Tidally Modulated Shorefaces

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Abstract
Tidally modulated shorefaces (TMS) are wave-dominated, but differ from conventional shorefaces in that sediments deposited in water depths equivalent to the upper, middle, and lower shoreface (depending upon the tidal range) are regularly subjected to variable wave processes, including swash–backwash, breaking-wave and surf processes, and shoaling waves during the tidal cycle. In upper macrotidal and megatidal settings, it is also possible that these shoreface deposits are subaerially exposed during low tide. TMS exhibit the morphology, seaward decrease in sediment caliber, and dominance of wave-generated sedimentary structures consistent with beach–shoreface settings. However, the sedimentological and ichnological structures of deposits exposed in the laterally extensive intertidal zone are more akin to those of the upper and lower shoreface, and not the beach. Four major differences permit the ready differentiation of tidally modulated shoreface successions from conventional shorefaces. (1) Sedimentary structures generated by swash–backwash (plane beds), surf and breaking waves (current ripples and trough cross-beds), shoaling waves (oscillation ripples), and storm waves (hummocky and swaly cross-stratification) are interbedded with one another across the shoreface. (2) Ebb-oriented tidal currents and surface runoff during the falling tide and at low tide deposit offshore-directed current ripples and combined-flow ripples in sandy sediments, or trough cross beds in gravel-dominated sediments. (3) Ichnologically, TMS exhibit a reduction in both the diversity of ichnogenera and density of burrowing across the entire shoreface profile. However, the incipient-trace associations are most similar to the Skolithos Ichnofacies in the upper shoreface–equivalent zone, and to the Cruziana Ichnofacies in the lower shoreface–equivalent zone. (4) The sedimentological and ichnological criteria commonly employed to identify the middle shoreface are spread out across the upper and lower shoreface, making this subenvironment difficult to differentiate.

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