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STATE OF FLORIDA
DEPARTMENT OF NATURAL RESOURCES
Tom Gardner, Executive Director





DIVISION OF RESOURCE MANAGEMENT
Jeremy A. Craft, Director




FLORIDA GEOLOGICAL SURVEY
Walter Schmidt, State Geologist








OPEN FILE REPORT 34





THE GEOLOGY AND GEOMORPHOLOGY OF FLORIDA'S COASTAL MARSHES

By

Frank R. Rupert and Jonathan D. Arthur


FLORIDA GEOLOGICAL SURVEY
Tallahassee, Florida
1990
















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The geology and geomorphology of Florida's Gulf coastal marshes
by
Frank R. Rupert, P.G. No. 149, and Jonathan D. Arthur, P.G. No. 1149.


GEOLOGY

Florida Is situated in the eastern Gulf of
Mexico sedimentary basin, a broad region of
sediment accumulation comprised of southern
Alabama, southern Georgia, Florida, Cuba, and
the Bahamas (Puri and Vernon, 1964). The
geologic framework of the state includes
thousands of meters of Mesozoic (250 to 67
million years ago) and Cenozoic (67 million years
ago to the present) marine limestones, dolomites,
sands, and clays. These sedimentary rocks in
turn rest on Precambrian (600 million years ago
and older), and Paleozoic (600 to 250 million
years ago) basement rocks, which lie at depths
In excess of about 1200 m (4000 ft) below land
surface.
The emergent portion of Florida and the
offshore continental shelf and slope areas are
comprised principally of Cenozoic marine
sedimentary rocks. Most of these rocks were
deposited in the shallow seas which covered the
Floridlan Platform sporadically during the last 66
million years. Over the millennia, younger rock
layers, or formations, were successively deposited
on top of the older layers. The result is that
Florida's rock units are stacked "layer cake" style
in the subsurface. Many of the layered formations
vary locally In thickness, however, or have been
tilted, downwarped, or modified by erosion since
the time they were originally deposited. This has
resulted In a somewhat complex geologic
structure underlying Florida's west coast. Figure
1 Is a generalized geologic cross section of the
west coast from Pensacola to St. Petersburg. It
illustrates the stratigraphy of the near surface
formations underlying the coastal marshes, and
will serve as a reference in the following
discussion of the geology of Florida's Gulf coast.
Data used in the construction of the cross section
were obtained from lithologic logs of well cores
and samples on file at the Florida Geological
Survey (FGS). The "W-" numbers shown on the
cross section represent FGS well accession
numbers.
The oldest rocks underlying Florida are known
only from samples brought to the surface during


the drilling of deep oil test wells. These
basement rocks typically range in age from Late
Precambrian (about 700 million years ago) to
Middle Mesozoic (about 150 million years ago).
In the eastern Florida panhandle and northern
peninsula, the basement rocks are igneous and
sedimentary in nature, and lie below 1200 m
(4000 ft) in depth; In southern Florida, the
basement rocks are generally more than 4500 m
(15,000 ft) deep, and are comprised primarily of
Middle Mesozoic Igneous rocks (Arthur, 1988).
Overlying the basement rocks are a series of
Mesozoic sedimentary rocks, including sands,
shales and limestones. Most are marine in
origin. Florida's two oil regions, the Jay trend in
northwestern Florida, and the Sunniland trend in
south Florida, produce oil from horizons within
these Mesozoic rocks.
A sequence of Cenozoic (predominantly
marine) sediments in turn overlie the Mesozoic
rocks. These rocks contain Florida's drinking
water aquifers and Industrial mineral deposits, and
comprise the visible portions of the state today.
Figure 2 is a generalized stratigraphic chart
summarizing the Eocene and younger Cenozoic
formations present under west coastal Florida.
The Paleocene rocks are marine limestones and
dolomites. These older rocks are not important
fresh water aquifers, and lie deeper than the
depths attained by most water wells. For the
purposes of this report, the discussion of the
stratigraphy will be limited to Middle Eocene and
younger sediments. Many of these sediments
crop out at the surface, and most directly affect
the geology and hydrology of Florida's Gulf
coastal marsh region. Data on the lithology,
depth, thickness, and occurrence of the
formations is derived from well logs on file at the
Florida Geological Survey.

