BULLETIN NO. 68


and 46). Locations of depocenters within
subjacent lithostratigraphic units occur in
roughly the same locale: the Ocala Limestone,
Suwannee Limestone and Hawthorn Group
deepen in this area (Plates 39, 41 and 43,
respectively), and the units thicken as well
(Plates 40, 42 and 44, respectively). This basin
is also observed in the surface of the Avon Park
Formation (Plate 38); however, due to lack of
well control, Miller's (1986) Middle Eocene
maps do not reflect this feature. These
observations suggest that the area experienced
continued subsidence and infilling from Middle
Eocene through at least Late Miocene.
Alternatively, the apparent depocenters may
have structural control owing to the proximity of
the "North Port" fault (Winston, 1996).

 Nocatee Member

 The Upper Oligocene (Brewster-Wingard et
al., 1997) Nocatee Member of the Arcadia
Formation is an interbedded sequence of quartz
sands, clays and carbonates all containing
variable amounts of phosphate (Scott, 1988) that
generally average five percent but locally can
reach ten percent or more. The unit is
predominately siliciclastic and generally
interbedded with lower percentages of
carbonate. Original macrofossil material is not
common in this unit; however, fossil molds of
mollusks, algae and corals are observed.
Diatoms are commonly found within the clay
units. Porosity of the Nocatee Member is
generally intergranular, with highly variable
permeability. Total porosity of five core
samples from the Nocatee Member average 27.5
percent (median =24.7 percent) and range from
20.4 percent to 35.4 percent.

 The subcrop extent of the Nocatee Member
includes west-central Polk County south to
Charlotte and Glades Counties and extends as
far west as central Sarasota County. The
northeastern limits of the unit are generally well
defined and comprise a stratigraphic pinchout
(e.g., Polk and Highlands Counties); however,
the southwest extent is more subjective as the
unit grades laterally into the undifferentiated
Arcadia Formation, or locally into the Tampa
Member. In an area extending south from


southwestern Polk County, the Nocatee Member
is conformably overlain by the Tampa Member
of the Arcadia Formation. Elsewhere in the
study area, the upper and lower limits of the
Nocatee Member are gradational into the
undifferentiated Arcadia Formation.

 The top of the Nocatee Member ranges in
elevation from greater than 50 ft (15.2 m) MSL
in west-central Polk County to depths exceeding
-600 ft (-183 m) MSL in the southeastern part of
the study area (Plate 47). Although the Nocatee
Member ranges in thickness to more than 240 ft
(73.2 m), it averages approximately 75 ft (22.9
m) thick (Plate 48).

 Gamma-ray activity within the Nocatee
Member is generally less than or equal to that of
the overlying Hawthorn Group units (i.e., Tampa
Member and Arcadia Formation; e.g., Figure 10
and Plate 26). Where the Nocatee Member is
underlain by (and generally grades into) the
undifferentiated Arcadia Formation, gamma-ray
logs are not as useful in distinguishing between
the two units. On the other hand, where the
Nocatee Member overlies the Suwannee
Limestone, the gamma-ray activity can be very
useful for distinguishing the two units.

 Although most of the Nocatee Member
correlates with the IAS/ICU, this
lithostratigraphic unit is hydraulically connected
to the UFA within part of DeSoto County
(Figure 8) and thus locally comprises the
uppermost UFA in those areas.

 The Nocatee Member was deposited on the
southeast edge of the carbonate bank prior to
and during deposition of the Tampa Member.
The Nocatee represents a higher energy, more
open near-shore environment and grades
westward into a very sandy facies of the
undifferentiated Arcadia Formation and
northwestward into the carbonate facies of the
Tampa Member (Scott, 1988).

 Tampa Member

 The Upper Oligocene to Lower Miocene
(Brewster-Wingard et al., 1997) Tampa Member
of the Arcadia Formation is white to yellowish
gray in color and ranges from a wackestone to