LONG-DISTANCE IMPACT OF PO DELTA GROWTH ON THE SHORE-PARALLEL ADRIATIC PRODELTA WEDGE, DURING THE LATE HOLOCENE

Mediterranean shelves exhibit extensive shore-parallel mud-dominated regressive wedges, up to 10’s m thick, formed during the modern sea level highstand under the influence of fluvial supply and marine processes. In the Adriatic epicontinental shelf, a laterally extensive mud wedge, up to 35 m thick and with a volume of ca. 180 km³, rests above the maximum flooding surface (5.5 cal kyr BP). The Adriatic mud wedge includes three genetically-related depocenters: 1) the Po delta and prodelta, 2) the central Adriatic prodelta wedge, fed from the Po and coalescing Apennine rivers, and 3) the Gargano subaqueous delta (GSD) nourished by shore-parallel sediment advection, 600 km south of the Po delta.

The huge outbuilding of the modern delta (up to 129 m/yr) occurred since the onset of Little Ice Age (ca. 1450 - 1850 AD) under anthropogenic forcing on the river regime and increasing supply driven by colder climatic conditions. The central Adriatic wedge and the GSD are composed of sigmoidal subunits with regional extent. In the central Adriatic mud wedge, sediment provenance characterization indicates that the modern Po delta progradation and basinward shift of the river mouth increased the amount of sediment available for long-distance transport by geostrophic circulation. The GSD has a submerged topset (ca. 25-28 m w.d.) and an exclusive occurrence of marine facies associations with a marked dominance of basin dynamics. Even if detached from any direct river supply, the GSD is part of, and genetically linked to, the late-Holocene Adriatic wedge, and accounts for about 1/7th of its total volume.

Within the Adriatic mud wedge, VHR seismics, AMS ¹⁴C dates and tephrochronology allowed to correlate the main intervals of outbuilding of the Po delta with the prograding subunits of the central Adriatic mud wedge and the GSD. Geometric or compositional changes from one subunit to the next reflect: a) changes of the relative importance of Po vs. Apennine supply, and b) possible changes in the intensity of the geostrophic circulation redistributing sediment along the coast. The most recent sigmoid within the GSD deposited during the Little Ice. At least in this interval, the construction of the GSD was in phase with that of its parent delta, rather than representing a product of its abandonment and cannibalization.