260
VV. Schvvarzacher
localities. Whether the three maxima which are particularly well developed in
Fig. 8B, can be correlated with three high stands is doubtful. Detailed stratigraphic work and lateral tracing of cycles (Beaudoin 1992) is necessary before
such comparisons can be conclusive.
8
Conclusions
The different origins of sequence and Milankovitch cycles strongly suggest that
they should be treated as different stratigraphic units. Sequences representing
first to third order cycles are believed to be due to tectonic processes which are
most likely to involve the transport of mantle material. Dynamic systems of this
type must be highly damped and therefore cannot sustain oscillations. The triggering of such sequences may well have been a random event. However, the record
of sea level changes in the past is still very incomplete. This is largely due to the
difficulty in estimating the magnitude of the eustatic component but also because
accurate timing in shallow basins with multiple sedimentation gaps is difficult.
Tracing sequence boundaries towards the centre of basins, together with the recording of Milankovitch cycles, can at least provide a more accurate time scale.
Shorter cycles may have had a variety of origins but near-periodic oscillations
with periods of 2000 to lO ka are almost certainly related to Milankovitch cycles.
Although the astronomical origin of short cycles may be difficult to prove, this
should always be attempted. Any cycle generated by an oscillating system is a potential time scale and is therefore of importance in the study of sequences. The
reference to orders of cycles without giving evidence of how its alleged duration
was determined and without examining the regularity of such "cycles", is not
productive in quantitative stratigraphic studies. It may not yet be possible to
prove the absolute synchroneity of global tectonic cycles, but if they are confused with Milankovitch cycles this will be even more difficult.
References
Beaudoin, B., 1992, The black cherts in the Scaglia bianca. Poster presentation, 29th. Int. Geol.
Congress (Kyoto).
Berger, A. L., and Loutre, M. E, 1991, Insolation values for the climate of the last 10 million
years. Quaternary Science Revievvs v.lO, p .297-317.
Berger,A. L., and Loutre, M. E, 1994, Astronomical forcing through geological time. In: de Boer,
P. L., Smith, D. G., eds., Spec. Publ.Ass. Sediment. v.19, p.15-24.
Bolton, E. vv., Maasch, K. A., Lilly,J. M., 1995. A vvavelet analysis of Plio-Pleistocene
climate indicators: A nevv vievv of periodicity evolution. Geophys. Res. Lett., v.20, No.30, p.
2753-2756.
De Boer. P. L., 1983, Aspects of Middle Cretaceous pelagic sedimentation in S.Europe. Geol.
Utraiectina 30.
Drummond, C. N., VVilkinson, B. H.,1993, Carbonate cycle stacking patterns and hierarchies
of orbitally forced eustatic sealevel change. Jour. Sed. Pet., v. 63, p. 369-377.
Elrick, M., 1995, Cyclostratigraphy of Middle Devonian carbonates of the Eastern Great Basin.
Jour. Sediment. Res., v. B65, p. 61-79.
VV. Schvvarzacher
localities. Whether the three maxima which are particularly well developed in
Fig. 8B, can be correlated with three high stands is doubtful. Detailed stratigraphic work and lateral tracing of cycles (Beaudoin 1992) is necessary before
such comparisons can be conclusive.
8
Conclusions
The different origins of sequence and Milankovitch cycles strongly suggest that
they should be treated as different stratigraphic units. Sequences representing
first to third order cycles are believed to be due to tectonic processes which are
most likely to involve the transport of mantle material. Dynamic systems of this
type must be highly damped and therefore cannot sustain oscillations. The triggering of such sequences may well have been a random event. However, the record
of sea level changes in the past is still very incomplete. This is largely due to the
difficulty in estimating the magnitude of the eustatic component but also because
accurate timing in shallow basins with multiple sedimentation gaps is difficult.
Tracing sequence boundaries towards the centre of basins, together with the recording of Milankovitch cycles, can at least provide a more accurate time scale.
Shorter cycles may have had a variety of origins but near-periodic oscillations
with periods of 2000 to lO ka are almost certainly related to Milankovitch cycles.
Although the astronomical origin of short cycles may be difficult to prove, this
should always be attempted. Any cycle generated by an oscillating system is a potential time scale and is therefore of importance in the study of sequences. The
reference to orders of cycles without giving evidence of how its alleged duration
was determined and without examining the regularity of such "cycles", is not
productive in quantitative stratigraphic studies. It may not yet be possible to
prove the absolute synchroneity of global tectonic cycles, but if they are confused with Milankovitch cycles this will be even more difficult.
References
Beaudoin, B., 1992, The black cherts in the Scaglia bianca. Poster presentation, 29th. Int. Geol.
Congress (Kyoto).
Berger, A. L., and Loutre, M. E, 1991, Insolation values for the climate of the last 10 million
years. Quaternary Science Revievvs v.lO, p .297-317.
Berger,A. L., and Loutre, M. E, 1994, Astronomical forcing through geological time. In: de Boer,
P. L., Smith, D. G., eds., Spec. Publ.Ass. Sediment. v.19, p.15-24.
Bolton, E. vv., Maasch, K. A., Lilly,J. M., 1995. A vvavelet analysis of Plio-Pleistocene
climate indicators: A nevv vievv of periodicity evolution. Geophys. Res. Lett., v.20, No.30, p.
2753-2756.
De Boer. P. L., 1983, Aspects of Middle Cretaceous pelagic sedimentation in S.Europe. Geol.
Utraiectina 30.
Drummond, C. N., VVilkinson, B. H.,1993, Carbonate cycle stacking patterns and hierarchies
of orbitally forced eustatic sealevel change. Jour. Sed. Pet., v. 63, p. 369-377.
Elrick, M., 1995, Cyclostratigraphy of Middle Devonian carbonates of the Eastern Great Basin.
Jour. Sediment. Res., v. B65, p. 61-79.