Geology Reference
In-Depth Information
1
Tidal Constituents of Modern
and Ancient Tidal Rhythmites:
Criteria for Recognition
and Analyses
Erik P. Kvale
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 poten-
tial for any locality on Earth can be conceptualized as the summation of the ampli-
tudes 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.
1.1
Introduction
Tidal rhythmites, small-scale sedimentary structures
that include thinly layered, fine grained sediments,
record, through the cyclic variations in the thicknesses
of successive laminae, changes in current velocities
associated with lunar/solar cycles. The thickness of a
