BULLETIN NO. 68


occurrence of hard-rock phosphate in the region
lend support to the proposal by Scott (1988) that
Hawthorn Group sediments once covered the
entire Florida peninsula. Upchurch (1992) also
suggests that Hawthorn sedimentation occurred
on the crest of the Ocala Platform.

 Undifferentiated Hawthorn Group sediments
are generally comprised of clay beds with
variable amounts of carbonate, quartz sand and
silt, phosphorite, organic material and minor
shell fragments. Maximum elevations of
undifferentiated Hawthorn Group sediments
exceed 100 ft (30.5 m) MSL along inland
uplands and ridges north of Pasco County. In
Pinellas County, these sediments generally occur
deeper than -25 ft (-7.6 m) MSL.
Undifferentiated Hawthorn Group sediments
occurring in Hillsborough, Pinellas and Pasco
Counties grade laterally southeastward into the
Peace River Formation. In central Pinellas
County undifferentiated Hawthorn Group
sediments exceed 100 ft (30.5 m) thick and
grade southward into the Arcadia Formation
(Plate 31).

 Gamma-ray response of the undifferentiated
Hawthorn Group sediments varies from low-
background counts where the unit is dominated
by clays, to broad and diffused patterns where
phosphorite is more abundant. Gamma-ray logs
are generally not very useful as a tool to
determine the contact with underlying and
overlying units.

 In the northern region, undifferentiated
siliciclastic sediments of the Hawthorn Group
comprise discontinuous semi-confinement
between the SAS and FAS. The lateral extent
and effectiveness of this lower-permeability
layer is difficult to define due to factors such as:
1) variable lithology (e.g., percent clay) and
thickness, 2) desiccation cracking of thin clay-
rich beds during long-term low-water table
conditions and 3) breaches due to fractures and
sinkholes. As a result, many areas in this region
are denoted on Plate 55 as "discontinuous basal
confinement of SAS." These same areas are
likewise labeled "discontinuous" on Plates 56 and 57.


Pliocene and younger Series

 Post-Hawthorn Group Sediments

 Sediments overlying Hawthorn Group and
older units in the study area are generally
comprised of varying percentages of
undifferentiated sand, shell and clay. Within the
eastern half of the study area, the Cypresshead
Formation dominates most uplands (Figure 2)
and is well exposed along the Lake Wales
Ridge. In the southern region, three Pliocene-
Pleistocene formations are recognized: the
Tamiami Formation, the Caloosahatchee
Formation and the Fort Thompson Formation.
The latter two formations are faunally rather
than lithologically based. As a result, Scott
(1992c) provides a conceptual framework to
include Caloosahatchee and Fort Thompson
sediments as part of the Okeechobee formation
(informal). The framework also includes
informal "Bermont formation" sediments.

 Although mapping post-Hawthorn Group
sediments is beyond the scope of this study,
general descriptions of the units are provided
herein. These undifferentiated surficial
sediments and formations generally comprise
the SAS. In the southern region, the lower
Tamiami Formation is considered part of the
IAS/ICU. In the cross sections (Plates 4-37),
undifferentiated sediments are broadly
classified as sand and clay (UDSC) or sand
and shell (UDSS). Existing lithologic
descriptions may identify "Pliocene -
Pleistocene" sediments (often referred to as
"PCPC"); however in the present study, these
sediments are labeled "UDSC" due to
difficulty in distinguishing the two
undifferentiated types as well as inconsistent
usage of PCPC. Gamma-ray activity in post-
Hawthorn Group sediments is highly variable
(Plates 4-37; Davis et al., 2001). In some
areas, however, a consistent peak at the base
of these sediments represents a phosphate lag
deposit reworked from Hawthorn Group
sediments (Scott, 1988; Green et al., 1995;
see also W-16303 [TR 7-4] and W-17057
[TR 7-2], Plate 16).