Page 116 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 7: SEQUENCE STRATIGRAPHY OF THE T FACTORY 107

and wind on the large-scale anatomy of platforms in the Ba- Systems tracts in T carbonates and their control by
hamas. On the left, windward, margin the rims stack ver- accommodation and production
tically and the gradual lateral migration of systems tracts
in response to sea level is interrupted. The prograding lee- Systems tracts of the standard model were defined ge-
ward margin on the right in Fig. 7.1 more closely resem- ometrically and subsequently interpreted in terms of rela-
bles the classical sequence model because loose sediment tive sea-level changes (see chapter 6). Very similar defini-
from offbank transport plays a bigger role. However, rim tions can be applied to tropical carbonates (Fig. 7.3) . One
building by reefs or lithified sand shoals is important even significant difference is that well-rimmed tropical platforms
in prograding margins: the constructional rims tend to oc- have essentially horizontal tops rather than seaward dip-
cur intermittently and as lenses; they are not resolved by ping shelf profiles.
the low-frequency seismics in Fig. 7.1 but can be seen in In chapter 6, the systems tracts of the standard model
high-resolution data of the Holocene (Fig. 7.2). The fluctuat- were interpreted as the balance of the rate of change of ac-
ing shelf-margins of these prograding platforms with their commodation and the rate of sediment supply (Fig. 6.12).
buried rims seem to yield the most reliable sea-level record In carbonates, the situation is analogous except that out-
in carbonate seismic stratigraphy. side sediment supply has to be replaced by G, the in-situ
growth and production of carbonate material. Sediment ge-
ometry and systems tracts are again controlled by two a pri-
ori independent rates: A’ = dA/dt, the rate of change in
accommodation as defined in chapter 6, and G’ = dG/dt,
the rate at which a platform produces sediment and builds
wave-resistant structures. The maximum rate of growth that
the system can sustain, the growth potential, varies across
the platform. At the very least one must distinguish be-
reef pinnacles
tween G r ’= the growth rate of the platform rim, and G p ’=
1 km
10 m the growth rate of the platform interior.
deep reef
20 m Based on the relationship between A’ and G’, five char-
30 m reef-veneered rock acteristic patterns can be distinguished (Fig. 7.4). The first
ridge (eolian?)
40 m
three situations correspond to the three systems tracts of
50 m Holocene sand
the standard model in Fig. 6.12. The last two situations are
60 m Holocene reef
typical for rimmed tropical carbonates. Empty bucket and
70 m Pleistocene rock
drowned platform tops are particularly important because
80 m
they yield diagnostic patterns for recognizing tropical plat-
forms in seismic data.
The importance of sediment supply on sequence anatomy
is illustrated by Fig. 7.1. The entire area forms a stable
HIGHSTAND TRACT
covers top, progrades

500 m
10 m
20 m
30 m
TRANSGRESSIVE TRACT
40 m starts below top, then covers top, LOWSTAND TRACT
Holocene sand
retrogrades below top, progrades
Holocene reef
Pleistocene rock
Fig. 7.3.— Terminology of systems tracts on rimmed carbonate
platforms. The platform margin of the preceding cycle serves as
Fig. 7.2.— Windward and leeward Holocene platform margins in
the Bahamas. Reefs are actively growing on the windward mar- a reference level. A systems tract whose top is lower than the top
gin (upper panel) and mostly buried in sediment on the leeward of the preceding platform is a lowstand tract, systems tracts that
margin because of offbank sediment transport (lower panel). After cover the top of the preceding platform are either transgressive
Hine and Neumann (1977). (Reprinted by permission of the AAPG tracts or highstand tracts, depending on whether they retrograde
whose permission is required for further use). or prograde. The tops of rimmed platforms tend to be very flat and
flooding of the rim often leads to abrupt backstepping of the plat-
form margin. After Schlager et al. (1994).