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In-Depth Information
kilometres
Fig. 5.
Detailed seismic line illustrating deformation beneath lower Tampa Bay. Style of deformation indicates collapse
from below due to deep-seated dissolution of older carbonates or evaporites (modifi ed fi g. 16, Berman
et al
., 2005).
crops out forming hardbottoms, which dominate
portions of the west-central Florida shelf (Locker
et al
., 2003). This lack of cross-shelf continuity
indicates that these sub-basins are not shelf valleys
that have been carved by rivers during sea-level
lowstands (Dalrymple
et al.
, 1994; Donahue
et al.
,
2003) in contrast to Brooks & Doyle's (1998) con-
tention that the sub-basin beneath Tampa Bay was
formed by palaeofl uvial incision.
Numerous cores from Tampa Bay (US Army
Corps of Engineers, 1969) as well as the surround-
ing onshore boreholes (Green
et al
., 1995; Florida
Geological Survey, 2005) indicate that overlying
the Arcadia Formation is the middle Miocene to
lower Pliocene Peace River Formation, which
constitutes the basin fi ll. This lithostratigraphic
unit is principally siliciclastic (>66%) with inter-
bedded quartz sands, clays and carbonates. The
Peace River Formation is a very complex unit
consisting of large amounts of fl uvio-deltaic
and coastal siliciclastic sediments to minor
amounts of restricted and open-marine carbonates
(Scott, 1988). The Hawthorn Group, as defi ned by
Scott (1988), is composed of the Arcadia and the
Peace River formations.
Suthard (2005) defi ned 11 different seismic
sequences from two major depocentres that
constitute the Peace River Formation and the
overlying Plio-Quaternary siliciclastic units
(Fig. 3). This seismic sequence mapping
reveals dominant north-northwest prograding
clinoforms (Fig. 3) indicating a source area from
the south-southeast - probably from the ances-
tral Manatee and Little Manatee Rivers (Fig. 6).
As mentioned above, the lowermost seismic
sequences are deformed as well as the seismic
basement indicating syndepositional deformation
during the late stage of basin formation and early
stage of sub-basin fi lling.
CHARLOTTE HARBOR SUB-BASIN
Approximately 150 km to the south of Tampa Bay,
located along Florida's Gulf of Mexico coastline,
lies Charlotte Harbor having very similar dimen-
sions (~725 km
2
), shape and average depth (~2.3 m)
as Tampa Bay (Fig. 1). Evans (1989) and Evans
et al
. (1989), using a grid of 800 km of analogue,
high-resolution seismic-refl ection data tied to 22
borehole sites around this estuary, found strong
similarities to what was later discovered beneath
Tampa Bay (Fig. 7). Additionally, later work by
Cunningham
et al
. (2003) in the Caloosahatchee
River and southern Charlotte Harbor provided
chronostratigraphic control through seismic
refl ection data and borehole analyses.
Charlotte Harbor is underlain by multiple
sub-basins, some having upwards of 100 m of
relief as shown by mapping the seismic base-
ment refl ection, which also constitutes the top
of the Arcadia Formation (Fig. 8). Evans
et al
.
(1989) and Evans & Hine (1991) concluded that
these underlying Tertiary carbonates had under-
gone extensive dissolution and collapse creating
isolated karst depressions that extend seaward
beneath the inner shelf. The same deformational
style seen beneath Tampa Bay occurs beneath
Charlotte Harbor where broad, high, folded
areas are separated by narrow, sinkhole-like sags
(Fig. 9). One fold in particular, reveals high-
angle faulting, fracturing or both, indicating
some degree of lithological induration prior to
deformation (Fig. 10).
