Morphodynamic evolution and stratal architecture of translating tidal point bars: Inferences from the northern Venice Lagoon (Italy)

The widespread distribution of tidal creeks and channels that undertake meandering behaviour in modern coasts contrasts with their limited documentation in the fossil record, where point-bar elements arising from the interaction between a mix of both fluvial and tidal currents are mainly documented. The sedimentary products of tidal channel-bend evolution are relatively poorly known, and few studies have focused previously on specific facies models for tidal point bars present in modern settings. This study improves understanding of tidal channel meander bends through a multi-disciplinary approach that combines analyses of historical aerial photographs, measurements of in-channel flow velocity, high-resolution facies analyses of sedimentary cores and three-dimensional architectural modelling. The studied channel bend (12 to 15 m wide and 2 to 3 m deep) drains a salt marsh area located in the north-eastern sector of the microtidal Venice Lagoon, Italy. Historical photographs show that, during the past 77 years, the bend has translated seaward ca 15 m. Results show that the channel bend formed on a non-vegetated mud flat that was progressively colonized by vegetation. Seaward translation occurred under aggradational conditions, with an overall migration rate of 02 to 03 m year 1, and was promoted by the occurrence of cohesive, poorly erodible outer bank deposits. Ebb currents are dominant, and translation of the channel bend promotes erosion and deposition along the landward and seaward side of the bar, respectively. Tidal currents show a clear asymmetry in terms of velocity distribution, and their offset pattern provides a peculiar grain-size distribution within the bar. During the flood stage, sand sedimentation occurs in the upper part of the bar, where the maximum flow velocity occurs. During the ebb stage, the bar experiences the secondary helical flow that accumulates sand at the toe of the bar. Lateral stacking of flood and ebb deposits has caused the formation of localized coarseningupward and fining-upward sedimentary packages, respectively.