Page 135 - Carbonate Sedimentology and Sequence Stratigraphy

126 WOLFGANG SCHLAGER

Maximum flooding surface. On tropical platforms and reefs,
Type 1 a level of maximum flooding usually can be defined as
the point where the facies trend changes from deepening
to shoaling. Sediments of the T factory are rather depth-
sensitive such that deepening and shoaling trends can be
established with fair certainty. Geometric identification of
the mfs as a level of downlap of highstand clinoforms is dif-
ficult or impossible because the platform tops are virtually
horizontal, bedding near-parallel and lapouts scarce. As a
consequence, maximum flooding levels commonly are indi-
Type 2 cated as intervals rather than surfaces. Techniques for deal-
ing with this “sequence stratigraphy of gradual change” are
discussed at the end of this chapter.

Pseudo-unconformities
In chapter6,it was pointed out that seismic unconformi-
ties and outcrop unconformities do not always match. It
seems that discrepancies between outcrop record and seis-
mic record are more frequent in carbonates, particularly
S Type 3 (drowning) tropical carbonates, than in siliciclastics. The following char-
acteristics of carbonates may be responsible for the differ-
ence.
E ➤ Carbonates can build steeper slopes because the shear
strength of muddy deepwater carbonates is higher than
z their siliciclastic counterparts and because the sedi-
E+S ments often lithify at or near the sea floor. Thus, the
exposure surface
total range of depositional slope angles in carbonates
marine erosion surface usually is wider than in siliciclastics.
t
➤ Carbonate slope angles may change rapidly laterally
and with time during the accumulation process. This
Fig. 7.24.— Conceptual model of the interplay of eustasy and is so because there are many local sediment sources,
subsidence. The same eustatic cycle creates different types of the production includes in-situ framework and a wide
sequence boundaries depending on the rate of subsidence. Top range of grains sizes, and rapid lithification impedes
panel: type-1 boundary forms if the rate of eustatic fall distinctly lateral transport that usually reduces variations in slope
exceeds the rate of subsidence. Middle panel: type-2 boundary angle of siliciclastics.
forms if the rate of eustatic fall is approximately equal to the rate ➤ Changes from the carbonate factory to surrounding sili-
of subsidence. Bottom panel: No exposure occurs if the rate of ciclastic domains may be particularly rapid because
eustatic fall is less than the rate of subsidence; however, the sys- there are many local carbonate production sites and
tem may be drowned or deeply flooded during the subsequent rise
when eustasy and rapid subsidence are in phase; this produces a limited lateral transport because of rapid cementation.
type-3 boundary. After Schlager (1999b). Seismic models of outcrops are one way to directly com-
pare outcrop observation and seismic image. The poten-
tial of outcrops in carbonate sequence stratigraphy was el-
Transgressive surface and maximum flooding surface egantly illustrated by Bosellini (1984, 1988) and soon fol-
lowed by studies presenting actual seismic models of large
Both surfaces (or intervals) are common on tropical reefs outcrops in carbonates (e.g. Middleton, 1987; Rudolph et al.,
or platforms albeit with certain differences to the siliciclastic 1989; Schlager et al., 1991; Biddle et al., 1992; Stafleu, 1994;
standard. Bracco Gartner and Schlager, 1999; Anselmetti et al., 1997;
Kenter et al., 2001). It is probably significant that the major-
Transgressive surface. It is highly variable, depending ity of these models show one or several lapout patterns in
on the extent of the lowstand tract. Where the lowstand the seismic model that were not present in the outcrop nor
tract is narrow and dominated by reef growth rather than in the impedance model that formed the basis of the seismic
accumulation of loose sediment, the transgressive surface model.
may have many meters of depositional relief as it runs up Figs 7.25, 7.26, 7.27, 7.28, 7.29, 7.30 present the basic
and down over isolated reef bodies. The early Holocene types and examples of these false lapouts, termed pseudo-
shows examples of this type. unconformities by Schlager et al. (1991). The pseudo-
unconformities include pseudo-toplap, pseudo-onlap and