An integrated approach to investigate intra-point bar grain-size variability: an example from the Holocene alluvial succession of the Venetian Plain (Italy)

Abstract:

Fluvial and fluvio-tidal meandering channels are widespread in coastal areas, where they generated deposits hosting most of the modern surficial aquifers. Understanding sediment distribution and connectivity within these deposits is essential to predict groundwater propagation, with relevant implications for aquifer management to face salt-water intrusion and pollutant propagation.

The present work focuses on two paleomeanders of the southern Venetian Plain, which is located in northeast Italy and was generated during Holocene transgression by aggradation of alluvial systems. By combining analyses of satellite images, geophysical data, and sedimentary cores, this work aims to define a 3D model depicting geometries of the related point-bar bodies, with a specific focus on along-bar sediment grain-size distribution.

The study paleochannel is ca. 25 m wide and defines two main bends: the first bend (B1) is a wide, strongly asymmetric bend, whereas the second one (B2) is an open, poorly asymmetric bend. The planform evolution of these bends was reconstructed by analyzing scroll-bar patterns, which are still visible from satellite images. B1 and B2 progressively expanded during their evolution, and B1 was affected by a marked downstream rotation of the bend apex during its final growth stage. Geophysical investigations (Frequency Domain Electro-Magnetometer) provide conductivity maps of the investigated areas, depicting 3D geometries of the sedimentary bodies based on a marked conductivity contrast between less conductive bar bodies and more conductive overbank deposits. Sedimentary data reveal that the overbank is dominantly muddy, whereas the bar bodies consist of sandy sediments. Comparison between grain-size and conductivity data allowed us to discuss the variability of conductivity values within point-bar bodies. Our integrated approach between geophysical and sedimentological data provides a link between planform evolution of fluvial meander bends and grain-size distribution within the related bar bodies, with direct implications to predict subsurface flow propagation within alluvial sedimentary bodies.