Bedforms in a tidally modulated ridge and runnel shoreface (Berck-Plage; North France): implications for the geological record

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.Keywords: waves / tides / ridge and runnel / bedforms / stratigraphy / Berck-Plage / France Résumé -Structures sédimentaires d'une zone intertidale en barre et bâches dominée par la houle (Berck-Plage ; Nord de la France) : implications pour le registre sédimentaire ancien. Les plages et avant-plages sous l'influence de la houle et modulées par l'action de la marée, correspondent à des environnements côtiers particuliers. Leurs géomorphologies générales ainsi que les structures sédimentaires associées résultent de l'action combinée de la houle et de la marée. L'identification de ces environnements atypiques dans des successions sédimentaires anciennes demeure difficile. L'analyse d'un environnement mégatidal semi-diurne actuel dans le Nord de la France (Berck-Plage) a permis de fournir de nouveaux critères de reconnaissance pour ce type de système côtier hybride dans le fossile. Une étude de terrain ainsi qu'une cartographie numérique ont été réalisées à marée basse au cours d'une marée de vives-eaux. La zone intertidale est caractérisée par une succession de plusieurs bancs de sables parallèles à la côte et qui sont définies comme des barres (topographie po...

Changes in hydrodynamic process dominance (wave, tide or river) in foreland sequences: The subalpine Miocene Molasse revisited (France)

“…In F10, the lateral changes from sandstone levels displaying planar stratifications and SCS to heterolithic facies displaying wave structures (combined wave‐flow ripples) suggest a ‘ridge and runnel’ environment (Vaucher et al ., , 2018b). This type of organization develops in tide‐modulated, wave‐dominated coasts (Short, ; Kroon & Masselink, ; Anthony et al ., ; Masselink et al ., ; Reichmüth & Anthony, ; Vaucher et al ., , ,b), where the intertidal zone consists of topographical highs (ridges) and lows (runnels). The ridges are formed by the swash and backwash during rising and falling tides.…”

“…The runnels are filled during high tides with a dominant wave action, thus explaining the dominance of wave sedimentary structures interstratified with fine‐grained heterolithics deposited at low tides. Moreover, changes in sea level during tidal cycles modify the size of wave orbitals impacting on the sediment floor as testified to by the frequent occurrence of downcutting in the bottomsets of the ripples (Vaucher et al ., ,b).…”

“…When marine sedimentation takes place, this configuration is responsible for: (i) amplification of tidal currents due to restriction of cross‐sectional area (Defant, ; Pugh, ; Besson et al ., ; Reynaud et al ., , , ; Longhitano et al ., ; Rossi et al ., 2017a); (ii) high amount of sediment supply through fluvial inputs because the seaway is bounded by a rising belt; and (iii) influence of waves when the fetch of the seaway is wide enough. Since the 1990s, modern mixed‐energy coasts have been investigated and processes are better constrained (Short, ; Masselink & Short, ; Li et al ., ; Yang & Chun, ; Anthony & Orford, ; Yang et al ., ; Masselink et al ., ; Vaucher et al ., ,b). Ancient mixed‐energy coastal environments are being increasingly documented in the literature (Vakarelov et al ., ; Ahokas et al ., ; Leva López et al ., ; Rossi & Steel, ; Vaucher et al ., ; Peng et al ., ; Zhang et al ., ).…”

Hutchison Medallist 1. Wave-Dominated to Tide-Dominated Coastal Systems: A Unifying Model for Tidal Shorefaces and Refinement of the Coastal- Environments Classification Scheme

“…Tidal influence on the geomorphology of beach-shoreface systems was explored in the 1980s and 1990s (Fig. 5; Short 1984Short , 1991Short , 1999Masselink and Short 1993;Masselink and Hegge 1995), but sedimentological models that incorporated expressions of tidal processes in the preserved character and architecture of shorefaces did not occur until the mid-to late-2000s with competing and, in some cases, complementary models for open-coast tidal flats (OCTF; Yang et al 2005Yang et al , 2006Yang et al , 2008b, tidally modulated shorefaces (TMS; Dashtgard et al 2009;Pemberton et al 2012;Vaucher et al 2018a), micro-meso tidal shorefaces (Vakarelov et al 2012), and tide-influenced shorefaces (Frey and Dashtgard 2011;. With the exception of the micro-meso tidal shoreface model (Vakarelov et al 2012), all models were developed in modern depositional settings.…”

“…The systems used to develop both the OCTF and TMS models experience numerous storms, and storm-influence is evident in preserved deposits in the form of HCS and the preservation of unbioturbated sediment. In TMS, tides are expressed in the interbedding of wave-formed sedimentary structures that result from the across-shore translation of wave zones (shoaling, surf, swash-backwash), and the accompanying tidal modulation of wave energy at any given location, with the rising and falling tide (Dashtgard et al 2009;Vaucher et al 2018a). For example, during high tide, swash-backwash will occur at the landward side of the beach while the lower intertidal zone will be subjected to shoaling waves and the offshore will be dominated by offshore currents (probably tidal currents).…”