Holocene deposits exhibit distinct, predictable and chronologically constrained facies patterns that are quite useful as appropriate modern analogs for interpreting the ancient record. In this study, we examined the sedimentary response of the Po Plain coastal system to short term (millennial scale) relative fluctuations of sea level through high resolution sequence stratigraphic analysis of the Holocene succession.Meters thick parasequences form the building blocks of stratigraphic architecture. Above the Younger Dryas paleosol, a prominent stratigraphic marker that demarcates the transgressive surface, Early Ho locene parasequences (#s 1 3) record alternating periods of rapid flooding and gradual shoaling, and are stacked in a retrogradational pattern that mostly reflects stepped, post glacial eustatic rise. Conversely, Middle to Late Holocene parasequences (#s 4 8) record a complex, pattern of coastal progradation and delta upbuilding that took place following sea level stabilization at highstand, starting at about 7 cal ky BP. The prominent transgressive surface at the base of parasequence 1 correlates with the period of rapid, global sea level rise at the onset of the Holocene (MWP 1B), whereas flooding surfaces associated with parasequences 2 and 3 apparently reflect minor Early Holocene eustatic jumps reported in the literature. Changes in shoreline trajectory, parasequence architecture and lithofacies distribution during the following eustatic highstand had, instead, an overwhelming autogenic component, mostly driven by river avulsions, delta lobe switching, local subsidence and sediment compaction. We document a ~1000 year delayed response of the coastal depositional system to marine incursion, farther inland from the maximum landward position of the shoreline. A dramatic reduction in sediment flux due to fluvial avulsion resulted in marine inundation in back barrier position, whereas coastal progradation was simultaneously taking place basinwards.We demonstrate that the landward equivalents of marine flooding surfaces (parasequence boundaries) may be defined by brackish and freshwater fossil assemblages, and traced for tens of kilometers into the non marine realm. This makes millennial scale parasequences, whether auto or allogenic in origin, much more powerful than systems tracts for mapping detailed extents and volumes of sediment bodies.The Holocene parasequences of the Po coastal plain, with strong age control and a detailed under standing of sea level variation, may provide insight into the driving mechanisms and predictability of successions characterized by similar depositional styles, but with poor age constraint, resulting in more robust interpretations of the ancient record.

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.

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Although general trends in transgressive to highstand sedimentary evolution of river-mouth coastlines are well-known, the details of the turnaround from retrogradational (typically estuarine) to aggradational-progradational (typically coastal/deltaic) stacking patterns are not fully resolved. This paper examines the middle to late Holocene eustatic highstand succession of the Po Delta: its stratigraphic architecture records a complex pattern of delta outbuilding and coastal progradation that followed eustatic stabilization, since around 7Á7 cal kyr BP. Sedimentological, palaeoecological (benthic foraminifera, ostracods and molluscs) and compositional criteria were used to characterize depositional conditions and sediment-dispersal pathways within a radiocarbon-dated chronological framework. A three-stage progradation history was reconstructed. First, as soon as eustasy stabilized (7Á7 to 7Á0 cal kyr BP), rapid bay-head delta progradation (ca 5 m year À1 ), fed mostly by the Po River, took place in a mixed, freshwater and brackish estuarine environment. Second, a dominantly aggradational parasequence set of beachbarrier deposits in the lower highstand systems tract (7Á0 to 2Á0 cal kyr BP) records the development of a shallow, wave-dominated coastal system fed alongshore, with elongated, modestly crescent beaches (ca 2Á5 m year À1 ). Third, in the last 2000 years, the development of faster accreting and more rapidly prograding (up to ca 15 m year À1 ) Po delta lobes occurred into 30 m deep waters (upper highstand systems tract). This study documents the close correspondence of sediment character with stratal distribution patterns within the highstand systems tract. Remarkable changes in sediment characteristics, palaeoenvironments and direction of sediment transport occur across a surface named the 'A-P surface'. This surface demarcates a major shift from dominantly aggradational (lower highstand systems tract) to fully progradational (upper highstand systems tract) parasequence stacking. In the Po system, this surface also reflects evolution from a wave-dominated to 3029 river-dominated deltaic system. Identifying the A-P surface through detailed palaeoecological and compositional data can help guide interpretation of highstand systems tracts in the rock record, especially where facies assemblages and their characteristic geometries are difficult to discern from physical sedimentary structures alone.