Page 166 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 9

Looking back, moving forward

Single-author books inevitably present a biased view of imentology. For instance, the adaptations of the standard
things. In this last section, I will add insult to injury by ex- model required for carbonates are a direct consequence of
posing some very personal thoughts on the future of car- the way carbonates are produced, and altered by early dia-
bonate sedimentology and sedimentary geology in general. genesis, as illustrated in chapters 7 and 8.
These thoughts are guided by basic patterns and lines of rea- The concept of sequence stratigraphy includes both the
soning that have emerged in the expose on the preceding technique of recognizing unconformity-bounded units as
pages. well as the identification of systems tracts. The differenti-
With regard to marine carbonates, this book – I hope – ation of shoal-water accumulations into prograding, retro-
has illustrated their intricate connection with the chemical grading and downstepping units is - in my opinion - a fun-
conditions and the biota of the ocean. The mode of precipi- damental property of the sediment record. It must be noted,
tation, notably the degree of biotic influence, determines not however, that only the downstepping units are reliable in-
only the texture and composition of carbonate sediments but dicators of relative sea-level change. Changes from progra-
to a significant extent also the anatomy of the accumulations dation to retrogradation and vice versa are ambiguous. On
and their distribution in space and time. The precipitation a subsiding substrate, progradation and retrogradation de-
modes, therefore, provide a basis for distinguishing the dif- pend on the balance between the rate of accommodation cre-
ferent production systems, or factories, introduced in chap- ation and the rate of sediment supply. Thus, a large change
ter 2. in sediment supply may override the rate of accommodation
The eminent role of chemical and biotic factors distin- creation caused by sea-level change (chapter 6, 7).
guishes carbonate systems from siliciclastic ones. In sili- Where do we go from here? The future of sedimentary
ciclastics, source and sink fundamentally depend on rates geology as a dynamic discipline may depend on how suc-
of uplift and subsidence as well as topographic gradients cessful we are in discovering general principles behind the
– i.e. factors largely controlled by tectonics. The siliclastic formation of sedimentary rocks and their stratigraphic suc-
accumulations themselves faithfully record hydrodynamic cessions.
conditions and sediment supply but they are rather insen- Sequences and systems tracts illustrate one such principle
sitive to many other environmental factors. Hydrodynamic that governs many sediment accumulations - scale invari-
factors being equal, a wave-dominated delta will look very ance in time and space. The principle was recognized early
0
much the same at the equator and at 60 latitude, in normal in sedimentology. For instance, Potter (1959) in his succinct
marine, hypersaline and fresh-water settings. The respec- summary of the emerging concept of facies models already
tive differences in carbonate accumulations are profound observed “that scale is not a critical factor”. In this book
and chapters 2 – 4 summarize important trends. (chapter 6), it was argued that sequences and systems tracts
The different environmental sensitivity of siliciclastics are invariant on length scales of at least 6 orders of magni-
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and carbonates must not distract from the fact that the tude (from 10 1to10 m). Time required to produce these
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two sediment families have a common basis in the hydro- scale-invariant sequence patterns may vary from 10 yto 10 8
dynamics of sedimentation and erosion. Therefore, many y, i.e. by five orders of magnitude if one includes the slow
rules apply to both families and this leads to effective cross- process of soil formation. For purely geometric aspects of
fertilization. In chapter 4, for instance, the siliciclastic shore- sequences, the scale-invariant domain is significantly larger,
8
to-shelf model was quite easily applied to the carbonate probably 10 −1 yto10 y.
ramp. The same holds for turbidites, debris flows, slumps Fig. 9.1 shows an example of scale-invariant sediment ge-
and many other phenomena. ometry that extends beyond the sequence domain: sediment
Sequence stratigraphy, discussed in chapters 6 – 8, also accumulations fed by a point source assume the shape of
constitutes a fundamental concept that applies to siliciclas- a delta with dendritic feeder channels. Van Wagoner et al.
tics and carbonates as well as sulfate evaporites, volcaniclas- (2003) explain them as energy-dissipation patterns, a geo-
tics and other sediments not examined here. Some modifi- metric expression of the principles of thermodynamics.
cations are required if one applies the siliciclastic standard The trio of topset-foreset-bottomset provides another ex-
model to other sediment families but usually the necessary ample of scale invariance, particularly if one considers it
modifications can be deduced from first principles of sed- together with the analogous classification of undaform-

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