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State of Florida
Department of Natural Resources
Tom Gardner, Executive Director




Division of Resource Management
Jeremy Craft, Director




Florida Geological Survey
Walt Schmidt, State Geologist and Chief






Open File Report 36



The Lithostratigraphy of the Hawthorn Group
of peninsular Florida

by

Thomas M. Scott

in

Phosphate Deposits of the World
Published by Cambridge University Press

by

William Burnett and S. R. Riggs


Florida Geological Survey
Tallahassee, Florida
1990






























Abstract
The Hawthorn Group has been a problematic unit since it was
named by Dall & Harris (1892). It is a complex unit consisting of
interbedded and intermixed carbonate and siliciclastic sediments
containing variable concentrations of phosphate. Scott (1988)
upgraded the Haw thirn to group tatus in Florida and delineated
its component formations.
The Hawthorn Group in northern Florida has been subdi-
vided, in ascending order, into the Penney Farms, Marks Head,
Coosawhatchie and Statenville Formations. These units range in
age from Early Miocene (Aquitanian) to Middle Miocene (mid
Serravalian) :HuddleiLun. pers. comm., 1983). Lithologically,
the Hawthorn Group in northern Florida is made up of a basal
carbonate with interbedded siliciclastics (Penney Farms). a com-
plexly interbedded siliciclastic-carbonate sequence (Marks
Head i. a s5liCt lSt unit A ith \'arL.i-le concentr:ton. of carbonate
in the matrix and individual beds (Coosawhatchie), and a
crossbedded, predominantly siliciclastic unit (Statenville). Phos-
phate grains are present throughout most of these sediments,
varying in concentration from absent to as much as 60% of the
sediment.
In southern Florida. the Hawthorn Group is comprised of, in
ascending order, the Arcadia and Peace River Formations. The
Tampa Formation or Limestone of former usage is included as a
lower member of the Arcadia Formation due to the Tampa's
limited area extent, lithologic similarities and lateral relationship
with the undifferentiated Arcadia, Similarly, the Bone Valley
Formation of former usage is incorporated as a member in the
Peace River Formation. The southern Florida Hawthorn Group
sediments range in age from Early Miocene (Aquitanian) to Early
Pliocene (Zanclean) (Hunter, pers. comm.. 1985). Lithologically.
the Arcadia Formation is composed of carbonate with varying
amounts of included and interbedded siliciclastics which are most
prevalent in the Nocatee Member (basal Arcadia). The Peace
River Formation is predominantly a siliciclastic unit with some
interbedded carbonates. The carbonates are often dolostone.
Phosphate grains are virtually ubiquitous in the south Florida
Hawthorn sediments with the exception of the Tampa Member
where it is often absent.
The Hawthorn sediments of peninsular Florida reflect a series
of sea-level events and phosphogenic episodes. These sediments
are characterized as being deposited in inner shelf, nearshore
environments. Erosion and reworking of pre-existing sediments
played an important role in the development of the Miocene
sediment packages.


Introduction
The sediments of the Hawthorn Group represent a dram-
atic change in sedimentation in Florida. From the end of the
Mesozoic to the beginning of the Neogene, carbonate sediments
with a very minor siliciclastic component dominated the deposi-
tional environments of peninsular Florida. During the Early
Miocene, siliciclastics, derived from the southern Appalachians,
filled the Gulf Trough and encroached into the carbonate-
producing environments of peninsular Florida. The carbonate-
producing environments were gradually pushed further south as
siliciclastic deposition increased. During the Miocene.
phosphogenesis became an important depositional-diagenetic
process. Phosphate grains occur within the Hawthorn sediments
in amounts ranging from trace to 60%. Palygorskite. sepiolite,
smectite and minor illite are the clay minerals associated with the
Hawthorn sediments.
The lithostratigraphy of the Hawthorn Group throughout
peninsular Florida provides the regional framework for at-
tempting to understand the complex nature of the phosphate-
bearing Miocene sediments.
There have been numerous investigations of the Hawthorn
sediments during the last century. Scott (1988) provides a
lengthy discussion of these previous investigations and as such
they will not be discussed in this paper.

