Seafloor undulation pattern on the Adriatic shelf and comparison to deep-water sediment waves

On the Adriatic shelf, a large field of seafloor and subsurface undulations (typically 300 m wide, 2.5 m high, and several km long, parallel to the bathymetric contour) affects the progradational clinoforms of the late Holocene prodelta wedge above a regional downlap surface that shows evidence of deformation and fluid escape. The undulations are associated with mud reliefs that occur farther seaward in elongated swarms perpendicular to the regional slope and to the crests of the undulations. A quantitative measurement of key morphobathymetric parameters (such as wavelength, height, asymmetry and wave-form indexes) was performed on 13 selected seismic-reflection profiles perpendicular to the undulations; these measurements are determined with respect to water depth or downslope distance from the shallowest undulation. The results highlight substantial differences from deep-water sediment waves; these differences include: (1) most undulations show a staircase morphology with sub-horizontal upslope limbs of 100 300 m delimited seaward by sharp edges and steep downslope limbs; (2) landward- or upslope-dipping limbs do not develop; (3) there is no consistent downslope decrease in wavelength or amplitude as opposed to most cases of sediment waves related to overflowing turbidity currents; (4) there is a consistent downslope loss in crest continuity approaching areas where mud reliefs and sediment deformation are evident at the seafloor; (5) many of the undulations show differences in angle of climb and in some cases a single undulation at the seafloor results from the burial and merger of more undulations in the underlying section.These differences compared to bottom current or turbidity current sediment waves are augmented by the observation that the Adriatic shallow-water undulations are also typically associated with gas-charged sediment, fluid escape, sediment deformation and localised failure planes. It is therefore suggested that seafloor undulations evolved in response to sediment deformation and were successively amplified by differential deposition from bottom currents crossing an irregular seafloor. In general, we feel that case-by-case solutions should be found to explain the origin and evolution of seafloor wavy features, based on the geologic/stratigraphic setting, the recognition of possible weak layers and the co-existence of a variety of other deformation geometries that may occur within or adjacent to the field of wavy features. The Adriatic example can reinforce recent suggestions that besides purely depositional bedforms there exist also wave-like deformation structures and multi-process sediment waves [Faugères et al., 2002. Multi-process generated sediment waves on the Landes Plateau (Bay of Biscay, North Atlantic), Marine Geology, 182, 279 302].