Page 124 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 7: SEQUENCE STRATIGRAPHY OF THE T FACTORY 115

formation of the highstand tract when platform environ- ➤ The shoalwater belts of lowstand tracts are much nar-
ments become increasingly restricted and prograding bod- rower than transgressive and highstand tracts. The lim-
ies of loose sediment bury many reefs. ited width is a geometric consequence of the intersec-
tion of the photic zone with a steep slope. For a plat-
◦
form with a top dipping at 0.1 and a slope dipping
Ecologic reef, geologic reef, seismic reef at 10 , the widths of the lowstand and highstand tracts
◦
differ by about two orders of magnitude. Consequently,
Chapters 2 and 3 emphasized the importance of reefs at the lowstand tract tends to be well flushed and normal
platform margins for the creation of flat-topped platforms marine, rich in reefs, hardgrounds and rocky shores;
so typical of tropical carbonates. The question of what is a sediment tends to be coarse and predominantly skele-
reef continues to fuel heated discussions among geologists. tal; ooids tend to be scarce because tidal currents are
Dunham (1970) introduced a novel perspective in this de- weak on the narrow shelf. The facies succession may
bate when he proposed to qualify the term by distinguishing be shoaling or deepening upward and the margin pro-
between ecologic reefs, built and bound by organisms, and grading or backstepping. There is no indication that
stratigraphic reefs where the binding may also be done by carbonate lowstand tracs are only prograding, nor is
cementation of loose sediment. This distinction has proved there a theoretical basis for such a postulate.
very useful but the increasing use of seismics to identify ➤ Transgressive systems tracts are characterized by the
reefs has complicated matters again. rate of accommodation creation exceeding the rate of
sediment supply. Consequently, facies are deepening
The “seismic reefs” defined by geometry and reflection
upward. Reef rims and sand shoals are narrow and
character do not correspond straighforwardly to either eco-
breached by wide passages, the platform interior is
logic or stratigraphic reefs. Seismics tends to overlook small
generally well flushed and normal marine; patch reefs
reefs and, in other circumstances, adds non-reef deposits to
spread far landward. In extreme situations, the rim may
the reef. Living parts of reefs are small by seismic standards,
be lacking altogether such that the platform temporar-
easily destroyed and quickly buried by their own debris.
ily represents a ramp that steepens at the distal end
The diagnostic criteria for identification of reefs in outcrop
to merge with the profile of the old highstand slope.
are far below normal seismic resolution (see Fig. 4.6). The
Mud is rare in the shallow-marine environments be-
seismic tool reveals reefs only where many generations of
cause of the scarcity of protecting barriers; however,
reef growth were stacked to thicknesses of tens to hundreds
deep flooding may increase water depth to below wave
of meters. Where the sites of reef growth shift laterally, reefs
base such that deeper-water muds accumulate on top of
do not stack to seismically recognizable structures but form
small lenses embedded in detrital sediment. These small winnowed early TST deposits.
constructions are readily recognized in the field but remain ➤ Highstand systems tracts are characterized by the rate
largely hidden in standard seismic data. of sediment supply exceeding the rate of accommoda-
tion creation. As a consequence, sand shoals and reefs
Equally important are situations where the seismic tool
shows more “reef” than is actually there in a stratigraphic at the margin become wider and more continuous. Fa-
cies successions are shoaling upward; the degree of re-
or ecologic sense. Reefs defined by geometry and reflec- striction increases in the platform interior and mud may
tion character normally include, beside ecologic reefs, also start to accumulate in the lagoon and in expanding tidal
the reef aprons on the landward side as well as the coarse,
flats. Lagoonal patch reefs tend to decrease upward.
poorly stratified talus on the fore-reef side (Fig. 7.13. In addi-
➤ Non-skeletal grains (ooids, peloids) tend to be more
tion, sand shoals interfingering with reefs may be included
abundant in transgressive and highstand tracts than in
in the seismic reef unit.
lowstand tracts because they benefit from the amplifi-
cation of tidal currents and the extensive winnowing
Rules of thumb on systems tracts and facies by waves on the vast, flooded platform top. This dis-
tribution pattern holds as long as carbonate precipita-
tion rates are in the same range as those of the modern
The facies characteristics of recent and ancient systems
Bahamas or the Persian Gulf. More rapid rates of pre-
tracts are rooted in two principles of carbonate deposition:
cipitation may lead to formation of ooids and hardened
the balance beween rate of sediment supply and rate of ac-
peloids on narrower platforms.
commodation creation, and the tendency of carbonate sys-
➤ The deposits of slope and basin commonly permit a dif-
tems to form flat tops and steep slopes. The shifting bal-
ferentiation into times of bank-top flooding and times
ance of rates explains the differences between transgressive
of bank-top exposure, i.e. highstand and transgressive
and highstand systems tract, the flat-top-steep-slope mor-
tracts on the one side, and lowstand tracts on the other.
phology sets the lowstand tract apart from transgressive
The key to this differentiation is grain composition for
and highstand tracts. Below follow some rules of thumb on instance abundance of ooids and platform mud. In this
sytems tract facies. Most of them are directly related to the instance, facies analysis is a better tool than the geome-
balance of rates and to morphology.