Page 107 - Carbonate Sedimentology and Sequence Stratigraphy

98 WOLFGANG SCHLAGER

Fractals - an alternative to orders in sequence stratigraphy parasequences and their type-3 boundaries may be formed
by fluctuations in sediment supply or by relative changes of
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Work on the sequence record in the domain of 10 to 10 6 sea level. The type-3 sequence boundary was already rec-
years has gone far enough to formulate an alternative to the ognized as an important type of unconformity by Vail and
concept of orders in sequence stratigraphy. Todd (1981, Fig. 3c) and by Schlager (1999b) who proposed
Examination of sediment anatomy indicates that many the name.
patterns relevant for the sequence model are invariant in As the terms parasequence and standard sequence had
a wide range of scales. Moreover, a strong case has been been defined by both the length of time represented by
made that the two basic controls of sequences - changes in the unit and the type of bounding surface, Schlager (2004)
sea-level and sediment supply - are fractals in the time do- introduced the terms S sequence and P sequence for se-
main. quences defined solely by the nature of their boundaries.
Based on these observations, I proposed the following The terms can be avoided and replaced by “standard se-
conceptual model (Schlager, 2004): The pattern of sequences quence” and “parasequence” if one defines the two se-
and systems tracts is scale invariant and a statistical fractal. quence types strictly by the nature of their boundaries and
In the arrangement of systems tracts and the nature of se- disregards the length of time they represent: Parasequences
quence boundaries two types are about equally likely in the are sequences bounded by flooding surfaces, standard se-
validity range of the model (Fig. 6.16, 6.17): quences are bounded by exposure surfaces. Throughout this
➤ Standard sequence, or S sequence , shows the succession:
book, these definitions will be applied. This usage is com-
highstand tract - sequence boundary - lowstand tract
mon (e.g. Nummedal et al. 1993,p. 55; J.F. Sarg, written com-
- transgressive tract - highstand tract. The sequence
munication), but not unanimous.
boundary is a type-1 or type-2 boundary caused by rel-
Conservative estimates of the validity range of the model
ative sea-level lowering (forced regression). 3 6 1 5
are 10 –10 y in the time domain, 10 –10 m in the hor-
➤ Parasequence, or P sequence , shows the succession: 0 3
izontal directions of space and 10 –10 minthe vertical.
highstand tract - sequence boundary - transgressive
Justification of these ranges is given below.
tract - highstand tract. The sequence boundary is a
The time range of the model can be justified as follows.
type-3 boundary, i.e. a flooding surface that overlies
The statistics on flooding versus exposure boundaries is
marine deposits without demonstrable evidence of ter- 4 5
based on cycles with durations of 10 –10 y. Physical pro-
restrial exposure or forced regression. cesses operate in similar fashion also on much shorter time
The crucial distinction between the two types is that stan- scales. However, the formation of recognizable exposure
dard sequences are bounded by surfaces of forced regression surfaces depends on soil processes whose rates are consid-
that require a relative fall in sea level. Parasequences are erably lower. Accumulation of organic matter, one of the
bounded by type-3 sequence boundaries, i.e. flooding sur- fastest soil processes, takes thousands of years to advance
faces that lack evidence of forced regression. Consequently,
Orders of Sequence Cycles FRACTAL MODEL OF SEQUENCES
Scale-invariant succession of P and S sequences
HST Standard-sequence
1. exposure boundaries
TST
LST
2.
HST

3. HST Para-sequence
flooding boundaries
TST
4. HST

5. Scale-invariance in time Scale-invariance in space
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-1
6. 10 - 10 yr 10 -10 m
0
5
8
10 3 10 4 10 5 10 6 10 7 10 yr soil rates tectonic setting 10 -10 m
Fig. 6.15.— Orders of stratigraphic sequences as defined by var- Fig. 6.16.— Scale-invariant model of stratigraphic sequences.
ious authors since 1977. In each category, the oldest publication Standard sequences, i.e. sequences bounded by exposure sur-
is on top. Differences are about 1/2 order at each boundary, in the faces, and parasequences bounded by flooding surfaces, are as-
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4 th -6 th orders even larger; opinions do not seem to converge with sumed to occur in about equal proportions in the range of 10 to
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time. After Schlager (2004), modified. 10 y. After Schlager (2004), modified.