Anatomy of the Holocene succession of the southern Venice lagoon revealed by very high-resolution seismic data

The southern portion of the Venice lagoon contains a relatively thick (up to 20 m) Holocene sedimentary body that represents a detailed record of the formation and evolution of the lagoon. New very high- resolution (VHR) seismic profiles provided a detailed investigation on depositional geometries, internal bounding surfaces and stratal relationships. These informations, combined with core analysis, allowed the identification of large- to medium-scale sedimentary structures (e.g. dunes, point bars), the corresponding sedimentary environment, and of retrogradational and progradational trends. In addition, the availability of dense seismic network produced a 3D reconstruction of the southern lagoon and the recognition of the along-strike and dip variability of the stratal architecture. Three main seismic units (H1–H3), separated by key stratal surfaces (S1-S3), form the Holocene succession in the southern Venice lagoon. This succession is bounded at the base by the Pleistocene/ Holocene boundary (the surface S1), which consists of a surface of subaerial exposure locally subjected to river incision. The lower part of the Holocene succession (up to 13 m thick) consists of incised valley fills passing upward into lagoon and then shallow-marine sediments (Unit H1), and therefore shows a deepening-upward trend and a retrogradational stacking pattern. A prograding delta and adjacent shorelines, showing internal clinoforms downlapping onto the top of Unit H1 (the surface S2), form the middle part of the Holocene succession (Unit H2, up to 7.5 m thick). Unit H2 is interpreted as a result of a regressive phase started about 6 kyr BP and continued until recent time. The upper part of the Holocene succession (Unit H3) consists of lagoonal deposits, including tidal channel and tidal and subtidal flat sediments, that abruptly overlie Unit H2. Unit H3 is thought to represent a drowning of the area primarily due to human interventions that created rivers diversion and consequent delta abandonment during historical time.