Middle Eocene Series
Avon Park Formation

The Avon Park Formation (Applin and Applin,
1944; Miller, 1986) is typically a cream to brown
to tan, fossiliferous, marine dolomite. It
commonly contains pasty limestone beds and


































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peat flecks and seams. The Avon Park
Formation was deposited in a shallow sea which
covered the area of present-day Florida about 54
million years ago. Numerous shallow-water
fossils are found in Avon Park Formation
sediments, Including heart urchins, sand dollars,
mollusks, foraminifera, and a "tutle grass-like"
marine plant, very similar to the species living off
the Big Bend coastline today.
The Avon Park Formation is the oldest rock
exposed at the surface In Florida. It occurs In
west-central Florida along the crest of a gentle,
northwest-southeast trending anticlinal feature
variously called the Ocala Uplift, Ocala Arch,
Ocala High, or Ocala Platform (Purl and Vemon,
1964; Scott, 1988), the origin of which is uncertain.
Eocene and younger rocks have been removed
by erosion over the structure.
The Avon Park Formation is exposed only in
small areas of central and southern Levy County
and northernmost Citrus County, corresponding
to the crest of the Ocala Platform (see Figure 1).
It dips westward, where it interfingers with the age
equivalent Usbon Formation under the western
Florida panhandle. Thickness of the unit In
west-central Florida is variable, but generally
ranges from 0 to 240 m (0 to 800 ft) statewide
(Chen, 1965).
Dolomite from the Avon Park Is quarried In
Citrus and Levy Counties for use in construction
materials. The Avon Park is also a unit of the
Floridan aquifer system, one of Florida's primary
drinking water aquifers. It is unconformably
overlain by marine limestone of the Upper
Eocene Ocala Group.

Upper Eocene Series
Ocala Group

The Late Eocene (38 to 41 million years ago)
Ocala Group (Puri, 1957) is composed of three
marine limestone formations. In ascending order,
these are the Inglis, Williston, and Crystal River
Formations, all named for towns in the respective
local outcrop areas of each formation. The
formations have traditionally been differentiated
on the basis of fossil content, and to a lesser
extent, lithology. For the purposes of this report,
these three units will be referred to collectively as
the Ocala Group.
The Ocala Group was deposited In a shallow,
Late Eocene sea. It is typically a white to cream,
abundantly fossiliferous, chalky to coqulnold


limestone. Microfossils are generally the most
common fossil forms present, especially the large
species of foraminifera Lepldocyclina,
Nummulites, and Heterostegina. Mollusks,
bryozoans, and echinoids are also common in
these sediments. As shown on Figure 1, the
Ocala Group also crops out in west central
Florida, along the crest of the Ocala Platform. It
dips generally westward and southwestward from
its outcrop area. The thickness of the Ocala
Group in west-central Florida averages about 60
m (200 ft), but Is locally variable, and generally
thins over the crest of the Ocala Platform.
Limestone of the Ocala Group is exposed
sporadically or is blanketed only by a thin veneer
of Pleistocene and Holocene sands in all of Dixie
County and portions of Lafayette, Gilchrist, Levy,
Citrus, Marion, and Hernando Counties.
Throughout this area it Is quarried as construction
material and road base.
The exposed surface of the Ocala Group
limestone forms a relatively flat-lying, gently
seaward-sloping, solution depression and
sinkhole-pocked plain over most of its outcrop
area. This plain continues offshore onto the
broad western Florida continental shelf.
Boulders, pinnacles, and small islands of Ocala
Group limestone are common along the Gulf
coastline of Dixie, Levy, and Citrus Counties
(Vemon, 1951). Many of the coastal marshes in
this area are developed in the thin calcitic muds,
silt, organs, and relict marine sands deposited in
solution depressions in the underlying limestone
(see Figure 3). The Ocala Group is an important
unit of the Floridan aquifer system. Along much
of the central west coast and within many of the
coastal marsh areas, the potentiometric surface of
the Floridan aquifer system is at or above land
surface. As a result, numerous seeps and small
springs feed freshwater into the marshes. Fow
from some of these springs is sufficient to form
small tidal creeks and tributaries through the
otherwise dense marsh grasses (Nettles, 1976).

