30 km inland

with respect to its present location (Castiglioni et al.,

1990; Bondesan et al., 1995a,b; Correggiari et al.,

1996; Amorosi and Milli, 2001). The Po Delta formed

after the present sea level highstand was attained,

represents the major component of the late Holocene

highstand systems tract (HST) and is genetically related

to the rapidly deposited mud wedge that accumulated

on the shelf for 600 km along the Adriatic coast of Italy

(Trincardi et al., 1996; Cattaneo et al., 2003).

2.2. Po Delta geomorphology

The Po Delta plain occupies a broad area extending

east of Ferrara and along a coastal area of ca. 120 km

between the Adige and Reno rivers (Fig. 1). Most of

the lower delta plain (ca. 1550 km2) is below mean

sea level and half of it is deeper than 2 m below sea

level (Bondesan et al., 1995a). The delta plain is

poorly supplied with sediments because all of the

branches of the Po River have major artificial levees.

The lower delta plain includes N–S elongated topo-

graphic highs (typically less than 2 m above sea level)

that correspond to ancient stranded beach ridges.

Fresh- or brackish-water deposits occur between the

ridges. The present-day delta is undergoing retreat and

is evolving towards a wave-dominated, cuspate mor-

phology (Dal Cin, 1983). Decreasing sediment load

over the last few decades was caused primarily by

intensive sand excavation within the river (up to

690�106 m3 between 1958 and 1981; Dal Cin,

1983; Cencini, 1998). Today, five active branches of

the Po have estimated water and sediment discharge

as follows (Fig. 1): Goro (8% water, 8% sediment

discharge), Gnocca (16%, 11%), Tolle (12%, 7%),

Pila (61%, 74%), Maestra (3%, 1%; data from Nelson

(1970)). Two additional regulated branches, south of

the modern delta, are Po di Volano and Po di Primaro

(occupied today by the diverted Reno River).

2.3. Subsidence

The Adriatic shelf is a shallow semi-enclosed basin

that corresponds to the most recent Apennine foreland

(Ori et al., 1986). Structural and seismic-stratigraphic

studies indicate that the coastal-plain area surrounding

the modern Po Delta has been affected by high rates of

subsidence and sediment compaction during the Plio-

Quaternary (Selli and Ciabatti, 1977; Pieri and

Groppi, 1981). Natural land subsidence (tectonic,

sediment load and compaction) is on the order of

1.1 mm yr�1 in the Po Delta and 2.5 mm yr�1 in

the area of Ravenna (Gambolati and Teatini, 1998).

This combined subsidence results in a relative sea

level rise that may be as much as 1–2 m at the scale

of the life span of an individual delta lobe. Model

predictions for the next 100 yr indicate rates of sub-

sidence of 0.5 mm yr�1 in the Venice lagoon, 2.5 mm

yr�1 in the Ravenna area, and as much as 5 mm yr�1

in the Po Delta (Gambolati and Teatini, 1998).

Anthropogenic subsidence, resulting mainly from

groundwater withdrawal and enhanced by gas produc-

tion on land and offshore, is up to 10 cm yr�1, locally

and over short intervals (Selli and Ciabatti, 1977;

Castiglioni et al., 1990; Bau et al., 2000).

2.4. Discharge regime

The Alpine and Apennine tributaries of the Po

River display distinctive characters. The Alpine tribu-

taries flow through former glacial valleys, while the

Apennine rivers include only a short stretch of high

altitude course and have lower courses with low gra-

dients. Tributaries from the Alps have high discharges

linked to snow melt; tributaries from the Apennines

provide larger amounts of sediment reflecting the

higher erodibility of the Apennines and the lack of a

depositional zone upflow of the junction with the Po

River (Tomadin and Varani, 1998). This picture is

further complicated by seasonal asynchronous flood

cycles affecting distinctive reaches of the Po catch-

ment (Tomadin and Varani, 1998), and resulting in a

bimodal river discharge with a peak in spring and one

in autumn (Correggiari et al., 2005).

The mean annual rainfall on the Po watershed is

1107 mm, but reaches twice this value if calculated for

the Apennine tributaries only (Correggiari et al.,

2005). The historical river discharge fluctuates

between 275 and 11,580 m3 s�1 with an average of

1480 m3 s�1 that correspond to a mean annual runoff

of 668 mm (Nelson, 1970; Po River Basin Authority,

2005). The greatest concentrations of suspended load

are reached in April and November, when the Apen-

nine tributaries reach peak discharge. The Western

10ms

20ms 0m

s

2ms

4ms

-30m

-20m

-10m

Porto Viro

Mesola

Adria

-30m

Ravenna

km0 10

SCARDOVARI 1BOREHOLE

maximummarine ingression

Adige River

Po River(Po Grande)

Po di Volano

extent of prodelta muddy w

edge

Venicelagoon ON LAND

Reno River, formerPo di Primaro

0-130

130-214

214-360

360-420

1500-2500

2500-3000

3000-4000

4000-5500

420-1500

yr BP1820-2000

Bronze Age

Roman Age

middle ages(5th-15th sec.)

1530-1590

1590-1736

1736-1820

yr AD

Iron Age

44˚

40'

45˚

00'

12˚ 20'12˚ 00'

12˚ 40'

N

LEVANTE

MAESTRA

GORO

GNOCCA

TOLLE

PILA

ancientbeach ridges

Comacchiolagoon

LOCATION FIG. 6

Fig. 1. Synthetic map of the Po Delta area showing the extent of the prodelta offshore (from Correggiari et al., 2001) and the phases of delta

progradation on land (based on Stella, 1887; Ciabatti, 1967; Bondesan, 2000; Stefani and Vincenzi, 2005). The main steps of delta progradation

are reported in shades of grey and marked by white lines corresponding to historical coastlines. The present coastline of the Po Delta shows

several bays, coastal inlets and lagoons, connected to the sea (like the Scardovari lagoon, where a long borehole is located). The names of Po

River mouths are in capital letters. The thickness of the late Holocene prodelta mud offshore is in milliseconds (ms). The black line on the map

shows the location of the composite profile parallel to the coast shown in Fig. 6.


Alps show a maximum in precipitation in late spring

to early summer (Nelson, 1970). A flux of 568 kg s�1

corresponds to an average concentration of suspended

sediment of 336 mg l�1 at Pontelagoscuro. The aver-

age monthly concentrations for April and December

are about 500 mg l�1, but during flood conditions

monthly averages of 3100 to 4350 mg l�1 were

reported by Nelson (1970); however, Vignati et al.

(2003) only measured a maximum of 940 mg l�1

during the flood of 1994. The sediment is 10.4

106m3 yr�1 (Hovius, 1998) and the sediment yield

is 214 t km�2 yr�1 (Syvitski et al., 2005).

Pontelagoscuro at the apex of the delta plain is

close to the first channel bifurcation and discharge

through this gauge corresponds to the total discharge

that is partitioned among the main delta outlets. The

recurrence of major flood events (with water and

solid discharges in the order of 10�103 m3 sec�1


and 24�106 t yr�1, respectively) shows a sub-dec-

adal variability (three-day average value from Gian-

dotti (1933) and annual mean value from IDROSER

(1983)). The change in river discharge pattern

occurred in the mid 20th century is illustrated in

Fig. 2: the average annual sediment load from 1918

to 1944 and from 1956 to 1987 is 11.5�106 t yr�1

at Pontelagoscuro station. Although the average

annual water discharge is similar for the period

before and after 1960, the average sediment load

decreases significantly in the second period, likely

in response to changes in agricultural pattern, refor-

estation over the past 50 yr and river-bed mining for

sand and gravel.