Structure
The Hawthorn Group is present throughout much of
peninsular Florida (Figs. 26.1, 26.2) except on top of the Ocala
Platform and the Sanford High where it is missing due to erosion
or non-deposition. The main structures which controlled the
deposition and present distribution of the Hawthorn Group are
shown on Figure 26.3. These features have been considered
structural in origin: however, there is considerable discussion
concerning their origin, whether they are depositional or struc-
tural. Nonetheless, the features were topographically expressed
on the surface underlying the Hawthorn Group. The two major
positive features were the Ocala Platform with the Central Flor-
ida Platform and the Sanford High with the St Johns and


26


The lithostratigraphy of the

Hawthorn Group of

peninsular Florida


T.M. SCOTT





T.M. Scott


TOP OF HAWTHORN GROUP


a 0 .0 20 .0 m M

APPROXIMATE LIMITS OF THE HAWTHORN GROUP
ICON INTERVAL SOFT.

CONTOUR INTERVAL SOFT.


0..;0


Fig. 26.1. Top of Hawthorn Group in Florida based on cores and well
cuttings.




Hawthorn Group: lithostratigraphy 327


200

ISOPACH OF THE HAWTHORN GROUP 300O
.. .a 400'
45.
APPROXIMATE LIMITS OF THE HAWTHORN GROUP
11 I 1 I Il i I f I1

CONTOUR INTERVAL SOFT.


Fig. 26.2. Isopach of the Hawthorn Group in Florida based on cores
and well cuttings.


i..,

c



r


n;.~FP~









0


NASSAU NOSE


i


Fig. 26.3. Structural features which affected the deposition and present
occurrence of the Hawthorn Group.


Brevard platforms (RiEs,. 1979). The major negative features
include the Jacksonville Basin, the Osceola Basin and the broad
Okeechobee Basin. These basins contain significant thicknesses
of Miocene sediments ranging from 250 ft (76 m) in the Osceola
Basin to 900 ft (275 m) in the Okeechobee Basin (Fig. 26.2).

Lithostratigraphy
Previous investigations considered the Hawthorn sedi-
ments as a single formation which displayed great variability
throughout Florida. The resulting confusion is evident in the
literature. Elevating the Hawthorn to group status eliminates
much of the confusion,


The sediments of the Hawthorn are lithologically defined by
the occurrence of variable amounts of phosphate, quartz sand.
dolomite and palygorskite. The Hawthorn Group is most easily
discussed in the context of northern and southern areas divided
by an east-west line between the southern erosional limit of the
Hawthorn on the Ocala PLit.-.-mn and the Sanford High. respec-
tively (see Fig. 26.4). Lateral and vertical variability is most
pronounced in north Florida.

North Florida
The Hawthorn Group in north Florida contains signifi-
cantly higher percentages of siliciclastic sediments than in south


T.M. Scott


JACKSONVILLE
BASIN


3 ,~~










EASTERN EASTERN A NW FLA.
NORTH SOUTH SE ANDE AND NORTHERN SOUTHERN
CAROLINA CAROLINA GEORGIA SW GA. FLORIDA FLORIDA
PLIOCENE TOWN FM. RAYSOR CYPRESSHEAD FM. MICCOSUKEE FM. CYPRESSHEAD FM. TAMIAMI FM, PLIOCENE
P YORK TOWN FMS. /DUPLIN FM. /CITRONELLE FM, I NASHUA FM.
REWORKED
UPPER SEDIMENT UPPER
m
PEACE
I1 COOSAW- COOSAW- RIVER
0 STATENVILLE C < g
0 HATCHIE HATCHIE > O STAT E > FM.
,,, -M, F M. F o I )
SF COOSAW-
PUNGO cc H HATCHIE

SFM. MARKS MARKS TORREYA MARKS FDIA
OLGCEN BEN FM. S N S N S N
HEAD FM. HEAD FM. FM. < HEAD FM.
S_______ NOCATEE MBR.
PARACHUCLA PARA- CHATTA- PENNEY I TAMPA MBR.
CHUCLA HOOCHEE AND FARMS 0
FM. FM. ST. MARKS fms. FM.

