Stratigraphic models for microtidal tidal deltas; examples from the Florida Gulf Coast


Davis, R.A.; Cuffe, C.K; Kowalski, K.A.; Shock, E.J.

Marine Geology 200(1-4): 49-60

2003


Extensive vibracoring of both flood- and ebb-tidal deltas along the central Gulf Coast of the Florida peninsula reveals a strong overall similarity with subtle distinctions between flood and ebb varieties. Although the coast in question is microtidal, the inlets range from tide-dominated to distinctly wave-dominated. Both types of tidal deltas overlie a muddy sand interpreted to have been deposited in a back-barrier environment. The sharp contact at the base of the tidal delta sequence is typically overlain by a thin shell gravel layer. The ebb-tidal delta sequence is characterized by fine quartz sand with shell gravel in various concentrations; coarse and massive at the margins of the main ebb channel, and finer and imbricated at the marginal flood channels. The flood-tidal deltas are characterized by the same facies but with a small amount of mud. Shelly facies on the channels on flood deltas are not as well developed as on the ebb deltas. The combination of the stratigraphic sequence and the lithofacies make tidal deltas readily identifiable in the ancient record. The differences between flood and ebb varieties are subtle but consistent.

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Marine
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200
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49-60
MARINE
GEOLOGY
INTERNATIONAL
JOURNAL
OF
MARINE
GEOLOGY
GEOCHEMISTRY
AND
GEOPHYSICS
www.elsevier.com/locate/margeo
Stratigraphic
models
for
microtidal
tidal
deltas;
examples
from
the
Florida
Gulf
coast
Richard
A.
Davis
Jr.
a
'
*
,
C.
Kelly
Cuffe
b
,
Katherine
A.
Kowalski
a
,
Eric
J.
Shock
a
a
Coastal
Research
Laboratory,
Department
of
Geology,
University
of
South
Florida,
Tampa,
FL
33620,
USA
b
California
Coastal
Commission,
725
Front
St.,
Suite
300,
Santa
Cruz,
CA
95060,
USA
Accepted
1
June
2003
Abstract
Extensive
vibracoring
of
both
fl
ood-
and
ebb
-tidal
deltas
along
the
central
Gulf
Coast
of
the
Florida
peninsula
reveals
a
strong
overall
similarity
with
subtle
distinctions
between
fl
ood
and
ebb
varieties.
Although
the
coast
in
question
is
microtidal,
the
inlets
range
from
tide
-dominated
to
distinctly
wave
-dominated.
Both
types
of
tidal
deltas
overlie
a
muddy
sand
interpreted
to
have
been
deposited
in
a
back
-barrier
environment.
The
sharp
contact
at
the
base
of
the
tidal
delta
sequence
is
typically
overlain
by
a
thin
shell
gravel
layer.
The
ebb
-tidal
delta
sequence
is
characterized
by
fine
quartz
sand
with
shell
gravel
in
various
concentrations;
coarse
and
massive
at
the
margins
of
the
main
ebb
channel,
and
finer
and
imbricated
at
the
marginal
fl
ood
channels.
The
fl
ood
-tidal
deltas
are
characterized
by
the
same
facies
but
with
a
small
amount
of
mud.
Shelly
facies
on
the
channels
on
fl
ood
deltas
are
not
as
well
developed
as
on
the
ebb
deltas.
The
combination
of
the
stratigraphic
sequence
and
the
lithofacies
make
tidal
deltas
readily
identifiable
in
the
ancient
record.
The
differences
between
fl
ood
and
ebb
varieties
are
subtle
but
consistent.
©
2003
Elsevier
B.V.
All
rights
reserved.
Keywords:
fl
ood
-tidal
delta;
ebb
-tidal
delta;
lithofacies;
microtidal;
hurricane;
fi
ning
-upward
sequences
1.
Introduction
Tidal
deltas
represent
a
primary
sediment
sink
on
barrier
island
coasts
and
are
also
among
the
most
preservable
depositional
environments
in
the
barrier
island
system.
They
are
important
ele-
ments
in
coastal
management,
and
are
reservoirs
for
production
of
oil
and
gas.
