236 Deep-Sea Sediments - Patterns, Processes, and Stratigraphic Methods
effects of episodic dissolution pulses within the post-Eocene carbonate sequence. The
basement reflectors are basaltic layers. The profiles of dissolved silicate indicate
reflux of silica to the sea floor from sediments as old as 10 million years. This loss of
silica is unfavorable for subsequent formation of chert.
The question why deep-sea chert deposits are concentrated in some geologic
periods and not in others cannot be answered at this time. Presumably, changes in the
global silica supply to the ocean (weathering processes, volcanism, ridge-crest hydrothermal activity) changed the amount of silica deposited, and changes in ocean
productivity changed the distribution patterns (see Sect. 8.6.2).
Certain ancient cherts, exposed as radiolarities in ophiolites and melanges of active margins (ribbon cherts), may be turbitides. Such cherts, it may be surmised,
could form because opal-rich sediments along active margins were redeposited by
turbidity currents into marginal basins, without much dilution from terrigenous materials (due to a high sealevel stand).
8.7 Stratigraphy and Dating
8.7.1 General Considerations. All absolute dating of deep-sea sediments ultimately
relies on the measurement of radioactive isotopes and their daughter elements (Appendix A8). However, almost all routine dating is done by these three stratigraphic
methods: determination of fossil content, determination of magnetic reversal sequence, and determination of sequence of chemical properties such as stable isotopes
of oxygen, carbon, and strontium. To these tools (biostratir;raphy, magnetostratir;raphy, chemostratir;raphy) may be added the counting of astronomical cycles observed (or suspected) in paleontological, sedimentological, or physical properties of a
pelagic section (cyclostratir;raphy or Fourier stratigraphy). In all cases except cyc1ecounting, dating is by correlation to a section considered to be well dated.
The best correlation, naturally, is achieved when several methods used simultaneously agree on the placement of a given section, with respect to the reference
section.
8.7.2 Aspects of Biostratigraphy. The workhorse of stratigraphic correlation in pelagic sediments is (micro-)paleontology, that is, the identification of fossils, notably
nannofossils, foraminifers, radiolarians, and diatoms. The stratigraphic distribution of
these fossils was worked out mainly in the last 40 years or so. The work was begun
by pioneers such as the American geologists M. N. Bramlette (1896-1977), A. R.
Loeblich, and H. Tappan, the Swiss paleontologist H. Bolli, and the AustralianAmerican paleontologist W. Riedel, among others, who realized the great potential of
microfossils for dating pelagic sequences. Nannofossils and radiolarians are useful
throughout the age range of deep-sea sediments (Fig. 8.17). Planktonic foraminifers
diversify sometime in the middle Cretaceous, and diatoms in the late Cretaceous.
The principle of stratigraphic correlation goes back to the beginnings of scientific
geology and paleontology; it is the observation that sedimentary sections in different
parts of the world show a similar sequence of different associations of fossils. Quite
effects of episodic dissolution pulses within the post-Eocene carbonate sequence. The
basement reflectors are basaltic layers. The profiles of dissolved silicate indicate
reflux of silica to the sea floor from sediments as old as 10 million years. This loss of
silica is unfavorable for subsequent formation of chert.
The question why deep-sea chert deposits are concentrated in some geologic
periods and not in others cannot be answered at this time. Presumably, changes in the
global silica supply to the ocean (weathering processes, volcanism, ridge-crest hydrothermal activity) changed the amount of silica deposited, and changes in ocean
productivity changed the distribution patterns (see Sect. 8.6.2).
Certain ancient cherts, exposed as radiolarities in ophiolites and melanges of active margins (ribbon cherts), may be turbitides. Such cherts, it may be surmised,
could form because opal-rich sediments along active margins were redeposited by
turbidity currents into marginal basins, without much dilution from terrigenous materials (due to a high sealevel stand).
8.7 Stratigraphy and Dating
8.7.1 General Considerations. All absolute dating of deep-sea sediments ultimately
relies on the measurement of radioactive isotopes and their daughter elements (Appendix A8). However, almost all routine dating is done by these three stratigraphic
methods: determination of fossil content, determination of magnetic reversal sequence, and determination of sequence of chemical properties such as stable isotopes
of oxygen, carbon, and strontium. To these tools (biostratir;raphy, magnetostratir;raphy, chemostratir;raphy) may be added the counting of astronomical cycles observed (or suspected) in paleontological, sedimentological, or physical properties of a
pelagic section (cyclostratir;raphy or Fourier stratigraphy). In all cases except cyc1ecounting, dating is by correlation to a section considered to be well dated.
The best correlation, naturally, is achieved when several methods used simultaneously agree on the placement of a given section, with respect to the reference
section.
8.7.2 Aspects of Biostratigraphy. The workhorse of stratigraphic correlation in pelagic sediments is (micro-)paleontology, that is, the identification of fossils, notably
nannofossils, foraminifers, radiolarians, and diatoms. The stratigraphic distribution of
these fossils was worked out mainly in the last 40 years or so. The work was begun
by pioneers such as the American geologists M. N. Bramlette (1896-1977), A. R.
Loeblich, and H. Tappan, the Swiss paleontologist H. Bolli, and the AustralianAmerican paleontologist W. Riedel, among others, who realized the great potential of
microfossils for dating pelagic sequences. Nannofossils and radiolarians are useful
throughout the age range of deep-sea sediments (Fig. 8.17). Planktonic foraminifers
diversify sometime in the middle Cretaceous, and diatoms in the late Cretaceous.
The principle of stratigraphic correlation goes back to the beginnings of scientific
geology and paleontology; it is the observation that sedimentary sections in different
parts of the world show a similar sequence of different associations of fossils. Quite