Plate Stratigraphy and CCO Fluctuations
267
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Depth (m)
Fig. 9.16. Determination of paleodepth of the CCO by backtracking of drill sites. Subsidence track
and sediment sequence of Site 137 Central Atlantic . Lithologic symbols (from left to right at base):
broken lines clays; dots calcareous ooze; random dashes (below 400 m depth) basalt. CCO is
reached near 3.6 km depth, 90 million years ago . [W. H. Berger, E. L. Winterer, 1974, Spec. Pub!.
Int. Assoc. Sedimentol. I: 11]
The changes of facies that are related to changes in oceanography emerge as a
result of "backtracking" of sites, which brings the sediments accumulated on a piece
of sea floor back to the original place of deposition, as a function of age. Horizontal
backtracking is done by reversing the path of a drilling site the appropriate pole of
rotation for the plate the site occupies, For some purposes it is sufficient to obtain the
paleolatitude (that is, the distance to the equator), Vertical backtracking is necessary
any time depth-sensitive properties are involved, such as carbonate content, sedimentation rate, or preservation of calcareous fossils.
The position of the CCD is, by definition, tied to depth of deposition, and the past
positions of the CCD in anyone region, therefore, have to be found by backtracking.
How exactly does the CCD fluctuate? What can this tell us about the changing
chemistry and fertility of the ocean? Before we can answer these questions, we must
reconstruct the depth levels at which the CCD was crossed, for a sufficient number of
drilling sites. To do this we use the generalized subsidence curve (Fig. 1.9) and
proceed as follows.
First, we identify the point which corresponds to the basement age of a given
deep-sea record, on the subsidence curve of Fig. 1.9. Then we walk back up the
267
~Ime->I ~ (K? ~s)
0
10
20
30
40
50
60
10
80
90
100
110
i
I
I
I
I
I
i
I
1
i
1
1
Boson
,'( ~ 105 mjlton yeo .. J
:3
,sw :3
/ ' 6
-~,.:.. ceo
E
,.-6
4
,··' S
4
~
,
~
. . ,
!
,. ,..-.;'
.
, . • ... 3
.'
5
.-... ~.'
5
-.--~--.-SompieS
2
:3
4
5
6 7 16SWS
o
100
200
300
400
Depth (m)
Fig. 9.16. Determination of paleodepth of the CCO by backtracking of drill sites. Subsidence track
and sediment sequence of Site 137 Central Atlantic . Lithologic symbols (from left to right at base):
broken lines clays; dots calcareous ooze; random dashes (below 400 m depth) basalt. CCO is
reached near 3.6 km depth, 90 million years ago . [W. H. Berger, E. L. Winterer, 1974, Spec. Pub!.
Int. Assoc. Sedimentol. I: 11]
The changes of facies that are related to changes in oceanography emerge as a
result of "backtracking" of sites, which brings the sediments accumulated on a piece
of sea floor back to the original place of deposition, as a function of age. Horizontal
backtracking is done by reversing the path of a drilling site the appropriate pole of
rotation for the plate the site occupies, For some purposes it is sufficient to obtain the
paleolatitude (that is, the distance to the equator), Vertical backtracking is necessary
any time depth-sensitive properties are involved, such as carbonate content, sedimentation rate, or preservation of calcareous fossils.
The position of the CCD is, by definition, tied to depth of deposition, and the past
positions of the CCD in anyone region, therefore, have to be found by backtracking.
How exactly does the CCD fluctuate? What can this tell us about the changing
chemistry and fertility of the ocean? Before we can answer these questions, we must
reconstruct the depth levels at which the CCD was crossed, for a sufficient number of
drilling sites. To do this we use the generalized subsidence curve (Fig. 1.9) and
proceed as follows.
First, we identify the point which corresponds to the basement age of a given
deep-sea record, on the subsidence curve of Fig. 1.9. Then we walk back up the