Given
their
impor-
*
Corresponding
author.
Fax:
+1-813-974-2654.
E-mail
address:
rdavis@chumal.cas.usf.edu
(R.A.
Davis
Jr.).
tance
in
both
the
modern
coast
and
the
ancient
stratigraphic
record,
it
is
critical
to
have
a
good
understanding
of
their
stratigraphy
and
geologic
development.
The
Gulf
Coast
of
peninsular
Florida
provides
an
excellent
setting
for
studying
these
sediment
bodies
in
detail
because
it
contains
tidal
deltas
having
both
variety
and
accessibility.
The
objec-
tive
of
this
part
of
a
comprehensive
investigation
of
this
barrier/inlet
system
is
to
produce
strati
-
graphic
models
for
tidal
deltas
that
are
applicable
throughout
the
subject
coast,
and
that
can
be
ap-
plied
to
other
parts
of
the
Gulf
Coast
and
be
-
0025
-3227/03/$
see
front
matter
©
2003
Elsevier
B.V.
All
rights
reserved.
doi:10.1016/80025-3227(03)00164-6
50
R.A.
Davis
Jr.
et
al.
/Marine
Geology
200
(2003)
49-60
yond.
The
synthesis
of
three
separate
investiga-
tions
of
both
fl
ood-
and
ebb-tidal
deltas
provides
data
from
three
fl
ood
-deltas
and
three
ebb
-deltas
that
can
be
summarized
into
models
that
repre-
sent
this
coastal
regime.
1.1.
Coastal
setting
The
barrier/inlet
system
of
the
Gulf
Coast
of
peninsular
Florida
is
a
low
-energy
(Tanner,
1960),
microtidal
(Davies,
1964)
coast
that
can
best
be
described
as
mixed
energy
(Davis
and
Hayes,
1984).
The
coast
to
the
north
is
a
tide
-
dominated,
open
coast
marsh
system
(Hine
et
al.,
1988)
and
south
of
this
barrier
system
is
a
tide
-dominated
coast
characterized
by
mangroves
(Davis
et
al.,
1992).
Although
this
coast
experien-
ces
a
maximum
tidal
range
of
about
90
cm,
it
displays
the
most
varied
barrier
island/tidal
inlet
morphologies
in
the
world
(Davis,
1989).
This
system
includes
both
wave
-dominated
and
mixed
-energy
or
drumstick
barriers,
and
the
entire
spectrum
of
tidal
inlet
and
tidal
delta
morpholo-
gies
(Davis,
1997).
1.2.
Coastal
processes
Processes
that
influence
this
barrier/inlet
system
are
dominated
by
tropical
storms
and
by
frontal
systems
that
pass
during
the
winter.
Prevailing
winds
are
gentle
and
have
a
southerly
component;
influenced
by
the
Bermuda
High
over
the
central
Atlantic
(Henry
et
al.,
1994).
These
southerly
winds
prevail
from
about
April
through
October
but
are
also
present
during
the
remainder
of
the
year.
Cold
fronts
move
across
the
Gulf
of
Mexico
from
the
northwest
and
pass
over
this
coast
from
late
October
through
mid
-March.
They
range
in
intensity
and
periodicity
but
they
typically
pass
at
about
7-10
day
intervals.
These
weather
systems
dominate
the
annual
coastal
process
climate
and
produce
a
southerly
regional
longshore
sediment
transport
(Hine
et
al.,
1988;
Davis,
1989).
Hurricanes
are
not
frequent
along
the
Florida
Gulf
Coast
(Henry
et
al.,
1994)
but
they
have
played
a
major
role
in
the
formation
of
inlets
and
associated
tidal
deltas.
Although
no
hurricane
has
made
landfall
in
the
study
area
since
1921,
the
passage
of
these
storms
through
the
Gulf
of
Mex-
ico
has
influenced
coastal
processes
in
this
area.
Examples
are
hurricanes
Elena
and
Juan
(1985),
Opal
(1995),
and
Josephine
(1996).
Each
caused
significant
sediment
transport
and
coastal
change
along
the
barrier/inlet
system
of
the
study
area.
