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Accretionary algal mound growth probably would
have been able to outpace the relative rise of sea
level in response to the combination of tectonic and
eustatic mechanisms (Schlager, 1981; Goldhammer
et al ., 1991). The majority of the mounds in the
subsurface are interpreted as accretionary build-
ups formed by a combination of sediment baffl ing,
binding and early marine cementation, and they
exhibit a general baffl estone fabric with little inter-
nal sediment. Growth of mounds vertically into
shallow water is indicated by the general decrease
in mud towards the tops of mounds, the extensive
brecciation, which is probably due to wave action,
and the fact that some of the mounds show evi-
dence of subaerial exposure. The regional thinning
of overlying evaporites confi rms that the mounds
were positive topographic features.
Lateral variability within the mound fi elds
observed in the subsurface, which varies from
tens to several hundred of metres, was described
by Grammer et al . (1996, 2000) as a function of
the distribution of individual mounds within
the fi eld. In outcrop at least, lateral variabil-
ity of the mounds is observed on two scales.
Well-developed, high-amplitude algal mounds
with good porosity and permeability occur in
coalesced units that are typically 300-400 m in
long dimension and that pinch out laterally into
muddy facies. These coalesced mound units
are what is most commonly discussed in sub-
surface examples and, when locally abundant,
make up the majority of the small algal fi elds in
the basin. The individual mounds that make up
these coalesced units vary from about 3 to 14 m
high and are spaced from 30 to 70 m apart. From a
reservoir standpoint, these individual mounds, in
general, have the best porosity and permeability
values near the core of the mound, with a general
increase in mud and overall decrease in porosity
and permeability towards the fl anks.
Grammer et al . (1996, 2000) suggested that one
explanation for the distribution of these smaller
coalesced mound units is that they formed on
local areas of slightly higher relief either on
regional-scale (i.e. fi eld scale) mud banks or
within structurally controlled lows. They sug-
gested that low-relief antecedent highs could then
have formed through the accumulation of smaller-
scale mud banks in both strike- and dip-parallel
orientations, similar to that described in modern
settings by Rine & Ginsburg (1985). In the above
scenario, Grammer et al . (1996, 2000) suggested
that individual mounds that locally display a near
sinusoidal distribution, such as those at 8-Foot
Rapids, could have been controlled by current
and/or wave action, but no example of how this
might occur was presented.
The Quicksands, southwest Florida Keys
The Quicksands is an area about 60 km west of
Key West, Florida, which consists of a high-
energy accumulation of non-oolitic carbonate
sands that has formed in response to tidal currents
fl owing across the shallow platform (Fig. 6). The
area ranges from approximately 2 to 10 m deep,
and extends over an area that is about 15 km wide
(E-W) and 10 km long (N-S). The Quicksands
have formed on an antecedent (Pleistocene) topo-
graphic high called the Marquesas-Quicksands
Ridge, which sits an average of about 10-15 m
higher than the surrounding west Florida plat-
form (Shinn et al . 1990). The southern end of the
Marquesas-Quicksands Ridge is located about
10 km in from the platform margin that borders the
deep water Straits of Florida further to the south
(Shinn et al ., 1990). The area of tidal channels and
tidal bar belts near Halfmoon shoal, located in the
southwesternmost part of the Quicksands com-
plex, is the primary location used for comparison
in this study (Fig. 7).
As reported in Shinn et al . (1990) and verifi ed by
our own observations, sand waves with amplitudes
of 5-7 m and wavelengths varying from about 150
to 250 m, cover the bottom of the main tidal chan-
nel just west of Halfmoon Shoal (Fig. 8). These
sand waves are forming in response to N-S tidal
fl ow, and occur in water depths of approximately
10-12 m. Based upon the results reported in Shinn
et al . (1990), as well as our own sampling, the sand
waves are made up primarily of Halimeda sands
(average 45%), with subordinate amounts of nor-
mal marine skeletal components (corals, molluscs,
benthic foraminifera, echinoids and red algae).
Halimeda is a codiacean green algae that is con-
sidered to be a modern example of at least some of
the phylloid algae (Heckel & Cocke, 1969; Scoffi n,
1987), especially Ivanovia , the most common
type found in the Paradox Basin mounds (Pray &
Wray, 1963). The calcifying algae have large,
potato chip-like plates which break off during
storms and/or are released after the organism dies
(Fig. 9). As discussed in Shinn et al . (1990), stud-
ies have shown that a single mature plant of the
type of Halimeda most prevalent in the Quicksand
sand bars ( Halimeda opuntia ), may grow as many
as 3000 new plates within a time span of about
4 months (Hudson, 1985).
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