*RIVER COOPER SUWANNEE
SUWANNEE SUWANNEE SUWANNEE
OLIGOCENE L BEND FM. LS. LS. OLIGOCENE


COOPER FM. OCALA GP. OCALA GP. OCALA GP. OCALA GP. UPPER
EOCENE SANTEE AVON
CASTLE HAYNE SANTEE LS. L. PARK AVON PARK FM. AVON PARK FM. AVON PARK FM. MIDDLE
FM.


Fig. 26.4. Generalized %rrlligraphic correlations from Florida to North
Carolina (modilied from unpublished Gulf Coast (COS: NA chart and
Allantic Coastal Plain chart, American Association of Petroleum
Geologists, 1983).






T.M. Scott


Florida since it is closer to the siliciclastic source area in the
Piedmont and southern Appalachians. In north Florida, the
Hawthorn is subdivided as shown in Figure 26.5. Definitions of
these formations including type sections, detailed lithologic de-
scriptions, stratigraphic relationships, structure and isopach
maps and discussions of the formational ages are presented in
Scott (1988).

Penney Farms Formation The basal, carbonate-rich
Hawthorn Group sediments have been assigned to the Penney
Farms Formation (Scott, 1988). The carbonates are variably
quartz sandy, phosphatic and clayey dolostones. Carbonate
beds are thicker and more'common in the lower portion of the
Penney Farms. Interbedded siliciclastic units become thicker
and more abundant upward in the formation. The siliciclastic
sediments consist of dolomitic, clayey, phosphatic quartz sands
and dolomitic, phosphatic, quartz sandy clays.
The Penney Farms Formation unconformably overlies the
Eocene Ocala Group throughout most of northern Florida.
Occasionallb it overlies the Oligocene Suwannee Limestone. It is
overlain unconformably by the Marks Head Formation (Figs.
26.6,26.7; Fig. 26.5 shows location ofcross-sections). The Penny
Farms reaches a maximum thickness of 230 ft (70 m) in the
Jacksonville Basin. Palaeontologic evidence suggests that the
Penney Farms is Early Miocene cEarly-Middle Aquitanian)
(Huddlestun, pers, comm., 1983).

Marks Head Formation The complexly interbedded
siliciclastics and carbonates overlying the Penney Farms Forma-
tion have been referred to as the Marks Head Formation (Scott,
1988). This unit is the most lithologically variable section of the
Hawthorn Group. The carbonates are variably quartz sandy,
clayey, phosphatic dolostones. The carbonates are interbedded
with dolomitic, phosphatic, clave:, quartz sands and dolomitic,
phosphatic, quartz sandy clays. Often the Marks Head clays
may contain only minor amounts of the other constituents.
The Marks Head Formation unconformably overlies the
Penney Farms Formation and unconformably underlies the
Coosawhatchie Formation (Figs. 26.6, 26.7). The Marks Head
reaches a maximum thickness of 130 ft (40 m) in the Jacksonville
Basin. Limited paleontologic evidence suggests a mid-late Early
Miocene iBurdiealiani age IHuddlestun, pers. comm., 1983).