The
hurricanes
of
1848
and
1921,
while
not
docu-
mented
in
detail,
were
the
most
prominent
storms
of
record
along
the
study
area.
They
opened
tidal
inlets,
Johns
Pass
and
Hurricane
Pass,
which
have
persisted
to
the
present
time
(Mehta
et
al.,
1976;
Lynch-Blosse
and
Davis,
1977).
Mean
annual
wave
height
along
the
coast
of
the
barrier
system
is
only
30-40
cm
and
the
mean
period
is
4-5
s
(Tanner,
1960;
Davis
and
Andro-
naco,
1987).
Frontal
systems
during
the
winter
produce
breakers
of
1
m
or
more.
These
storms
also
generate
waves
that
move
through
the
surf
zone
at
angles
of
up
to
20°
with
the
shoreline
(Wang,
1995;
Wang
et
al.,
1998)
producing
long
-
shore
currents
that
reach
velocities
of
nearly
1
m/
s.
Such
conditions
cause
rapid
transport
of
sedi-
ment
through
the
surf
zone
but
the
bimodal
wind
and
wave
approach
directions
result
in
a
modest
annual
littoral
transport;
typically
less
than
50,000
m
3
(Mehta
et
al.,
1976).
2.
Study
areas
This
report
represents
a
synthesis
of
three
mas-
ters
theses
by
Cuffe
(1991),
Kowalski
(1995)
and
Shock
(1994)
at
fi
ve
different
inlets.
Three
fl
ood
-
tidal
deltas
and
three
ebb
-tidal
deltas
are
in-
cluded.
The
inlets
and
their
included
elements
are
Hurricane
Pass
(flood
and
ebb
deltas),
Johns
Pass
(flood
delta),
New
Pass
(ebb
delta),
Big
Sar-
asota
Pass
(ebb
delta),
and
Midnight
Pass
(flood
delta)
(Fig.
1).
Two
of
these
inlets
(Hurricane
Pass
and
Johns
Pass)
were
cut
by
hurricanes
dur-
ing
historical
time,
but
the
origin
and
age
of
the
other
tidal
inlets
are
unknown.
All
of
the
fl
ood
-tidal
deltas
are
multilobate.
Only
Johns
Pass
displays
distinct
channels
that
separate
individual
lobes
and
only
Hurricane
Pass
has
a
completely
subtidal
fl
ood
delta.
The
ebb
-tidal
deltas
range
in
configuration
but
all
are
significantly
wave
influenced
with
a
distinct
R.A.
Davis
Jr.
et
al.
I
Marine
Geology
200
(2003)
49-60
51
83°
82°
81°
W
28°
N
27°
N
26°
N
Anclote
Key
0
_HURRICANE
PASS
-
--71)0
JOHN'S
PASS
->
Tarpon
Springs
.1/
Clearwater
o
ff
,/
Tampa
Tr
WEST
-
CENTRAL
FLORIDA
GULF
COAST
Bradenton
NEW
PASS
>
Sarasota
BIG
SARASOTA
PASS
MIDNIGHT
PASS
Gulf
of
Mexico
STUDY
ARE
Punta
Gorda
•Fort
Myers
Naples
0
Cape
Romano
28°
N
27°
N
26°
N
83°
82°
81°
W
Fig.
1.
Location
map
of
study
area
along
the
west
-central
Gulf
Coast
of
the
Florida
Peninsula
showing
the
five
inlets
that
con-
tain
the
tidal
deltas
included
in
this
report.
asymmetry
in
the
direction
of
dominant
longshore
drift.
The
area
of
the
fl
ood
deltas
ranges
from
3.37
X
10
5
m
2
to
4.42
X
10
5
m
2
,
and
that
of
the
ebb
deltas
is
from
4.4k
X
10
5
m
2
to
31.05
X
10
5
m
2
(Table
1).
3.
Data
collection
and
analysis
The
primary
means
for
collecting
stratigraphic
data
from
these
tidal
deltas
is
through
vibracores.
The
technique
for
coring
and
recovery
followed
modifications
of
that
originally
described
by
La-
nesky
et
al.
