MODERN AND ANCIENT TIDES

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Oceanic tides are capable of generating currents that erode, transport, and deposit sediments. Tidal processes were both significant and widespread throughout much of the Earth's history. Ancient tidal rhythmites have been found on every continent in the world except Antarctica. In modern environments, tidal rhythmites occur in deposits associated with
These processes can be recorded in small-scale sedimentary structures referred to as "tidal rhythmites" that include thinly layered, fine grained sediments. The tidal influence on the origin tide-dominated deltas, tidal embayments, and estuaries. The recognition of ancient tidal rhythmites has important implications for academic and economic reasons. They
of these rhythmites is indicated by the cyclic variations in the thicknesses of successive laminae in response to changing current velocities associated with lunar cycles. The thickness of a can be used for reconstructing ancient paleogeographies and paleoclimates, estimating paleotidal ranges, understanding sand transport within tide-dominated basins, and IN Y
D
IA IT
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 determining lunar-retreat rates through time. The figures below illustrate (from left to right): A diagram and explanation of the tidal theory of the six main governing tidal NA
UNIVE
RS

of days, months, or years, changes in tidal current strengths associated with various lunar cycles are mirrored by the change in thicknesses of the vertically stacked laminae. periodicities that can be detected in rhythmite successions; a bar chart of tidal height data (high tide elevations) from a modern, real-world setting that shows how the
astronomical effects are reflected in cyclic changes in daily high tides; a core from an ancient tidal rhythmite succession showing how these cyclic tidal effects might be
manifested in a laminated tidal rhythmite; and a bar chart of laminae thicknesses interpreted in the context of the modern tidal cycle.

TIDAL THEORY MODERN TIDES ROCK CORE ANCIENT TIDAL RECORDS FROM CORE
PENNSYLVANIAN (MORROWAN) PENNSYLVANIAN (MORROWAN)
SEMIDIURNAL

SUN 4.5
(12.42 HOURS)

S - Subordinate Tidal Deposit

EARTH (equatorial view) Dominant 4.0 D - Dominant Tidal Deposit

Increasing Relative Height of High Tide

Dominant
Dominant Dominant 3.5
Subordinate Subordinate Subordinate
The interaction of tidal forces from the Moon and Sun Subordinate

Lamina Thickness (mm)

3.0
produce two oceanic bulges on opposite sides of the DOMINANT
Earth. The rotation of a point on the Earth through these 2.5 D
bulges once a day produces two tides (the semidiurnal
S
tide). Typically, these tides are not equal (termed diurnal
inequality), as one is higher (dominant) than the other MOON 2.0
D
(subordinate) because of nonparallelism between the
1.5
Moon’s orbital plane and the Earth’s equatorial plane. S
This angular difference is termed lunar declination.
2 cm 1.0
up
SUBORDINATE 0.5

TIDAL BULGE Morning

12 Evening
24
Morning
36
Evening 48
0.0
High Tide High Tide High Tide High Tide 0 10 20 30 40 50 60 70 80 90
Hours
Lamina Number

Lunar Phase PENNSYLVANIAN (MORROWAN) PENNSYLVANIAN (MORROWAN)

EARTH (polar view) New 1st Full 3rd New 1st Full 3rd New 1st Full
Qtr. Qtr. Qtr. Qtr. Qtr.
4.5
NEAP N - Neap Tides S - Spring Tides N - Neap Tides S
(1st quarter)
(29.53 DAYS)

S - Spring Tides N S N S N S 4.0 S - Spring Tides

SYNODIC

S
S
Daily high tides are higher when the Earth, Moon, and
SPRING S
3.5
S

Increasing Relative Height of High Tide

Sun are nearly aligned (full or new moon) and lower (new)

Lamina Thickness (mm)

S S S 3.0
when the Sun and Moon are at right angles to the Earth S S
(1st or 3rd quarter phase). Tides during full or new moon
2.5
are referred to as spring tides and tides at quarter
S
phases are referred to as neap tides. The neap-spring
tidal period is related to the changing phases of the N 2.0 S
N N
Moon associated with the half-synodic month. The N
synodic month (new moon to new moon) has a modern N 1.5 N
period of 29.53 days and contains two neap-spring N
cycles. 1.0 N
SPRING N N
(full) 0.5
up
0.0
0 10 20 30 40 50 60 70 80 90
NEAP 0 20 40 60 80 100 120 140 160
Lamina Number
High Tide Number
(3rd quarter)

Lunar Phase PENNSYLVANIAN (MORROWAN) PENNSYLVANIAN (MORROWAN)

New 1st Full 3rd New 1st Full 3rd New 1st Full
Qtr. Qtr. Qtr. Qtr. Qtr.
(SEMIDIURNAL -

4.5
C - Equatorial Passage of Moon
Tides(Crossover)
NORTH OF EQUATOR S - Spring
C - Equatorial Passage of Moon (Crossover)
C
TROPICAL

No - Moon at Maximum Northern Declination C C C

The tidal force also depends on the declination of the N - Neap Tides
27.32 DAYS)

C So - Moon at Maximum Southern Declination 4.0

Moon. The period of the variation in declination is called C
the tropical month -- the interval of time it takes the 3.5

Increasing Relative Height of High Tide

Moon to complete one full orbit from its maximum C
northern declination to its maximum southern declination C No So C

Lamina Thickness (mm)

