Ontogeny of a subtidal point bar in the microtidal Venice Lagoon (Italy) revealed by three-dimensional architectural analyses

Sedimentological and architectural features of meandering subtidal channels are relatively unexplored, and their deposits are commonly investigated based on facies models set up for intertidal meandering channels. The Venice Lagoon (northern Adriatic Sea, Italy) is affected by a micro-tidal regime and hosts a dense network of active and buried tidal channels. It represents an excellent natural laboratory to improve the current knowledge on subtidal meander morphodynamics and related deposits. In this study, the integration of high-resolution geophysical images and core data allows reconstruction of the architectural three-dimensional model of a meandering subtidal palaeochannel, which is buried below a modern subtidal flat. The study palaeochannel was 35 m wide and 3 m deep, and formed three adjacent meander bends and related point bars. A detailed three-dimensional architectural reconstruction was carried out for deposits associated with one of these meander bends, that was crossed by a minor, low-sinuosity channel with two minor bank-attached bars. This reconstruction highlights that the study point bar has a horseshoe shape, which arose from the onset of bar accretion from an already-sinuous channel. Reconstructed growth stages of the studied bends show that point-bar accretion can follow different styles of planform transformation, also experiencing simultaneously landward (or seaward) deposition according to the dominant flow direction (i.e. local tidal asymmetry). The analyses show that planform transformations occurred in parallel with elevation changes of the related channel thalweg, which shaped pools with geometry varying with the radius of curvature of the bend. The present study highlights the relevance of high-resolution three-dimensional reconstructions to link palaeomorphodynamic processes with related sedimentary products.