BULLETIN NO. 68


Figure 12. Selected diagnostic fossils common within the Avon Park Formation. From left to right:
Cushmania americana (Dictyoconus americanus) (bar = 0.5 mm; Rupert, 1989), Fallotella
(Coskinolina) floridana (bar = 1 mm; photo courtesy of Jonathan Bryan) and Neolaganum
(Peronella) dalli (bar = 15 mm; photo courtesy of Invertebrate Paleontology, Florida Museum of
Natural History [IP/FMNH]).


to dolomitization/leaching (e.g., Plate 27; W-
17056 [ROMP 9]). In rare instances, selective
dolomitization occurs where the original
limestone matrix is dolomitized, however, the
faunal assemblage foraminiferaa and echinoids)
remain as calcite.

 Gamma-ray response in the Avon Park
Formation is generally due to variable dolomite
and organic content. In many wells, high
gamma-ray activity at or near the top of the
Avon Park Formation corresponds to thin layers
of organic material (e.g., Plate 16; W-16303
[ROMP TR 7-4]; Figure 10). Similar
observations have been made east of the study
area (Davis et al., 2001). In cases where the top
of the Avon Park Formation has been
dolomitized or recrystallized, this organic-
associated gamma-ray peak tends to be a
broadened, more subdued signal. The degree to
which the signal remains is likely a function of
the extent of recrystallization and amount of
organic impurities remaining in the carbonate.
Relative to the overlying Ocala Limestone, the
Avon Park Formation gamma-ray signature has
higher background count rates and is more
variable throughout the unit. In general, a
relatively higher gamma-ray peak is observed at
the top of the Avon Park Formation throughout
the study area, except for the west-central


region, including northern Hillsborough and
Pinellas Counties as well as eastern Polk and
northern Manatee Counties. Several gamma-ray
logs from wells in the study area also exhibit a
peak at depths greater than 350 ft (106.7 m)
below the top of the unit. Preliminary analysis
attributes this peak to organic content.

 All major parts of the FAS (the UFA, MFCU
and LFA) can include the Avon Park Formation.
The "Avon Park permeable zone" (Reese and
Richardson, 2008) generally occurs 200 to 400 ft
(60.9 m to 121.9 m) beneath the top of the Avon
Park Formation in the study area, and is
analogous to the "Avon Park highly permeable
zone" reported by Hutchinson (1992) for
southwestern Florida. Reese and Richardson
(2008) map this permeable zone throughout
most of the southern half of peninsular Florida,
describing it as a thick dolostone unit with
interbedded limestone and dominated by fracture
porosity that may occur in either the UFA or the
LFA.

 With increasing depth in the Avon Park
Formation, interbedded and intergranular
evaporites (gypsum/selenite and anhydrite)
reduce formation porosity and permeability
throughout much of the region. Water-quality
data, (Sacks and Tihansky, 1996; Jack Hickey,