(1979).
Most
of
the
coring
was
accom-
plished
utilizing
a
pontoon
coring
barge
similar
to
that
described
by
Stone
and
Morgan
(1992).
Core
recovery
ranged
from
less
than
1
m
to
more
than
5
m
in
length
(Table
2).
Coring
of
fl
ood
deltas
was
generally
more
efficient
than
on
ebb
deltas
because
of
the
tendency
for
waves
to
sort
and
pack
sediments
thereby
making
penetration
diffi-
cult
on
the
open
coast.
Flood
deltas
also
tend
to
have
more
mud
content
which
facilitates
core
pen-
etration.
This
coring
program
resulted
in
a
total
of
89
cores,
distributed
as
shown
on
Table
2.
Cores
were
split
and
visually
described
using
standard
techniques.
They
were
photographed
Table
1
Areas
of
each
tidal
delta
Tidal
Delta
Type
Area
(m
2
)
Hurricane
Pass
Hurricane
Pass
Johns
Pass
New
Pass
Big
Sarasota
Pass
Midnight
Pass
Flood
Ebb
Flood
Ebb
Ebb
Flood
4.42
X10
5
4.15
X10
5
3.37
X10
5
9.65
X10
5
31.05
X
10
5
4.13
X105
52
R.A.
Davis
Jr.
et
al.
I
Marine
Geology
200
(2003)
49-60
Table
2
Vibracores
collected
from
each
tidal
delta
Location
No.
of
Cores
Range
of
Length
(m)
Hurricane
Pass
(flood)
13
Hurricane
Pass
(ebb)
14
Johns
Pass
(flood)
18
New
Pass
(ebb)
9
Big
Sarasota
Pass
(ebb)
15
Midnight
Pass
(flood)
20
0.21-4.99
0.42-4.00
2.20-4.30
2.00-4.20
1.85-3.65
0.40-3.90
and
sampled
for
textural
analysis.
Lacquer/fiber-
glass
peels
were
made
of
several
cores
to
highlight
sedimentary
structures.
These
data
provided
the
basis
for
the
designation
of
the
lithofacies
de-
scribed
in
the
following
section.
4.
Lithofacies
The
tidal
delta
sequences
along
the
west
-central
Florida
coast
contain
four
lithofacies;
three
com-
pose
the
tidal
deltas
themselves
and
the
fourth
underlies
the
tidal
deltas
(Table
3).
The
three
fa-
cies
of
the
tidal
deltas
are
transitional
among
each
other
with
the
two
major
variable
elements
being
fi
ne
quartz
sand
and
shell
gravel
(SG).
4.1.
Muddy
shelly
sand
(MSS)
This
facies
has
up
to
25%
mud
and
20%
shell
mixed
with
fi
ne,
well
-sorted
quartz
sand.
Locally,
in
the
basal
portion
of
the
facies,
the
mud
in-
cludes
up
to
5%
particulate
organic
matter.
The
shell
is
dominantly
bivalves
and
consists
of
both
Table
3
Lithofacies
characteristics
fragments
and
whole
shells
which
are
randomly
arranged
(Fig.
2D).
Shell
material
decreases
up-
ward
in
the
facies
and
the
shell
material
near
the
top
is
abraded
and
bored;
not
as
fresh
in
appear-
ance
as
that
in
the
lower
portion
of
the
facies.
The
vast
majority
of
this
facies
is
characterized
by
bioturbated,
MSS.
Few
distinct
burrows
are
pre-
served
and
articulated
bivalves
are
present
but
uncommon.
Distinct
shell
beds
are
rare
due
to
prevalent
burrowing.
This
facies
is
found
beneath
some
combination
of
the
three
clean,
tidal
delta
facies
and
reaches
several
meters
in
thickness.
The
contact
with
the
overlying
mud
-free
facies
is
sharp
at
all
locations.
Where
this unit
is
completely
penetrated
the
underlying
unit
is
either
iron
-stained,
Pleistocene
sand
or
Miocene
Tampa
limestone.
4.2.
