Unfathomable: The shifting sand of wave base

Abstract

ABSTRACT Notions of fair-weather, storm, and swell wave bases are ubiquitous in interpretations of wave-dominated siliciclastic shelves, carbonate ramps, and mixed-systems deposits that are present throughout the geologic record. A review of literature, observations of several modern and ancient depositional systems, and numerical hydrodynamic models reveal the roles of sediment grade, bathymetric irregularities, and depositional gradient on the variable depths to which waves leave a sedimentologically discernible record. They reveal that wave-induced horizontal particle velocities and estimates of wave effectiveness form a continuous spectrum with depth, with a lack of distinct subdivision into fair-weather and storm conditions. Although commonly ascribed to “fair-weather wave base,” the depth above which sediment is persistently agitated or winnowed also is shaped by bathymetric gradient, direction of wave approach, tides, and currents, as well as sediment grade. Similarly, even with identical waves, the maximum depth of initiation of sediment movement, e.g., effective wave base, is not directly comparable among shelves, or even within the same shelf through time, because hydrodynamic processes are encoded differently by shelves of variable morphology and sedimentology. The numerical models further suggest possible geomorphology–hydrodynamics–sediment linkages. Relative to steeper shelves impacted by identical waves, shelves of shallower gradient favor lower-energy seafloor conditions, likely accompanied by accumulation of finer or muddier sediment, thinner sandy shoreface accumulations, or both. Given that many stratigraphic accumulations from the parasequence to composite-sequence scale steepen upward with time, this conceptual model predicts an apparent increase in wave energy on the seafloor through time for individual progradational shorelines, even with constant waves.