FLORIDA GEOLOGICAL SURVEY


Limestone as three megacycles each composed
of several shallowing upward cycles: 1) the
outer ramp characterized by skeletal-rich, grain
supported to muddy, open-marine, shallow and
deep-ramp facies, 2) shallow ramp facies -
composed of wave dominated skeletal banks and
shoal complexes and shallow and deep subtidal
lagoonal deposits, and 3) restricted marine -
deposition in a restricted marine, brackish
lagoon and mud-rich tidal flat environment.

Oligocene-Pliocene Series

 Hawthorn Group

 Hawthorn Group sediments range in age
from mid-Oligocene (Brewster-Wingard et al.,
1997) to Early Pliocene (Scott, 1988; Covington,
1993; Missimer et al., 1994) and generally
consist of phosphatic siliciclastics (sands, silts
and clays) and carbonates. Trace amounts of
pyrite occur throughout the Hawthorn Group
section in southwestern Florida (Lazareva and
Pichler, 2007). In the study area, the Hawthorn
Group consists of the Arcadia Formation, the
Peace River Formation and undifferentiated
sediments, all of which generally lie
unconformably above the Suwannee Limestone
and unconformably beneath undifferentiated
Pliocene and younger sands, shells and clays.
Benthic foraminifera characteristic of the
Hawthorn Group include Archaias sp., Sorites
sp., Amphistegina lesson and Cassigerinella
(Cassidulina) chipolonsis (Figure 15).
Predominant formational members of the
Hawthorn Group present in the study area
include the Tampa and Nocatee Members
(Arcadia Formation) and the Bone Valley
Member (Peace River Formation). The extent of
all Hawthorn Group sediments (Plate 43)
generally includes those areas where
undifferentiated confining beds of the IAS/ICU
(Plate 56) are present beyond the mapped extent
of the Arcadia and Peace River Formations
(Plates 45 and 51, respectively) such as Marion
County, Pinellas County and central Pasco
County. The maximum observed thickness of
the Hawthorn Group exceeds 825 ft (251.5 m) in
south-central Charlotte County (Plate 44).

 The Hawthorn Group was deposited in a
shallow marine to nonmarine fluvial and deltaic
environment that prograded over the older


carbonate platform (Scott, 1988; Ward et al.,
2003). Similar to other units mapped in this
study, the top of the Hawthorn Group can
demonstrate variable local relief, as exhibited by
its irregular erosional and karstic surface (Berndt
et al., 1998). Based on mineralogy of Hawthorn
Group sediments, incipient stages of
phosphogenesis occurred during the Late
Oligocene during deposition of the lower
Arcadia Formation (Brewster-Wingard et al.,
1997). Sea-level fluctuations strongly
influenced deposition and exerted a major
control on phosphogenesis and sedimentation
(Riggs, 1979a, 1984; Compton et al, 1993).
During sea-level transgressions a large part of
the Florida platform was submerged.
Meandering of the Gulf Stream resulted in
upwelling over the platform, which increased
organic productivity and enhanced
phosphogenesis in the shallow waters of the
shelf (Compton et al, 1993). Maximum
phosphorite precipitation is thought to have
occurred in shallow-water coastal and nearshore
shelf platforms or other submarine topographic
highs (Riggs, 1979a). Sea-level fluctuations and
ocean currents facilitated transport, deposition
and concentration of phosphate grains (Scott,
1988, 1992b). Primary depositional features
such as graded bedding and cross beds provide
evidence of this high-energy depositional
environment (Scott, 1988). The height of
phosphate deposition and reworking was
synchronous with Peace River Formation
deposition (Middle to Early Pliocene).

Arcadia Formation

 The Upper Oligocene (Brewster-Wingard et
al., 1997) to Middle Miocene Arcadia Formation
is comprised of a yellowish gray to white,
variably sandy (quartz and phosphorite)
carbonate with interbeds of siliciclastic-
dominant sediments. Although limestone is
present, dolostones are most common, ranging
in grain size from microcrystalline to medium
sand, with the more coarse material being
sucrosic. Minor clays and chert beds (some
comprised of silicified clay) also occur
(Upchurch et al., 1982; Scott, 1988). Porosity
types include intergranular and moldic.