FLORIDA GEOLOGICAL SURVEY


Suwannee Limestone (Figure 8).

 Gamma-ray activity in the Arcadia
Formation is distinctive, with strong gamma-ray
peaks characterizing the upper undifferentiated
part of the unit (e.g., W-16784 [ROMP 33],
Plates 16 and 37; Figure 10). Lithostratigraphic
members within and below the formation are
characterized by significantly weaker gamma-
ray responses. For example, where the
undifferentiated Arcadia Formation overlies the
Suwannee Limestone, the gamma-ray response
for the older unit is often contrastingly low in
gamma-ray activity (e.g., W-15683 [TR 3-3],
Plate 19 and W-15333 [TR 2-1], Plates 19, 28
and 37). Although the high gamma-ray activity
sequence in the Arcadia Formation is distinctive,
it is not always useful as a diagnostic tool for
identifying the upper formational boundary.
Phosphate lag deposits locally comprising the
base of the Peace River Formation have gamma-
ray peaks as high as that of the Arcadia
Formation.

 Deposition of the Arcadia Formation is
somewhat unique owing to its composition of
mixed carbonate and siliciclastic sediments. In
most depositional environments, an influx of
siliciclastic sediments usually inhibits the
production of carbonates. During the Oligocene,
siliciclastics began to deposit along the Florida
Platform (Hammes, 1992; Missimer, 2002).
This influx of siliciclastics, during low sea-level
stands, began to slowly bypass the Georgia
Channel System (Huddlestun, 1993) as it filled.
By Late Oligocene the lower part of the Arcadia
Formation was being deposited and a more
continuous influx of quartz sand was occurring
(Missimer and Ginsburg, 1998). These
siliciclastics were transported south several
hundred kilometers to the southern part of the
Florida carbonate platform by longshore currents
(Scott, 1988; Missimer and Ginsburg, 1998).
The rate of siliciclastic transport was initially
episodic and sufficiently low to minimize
interruption of the production of carbonates.
Differences in shoreline positions caused by
fluctuating sea levels allowed siliciclastics to
mix over a broad area. Mixing of the carbonates
and siliciclastics was achieved by tidal transport,
storms, longshore currents, bioturbation and
aeolian processes (Scott, 1988; Missimer and


Ginsburg, 1998; Missimer, 2002). Missimer and
Ginsburg (1998) list three important factors that
allowed the homogenized, co-deposition of
Arcadia Formation carbonates and siliciclastics
to occur: 1) a relatively slow rate of siliciclastic
sediment influx, 2) the lack of mud or clay, and
3) marine transport without river or stream
transport. Freshwater input would have caused
an increase in finer-grained siliciclastics (e.g.,
silts and clays), increased turbidity and
decreased salinity, all of which would have
diminished carbonate production.

 Noteworthy topographic features occur along
the surface of the Arcadia Formation. In the
Tampa Bay area, interpreted seismic data
indicate subsurface relief of up to -197 ft (60 m;
Hine et al., in press). These features are
attributed to "spatially restricted, semi-enclosed,
siliciclastic-filled karst" that may coalesce into
larger collapse systems (Hine et al., in press). A
karst basin identified in their study due south of
the Pinellas County peninsula in Tampa Bay
correlates well with a trough extending
northward into the peninsula (Plate 45; see also
Tampa Member surface map, Plate 49).

 Along the southern part of the Lake Wales
Ridge, the Arcadia Formation deepens sharply
(Plate 45). This elongate depression (or trough)
also occurs in the overlying Peace River
Formation (Plate 51). Although topographic
relief of the troughs are similar (-175 to -200 ft;
-53 to -61 m), the Arcadia Formation exhibits
little evidence of thinning, unlike the Peace
River Formation (Plate 52). Periods of erosion
(scouring?) or non-deposition owing to sea-level
fluctuations and paleo-ocean currents are likely
factors contributing to the origin of this trough.
Miocene-Pliocene structural control of the
feature is not indicated by the thickness of either
formation. The thickness of post-Hawthorn
Group sediments (see Plate 55 for
approximation) suggests that the trough may
have become a depoaxis for Pliocene-
Pleistocene siliciclastics.

 A third significant topographic feature occurs
in south-central Charlotte County, where the
surface of the Arcadia Formation deepens to
more than 200 ft (60.9 m) MSL and notably
thickens to more than 700 ft (213 m; Plates 45