2.5. Oceanography

The Adriatic Sea is a shallow epicontinental basin

dominated by a cyclonic thermohaline circulation

with strong seasonal variability (Malanotte-Rizzoli

and Bergamasco, 1983). The Adriatic has a microtidal

35

30

25

20

15

10

5

0

1918

1922

1926

1930

1934

1938

1942

1944

data

gap

period 1918-1960avg annual sed. load (tons x 106)avg annual water discharge (km3)

Ann

ual S

edim

ent l

oad

(ton

s x

106 )

Fig. 2. Time series of averaged annual sediment load (istogram) and annual

1918 and 1987 AD with a data gap between 1945 and 1955 AD. The scale

diagram, respectively. The year 1960 AD marks a sharp decrease in sedimen

regime and is wave-influenced, with storm-driven

waves that may exceed 9 m in amplitude (Cavaleri,

2000). Fresh water input from rivers is 5700 m3 s�1,

more than half of which enter the northern Adriatic,

and affects the basin circulation causing heat loss and

low-salinity water gain (Artegiani et al., 1997a,b).

River runoff and wind forcing vary strongly, both

seasonally and inter-annually (Nittrouer et al., 2004,

Sherwood et al., 2004). The Po River runoff accounts

for ca. 1 /4 (in winter) to 1 /3 of the total river runoff

(Raicich, 1994; Kourafalou, 1999).

Scirocco is the dominant storm wind blowing from

the SE along the major axis of the basin. During

winter, the strong katabatic Bora wind, coming from

the NE, generates two large gyres that affect the whole

water column with a southeastwards component along

the Italian coast (Zore Armanda and Gacic, 1987;

Gacic et al., 1999). In the present conditions, the

outbulge morphology of the Po Delta affects the

evolution of buoyant river plumes by enhancing

their ability to reach offshore areas, where the geos-

90

80

70

60

50

40

30

20

10

0

1956

1960

1964

1968

1972

1978

1982

1986

period 1961-1987

48.29.3

49.613.3

Ann

ual W

ater

Dis

char

ge (

km3 )

water discharge (black line) for the Po River for the period between

s and units for each of these variables are on the left and right of the

t load likely caused by increased human impact in the Po catchment.


trophic current is able to advect sediment to the south

(Tomadin, 1979; Cattaneo et al., 2003).

3. Materials and methods

This paper is based on the interpretation of a dense

grid of VHR seismic profiles (mostly CHIRP-sonar

profiles accompanied by older 3.5-kHz and UNI-

BOOM profiles) in water depths ranging between 30

and 5 m. This dataset allows resolving offshore seis-

mic units of minimum thickness of 1 m. Stratigraphic

data on the youngest prodelta lobes are integrated by

sediment budget calculations based on bathymetric

and hydrologic studies carried out by the Ufficio

Idrografico del Po since the beginning of the 20th

century (Visentini and Borghi, 1938; Visentini, 1940).

The volume of the three most recent Po prodelta lobes

are calculated by combining direct measurements of

the thickness distribution from seismic profiles, off-

shore, and interpolation across the lower delta plain

based on the relative position of dated shorelines. In

making these estimates, an uncertainty is related to the

unknown dip of the delta foresets and the depth of the

foundation surface on which each prodelta lobe is

built. However, such foundation surface cannot be

deeper than the ravinement surface (27–30 m)

detected on seismic profiles, offshore, and in bore-

holes on the delta plain (Amorosi et al., 2005; Stefani

and Vincenzi, 2005). Geochronological control comes

from sediment cores, discussed in previous studies,

and correlation to dated historical geomorphological

and bathymetric maps. This information is comple-

mented by AMS 14C dates (reported in calibrated

years BP) discussed in previous studies (Correggiari

et al., 2005).

The time steps chosen for this work are dictated by

the availability of historical maps and a preliminary

quality control on those that are available: we use the

maps from Visentini and Borghi (1938) for 1811,

1860, 1905 and 1932 AD. An additional map for

1886 was constructed from a very accurate grid of

single-point bathymetric measurements from Stella

(1887). All contour maps are geo-referenced to a

common set of reference points, that likely remained

fixed in position (villages, houses, artificial levees),

and superimposed onto modern bathymetric data col-

lected during our cruises.

4. Results

The modern Po Delta system formed during the

last ca. 500 yr and records the fastest interval of delta

growth with a shoreline progradation of about 30 km

(Fig. 3). The modern Po Delta system includes an

extensive delta plain, a wave-influenced delta front

with five main outlets, and a broad asymmetric pro-

delta. This prodelta can be subdivided in elementary

prodelta lobes defined as the main depocentres that

can be traced as identifiable seismic-stratigraphic

units at the termini of the main river outlets. As a

whole, the Po River prodelta is thicker and broader in

the south because individual prodelta lobes emanating

from the southward oriented delta outlets are more

easily preserved compared to those advancing episo-

dically to the NE. The prodelta lobes overlap laterally

and can be traced offshore on distances that vary with

the sediment flux from land. A comparison of old

maps available for the delta area allows to readily

identify the significant changes in the relative impor-

tance of individual prodelta lobes, defining phases of

lobe advance and lobe retreat (Figs. 3,4). Offshore,

prodelta lobes can be traced as identifiable seismic-

stratigraphic units by using very high resolution

CHIRP-sonar profiles (Fig. 4). Using this tool and

the seismic-stratigraphic approach, significant varia-

tions in prodelta lobe geometry and acoustic facies

can also be reconstructed (Figs. 5,6).

4.1. Summary of the pre-Modern Age Po Delta history

Landward of the modern Po Delta, older subdeltas

were identified by correlation of borehole data (Amor-

osi et al., 1999; Amorosi and Milli, 2001). These

systems advanced in shallow embayments. In the

most landward positions, at least 30 km landward of

the present shoreline, the base of the delta deposits

consists of clay and peat deposits dated ca. 5300 cal.

years BP overlying extensive lagoon deposits dated

ca. 6000 cal. years BP (Amorosi and Milli, 2001).

Since the Bronze Age (ca. 3500 cal. years BP), the

Po Delta occupied a broad stretch of coastal area

extending from Ravenna to Adria with several river

branches and outlets that evolved through natural

avulsions (Figs. 1,3; Ciabatti, 1967). Until the Middle

Ages, the Po River discharge was dispersed through

several distributary outlets that nourished cuspate del-

7.5

9

15m

1.5

04.5

6

3

15

4.5

31.5

2002 ADisobaths

-25m

-2m

0

20

10

30

C D E1530

1592

1685

1736

1811

2002

yr A

D

modern seafloor

km0 10

m

Sca

rdov

ari1

seafloor ca. 1500 AD

1530

1592

1685

1736

1811

2002

1886

yr A

D0

20

28

10

km0 10

modern seafloor

A B

ravinement surface + mfs

m

km0 10

1530

AD

coas

tline

1592

AD

1685

AD

1736

AD

1811

AD

N

A B

C

D

E

seawardextent ofPo prodelta

prodeltathickness

(m)

Legend 1592

AD

1736

AD

Scardovari1 boreholecoastline progradationcoastline retreat

Fig. 3. Schematic sections of the Po Delta based on dated shorelines, depth of the base of the delta (from boreholes and seismic profiles

offshore) and inferred steepness of the clinoforms (based on modern delta-front and prodelta morphology). Inset map shows the ages of the

shorelines, the thickness of the modern delta (post 1600 AD; from Stella, 1887; Nelson, 1970; Bondesan, 2000), the �2 and �25 m modern

contour lines and the location of the Scardovari1 borehole (Roveri et al., 2001; Correggiari et al., 2005).


tas; this cuspate morphology, with well-preserved

beach ridges, suggests that these deltas were wave-

dominated (Visentini, 1931; Bondesan et al., 1995a,b;

Correggiari et al., 2005).

