FLORIDA GEOLOGICAL SURVEY


 Th Undifferentiated Hawthorn Group The undifferentiated Hawthorn Group occurs at
or near the surface near the southern flank of the Ocala Platform from Gilchrist County
southward to Pasco County with isolated occurrences in Pinellas County. Correlation of these
sediments to the formations of the Hawthorn Group exposed to the east and in the subsurface is
uncertain. There is little to no phosphate present in these sediments and fossils are rare. Ages
have not been documented but stratigraphic position suggests inclusion in the Hawthorn Group.
These sediments may be residual from the weathering and erosion of the Hawthorn Group. The
Hawthorn Group sediments on the Brooksville Ridge are deeply weathered and in some outcrops
look like Cypresshead Formation siliciclastics.
 The undifferentiated Hawthorn Group sediments are light olive gray and blue gray in
unweathered sections to reddish brown in deeply weathered sections, poorly to moderately
consolidated, clayey sands to silty clays and relatively pure clays. These sediments are part of
the intermediate confining unit/aquifer system and provide an effective aquitard for the FAS,
except where perforated by karst features.
 Hard-rock phosphate deposits are associated with the undifferentiated Hawthorn Group
sediments on the eastern flank of the Brooksville Ridge. The hard rock phosphate deposits were
formed by the dissolution of phosphate in the Hawthorn sediments and redeposition in karst
features.

 Tab Alum Bluff Group West of the Apalachicola River, the Hawthorn Group is
replaced by the Alum Bluff Group. The Alum Bluff Group includes the Chipola Formation, Oak
Grove Sand, Shoal River Formation, Choctawhatchee Formation and the Jackson Bluff
Formation (Huddlestun, 1984; Braunstein et al., 1988). The formations included in this group
are generally defined on the basis of their molluscan faunas and stratigraphic position (Schmidt
and Clark, 1980). Puri (1953) described sediment facies as they relate to the formations of the
Alum Bluff Group These sediments are lithologically distinct as a group, not as individual
units. Brooks (1982) mapped much of the Alum Bluff Group as the Shoal River Formation. The
Alum Bluff Group crops out or is beneath a thin overburden in the western panhandle from river
valleys in Okaloosa County eastward to western Jackson County.
 The Alum Bluff Group consists of clays, sands and shell beds which may vary from
fossiliferous, sandy clays to unfossiliferous sands and clays and occasional carbonate beds
(Huddlestun, 1984). Mica is a common constituent and glauconite and phosphate occur
sporadically. Induration varies from essentially nonindurated in sands to well indurated in
carbonate lenses. Colors range from cream to olive gray with mottled reddish brown in
weathered sections. Sand grain size varies from very fine to very coarse with sporadic
occurrences of gravel. These sediments generally have low permeabilities and are part of the
intermediate confining unit/aquifer system.

 Trm Residuum on Miocene sediments The undifferentiated Miocene residuum,
mapped on parts of the Chattahoochee "Anticline", characteristically consists of reddish brown,
variably sandy clay with inclusions of variably fossiliferous, silicified limestone. The residuum
includes Lower to Upper Miocene and younger weathered sediments.

 Miocene Pliocene Series