Millennial-scale depositional cycles from the Holocene of the Po Plain, Italy

The Holocene depositional history of southeastern Po Plain on time scales of 10 3 yr is reconstructed, based upon integrated sedimentological and micropalaeontological analyses of nine continuously-cored boreholes, about 40 m deep. Major palaeoenvironmental changes include the rapid landward migration of a barrier-estuary–lagoon system during the Early–Middle Holocene (transgressive systems tract—TST), followed by extensive delta progradation in the last 6000 yr (highstand systems tract—HST). Detailed facies analysis of cores combined with the identification of 12 microfossils (benthic foraminifer and ostracod) associations allow an ultra-high-resolution sequence–stratigraphic framework to be reconstructed. Particularly, eight small-scale, high-frequency cycles, about 3–5 m thick and spanning intervals of time of about 1000 yr, can be physically traced throughout the study area. Interpretation of these cycles, which are invariably bounded by sharp flooding surfaces and generally show internal shallowing-upward trends (parasequences), indicates that relative sea-level changes during the Holocene were episodic and punctuated by rapid phases of sea-level rise, followed by periods of stillstand (or decreasing sea-level rise). From seaward to landward locations, parasequence boundaries document beach-barrier migration, bay-head delta abandonment and increasing accommodation in the coastal plain. The ensuing phases of sea-level stillstands resulted in the progressive filling of the newly formed accommodation space, through beach progradation, extensive mud deposition in behind-barrier lagoonal (estuarine) and marsh deposits, and aggradation in bay-head delta systems at the head of estuaries. Eustacy appears to be the major controlling factor of the retrogradational stacking pattern of parasequences within the TST. By contrast, a complex interplay of eustacy, sediment supply and subsidence, with an increasing influence of autocyclic mechanisms, such as channel avulsion and delta lobe abandonment, controlled facies architecture within the HST. The maximum flooding surface cannot be assumed to be synchronous, its timing being strongly dependent upon local variations in sediment influx and subsidence. This study shows that the micropalaeontologic characterization of mud-prone (coastal plain and estuarine) successions in terms of water depth and salinity can lead to very accurate sequence–stratigraphic interpretations, allowing identification of parasequence boundari