The Po di Primaro was a large cuspate (wave-

dominated) delta detectable on historical maps and

traced offshore in high resolution seismic profiles

into a set of buried sandy clinoforms (Correggiari et

al., 2005; Fig. 1). Seismic profiles and sediment cores

through the Middle Age Po di Primaro delta show

sandy clinoforms that downlap seaward on older trans-

gressive deposits in water depths of about 25 m (Cor-

reggiari et al., 2005), and are draped by fine-grained

sediment dated ca. 1200 AD (ca. 750 cal. years BP),

indicating deposition after the end of the Middle Ages

(Amorosi et al., 1999). Between 1150 and 1200 AD

(800 and 750 yr BP), a major avulsion (Rotta di

Ficarolo; see Correggiari et al. (2005)) placed the

main trunk of the Po in its northernmost position and

the Po di Primaro became less important (Toniolo,

1924; Visentini, 1931). The late Middle Ages and

Renaissance (800–450 yr BP) were a prolonged inter-

val of climatic deterioration with more humid summers

and colder winters, heralding the onset of the Little Ice

Age (Veggiani, 1990; Grove, 2001). River floods

increased in frequency and magnitude and resulted in



repeated episodes of flooding of the Po alluvial plain.

Recurrent avulsions affected the entire Po alluvial and

delta plain and also affected several Apennine tribu-

taries of the Po (Veggiani, 1990). Following the aban-

donment of the Po di Primaro delta, active deposition

shifted further north and resulted in the growth of

wave-dominated late Middle Ages and Renaissance

deltas (Correggiari et al., 2005). On seismic profiles,

an extensive tabular unit with plane-parallel reflectors

forms the foundation of the modern Po prodelta and

extends to the south draping the abandoned Po di

Primaro prodelta (Figs. 5,6).

4.2. Evolution of the Modern Age Po Delta

The Modern Age multi-lobe Po Delta onset after

the Porto Viro diversion (1600–1604 AD, ca. 350 yr

BP) endorsed by the Venice Republic. This diversion

took place after a 100 yr interval of engineering

activities on both the river and the delta (Visentini

and Borghi, 1938). After this diversion, historical

maps show a dramatic change in coastal morphology

with the growth of the Po Delta and a complete

reshaping of the northern Adriatic shoreline (Fig.

7). This supply dominated phase of the Po Delta

has no equivalent in the previous history of the

delta and impacted the entire Adriatic dispersal sys-

tem, down to the Gargano peninsula (Cattaneo et al.,

2003). Considered as a whole, this is a sustained

phase of delta construction; however, even during

this phase individual lobes underwent brief intervals

of significant retreat when abandoned or regulated.

Unprecedented rates of delta construction (Visentini,

1940) were likely brought about by a climatic change

that lead to increased runoff and sediment load during

the Little Ice Age (ca. 1450–1850 AD; 500–100 yr

BP). This change in runoff was accompanied by

increased anthropogenic forcing on the Po River

regime, that included rapid deforestation and con-

struction of artificial levees, enhancing the seaward

out-building of the delta (Veggiani, 1990; Correggiari

et al., 2005).

Fig. 4. Above: 1886 AD map with reported the bathymetric contour of the

The thickness of the Po di Tolle prodelta lobe derived from seismic-reflecti

showing phases of progradation/retrogradation of the Po di Tolle lobe. This

the end of the Little Ice Age). The Scardovari1 borehole constrains the ag

coinciding with the downlap surface at the base of the prodelta. A Chirp-s

lobe. Note that the younger Po di Pila lobe laps out on the retreating Po

The Modern Age Po Delta outbuilding, including

short phases of retreat of individual lobes, is best

reconstructed from historical maps since the 18th

century (Figs. 4,7,8). In summary, between 1604

AD and 1750 AD, the Po Delta advanced dominantly

toward the southeast at 86 m yr�1; between 1750 and

1820, Po di Goro-Gnocca advanced to the southeast

at 129 m yr�1; between 1811 and 1840, the Po di

Maestra prograded northwards at 60 m yr�1; between

1840 and 1886 several lobes co-evolved with distinct

and variable relative importance, and Po di Tolle

became dominant, advancing at 60 m yr�1. Since

1886 AD, following the end of the Little Ice Age,

Po di Pila became dominant and advanced at a rate of

47 m yr�1 (Visentini and Borghi, 1938; Nelson, 1970;

Bondesan and Simeoni, 1983; Mikhailova, 2002).

4.3. Very high resolution anatomy of modern Po

prodelta lobes

Closely spaced CHIRP-sonar profiles in the Mod-

ern Age Po prodelta reveal a complex geometric

relationship among the subaqueous components of

individual prodelta lobes (Figs. 5,6). Two geologic

sections along quasi-orthogonal directions of delta

construction document the asymmetric nature of the

Modern Age Po Delta system (Fig. 3): the section

along the main direction of delta growth (south) docu-

ments average rates of coastline advance in the order

of 100 m yr�1, at least 4 times higher than that along

sections towards the east or northeast. In summary,

since 1811 the delta shows no net construction toward

the northeast, despite a number of ephemeral phases

of delta advance in this direction are documented by

old cartography. As a consequence, the modern bathy-

metry in this region coincides with that of 1811.

Toward the south, the modern Po Delta advanced in

comparable water depths (about 28 m), but the section

also shows the progressive lap-out onto the older

subaqueous lobe of the Po di Primaro delta. Despite

the inevitable simplifications, the comparison of these

two sections indicates that both the Modern Age Po

same year and the present-day �2 m contour line, for comparison.

on profiles is reported in metres. Below: Morpho-bathymetric profile

lobe underwent considerable retreat since 1886 AD (approximating

e of the deposits below and above the ravinement surface (rs), here

onar profile offshore images the seaward termination of Po di Tolle

di Tolle lobe with a marine onlap.

Fig. 5. Chirp-sonar profiles perpendicular to the coastline illustrating the internal geometry of the Po prodelta lobes above the basal ravinement

surface (rs), that corresponds also to the maximum flooding surface, mfs, see Correggiari et al. (2005). Each prodelta lobe is named after the

river mouth of origin. The surfaces separating prodelta lobes are distinctive seismic reflectors that can be correlated laterally. The dashed line on

the map shows the location of the composite profile parallel to the coast shown in Fig. 6.


Fig. 6. A composite Chirp-sonar profile sub-parallel to the present-day coastline of the Po Delta summarizes the stratigraphic relationship among individual prodelta lobes (profile

location is on Fig. 5). The pre-Modern Age Po di Primaro lobe lays in the South (left in this figure) and is partially draped by a weakly reflective stratigraphic unit that corresponds to

the growth of the late Middle Ages and Renaissance cuspate deltas located north of Primaro (marked by the beach ridges reported on Fig. 1). The Modern Age lobes of Maestra, Pila,

Tolle and Goro-Gnocca record the last few centuries of delta growth. Vertical exaggeration is extreme (about 500 times) to enhance the stratigraphic relations among delta lobes.

A.Correg

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eGeology222–223(2005)49–74

59


Delta and the prodelta are markedly asymmetric with

a dominant sediment accumulation toward the SE.

