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

Transcription

1 Marine Geology (25) The modern Po Delta system: Lobe switching and asymmetric prodelta growth Annamaria Correggiari *, Antonio Cattaneo 1, Fabio Trincardi ISMAR (CNR), v. Gobetti 11, 4129 Bologna, Italia Accepted 15 June 25 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 3 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 tyr 1 for Po di Pila and Po di Goro-Gnocca lobes. This estimate is remarkably consistent with the total sediment load of tyr 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 shoreparallel 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 (ranging from 1 to 3 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 25 Elsevier B.V. All rights reserved. Keywords: Po Delta; prodelta lobes; subaqueous channels; seismic stratigraphy; Adriatic; Little Ice Age * Corresponding author. Tel.: ; fax: address: anna.correggiari@bo.ismar.cnr.it (A. Correggiari). 1 Present address: Ifremer-Brest-GM, BP Plouzané, France /$ - see front matter D 25 Elsevier B.V. All rights reserved. doi:1.116/j.margeo

2 5 A. Correggiari et al. / Marine Geology (25) 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, 21). In particular, late Holocene prodelta systems are potential archives for reconstructing these short-term environmental changes over the last few millennia. Exploiting 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 depositional unit. A significant improvement in our understanding of delta and prodelta systems may derive from an integration of geophysical records and precisely 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., 24). In the Mediterranean, fine-grained prodelta deposits are typically 2 3 m thick and can be used to complement reconstructions of the evolution of their parent delta systems (Bellotti et al., 1994; Cattaneo et al., 23; Correggiari et al., 25). 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. 5 yr, includes an extensive prodelta composed of multiple prodelta lobes each extending to the south and coalescing into an undifferentiated distal prodelta system (Correggiari et al., 25). 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, 197; Cencini, 1998; Castiglioni et al., 1999; Roveri et al., 21). The overall evolution of the Po Delta system during the last few thousands of years was presented by Correggiari et al. (25) defining the major phases of delta and prodelta outbuilding since the Bronze Age (35 yr BP) and emphasizing the shore-parallel component of prodelta growth based on reconstructions of sedimentary facies and geochronological information (mainly AMS 14 C 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 5 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 relationships among prodelta lobes to reconstruct the major phases of delta and prodelta outbuilding during the last 5 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 prodelta construction; and 4) the possible differences in style of sediment transport during distinctive phases of delta growth. We anticipate that thickness reconstructions from seismic profiles, offshore, only record the net accumulation that is preserved after possible episodes 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 74,5 km 2, of which 3,79 km 2 are above 2 m in elevation and the remainder is an aggrading alluvial plain (Nelson, 197). About half of the Po plain is below 5 metres of elevation. The Po River is 691 km long and has a relatively steep gradient upstream of Pontelagoscuro (located 1 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 38

3 A. Correggiari et al. / Marine Geology (25) km 2. At the time of maximum marine transgression (ca. 5 6 kyr BP; Trincardi et al., 1996; Cattaneo et al., 23), the shoreline was on the order of 3 km inland with respect to its present location (Castiglioni et al., 199; Bondesan et al., 1995a,b; Correggiari et al., 1996; Amorosi and Milli, 21). 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 6 km along the Adriatic coast of Italy (Trincardi et al., 1996; Cattaneo et al., 23) Po Delta geomorphology The Po Delta plain occupies a broad area extending east of Ferrara and along a coastal area of ca. 12 km between the Adige and Reno rivers (Fig. 1). Most of the lower delta plain (ca. 155 km 2 ) 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 topographic 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 morphology (Dal Cin, 1983). Decreasing sediment load over the last few decades was caused primarily by intensive sand excavation within the river (up to m 3 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 (197)). Two additional regulated branches, south of the modern delta, are Po di Volano and Po di Primaro (occupied today by the diverted Reno River) 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 1 yr indicate rates of subsidence of.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 production on land and offshore, is up to 1 cm yr 1, locally and over short intervals (Selli and Ciabatti, 1977; Castiglioni et al., 199; Baù et al., 2) Discharge regime The Alpine and Apennine tributaries of the Po River display distinctive characters. The Alpine tributaries flow through former glacial valleys, while the Apennine rivers include only a short stretch of high altitude course and have lower courses with low gradients. 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 catchment (Tomadin and Varani, 1998), and resulting in a bimodal river discharge with a peak in spring and one in autumn (Correggiari et al., 25). The mean annual rainfall on the Po watershed is 117 mm, but reaches twice this value if calculated for the Apennine tributaries only (Correggiari et al., 25). The historical river discharge fluctuates between 275 and 11,58 m 3 s 1 with an average of 148 m 3 s 1 that correspond to a mean annual runoff of 668 mm (Nelson, 197; Po River Basin Authority, 25). The greatest concentrations of suspended load are reached in April and November, when the Apennine tributaries reach peak discharge. The Western

