Stratal patterns and sedimentary facies in tidal point bars

Tidal channels play a key role in the evolution of coastal environments and commonly dominate tidal landscapes. These channels commonly show a clear meandering pattern which shapes the architectural geometries of sedimentary successions accumulated in tidal coastal realms. Nevertheless, a limited number of studies analysed the morphodynamic evolution of tidal meanders and the related point bar sedimentary bodies, whose internal architecture and sedimentary facies distribution remain still poorly explored, as highlighted by their scarce documentation in the rock record. The capability of reading the signature of tidal processes in sedimentary successions represents a powerful tool for paleo-environmental reconstructions. Detecting tidal channel deposits will contribute to studies on Holocene deposits and ancient rocks, with implications spanning from adaptation of natural systems to anthropic pressure to subsurface exploration for georesources. The present work aims at improving our understanding of the relationship between the evolution of tidal meander bends and the related sedimentary products, through the investigation of different research issues (RI), including: i) the influence of vertical aggradation and substrate compaction in shaping geometries of tidal point bars (RI1); ii) the reconstruction of three-dimensional architecture of tidal point bars, through a numerical modelling approach, developed under different aggradational conditions and planform transformation styles (RI2); iii) the relationship between mechanisms of bar growth and internal facies arrangements (RI3); and iv) the analysis of piracy-controlled geometries of point bar bodies (RI4). These research issues have been carried out in different tidal settings, including microtidal (Venice Lagoon, Italy – RI1, 2, and 4) and macrotidal (Bay of Mont Saint Michel, France – RI3) environments; both in modern (RI1, 2, 3, and 4) and ancient record (Eocene deposits of the Tremp-Graus Basin, Spain – RI4). A multidisciplinary approach has been adopted for modern examples, with different methodologies encompassing remote sensing techniques (i.e. historical aerial photos and LiDAR topographic time-series analysis), geomorphological field observations, sedimentary core analysis, and 3D forward stratigraphic modelling. The analysis of the ancient case study examples relies on classical outcrop sedimentology approach, which included line-drawing of photomosaics, bed-by-bed logging, and collection of paleo-flow measurements. The main results from this work highlighted that: I) geometries of tidal point bars can vary following aggradational conditions of surrounding overbank areas, along with changes in local accommodation space and channel discharge. These are, in turn, influenced by differential substrate compaction and channel network evolution; II) 3D geometries of point bars developed by slowly migrating tidal channels under aggradational conditions, differ from tabular bodies envisaged by traditional point bar models. Indeed, their geometries are shaped by the planform and vertical shift of channel bends; III) tidal meanders can expand alternating accretionary stages along seaward and landward side of point bars. In this contest, large amounts of mud-rich deposits are stored in their axial zone, where rhythmic tidal deposition is better recorded; IV) occurrence of densely-drained tidal networks prevents channel bends to freely meander and causes interaction with adjacent channels triggering piracies. These piracies cause premature deactivation of sinuous channels and hinder the development of laterally extensive point bar bodies.