Page 100 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 6: FUNDAMENTALS OF SEQUENCE STRATIGRAPHY 91

formity the sense of Vail et al. (1977) and needs pattern, too, is common in the geologic record, particularly
to be recognized and eliminated before sequence in epochs with pronounced glacio-eustasy.
analysis is done (Fig. 6.8, 6.9 and 6.10). Carbonate- Application of the concept of parasequence and simple
siliciclastic transitions and sand-shale transitions sequence requires that a clear distinction be made between
are particularly prone to this effect. marine flooding surfaces reflecting abrupt change from shal-
Recognition of pseudo-unconformities still is in its in- low to deep, and exposure surfaces reflecting change from
fancy. Vail et al. (1977) spotted the phenomenon (Fig. 6.8) shallow marine to exposure to deep flooding. In carbon-
but did not think it was serious. I believe it deserves some ates, this can be quite cumbersome as the upper suprati-
attention. In seismic models of outcrops the true nature dal zone already develops fresh-water diagenetic overprints
of the interfingering can often be revealed by increasing and terrestrial exposure surfaces need not appear as dra-
the wave frequency. Before the pattern is completely re- matic breaks in the record. As a consequence, this distinc-
solved, a series of short, en-echelon reflections appears in tion has been largely ignored in the literature such that the
the transition zone. Complex-trace attributes such as instan- Pleistocene glacial-interglacial cycles of the Bahama Banks
taneous phase and reflection strength offer additional possi- and other extant platforms were classified as parasequences
bilities to recognize pseudo-unconformities (Bracco-Gartner even though they are bounded by exposure unconformities
and Schlager, 1999). during which sea level stood 100 m or more below the plat-
form top (e.g. Kievman, 1998).
Parasequence and simple sequence
Sea level
The parasequence was originaly defined as “ ... relatively con-
formable succession of genetically related beds ... bounded Sea level is a crucial element in sequence stratigraphy.
by marine flooding surfaces or their correlative surfaces” According to the standard model, it is the principal cause
(Van Wagoner et al., 1988, p. 39). The marine flooding sur- of stratigraphic sequences and it certainly represents one
face was defined by the same authors as “a surface ... across of the most important environmental boundaries in sedi-
which there is evidence of an abrupt increase in water depth. mentology. Sea level is easy to visualize but measuring the
This deepening is commonly associated by minor subma- elevation of the sea surface and its change with time is a
rine erosion (but no subaerial erosion or basinward shift in formidable task.
facies) ... ”.
Relative sea level refers to sea level measured relative to a
The simple sequence was defined by Vail et al., 1991, p. 630) fixed point on land (e.g. Revelle et al., 1990). In ocean en-
as a unit that “has the stratal and lithologic characteristics of gineering, this fixed point is normally chosen at the land
a sequence, but its duration is that of a parasequence”. The surface near the coast. Sequence stratigraphy choses a
authors indicate that “simple sequences are picked where fixed point in the sediment pile, preferably near or at its
possible, but in most cases simple sequence boundaries are base (Posamentier et al., 1988, p. 110; Emery et al. 1996,
difficult to identify, therefore the more readily recognizable p. 16). Many stratigraphers and sedimentologists take,
parasequence boundaries are used.” explicitely or by tacit assumption, the sea bottom as the
There can be little doubt that shoaling successions of the fixed point for measuring relative sea level (Hallam, 1998).
parasequence type are a common feature of the sediment The difference between this common stratigraphic practice
record. Van Wagoner et al. (1988) point out that they are and the approach of sequence stratigraphy is fundamen-
best developed in deposits of the coastal plain, the nearshore tal. Relative sea-level changes determined by the sequence-
zone and the shelf. They are difficult to define in fluvial stratigraphic approach represent the sum of eustatic and
and deep-water deposits. This distribution points to the ori- tectonic movements, relative sea-level changes measured
gin of the shoaling successions: they reflect sedimentation from the sea floor yield the sum of eustasy, tectonics, sedi-
in settings with limited accommodation where the sedimen- ment accumulation and sediment compaction. By sequence-
tation pile gradually approaches the accommodation limit ( stratigraphic standards, a 1,000 m-pile of shoalwater lime-
in carbonates the upper supratidal zone). There is no sig- stones deposited at sea level has recorded a 1,000-meter rise
nificant fall of relative sea level and the sea transgresses the of relative sea level (probably largely caused by regional
flats again by gentle inundation with only minor erosion. subsidence); by common stratigraphic standards, the lime-
Shoaling cycles of this kind are a well-established pattern in stone pile indicates no relative change of sea level (or only
shoal-water carbonates (chapter 3). However, shoaling cy- minute oscillations indicated by alternation of supratidal
cles bounded by flooding surfaces are not the only pattern and subtidal facies).
in shoal-water carbonates. Symmetrical shoaling-deepening
cycles, deepening cycles or oscillating successions without Eustatic sea level. When he introduced the term, Suess (1888)
distinct shoaling or deepening trends are quite common (e.g. called eustatic changes those that originated in the sea and
Enos and Samankassou, 1998). contrasted them with sea-level changes that were caused by
In contrast to the parasequence, the simple sequence re- uplift or subsidence of the surface of the solid Earth. Nowa-
quires falls of relative sea level as bounding events. This days, eustatic sea level is defined as the elevation of the