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layer washed 10 km in through the broad mangrove
swamp along 10 km of the coast in less than
45 minutes. Except right at the coast, the grains
were less than 200 μm and only gradually became
fi ner inland (like the Hurricane Kate layer across
the Caicos tidal fl at, this Hurricane Andrew
layer was mostly a fi ne grainstone, as most of
the mud washed off as the surge moved back
to the sea). Fortunately the layer had a few per
cent quartz grains in it. Within a year, mangrove
rootlets from the surviving mangroves began to
grow up into the overlying layer. This root bio-
turbation, combined with dissolution of the car-
bonate particles by tannic and humic acids has
essentially obliterated this dramatic layer in only
about 10 years. Fine quartz sand is dissemin-
ated throughout this mangrove peat sequence
along the southwest coast of Florida. This is
all that remains of the story of repetitive major
hurricane sedimentation events there.
Layering is better preserved where individual
depositional units are thicker, coarser grained, or
happen frequently. Shepard (1960) recorded pre-
served layering in the central Texas lagoons only
in association with river or tidal deltas, which
served as a focus of more frequent or thicker
sedimentation events. Perlmutter (1982) found
preserved layering in the 10,000 Islands of south
Florida mainly where hurricane fl ood surges
through tidal channels formed broad, low deltas
in the interior lagoons.
Many carbonate sedimentologists have focused
on the enormous role of biological processes in
forming carbonate sequences, and early on it
seems that physical processes were played down
except in the more energetic settings, such as
ooid sand shoals. Ball (1967) did point out that
the Safety Valve muddy tidal bar belt along the
seaward side of Biscayne Bay (south Florida) was
the low-energy setting to which sediment could
be transported, deposited and accumulate. But in
reading
Carbonate Sediments and their Diagenesis
(Bathurst, 1975), the mud banks in Florida Bay
were portrayed as a biological wonderland in
which carbonate sediment was produced by cal-
careous green algae, trapped by the sea grasses
and grew carbonate banks. In fact these banks,
which grow into the intertidal zone, are layered,
physically deposited features in which the lay-
ers are a product of energy events, predominantly
winter storms (Wanless & Tagett, 1989). They are
actively migrating in central Florida Bay, building
on the south and west fl anks at rates of 1-2 cm yr
or more (Boscence, 1989).
SEA-LEVEL-DRIVEN PULSES OF
SEDIMENTATION
Most Holocene and Pleistocene dune ridges in
the Bahamas are predominantly built of physi-
cally layered strata, whereas nearly all modern
coastal dune ridges today are mostly stabilized
by vegetation and do not appear to be producing
layered sequences, because the sand as it gradu-
ally accumulates is bioturbated by root processes.
There seem to be two answers to this dilemma,
both potentially correct. First, some of these
ridges are formed during and just following major
storm events as large volumes of sediment are
scoured and recycled. Some Pleistocene ridges in
the Bahamas have low-angle beach laminae with
beach fenestrae (bubble casts) extending up an
individual layer a vertical distance of more than
6 m, a representation of storm wave run up.
Second, coastal and shallow-marine environ-
ments exhibit pulses of growth. In the Berry Islands
and elsewhere in the Bahamas, islands record
rapid pulses of growth during times of rising sea
level as large volumes of coastal and shelf sediment
became exposed and unstable. Waves and currents
rapidly erode and recycle this sediment bankward
producing pulses of island or dune-ridge growth.
These thick layered sequences are then followed
by a time of vegetative stabilization, bioturbation
of the upper portion, and minor trapping of sand
that is blown or washed in. A similar process is
happening along sections of the southwest Florida
coast today. Probably with the help of 23 cm rela-
tive rise of sea level during the past 70 years, the
marl (fi rm carbonate mud) and organic peat coast
is rapidly eroding (200-400 m since the earliest
1928 aerial photographs) and large volumes of
sediment are being redistributed (see Vlaswinkel
& Wanless, 2009). Natural and cut tidal channels
are feeding a larger tidal prism and thus bringing
increased volumes of sediment-laden tidal water
into coastal lakes and estuaries. Rapidly forming
fl ood tidal mud deltas are fi lling these lakes and
bays at rates of 1-20 cm yr
. Organic content of
these carbonate sediments is up to 40% (in con-
trast to 2-10% in most of the Florida Bay mud
banks). There is a burrowing infauna and aggres-
sive ibis (deep prodding) wading bird community
across most of the deltas. Nevertheless layering is
commonly preserved as distinct mud and organic
laminae from daily and winter storm fl ood tide sedi-
mentation and millimetre to centimetre fi ne-sand
and mud layers from historical hurricanes. This
rapid pulse of coastal sedimentation in response
