Unfathomable: The shifting sand of wave base

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.

Paleogeographic Reconstructions of an Ocean Margin on Mars Based on Deltaic Sedimentology at Aeolis Dorsa

“…Sand sheets have been observed in the Gulf of Mexico related the reworking of more heterogeneous deposits and associated with delta deposits (Suter & Berryhill, 1985). The winnowing of mud from sand as well as sieving of mud into a sandy subsurface is known to occur with wave reworking of sediment (Rankey & Appendini, 2022; Wu et al., 2018), and could suggest the ocean was at least seasonally ice free. Several of the exhumed deltas appear to abut these topographic contacts (Figure 2a), but we do not know if delta deposits continue into the plana.…”

Comment on Brandano et al. (2022) – Introduction of ‘Understanding carbonate factories through palaeoecological and sedimentological signals – Tribute to Luis Pomar’, Sedimentology, 69, 5–23

“…The hydrodynamic threshold for the different carbonate factories will vary as the factories will have different sediment production strategies, for example, compare T-factories with their reefs at the platform margin with C-factories with loose transportable grains (James, 1997;Schlager, 2005;Reijmer, 2021), or with M-factories with their upper-slope dominated sediment production realm (Della Porta et al, 2003;Kenter et al, 2005). The interpretation of features associated with physical accommodation or hydrodynamic thresholds, for example, storm wave base or fair-weather wave base, must be made with the utmost care as the physical sedimentology and stratigraphy of these processes in modern environments may be comparable (Peters & Loss, 2012), and merely may be the expression of variations in geomorphologyhydrodynamics-sediment linkages (Neal et al, 2021;Rankey & Appendini, 2022). Brandano et al (2022;p.13, top left column) propose to use sediment sorting and grain roundness as sources of information about the hydraulic energy of the depositional environment.…”

An equation for deriving spatial variations in carbonate production rates from sediment deposition rates and dilution: Application to Santa Maria Island, Azores

“…This idea was suggested to explain greater bioclast content of a core from the southerly shelf of Faial compared to one from its northerly shelf (Valente, 2020). Wave-induced particle velocities vary greatly though at any time tend to decline in an exponential-like manner with depth (Rankey and Appendini, 2022). In the NW shelf sector of Santa Maria, particle movements capable of disturbing the sediments appear to be rare deeper than 100 m (Zhao et al, 2022).…”