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

Abstract

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 threedimensional reconstructions to link palaeomorphodynamic processes with related sedimentary products.

Hydrodynamic controls on sedimentary facies of tidal point bars: A case study in the Georgia coastal plain, USA

“…The periodic high discharge, water turbidity and sedimentation rates, combined with low salinity levels in fluvially influenced channels are conditions unfavourable for infauna colonization (Pearson & Gingras, 2006; MacEachern & Gingras, 2007; Sisulak & Dashtgard, 2012; Johnson & Dashtgard, 2014; La Croix & Dashtgard, 2015; Shchepetkina et al ., 2016). In contrast, strictly tidal conditions are more conducive for burrowing organisms due higher and less varying saline conditions (Sisulak & Dashtgard, 2012; Johnson & Dashtgard, 2014; Brivio et al ., 2016; Ghinassi et al ., 2019; Bellizia et al ., 2021). The observations in this study are corroborated by Wang et al .…”

Two-dimensional model of flow and transport in porous media: Linking heterogeneous anisotropy with stratal patterns in meandering tidal channel deposits of the Venice Lagoon (Italy)

“…The prototype case deposits are buried below 1 m of tidal-flat deposits located close to the S. Francesco del Deserto island, where water depth is ca. 1 m. These deposits were formed in the late Holocene by a 35 m-wide paleochannel (Madricardo et al, 2007) and consist of three adjacent point-bar bodies, named PB1, PB2 and PB3 which are on average 2.5 m thick (Bellizia et al, 2022). The channel belt axis of the paleochannel system is WNW-ESE oriented (fig.…”

“…Sedimentary cores show that bar deposits mainly consist of laminated silt and very fine sand with 0.5 -1 mm-thick muddy laminae. Channel fill and over-bank deposits are made of homogeneous mud (Madricardo et al, 2007;Bellizia et al, 2022). A detailed 3D model based on seismic data (Bellizia et al, 2022) allowed us to depict the effective geometry of bar bodies and to identify four major evolutive stages of the study channel.…”

“…Channel fill and over-bank deposits are made of homogeneous mud (Madricardo et al, 2007;Bellizia et al, 2022). A detailed 3D model based on seismic data (Bellizia et al, 2022) allowed us to depict the effective geometry of bar bodies and to identify four major evolutive stages of the study channel. Reconstructed geometry of bars shows that they started to accrete from a sinuous channel, as clearly shown by the arcuate shape of bar PB1, which encloses homogeneous over-bank mud (fig.…”

“…The present work investigates modes of 2D flow within deposits of an ancient tidal channel of the Venice Lagoon (Italy) using numerical models of flow and transport accompanied by a high resolution characterization of stratal geometries (Bellizia et al, 2022), spatial distribution of sediment properties, and reconstruction of anisotropy directions. A detailed geological description is less important in areas filled with fine sediments where no flow of practical interest occurs, and so our focus is mainly concentrated on the tidal channel.…”