Early Holocene transgressive palaeogeography in the Po coastal plain (northern Italy)

To understand the complex stratigraphic response of a coastal depositional system to rapid eustatic rise and sediment inputs, the evolution of the Adriatic coastline and Po River system, during the post-glacial (Holocene) transgression, was investigated. The landward migration and evolution of a wave-dominated estuary was mapped, based on an extensive data set comprising 14 boreholes, 28 core descriptions and 308 piezocone tests, chronologically constrained between 11·5 and 7·0 kyr bp by 137 radiocarbon dates. Palaeogeographic maps reveal temporal differences in retrogradational geometries and mechanisms that likely underpin shoreline retreat. The Po estuary initially developed within a shallowly incised valley and then spread onto the interfluves. Between 11·5 and 9·2 kyr bp the Po fluvial system became avulsive/distributive and wetlands developed in topographically depressed areas. The shoreline retreated at a mean rate of ca 10 m year−1, between 9·2 kyr and 7·7 kyr bp, following a stepped trajectory at the centennial scale. After 7·7 kyr bp, bayhead deltas started to prograde and partially filled the estuary. The overall stratigraphic architecture is interpreted to reflect the sedimentary response of the coastal depositional system to the main pulses of early Holocene eustatic rise. The influence of antecedent topography, partly due to local subsidence, was dominant at the time of initial transgression. Basin morphology influenced sediment dispersal and partitioning. Sediment supplied by the Po River was trapped within the estuary, whereas coastal sand bodies at the estuary mouth were fed by alongshore currents and by reworking of older barriers. High-resolution age control that ties facies evolution to independently constrained eustasy provides direct data to test models of short-term coastal retreat under conditions of relative sea-level rise, and makes this case study a useful analogue for the interpretation of ancient marginal-marine, retrogradational systems where only stratal geometries are available.