Shell
gravel
(SG)
Although
there
is
some
gradation
in
abundance
of
shell
in
this
facies
as
compared
to
the
SS,
the
nature
and
fabric
of
the
shell
component
is
dis-
tinctly
different.
This
facies
has
at
least
25%
shell
and
commonly
it
is
about
50%.
The
SG
is
com-
posed
primarily
of
whole
bivalve
shells
that
are
up
to
3cm
in
diameter.
The
shells
are
neither
sorted
nor
layered
as
viewed
in
the
cores
(Fig.
2C).
It
is
possible
that
because
of
their
size,
the
layering
has
been
destroyed
by
the
coring
process.
As
in
the
other
facies,
the
quartz
sand
is
fi
ne
and
well
-
sorted.
Maximum
thickness
of
this
facies
is
at
least
3.0m
and
it
is
confined
to
areas
adjacent
to
the
main
channel
of
ebb
-tidal
deltas.
Facies
Sediment
Biota
Structures
Quartz
Sand
(WSS)
Shelly
Qtz.
Sand
(SS)
Shell
gravel
(SG)
Muddy
Shelly
Qtz.
Sand
(MSS)
fine,
sorted
qtz.,
1-2%
fi
ne
Donax
variabilis,
Lucina
floridiana,
Tellina
sp.
shell,
rare
mud
fine,
sorted
qtz.
<25%
shell
grave
fine,
sorted
qtz.
>
25%
SG
fine,
sorted
qtz.
abundant
SG,
abundant
mud
D.
variabilis,
L.
floridiana,
Chione
cancellata,
Tellina
sp.
Anadara
trans
versa,
D.
variabilis,
C.
cancellata,
Tellina
sp.
Argopecten,
Bulla,
Lucina,
Tellina,
C.
cancellata
laminations,
rare
(lasers,
shell
laminae
laminations,
beds
of
shell
fragments
massive,
small
whole
shells
bioturbated,
some
distinct
burrows
R.A.
Davis
Jr.
et
al.
/Marine
Geology
200
(2003)
49-60
53
CM
10
1
.'e
'tt
k.
41
0
il
Fig.
2.
Photographs
showing
each
of
the
lithofacies
included
in
the
tidal
delta
stratigraphic
models;
oldest
to
youngest:
(D)
muddy
shelly
quartz
sand,
(C)
shelly
quartz
sand,
(B)
sandy
SG,
and
(A)
quartz
sand.
4.3.
Shelly
sand
(SS)
This
facies
is
characterized
by
up
to
25%
shell
mixed
with
fi
ne,
quartz
sand
(Fig.
2B).
There
is
shell
debris
scattered
throughout
the
unit
but
it
occurs
in
highest
concentrations
as
distinct
shell
layers.
Nearly
all
shell
is
bivalve
debris
and
is
less
than
1
cm
in
diameter.
The
shell
mode
itself
is
well
-sorted
and
shell
fragments
tend
to
be
well
-
imbricated.
There
is
more
shell
and
the
shell
frag-
ments
are
more
bedded
in
the
ebb
-tidal
deltas
as
compared
to
the
fl
ood
-tidal
deltas.
This
facies
is
up
to
2.0
m
thick
and
is
not
laterally
extensive.
4.4.
Quartz
sand
(WSS)
The
most
homogenous
of
the
lithofacies
is
the
quartz
sand.
It
is
composed
of
fi
ne,
well
-sorted
sand
(Fig.
2A)
that
typically
is
free
of
mud
and
shell
although
both
may
be
present
locally
in
54
R.A.
Davis
Jr.
et
al.
I
Marine
500m
Lido
Key
Outer
limit
i
of
ebb
delta
Control
point
t
r
lm
3m
3m
-
67
Siesta
Key
I
Fig.
3.
Isopach
map
showing
the
thickness
of
the
ebb
tidal
delta
at
Big
Sarasota
Pass.
small
amounts.
This
shell
component
is
present
in
distinct
layers
in
which
the
shell
fragments
are
less
than
1
cm
in
diameter.
Mud
may
be
present
as
scattered
fl
asers
or
as
burrow
linings
in
the
lower
Geology
200
(2003)
49-60
portion
of
the
facies.
