Geology Reference
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margin facing south into the Straits of Florida,
a new siliciclastic shelf and slope system was
formed. Fluvial-deltaic sedimentary processes
merged into cross-shelf and downslope sedimen-
tary processes, again all pulsed by variations in
sea-level and climate (windiness, storminess,
rainfall) as well as oceanographic processes
(e.g. Florida Current and Loop Current activity).
the Long Key Formation (Guertin
et al
., 1999) and
is the time-transgressive equivalent of the
upper Peace River in the Charlotte Harbor-
Caloosahatchee River area.
As Warzeski
et al
. (1996) and Cunningham
et al
. (2003) point out, the aggradation of this
siliciclastic system provided the substrate for
the return of shallow-water carbonates in the
Pleistocene. By the late Pliocene-early Pleistocene
boundary, a carbonate-dominated depositional
environment returned capping the siliciclastics
from the Everglades across the Florida Keys to
the top of the Pourtales Terrace (Guertin, 1998;
Guertin
et al
., 1999, 2000; Multer
et al
., 2002;
McNeill, personal communication, 2006). These
carbonates are overlain by the carbonate Key
Largo Formation for which the initiation age is
uncertain (D.F. McNeill, personal communication,
2006). The aggradation of the siliciclastic system
was essential in creating a widespread shallow-
water environment to support a vibrant carbonate
factory. But, there must have been a concurrent
reduction in the siliciclastic transport as well. We
can only speculate that rainfall and runoff (and
resulting siliciclastic sediment transport in local
rivers) must have been reduced during this time
allowing the carbonate factory to fl ourish.
High-sediment discharge local rivers?
Since there is no evidence for an extensive long-
distance palaeofl uvial transport by a single large
river fl owing down peninsular Florida from the
north, the sub-basins beneath Tampa Bay and
Charlotte Harbor must have fi lled in by local,
short-length, high-sediment discharge rivers and
streams. With no bay-head deltas or sediment-
choked areas at the head-of-tides in today's rivers
discharging into Tampa Bay or Charlotte Harbor,
the rivers of the late Miocene to Pliocene must
have had a higher sediment discharge than pres-
ent. We postulate that the warm period during
the Pliocene (Willard
et al
, 1993; Poore & Sloan,
1996; Dowsett
et al
., 1996) stimulated local
thunderstorm activity over peninsular Florida,
thus increasing rainfall, runoff and sediment
discharge. Increased sea-surface temperatures
in the waters surrounding peninsular Florida
(~+2-4°C; Dowsett
et al
., 1996, their Fig. 1) and
increased heating of the land mass would have
stimulated the local sea-breeze effect, built larger
and more vigorous thunderstorms and could have
prolonged the thunderstorm season beyond the
4-5 months seen today.
Additionally, the sea-level highstand of the
very late Miocene and early-mid-Pliocene lasted
several million years (~5.5-3 Ma) and may have
been as much as +35 m higher than present sea
level (PRISM reconstruction of Dowsett & Poore,
1991). As such, there was suffi cient time for the
fi lling of these peripheral sub-basins and for
the major fl uvial deltaic system to migrate from
the southern end of the Lake Wales Ridge com-
plex to the south Florida margin thus covering the
pre-existing carbonate ramp. Perhaps, maximum
deltaic progradation occurred during multiple,
brief intervals or pulses of enhanced sediment
discharge during the falling stages of higher-
frequency, sea-level events. However, by the late
Pliocene, the massive siliciclastic infl ux seems
to have slowed or stopped in south Florida (D.F.
McNeill, personal communication, 2006). In south
Florida, this ~150 m thick siliciclastic unit is called
Geological signifi cance of semi-enclosed
karst sub-basins
The siliciclastic-fi lled sub-basins underly-
ing Tampa Bay and Charlotte Harbor occur in a
mid-carbonate platform setting and not along a
carbonate margin where siliciclastics have been
known to accumulate (e.g. Belize - Ferro
et al
.,
1999; Great Barrier Reef - Dunbar
et al
., 2000).
Rather than incised valley fi lls or reef-margin,
backfi lled basins, they represent spatially
restricted, semi-enclosed siliciclastic-fi lled karst
features. Kerans (personal communication, 2005)
has indicated that there are large-scale, karst
collapse systems with up to 320 m of vertical
extent and 100-200 m width in the lower
Ordovician El Paso Group of west Texas. Kerans
(personal communication, 2005) also points out
35 m vertical relief, 100 m diameter cave col-
lapses of mid-Permian age in the Sierra Diablo
Range (Texas). Both of these ancient examples
are similar scales to the individual warps, folds
and sags seen beneath Tampa Bay and Charlotte
Harbor.
However, neither of these ancient examples
replicates the spatial scale of those seen in the