Coosawhatchie Formation The uppermost sediments of
the Hawthorn Group in much of north Florida have been placed
in the Coosawhatchie Formti i on (Scoi, 1988). Sediments of the
Coosawhatchie are characteristically less variable than the
Marks Head, consisting of very quartz sandy, phosphatic, clayey
dolostones to dolomitic, phosphatic, clayey quartz sands. The
base of this formation in the Jacksonville Basin often is a
dolomitic, silty clay.
The Coosawhatchie Formation unconformably overlies the
Marks Head and is unconformably overlain by post-Hawthorn
undifferentiated sediments (Figs. 26.6, 26.7). The thickest
Coosawhatchie section encountered was 222ft (68m) in the
Jacksonville Basin. Paleontologic evidence suggests a Middle
Miocene (Early Serravalian) age (Huddlestun, pers. comm.,
1983).


Statenville Formation In a limited area of north Florida.
the Statenville Formation replaces the Coosawhatchie Forma-
tion as the uppermost Hawthorn Group unit (Scott, 1988). The
Statenville consists of thin-bedded, often cross-bedded, clayey,
phosphatic, quartz sands interbedded with thin dolostone beds
and clays. The phosphate content is great enough in limited areas
to be an economically important deposit (e.g. Occidental Petro-
leum's north Florida deposit).
The Statenville Formation unconformably overlies the
Marks Head in some areas and conformably overlies the lower
part of the Coosawhatchie in other areas. The Statenville is
laterally equivalent to at least the upper part of the
Coosawhatchie. It is unconformably overlain by post-Haw-
thorn undifferentiated sediments. The thickest known section of
Statenville is 87 ft (26 m). Correlation of the Statenville with the
Coosawhatchie suggests a Middle Miocene age (Huddlestun,
pers. comm., 1983). There also is a zone of reworked Statenville
exposed in the north Florida phosphate pits that contains a Late
Miocene vertebrate fauna (Webb, pers. comm., 1985).

South Florida
The Hawthorn Group of southern Florida generally con-
sists of a basal carbonate unit and an upper siliciclastic unit and
is subdivided as shown in Figure 26.4. Complete definitions of
the formations are available in Scott (1988).

Arcadia Formation The lower Hawthorn carbonate sec-
tion in south Florida has been assigned the name Arcadia
Formation (Scott, 1988). The carbonates are characteristically
quartz sandy, phosphatic. sometimes clayey dolostones to
limestones. Quartz sandy, non-phosphatic-slightly phosphatic
limestones predominate in the Tampa Member (Fig. 26.4). Phos-
phatic siliciclastic beds occur sporadically throughout the Arca-
dia Formation, becoming the dominant lithology only in the
Nocatee Member (Fig. 26.4).
The Arcadia Formation unconformably overlies the Eocene
Ocala Group, Crystal River and Williston Formations, in the
north-central and northeastern portions of south Florida. It
unconformably overlies the Oligocene Suwannee Limestone
elsewhere in this area. The Arcadia is overlain unconformably by
the Peace River Formation (Figs. 26.8 and 26.9). The Arcadia
reaches a maximum thickness of nearly 600 ft (183 m) in the
Okeechobee Basin. Few datable fossils have been found in the
Arcadia. Limited data suggest that this Formation ranges in age
from very earliest Miocene (early Aquitanian) to late Early
Miocene (Late Burdigalian) (Hunter, pers. comm.. 1985).

Peace River Formation The upper Hawthorn siliciclastic
section in south Florida has been assigned to the Peace River
Formation (Scott, 1988). The siliciclastics are typically
dolomitic. phosphatic, clayey quartz sands. Clay beds occur
infrequently throughout the section. Carbonate beds are com-
mon within the Peace River Formation and are generally quartz
sandy, phosphatic, clayey dolostones.
The Peace River Formation unconformably overlies the Ar-
cadia and is unconformably overlain by post-Hawthorn undif-
ferentiated sediments (Figs. 26.8 and 26.9). The Peace River







Hawthorn Group: litho:stratii ra iph,.


miles


80 km


Fig, 26.5. Subdivision of north and south Florida and location of cross-
sections.


