MORPHODYNAMICS AND STRATAL ARCHITECTURE OF A TRANSLATING TIDAL POINT BAR (VENICE LAGOON, ITALY)

The present study contributes at improving our understanding of tidal meander bends in saltmarsh settings, through a sedimentological study of an active bend of the Venice Lagoon (Italy). The study channel, about 15-18 m wide... [ view full abstract ]

The present study contributes at improving our understanding of tidal meander bends in saltmarsh settings, through a sedimentological study of an active bend of the Venice Lagoon (Italy). The study channel, about 15-18 m wide and 2-3 m deep, forms a bend with a radius of curvature of about 17 m. ADCP measurements in the bend apex zone show a maximum speed of about 0.25 m/s and highlight that the zone of maximum flow velocity is located along the inner and outer bank during the flood and ebb stages, respectively. The analysis of historical photos shows that during the past 70 years the bar shifted seaward about 10 m without increasing its sinuosity. Sedimentological core analyses show that the study bar covers a sub-horizontal, 5-10 cm thick, shell-rich layer consisting of mud-free medium sand, which accumulated as a bypass lag in the deepest part of the channel. Overlying bar deposits increase in thickness from 1.3 to 2.0 m moving from the landward to the seaward side of the bend. Bar deposits show an overall fining-upward trend from medium sand to silt and are characterized by seaward-dipping (10-15°) beds. Bar sand is commonly massive because of the intense bioturbation, although locally plane-parallel stratifications can be observed. Bar sand is covered by muddy saltmarsh deposits, which are 0.7 m thick in the seaward side of the bar and thin seaward to pinch out on the downstream side. This mud consists of organic-rich, tidal flat mud grading upward into saltmarsh, oxidized mud. Sedimentary cores from the outer bank area, show that the channel cut through 1.7 m thick saltmarsh to tidal flat mud overlaying 0.3 m muddy sand tidal flat deposits.
Seaward shift associated with no changes in bend sinuosity shows that point bar translation can occur also in tidal bends. Translation of the study meander causes erosion along the landward side of the bar an deposition along its seaward side, and, as documented for fluvial bars, the occurrence of cohesive, non-erodible deposits along the outer bank seems to be a key forcing for promoting bar translation. Bar translation occurs here under aggradational conditions, as attested by the seaward thickening of point bar deposits and related thickening of overlying salt marsh mud.