Ancient tides and physiography: time to update sequence-stratigraphic models?

Tidal dynamics are dependent on the physiography (geometry and bathymetry) of a basin and its latitude. By studying the impact that changes in physiography have on tides, scientists can improve their understanding of sedimentary processes that occurred in palaeo-seas, and incorporate potential variations in tidal dynamics in response to relative sea level changes to their interpretation. We developed a workflow that everyone can use to model the propagation of tides in ancient basins, to understand the evolution of tidal dynamics in a basin with respect to change in its physiography, or to test the feasibility of certain geological interpretations. This methods was tested in the epicontinental Upper Jurassic Sundance and Curtis Seas, which extended from today’s British Columbia to today’s east-central Utah, where tidal currents strongly impacted the deposition of certain sedimentary units preserved in the rock record. The numerical simulations of tides were run in the open-source Delft3D software by Deltares, and one of the initial input that the software requires is palaeobathymetric data. Because palaeobathymetry data might not exist for the study of ancient seaways, we developed a simple Python code that extracts the colour-value of every pixel in a georeferenced image, before attributing it a depth value depending on how light- or dark-colour the pixel was: the lighter-coloured the pixel is, the shallower the attributed depth is (and vice-versa). The results of numerical modelling of tidal propagation show that changing the palaeophysiographic configuration of the basin controls both the magnitude and the location of tidal amplification. Some palaeophysiographic configurations resulted in an overall increase in tidal amplitude in the seaway (with various degrees of amplification) but potentially link to the onset of resonance in (part of) the basin, whereas other palaeophysiographic configurations led to a general decrease of the tidal amplitude. Further, certain palaeophysiographic configurations resulted in a heterogeneous spatial distribution of the tidal amplification across the basin as additional tidal harmonics only appeared in certain parts of the basin, but were absent from others. This results in the resurgence of tidal amplification or dampening with different periods at different locations, with increasing spatial variations the deeper the basin is. As a result, the stacking pattern of the various architectural elements would differ from one margin of the basin to the other, despite a similar relative sea-level history. Consequently, the interpretation of the relative sea-level history across an entire basin necessitates caution when tides are one of the main process active at the time of deposition, especially when data points are scarce or limited to a portion of the basin only.