Formation reaches a maximum known thickness of 650 ft
(198 m) in the Okeechobee Basin. Faunal :-.idence sueye-is that
the Peace River Formation ranges in age from Middle Miocene
(Serravalian) to no younger than earliest Pliocene (earliest
Zanclean) (Hunter. pers. comm., 1985).
Scott (1988) reduced the status of the former Bone Valley
Formation to Member. The status reduction was due to the
limited areal extent of this unit and the liihologic relationship
and demonstrated time equivalence to the undifferentiated
Peace River Formation. Lithologically, the most important fac-
tor for separating the Bone Valley from the remainder of the
Peace River Formation is the occurrence of phosphate gravel in
the Bone Valley Member. Phosphaie gravel and sand are mixed
with quartz sand and clay in proportions that vary from bed to
bed vertically, and within beds laterally. This unit is the main


o ..~'dot


phosphate-producing horizon in the Central Florida Phosphate
District.
The boundaries of the Bone Valley Member are complex. In
some areas of the district it unconformably overlies the Arcadia
Formation while in other areas the Bone Valley coniforma bii. to
unconformably overlies the undifferentiated Peace River
Formation. The Bone Valley Member is unconformably
overlain by post-Hawthorn undirTereniui.ed sediments (Figs.
26.8 and 26.9). The maximum recognized thickness for this unit
is approxim maeln 50 ft (15 m). The age of the Bone Valley NMem-
ber is derived entirely from vertebrate fossils collected from this
until MacFadden & Webb (1982) and Webb & Crissinger (1983)
suggest that the Member ranges in age from as old as latest Early
Miocene (latest Burdigalian) to as young as Early Pliocene
(Zanclean).








T.M. Scott


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Fig. 26.6. Cross-section A-A' showing the depositional and erosional
pinchout of Hawthorn Group sediments southward from Jacksonville
Basin onto the Sanford High.



Geologic history
During the Miocene, peninsular Florida was subjected to
numerous major and minor sea-level fluctuations and penodi-
cally subaerially exposed. The sediments of the basal Hawthorn
Group are the first record of Miocene transgressions upon the
eroded, karstic, limestone surface of Florida. These sediments,
carbonates and siliciclastics of the Penney Farms and lower
Arcadia.Formations, contain significant but not economi-
cally important amounts of apatite, indicating the Miocene
phosphogenic episode had begun.
Carbonate sediments dominated deposition in the Early
Miocene, although they contained a sia nifican siliciclastic com-
ponent. The siliciclastics present in the earliest Miocene repre-
sent the first recorded Cenozoic influx of siliciclastics onto the
carbonate bank of peninsular Florida. This major shift in sedi-


mentation may have been the result of eperiogenic uplift in the
Appalachians. possibly coupled with a climatic change. The
dramatic influx of siliciclastic sediments filled the Gulf Trough
which had effectively isolated the carbonate bank from
siliciclastic input from at least Early Paleocene through Late
Oligocene. Siliciclastic deposition was very important in north-
ern Florida in the Lower Miocene but did not dominate in
southernmost Florida until possibly as late as early Late Mio-
cene (Scott, 1983). Phosphate occurs as discrete apatite grains
and as rims on and replacement of carbonate intraclasts. Sedi-
ments deposited during the early part of the Lower Miocene
(Early Aquitanian) probably did not completely cover the Ocala
Platform or the Sanford High.
Sediments deposited in the mid-Lower Miocene (Late
Aquitanian-early Burdigalian) have not been recognized in


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Hawthorn Group: lithostratigraphy


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SCALE 1 250,000
5 5 10 MIL
5 0 5 70 15km











Fig. 26.7. Cross-section B-B' showing the relationship of the
Statenville Formation to the remainder of the Hawthorn Group in
north Florida.



