Milankovitch Cycles and Sequences:
Two Different Stratigraphic Tools
W. Schwarzacher
1
Introduction
With the introduction of sequence stratigraphy, the word cycle has again become a term which is frequently used. Cycles have been classified in a very arbitrary way into orders of different length and several different orders can occur
together in the stratigraphic record.
The study of repetition is important. Sedimentary series always contain a limited number of rock types and therefore random deposition will sooner or later
lead to repetition. Repetitions however, can also be the outcome of a more orderly system. If one or more of the lithologies leads with preference to a definite successor, then repetition will occur regularly. Such cycles are generated by an oscillating system. There are therefore, two quite different groups of cycles caused
either by oscillating systems or by random impulses. This difference is very important in explaining the origin of sediment sequences. We will discuss two
types of stratigraphic cycles: sequences in their original definition as unconformity-bound units and Milankovitch cycles, which are genetically defined as
being the result of orbital variations of the earth. Milankovitch cycles can form
sequences, but this is not necessarily the case. Sequences, by implication, are
formed by variations in the sea level, but we know very little about the causes of
such variations. The astronomical mechanisms of insolation changes that cause
the Milankovitch cycles, on the other hand, are relatively well known, but much
less is known of how they will eventually produce stratigraphic cycles.
The differences between the two types of "cycles" have some very practical
implications in stratigraphy. The most important advantage of Milankovitch cycles is that they represent constant or nearly constant time markers. Also, because they are the result of periodic or near-periodic changes in the sun planet
system, they must have a worldwide effect, even if they are not recorded everywhere. The major disadvantage of Milankovitch cycles is the difficulty in recognising them. The most important advantage of sequences is their clear definition
in stratigraphic terms which allows an accurate identification of sequence
boundaries. Also useful is the relatively long time they represent, which makes
interbasin correlation possible. With Milankovitch cycles, this has only been
successful so far in the Pleistocene. Disadvantages are that we do not know a
Two Different Stratigraphic Tools
W. Schwarzacher
1
Introduction
With the introduction of sequence stratigraphy, the word cycle has again become a term which is frequently used. Cycles have been classified in a very arbitrary way into orders of different length and several different orders can occur
together in the stratigraphic record.
The study of repetition is important. Sedimentary series always contain a limited number of rock types and therefore random deposition will sooner or later
lead to repetition. Repetitions however, can also be the outcome of a more orderly system. If one or more of the lithologies leads with preference to a definite successor, then repetition will occur regularly. Such cycles are generated by an oscillating system. There are therefore, two quite different groups of cycles caused
either by oscillating systems or by random impulses. This difference is very important in explaining the origin of sediment sequences. We will discuss two
types of stratigraphic cycles: sequences in their original definition as unconformity-bound units and Milankovitch cycles, which are genetically defined as
being the result of orbital variations of the earth. Milankovitch cycles can form
sequences, but this is not necessarily the case. Sequences, by implication, are
formed by variations in the sea level, but we know very little about the causes of
such variations. The astronomical mechanisms of insolation changes that cause
the Milankovitch cycles, on the other hand, are relatively well known, but much
less is known of how they will eventually produce stratigraphic cycles.
The differences between the two types of "cycles" have some very practical
implications in stratigraphy. The most important advantage of Milankovitch cycles is that they represent constant or nearly constant time markers. Also, because they are the result of periodic or near-periodic changes in the sun planet
system, they must have a worldwide effect, even if they are not recorded everywhere. The major disadvantage of Milankovitch cycles is the difficulty in recognising them. The most important advantage of sequences is their clear definition
in stratigraphic terms which allows an accurate identification of sequence
boundaries. Also useful is the relatively long time they represent, which makes
interbasin correlation possible. With Milankovitch cycles, this has only been
successful so far in the Pleistocene. Disadvantages are that we do not know a