Geology Reference
In-Depth Information
16.3.1 Shorter-Term Tidal Cycles
The moon also undergoes changes in declination
relative to the earth and this is the “tropical” period
(see Kvale and Archer
1991
; Archer
1996a
). Extraction
and delineation of tropical periodicities require a rhyth-
mite that has essentially continuous preservation of all
tidal events, which are relatively uncommon. A number of
such near-continuous cycles, however, has been docu-
mented in Pennsylvanian rhythmites of the EIB and
WIB (Kvale et al.
1989
; Archer
1996a
).
Another prominent shorter-term tidal cycle includes
changes in lunar distance from the earth as related to
the varying eccentricity of the lunar orbit. During lunar
apogee, the moon is farther from the earth. During
perigee, the moon is significantly closer to the earth.
When lunar perigee closely corresponds to new or full
moon (syzygy), tidal ranges can be considerable ampli-
fied. Conversely, during the preceding or following
lunar apogee the neap-spring tides are significantly
reduced. Apogean-perigean periods can be very dis-
tinctive in vertically accreted tidal facies, particularly
when they are in phase with neap-spring periods. Gray
shale facies in the EIB can exhibit this combined effect
(Fig.
16.7c
).
Many examples of Proterozoic and Phanerozoic
rhythmites also exhibit these combined periodicities
(e.g. Archer
1996a
). It is probably not unusual that such
combined cycles occur within many laminae-thickness
series. In an analysis of modern tides, Wood (
1986
)
pointed out that the co-occurrence of spring tides and
perigee would result in unusually high tides, which he
termed “perigean-spring tides.” Because of higher tidal
velocities, rhythmites deposited during perigean-spring
tides could be thicker and more rapidly accreted.
A combination of these factors could greatly increase
the preservational potential of rhythmites. Although,
apogee-perigee cycles have been interpreted in many
ancient rhythmites (Archer
1996a
), there are few modern
examples. One example has recently been documented
from tidal flats in Turnagain Arm, Alaska, a hypertidal
estuary (Greb and Archer
2006
).
Within the rhythmites and texturally-banded facies
discussed herein, a variety of tidal cycles have been
described. At the finest scale, banded facies exhibit
well-defined lamina or beds that were formed during
the subdaily to daily (semidiurnal to diurnal) rise and
fall of lunar tides. Pairing of a thicker lamina with an
overlying thinner lamina is common (Fig.
16.6a
) in the
EIB (e.g. Kvale et al.
1989
).
Tidal lamina pairing, termed “doublets” or “cou-
plets” (Kvale et al.
1989
; Kvale and Archer
1991
).
Occurrence of couplets will be best developed within a
tidal system that exhibits a mixed, predominantly
semidiurnal regime. This type of tidal system has a
well-developed diurnal inequality such that a higher-
high and lower-high tide occur each tidal day.
The higher-high tide produces a thicker lamina than
the lower-high tide, resulting in a laminae couplet.
Predominantly semidiurnal systems are generally not
able to produce couplets because each high tide is
essentially of the same height. Preservation of succes-
sive couplets indicates that the original sediments were
deposited within a setting that had a strong asymmetry
between flood- and ebb-tidal velocities, as occurs in
many modern tidal settings. Statistical techniques have
been proposed that can determine if the couplets are
statistically significant (De Boer et al.
1989
; Tessier
1993
) and thus support a tidal-depositional interpre-
tation. Detailed analyses of ripple-scale features in
Pennsylvanian rhythmites from such units indicate that
any depositional effects of the subordinate tide, which
could be either the flood or the ebb tide, are much
reduced (Kvale and Archer
1991
).
In addition to simple two-part couplets, rhythmites
have been described that are actually composed of
pairs of couplets. Such complex rhythmites, termed
four-part rhythmites (Archer et al.
1995a
), preserve
both ebb- and flood deposition in a twice daily (semi-
diurnal) tidal system.
Perhaps the most common and prominent perio-
dicity in texturally banded, cyclic rhythmites is the neap-
spring cycle (Fig.
16.6a
,
16.7c, d
). This cycle relates to
changes in lunar phase as observed from the earth.
During new or full moon (syzygy), a linear alignment
within the earth-moon-sun system results in higher, or
spring, tides. The synodic month is the duration of one
lunar orbit around the earth. There are two periods of
spring tides separated by periods of lower, neap tides.
16.3.2 Longer-Term Cycles
in Similar-Appearing Rhythmites
A sample of rhythmites can appear to contain neap-
spring cycles, but simple similarity doesn't preclude
that the observed cycles might represent an entirely
different magnitude of periodicity. As an example,
