BULLETIN NO. 68


faults in the Suwannee Limestone exists to some
degree within two areas. The feature most
supported by the data presented herein is an
inferred northwest-striking fault in northwestern
Polk County (Figure 3). The Suwannee
Limestone thickness map (Plate 42) indicates an
abrupt change in thickness; wells reflecting
more than 100 ft (30.5 m) of the unit are
proximal to wells that contain no Suwannee
Limestone even though the wells are deep
enough to have encountered the unit (assuming
similar regional dip). The strike and polarity of
this particular feature, indicated as an inferred
fault on Plate 41 and 42, roughly agrees with a
fault proposed by Pride et al. (1966). Northeast
of the fault, the Suwannee Limestone is reported
to occur as exposed remnant boulders in Sumter
County (Campbell, 1989).

 A second inferred fault may occur along the
updip limit of the Suwannee Limestone in
northeastern Hernando County (Figure 3).
Vernon (1951) reports a fault intersecting the
"Inglis Member" in the area, with the upthrown
side to the northeast. Data represented in Plates
41 and 42 support the location and polarity of
Vernon's (1951) fault for the Suwannee
Limestone. Thicknesses greater than 50 ft (15.2
m) terminate along the northeastern subcrop
limit of the unit (Plate 41 and 42). In the
Charlotte Harbor area, the "North Port" fault
(Winston, 1996) may have affected the
Suwannee Limestone surface and thickness,
similar to that of the Ocala Limestone and Avon
Park Formation. Other faults and lineaments are
reported in this area (Hutchinson, 1991;
Winston, 1996; Michael Fies, personal
communication, 2007) suggesting a complex
geologic setting.

 The Suwannee Limestone is characterized by
a gamma-ray log response (i.e., activity) that is
generally more variable within the lower half of
the unit (e.g., Plate 11 and 12; Figure 10).
Relative to the Ocala Limestone, it has an
overall higher background rate and exhibits
much more variability. This variability is likely
due to higher amounts of dolomite, organic
material and other non-calcitic constituents in
the Suwannee Limestone relative to the Ocala
Limestone. Although the gamma-ray log is
generally useful for providing corroborative


evidence for the lithostratigraphic boundary
between the Eocene Oligocene carbonates, use
of the logs for determination of the upper
boundary of the Suwannee Limestone is not
always as straightforward. For example, where
the Tampa Member (Arcadia Formation) is in
contact with the Suwannee Limestone, gamma-
ray signatures for the two units are quite similar,
both in their background count rates and
distribution of peaks (e.g., Plate 31, W-15204
[TR14-2] and Plate 33, W-16740 [ROMP 39]).

 A generally consistent pattern in the
Suwannee Limestone gamma-ray logs,
especially for wells in Gulf-coastal counties, is
the presence of a 50- to 100-ft (15.2 to 30.5 m)
thick interval of high gamma-ray activity within
the central to lower parts of the Suwannee
Limestone. This interval varies in thickness and
depth and apparently does not correlate with a
given stratigraphic horizon. Inspection of
lithologic logs suggests that this high gamma-
ray activity zone is associated with dolomite
and/or minor organic content.

 The Suwannee Limestone, where present,
comprises most of the FAS surface; exceptions
being where hydraulic continuity exists between
the Tampa Member (Arcadia Formation) and
Suwannee Limestone in Pasco, Pinellas, most of
Hillsborough and northern Manatee Counties
(Figure 8). Along the updip limit of the Tampa
Member in Pasco County, the top of the FAS
includes the Tampa Member (where present)
and the Suwannee Limestone (Figure 8).
Grainstones within the Suwannee Limestone are
among the most permeable zones in the UFA.

 Suwannee Limestone deposition occurred in
shallow open marine to peritidal environments
on the Florida Platform (Cander, 1994) until the
Late Oligocene sea-level low stand (Hammes,
1992). During deposition of the unit in the study
area, the Georgia Channel System (Huddlestun,
1993) acted as a barrier to a southward influx of
plastic sediments from the Appalachian
Mountains. Deposition of the predominantly
skeletal lithologies was cyclic and controlled by
the pre-existing topography as well as
fluctuating sea level. Restricted marine facies
and skeletal shoal facies developed on previous
highs and deeper subtidal facies occurred in the
lows. Hammes (1992) describes the Suwannee