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

Abstract

Coastal depositional systems are normally classified based on the relative input of wave, tide, and river processes. While wave- through to river-dominated environments are well characterized, environments along the wave-to-tide continuum are relatively poorly understood and this limits the reliability and utility of coastal classification schemes. Two tidal shoreface models, open-coast tidal flats (OCTF) and tidally modulated shorefaces (TMS), have been introduced for mixed wave-tide coastal settings. Following nearly two decades of research on tidal shorefaces, a number of significant insights have been derived, and these data are used here to develop a unified model for such systems. First, OCTFs are components of larger depositional environments, and in multiple published examples, OCTFs overlie offshore to lower shoreface successions that are similar to TMS. Consequently, we combine OCTFs and TMSs into a single tidal shoreface model where TMS (as originally described) and TMS-OCTF successions are considered as variants along the wave-tide continuum. Second, tidal shoreface successions are preferentially preserved in low- to moderate-wave energy environments and in progradational to aggradational systems. It is probably difficult to distinguish tidal shorefaces from their storm-dominated counterparts. Third, tidal shorefaces, including both TMSs and OCTFs, should exhibit tidally modulated storm deposits, reflecting variation in storm-wave energy at the sea floor resulting from the rising and falling tide. They may also exhibit interbedding of tidally generated structures (e.g. double mud drapes or bidirectional current ripples), deposited under fairweather conditions, and storm deposits (e.g. hummocky cross-stratification) through the lower shoreface and possibly into the upper shoreface.The development of the tidal shoreface model sheds light on the limitations of the presently accepted wave-tide-river classification scheme of coastal environments and a revised scheme is presented. In particular, tidal flats are components of larger depositional systems and can be identified in the rock record only in settings where intertidal and supratidal deposits are preserved; consequently, they should not represent the tide-dominated end-member of coastal systems. Instead, we suggest that tide-dominated embayments should occupy this apex. Tide-dominated embayments exhibit limited wave and river influence and include a wide range of geomorphological features typically associated with tidal processes, including tidal channels, bars and flats.

Middle Jurassic multi‐scale transgressive–regressive cycles: An example from the Lusitanian Basin

“…Laminated and bioturbated siltstone is usually deposited in offshore settings (i.e. below fair‐weather and storm wave base levels) and is generally associated with heterolithic and hummocky cross‐stratified sandstone (Clifton, 2006; Dashtgard et al, 2021; Plint, 2010). Thalassinoides , Ophiomorpha and R. commune occur in many marginal and marine palaeoenvironments.…”

“…Storms probably form hummocky cross‐stratification and planar lamination in the lower shoreface (Dashtgard et al, 2021; Jelby et al, 2020; Leckie & Walker, 1982; MacEachern et al, 2010; Plint, 2010; Plint & Nummendal, 2000; Walker & Plint, 1992). Horizontal lamination develops in fine to very fine sand due to the high velocity of the flow and immediately above the erosional base of a storm bed (Arnott & Southard, 1990; Cheel, 1991; Plint, 2010).…”

“…The sharp and erosional bottom of the sandstone bodies suggests episodes of relatively high‐energy wave action and erosive currents that scoured a cohesive muddy substrate and created gutter casts during storm events (Myrow, 1992; Pérez‐López, 2001; Plint, 1996). The scours filled with hummocky cross‐stratified sandstone suggest deposition in the lower shoreface (Dashtgard et al, 2021; Jelby et al, 2020). This sandstone bed can be classified as simple hummocky (Jelby et al, 2020).…”

“…The predominance of trough cross‐stratification and horizontal lamination in sandstone suggests a progressive upward increase in traction processes (Plint, 2010). The dominance of suspension‐feeding behaviours ( Skolithos Ichnofacies) over a few deposit‐feeding ethologies ( Cruziana Ichnofacies) associated with those sedimentary structures is suggestive of deposition above the fair‐weather wave base in upper to middle shoreface settings (Dashtgard et al, 2021; MacEachern et al, 2010; Pemberton et al, 2012). Bioclastic sandstone and conglomerate suggest the mixing and deposition of terrigenous material and skeletal fragments due to the reworking of coeval carbonate deposits by waves or currents (Chiarella & Longhitano, 2012).…”

Non‐palimpsested crowded Skolithos ichnofabrics in a Carboniferous tidal rhythmite: Disentangling ecological signatures from the spatio‐temporal bias of outcrop

“…The positioning of the rhythmite immediately above a transgressive surface onto the Acre Coal, within a succession that locally also contains evidence for fluvio‐deltaic and storm‐induced hummocky cross‐stratification (e.g. Reynolds, 1992; Booth et al ., 2020), suggests deposition within the shallow subtidal realm (Dashtgard et al ., 2021). The restricted vertical and lateral extent of the rhythmite is compatible with suggestions that deposition occurred in an interdistributary bay, between some of the many localized delta lobes that typify facies models for the cyclothems of the Yoredale Group (e.g.…”