4 52 A. Correggiari et al. / Marine Geology (25) ' 45 ' 12 ' N km 1 maximum marine ingression Adige River Po River (Po Grande) Po di Volano Adria Venice lagoon Porto Viro Mesola -1m 12 2' -2m LEVANTE GORO MAESTRA GNOCCA ON LAND yr BP yr AD middle ages (5 th -15 th sec.) Roman Age 25-3 Iron Age 3-4 Bronze Age 4-55 PILA TOLLE -3m SCARDOVARI 1 BOREHOLE LOCATION FIG ' extent of prodelta muddy wedge ancient beach ridges Reno River, former Po di Primaro Comacchio lagoon -3m 2ms Ravenna 1ms 4ms 2ms ms Fig. 1. Synthetic map of the Po Delta area showing the extent of the prodelta offshore (from Correggiari et al., 21) and the phases of delta progradation on land (based on Stella, 1887; Ciabatti, 1967; Bondesan, 2; Stefani and Vincenzi, 25). 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, 197). A flux of 568 kg s 1 corresponds to an average concentration of suspended sediment of 336 mg l 1 at Pontelagoscuro. The average monthly concentrations for April and December are about 5 mg l 1, but during flood conditions monthly averages of 31 to 435 mg l 1 were reported by Nelson (197); however, Vignati et al. (23) only measured a maximum of 94 mg l 1 during the flood of The sediment is m 3 yr 1 (Hovius, 1998) and the sediment yield is 214 t km 2 yr 1 (Syvitski et al., 25). 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 11 3 m 3 sec 1

5 A. Correggiari et al. / Marine Geology (25) and tyr 1, respectively) shows a sub-decadal variability (three-day average value from Giandotti (1933) and annual mean value from IDROSER (1983)). The change in river discharge pattern occurred in the mid 2th century is illustrated in Fig. 2: the average annual sediment load from 1918 to 1944 and from 1956 to 1987 is tyr 1 at Pontelagoscuro station. Although the average annual water discharge is similar for the period before and after 196, the average sediment load decreases significantly in the second period, likely in response to changes in agricultural pattern, reforestation over the past 5 yr and river-bed mining for sand and gravel 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 regime and is wave-influenced, with storm-driven waves that may exceed 9 m in amplitude (Cavaleri, 2). Fresh water input from rivers is 57 m 3 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., 24, Sherwood et al., 24). 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- period period avg annual sed. load (tons x 1 6 ) avg annual water discharge (km 3 ) Annual Sediment load (tons x 1 6 ) Annual Water Discharge (km 3 ) data gap Fig. 2. Time series of averaged annual sediment load (istogram) and annual water discharge (black line) for the Po River for the period between 1918 and 1987 AD with a data gap between 1945 and 1955 AD. The scales and units for each of these variables are on the left and right of the diagram, respectively. The year 196 AD marks a sharp decrease in sediment load likely caused by increased human impact in the Po catchment.

