TIDAL CHANNEL AND MUDFLAT SEDIMENTATION PROCESSES AND RATES, LAGOON OF VENICE, ITALY

The Venice lagoon is known for subsidence, relative sea level rise, and possible channel meander migration as indicated by sub-bottom profiles and related erosional and depositional characteristics of predominantly silty sediments. Recently, 13 cores were collected for the purposes of determining mudflat deposition rates and quantifying the history of channel meanders. Grain size analysis of 45 sub-samples reveals sediments to be largely comprised of silt with lesser percentages of sand and clay partly dependent on the location in the lagoon. It is thus possible to make a clear distinction between the sub-environments of channels (sandy) and mudflats (clayey silts). Plant material found preserved in eight cores enabled AMS 14C dating of 18 sub-samples ranging in age from 5370 BC to 1450 AD, all of which appear to be consistent with environmental expectations and as plotted on the relative sea level rise curve. Vertical sediment accumulation rates for three cores range from 5.8 cm/100 yrs to 23.2 cm/100 yrs (mean of 14 cm/100 yrs). Channel meander migration rates from seven cores range from 6.7 m/100 yrs to 21.7 m/100 yrs (mean of 11.9 m/100 yrs). Sub-bottom profiles reveal dipping beds of point bar growth along modern channels and are a preserved part of the channel meander migration history. The grain size distribution, radiocarbon dates, and sub-bottom profiles as well as comparisons to the sea level curve show a remarkable internal consistency supporting the developmental history of migrating tidal channel meanders and also mudflat aggradation at the rate of relative sea level rise. This analysis refines the sedimentary framework model of the depositional environments within the lagoon. It clearly enriches the understanding of lagoon dynamics and serves as a guide for environmental predictions, coastal zone management, future archaeological excavations and explanations of early human habitation. The processes found in Venice are expected to be active in other tidal lagoon settings.