FLORIDA GEOLOGICAL SURVEY


Figure 11. Helicostegina gyralis, a foraminifer common within the Oldsmar Formation (bar = 1 mm).


 The contact relations between the Oldsmar
and Avon Park Formations are generally subtle
and difficult to identify within the study area,
which supports the interpretation that the contact
is possibly conformable (Braunstein et al.,
1988). In general, the contact grades from a
dolostone to a chalky white limestone. In many
cases, however, these characteristics are not
present. A useful faunal indicator of the
transition into the Oldsmar Formation is the
appearance of abundant Helicostegina gyralis
(Figure 12; Miller, 1986). Although this
foraminifer does not exclusively occur in the
Oldsmar Formation, it generally appears in
lowermost Avon Park Formation and increases
in abundance in the Oldsmar Formation
carbonates.

 The Ocala Limestone unconformably
overlies the Avon Park Formation throughout
nearly the entire study area; exceptions include
parts of Levy and Citrus Counties (Avon Park
Formation exposures) or where the Ocala
Limestone is absent in the subsurface
(northwestern Osceola County). The contact
between these two units is readily apparent in
many up-dip locations and difficult to determine
down dip. The more obvious contact relations
occur where: 1) lithology of the Avon Park
Formation is tan to brown dolostone, overlain by
white to cream limestone of the Ocala
Limestone (e.g., W-16456 [ROMP 49], Plates


14 and 33; W-9059, Plates 23 and 34) and 2) in
the case where both units are limestone, the
uppermost Avon Park Formation is grain-
supported and contains disseminated organic or
thin (less than 2 inches; ~ 5 cm) beds of peat (in
some cases varying toward lignite), whereas the
lowermost Ocala Limestone is finer grained and
skeletal (e.g., W-720, Plates 6 and 29; W-12943
Plates 12 and 21). Although the Avon Park
Formation is not defined based on bio-
assemblages, additional contact indicators
include the appearance of diagnostic
foraminifera and echinoids listed above, as well
as abundance of coralline algae.

 Throughout most of the southwestern part of
the study area, including nearly all of Manatee
and Sarasota Counties, the contact between the
Avon Park Formation and the Ocala Limestone
is dolomitized (e.g., Plate 18) and as a result,
formation boundary delineation is difficult. In
this situation, subtle characteristics can be used
to delineate the two units. Dolomitization of the
limestones with slightly varying textures may
yield differences in dolostone textures and
dolomite grain sizes. Fossil molds are another
useful indicator, because fossil molds of
diagnostic fossils may help narrow the
uncertainty. For example, narrow, discoid vugs
may represent prior Lepidocyclina sp. whereas
smaller cone-shaped vugs may represent where
Cushmania or Fallotella sp. were present prior