Oligocene Series
Suwannee Limestone

The Oligocene (33 to 25 million years old)
Suwannee Limestone (Cooke and Mansfield,
1936) is a white to tan, fossiliferous, commonly
dolomitic marine limestone, named after
exposures along the Suwannee River in northern
Florida. As with the preceding Eocene










formations, the Suwannee was probably
deposited in a shallow temperate sea. It contains
abundant microfossils, mollusks, echinolds,
bryozoans, and rare corals. Early Indians of the
region used the abundant chert occurring in both
the Ocala Group and Suwannee Umestone to
fashion tools and weapons. The Suwannee
Umestone has been truncated against the flanks
of the Ocala Platform (Figure 1), and is exposed
at the surface at the northern and southem ends
of this feature. It occurs as the surflcial unit in
eastern Jefferson County, most of Taylor County,
western Hemando County, and northem Pasco
County. In these areas, the Suwannee Umestone
forms a flat-lying, karstic plain which extends
seaward onto the continental shelf. Boulders and
pinnacles of Suwannee Umestone are common
along the low energy coasts of Jefferson and
Taylor Counties. The Suwannee Umestone is the
upper unit of the Floridan aquifer system in its
outcrop area. Small fresh water springs and
seeps commonly occur In the marshes of
southern Jefferson and Taylor Counties, some
forming creeks or contributing to the flow of rivers
such as the Aucila and Econfina.

Miocene Series

The Miocene Epoch marked a significant
change in the depositional regime of the Florida
peninsula. Previously a shallow carbonate bank,
Florida experienced an influx of continental
silclciastic sediments during the Miocene from
the continental mainland to the north. This was
due In large part to the closing of a feature called
the Suwannee Straits or Gulf Trough, which
trended southwestward across the eastern Florida
panhandle (Figure 1). The Gulf Trough was
thought possibly to have been an ancient
channel or seaway, existing from the Cretaceous
Era through the end of the Oligocene Epoch (Dall
and Harris, 1892; Chen, 1965). It probably
connected two major depositional basins, the
Southeast Georgia Embayment and the
Apalachicola Embayment. Currents within this
paleo-strait may have functioned as both a
zoological and a sedlmentologlcal barrier between
the carbonate banks and Islands of peninsular
Florida and the continental mainland.
Phosphogenesis also occurred on a large
scale beginning In the Miocene. Many Miocene
formations contain phosphate grains, ranging
from silt to pebble size. Florida's economic


phosphate deposits are concentrated In the
Miocene Hawthorn Group sediments in the
Central Florida Phosphate District of Polk and
surrounding counties and in the North Florida
Phosphate District in Hamilton County.
The Miocene formations in Florida are
primarily marine In origin. In contrast to the.
nearly pure calcium carbonate limestones of the
Eocene, the Miocene and younger sediments,
Including the carbonates, typically contain
terrigenous quartz and heavy mineral sands and
days and phosphate. Starting In the Lower
Miocene, a massive influx of river-borne
continental sediments poured into the seas
covering present day peninsular Florida (Scott,
1988). The Miocene seas reworked and
deposited these sediments in a broad blanket
over the carbonates of earlier epochs. In the
western panhandle, a series of marine limestones
and shelly sands were laid down. The Miocene
seas supported a rich marine fauna, as shown by
the numerous fossils found in these sediments.
Based on fossils, paleoenvironments ranged from
shallow nearshore and lagoon to deepwater
continental shelf. Many of Florida's commercial
mineral deposits, including phosphate and fuller's
earth, were formed In these ancient sea floor
sediments,

Lower Miocene Series
St. Marks Formation

The St. Marks Formation (Purl and Vernon,
1964) is exposed along the northern Big Bend
coastline in Wakulla County. It Is the only
Miocene carbonate unit exposed in the coastal
marsh zone. This formation Is a white to very
pale orange to light gray, quartz sandy,
fossiliferous marine limestone. Foraminifera and
mollusks are the dominant fossil forms, generally
present as molds. Along the coast the St. Marks
Formation contains abundant chert, which
probably supplied the paleoindlans of the region
with tool and spear point material. The St. Marks
Formation occurs near or at the surface in most
of south central and south eastern Wakulla
County. Here it forms a slightly seaward-slop-
Ing, karstic plain called the Woodville Karst Plain.
Thickness varies from 0 m at the
Wakulla-Jefferson County line to over 30m (100
ft) along the eastern flank of the Apalachlcola
Embayment. Extensive salt marshes, developed
in the thin sand and mud veneer overlying the St.










