BULLETIN NO. 68


 Similar to basal Peace River Formation lag
deposits, reworked Miocene-Pliocene sediments
may also yield a phosphate lag deposit at the
base of overlying (e.g., post-Hawthorn Group)
sediments. Units superjacent to the Peace River
Formation include the Tamiami, Ft. Thompson
and Caloosahatchee Formations. In most cases,
the Peace River Formation is readily
distinguished from these overlying
sand/shell/carbonate lithofacies.

 Lateral facies transitions of the Peace River
Formation are most evident along the
northwestern extent of the unit in Hillsborough
and Polk Counties (Plate 51). In this area,
sediments characteristic of the Peace River
Formation grade into clay-rich and phosphate-
poor sediments of the undifferentiated Hawthorn
Group (e.g., Plate 13).

 Maximum elevations of the Peace River
Formation occur in the vicinity of the Polk
Upland and Lakeland Ridge, where the unit
exceeds 125 ft (38.1 m) MSL (Plate 51). The
maximum observed depth of the Peace River
Formation exceeds -200 ft (-60.9 m) MSL along
the Lake Wales Ridge. Thicknesses range to
over 120 ft (36.6 m) along the southeastern third
of the unit's mapped extent (Plate 52).

 Gamma-ray activity in the Peace River
Formation is highly variable. In some areas, due
to the high-phosphorite content in the sediments,
strong gamma-ray peaks are readily observed in
contrast to lower gamma-ray activity of the
Arcadia Formation (e.g., W-16576 [ROMP DV-
1], Plate 33). The opposite occurs as well,
where gamma-ray activity in the Peace River
Formation is lower than that of the Arcadia
Formation (Figure 10). Where the Peace River
Formation lies above Eocene carbonates, the
difference is also pronounced, with the younger
unit exhibiting a stronger gamma-ray signal
(Plate 20). In many wells, a lack of gamma-ray
contrast between the Peace River Formation and
the Arcadia Formation is observed (W-16740
[ROMP 39], Plate 33). In wells where the base
of Peace River Formation contains a reworked
phosphate lag deposit, a characteristically strong
gamma-ray peak is observed (e.g., W-15938,


Plate 22). The unit may also be overlain by a
similar lag deposit within undifferentiated post-
Hawthorn Group sediments.

 The Peace River Formation is a regional
confining to semi-confining lithostratigraphic
unit within the upper part of the IAS/ICU.
North of central Hillsborough and Polk
Counties, the Peace River Formation and
undifferentiated Hawthorn Group sediments
comprise a low-permeability confining to semi-
confining facies of the IAS/ICU. South of this
region, permeable, water-producing zones exist
within interlayered carbonate lenses (e.g., Ryder,
1985, Torres et al., 2001). In some areas, the
uppermost sediments of the Peace River
Formation are in hydraulic connection with
overlying sands due to a low-to-absent clay
content (e.g., Plate 17, W-14382 [ROMP 23]).
As a result, the Peace River Formation may
comprise the lower part of the SAS. Clay-poor
sediments occur within the uppermost Peace
River Formation in eastern Sarasota County and
western Manatee County (Tom Scott, personal
communication, 2006).

 The Middle Miocene Lower Pliocene Peace
River Formation sediments characterize a
complex depositional environment strongly
influenced by sea-level fluctuations (Missimer et
al., 1994). The northern extent of the unit was
deposited in a shallow marine, deltaic to
brackish water environment while further south
open marine conditions prevailed (Scott, 1988).
Carbonate deposition in the unit was
periodically restricted by a flood of siliciclastics
from the north and a rise in sea level (Scott,
1988). Missimer (2001) suggests that the Peace
River Formation immediately south of the study
area was deposited in a variety of depositional
environments ranging from inner ramp to deltaic
to beach and can be explained by shoaling
upward or lateral accretion of sediment. Sea-
level transgressions or highstands appear to
favor phosphogenesis, while reworking of the
sediments during sea-level regressions or
lowstands concentrate the phosphorite (Compton
et al., 1993). Phosphorite concentrations are
considered economic ore deposits in the central
region and are locally mined.