Point-bar brink and channel thalweg trajectories depicting interaction between vertical and lateral shifts of microtidal channels in the Venice Lagoon (Italy)

Abstract

ABSTRACT Although in tidal point bars, the point-bar brink (i.e. the break-in-slope between bar top and bar slope) and the channel thalweg (i.e. the deepest part of the channel) are thought to shift horizontally toward the outer bank, the occurrence of stable to slow-migrating channels in high-aggradational settings, e.g. salt marshes, is likely to promote a mixed, latero-vertical shift of meandering channel systems. The present study focuses on the trajectories of point-bar brink and channel thalweg of eight point bars located in the salt marshes of the Venice Lagoon, in order to understand how vertical aggradation can interact with lateral migration to shape their stratal geometries. High resolution facies-analysis carried out on closely-spaced sediment cores, collected along the bar axis, allowed us to define specific trajectories, which were classified either as ascending or descending, and linear or non-linear. All the analyzed brink trajectories are ascending, and show evidence of lateral shift of the bar brink under aggradational conditions of surrounding marshes. Development of non-linear brink trajectories is linked with changes in the ratio between vertical and lateral shift rates of the brink, which is in turn dictated by changes in local base level due to substrate compaction. Conversely, the thalweg trajectories can be either ascending or descending, reflecting an interaction between rates of lateral shift and aggradation/degradation of the channel floor. The brink and thalweg can either shift consistently (e.g., both trajectories are ascending) or incongruously (e.g., ascending brink vs. descending thalweg trajectory), reflecting different attitudes of the channel to maintain or increase its cross-sectional area.