Marks Formation limestone, border the southern
edge of Wakulla County.
The St. Marks Formation is the uppermost unit
of the Floridan aquifer system in the eastern
panhandle. In southern Wakulla County, the
potentiometric surface of this aquifer is at or
above the land surface elevation. Many small
fresh water springs are scattered through the
coastal marshes, and several submarine springs
are situated on the offshore extension of the karst
plain. The St. Marks pinches out against the
underlying Suwannee Limestone to the east in
Jefferson County (Figure 1), and dips generally to
the west-southwest. To the west under Franklin
and Gulf Counties it becomes indistinguishable
from the Bruce Creek Limestone. It interfingers to
the north and possibly under the western
panhandle with the age-equivalent Chattahoochee
Formation.

Hawthorn Group
Arcadia Formation

The Lower Miocene Arcadia Formation of the
Hawthom Group (Scott, 1988) underlies the
west-central Gulf coast from approximately New
Port Richey to Sarasota (Figure 1). This unit is
comprised primarily of white to yellowish-gray to
light olive-gray limestone and dolomite containing
variable amounts of quartz sand, clay, and
phosphate grains. It unconformably overlies the
Suwannee Umestone, and Is overlain by
undifferentiated Pleistocene quartz sands.
Hawthorn Group sediments are absent along the
panhandle coast.
Hawthorn Group sediments serve as an
Intermediate confining unit to the underlying
Floridan aquifer system. Locally, carbonates
within the Hawthorn Group may also function as
an intermediate freshwater aquifer system, or in
some areas, are in hydrologic continuity with the
underlying Floridan aquifer system and
considered part of the Floridan aquifer.
The Hawthorn Group does not crop out in the
vicinity of the coastal marshes. It is probably
eroding offshore however, as pebbles and
cobbles of Hawthorn Group carbonate are often
found washed up on Pinellas County beaches.


Middle Miocene
West of Wakulla County in the panhandle, the
Miocene units dip and thicken southwestward


into the Apalachicola Embayment (Figure 1).
Both the Lower Miocene St. Marks Formation and
the overlying Middle Miocene Bruce Creek
Limestone dip into the trough of the embayment,
reaching a maximum local depth and thickness
under the axis of the basin. These units shallow
again on the western side of the embayment,
and attain a depth and thickness similar to that
on the eastern edge of the embayment.

Bruce Creek Limestone

Bruce Creek Limestone was the name given
by Huddlestun (1976) to a white to light
yellowish-gray, Middle Miocene marine limestone
underlying part of panhandle Florida. It extends
from the eastern edge of the Apalachicola
Embayment westward, with generally southwest
dip. The Bruce Creek Limestone is commonly
quartz sandy, phosphoritic, and both micro- and
macro-fossiliferous. Fossils are generally
preserved as molds. This formation is primarily a
subsurface unit, and does not crop out in the
vicinity of the coastal marshes. The Bruce Creek
Limestone thickens to over 60 m (200 ft) in the
Apalachicola Embayment. Down-dip, it is
indistinguishable from the underlying St. Marks
Formation. In the central and western panhandle,
the Bruce Creek Limestone comprises the
uppermost unit of the Floridan aquifer system.