This picture is further complicated if the history of

individual distributary outlets and prodelta lobes is

-20m

-15m

-10m

-5m

-30m

-25m

1.5m

34 65

2

1530

AD

coa

stlin

e

1736

AD1685

AD

2002

AD

coas

tline

km0 10N

Cà Venier

Po di Maestra

Po di Pila

Po di Tolle

Po di Gnocca

Po diVolano

Po di LevantePortoViro

Gorolagoon

Adige R.P

o di Goro

GORO-GNOCCAPROFILE

PV

PV

-2m

-10m-15m

-20m

-25m-5m

1.5m34.5

67.5

9

10.5

12

1.5m

9

3

1530

AD

coa

stlin

e

1685

AD

1736

AD

1886

AD

coa

stlin

e

Po di Maestra

Po di Pila

Po di TollePo di Gnocca

Po diVolano


Gorolagoon

Adige R.

Po di G

oro

km0 10

N

TOLLE

PR

OFILE

-2m w.d. 2002AD

-25m w.d. 2002AD

PV

A

C

considered. A composite seismic profile along a

shore-parallel direction (in water depths between 15

and 25 m (Fig. 6) illustrates the stratigraphic relation

among five depositional elements above the sub-hor-

-20m

-15m

-10m

-5m

-30m

-25m

1.5m

34.5

67.5

910.5

1530

AD

coa

stlin

e

1736

AD1685

AD

2002

AD

coas

tline

km0 10N

Cà Venier

PILAPROFILE

Po di Maestra

Po diPila

Po di Tolle

Po di Gnocca

o diolano

Po di Levanteortoiro

Gorolagoon

Adige R.

Po di G

oro

1.5m

9

3

-20m

-15m

-10m

-5m

-30m

-25m

2002

AD

coas

tline

km0 10N

Cà Venier

Po di Maestra

Po di Tolle

Po di Gnocca

Po diVolano

Po di Levanteortoiro

Gorolagoon

Adige R.

Po di G

oro

GORO-GNOCCALOBE

Po diPila

PILALOBE

TOLLE

LOBE

seawardextent ofPo prodelta

B

D


izontal transgressive ravinement surface, here coincid-

ing with the maximum flooding surface (rs+mfs, in

Fig. 6). The oldest depocentre corresponds to the

growth of the Po di Primaro in the south. The weakly

reflective unit above it correlates to the wave-domi-

nated deltas that formed north of Primaro before the

onset of the Modern Age delta. This weakly reflective

unit extends as a thin tabular deposit at the base of the

three younger prodelta lobes: the slightly older Po di

Tolle (in a central position), and the overlying Po di

Goro-Gnocca (to the southwest) and Po di Pila (to the

northeast). Each of these three lobes presents a dis-

tinctive internal architecture that likely reflects differ-

ences in discharge regime and in the exposure to

storms and marine currents (Fig. 6). In the next sec-

tion we focus on the detailed reconstruction of three of

such prodelta lobes, namely the Po di Tolle, the Po di

Goro-Gnocca, the Po di Pila and the Po di Maestra.

4.3.1. Po di Tolle prodelta lobe

The Po di Tolle was the dominant distributary of

the Po Delta at the end of the Little Ice Age. Ancient

geo-referenced maps through the 19th century (Figs.

4,7A,8) indicate that the Po di Tolle lobe advanced

significantly between 1811 and 1886, advancing at

60 m yr�1 between 1840 and 1886 AD (Visentini

and Borghi, 1938; Fig. 4). Fig. 7A shows the thick-

ness distribution of Po di Tolle and Po di Maestra

prodelta lobes (conservatively defined as the area

characterized by a thickness greater than 1.5 m). Po

di Tolle lobe extends over 120 km2 and exceeds 10 m

in thickness only over about 10 km2. Assuming a dip

of 18 for the basal foresets (not detectable in the

southwest area), a conservative volume estimate for

the lobe is ca. 0.5 km3.

The Scardovari1 borehole (Roveri et al., 2001;

Trincardi and Argnani, 2001) located in the lagoon

between Po di Gnocca and Po di Tolle recovered

about 30 m of shoaling upward deltaic deposits

above a 5 m-thick transgressive continental and mar-

Fig. 7. A) Thickness map of the Po di Tolle and Po di Maestra prode

bathymetric and coastline map of 1886 AD, when a major retreat of this lo

Pila lobe is shown on the modern bathymetry of the Po prodelta. The majo

(see Fig. 8). The thickness plotted on the map above refers to this latter pha

reduced compared to the Po di Pila and Po di Tolle lobes and the extent of

advection component and a more sheltered position with respect to the dom

and thickness (in metres) of the four lobes shown separately in A, B and C.

they are older and partially buried by the other two lobes. The extent of t

ine record. AMS 14C dates constrain the age of the

sandy and bioclastic transgressive record between

11,000 and 9000 BP cal. years, and yielded 750

cal. years BP (i.e. 1200 AD) at 27 m below the

core top (Correggiari et al., 2005). These dates are

consistent with the ancient cartographic data and

indicate that prodelta deposits reached this site and

advanced seaward in relatively recent times (Fig. 4).

In the lower delta plain, the Po di Tolle outlet

branches into several distributary channels (Fig. 9)

likely because artificial levees extend less far down-

river than in all other Po outlets, allowing a more

bnaturalQ behaviour of the outlet. A comparison of

historical maps with the present-day bathymetry

indicates that the Po di Tolle system underwent

retreat since 1886. This retreat was accompanied

by partial erosion offshore (Bondesan and Simeoni,

1983), but a distinctive mounded deposit can still be

recognized on seismic profiles (Fig. 9). This deposit

downlaps seaward on older units and displays a bi-

directional downlap on shore-parallel profiles.

The Po di Tolle prodelta lobe shows the best evi-

dence of subaqueous distributary channels in the pro-

delta. A set of CHIRP-sonar profiles parallel to the

bathymetric contour indicates that these subaqueous

channels are between 150 and 300 m wide and

between 2 and 4 m deep, have markedly erosional

bases and are filled by deposits with acoustically

transparent seismic facies. These subaqueous channels

likely evolved as cut-and-fill features during the over-

all construction of the downlapping lobe and the pre-

sence of discontinuous seismic reflectors suggest a

multi-phase history of erosion and fill. Preservation

of steep incision flanks suggests that erosion was

followed immediately by deposition of the transparent

facies. The channel fill does not prevent the penetra-

tion of the acoustic signal, indicating that its lithology

is likely fine-grained and homogenous (Fig. 9). Cores

through the upper part of the acoustically transparent

channel fill yielded homogenous and structure-less

lta lobes based on seismic profiles offshore, superimposed on the

be initiated. B) The shore-parallel thickness distribution of the Po di

r phases of advance are 1685–1736, 1860–1886 and after 1905 AD

se only. C) The thickness of the Po di Goro-Gnocca prodelta lobe is

the deposit is broader and more uniform, likely reflecting a reduced

inant southerly current compared to the other two lobes. D) Overlap

The Po di Tolle and Po di Maestra lobes have a dashed line because

he Po Delta and prodelta is in grey.

0

10

20

5

15

25

1811

1860 20

0219

05

1860

1886

1932

1685

1736

1 2 3 4km0

1932

1886

1905

1886

modernseafloor

(2002AD)

yr A

D

CGORO - GNOCCA

Cà

Zul

iani

Cà

Ven

ier

0

10

20

30

5

15

25

1685

1811

1860

1886

1736

1905

1932

PILA

1 2 3 4km0

2002

modernseafloor

(2002AD)

yr A

D

B

0

10

20

30

5

15

2518

11

1860

1886

1736

1905

1932

TOLLE

1 2 3 4km0

Cà

Zul

iani

yr A

D

modern seafloor (2002AD)

2002

A

Fig. 8. Bathymetric profiles based on the position of dated shorelines showing phases of progradation/retrogradation of the Po di Tolle (A), Po di

Pila (B), and Po di Goro-Gnocca (C). Location in Fig. 7. Black dots near the date denote delta progradation, open circles indicate retreat. In the Po

di Goro-Gnocca, coastlines and bathymetric profiles between 1860 and 2002 almost coincide while deposition has occurred in the prodelta lobe.


sandy silt (Correggiari et al., 2005). Three stacked cut-

and-fill units are visible in the Po di Tolle lobe, and

appear progressively more confined landward suggest-

ing a progressive reduction in river discharge (Fig. 9).

