BULLETIN NO. 68


gamma-ray logs may exhibit a slightly higher
and more sporadic signature (Arthur et al.,
2001a). Many peaks in the gamma-ray logs for
this unit correlate with the presence of trace
organic or pyrite (e.g., W-12640 [ROMP 59];
Plate 21 and 32).

 In peninsular Florida, Ocala Limestone
deposition is interpreted to have occurred on a
homoclinal distally steepened carbonate ramp in
shallow (probably less than 10 m [32.8 ft])
subtidal to intertidal, somewhat high-energy,
open marine environment (Randazzo et al.,
1990; Cander, 1994; and Loizeaux, 1995). The
Ocala Limestone represents a more constant and
stable depositional environment than the
underlying Avon Park Formation (Randazzo et
al., 1990). Specifically, deposition of the Ocala
Limestone occurred during a long-term eustatic
high stand representing an overall transgressive
sequence of sedimentation with shoreline
progradation (Randazzo et al., 1990; Loizeaux,
1995). The lower Ocala Limestone represents a
deepening upward sequence with the sediments
becoming muddier through time and then
shoaling in the uppermost Ocala (Randazzo et
al., 1990). This appears to coincide with the
post-middle Eocene marine transgressions
followed by a fall in eustatic sea level at the end
of late Eocene (Loizeaux, 1995).

Oligocene Series

 Suwannee Limestone

 The Lower Oligocene Suwannee Limestone
was first identified by Cooke and Mansfield
(1936). This formation ranges from light-gray
to white, variably moldic packstones and
grainstones. Carbonate grains are generally
miliolids and peloids. Small amounts of sand (<
3 percent) and clay generally occur within the
uppermost part of the unit when overlain by
Hawthorn Group sediments. South of the study
area, however, Missimer (2002) reports local
sand beds in the Suwannee Limestone. Trace
amounts of pyrite and organic (finely
disseminated and as laminations) also occur
throughout this formation (Arthur et al., 2001b;
Price, 2003). The Suwannee Limestone is also
variably dolomitized. For example, Green et al.,


(1995) noted a relatively consistent dolostone or
dolomitic limestone bed (10-20 ft thick [-3-6
m]) occurring within the lower third of the unit
in the central part of the study area. Chert is
present throughout the unit, especially within the
updip part of the unit. In Hernando County, for
example, partial silicification of the limestone is
observed, leaving echinoids in their original
calcite form. Sedimentary structures include
cross bedding and bioturbation (Budd, 2002).
Porosity types include moldic and intergranular.
Measured total porosity, based on analysis of 29
samples, averages 36.3 percent (median = 37.1
percent) and ranges from 2.3 percent to 55.8
percent. Fossils within the Suwannee
Limestone include mollusks, gastropods,
echinoids (most commonly the index fossil
Rhyncholampus gouldii), abundant miliolids and
other benthic foraminifera including Fallotella
(Dictyoconus) cookei, Discorinopsis gunteri
(Figure 14) and Fallotella (Coskinolina)
floridana (Figure 12).

 For the most part, the Suwannee Limestone
unconformably overlies the Ocala Limestone
and is unconformably overlain by Hawthorn
Group sediments; however, there exists some
question regarding the lateral extent of both
unconformities. The contact between the
subjacent Ocala Limestone and the Suwannee
Limestone can be locally gradational, showing
some evidence of interbedding. In some areas,
the lower Suwannee Limestone increases in
carbonate mud content, which can be difficult to
distinguish from the sometimes chemically
weathered, chalky upper Ocala Limestone.
Torres et al. (2001) and Brewster-Wingard et al.
(1997) have also noted that the boundary is often
difficult to pinpoint. These researchers, as well
as the authors of this report, suggest that
foraminifera are often useful to distinguish the
units.

 The upper contact of the Suwannee
Limestone is locally gradational with the
overlying Tampa Member (Arcadia Formation)
in northeastern Hillsborough and Pinellas
Counties. Researchers faced with the difficulty
of making formation picks in this area have
come to informally refer to this transitional zone
as "SuwTampaHaw" (Tom Scott, personal
communication, 2004). The Suwannee
Limestone is also locally overlain by green clays