BULLETIN NO. 68


Caloosahatchee Formation and reports a Late
Pliocene to Early Pleistocene age. The
Caloosahatchee Formation consists of marls
composed primarily of quartz sand, silt and
shells with varying amounts of carbonate in the
matrix. It varies from poorly indurated to well
indurated and the fauna is varied and often well
preserved. Usually moderately to abundantly
fossiliferous, some sands are almost or
completely barren of fossils (DuBar, 1958).
Freshwater limestones are commonly present.

 The extent of the Caloosahatchee Formation
is shown on previous geologic maps by Cooke
(1945) and Vernon and Puri (1964). The contact
between the subjacent Tamiami Formation and
the Caloosahatchee Formation is generally
unconformable. The Tamiami Formation was
subjected to significant subaerial erosion and the
deposition of the Caloosahatchee Formation
gradually filled in old "Tamiami valleys"
(DuBar, 1958). The Caloosahatchee Formation
comprises part of the SAS. In most
hydrogeological investigations, the
Caloosahatchee Formation is not differentiated
from the Fort Thompson Formation (Scott,
1992a). The depositional environment was
subtropical with predominantly carbonate
deposition and a coastal influx of quartz sand.
Tropical and subtropical mollusks and corals
abundant in the unit reflect an environment
similar to the present area between Cape Sable
and Florida Bay (Missimer, 2001).


Fort Thompson Formation

 Sellards (1919) proposed the name Fort
Thompson Beds, which were informally
designated a formation by Cooke and Mossom
(1929). DuBar (1958) recognized the unit as
upper Pleistocene. The Fort Thompson
Formation is a sandy limestone deposited under
freshwater and marine conditions. The sand is
fine to medium grained and is interlayered with
shell beds and limestones. The shell beds are
slightly indurated to unconsolidated and variably
sandy (Scott, 1992b; Berndt et al., 1998). A
characteristic Fort Thompson marine fossil is
Chione elevata and Helisoma scalare (Figure
17) is typical of freshwater beds in the unit
(DuBar, 1958).

 The Fort Thompson Formation is thin, does
not exceed 30 ft (9.1 m) in thickness, and has an
unconformable relationship to the variable units
above and below. It is most commonly
underlain by the Caloosahatchee Formation or
the Tamiami Formation (DuBar, 1958). The
Fort Thompson Formation is part of the
undifferentiated sediments in southern Florida
(Scott, 1992b) and comprises part of the SAS.
The extent of the Fort Thompson Formation is
shown on geologic maps by Cooke (1945) and
Vernon and Puri (1964). On the most recently
published geologic map of Florida (Scott et al.,
2001), the Ft. Thompson and Caloosahatchee
Formations are mapped as undifferentiated
Tertiary Quaternary shell-bearing sands
(TQsu; Figure 2).


Figure 17. Characteristic Ft. Thompson Formation fossils Chione elevata (left) and Helisoma
scalare (right; photos courtesy of IP/FMNH); bar = 12 mm.