Page 119 - Carbonate Sedimentology and Sequence Stratigraphy

110 WOLFGANG SCHLAGER

of the vast shallow lagoons of mature platforms. Transgres- Transgressive tract (Figs 7.6, 7.7, 7.9a). In most parts of the
sive tracts and highstand tracts, on the other hand, inherit Bahamas and southern Florida, sea level has flooded the
the vast shallow environments of the platform top from the platform top and sedimentation clearly trails the rising
preceding highstand. In the transgressive tract, the retro- sea. Along the platform margin, reefs have stepped back
grading platform margin implies that sediment supply lags repeatedly in the past 10,000 yr and the present barriers lie
behind the rate of accommodation creation by relative sea- 200 - 1,000 m bankward of the marginal escarpment (Hine
level rise; as a consequence, water depth increases and open- and Neumann, 1977). These barriers are discontinuous
marine conditions expand on the platform top during the with only a fraction of the platform perimeter occupied
transgressive stage. Conversely, progradation of the high- by shallow reefs (rim index <0.25). Sand shoals, mainly
stand tract indicates that the rate of supply exceeds the rate composed of oolite, are widespread. Much like the reefs,
of change in accommodation; water depth decreases and these shoals are rather ineffective barriers; typical are
circulation on the platform top tends to become restricted. tidal-bar belts that extend 20 – 30 km onto the platform
Progradation of the highstand tract may be bi-directional, but whose seaward tips do not reach the platform edge.
filling the empty lagoon and expanding the platform sea- Channels between the sand bars usually are wider than
ward (see Fig. 3.19; Saller et al., 1993). the sand bars themselves. The sediment covering the vast
interior of the Bahama Banks is dominated by pellets,
ooids and grapestone lumps; skeletal grains generally
Systems-tract facies of modern Bahamas and Florida are less than 15%; mud is less than 10%, higher concen-
trations occur only in the lee of islands (Bathurst, 1971).
Hardgrounds are common. In large areas, the Pleistocene
In order to test these tentative conlusions and examine the surface lies bare or is thinly covered by corals and sponges,
facies in more detail, we turn to modern environments of the and sometimes studded with patch reefs (“transgressive
Florida-Bahama platforms. If one follows the definitions on surface”). Other hardgrounds occur within the Holocene
Fig. 7.3, all three systems tracts are currently developed on section where sedimentation was interrupted for extended
the platforms (Figs 7.6, 7.7). This may seem surprising at periods by currents and waves (e.g. on the eastern lobe
first because eustatic sea level movement has been practi- of Great Bahama Bank). In the sediment cover, the facies
cally the same in the entire area. However, the effects of in- succession is deepening upward, with mangrove peat and
herited topography and variations in sediment supply suf- muddy deposits of restricted lagoons at the base and open-
fice to differentiate the record. marine, winnowed sand on top (Enos, 1977 for Florida area).

Lowstand tract (Figs 7.6, 7.7 7.8). What I consider analogues Highstand tract (Figs 7.6, 7.7, 7.9a, 7.10). Even though the
of lowstand tracts are narrow carbonate shelves that rim the Holocene eustatic rise of sea level must have been rather
exposed limestones of the last interglacial highstand tracts.
uniform across the stable Florida-Bahama area, the sediment
The width of these shelves varies from about 100 m to over
cover does not uniformly exhibit transgressive character-
2,000 m. The sediment cover consists of the coralgal lithofa-
istics. Where currents and waves sweep the sediment to-
cies of Bathurst, (1971, p. 108): skeletal sands, hardgrounds
gether or where carbonate production is exceptionally high,
and reefs (fringing reefs, barrier reefs protecting narrow la-
sedimentation has caught up with the rising sea. In these
goons, and patch reefs in the lagoons). Most of the “out-
areas, sediments have filled the available accommodation
lier reefs” mapped by Lidz et al. (2003) also fall in this cate- space and started to prograde laterally. Geometrically these
gory. The biota is dominated by algae, corals, molluscs and parts of the Holocene cover are highstand systems tracts.
foraminifers, derived from the reefs and from the sea-grass Examples include the oolite shoal of Joulters Cays and the
communities on the flat sea floors. Very fine sand and mud tidal flats SW of Andros.
amount to ca. 10%, pellets to 25% (Bathurst, 1971). Ooids The shoals of Joulters Cays differ significantly from their
are rare. A possible exception are certain beaches on Caicos counterparts in a transgressive setting. At Joulters, the sand
Bank (Wanless and Dravis, 1989), where precipitation seems shoal forms a nearly continuous, margin-parallel barrier
to be so rapid that ooids form on the shore face before they whose crest is several kilometers wide and reaches in the
are ejected from this narrow belt of favorable conditions. intertidal zone, locally topped by beach-dune islands (Har-
Chemical conditions have not been studied; however, it is ris, 1979). Tidal channels are narrow, widely spaced and
likely that the process depends on backflow of hypersaline frequently choked by migrating sand on either end. Cores
lagoon waters and thus ultimately requires a flooded plat- show that the intertidal shoal has been established on top
form top - a condition not met during genuine lowstands of shallow-marine pellet sand. In the last 1,000 y, islands
of sea level. Most of the Bahamian coralgal shelves are formed and prograded seaward. Similar progradation of
sediment-starved. However, the environment is highly pro- beach-dune complexes was described from other Bahamian
ductive and will quickly catch up with sea level and start to platforms (Wanless and Dravis, 1989).
prograde. Progradation can be expected to be slow because More significant progradation took place on the tidal flats
the system faces a high, steep slope. of SW Andros. The belt S of Williams Island prograded 15 –
20 km in the past 6,000 y while the flats N of Williams Island