BULLETIN NO. 68


 "Venice Clay"

 The Venice Clay is an informal unit
originally considered part of the lower Tamiami
Formation (Pliocene); however, microfossil data
suggest an age of Early to Middle Miocene
(Scott, 1993). Scott (1992b) suggests informal
placement of the Venice Clay in the upper half
of the Arcadia Formation based on subjacent and
suprajacent lithologies and preliminary fossil
evidence. The Venice Clay is gray-green
magnesium-rich clay, variably dolomitic with
minor amounts of quartz sand and silt. The unit
rarely contains phosphorite and becomes
increasingly silty toward its upper and lower
contacts (Campbell et al., 1993).

 The subcrop extent of the Venice Clay
includes Sarasota County and adjacent parts of
Manatee, DeSoto and Charlotte Counties; the
unit may also extend offshore (Barr, 1996).
Based on data collected in the present study, the
top of the unit generally occurs between -10 ft
MSL and -100 ft MSL (-3.1 to -30.5 m). Barr
(1996) reports that thickness of the unit ranges
up to approximately 30 ft (9.1 m). Gamma-ray
activity is diagnostically very low for this unit,
(note clay beds near top of the Arcadia
Formation in W-15683 [TR 3-3]; Plate 19),
suggesting the mineral assemblage does not
include abundant potassium-rich illite-group
clays. The Venice Clay was likely deposited in
a quiet shallow water marine environment -
possibly an estuary (Tom Scott, personal
communication, 2004).

 The Venice Clay acts as a confining unit in
the upper part of the IAS/ICU. Specifically,
Barr (1996) suggests that it comprises the
confining unit below "permeable zone 1."
Owing to its limited thickness and aerial extent,
as well as recent mapping by Barr (1996), the
Venice Clay is not mapped in the present study.

Peace River Formation

 The Middle Miocene to Lower Pliocene
(Scott, 1988; Covington, 1993) Peace River
Formation is comprised of yellowish gray to
olive gray, interbedded sands, clays and
carbonates with the siliciclastic component


being dominant (Scott, 1988). The relative
abundance of carbonate beds generally increases
toward the south, especially near the base of the
unit. Variable amounts of phosphate sand and
gravel are interspersed throughout the unit;
however, they are most common within the
uppermost beds. The Peace River Formation
contains a diverse fossil assemblage of marine
and terrestrial fauna (e.g., shark teeth and
vertebrae, ray spines, horse teeth, dugong and
whale ribs, etc.), especially within the Bone
Valley Member (Figure 16). Porosity types in
the formation are generally intergranular, except
in the carbonate-rich zones, where moldic
porosity is also present. Only two total porosity
analyses of Peace River Formation samples have
been measured in this study. The results, 34.4
percent and 39.4 percent, should not be taken as
representative of the unit given its diverse
lithology.

 Lithologic characteristics of the Peace River
Formation are generally consistent; however, the
carbonate component becomes more prevalent
from north to south as the unit thickens.
Throughout most of its extent, the Peace River
Formation does not contain shell material, with
possible exception of southeast DeSoto County,
where barnacles are present within the unit
(Green et al., 1999). These barnacle-rich
sediments may be the equivalent of "unit 11"
from Petuch (1982). Missimer (2001) reports
shell material in the Peace River Formation
south of Charlotte County. In the same region,
calcareous nannofossils occur in the unit
(Covington, 1993).

 The Peace River Formation generally has an
unconformable contact with the underlying
Arcadia Formation. In an isolated area in north-
east-central Hillsborough County, the Peace
River Formation directly overlies the Tampa
Member (Plates 13 and 33). The Peace River
Formation also has an unconformable contact
with the underlying Ocala Limestone in northern
Polk County (Plates 11 and 34; W-14389
[ROMP 76]). In this area, reworked Peace River
Formation sediments may occur unconformably
above the Avon Park Formation where the Ocala