Depositional patterns in the late Holocene Po delta system

ABSTRACT: The late Holocene Po delta system is fed by the Po river, which drains the E-W-trending Po plain (catchment area 74,500 km2) and enters the Adriatic epicontinental sea. The Po delta system includes an extensive delta plain, a wave-influenced delta front, and a broad asymmetric prodelta deposit. The prodelta shows a steeper and shorter slope to the north and a very gentle slope to the south, where the thick Po prodelta merges into the late Holocene Adriatic mud wedge. The growth of the late Holocene Po delta started 5.5 kyr BP, after the present sea-level highstand was attained. The Po delta was characterized by alternating phases of rapid advance and abandonment of its multiple deltaic lobes. This delta history reflects the forcing of high-frequency climate change, autocyclic avulsions, and anthropogenic factors, acting on variable, but typically short, time scales. The Po system has been investigated through: (1) a review of historical cartography extending back several centuries; (2) integrated surveys of VHR seismic profiles recorded offshore of the modern delta from water depths as shallow as 5 m to the toe of the prodelta in about 30 m; and (3) sedimentological and geochronological data from precisely positioned sediment cores. Historical accounts and cartography show that avulsion was an important process in Po delta building, during highstand progradation; with reference to the position of the present-day Po delta, older elements developed both to the north and to the south. During the late Bronze Age (ca. 3 kyr BP), the Po river developed two main branches: the Po di Adria, in the northern sector, and the Po di Spina, to the south. During the Roman Age (ca. 2.5-1.5 kyr BP) two other branches became dominant, both in the southern sector: the Po di Eridano and the Po di Olano. During the Middle Ages, Po di Primaro activated slightly south of the Po di Eridano, and at the same time the Po di Ariano branch developed to the north. The Ficarolo avulsion (12th century, ca. 800 yr BP) shifted the main distributary channel farther to the north, activating the Po delle Fornaci, and threatening the Venice lagoon with siltation over the following centuries. The Porto Viro cut by the Venice Republic (1600-1604 AD, ca. 350 yr BP) redirected the northern branch of the Po to a southern outlet (Po Grande), initiating the rapid formation of the Modern Po delta with a protruding morphology on the western Adriatic coast. The rapid outbuilding of the Modern Po delta with rates up to 129 m yr-1 reflects the impact of the Little Ice Age (1450-1850 AD, ca. 500-100 yr BP), probably enhanced by anthropogenic forcing of the river regime. The reconstruction of the stratigraphy of the Po prodelta and the correlation to geomorphologic and historical data in the delta plain show: (a) a marked asymmetry of the delta-prodelta system with significant down-drift sediment dispersal to the south of each individual delta mouth and asymmetric prodelta lobes; (b) short-term shifts from supply-dominated to wave-dominated delta lobes; and (c) the presence of two markedly distinct depositional elements in the prodelta. The two main kinds of prodelta deposits are: (1) shingled depositional prodelta lobes characterized by laterally continuous seismic reflectors showing evidence of variably preserved flood deposits in core, and (2) depositional prodelta lobes accompanied by "cut-and-fill" features typically detected offshore of short-lived, very active distributary channels characterized by massive silt in core. In the first kind of prodelta deposit, individual lobes onlap each other laterally, reflecting changes in the relative importance of their feeding systems over short intervals. In the second case, seismic profiles indicate that the channel-like forms have sharp bases and record repeated episodes of cut and fill while the entire lobe is building. The channel-like features typically occur in clusters, with individual widths ranging from 100 to 300 m and depths up to 4-5 m, filled with massive silt to very fine sand. The short-lived episodes of cut and fill are hypothetically attributed to episodic flood events of catastrophic reactivation of distinctive Po river branches driven by sudden increases in river discharge of natural origin or by human intervention.