Geology Reference
In-Depth Information
lamina is directly and positively related to tidal current
strength, which in turn is directly and positively related
to the magnitude of the daily rise and fall of the tide
(tidal range). Over periods of days, months, or years,
changes in tidal current strengths associated with
various lunar/solar cycles are mirrored by the change
in thicknesses of the vertically stacked laminae.
Modern and ancient tidal rhythmites have been found
on every continent in the world except Antarctica. In
modern environments, tidal rhythmites occur in depos-
its associated with tide-dominated deltas, tidal embay-
ments, and estuaries. Tidal rhythmites can be used for
reconstructing ancient paleogeographies and paleocli-
mates (e.g. this chapter, Hovikoski et al.
2005
; Kvale
et al.
1994
), estimating paleotidal ranges (e.g. Archer
1995
; Archer and Johnson
1997
), understanding chan-
nel migration in the fluvio-estuaring transition (Choi
2010
) determining lunar-retreat rates through time (e.g.
Williams
1989
; Kvale et al.
1999
), and most recently,
have been used to infer the major tidal constituents
associated with the tides that deposited them (e.g.
Kvale
2006
). In order to understand tidal rhythmites,
however, one has to understand how tides are generated
and what controls their genesis.
The impact of diurnal, semidiurnal, and semimonthly
(neap-spring) tidal cycles on sediment deposition has
been well documented since the early 1980s (e.g. Visser
1980
; Boersma and Terwindt
1981
; Allen
1981
). For
many geologists these became benchmark papers when
they were published because they showed how deposi-
tional packages within sedimentary successions can be
linked to a tidal origin. However, it was the discovery
of modern and ancient tidal rhythmites in the late 1980s
and 1990s that showed that a hierarchy of tidal cycles,
beyond simple semidaily, daily or fortnightly events,
could be preserved in the rock record (e.g. Kvale et al.
1989
; Williams
1989
; Dalrymple and Makino
1989
;
Archer et al.
1991
; Kvale et al.
1994
; Miller and
Eriksson
1997
). Tidal cycles associated with monthly,
semiannual, annual (usually includes a significant sea-
sonal climatic component), and even an approximately
18-year cycle have been identified from ancient tidal
rhythmites.
Studies, however, showed that the understanding of
one of the most basic of the tidal cycles, the neap-spring
or fortnightly tidal cycle, by most geologists, and
apparently many oceanographers, and astronomers as
well, was over-simplified. Many college-level textbooks
today continue to propagate a basic misunderstanding
of the neap-spring cycles and the origin of oceanic
tides in general (e.g. Duxbury et al.
2002
).
The intent of this chapter is neither to outline a
history of the study of tides and tidal deposits nor to
document the current state of knowledge regarding
the history of the Earth-Moon system. These issues
are treated in some detail in Klein (
1998
), Rosenberg
(
1997
), Williams (
2000
), and Coughenour et al.
(
2009
). Rather, it is to explain some basic tidal theory
and show how a more complete knowledge of ancient
tides can be extracted from the rock record. Most of
the information contained within this chapter is dis-
tilled from two summary papers: Kvale et al. (
1999
)
and Kvale (
2006
).
To truly understand tidal systems and, in particular,
the genesis of tidal rhythmites it is useful to understand
both an equilibrium tidal model and a dynamic tidal
model. The former explains the driving forces behind
the formation of tides and is commonly taught to
geology, oceanography, and astronomy undergraduates,
whereas the later, more accurately explains real-world
tides and is more useful in interpreting the rock record.
An understanding of both models is essential to anyone
who studies tides and tidal deposits, and both will be
discussed.
1.2
Equilibrium Tidal Theory
Most geologists understand tidal periodicities in the
context of equilibrium tidal theory. Tides are generated
by the gravitational forces of the Moon and, to a lesser
degree, the Sun on the Earth. The Moon accounts for
approximately 70% of the tide-raising force because of
its proximity to the Earth. In an equilibrium world, the
Earth is covered by an ocean of uniform depth that
responds instantaneously to changes in tractive forces
(MacMillan
1966
). The equilibrium model can be used
to explain five of the six tidal periodicities that have
been commonly detected in rhythmite successions.
These six cycles are illustrated in Figs.
1.1
-
1.6
(previ-
ously illustrated in Kvale et al.
1998
). A seventh cycle
known as the “nodal cycle”, an approximately 18 year-
tidal cycle, and very well documented by Miller and
Eriksson (
1997
) within the Pride Shale, a lower
Carboniferous succession found in West Virginia, is
not illustrated here.
The figures each illustrate (from upper left to lower
right): A diagram and explanation of the equilibrium