Middle Miocene Through Pliocene Series
Intracoastal Formation

Huddlestun (1976) applied the name
Intracoastal Formation to a soft, yellowish-gray to
olive green, sandy, highly microfossiliferous,
argillaceous marine limestone underlying the
coastal area of west Florida. This formation
extends from easternmost Franklin County
westward, and continues to approximately the
Santa Rosa County line (Schmidt, 1984). The
Intracoastal Formation is predominantly a
subsurface unit, dipping and thickening to the
west-southwest into the Apalachicola Embayment.
It approaches 100 m (300 ft) in thickness in the
trough of the Apalachicola Embayment. The unit
shallows and thins west of the embayment, then
thickens and dips westward towards the
Pensacola area. Analysis of the microfossils
present indicate an age range for the Intracoastal
Formation of Middle Miocene (down dip) to Late
Pliocene (in the up-dip portions), with a hiatus










separating the age extremes. Locally it occurs
close to the surface in the central panhandle, but
does not crop out In the coastal marsh region.

Pensacola Clay

The Pensacola Clay (Marsh, 1966) Is a pale,
yellowish-brown to olive-gray, dense, silty,
commonly quartz-rich sandy clay. This unit Is
restricted to the subsurface, occurring under the
panhandle from approximately Okaloosa County
westward into Alabama. It thickens rapidly to the
south and west (see Figure 1), reaching a
maximum thickness of about 150 m (500 ft) under
Pensacola Bay (Clark and Schmidt, 1982). The
Pensacola Clay is considered Late Miocene in
age, overlying the Bruce Creek Limestone, and is
interflngered locally with the Intracoastal
Formation or the Miocene Coarse Clastics (Clark
and Schmidt, 1982).

Miocene-Pliocene Coarse Clastics

Marsh (1966) proposed the name Miocene
Coarse Clastics for a series of sands, gravel,
days, and shell beds underlying the western
panhandle. These sediments are largely
comprised of light-gray to pale-yellowish-brown
quartz sand and gravel, with minor clay and
marine mollusk shells. The Miocene-Pliocene
Coarse Clastics occur from western Okaloosa
County westward through Santa Rosa and
Escambia Counties. Thickness of this unit
reaches nearly 150 m (500 ft) under Santa Rosa
County. The coarse clastics overlie the
Pensacola clay and Intracoastal Formations, and
locally may be contemporaneous (Late Miocene
to Early Pliocene In age) with portions of both
formations; they are overlaln by undifferentiated
Pleistocene sand (Clark and Schmidt, 1982).

Chlpola Formation

The Chipola Formation (Puri and Vemon,
1964) is present at depth under the coastal
marshes in the vicinity of Gulf County, along the
axis of the Apalachlcola Embayment.
Lithologlcally, it is comprised of a yellowish-gray
to light-gray, quartz sandy marine limestone.
Foraminifera and mollusks are typically the most
abundant fossils. Thickness of this unit reaches
about 15 m (50 ft) In the central Apalachlcola
Embayment, thinning and pinching out to the


west and east of the embayment. In the down-
dip coastal and offshore areas, the Chipola
Formation overiles the Intracoastal Formation and
is considered to be late Pliocene In age (Schmidt,
1984). It Is unconformably overlain by the
Jackson Bluff Formation.

Jackson Bluff Formation

The Late Pliocene Jackson Bluff Formation
(Purl and Vemon, 1964) consists of tan to
oranglsh-brown to grayish green sandy, clayey
shell beds. It Is restricted in occurrence to the
central Apalachlcola Embayment, where the unit
reaches about 30 m (100 ft) in thickness. The
Jackson Bluff Formation overlies the Chipola and
Intracoastal Formations, and Is in turn overlain by
undifferentiated Pleistocene and Holocene sands.
The Jackson Bluff Formation does not crop out in
the coastal marshes.

Pleistocene and Holocene Series

A series of undifferentiated Pleistocene (1.8
million to 10,000 years old) and Holocene (10,000
years and younger) quartz sands, clayey sands,
and sandy clays blanket the older formations
along much of Florida's west coast. These
sediments are composed largely of reworked
relict Pleistocene marine sands and Holocene
alluvium, calcitic muds, and organic.
The Pleistocene Epoch, or "Ice Age", was
characterized by four great glacial periods.
During this epoch, global temperatures cooled,
and huge Ice sheets grew southward from
Canada. Large quantities of seawater were
consumed to build the glaciers, and sea level
dropped as much as 130 m (400 ft). Each glacial
period was punctuated by warmer Interglacial
periods during which the sea level rose, at times
to over 30 m (100 ft) above modem level. As the
Pleistocene seas transgressed Inland, wave and
current activity eroded, reworked, and
redeposited the sands of earlier formations. At
the same time, rivers and an active southward-
moving littoral drift system brought new plastic
sediments into Florida. The Big Bend Area was a
drowned karst coastline during the Pleistocene
sea-level highstands. Planing by wave action and
in-filling of the karst features with sand resulted in
the flat, seaward-sloping plain characterizing this
region today. Figure 3 illustrates a typical near-
surface cross-section in the coastal marsh zone