4.3.2. Po di Pila prodelta lobe

The Po di Pila became dominant after 1886 AD,

following the end of the Little Ice Age, and also in

response to the artificial E–W straightening of the

main feeding river trunk. The Po di Pila lobe

advanced at rates up to 47 m yr�1 (Visentini and

Borghi, 1938), but has undergone partial retreat over

the last few decades in response to damming, river

excavation and artificial breaching during peak

floods to reduce delta plain inundation. Beside

these human impacts, Cencini (1998) also docu-

Fig. 9. CHIRP-sonar profiles (spaced about 500 m) roughly parallel to the shoreline showing subacqueous channels and channel fills in the Po di Tolle prodelta lobe. These

channels are sharp-based, have acoustically transparent fill and record repeated phases of erosion and deposition. The older channels extend farther seaward compared to the last

generation. Key reflectors (either black or white lines depending on the dominant background) are traced to indicate the maximum flooding surface (dashed line at the base of

prodelta lobes) here coinciding with the ravinement surface (mfs+rs), the base of Po di Tolle lobe (dotted line), and the base of Po di Pila and Po di Goro-Gnocca lobes

(continuous line).

A.Correg

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al./Marin

eGeology222–223(2005)49–74

63


ments the relevance of land reclamation particularly

between 1870 and 1960 AD. The Po di Pila pro-

delta lobe advanced directly onto the much older

transgressive record following a significant hiatus.

This lobe reflects phases of advance and retreat

accompanied by consistent net sediment transport

to the south. The best control on thickness variations

comes from the last century (net difference in bathy-

metric surveys between 1905 and 1953; Fig. 10,

modified from Visentini and Borghi (1938), and

3.54.54.5

3.5

3.5

1.50.5

0-0.5

1.5

1.5

0.5

2.5

0.50.5

1.5

1.5

1.5

1.501.51.5

1.5

2.5

0

10.5

3m

6m

1992AD bathymetry

6m3m

1530

AD

coas

tline

1685

AD

1736

AD

1905

AD

coas

tline

Ancient coastlines (yr AD)

Pila and Goro-Gnoccaprodelta lobe thickness (m)

Po di Maestr

a

Po di PilaPo di Tolle

Po di Gnocca

Po di Goro

Po di Volano


Scardovari

lagoonGorolagoon

Adige R.

Po G

rande

-2 m

-25

m

retreat/erosion

retreat / erosion

km0 10N

LEGEND

0 m

0.5 m and steps of 1 m(1.5; 2.5; etc) = deposition

-0.5 m and steps of -1 m(-1.5; -2.5; etc) = erosion

Difference in bathymetry 1953-1905AD:

Fig. 10. Map reporting the net bathymetric difference from 1905 to

1953 AD (modified from Bondesan and Simeoni, 1983). Within this

time interval, positive values (shaded areas) indicate deposition,

negative values (white areas offshore) denote erosion. The areas

of maximum deposition in this interval correspond to the Po di Pila

and Po di Goro-Gnocca prodelta lobes (thick lines).

Bondesan and Simeoni (1983)). These data indicate

that the net deposition at the Po di Pila mouth was

18.5 m in 48 yr (Figs. 7B,8). The thickness distribu-

tion of this delta lobe exceeds 10 m over an area of

ca. 25 km2 and 1.5 m over an area of ca. 200 km2

(Fig. 7B). A conservative volume estimate is there-

fore ca. 1.5 km3.

On seismic-reflection profiles, the internal reflec-

tor geometry and lateral distribution of the Po di Pila

prodelta lobe around the feeder channel is markedly

asymmetric (Fig. 6). A rapid thinning of the deposit

(from 15 m to nil) occurs in the northeast and east

directions and is accompanied by internal reflectors

downlapping onto older condensed prodelta deposits

with a maximum inclination of ca. 18 (Fig. 6).

Toward the south, the Po di Pila lobe shows a

gradual downlap onto the slightly older Po di Tolle

lobe. This downlap occurs at very low angle on a

surface that is transverse to the modern coast. Down-

lap terminations are particularly clear where the

underlying Po di Tolle lobe is characterized by a

mounded upper surface (Figs. 6,9).

4.3.3. Po di Goro-Gnocca prodelta lobe

At a seismic-stratigraphic scale, the last outbuild-

ing phase of the Po di Pila is coeval to the reactiva-

tion of the Po di Goro-Gnocca lobes that are

advancing onto the previous Po di Tolle system,

although at a reduced rate compared to Po di Pila

(Fig. 6). The Po di Goro-Gnocca outlets are sub-

parallel and closely spaced making it relatively dif-

ficult to disentangle individual prodelta lobes off-

shore. Both distributaries were active since the

onset of the modern Po Delta progradation, as

shown by the substantial seaward advance of the

bathymetric profile between 1685 and 1886 (Figs.

7C,8). In 1770 a dyke was constructed on Po Grande

to divert the Po di Gnocca branch to the south

(Visentini and Borghi, 1938). From 1886 to 2002

the sediment supplied through the Po di Goro-

Gnocca outlets fed a large coastal sand spit elon-

gated to the southwest (Simeoni et al., 2000). Off-

shore, VHR seismic profiles identify the Po di Goro-

Gnocca prodelta lobe over an area of ca. 250 km2

(Fig. 7C). The lobe pinches out to the south with a

downlap termination onto the older Po di Primaro

deposit, and to the northeast, against the Po di Tolle

prodelta lobe (Fig. 6). Based on its thickness dis-


tribution, an estimate that the volume of the Po di

Goro-Gnocca lobe is in the order of 0.6 km3.

4.3.4. Most recent depositional unit in the Po prodelta

CHIRP-sonar profiles allow identification of an

upper unit within the Po di Pila prodelta, where

recent flood events accumulated over the last few

decades (Palinkas et al., 2005). This deposit,

resolved in seismic-reflection profiles only where

thicker than about 1 m, has a shore-parallel distribu-

tion to the southwest, like the older seismic-strati-

graphic units that compose the prodelta (Correggiari

et al., 2005, their Fig. 15). Locally, and depending

on the underlying prodelta morphology, the upper-

most muddy deposit shows a marine onlap termina-

tion in the landward direction. Barmawidjaja et al.

(1995) confirm this evidence documenting an

increase of sediment accumulation rates in the distal

prodelta during the last few decades. Particulate Al

(20–200 mg l�1) concentrations in the bottom waters

and 210Pb sediment accumulation rates also indicate

southward dispersal (Price et al., 1999).

4.3.5. Quantitative estimate of sediment accumulation

in the most recent Po Delta lobes

The volumes of the Po di Pila and Goro-Gnocca

lobes are estimated from the interval since 1886 AD.

In the shallow areas that cannot be investigated by

seismic profiles, the base of each lobe is interpolated

assuming a clinoform steepness similar to that

observed in the present-day prodelta and may there-

fore be inaccurate. However, the precision in this kind

of estimate is limited by the uncertainties in the

knowledge of the actual clinoform dips on land. Our

estimate of the delta growth rates relies on the dates of

the successive shorelines and on the assumption that

the clinoform dip, on average, resembles the range of

dips that can be observed today.