It
is
more
abundant
in
fl
ood
deltas
than
in
ebb
deltas.
The
uniform
nature
of
the
sand
is
such
that
no
vertical
trends
in
grain
size
are
detectable.
The
appearance
of
the
sand
is
massive
to
plane
-lami-
nated
with
the
laminations
being
best
displayed
by
the
presence
of
minor
amounts
of
fi
ne
shell.
Thickness
of
this
facies
exceeds
5.0
m
in
the
ebb
delta
at
Big
Sarasota
Pass
(Fig.
3)
but
is
typically
less
than
2.5
m
thick
in
the
other
ebb
and
fl
ood
tidal
deltas
(Fig.
4).
This
facies
is
found
through-
out
both
fl
ood
and
ebb
deltas
and
is
the
most
widespread
of
the
tidal
delta
facies.
5.
Distribution
and
environmental
interpretation
of
facies
Coring
in
modern
tidal
deltas
provides
direct
information
on
the
environmental
significance
of
the
lithofacies
and
their
geographic
and
strati
-
graphic
distribution
(Figs.
5-10).
The
fact
that
two
of
these
inlets
were
formed
by
hurricanes
during
historical
time
further
permits
the
details
of
the
environmental
significance
of
the
tidal
delta
facies
to
be
documented.
Aerial
photographs
also
aided
with
these
interpretations.
This
was
partic-
ularly
beneficial
in
the
case
of
Hurricane
Pass
which
was
formed
in
1921
and
for
which
aerial
Honeymoon
Island
main
channel
1
0
250m
Contours
in
meters
Outer
limit
of
ebb
delta
Control
point
Fig.
4.
Isopach
map
showing
the
thickness
of
the
ebb
tidal
delta
at
Hurricane
Pass,
formed
in
1921.
R.A.
Davis
Jr.
et
al.
/Marine
Geology
200
(2003)
49-60
55
West
Caladesi
Id
Fast
HP
-
1
HP
-
3
HP
-
5
HP
-
8
S.L.
4
0
Quartz
Sand
Muddy
Shelly
Sand
Limestone
I I I I
0.5
1.0
km
2
3
Fig.
5.
Stratigraphic
cross
section
of
the
ebb
delta
at
Hurricane
Pass.
4m
photography
is
available
from
1926
to
the
spread
lithofacies
throughout
this
barrier
system
present.
(e.g.
Davis
and
Kuhn,
1985;
Evans
et
al.,
1985;
Gibbs
and
Davis,
1991;
Davis
et
al.,
1992).
It
is
at
5.1.
Muddy
shelly
quartz
sand
(
MSS)
least
a
few
meters
thick,
it
is
continuous
(Figs.
5-
10)
and
various
shells
in
it
have
been
dated
at
4-6
This
basal
Holocene
facies
is
the
most
wide-
kyr
BP
(Gregory,
1984;
Davis
and
Kuhn,
1985;
Southwest
Northeast
S.L.
I
I I I
NP
-
6
NP
-
2
NP
-
5
Shell
Sand
Quartz
Sand
X
Shell
0
Gravel
2
—3
Limestone
4m
i
I
I
4m
0
0.5
1.0
1.5
km
Fig.
6.
Stratigraphic
cross
section
of
the
ebb
delta
at
New
Pass.
56
R.A.
Davis
Jr.
et
al.
I
Marine
Geology
200
(2003)
49-60
West
S.L.
BR:
-
8
BSP
-
2
BSP
-
7
East
Swash
Platform
Quartz
Sand
So'
:
-
'•-•••••—•,—"^
"'J
r ' .
Shell
Gravel
e .
0
A
"
V
V
Main
Ebb
Channel
A
0.5
-
2
1.0
km
1.0
km
Fig.
7.
Stratigraphic
cross
section
of
the
ebb
delta
at
Big
Sarasota
Pass.
Gibbs
and
Davis,
1991),
placing
it
about
mid
-Ho-
locene
in
age.
The
basal
part
of
the
unit
contains,
at
most,
a
few
marine
shells
and
has
a
relatively
high
content
of
organic
matter
including
recognizable
plant
fragments.