_~_Ul___l~_ I____ ___~___I__











































S 100 METERS

0 300 FEET


VERTICAL EXAGERATION APPROXIMATELY 110 TIMES TRIE SCALE







Figure 3. Generalized near-surface cross section
in the coastal marshes, west-central Florida
(modified from Hine and Belknap, 1986).


EOCENE/OLIGOCENE LIMESTONE
Ir . . I . . . .
.........................
...........

. . . .
. . . I . .




ii










scarps and terraces do exist in the Gulf Coast
region; however, further study is required in order
to more accurately define their occurrence and
extent.
Pliocene-Pleistocene sea-level fluctuations are
the primary reason for the presence of these
terraces (Winker and Howard, 1977a, 1977b).
During the Pleistocene "Ice Age" when the
fluctuations became more prevalent, four major
glacial cycles occurred due to major climatic
changes. As glaciers spread over the continents,
sea-level dropped. During periods of melting or
interglacial periods, sea-level rose. Healy (1975)
and MacNell (1950) suggested that during
subsequent (younger) interglacial periods, the
seas stood at levels below that of the prior event,
thus preserving the earlier formed terraces and
scarps. Several authors have recognized that
glacial melting alone cannot account for the
present-day anomalously high elevations of the
older marine terraces. Tanner (1968, 1985) and
Winker and Howard (1977a, 1977b) document
evidence for regional uplift during this time.
In a study entitled "The Geomorphology of the
Florida Peninsula," White (1970) subdivided the
region Into a series of uplands and lowlands.
Most of the present study area lies within a
single, major physiographic province: the Gulf
Coastal Lowlands (Figure 4). This province
stretches from the western Florida panhandle to
southern peninsular Florida and averages 40
kilometers (25 miles) in width. Various highlands,
ridges or scarps constitute the Inland limits of the
Gulf Coastal Lowlands, whereas the present-day
shoreline marks the seaward boundary. Unlike
eastern coastal areas of Florida, this geomorphic
province does not correspond to any specific
marine terrace delineated by Healy (1975). The
Gulf Coastal Lowlands contain various erosional
and depositional landforms that occurred in
response to Pliocene-Pleistocene sea-level.
fluctuations. Several relict bars, spits and
terraces are superimposed on the modem
topography of the region. Subdivisions within the
Gulf Coast Lowlands include Gulf Barrier Chains,
Coastal Swamps, Coastal Lagoons and Estuaries.
Using White's (1970) terminology, the Gulf
Coastal Swamps include areas where a
deficiency exists In the sand budget for building
beaches. These low-lying areas correspond to
areas on topographic maps where the swamps
are immediately adjacent to the coast. No
distinction is made between salt marshes and
fresh water swamps. Where Isolated patches of