Assuming a density of 2.65 g cm�3 and an 80%

porosity, the volume of the Po di Pila lobe is

1.47�109 m3, and the total sediment accumulation

is in the order of 779�106 tons, and the Po di Goro-

Gnocca lobe has an estimated volume of 0.558�109

m3 and a total sediment accumulation of 296�106

tons. This gives an average mass accumulation rate of

6.8�106 tons yr�1 for Po di Pila and 2.6�106 tons

yr�1 for Po di Goro-Gnocca. These estimates are

consistent with sediment supply rates measured at

Pontelagoscuro station (closure point approximating

the apex of the delta plain) from the IDROSER (1983)

and Eurodelta databases, which give a 57-yr average

of sediment load of 11.5�106 tons yr�1. This value

includes sediment delivered to other, less active, lobes

or bypassing the proximal prodelta and advected to

the SE by the dominant long-shore current system.

5. Discussion

River sediments accumulate in deltaic depositional

sequences defining alternated regressive (construc-

tional) and transgressive (destructive) phases; this

alternation defines delta cycles (sensu Scruton, 1960;

see also Amorosi et al., 2005). Popular delta models

developed over the last 30 yr emphasize variations in

the proportion of wave-, tide-, and river-influence as

primary controls on transgressive/regressive cycles,

delta morphology and facies assemblage (Wright and

Coleman, 1973; Galloway, 1975; Orton and Reading,

1993). Where deltas are observed at very high spatial

and temporal resolution, this tripartite subdivision

proves more difficult to apply because multiple,

quasi-synchronous, delta lobes may be characterized

by contrasting discharge regimes and orientation rela-

tive to the coast and prevailing direction of wave and

currents (McManus, 2002; Trincardi et al., 2004; Cor-

reggiari et al., 2005).

Based on the data presented, five main aspects of

prodelta lobe stratigraphy and sediment dynamics are

discussed: 1) the growth of individual prodelta lobes

as a function of sediment supply fluctuations, includ-

ing artificial channel regulation; 2) the asymmetry of

prodelta lobes and the implications on 3D prodelta

architecture; 3) the impact of short phases of erosion

retreat; 4) the implications in high resolution geochro-

nology of delta and prodelta systems; and 5) the origin

of shallow water channels within prodelta lobes.

5.1. Modern Po Delta system: growth patterns and

volume variations

Estimates of growth pattern and volume variation

in prodeltas should take into account volume reduc-

tions induced by sediment compaction and erosion.

The best estimates of the depth of erosion caused by

a reduction in sediment discharge to a prodelta lobe


comes from Fig. 10, where areas of net accumula-

tion vs. erosion are defined by subtracting two

bathymetric surveys from 1905 and 1953 (Bondesan

and Simeoni, 1983). This map shows the areas of

net accumulation (Po di Pila and Po di Goro-

Gnocca) and of net erosion (Po di Maestra and

Po di Tolle) where fixed river outlets receive

decreased sediment loads becoming vulnerable to

headland retreat.

The depositional complexity deriving from the

changing relative importance of quasi-contempora-

neous delta lobes is best illustrated by the shoreline

variations from 1811 to 1935 compiled by Visentini

and Borghi (1938). A set of 54 bathymetric transects

perpendicular to the coast, repeated between 1811

and 1935 (Fig. 11B), documents the changes in sedi-

ment volumes (in 106 m3) of the main delta lobes

with time steps of 25 to 50 yr (Visentini and Borghi,

1938; Fig. 11C). Each lobe records alternating retreat

and advance on a decadal time scale. Fig. 11 shows

clearly the phase relationship between lobe advance

at one site and lobe retreat in an adjacent site. This is

the case of the interval 1811–1860 when the Po di

Pila lobe loses more than 300 106 m3 while the Tolle

lobe advances substantially. In other cases, two or

more lobes advance contemporaneously, though at

different rates. The cumulative volume variations

between 1811 and 1935 (Fig. 11D) show that the

southern half of the delta has substantially advanced,

regardless of the short-term variations illustrated in

Figs. 8,11C.

5.2. Asymmetry of the Po Delta and prodelta system

Bhattacharya and Giosan (2003) recently intro-

duced the concept of the asymmetric delta to

account for the complexity of wave-influenced del-

tas. The modern Po Delta system, including its thick

prodelta, represents a particular case of asymmetric

delta. Indeed, the subaerial delta is more developed

Fig. 11. A) Reference map of the Po Delta. B) Reference map of Visen

Quantitative estimate of sediment volume variations of the Po prodelta lob

up, to south) measured in four time intervals as indicated (modified from

entire interval between 1811 and 1935 AD. Note that the Po di Pila lobe

values) in the North, and net progradation in the South. The southern half

variations in discharge at each delta outlet. In C and D, shaded grey area

number of bathymetric section marked in B.

south of the main trunk (the Po Grande; Figs. 1,3).

This asymmetry may well reflect the impact of

diversions and other continued human intervention

and flow regulations (Correggiari et al., 2005). A

semi-quantitative estimate of this asymmetric sedi-

ment partitioning in the delta is given in Fig. 11 and

shows highest accumulation south of Tolle and

Goro-Gnocca outlets. In other examples of asym-

metric deltas, the growth of individual prograda-

tional lobes may even result in the formation of

sheltered bbay-likeQ areas (Bhattacharya and Giosan,

2003).

In the Po system, not only the overall prodelta

distribution is asymmetric but also individual pro-

delta lobes grow as asymmetric deposits that appear

dominated by shore-parallel, southward, advection of

finer-grained sediment. This preferential advection of

sediment can be observed at the scale of the entire

Modern Age delta (Figs. 3,11D), or parts of it, like

the most recent deposit of the Po di Pila (Fig. 7B). In

a multi-lobe delta such as the Po, each prodelta lobe

tends to generate relief on the sea floor, particularly

in a downdrift location, and this trend limits the

available space for any younger prodelta lobe. The

shore-normal extent of prodelta lobes tends to

broaden downdrift of the feeding delta outlet and to

pinch out against pre-existing prodeltas lobes. The

cartoon of Fig. 12, illustrates schematically two main

kinds of lap out termination in distal prodelta lobes.

In the first case, deposition in a distal prodelta lobe

reflects lateral advection by bottom currents against

the clinoform of a pre-existing prodelta lobe located

downdrift (Fig. 12 A). In this case the lap out termi-

nation of the younger distal prodelta lobe is a case of

marine onlap. In case B of Fig. 12, instead, the

presence of a distal tabular drape deposit, associated

with an older prodelta lobe, limits the accommoda-

tion that remains available for a younger prodelta

lobe to prograde. The older tabular deposit can be

rather thick, like in the case of the post-Primaro unit

tini (1940) with location of the bathymetric sections measured. C)

es calculated from the difference of bathymetric profiles (from north,

Visentini, 1940). D) Cumulative value of sediment volume over the

shows a marked change with dominant retreat and erosion (negative

of the delta has substantially advanced, regardless of the short-term

s highlight the position of the main Po Delta lobes referred to the


of the modern Po Delta (Fig. 6 and Correggiari et al.,

2005). It is possible to represent such complex rela-

tionships between prodelta lobes in space (Fig. 6) and

-10-15

-20

-25m

coastline and bathymetry 1886 AD

-2 -5 -25m

1736

AD

1530

AD

1685

AD

old coastline

Tagliodi Po

Po di Maestra

Po di Pila

Po di Tolle

Po di Gnocca

Po di Goro

Po di Volano


Mesola

GoroScardovari

lagoon

Gorolagoon

Adige River

Po di Goro

Po Grande

Adria

Chioggia

Venicelagoon

-2 m

w.d

.

