FLORIDA GEOLOGICAL SURVEY


 Bone Valley Member

 The Middle Miocene to Lower Pliocene
(Webb and Crissinger, 1983) Bone Valley
Member of the Peace River Formation has a
limited areal extent, centered in southwestern
Polk County (Plate 53). It occurs in the Central
Florida Phosphate District which is among the
world's largest economic phosphorite deposits
(Freas and Riggs, 1968). Due to mining, most
of the Bone Valley Member sediments have
been removed. Although similar in lithology to
the Peace River Formation, the Bone Valley
Member contains greater amounts of
phosphorite (more than 30 percent by volume)
that is coarser-grained, ranging up to gravel-size
nodules. Phosphorite occurs as carbonate-
fluorapatite (francolite) nodules, peloids, fecal
pellets, intraclasts and grain coatings. Some
pebble-sized grains show evidence of reworking,
boring structures and multiple stages of
phosphatization. The non-phosphorite
component of the Bone Valley Member is
comprised of quartz sand with clay (e.g.,
palygorskite and sepiolite; Scott, 1988)
generally exceeding 20 percent by volume.
Carbonate beds are not present; however
limestone and dolostone cobbles and larger
fragments are observed (Tom Scott, personal
communication, 2005).

 An extremely diverse fossil assemblage
exists within the unit (Webb and Crissinger,
1983) ranging from dugong and whale ribs,
shark teeth and turtle scutes to petrified wood
and teeth from horses and alligators (Figure 16).
Thicknesses of the Bone Valley Member exceed
40 ft (15.2 m) (Plate 54). In terms of hydrologic
function, the unit is a localized, yet efficient
confining unit within the IAS/ICU. On the other
hand, due to the significant economic value of
phosphorite deposits, the unit has been
extensively mined thereby reducing or
eliminating local confinement.

 Owing to the high phosphorite content,
gamma-ray log intensities for Bone Valley
Member sediments are very high. For example,
the truncated, high-intensity gamma-ray peak at
the top of the Peace River Formation for W-


14385 (ROMP 45; Plate 22) represents Bone
Valley Member sediments.

 Many questions exist regarding the genesis
of phosphate in Florida. Given the diverse fossil
assemblage, it reflects near-shore marine
conditions and is unlike other large phosphate
deposits of the world. Parts of the Bone Valley
Member were deposited in a high-energy
nearshore environment (topographic highs)
while other parts were deposited in a shallow
marine environment such as an embayment or
lagoon (Scott, 1988). The stratigraphy of the
unit is complicated by rapid facies changes and
post-depositional erosion, redeposition and
weathering (Freas and Riggs, 1968; Webb and
Crissenger, 1983).

Hawthorn Group (Undifferentiated)

 Undifferentiated Hawthorn Group sediments
lie unconformably above Eocene and Oligocene
carbonates and unconformably below
undifferentiated Pliocene and younger sands and
clays along the upland geomorphic provinces
within the northern region, as well as in parts of
Pinellas, Hillsborough and Polk Counties. In
Marion, Sumter and Lake Counties, sediments
mapped as undifferentiated Hawthorn Group
may be comprised of one or more of the
following lithostratigraphic units (from oldest to
youngest): the Penney Farms Formation, the
Marks Head Formation and the Coosawhatchie
Formation (Scott, 1988). These formations are
not delineated owing to their limited extent and
stratigraphic pinch-out along the northeastern
part of the study area. Moreover, Scott (1988)
reported on the difficulty of distinguishing the
uppermost Coosawhatchie Formation from
undifferentiated Hawthorn Group sediments in
central Florida. In the north-central part of the
study area, undifferentiated Hawthorn Group
sediments occur along the Brooksville Ridge
(e.g., W-6903, Plate 5 and W-15933, Plate 9) as
well as within karst features and isolated lenses
flanking the ridge. Vernon (1951) describes
"Miocene Hawthorn formation" sediments along
the Brooksville Ridge in Citrus County as
greenish-gray montmorillonitic clays. The
distribution of these sediments, as well as the