swamp are separated from the coast by dry land,
the term "coastal lagoons" is applied (White,
1970). This generalized terminology is being
revised based on vegetation, soil type and
topography in concurrent investigations (C.L.
Coultas, personal communication, 1990).
White (1970) presented a two-fold classification
for the Gulf Coast of peninsular Florida. Within
the study area, this includes a coastal salient that
extends from Tampa Bay north to the southern
part of Pasco County, and a coastal reentrant
spanning from this point north toward Apalachee
Bay. The salient is characterized by a relatively
steep offshore profile which allows wave energy
to transport, deposit and erode sand along the
shoreline. This coastal area contains many relict
barrier bars, beach ridges and lagoons. In
contrast, the reentrant offshore profiles have
gentle slopes and are floored by limestone. White
(1970) suggested that these broad shallow
profiles sufficiently dissipate wave energies to
account for the sand-starved marshy coasts. The
sand deficiency Is also due to a lack of sediment
transported by the Suwannee River. The very
low-energy reentrant coast Is virtually free of
relict shoreline features.
Along the Florida panhandle coastline, White's
(1970) generalization concerning the peninsula's
offshore profiles Is also applicable. Westward
from the west end of Apalachee Bay, the profile
is either as steep as or steeper than in the Tampa
Bay area. The entire panhandle coastline
consists of spits, lagoons, beach ridges and
offshore barrier islands. An even steeper offshore
slope (ramp) in the central panhandle region has
precluded barrier island formation offshore of
Walton and the west half of Bay counties (Tanner,
1960). Puri and Vernon's (1964) geomorphic
map of the region, which uses White's criteria for
landform definition, shows no coastal swamps.
Tanner (1960) presented data which quantify
energy levels within the study area. Rather than
focussing on shelf slope, he reported average
annual breaker heights to estimate wave energy.
Energy levels of Tanner (1960) are shown on
Figure 4. The "zero energy" coast corresponds to
the coastal swamps and marshes in the Big Bend
area. The lack of both wave energy and thus
sediment transport and deposition are the two
most significant factors that have allowed
swamp/marsh development in the region.
In addition to the regional geomorphic
characteristics of the study area, two localized
features are noteworthy, both of which pertain to










along the west-central Florida coast. The larstic,
irregular surface of the underlying limestone is
infilled with Pleistocene and Holocene sediments,
with a few limestone pinnacles exposed at the
surface. The younger sands, muds, and silts form
a substrate for many of the coastal marshes in
this area.
Relict dunes, bars, and barrier Island sand
bodies, left by retreating Pleistocene seas, also
are common features today along much of
Florida's central and northem Gulf coasts. In the
central panhandle, the ancestral Apalachicola
River meandered over a large area of Bay and
Gulf Counties during the Pleistocene and
Holocene, leaving In its wake relict levee deposits
and alluvium. Much of the surficial and near-
surface sediments in the central and western
panhandle coastal areas are a mixture of marine
and alluvial sands and clays.
The Pleistocene deposits are thinnest in the
Big Bend area, where limestone Is near or at the
surface. West of Wakulla County, these
Pleistocene sediments thicken to nearly 60 m (200
ft) near the Alabama state line.
Holocene sand deposition continues today.
Accumulation of these deposits Is primarily
concentrated along the banks, bottoms, and
mouths of the major Gulf coast rivers, such as the
Apalachicola, Ochlockonee, Aucilla, Suwannee,
and Withlacoochee. In portions of the central
west coast and Big Bend areas, a Holocene
Intertidal calcitic mud often overiles the
Pleistocene sand (Nettles, 1976). Organics
derived from decaying marsh grasses are
Intermixed with sandy typically forms the surface
layer In the coastal marshes (Hine and Belknap,
1986).

GEOMORPHOLOGY

The geomorphology of Florida's Gulf Coast
has been shaped primarily by coastal processes
and sea-level changes during the past five million
years. These sea-level changes have caused
shifts In ground water levels which in turn have
enhanced the development of karst landforms
during this time. The karst features have thus
altered some of the original shoreline erosional
and depositional features within the Gulf Coast
region. Discussion of these two Interrelated
geomorphic features is limited to an Inland
distance of 16 kilometers (10 miles), spanning the
study area from Pensacola to St. Petersburg. In


