Late Quaternary depositional architecture of Po and Tevere river deltas (Italy) and worldwide comparison with coeval deltaic successions

Po and Tevere rivers form two of the most important deltas of the whole Mediterranean area. Detailed investigations have been carried out in the last decade beneath the present delta plains of these two river systems using borehole data correlation, With the aid of radiocarbon dates and other dating methods (pollens, archaeology) a detailed sequence stratigraphic framework has been constructed in both areas, especially for the last glacial-interglacial cycle. Regional subsidence, combined with short-term sea-level fluctuations of relatively high amplitude, played a fundamental role in creating/subtracting accommodation during the long phase of falling sea level, which followed the Tyrrhenian (substage 5e) transgressive event. A discontinuous record of this phase is present in the subsurface of the Po Plain, whereas poor or no preservation has been detected in the Tevere area. Progradational lowstand wedges accumulated during the last glacial maximum mostly in the central part of the basins, far from the present deltas, and are lacking in the study areas, where a hiatal surface is invariably recorded below the transgressive deposits. Glacioeustasy exerted a major control on sedimentation in both areas during the latest Pleistocene-Holocene sea-level rise. Infilling of fluvial valleys and rapid landward migration of barrier-lagoon systems, in the context of wave-dominated estuaries, characterized the transgressive phase in both Po and Tevere systems. In-place drowning and transgressive submergence mechanisms of barrier migration were effective predominantly between 13.5 and 9 ka Bp. In contrast, shoreface retreat was dominant in the late transgressive phase. The maximum flooding surface (MFS) can be traced physically from continental to marine deposits, and has similar characteristics in the Po and Tevere river systems, being marked by peat layers or lagoonal deposits at landward locations and by distinctive lithological characteristics and micro- and macrofossils. assemblages basinwards. Above the MFS, the early highstand systems tract records initially the filling of the lagoons, followed by the rapid progradation of wave-dominate delta systems and flanking strandplains. Autocyclic processes, such as distributary channel avulsion, delta lobe abandonment, and local subsidence due to sediment compaction were the fundamental controlling factors on sedimentation during deposition of the HST. The stratigraphic architecture of the post-glacial Po and Tevere delta complexes closely resembles the basic facies successions documented from coeval deposits of wave-dominated and wave-influenced clastic coasts, both locally within the Mediterranean area and in other parts of the world. Our study emphasizes, however, some additional features and major differences with previously published depositional models, including (i) the development of backstepping wave-dominated estuaries during transgression; (ii) consistent changes through time in the mechanism of barrier migration; (iii) a later construction of the delta systems, with respect to what traditionally considered. (C) 2001 Elsevier Science B.V. All rights reserved.