Geology Reference
In-Depth Information
streams that are supported by small, local, upland
drainage basins. So, the cause of their estuarine
nature appears to be enigmatic.
Seismic data presented in this paper indicate
that these shallow (average depth ~2-4 m) estuaries
are underlain by karstic, semi-enclosed sub-basins
that have as much as 100 m of subsurface relief.
The sub-basins are defi ned as distinct structures
that are part of a larger sedimentary basin system -
in this case, the entire complex of basins containing
siliciclastic fi ll on the Florida Platform. The sub-
basins (tens of kilometres wide) beneath Tampa
Bay and Charlotte Harbor are really sub-basin
complexes with sub-basins within smaller sub-
basins (kilometres wide). Moreover, at even a
higher resolution spatial scale these sub-basins
reveal signifi cant deformation in the form of
folds, warps and sags in the deeper seis-
mic sequences and the seismic basement
(hundreds of metres wide). Coring indicates
that these basins have been fi lled with mostly
siliciclastic sediments. As a result, these sub-
basins present an unusual relationship between
carbonates and siliciclastics that has not been
previously described.
The new evidence indicates that Tampa Bay
and Charlotte Harbor are important siliciclas-
tic repositories and represent a type of mid-
carbonate platform accommodation not widely
recognized for siliciclastic deposition. This
stands in stark contrast to the neighbouring
Bahama Banks, which do not reveal any mid-
platform basins in the shallow subsurface pro-
viding accommodation in this manner (Eberli &
Ginsburg, 1987; Ginsburg, 2001). These platforms
do contain large, buried linear seaways, are much
larger scale than these Florida sub-basins, do not
appear to be karst-related and reveal no deforma-
tion. The purpose of this paper is to demon-
strate the scale, geometry and infi lling facies of
these sub-basins and to link them to the larger
Neogene-Quaternary siliciclastic fl ux onto the
Florida Platform.
trademark of Florida's tourist-driven economy,
and are world renowned as a result. Additionally,
the principal geomorphology of central peninsular
Florida consists of palaeoshorelines, terraces and
scarps composed of a siliciclastic veneer formed
on underlying lower Neogene and older carbon-
ates (White, 1970; Winkler & Howard, 1977). Even
early sediment distribution studies of the adjacent
seafl oor indicate that quartz-rich sediments extend
some 40 km out onto the west-central Florida shelf
and to the upper slope on the east Florida shelf
(Gould & Stewart, 1956; Doyle & Sparks, 1970;
Hine, 1997). However, the breadth and extent
of these siliciclastic sediments comprising the
subsurface of peninsular Florida was not fully
mapped in these early studies.
Strip mining to acquire phosphate-rich sedi-
ments as well as borehole geology driven by the
search for groundwater and hydrocarbons show
that central peninsular Florida is underlain by
a complex array of quartz-rich lithostratigraphic
units dominated by the Oligocene-to-Pliocene
Hawthorn Group (Riggs, 1979; Scott, 1988, 1997).
However, the extent of siliciclastic sediments
underlying the Pleistocene and modern carbonate-
dominated terrain of southernmost peninsu-
lar Florida (Enos & Perkins, 1977) remains more
enigmatic. Potentially, as these units are capped
by limestone, most remain unseen at the surface
and have been largely unstudied for some time as
a result. Missimer & Gardner (1976) and Enos &
Perkins (1977), for example, began to recognize
the broad extent by which quartz-rich sediments
were distributed in the subsurface of southern-
most Florida.
This work was considerably advanced by the
South Florida Drilling Project headed by R.N.
Ginsburg. The project was conceptualized in the
late 1980s (Ginsburg
et al
., 1989) and commenced
in 1993. Indeed, this scientifi c effort demonstrated
that there had been a signifi cant remobilization
of quartz-rich sand and even gravels during the
late Miocene through early Pliocene. Missimer &
Ginsburg (1998) pointed out that even during the
late Oligocene, the Arcadia Formation in south-
central Florida was interbedded and mixed with
numerous metre-scale units consisting of up to
80% siliciclastic sediments.
The late Miocene to early Pliocene remobi-
lization produced a ~150 m-thick succession
of siliciclastics (Cunningham
et al
., 1998) that
extended from the Lake Okeechobee area in
south-central peninsular Florida, running beneath
the Florida Everglades and Florida Keys, and
GEOLOGICAL BACKGROUND
Ever since the fi rst geologist walked Florida's
beaches (Vaughan, 1910), it has been obvious
that the Florida Platform had received substan-
tial quantities of quartz-rich sand in its geological
past. Some of these notable siliciclastic shore-
lines that have become classic localities in coastal
geology (Davis
et al
., 1992; Davis, 1997) are a
