Tidal Constituents of Modern and Ancient Tidal Rhythmites: Criteria for Recognition and Analyses

Abstract

The origin of oceanic tides is a basic concept taught in most introductory college-level sedimentology, geology, oceanography, and astronomy courses. Tides are commonly explained in the context of the equilibrium-tidal theory model. The equilibrium model explains tides in the context of changes in two hemisphere-opposite tidal bulges through which the Earth spins. The position and size of these tidal bulges relative to the Earth’s equator is largely controlled by the phases of the Moon and changes in declination and orbital distance of the Moon in its orbit around the Earth. While explaining the driving forces that cause tides, the equilibrium model does not explain most of the tides observed in the Earth’s oceans. A complete explanation of the origin of tides must include a discussion of dynamic tidal theory. In the dynamic tidal model, tides resulting from the motions of the Moon in its orbit around the Earth and the Earth in its orbit around the Sun are modeled as products of the combined effects of a series of phantom satellites. The movement of each of these satellites, relative to the Earth’s equator, creates its own tidal wave that moves around an amphidromic point. Each of these waves is referred to as a tidal “constituent” or “species.” The geometries of the ocean basins determine which of these constituents are amplified. Thus, the tide-raising potential for any locality on Earth can be conceptualized as the summation of the amplitudes of a series of tidal constituents specific to that region. A better understanding of tidal cycles opens up remarkable opportunities for research on tidal deposits with implications for, among other things, a more complete understanding of the tidal dynamics responsible for sediment transport and deposition, tectonic-induced changes in paleogeographies, and changes in Earth–Moon distance through time.