2002

AD

-25

m w

.d.

2002

AD

km0 10

N

A

-10

4

8

12

16

20

24

28

32

36

40

44

48

52

2

6

10

14

18

22

26

30

34

38

42

46

50

54

-150 -100 -50 0 50 100 1500

4

8

12

16

20

24

28

32

36

40

44

48

52

106 m32

6

10

14

18

22

26

30

34

38

42

46

50

54

NU

MB

ER

S R

EF

ER

TO

TR

AK

S O

N M

AP

B

1811-1860

1860-1886

1886-1905

1905-1935

sediment volumeC

A

also in time, in a chronostratigraphic diagram based

on the same seismic line (Fig. 13, discussed more in

detail below).

12

34

56

7

8

9

19

25

26

42

33

21

20

29

-10-15

-20

-25m

coastline and bathymetry 1886 AD

-2 -5 -25m

Tagliodi Po

Po di Maestra

Po di Pila

Po di Tolle

Po di Gnocca

Po di Goro

Po di Volano


Mesola

GoroScardovari

lagoon

Gorolagoon

Adige River

Po di Goro

Po Grande

dria

Chioggia

Venicelagoon

-2 m

w.d

.

2002

AD

-25

m w

.d.

2002

AD

km0 10

N

Pila lobe

Tolle lobe

Goro-Gnocca lobe

20000 -50 0 50 100 150 250

Maestra lobe

1811 to 1935 ADcumulative

sediment volume106 m3

from Visentini, 1938

D

B

Case A

PHASE I - lobe progradation

PHASE II - advection from source located updrift and marine onlap onto older lobetime

Legend

sea surface older deposits

younger deposits

Case B

PHASE I - advection and marine onlap, accommodation is "consumed"

PHASE II - lobe progradation on top of a risen foundation surfacetime

main direction of sediment transport

direction of shelf currents

Fig. 12. Idealized stratigraphic schemes showing the geometric and

phase relationship between progradational prodelta lobes and land-

ward-onlapping prodelta deposits suggesting deposition by shore-

parallel bottom currents. A: the lobe located bupdriftQ activates afterthe lobe located downdrift is abandoned. This is the case of the

younger Po di Pila deposits lapping out onto the slightly older Po di

Tolle lobe. B: the younger lobe is located downdrift of the aban-

doned lobe and, consequently, finds a reduced accommodation in its

toe region. This is the case of the Po di Goro-Gnocca prodelta lobe

downlapping onto the abandoned Po di Tolle lobe.


5.3. Role of sediment erosion in prodelta lobe shaping

Combined seismic stratigraphy and historical data

indicate that, at any stage, the growth of the modern

Po Delta occurred through the contemporaneous

advance of two or more delta lobes (Figs. 6 and 11).

Since avulsion is no longer possible, due to human

control, any change in delta morphology and strati-

graphy results only from changes in sediment flux to

individual prodelta lobes rather that from delta lobe

switching. The observed changes in delta morphol-

ogy, over this relatively short interval, reflect primar-

ily changes in the partitioning of river discharge

among the different delta outlets (Figs. 7 and 11).

Under these semi-artificial conditions, lobe retreat

takes place not in response to avulsion processes,

but rather because any dearth in supply to a specific

delta mouth results inevitably in lobe retreat and

downdrift sediment redistribution since, on short-

term, the overall oceanographic conditions remain

constant. This tendency has two consequences: 1) it

enhances the tendency of the delta to become asym-

metric, as erosion and transport result in preferential

accumulation to the south (downdrift), and 2) it results

in the deposition of super-expanded sections inter-

rupted by very subtle, but important, diastemic sur-

faces that may be difficult to detect at sediment-core

scale.

When the delta and prodelta lobe that becomes

abandoned is located up current of a new advancing

lobe, the sediment that is cannibalized from the older

lobe may significantly contribute to the new one,

reflecting the importance of shore-parallel sediment

advection. When, instead, the lobe that becomes aban-

doned is located down current with respect to the new

lobe, a sheltering effect allows a greater preservation

of the old lobe. Our data document how lobe canni-

balization and downdrift transport play a key role in

the construction of the delta system. The example of

Po di Pila lobe is typical of the role exerted by

sediment erosion and redistribution.

5.4. Chronostratigraphy of Po prodelta lobes

The geometric relationship between prodelta lobes

is best imaged on shore-parallel profiles showing in

detail the lap out patterns of seismic reflectors (Fig. 6).

Schematic chronostratigraphic diagrams (Wheeler dia-

grams, originally developed to represent stratigraphic

relations among sequences encompassing millions of

years) help clarify the complex relation among delta

lobes and between the delta as a whole and the units

underlying it. Fig. 13 summarizes the chronological

information available in the area (limited 14C dates

and ages of prodelta lobes inferred from their position

relative to dated shoreline from historical maps). Two

complementary chronostratigraphic diagrams are pro-

vided for the entire Holocene (ca. 11,000 cal. years;

mfs=rs TST

5 km

Max seaward extent of Primaro lobe

Renaissance lobes

Modern-Age Po delta lobes

Pila lobeGoro-Gnocca lobe

Tolle lobe

Maestra lobe

Primaro lobe

-10 m

-20

-30

Adige delta lobe(not shown above)

DE

PT

H

5001500250035004500

6500 5500

cal. yr BP

750085009500

10500

TST

mfs condensed interval

HST

0100200300400500600700

800900

yr BP

1950

1850

1750

1650

1550

1450

1350

1250

1150

1050

yr AD

TIM

ET

IME

(ex

pand

ed)

non deposition

non deposition

erosion or non deposition

Renaissance lobes Modern-Age Po delta lobesPrimaro lobe



Primaro lobe

Renaissance lobes

Tolle lobe



A

B

C

Maestra lobe

Pila lobeGoro-Gnocca lobe

Litt

le Ic

e A

ge

Fig. 13. A, B Chronostratigraphic diagrams based on the seismic profile shown in Fig. 6 (and reported here in C) with age constrains from dated

old maps and 14C dates. Progressively lighter grey shading represents progressively younger prodelta lobes; the white areas depict intervals of

non-deposition or erosion and time lines are horizontal. Exploiting all the very high stratigraphic resolution available, the reconstruction on a

100-yr scale (A) allows appreciating the stacking pattern of the prodelta lobes and identifying very minor but diagnostic hiatuses among them.

This representation also indicates that the most complete stratigraphic record in this setting would have to be a composite from cores taken

through each lobe for each time step. At a lower (500 yr) resolution (B) only the overall progradational trend is captured and deposition results

apparently more continuous.


Fig. 13B) and the last 1000 yr (Fig. 13A). In the

former diagram (Fig. 13B), a prolonged interval of

erosion records most of the relative sea level rise, and

no deposition characterizes most of the last ca. 5500

cal. years of sea level highstand. The complex history

of the Po Delta in the last 500 yr is not resolved unless

the chronological resolution is increased to quantify

short intervals and restricted areas characterized by


erosion or lack of sedimentation. Note that the erosion

documented atop Po di Tolle lobe is accompanied by

deposition on both Po di Pila and Po di Goro-Gnocca

lobes.

In the case of several Mediterranean deltas, accu-

rate historical cartography (Provansal et al., 1999;

Correggiari et al., 2005) allows defining the age of

individual prodelta lobes with resolutions that are

better than a few decades. For the late stages of

delta growth, a relative chronology is reconstructed

based on the stratigraphic superposition of prodelta

lobes, on seismic profiles, and on reconstructed shor-

elines. With time steps of 50 yr, a chronostratigraphic

diagram can summarize shifts or advance/retreat

phases of individual delta lobe through since the

onset of the Little Ice Age (Fig. 13A).

