6.4 SEDIMENTARY MODELS, INCREMENTS, AND CYCLES
289
1994; House and Gale, 1995). Since the work of Milankovitch (1941) it has been recognized that extraterrestrial processes impact sedimentation. Three types of Milankovitch cycles are recognized (Fig. 6.85). The precession of the earth's axis has a cycle with
two peaks of 19-23 ka, the obliquity, or tilt of the earth's axis, has a cycle of 41 ka, and
the eccentricity of the earth's orbit changes shape in 100- and 400-ka cycles (House and
Gale, 1995). Each one of these impacts the earth system, the various signals combining to produce what may appear random to the human eye. Many studies of cyclic sequences have distinguished hierarchical systems of "mega cyclothems," "cyclothems,"
and "microcyclothems." Not all of these are imaginary. There are a number of mathematical techniques for statistically testing a section for cyclicity and for determining cycle wavelength (Krumbein, 1967; Merriam, 1967). Gale (1998) has reviewed the
diverse statistical gymnastics that may be employed to separate the various astronomical cycles from the earthly residual signal (Fig. 6.86).
In conclusion it is commonplace to find predictable sequences within genetic increments of strata. Cyclicity is the repetition of these sequences. Autocyclic sedimentation
is an integral part of several sedimentary models. Allocyclic sedimentation occurs from
time to time in almost all deposits irrespective of environment; it is caused by a variety
of mechanisms. More than one cyclic process may control the deposition of a sedimentary section, including autocyclic, terrestrial, and extraterrestrial mechanisms.
i
l
t'
|
!
|
r
|
|
A
el00 ]
TILT
._,.,I
P23 LLI
O
P18
-2
B
0
SUM /
t
2
~. -2
./~
r=0.71
S
L,, o 0
DATA
00
2
-2
D
0
RESIDUALS
2
|
0
100
200
300
400
500
600
700
800
Time (000 s of years BP)
Fig. 6.86. Variations in the earth-sun system. (A) Oscillations caused by eccentricity, obliquity, and precession. (B) The sum of the four signals, a measure of solar energy received by the earth. (C) The oxygen isotope record, a manifestation of the Milankovitch signal. (D) The residual products after deducting B from C.
[From Gale, A. S. 1998. Cyclostratigraphy. In "Unlocking the Stratigraphic Record" (P. Doyle and M. R. Bennett, eds.), pp. 195-220, Fig, 7.3. Copyright John Wiley & Sons Limited. Reproduced with permission.]
289
1994; House and Gale, 1995). Since the work of Milankovitch (1941) it has been recognized that extraterrestrial processes impact sedimentation. Three types of Milankovitch cycles are recognized (Fig. 6.85). The precession of the earth's axis has a cycle with
two peaks of 19-23 ka, the obliquity, or tilt of the earth's axis, has a cycle of 41 ka, and
the eccentricity of the earth's orbit changes shape in 100- and 400-ka cycles (House and
Gale, 1995). Each one of these impacts the earth system, the various signals combining to produce what may appear random to the human eye. Many studies of cyclic sequences have distinguished hierarchical systems of "mega cyclothems," "cyclothems,"
and "microcyclothems." Not all of these are imaginary. There are a number of mathematical techniques for statistically testing a section for cyclicity and for determining cycle wavelength (Krumbein, 1967; Merriam, 1967). Gale (1998) has reviewed the
diverse statistical gymnastics that may be employed to separate the various astronomical cycles from the earthly residual signal (Fig. 6.86).
In conclusion it is commonplace to find predictable sequences within genetic increments of strata. Cyclicity is the repetition of these sequences. Autocyclic sedimentation
is an integral part of several sedimentary models. Allocyclic sedimentation occurs from
time to time in almost all deposits irrespective of environment; it is caused by a variety
of mechanisms. More than one cyclic process may control the deposition of a sedimentary section, including autocyclic, terrestrial, and extraterrestrial mechanisms.
i
l
t'
|
!
|
r
|
|
A
el00 ]
TILT
._,.,I
P23 LLI
O
P18
-2
B
0
SUM /
t
2
~. -2
./~
r=0.71
S
L,, o 0
DATA
00
2
-2
D
0
RESIDUALS
2
|
0
100
200
300
400
500
600
700
800
Time (000 s of years BP)
Fig. 6.86. Variations in the earth-sun system. (A) Oscillations caused by eccentricity, obliquity, and precession. (B) The sum of the four signals, a measure of solar energy received by the earth. (C) The oxygen isotope record, a manifestation of the Milankovitch signal. (D) The residual products after deducting B from C.
[From Gale, A. S. 1998. Cyclostratigraphy. In "Unlocking the Stratigraphic Record" (P. Doyle and M. R. Bennett, eds.), pp. 195-220, Fig, 7.3. Copyright John Wiley & Sons Limited. Reproduced with permission.]