No 3.0
and then return. The effect of the tropical month in most So
So C
semidiurnal systems is to cause the diurnal inequality of C
2.5
the tides. Ideally, diurnal inequality is greatest when the No
Moon is at its maximum declination (as shown at right) C
C 2.0
and is reduced to zero when the Moon is over the
equator, producing a crossover in the modern tidal data
(modern tides panel). The current length of the tropical 1.5
month is 27.32 days (two days shorter than the synodic
month -- see synodic row above). Because of this, 1.0
equatorial passages of the Moon (crossovers) occur
faster than the generation of spring tides as indicated by 0.5
the modern and rock tide data.
SOUTH OF EQUATOR up 0.0
0 20 40 60 80 100 120 140 160 0 10 20 30 40 50 60 70 80 90
High Tide Number Lamina Number

PENNSYLVANIAN (ATOKAN) PENNSYLVANIAN (ATOKAN)

C - Equatorial Passage of Moon (Crossover)
No - Moon at Maximum Northern Declination C - Equatorial Passage of Moon (Crossover)
NORTH OF EQUATOR So - Moon at Maximum Southern Declination
TROPICAL

So So C C C C 3.0
C
27.32 DAYS)

C C C C C So C
(DIURNAL -

In many dominantly diurnal systems (primarily one tide No C C C C C C C C

Increasing Relative Height of High Tide

per day), the tropical period described above is No No So

Lamina Thickness (mm)

responsible for generating neap-spring cycles. In C
No
contrast to the synodic system, tides in a tropical system
behave as though the Sun’s gravitational effects are No
2.0
dampened. In such cases, the dominant tidal force
depends on the declination of the Moon relative to the
Earth’s equator with the force being greatest when the
Moon is most directly over the site in question. In these
systems, the predicted and ancient tide data reveal that
equatorial passages of the Moon (crossovers) occur in 1.0
phase with the generation of neap-spring tides, in
contrast to the variable relationship shown in the tropical
(semidiurnal) row above.
2 cm
up
SOUTH OF EQUATOR
0.0
0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140
High Tide Number Lamina Number

MISSISSIPPIAN (CHESTERIAN) MISSISSIPPIAN (CHESTERIAN)

ANOMALISTIC

Another tidal effect arises from the changing distance of

minor axis PF - Phase Flip (occurs on minor axis A - Spring Tide at/near Apogee 30.0
P
of lunar orbit) (high spring tides) P - Moon at/near Perigee
(27.55 DAYS)

the Moon relative to the Earth during the lunar orbit.

P - Spring Tide at/near Perigee A - Moon at/near Apogee
Because the lunar orbit forms an ellipse, with the Earth (low spring tides) 25.0
Increasing Relative Height of High Tide

slightly offset from the center, the Moon alternates PF PF P

Neap
Phase Flip
between perigee (closest approach to the Earth) and P

Neap-Neap Thickness (cm)

Neap

Neap

Neap

Neap

Neap

Neap

Neap

Neap

Neap
P
apogee (farthest distance from the Earth). During the PERIGEE P P
P P
P P
P PHASE FLIP 20.0 P
lunar synodic month there will be two spring tides (see P A P A
A P P A A P A P A P
synodic row above). However, these spring tides A A A A A
A A A
commonly will be of unequal magnitude producing high APOGEE P
P
spring and low spring tides, which correspond to spring major axis 15.0 A
A
tides during or near perigee (high spring) and spring A
tides during or near apogee (low spring). The semi- A A
10.0
monthly inequality of the spring tides disappears when
the Moon lies along the minor axis of the lunar orbit and Eroded A
Eroded
the difference in lunar distance is minimized during
subsequent spring tides. The time it takes for the Moon 5.0
to move from perigee to perigee is called the anomalistic up
month, which is at present 27.55 days.
0.0
0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28
High Tide Number Neap-Neap Number

PENNSYLVANIAN (MORROWAN-ATOKAN) PENNSYLVANIAN (ATOKAN)

SEMIANNUAL

Semiannual Tidal Trend 9.0

Half-Year 7
(182.6 DAYS)

8.0 Half-Year
Half-Year Half-Year
Spring Tides 5
Neap Spring Neap Neap Spring Neap
Increasing Relative Height of High Tide

7.0
Lamina Thickness (mm)

6.0 6
The synodic, tropical, and anomalistic periods have
slightly different values. Because of this, these periods PERIGEE Spring Tide
5.0 Number
will interfere constructively twice each year causing tidal
forces at these times to reach a maximum (as shown by 56
4.0 8
the dashed line in the modern tides panel). The date of 7
4
this tidal maximum is a function of latitude (related to the 6 5
6 3
4
3.0 9 7 2
declinational effects of the Moon and Sun). 8 2
4 89 3
new moon 5
10
3
78
9
2.0 11 1 11 10 1
4 12 2 10 1 9
123
2 cm 1.0
up
0.0
Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. 0 100 200 300 400 500 600 700 800 900
Month Lamina Number

By
Cite: Kvale, E.P., Sowder, K.H., and Hill, B.T., 1998, Modern and ancient
Erik P. Kvale, Kimberly H. Sowder, and Barbara T. Hill tides. Poster and explanatory notes, SEPM (Society for Sedimentary
Indiana Geological Survey Geology), Tulsa, OK, and Indiana Geological Survey, Bloomington, IN.

1998
copyright 1998 SEPM (Society for Sedimentary Geology) and the Trustees of Indiana University