Analysis of Holocene sedimentary features on the Adriatic shelf from 3D very high resolution seismic data (Triad survey)

Very High Resolution (VHR) 3D seismic data were collected on the Adriatic shelf offshore the city of Ortona in order to define the small-scale geometric complexity of late Holocene deposits. Three groups of units are distinguished from the bottom up: (1) units U1 to U3 forming the Transgressive System Tract (TST); (2) units U4 and U5 representing the condensed interval deposited on the Maximum flooding surface (MFS) and the basal unit of the High stand System Tract (HST); and (3) U6 to U10 forming the progradational units of the HST. The muddy HST deposits show seafloor and subsurface sedimentary features over up to 40% of its extent on the Adriatic shelf. In the area offshore Ortona, the HST shows shore-parallel undulations in water depth of ca. 30–70 m and mud reliefs in water depth of ca. 70–110 m. The mud reliefs are subcircular in plan view and occur in swarms, perpendicular to the regional contour, that are connected to each other by shore-parallel junctions. The genesis of these features is interpreted as a combination of two different mechanisms: (1) sediment deformation in relation with the condensed section; and (2) depositional process related to shore-parallel currents interacting with topographic irregularities. The evolution of the HST sedimentary pattern is closely related to a “key unit” (U5) that influences both the seaward transition from linear to complex features and the stacking pattern.