Piracy-controlled geometry of tide-dominated point bars: Combined evidence from ancient sedimentary successions and modern channel networks

Establishing critical comparisons between fluvial- and tidal-channel morphodynamics is a major goal in the study of coastal landscapes. Freely migrating meandering rivers are known to produce laterally extensive, sand-prone point-bar bodies which commonly exhibit width:thickness ratios up to 250. Meandering channels are widespread in tidal channel networks draining intertidal plains, where they exhibit planform dynamics similar to their fluvial analogues. However, tidal networks are characterized by a high channel density that likely hinders tidal meanders from migrating laterally for long distances without interacting with other channels. In order to better understand how the interaction between adjacent meandering channels controls the development of tide-dominated point bars, two point-bar bodies from the Eocene Castigaleu Formation (Spain) are investigated and compared with the geometry of a modern tidal point bar of the northern Venice Lagoon (Italy). The Eocene bars are characterized by a low width:thickness ratio (<30), interpreted as the result of premature deactivation of their parent-channel migration. Such deactivations were likely related to avulsive piracy operated by adjoining tide-dominated channels, and likely prevented the parent channel bend from depositing a laterally extensive, tabular sand body along its inner bank. This analysis is corroborated by direct time-lapse observations of similar dynamics in modern tidal networks, and from morphodynamic and sedimentological evidence from recent deposits of the Venice Lagoon. We conclude that, in densely drained tidal networks, meander bends often interact with adjacent channels, thereby triggering piracy events and premature channel abandonments that generate point-bar architectures distinct from those produced in fluvial meandering rivers.