Page 142 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 7: SEQUENCE STRATIGRAPHY OF THE T FACTORY 133

TWO NEOGENE CASE STUDIES IN CARBONATE sand shoals. The tropical climate is dominated by easterly-
SEQUENCE STRATIGRAPHY northeasterly trade winds. The sequence-stratigraphic stud-
ies concentrated on a margin of Great Bahama Bank that
Northwestern Bahamas faces leeward with respect to the trades and borders a basin
that lies at the juncture of two seaways - the Florida Straits
The northern part of the Bahama archipelago has become and Santaren Channel. Both seaways are swept by strong
a model and standard for the interpretation of tropical northward currents that merge in the study area (Fig. 7.33).
carbonates. Chapters 2 and 3 repeatedly alluded to this They are part of the clockwise subtropical gyre of the North
role. The area also has become an important calibration Atlantic. The gyre belongs to the surface circulation of the
point for carbonate sequence stratigraphy by an assembly ocean (Fig. 1.4) but the western boundary currents are very
of research wells drilled along an industry seismic line and voluminous and therefore fill the Bahamian channels to the
its offshore extension by the Ocean Drilling Program. Two bottom. Consequently, sediment accumulations in the study
platform wells were contributed by the Bahama Drilling area are shaped by the interplay of carbonate production,
Project (Ginsburg, 2001), five wells on the slope and in the leeward transport at the bank top, sea level, and ocean cur-
basin by the Ocean Drilling Program Leg 166 (Eberli, Swart, rents that sweep the slopes and basin floors.
Malone et al. , 1997).
The sequence stratigraphy established by Eberli (2000),
Anselmetti et al. (2000), Eberli et al. (2001, 2002) yielded a
Setting. The carbonate platforms of southern Florida and
very consistent pattern. Unconformity-bounded packages
the Bahamas are part of the passive margin of North Amer-
were identified in the seismic data and traced from platform
ica, a mature margin that rifted in the Jurassic and whose
subsidence rate had slowed to few tens of meters per My in to basin (Fig. 7.34). Via check-shots and logs, the boundaries
the Plio–Peistocene (McNeill et al., 2001). The carbonate fa- were tied to the seven wells on the transect. Correlation
cies indicate tropical platforms, mostly rimmed by reefs or among the wells was accomplished by lithostratigraphic,
biostratigraphic and magnetostratigraphic techniques. This
exceptionally detailed case study produced several impor-
tant results.
➤ Correlations by tracing seismic reflectors and by rock
stratigraphy are consistent – the time lines proposed by
the various techniques never cross (Fig. 7.35).
➤ This very satisfactory correlation was achieved by con-
straining seismic analysis by age dates and lithologic
observations from the cores. Pure seismic correlation
from platform to basin would face big uncertainties be-
cause of repeated merging and splitting of reflections.
Currents are the principal cause of this problem – they
generate sediment drifts and scours on the basin floor
and episodically cut major hiatuses in the slope de-
posits (Fig. 7.37). On the other hand, currents are prob-
ably in phase with sea-level fluctuations such that low-
stands coincide with more intensive erosion because of
the reduced cross section of the channels (Eberli et al.
2001, p. 258).
➤ Sequences are bounded by exposure surfaces in the
platform domain and by abrupt facies changes or hia-
tuses marked by hardgrounds in the deep-water do-
main. Million-year long hiatuses and pervasive ma-
rine cementation or authigenic mineralization of hard-
grounds almost certainly reflect current activity. The
periplatform setting normally is very well supplied
with sediment and the long intervals of non-deposition
suggest an external cause.
➤ Lowstand wedges are clearly recognizable but they are
small and few have distinct platform margins (Fig.
Fig. 7.33.— Location map of western Great Bahama Bank and 7.34).
surroundings showing boreholes and seismic profilesaswellas ➤ Highstand shedding dominates this leeward slope.
shoal-water carbonate environments (shaded). After Eberli et al. Most progradation occurs during highstands where
(2002). Copyright 2002, reprinted with permission from Elsevier. aggradation is small (Eberli et al. 2001, p. 260). In this