Geology Reference
In-Depth Information
Detailed documentation of many of these aspects or
areas is well beyond the scope of this chapter; inter-
ested readers are referred to the primary literature on
the topics as cited. Similarly, this discussion omits
other carbonate peritidal systems with marked differ-
ences with the Bahamas, such as Shark Bay, Western
Australia (Logan et al.
1970,
1974
) , or Florida Bay
(Enos and Perkins
1979
; Enos
1989
) and does not con-
sider sandy carbonate fl ats such as those of Joulter
Cays or Shroud Cay in the Bahamas (Harris
1977
, see
Chap. 20, this volume ).
highs and islands - areas typically sheltered from
prevailing wind and wave energy. At a more local
scale, irregular bedrock topography, especially bedrock
highs that extend above sea level (such as on Crooked-
Acklins and southwest Andros; see below) within a
tidal-fl at complex, can exert a pronounced infl uence on
local geomorphic and facies patterns.
Tidal range
is an important control on carbonate
tidal fl ats, regulating the volume of water and, indi-
rectly, sediment passed onto and off the tidal fl ats, and
exerting an important infl uence on tidal hydrodynam-
ics, geomorphology, and the distribution of fl ora and
fauna. Most modern carbonate tidal fl ats occur in
microtidal settings, with tidal range less than 2 m. For
example, in the Bahamas near the open ocean, the
semi-diurnal tides have spring tidal amplitudes of
<1.2 m, but range in the protected platform interiors
and the tidal fl ats on the west side of Andros Island are
~46 cm at the mouth of creeks, decreasing to 29 cm in
the inner tidal-fl at ponds (Hardie
1977
) . Likewise,
open-ocean spring-tidal range in the southern Arabian
Gulf can exceed 2 m, but decreases into the lagoons
and onto the sabkha complex (Purser
1973
) . Despite
its potential signifi cance, the infl uence of tidal range
on the character of carbonate tidal fl ats has not received
systematic study, and objective criteria for recognition
of tidal range in the geologic record have not been
developed.
Sediments from terrestrial sources are negligible in
most humid carbonate tidal fl ats, and unlike many car-
bonate systems, sediment is not produced
in situ
to any
great extent. As a result, these areas are intimately tied
to their nearshore subtidal sediment sources
.
There are
few detailed studies of the infl uence of the nearshore
subtidal realm on tidal fl ats, although in a notable
exception, Gebelein (
1977
) estimated that up to 94%
of the sediment on the Cape Sable tidal-fl at complex of
southwestern Florida was generated from offshore
sources. Comparable qualitative statements are made
by Shinn et al. (
1969
) and Hardie (
1977
) concerning
the Andros Island tidal fl ats, and the trend is probably
similar for Caicos and Crooked-Acklins tidal fl ats as
well. In contrast, arid tidal fl ats such as those in the
Arabian Gulf may have markedly different sediment
dynamics. In these areas, eolian quartz sand from ter-
restrial sources can add considerable sediment to the
tidal fl at system (Shinn
1973a,
b
) . Nonetheless, the
lack of supply from terrestrial or onshore sources is
distinct from many siliciclastic systems.
19.2
Distribution of Carbonate
Tidal Flats
Although Holocene carbonate tidal fl ats are much less
spatially expansive than their ancient epeiric counter-
parts, several important and interlinked factors that
collectively infl uence the distribution, geomorphology,
and sedimentologic character are probably similar. At
a fundamental level, tidal fl ats are broad, nearly planar
areas near mean sea level, alternately fl ooded and
exposed by tides, and formed by accumulation of gen-
erally muddy sediment transported and deposited in
the absence of appreciable wave energy. A primary
condition for tidal fl at development is a shallow-water
setting with low energy. Within this context, several
factors infl uence the nature of tidal fl ats, including
geologic infl uences (bedrock confi guration, coastline
orientation), tidal amplitude, winds (daily, related to
the passage of cold fronts, or tropical depressions), cli-
matic setting, and sediment source. Of course, sea-
level change also markedly infl uences these systems;
this aspect is discussed in some detail in a later section,
in the context of the evolution and stratigraphic record
of Holocene tidal fl ats.
Geologic framework and history controls the orien-
tation of the shoreline, the topography from which
tidal fl ats nucleate and evolve, and the characteristics
of the nearshore shallow-marine system. The orienta-
tion and geographic setting of the coastline, for exam-
ple, are among the most marked broad-scale controls
on tidal-fl at distribution and character because they
infl uence waves and tides, the physical mechanisms
that drive energy and sediment fl uxes impacting the
coast. Accordingly, tidal fl ats in the Bahamian
Archipelago are most extensive on the western or
southern platformward fl anks of Pleistocene bedrock