Florida. If sediments of this age are to be found onshore, it may
be in the thick Arcadia Formation section in south Florida's
Okeechobee Basin. It is not known whether the absence of these
sedimcnri is due to non-deposition or erosion, but it is helpful to
note that they are not recognized in Georgia. South Carolina or
North Carolina.
Lower Miocene (Mid-Burdigaliani sediments of the Marks
Head and upper Arcadia form irons were deposited on the mid-
Lower Miocene unconformity. Siliciclastic sedimentation domi-
nated in northern Florida while carbonate deposition, mixed
with variable amounts of siliciclastics. continued in southern
Florida. Once again apatite is present. but not in economically
important quantities. In north Florida, the Lower Miocene
sediments were probably not deposited over the Ocala Platform


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or the Sanford High. However, the present distribution and
thickness of these sediments suggest that more of the structures
were covered than earlier in the Early Miocene.
Middle Miocene sediments disconformably overlie the
Lower Miocene throughout much of the State. The
unconformity is often marked by a bored, phosphatized. well-
lithified carbonate bed or a rubble resulting from the rip-up of
this bed. Deposition of the mid-Miocene sediments mas have
begun as early as very early Middle Miocene (Langhian) (Sn\-
der. pers. comm.. 1985) to mid-Middle Miocene (Early
Serravalian) (Huddlestun. pers.comm.. 1985). The deposition of
the Middle Miocene Hawthorn sediments covered the entire
peninsula (Scott, 1988). There, sediments were later removed by
erosion from the crest and upper flanks of the Ocala Platform.


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T.M. Scott


leaving only isolated outliers as evidence of at least part of its
former extent.
The greatest accumulation of phosphate in Florida appears
to have occurred during this Middle Miocene depositional cycle.
Much of the phosphate currently being mined in northern, and a
portion of that mined in central Florida, is thought to have been
deposited in the Middle Miocene. These phosphorites were
deposited during a high stand of sea level in response to the
impingement of topographically-induced upwellings (Riggs,
1984) onto the Ocala Platform and lesser positive features.
Subsequent reworking of the phosphorites has also been quite
important in development of the deposits. The Bone Valley
Member of the Peace River Formation provides the best
example of the importance of this reworking in that it contains
large quantities of phosphate gravel reworked from pre-existing
phosphate deposits. The reworking of these deposits has been
multicyclic, occurring primarily from early Middle Miocene
through Early Pliocene.
Upper Miocene sediments in the Hawthorn Group occur
primarily as zones of reworked, older sediments dated by the
occurrence of vertebrate remains (Webb & Crissinger. 1983).
These occur at the top of the Statenville Formation in northern
Florida and in the Bone Valley Member of the Peace River
Formation in southern Florida. Although it has not yet been
fully documented, other sediments of Late Miocene age
(Tortonian) occur in southern Florid. in the Peace River Forma-
tion. These sediments contain only minor (<5%) concentra-
tions of phosphate grains.
Sediments of the uppermost Bone Valley Member and the
informal Wabasso beds (Huddlestun et al,, 1982) are the only
documented occurrences of Pliocene-age sediments in the Haw-
thorn Group. The Bone Valley sediments are reworked from
older Hawthorn beds and deposited in fluvial to tidal channels.
The Wabasso beds were deposited under more open marine
conditions (Huddleston, pers. comm., 1985). Deposition of the
youngest Hawthorn Group sediments (the Wabasso beds) ended
in late Early Pliocene.


The amount of phosphate present in the post-Middle Mio-
cene Hawthorn Group sediments (except the Bone Vallgy Mem-
ber) is considerably less than in the Lower and Middle Miocene
portion. averaging < 3%. This suggests that with the lowering of
sea level in the Late Miocene, the major Miocene phosphogenic
episode ended. Minor amounts of phosphate may have been
deposited in these younger sediments; however, much of it could
be the result of reworking of the older phosphatic sediments.




References
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Dall, W.H. & Harris, G.D. (1892), Correlation paper-Neocene. US
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Scott. TM. (1983). The Hawthorn Formation of northeast Florida: Part
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Scott. T.M. (1988). The lithostratigraphy of the Hawthorn Group (Mio-
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