Abstract
Tidally modulated shoreface (TMS) corresponds to peculiar costal environments. The general morphology and the expressed bedforms are provided by the interplay of both waves and tides. The recognition of TMS in the fossil record still remains a difficult task. The study of one mega-tidal modern TMS in the north of France (Berck-Plage) provides new key criteria to identify this kind of coastal system in the rock record. Field investigation and digital mapping were realized at lowest tide during spring tide under fair-weather condition. The intertidal zone is characterized by a succession of several sand banks shore parallel separated by topographic lows that are defined as ridges and runnels. Seven distinct dominant bedforms are recognized: 3D current ripples, 3D asymmetrical ripples, 2D symmetrical ripples, 2D small symmetrical dunes, 2D large symmetrical dunes, 3D symmetrical dunes and plane beds. The upper stage plane bedding mainly composed the ridges while the six other bedforms are commonly found within the runnels or on the flanks of the ridges. Comparison of the bedforms of Berck-Plage with previous experimental studies on bedforms genesis proves that the necessary flow parameters for generating these bedforms belong to an oscillatory flow except for the 3D current ripples, which are formed by a unidirectional flow. This study confirms the dominance of oscillatory structures through the intertidal zone in a mega-tidal context and show that wave processes are more powerful than tide processes for bedform generation although during fair weather conditions. Based on the timing of genesis, the description and the repeated pattern of distribution of bedforms between two ridges is highlighted thus helping to propose a theoretical facies sequence for an intertidal zone characterized by ridges and runnels applicable to ancient sedimentary successions.
Highlights
Waves and tides are the two main processes acting either separately or together on the formation of sedimentary structures, their repartitions and the geomorphology of coastal systems
Even if recent studies have shown how the co-influence of waves and tides can be recognized in the rock record at different scales, from the sedimentary structures to the general stratigraphic succession in sedimentary sequences (e.g. Basilici et al, 2011, 2012; Vakarelov et al, 2012; Rossi and Steel, 2016; Smosna and Bruner 2016; Vaucher et al, 2017) there is a lack of study on modern environment providing key criteria for the recognition of ancient hybrid sedimentary successions
It allows providing the spatial repartition of the different bedforms in order to reconstruct an idealized temporal succession of sedimentary structures expected in Tidally modulated shoreface (TMS)
Summary
Waves and tides are the two main processes acting either separately or together on the formation of sedimentary structures, their repartitions and the geomorphology of coastal systems. Yang and Chun, 2001; Yang et al, 2005, 2006, 2008a, b; Dashtgard et al, 2009, 2012) and the double influence (waves and tides) was clearly pointed out by sedimentological and/or ichnological evidences These studies on modern environments are mainly based on boxcore analysis, giving a vertical (temporal) view to the observations but do not allow to understand the timing of formation of each bedform as well as their lateral evolution. In order to constrain the spatial distribution of sedimentary bodies and their associated bedforms in a tidally modulated modern shoreface (TMS; sensu Dashtgard et al, 2009), the Berck-Plage mega-tidal beach in northern France was investigated It allows providing the spatial repartition of the different bedforms in order to reconstruct an idealized temporal succession of sedimentary structures expected in TMS. These peculiar beaches were subject to numerous publications that mainly describe their coastal dynamics
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