Page 108 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 6: FUNDAMENTALS OF SEQUENCE STRATIGRAPHY 99

significantly (e.g. Birkeland, 1999, p. 215). Similarly, the eas- basins or continents exist for 100 My or more without signif-
ily preserved calcrete crusts form at rates of several mil- icant episodes of deformation or magmatism. Consequently,
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limeters per 10 y (e.g. Robbin and Stipp, 1979). Finally, the record of cycles in the 10 y-range is largely derived
modelling of rock-water interaction suggests that signifi- from flooding and exposure of continental interiors whereas
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cant decrease in bulk-rock δ Crequires 10 –10 y(Yang, data on shorter cycles mainly are from ocean margins. This
2001). Based on these indications, I assume that the forma- difference in setting may explain the characteristics of first-
tion of a preservable exposure record, even in carbonates, order cycles reported by Vail et al. (1991) and Duval et al.
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takes about 10 year. This number is taken as the short time (1998).
limit of the model because it assumes that the occurrence The spatial range of the model is essentially a conse-
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of parasequences and standard sequences is about equally quence of the time limits. In the range of 10 -10 y one typ-
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likely. Among sequences shorter than 10 y, parasequence- ically observes sequence packages that range in thickness
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1
0
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cycles are likely to dominate simply because there was in- from 10 -10 m and extend horizontally from 10 -10 m.
sufficient time to develop recognizable exposure surfaces. The larger horizontal extent is a characteristic of nearly all
One question that still needs to be sorted out is the dif- sedimentary bodies and reflects the fact that at the surface
ferent preservation potential of exposure events in carbon- of the Earth vertical gradients generally exceed horizontal
ates and siliciclastics. Hard soil crusts are uncommon in si- ones by several orders of magnitude.
liciclastics and sediment lithification is much slower than in
carbonates. Thus, soils and other vestiges of subaerial con- Support for the fractal nature of sequences
ditions may easily be washed away during the subsequent
transgression. The situation is different during long expo- The conceptual model introduced above draws support
sure where deeply incised valleys may form and preserve from three areas - sediment anatomy, the nature of sediment
the record of exposure by their morphology and by the pres- supply and sea-level fluctuations, and the distribution of
ence of soils in protected pockets. parasequences and standard sequences in time.
The upper time limit for the fractal model is set by lack Sediment anatomy, i.e. the external geometry and internal
of data. As Fig. 6.17 includes no cycles longer than ten mil- depositional structures of sediment bodies, is scale invari-
lion years, it seems prudent to limit the validity range to se- ant in a wide range of scales in time and space. (In fact
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quences shorter than 10 y. Beyond this limit set by the data many features are scale-invariant in a much wider range
base, tectonics may set yet another limit. Few margins of of scales than considered here). Without characteristic ob-
jects for scale, we could not tell if the delta-canyon system
in Fig. 6.18 is tens of centimeters or hundred kilometers
mean
wide, if it formed within hours or hundreds of thousands
of years. Similar statements can be made for avalanche
foresets, braided and meandering streams and many other
depositional patterns. Thorne (1995) illustrated the scale-
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Sequences of 1.10 yr and longer invariance of prograding clinoforms and developed a scale-
(N=80)
invariant quantitative model for them. Van Wagoner et al.
(2003) proposed that delta-shaped accumulations fed by a
mean
6
Sequences shorter than 10 yr
(N=682)

140 100 60 N 0 % exposure surfaces 80 100

Fig. 6.17.— Standard sequence boundaries (orange) and
parasequence boundaries (blue) in well-documented carbonate
rocks. Each bar represents a measured section with number of
analysed surfaces on the left. Top panel: cycles longer than 1 My.
Bottom panel: cycles shorter than 1 My. Sources: Buchbinder et al.
(2000); D’Argenio et al. (1997); D’Argenio et al. (1999); Egenhoff et
al. (1999); Föllmi et al. (1994); Hillgärtner (1999); Immenhauser et Fig. 6.18.— Scale-invariance of depositional anatomy is ele-
al. (2001); Immenhauser et al. (2004); Minero (1988); Saller et al. gantly demonstrated by this valley-delta system described by Posa-
(1993); Strasser and Hillgärtner (1998); Van Buchem et al. (2000); mentier et al. (1992a). As objects of characteristic size are absent,
Van Buchem et al. (1996); Wendte et al., (1992); Wendte and Muir, the dimensions of the system cannot be estimated. (The delta is
(1995). After Schlager (2004), modified. about 2 meters across).