An integrated approach to determine three‐dimensional accretion geometries of tidal point bars: Examples from the Venice Lagoon (Italy)

Abstract

Low rates of lateral migration (centimetres to decimetres per year) combined with relatively high rates of vertical accretion (millimetres to centimetres per year) recorded in microtidal channels of the Venice Lagoon (Italy) give rise to point-bar geometries and internal facies arrangements that differ substantially from widely accepted models of point-bar sedimentary architecture. In this study, field data from the Venice Lagoon are combined with a three-dimensional forward stratigraphic model, the 'Point-Bar Sedimentary Architecture Numerical Deduction' (PB-SAND), to predict the stratal geometries of point bars formed in aggradational settings. The PB-SAND uses a combined geometric and stochastic modelling approach that can be constrained by field evidence. The model applied determines the geometry of four point bars generated by 9 to 11 m wide channels cutting through salt marshes. An iterative best-fit modelling approach has been used to obtain multiple simulations for each case study, each of which fits the observations derived from the analysis of time-series historical aerial photographs and 44 sedimentary cores. Results demonstrate how the geometry of the bars is determined by the development of two key stratal surfaces: the point-bar brink and channel-thalweg surfaces. These surfaces are defined by the progressive translation and vertical shift of the point-bar brink (i.e. break of slope between bar top and bar slope) and the channel thalweg (i.e. deepest part of the channel) during bar evolution. The approach is used to: (i) reconstruct three-dimensional point-bar geometries; (ii) propose alternative reconstructions; (iii) provide insight to drive the acquisition of additional data to better constrain the proposed models; and (iv) provide insight into the mechanism of bar growth for slowly migrating channels in settings subject to relatively high rates of aggradation. This study highlights how interaction between styles of planform transformation and latero-vertical shifts of meandering channels can determine the geometry of related sedimentary bodies. Keywords Aggradation, channel-thalweg trajectory, point-bar brink trajectory, tidal meander, Venice Lagoon. negligible with respect to lateral migration processes. Hence, point bars are commonly described as purely laterally accreting bodies

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

“…The subsequent gentle rise of the thalweg during stage 3 was probably related to the maintenance of its formative depth, which was reached at the end of stage 2. During this stage, sediments are accumulated in the channel to maintain a constant equilibrium depth in the frame of keeping dynamic equilibrium conditions (sensu Allen, 2000) with the surrounding aggradational tidal flats (Cosma et al, 2019(Cosma et al, , 2021. This process could have also been enhanced by the activation of the minor channel, which connected bends 1 and 3 causing a local drop in the strength of erosional forces within bend 2 (cf.…”

“…Distinctive sedimentary features of these bar deposits are classically considered to be (Allen, 1982): (i) a shell‐rich channel lag; (ii) bedforms diagnostic of bidirectional flows; (iii) alternation between sandy and silty layers; and (iv) intense bioturbation. Recent studies provided new insights on the morphometry and sedimentology of intertidal meanders (Fagherazzi & Gabet, 2004; Pearson & Gingras, 2006; Choi, 2011; Hughes, 2012; Choi et al ., 2013; Choi & Jo, 2015; Brivio et al ., 2016; Ghinassi et al ., 2018b; Cosma et al ., 2019, 2021; Finotello et al ., 2019), revealing that: (i) intertidal‐meander migration rates are comparable to those of their fluvial counterparts, once properly normalized by the channel width (Finotello et al ., 2018); (ii) geometry of intertidal point bars is the result of the interaction between channel‐migration rate and aggradation rate of the surrounding marsh platform (Brivio et al ., 2016; Cosma et al ., 2019, 2021); (iii) morphometry of intertidal meanders can differ from that of their fluvial counterparts (Finotello et al ., 2019); and (iv) complex flow configurations, due to the occurrence of mutually evasive currents and recirculation zones, can prevent portions of bar deposits from experiencing bidirectional currents, thereby inhibiting the formation of bedforms retaining the signatures of bidirectional flows (Dalrymple & Choi, 2007; Li et al ., 2008; Choi et al ., 2013; Finotello et al ., 2020a).…”

Modeling the Role of Compaction in the Three‐Dimensional Evolution of Depositional Environments

“…In this contribution aimed at pointing out the features of the novel subsurface model, ω has been specified through simplified functions. In our viewpoint NATSUB3D represents a module that could be effectively coupled to surface morphodynamic simulators, for example, DELFT3D (https://oss.deltares.nl/web/delft3d), TELEMAC‐MASCARET (http://www.opentelemac.org/), or other research simulators addressing the evolution of more specific depositional environments but neglecting soil compaction (Cosma et al., 2021; Kim et al., 2009). This will improve the available tools to understand the dynamics of depositional landscapes, as well as increase our ability to quantify processes that drive their evolution.…”

Controls on fluvial meander‐belt thickness and sand distribution: Insights from forward stratigraphic modelling

“…Even though the modelling approach allows evaluating the impact of aggradation on sand volume and distribution ( cf . Cosma et al ., 2020; Yan et al ., 2020), channel‐bed aggradation was not simulated in this study to enable isolation of the effects of planform evolution. The process of vertical aggradation would generally increase the overall thickness of point‐bar deposits and overall sand volume and fraction (Willis & Tang, 2010; Ghinassi et al ., 2014).…”