Despite differences in the absolute sediment flux

to, and resulting thickness of, individual lobes, the

chronostratigraphic diagram captures two main char-

acters of the Modern Age and Recent Po Delta: 1) the

shift of the entry points tends to generate laterally

juxtaposed delta lobes that only partially stack onto

each other; and 2) at this very short scale of observa-

tion, the main changes in the growth patterns of the Po

Delta do not coincide with the onset or shutdown of

individual lobes, but rather record short-lived changes

in the relative importance of each system. The case of

the Po di Pila, Po di Tolle and Po di Goro-Gnocca

prodelta lobes during the 19th century is a clear

example and can be depicted on a Wheeler diagram

by reporting the location of the depocentre for each

lobe based on the physical correlation of correlative

seismic units (Fig. 13A). At the base, the diagram

shows the Po di Primaro prodelta lobe; when this

prodelta lobe was active, no deposition is documented

in the other areas further north along the chosen tract,

because deposition from other outlets was confined

further onshore.

5.5. Submarine channels in prodelta lobes

Channel fill features are not commonly detected in

modern prodelta environments, partly because, if pre-

sent, they occur in shallow waters where geophysical

surveys yield poor quality data (Correggiari et al.,

2005). Off the Po Delta, seismic profiles show cut-

and-fill features off the Po di Tolle mouth, within a

depositional lobe that records a rapid phase of delta

outbuilding at the end of the LIA (Fig. 9). Other

possible occurrences are in a poorly preserved rem-

nant of the Po di Maestra prodelta lobe, in the North,

and in the topset of the recent-most unit of the Po di

Primaro (Correggiari et al., 2005, their Fig. 11C). In

all cases, these features form during short intervals of

increased water discharge and sediment load during a

phase of overall outbuilding of the prodelta lobe. In

the case of the Po di Tolle prodelta lobe, seismic

stratigraphy suggests that three distinct cut-and-fill

phases occurred during a brief interval of prodelta

lobe advance (Fig. 9). The bases of the channels are

as deep as 24 m below mean sea level with each

individual cut-and-fill phase about 4 m thick and

composed of homogenous silty deposits, at least in

the upper part that was cored (Correggiari et al.,

2005). It is possible that hyperpycnal flows generated

a set of shallow subaqueous channels off the river

mouth through erosion and fill (Correggiari et al.,

2005), as it was observed elsewhere (Fulthorpe and

Austin, 2004). This mechanism was documented in

other prodelta environments, when river flows enter a

basin with a higher density than the ambient body of

water (Wright et al., 1986, 1988). The Po is among

rivers that seldom produce hyperpycnal flows (Milli-

man and Syvitski, 1992; Syvitski et al., 2005).

Whether, and over what distance, these flows erode

the seafloor is hotly debated (Mulder and Syvitski,

1995). In the case of the Po Delta, it is difficult to

prove a physical connection of the observed channels

with terminal distributary channels in the subaerial

delta, because of a lack of data in waters shallower

than 10 m. Anthropogenic causes (artificially increa-

sed water discharge) or jokulhlaups-like natural events

(perhaps generated by breaching of natural dams at

the end of the Little Ice Age) may have led to cata-

strophic increases in discharge and sediment load

capable of generating hyperpycnal flows across the

outbuilding prodelta. During the Fall-2002 Po River

flood, sediment concentration higher than 10 mg l�1

were considered close to the critical value for gener-

ating density currents (Nittrouer et al., 2004). In the

case of Po di Primaro, the formation of subaqueous

channels corresponds to the reactivation of the delta

when the Reno River was diverted into the old Po

branch of Primaro (dated 1763 AD; Correggiari et al.,

2005), confirming the role of man-induced sudden

changes in water discharge.


6. Conclusions

The modern Po Delta is a composite depositional

system that includes several lobes built over an area of

380 km2 during the last 500 yr. Individual prodelta

lobes (the subaqueous part of a delta lobe) underwent

constructional and retreat phases forced by diversions

or, more commonly, short-term changes in sediment

supply rather than avulsion. Combining the compila-

tion of historical cartography on the Po Delta plain

with VHR seismic-stratigraphic data in the prodelta,

supports the following conclusions:

1) The Modern Age Po Delta is the result of a marked

phase of river-dominated growth, where increased

sediment flux derived from climatic change and by

human impact both on the catchment (deforesta-

tion) and on the delta (diversions to the south

accompanied by construction of artificial levees

to maintain a fixed channel network).

2) The Modern Age delta shows an asymmetric thick-

ness distribution and facies partitioning reflecting

the fact that lobes emanating from the southern part

of the delta advance faster or are better preserved.

The distribution of the prodelta lobe reinforces this

asymmetry showing greater extent and thicker

deposits to the south, consistent with the dominant

alongshore dispersal by waves and currents.

3) The distributary branches of the delta have become

fixed in response to the construction of artificial

levees and avulsion is no longer possible. As a con-

sequence, the growth of the Modern Age Po Delta

reflects repeated changes in the relative importance of

individual delta lobes that advance/retreat during

each interval, depending on the balance between

oceanographic regime and sediment supply.

4) When the delta lobe that becomes abandoned is

located up current of the new lobe, a significant

component of the sediment that is cannibalized

from the old apparatus is delivered to the new

one, reflecting the importance of shore-parallel

sediment advection. When the lobe that becomes

abandoned is down current of the new lobe, a

sheltering effect allows a greater preservation of

the old one and a marine onlap forms on top of the

abandoned lobe.

5) Distal prodelta deposits are more uniform with

extensive, sub-parallel or seaward-converging,

reflector packages. Beyond 25 m water depth,

no individual source can be identified based on

seismic-stratigraphic criteria (localized thickness

increases of distinctive seismic units). Distal

deposits tend to drape older low-angle deposi-

tional relief and act as significant permeability

barriers.

6) Reflector geometries and facies assemblages within

the Modern Age Po prodelta record two distinctive

styles of deposition: the first consists of laterally

continuous and weakly converging reflector

packages and alternating fine sand or silt flood

layers observed in cores (Correggiari et al.,

2005), suggesting deposition from hypopycnal

flows; the second is characterized by the formation

of shore-normal cut-and-fill features (4 m deep and

as much as 300 m wide, extending seaward to the

20 m isobath) filled with massive silt during phases

of major prodelta lobe construction and suggest

deposition from hyperpycnal flows.

7) A crude estimate of the sediment accumulated in

the last ca. 120 yr for the most active Po di Pila

and Goro-Gnocca lobes is 779�106 and 296�106

tons, respectively. Converted in sediment accumu-

lations rates, these values give 6.8�106 and

2.6�106 t yr�1, for each lobe. These estimates

are consistent with the 57-yr average sediment load

measured at Pontelagoscuro (11.5�106 t yr�1)

because these lobes make 92% of the total Po

sediment load.

Acknowledgments

Data from the recent Po River floods and the last 90

yr of discharge data collected by Magistrato del Po di

Parma were kindly made available by Dr. Allodi.

Support to this research came from the European

Commission through the projects EURODELTA (Con-

tract N8: EVK3-CT-2001-20001) and EUROSTRATA-FORM (Contract N8: EVK3-CT-2002-20079). We

would like to thank J.P.M. Syvitski, guest editor of

this special volume, and reviewers Phil Hill and T.

Tornqvist for their comments and advices. Finally,

this is also an occasion to acknowledge all the partici-

pants of the COMDELTA conference and in particular

our French hosts in Aix en Provence. This is ISMAR

(CNR)- Bologna contribution no. 1459.


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