Sedimentology, ichnology and hydrodynamics of strait‐margin, sand and gravel beaches and shorefaces: Juan de Fuca Strait, British Columbia, Canada

Abstract

On the south‐west coast of Vancouver Island, Canada, sedimentological and ichnological analysis of three beach–shoreface complexes developed along a strait margin was undertaken to quantify process–response relations in straits and to develop a model for strait‐margin beaches. For all three beaches, evidence of tidal processes are expressed best in the lower shoreface and offshore and, to a lesser extent, in the middle shoreface. Tidal currents are dominant offshore, below 18 m water depth (relative to the mean spring high tide), whereas wave processes dominate sediment deposition in the nearshore (intertidal zone to 5 m water depth). From 18 to 5 m water depth, tidal processes decrease in importance relative to wave processes. The relatively high tidal energy in the offshore and lower shoreface is manifest sedimentologically by the dominance of sand, of a similar grain size to the upper shoreface/intertidal zone and, by the prevalence of current‐generated structures (current ripples) oriented parallel to the shoreline. In addition, the offshore and lower shoreface of strait‐bound beach–shoreface complexes are recognized ichnologically by traces typical of the Skolithos Ichnofacies. This situation contrasts to the dominantly horizontal feeding traces characteristic of the Cruziana Ichnofacies that are prevalent in the lower shoreface and offshore of open‐coast (wave‐dominated) beach–shorefaces. These sedimentological and ichnological characteristics reflect tidal influence on sediment deposition; consequently, the term ‘tide‐influenced shoreface’ most accurately describes these depositional environments.

Trace fossils, sedimentary facies and parasequence architecture from the Lower Cretaceous Mulichinco Formation of Argentina: The role of fair-weather waves in shoreface deposits

“…Shoreface variations attributed to overall storm intensity, storm frequency, and relative water depth result in different taphonomic pathways facilitating categorization of three major types of shorefaces; (1) strongly storm-dominated, (2) moderately storm-affected, and (3) weakly stormaffected (Buatois and Mángano, 2011;Pemberton et al, 2012). Two types of tidal shorefaces have also been defined by different hydrodynamic conditions: (1) tidally influenced shorefaces (TIS), and (2) tidally modulated shorefaces (TMS) (Dashtgard et al, 2009;Frey and Dashtgard, 2011;Pemberton et al, 2012).…”

A facies‐based depositional model for ancient and modern, tectonically–confined tidal straits

“…In other straits with low‐gradient margins, this zone is expressed by shoreface deposits or wave‐dominated deltas (e.g. Frey and Dashtgard, ).…”

Interplay of tectonism and eustasy during the Early Permian icehouse: Southern Sydney Basin, southeast Australia

“…(). It also includes features that are similar to those recorded in tidally influenced settings, such as the increase in the grain size of the sand from the upper to the lower shoreface, which is a common trend in such settings (Frey & Dashtgard, ).…”

“…These settings are affected by strong wave energies and by strong tides, with the latter facilitating the lateral migration of wave zones along the depositional profile of the beach and shoreface (Dashtgard et al, ; Dashtgard et al, ). Despite their common occurrence in the modern environments (e.g., Anthony, Levoy, Monfort, & Degryse‐Kulkarni, ; Levoy, Monfort, & Larsonneur, ), our knowledge about the impact of tides on the shoreface relies on few modern and ancient examples (Dashtgard & Gingras, ; Dashtgard, Gingras, & Butler, ; Frey & Dashtgard, ), and the available criteria for the recognition of TMS are restricted to a single modern (Dashtgard et al, ) and a single ancient example (Ainsworth et al, ). The research of Ainsworth et al .…”