Geology Reference
In-Depth Information
Fig. 1.12
Tradewater Formation, (
a
) Photo of the Abbott
sandstone outcrop. This is part of a much more extensive
dune mesoform. Examples of dominant (
D
) and subordinate
(
S
) semidiurnal foresets are labeled. Rock hammer for
scale (
lower part
of photo) (
b
)
Bar chart
showing foreset
(depositional event) thickness variability with spring tides (
S
),
neap tides (
N
) and lunar crossover (
arrows
) events labeled.
Notice the semimonthly inequality of the spring tides related
to perigee and apogee effects. Also note that the lunar
passages of the equator (
arrows
) track the neap tide deposits
fairly closely suggesting that the neap-spring cycles are in
phase with the tropical month
distance through time (Williams
1989
; Kvale et al.
1999
) and has even been suggested as a way to better
understand the impact that tides have on biological
systems (Kvale
2006
). It has also been used to model
tidal basin dynamics for determining the importance of
tidal facies within a basin or region (e.g. Ericksen and
Slingerland
1990
; Wells et al.
2007
). In the Abbott
example, an interpretation of neap-spring cyclicity could
be done with both the equilibrium and dynamic model,
but interpretation of the relative importance of the M
2
,
1.5
Summary and Implications
The equilibrium tidal model is very useful for explain-
ing the gravitational forces that generate tides on the
Earth. However, it is an over-simplification and does not
explain the tides in most of the oceans of the world. To
explain real-world tides requires a basic understanding
of the dynamic tidal model. The dynamic tidal model
has been used to estimate changes in the Earth-Moon