that the focus herein Is on the recent geologic
past, modern offshore landforms such as spits,
barrier Islands and bars are not discussed.
Up to six marine terraces are reported to occur
within the region of Interest (Cooke, 1945; Healy,
1975). As defined by Garner (1974), a marine
terrace Is a surface of erosion or deposition
formed along a coast by wave action. These
terraces represent the floors of ancient shallow
seas and are situated in a step-like manner,
roughly parallel to the present-day coast. Each
"step" Is a topographic break ranging from a
gentle slope to a sharp Incline. Where sharp,
these changes In topography which separate two
terraces are well preserved seaward-facing wave
cut scarps, some of which are regional in extent
and contain up to 15 meters (50 feet) of relief
(e.g. the Cody Scarp, Purl and Vernon, 1964,
p.11). Although there are few of this magnitude
proximal to the Gulf Coast, coastal terraces and
scarps appear to control surface drainage In
localized areas. For example, Healy's (1975)
Silver Bluff terrace generally coincides with the
location of several Gulf coastal swamps.
Early marine terrace studies have applied
physiographic and geomorphic evidence (White,
1970), aerial photography (Vernon, 1951),
sedimentology/stratigraphy (Altschuler and
Young, 1960; Pirkle and others, 1970), field
mapping (Parker and others, 1955) and fossil
evidence (Alt and Brooks, 1965). Although
conclusions drawn for localized areas are well
constrained by these data, studies based on
elevation correlations over large areas (e.g.
MacNeil, 1950; Healy, 1975) should be considered
with reservation. Given the possibility of
regional Pllocene-Pleistocene warping (Winker
and Howard, 1977a, 1977b), the predominance of
karst development in the region (White, 1970) as
well as other erosional processes, some of these
long-distance correlations may not be valid. For
example, a present day low-lying area may
appear to correlate with a certain low elevation
terrace, whereas in fact it may have been an
upland or higher elevation terrace which was
subsequently lowered by subsurface limestone
dissolution. On the other hand, a "young", low
elevation terrace may have undergone
epelrogenic uplift and now topographically
correlates with a higher elevation, older terrace.
Thus, the correlation between
swamps/marshlands and the Silver Bluff terrace
may be an artifact of the manner in which some
terraces have been delineated. Ancient marine


































o00Ck494


0.


N EXPLANATION
RH IHIOHLAND
o: -n COASTAL
SCALE


S

SWAMPS LOW


Figure 4. Geomorphic features along study area
modified from White (1970) and Purl and Vernon
(1964). Wave energy zones are from Tanner
(1960).


11




ii










karstiflcation. The first of these can be seen on
Tanner's (1960) map which classifies a shoreline
region of Citrus County as "drowned karst"
(Figure 4). This area, discussed in detail by Hine
and Belknap (1986) is called the Ozello Marsh
Archipelago and consists of several limestone
cored marsh Islands. The outer islands are
separated either by creeks or salt marsh
vegetation whereas the interior is pocked with
circular ponds. Various karstification processes
have controlled the shape and orientation of
these water bodies (Hine and Belknap, 1986).
Figure 3 illustrates the undulatory nature of
bedrock limestone in this area due to dissolution.
The Woodville Karst Plain (Hendry and Sproul,
1966) is a subdivision of the Gulf Coastal
Lowlands and is located north of the coastal
swamp belt in Wakulla County. Portions of the
Woodville Karst Plain extend into Jefferson and
Leon Counties where it is bounded to the north
by the Northern Highlands (Purl and Vernon,
1964). Permeable sands form a veneer over a
shallow, southward dipping limestone bedrock.
Dissolution of the underlying bedrock, which has
probably been occurring ever since the area has
been above sea level, has caused subsidence.
This somewhat localized depression, and its
prevalence of shallow sand filled sinkholes
characterizes the Woodville Karst Plain. Although
surrounding areas are also underlain by
limestone, dissolution has not been prevalent
because the overlying sediments contain clays
which both buffer the acidic rainwater and
reduce the amount of percolation. Geologic
variables such as those characterizing the
Woodville Karst Plain bedrock type, sediment
composition, and sea level history are significant
in the development of Florida's Gulf Coast
geomorphology.

ACKNOWLEDGEMENTS

The authors would like to thank Ken
Campbell, Dr. Walt Schmidt and Dr. Thomas Scott
of the Florida Geological Survey, and Dr. William
F. Tanner of Florida State University for critically
reviewing the manuscript of this report.

REFERENCES

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Survey Professional Paper 221-F.
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west coast of Florida (thesis], Department
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and Vemon, R. 0., 1964, Geology
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1977b,
Pllo-Plelstocene paleogeography of the
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27:409-420.


































o00Ck494


0.


N EXPLANATION
RH IHIOHLAND
o: -n COASTAL
SCALE


S

SWAMPS LOW


Figure 4. Geomorphic features along study area
modified from White (1970) and Purl and Vernon
(1964). Wave energy zones are from Tanner
(1960).


11










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