6 54 A. Correggiari et al. / Marine Geology (25) trophic current is able to advect sediment to the south (Tomadin, 1979; Cattaneo et al., 23). 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 3 and 5 m. This dataset allows resolving offshore seismic 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 2th century (Visentini and Borghi, 1938; Visentini, 194). The volume of the three most recent Po prodelta lobes are calculated by combining direct measurements of the thickness distribution from seismic profiles, offshore, 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 3 m) detected on seismic profiles, offshore, and in boreholes on the delta plain (Amorosi et al., 25; Stefani and Vincenzi, 25). Geochronological control comes from sediment cores, discussed in previous studies, and correlation to dated historical geomorphological and bathymetric maps. This information is complemented by AMS 14 C dates (reported in calibrated years BP) discussed in previous studies (Correggiari et al., 25). 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, 186, 195 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 collected during our cruises. 4. Results The modern Po Delta system formed during the last ca. 5 yr and records the fastest interval of delta growth with a shoreline progradation of about 3 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 prodelta. 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 episodically 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 importance of individual prodelta lobes, defining phases of lobe advance and lobe retreat (Figs. 3,4). Offshore, prodelta lobes can be traced as identifiable seismicstratigraphic units by using very high resolution CHIRP-sonar profiles (Fig. 4). Using this tool and the seismic-stratigraphic approach, significant variations in prodelta lobe geometry and acoustic facies can also be reconstructed (Figs. 5,6) Summary of the pre-modern Age Po Delta history Landward of the modern Po Delta, older subdeltas were identified by correlation of borehole data (Amorosi et al., 1999; Amorosi and Milli, 21). These systems advanced in shallow embayments. In the most landward positions, at least 3 km landward of the present shoreline, the base of the delta deposits consists of clay and peat deposits dated ca. 53 cal. years BP overlying extensive lagoon deposits dated ca. 6 cal. years BP (Amorosi and Milli, 21). Since the Bronze Age (ca. 35 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 A. Correggiari et al. / Marine Geology (25) m 1 A 153 Legend Scardovari1 borehole coastline progradation coastline retreat yr AD modern seafloor B A 153AD coastline 1592AD C 1736AD 1592AD 1685AD D AD -2m -25m 15m 22 AD isobaths B km 1 N seaward extent of Po prodelta m km ravinement surface + mfs C D E yr AD 1811AD E modern seafloor prodelta thickness (m) 1 2 Scardovari1 3 km 1 seafloor ca. 15 AD 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 16 AD; from Stella, 1887; Nelson, 197; Bondesan, 2), the 2 and 25 m modern contour lines and the location of the Scardovari1 borehole (Roveri et al., 21; Correggiari et al., 25). tas; this cuspate morphology, with well-preserved beach ridges, suggests that these deltas were wavedominated (Visentini, 1931; Bondesan et al., 1995a,b; Correggiari et al., 25). The Po di Primaro was a large cuspate (wavedominated) delta detectable on historical maps and traced offshore in high resolution seismic profiles into a set of buried sandy clinoforms (Correggiari et al., 25; Fig. 1). Seismic profiles and sediment cores through the Middle Age Po di Primaro delta show sandy clinoforms that downlap seaward on older transgressive deposits in water depths of about 25 m (Correggiari et al., 25), and are draped by fine-grained sediment dated ca. 12 AD (ca. 75 cal. years BP), indicating deposition after the end of the Middle Ages (Amorosi et al., 1999). Between 115 and 12 AD (8 and 75 yr BP), a major avulsion (Rotta di Ficarolo; see Correggiari et al. (25)) 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 (8 45 yr BP) were a prolonged interval of climatic deterioration with more humid summers and colder winters, heralding the onset of the Little Ice Age (Veggiani, 199; Grove, 21). River floods increased in frequency and magnitude and resulted in

