Very-high resolution analysis of a transgressive deposit in the Northern Adriatic Sea (Italy)


Giorgia MOSCON, Annamaria CORREGGIARI, Cristina STEFANI, Alessandro FONTANA, Alessandro REMIA
The Adriatic Sea is characterized in the northern and central shelf by different generation of isolated transgressive bodies formed and drowned in-place during the last relative sea-level rise. The continental shelf is characterized by a low gradient and, within the transgressive deposits the episodic variations in sea level and sediment supply caused the formation of backstepping parasequences. The transgressive bodies have been studied in detail because they have considerable amount of sorted sand exploitable for beach nourishment. A transgressive deposit, located south of the Po Delta, offshore Ravenna at depth of 34-35 m, has been investigated with a total coverage of very-high resolution (VHR) seismic profiles and high number of cores in order to understand the stratigraphic evolution during the late Quaternary sea-level rise. The transgressive body is composed of four seismic units separated by high-amplitude and high continuity reflectors, corresponding to peat and organic-rich layers indicating different depositional phases. The core analysis confirmed the presence of different units showing sediment variation from clay with peat-layer at the base to fine-grained sand at the top of the deposit. Furthermore, the core analysis gave information about the thickness of the deposit. The thickness of the transgressive body is from 3 to 5 m and each unit varies from 1 to 2 m. In particular, the sand portion reached a maximum thickness of 1.5 meters. The seismic analysis and the digital elevation model constructed for key surfaces highlighted the channel system direction was towards ENE during Last Glacial Maximum and during sedimentation of Unit 1 and 2, while it shifted toward ESE during the formation of Unit 3. Moreover, to constrain the chronology of the sedimentary evolution, some peat and organic-rich samples, have been dated with 14C method. These organic horizons are evident seismic reflectors and correspond to key surfaces. They are characterized by brackish lagoon facies and could be a proxy indicator for the relative paleo-sea level. Along with the previous data, they indicated that the sedimentation of the studied transgressive body occurred around 10,000 cal. a BP.
Quaternary, sea-level rise, sequence stratigraphy, beach nourishment, CHIRP-sonar
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