BULLETIN NO. 68


communication). It is likely that this factor has
biased identification of mudstones in lithologic
descriptions that may technically be fine-grained
grainstones.

 During archiving of borehole cuttings,
samples are gently washed in a 63 micron sieve
to remove any drilling mud (e.g., silt and clay-
sized material). When describing lithologic
characteristics of borehole cuttings, care was
taken to inspect the washed and unwashed
archival fractions of the samples. In many
cases, especially for older wells, the washed
sample fraction may under-represent the clay
fraction of the sample. For example, cuttings
representing the sandy clayey Nocatee Member
(Arcadia Formation) may have been washed to
the degree that only sand remains in the archived
sample. In such cases, the unwashed sets of
samples provide a better representation of the
original clay-rich lithology.

 The descriptions are coded within the
aforementioned Microsoft AccessTM database -
FGS Wells. This database is undergoing
continued enhancements including migration to
a more robust enterprise-level platform. These
and other lithologic descriptions are available
from the Florida Geological Survey web site:
http: //www.dep.statefl.us geology/.


 Delineation of Boundaries

Formations/Members

 Formation and member boundaries were
determined for all described samples and for
cores, cuttings and geophysical logs from an
additional -600 wells. Florida Geological
Survey published and unpublished data (e.g.,
Stewart, 1966; Hickey, 1982, 1990; Johnson,
1986; Miller, 1988; Scott, 1988; Campbell,
1989; DeWitt, 1990; Campbell et al., 1993,
1995; Clayton, 1994, 1999; Green et al., 1995,
1999; Sacks, 1996; Arthur et al., 2001a; Gates,
2001; Missimer, 2002; and O'Reilly et al., 2002)
provided lithostratigraphic and hydrostratigraphic
boundary information on an additional -200
wells. Gamma-ray logs and fossil assemblages
are used only to supplement the lithologic data


in the determination of the boundaries. Where
uncertainty exists regarding the exact position of
the formation boundary, or where the boundary
is inferred within an interval of poor or no
sample recovery, a dashed rather than solid line
is shown on the cross sections. Dashed contacts
are also drawn where only a gamma-ray log was
used and no samples were available for
inspection. In cases where sample quality is
poor, as is often true with cuttings, the gamma-
ray logs become more important in the
determination of formation boundaries.

 Uncertainties in lithostratigraphic unit
boundaries were recorded in a database of
elevations and thicknesses. These uncertainties
exist for several reasons. In the case of
inspecting cores, it is not uncommon for two
experienced geologists to disagree on a
formation boundary, especially when it is subtle
or gradational. Moreover, the core may have
poor recovery, resulting in missing intervals.
Regarding cuttings, samples often contain
borehole cavings, whereby the sampled interval
contains sediment or rock fragments from
overlying units. For example, in an extreme
case, dolostone cuttings from the Avon Park
Formation may contain phosphatic sands from
the upper Hawthorn Group. As a result, it is not
uncommon for a formation boundary estimation
based on cuttings to include an uncertainty range
on the order of 20 ft (6.1 m) based solely on
sample quality. Other uncertainties with respect
to mapped unit elevations also exist (see Map
Development and Data Management, p. 24).

 Table 2 summarizes the lithostratigraphic
units shown on the maps. The same units are
also shown on the cross sections. For the
purposes of this study, post-Hawthorn units are
depicted as Pliocene-Pleistocene sediments
(undifferentiated) and Pleistocene Holocene
undifferentiated sand and shell or sand and clay
(UDSS or UDSC, respectively).

Aquifer Systems

 Delineations of hydrostratigraphic units in
this report are based on the following: 1)
available hydrogeologic data collected during
drilling, 2) borehole geophysical logs,