8 56 A. Correggiari et al. / Marine Geology (25) 49 74

9 A. Correggiari et al. / Marine Geology (25) repeated episodes of flooding of the Po alluvial plain. Recurrent avulsions affected the entire Po alluvial and delta plain and also affected several Apennine tributaries of the Po (Veggiani, 199). Following the abandonment 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., 25). 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) Evolution of the Modern Age Po Delta The Modern Age multi-lobe Po Delta onset after the Porto Viro diversion ( AD, ca. 35 yr BP) endorsed by the Venice Republic. This diversion took place after a 1 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 system, down to the Gargano peninsula (Cattaneo et al., 23). 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, 194) were likely brought about by a climatic change that lead to increased runoff and sediment load during the Little Ice Age (ca AD; 5 1 yr BP). This change in runoff was accompanied by increased anthropogenic forcing on the Po River regime, that included rapid deforestation and construction of artificial levees, enhancing the seaward out-building of the delta (Veggiani, 199; Correggiari et al., 25). 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 164 AD and 175 AD, the Po Delta advanced dominantly toward the southeast at 86 m yr 1 ; between 175 and 182, Po di Goro-Gnocca advanced to the southeast at 129 m yr 1 ; between 1811 and 184, the Po di Maestra prograded northwards at 6 m yr 1 ; between 184 and 1886 several lobes co-evolved with distinct and variable relative importance, and Po di Tolle became dominant, advancing at 6 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, 197; Bondesan and Simeoni, 1983; Mikhailova, 22) Very high resolution anatomy of modern Po prodelta lobes Closely spaced CHIRP-sonar profiles in the Modern 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) documents average rates of coastline advance in the order of 1 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 bathymetry in this region coincides with that of 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 Fig. 4. Above: 1886 AD map with reported the bathymetric contour of the same year and the present-day 2 m contour line, for comparison. The thickness of the Po di Tolle prodelta lobe derived from seismic-reflection profiles is reported in metres. Below: Morpho-bathymetric profile showing phases of progradation/retrogradation of the Po di Tolle lobe. This lobe underwent considerable retreat since 1886 AD (approximating the end of the Little Ice Age). The Scardovari1 borehole constrains the age of the deposits below and above the ravinement surface (rs), here coinciding with the downlap surface at the base of the prodelta. A Chirp-sonar profile offshore images the seaward termination of Po di Tolle lobe. Note that the younger Po di Pila lobe laps out on the retreating Po di Tolle lobe with a marine onlap.

10 58 A. Correggiari et al. / Marine Geology (25) 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. (25). 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.

11 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 5 times) to enhance the stratigraphic relations among delta lobes. A. Correggiari et al. / Marine Geology (25)

12 6 A. Correggiari et al. / Marine Geology (25) 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 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- 153AD coastline 1886AD coastline Adige R. Porto Viro Po di Volano -5m Po di Levante Po di Goro -2m -1m 1685AD 1.5m 3 9 Po di Maestra Po di Gnocca Goro lagoon 1736AD -2m w.d. 22AD -15m Po di Tolle m m m 12 Po di Pila -25m w.d. 22AD N km 1 TOLLE PROFILE A 153AD coastline Adige R. 22AD coastline Po di Levante Porto Viro Po di Volano Po di Goro Po di Gnocca Goro lagoon -5m -1m Cà Venier 1685AD 1736AD -15m Po di Tolle Po di Maestra -2m Po di Pila N km 1-25m m -3m PILA PROFILE B Adige R. N km 1 C Adige R. N km 1 D Po di Levante Porto Viro Po di Maestra Po di Levante Porto Viro 1.5m 3 9 Po di Maestra seaward extent of Po prodelta 153AD coastline Po di Volano 22AD coastline Po di Goro 2 1.5m -5m -1m Goro lagoon AD -15m Cà Venier Po di Gnocca Po di Tolle 1736AD -2m -25m Po di Pila GORO-GNOCCA PROFILE -3m Po di Volano 22AD coastline Po di Goro -5m -1m Goro lagoon Po di Gnocca -15m Cà Venier Po di Tolle -2m Po di Pila -25m TOLLE LOBE PILA LOBE GORO-GNOCCA LOBE -3m

13 A. Correggiari et al. / Marine Geology (25) izontal transgressive ravinement surface, here coinciding 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-dominated 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 distinctive internal architecture that likely reflects differences in discharge regime and in the exposure to storms and marine currents (Fig. 6). In the next section 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 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 6 m yr 1 between 184 and 1886 AD (Visentini and Borghi, 1938; Fig. 4). Fig. 7A shows the thickness 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 12 km 2 and exceeds 1 m in thickness only over about 1 km 2. Assuming a dip of 18 for the basal foresets (not detectable in the southwest area), a conservative volume estimate for the lobe is ca..5 km 3. The Scardovari1 borehole (Roveri et al., 21; Trincardi and Argnani, 21) located in the lagoon between Po di Gnocca and Po di Tolle recovered about 3 m of shoaling upward deltaic deposits above a 5 m-thick transgressive continental and marine record. AMS 14 C dates constrain the age of the sandy and bioclastic transgressive record between 11, and 9 BP cal. years, and yielded 75 cal. years BP (i.e. 12 AD) at 27 m below the core top (Correggiari et al., 25). 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 downriver 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 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 bidirectional downlap on shore-parallel profiles. The Po di Tolle prodelta lobe shows the best evidence of subaqueous distributary channels in the prodelta. A set of CHIRP-sonar profiles parallel to the bathymetric contour indicates that these subaqueous channels are between 15 and 3 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 overall construction of the downlapping lobe and the presence 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 penetration 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 Fig. 7. A) Thickness map of the Po di Tolle and Po di Maestra prodelta lobes based on seismic profiles offshore, superimposed on the bathymetric and coastline map of 1886 AD, when a major retreat of this lobe initiated. B) The shore-parallel thickness distribution of the Po di Pila lobe is shown on the modern bathymetry of the Po prodelta. The major phases of advance are , and after 195 AD (see Fig. 8). The thickness plotted on the map above refers to this latter phase only. C) The thickness of the Po di Goro-Gnocca prodelta lobe is reduced compared to the Po di Pila and Po di Tolle lobes and the extent of the deposit is broader and more uniform, likely reflecting a reduced advection component and a more sheltered position with respect to the dominant southerly current compared to the other two lobes. D) Overlap and thickness (in metres) of the four lobes shown separately in A, B and C. The Po di Tolle and Po di Maestra lobes have a dashed line because they are older and partially buried by the other two lobes. The extent of the Po Delta and prodelta is in grey.

14 62 A. Correggiari et al. / Marine Geology (25) Cà Zuliani TOLLE yr AD modern seafloor (22AD) A km 5 1 Cà Venier 1685 PILA Cà Zuliani yr AD B km modern seafloor (22AD) yr AD GORO - GNOCCA 1886 C 1 15 modern seafloor (22AD) km 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 186 and 22 almost coincide while deposition has occurred in the prodelta lobe. sandy silt (Correggiari et al., 25). Three stacked cutand-fill units are visible in the Po di Tolle lobe, and appear progressively more confined landward suggesting a progressive reduction in river discharge (Fig. 9) 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-

15 Fig. 9. CHIRP-sonar profiles (spaced about 5 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. Correggiari et al. / Marine Geology (25)

16 64 A. Correggiari et al. / Marine Geology (25) ments the relevance of land reclamation particularly between 187 and 196 AD. The Po di Pila prodelta 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 bathymetric surveys between 195 and 1953; Fig. 1, modified from Visentini and Borghi (1938), and Adige R. Porto Viro 153AD coastline 1685AD Po di Volano 195AD coastline Po Grande -2 m 1736AD LEGEND Po di Levante 1.5 Goro lagoon 6m 3m Po di Gnocca Po di Goro AD bathymetry Ancient coastlines (yr AD) Pila and Goro-Gnocca prodelta lobe thickness (m).5 Po di Maestra Po di Pila Po di Tolle Scardovari lagoon N km m m m retreat / erosion retreat/ erosion Difference in bathymetry AD:.5 m and steps of 1 m (1.5; 2.5; etc) = deposition m -.5 m and steps of -1 m (-1.5; -2.5; etc) = erosion Fig. 1. Map reporting the net bathymetric difference from 195 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 distribution of this delta lobe exceeds 1 m over an area of ca. 25 km 2 and 1.5 m over an area of ca. 2 km 2 (Fig. 7B). A conservative volume estimate is therefore ca. 1.5 km 3. On seismic-reflection profiles, the internal reflector 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. Downlap terminations are particularly clear where the underlying Po di Tolle lobe is characterized by a mounded upper surface (Figs. 6,9) Po di Goro-Gnocca prodelta lobe At a seismic-stratigraphic scale, the last outbuilding phase of the Po di Pila is coeval to the reactivation 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 subparallel and closely spaced making it relatively difficult to disentangle individual prodelta lobes offshore. 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 177 a dyke was constructed on Po Grande to divert the Po di Gnocca branch to the south (Visentini and Borghi, 1938). From 1886 to 22 the sediment supplied through the Po di Goro- Gnocca outlets fed a large coastal sand spit elongated to the southwest (Simeoni et al., 2). Offshore, VHR seismic profiles identify the Po di Goro- Gnocca prodelta lobe over an area of ca. 25 km 2 (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-

17 A. Correggiari et al. / Marine Geology (25) tribution, an estimate that the volume of the Po di Goro-Gnocca lobe is in the order of.6 km 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., 25). This deposit, resolved in seismic-reflection profiles only where thicker than about 1 m, has a shore-parallel distribution to the southwest, like the older seismic-stratigraphic units that compose the prodelta (Correggiari et al., 25, their Fig. 15). Locally, and depending on the underlying prodelta morphology, the uppermost muddy deposit shows a marine onlap termination 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 (2 2 mg l 1 ) concentrations in the bottom waters and 21 Pb sediment accumulation rates also indicate southward dispersal (Price et al., 1999) 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 therefore 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 8% porosity, the volume of the Po di Pila lobe is m 3, and the total sediment accumulation is in the order of tons, and the Po di Goro- Gnocca lobe has an estimated volume of m 3 and a total sediment accumulation of tons. This gives an average mass accumulation rate of tons yr 1 for Po di Pila and 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 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 (constructional) and transgressive (destructive) phases; this alternation defines delta cycles (sensu Scruton, 196; see also Amorosi et al., 25). Popular delta models developed over the last 3 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 relative to the coast and prevailing direction of wave and currents (McManus, 22; Trincardi et al., 24; Correggiari et al., 25). 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, including 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 geochronology of delta and prodelta systems; and 5) the origin of shallow water channels within prodelta lobes Modern Po Delta system: growth patterns and volume variations Estimates of growth pattern and volume variation in prodeltas should take into account volume reductions 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

18 66 A. Correggiari et al. / Marine Geology (25) comes from Fig. 1, where areas of net accumulation vs. erosion are defined by subtracting two bathymetric surveys from 195 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-contemporaneous 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 sediment volumes (in 1 6 m 3 ) of the main delta lobes with time steps of 25 to 5 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 when the Po di Pila lobe loses more than m 3 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 Asymmetry of the Po Delta and prodelta system Bhattacharya and Giosan (23) recently introduced the concept of the asymmetric delta to account for the complexity of wave-influenced deltas. The modern Po Delta system, including its thick prodelta, represents a particular case of asymmetric delta. Indeed, the subaerial delta is more developed 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., 25). A semi-quantitative estimate of this asymmetric sediment partitioning in the delta is given in Fig. 11 and shows highest accumulation south of Tolle and Goro-Gnocca outlets. In other examples of asymmetric deltas, the growth of individual progradational lobes may even result in the formation of sheltered bbay-likeq areas (Bhattacharya and Giosan, 23). In the Po system, not only the overall prodelta distribution is asymmetric but also individual prodelta 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 termination 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 accommodation 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 Fig. 11. A) Reference map of the Po Delta. B) Reference map of Visentini (194) with location of the bathymetric sections measured. C) Quantitative estimate of sediment volume variations of the Po prodelta lobes calculated from the difference of bathymetric profiles (from north, up, to south) measured in four time intervals as indicated (modified from Visentini, 194). D) Cumulative value of sediment volume over the entire interval between 1811 and 1935 AD. Note that the Po di Pila lobe shows a marked change with dominant retreat and erosion (negative values) in the North, and net progradation in the South. The southern half of the delta has substantially advanced, regardless of the short-term variations in discharge at each delta outlet. In C and D, shaded grey areas highlight the position of the main Po Delta lobes referred to the number of bathymetric section marked in B.