This
is
interpreted
to
represent
a
par
-
North
0
2
3
4m
HP
30
S.L.
HP
21
HP
15
alic
vegetated
environment
similar
to
the
com-
bined
salt
marshes
and
mangrove
mangal
habitats
that
inhabit
some
of
the
study
area
at
the
present
time.
All
of
the
characteristics
of
this
facies
suggest
a
protected,
low
-energy
environment
similar
to
the
HP
16
HP
-
31
South
•:
.:•.
Caladesi
,...
......
..•
.
Id
.-.-......
'-'!•-•••••-..-..,....-.-.,..:••
••.-
-
.
.
,
.
.
.
'
-•
,
:
„.................;„
.-.....-
.
........-......y..7...
:
...,
.....
Sand
Muddy
....---
Quartz
Sand
Limestone
0
0.5
I I
1.0
km
Fig.
8.
Stratigraphic
cross
section
of
the
flood
delta
at
Hurricane
Pass.
0
-
2
-
3
4m
R.A.
Davis
Jr.
et
al.
/Marine
Geology
200
(2003)
49-60
57
present
back
-barrier
environment
along
this
coast.
Although
the
radiometric
dates
obtained
for
this
unit
are
much
older
than
known
ages
for
the
bar-
riers
(Stapor
et
al.,
1988),
it
is
possible
that
older,
more
Gulfward
barriers
were
present
at
this
time
or
that
the
facies
represents
a
low
-energy
open
coast
such
as
now
exists
to
the
north
of
the
study
area.
New
information
on
radiocarbon
ages
for
beach
and
backbarrier
deposits
at
Siesta
and
Casey
Keys
(Fig.
1)
indicate
that
older
barriers
were
present
in
that
area
(Spurgeon
et
al.,
2003;
Davis
et
al.,
2003).
5.2.
Shell
gravel
(SG)
This
facies
is
the
most
geographically
restricted
of
any
of
the
lithofacies
present
in
the
tidal
deltas
(Figs.
5-7).
It
may
be
at
least
3m
thick
and
it
decreases
in
thickness
away
from
the
ebb
channel.
It
is
present
in
both
New
Pass
and
Big
Satasota
Pass
but
was
not
recovered
in
any
of
the
cores
from
Hurricane
Pass
(Fig.
1).
There
is
a
thin
layer
of
this
facies
on
the
fl
oor
of
one
of
the
channels
in
the
Johns
Pass
fl
ood
delta
(Fig.
9).
The
nature
and
location
of
this
facies
within
the
ebb-tidal
deltas
indicate
that
it
is
a
channel
-mar-
gin
lag
deposit,
primarily
on
the
ebb
deltas.
The
concentration
of
large
pieces
and
whole
shells,
the
thickness
and
the
presence
of
similar
deposits
along
the
sides
and
fl
oor
of
the
present
channels
supports
this
interpretation.
These
shell
concen-
trations
are
the
result
of
tidal
currents
that
re-
move
the
sand
and
fi
ne
shell
thereby
concentrat-
ing
the
larger
shell
material.
5.3.
Shelly
quartz
sand
(SS)
The
shelly
quartz
sand
facies
is
present
on
both
fl
ood
and
ebb
deltas
but
is
most
prominent
in
ebb
deltas
(Figs.
6-10).
The
distribution
is
fairly
wide-
spread
but
shows
affinity
for
the
small
fl
ood
-dom-
inated
channels
on
ebb
deltas
such
as
in
New
Pass
and
Big
Sarasota
Pass
(Figs.
6
and
7).
It
is
also
present
on
the
fl
ood
deltas
(Figs.
8-10).
The
en-
vironment
of
deposition
combines
wave
-generated
currents
with
shallow
sandy
areas
causing
plane
bed
conditions
that
produce
isolated
shell
layers.
These
shell
lag
concentrations
are
associated
with
North
SP
-
N
JP
-
12
4m
S.L.
South
JP
-
18
JP
-
5
JP
-
13
an
Muddy
Quartz
Sand
Clayey
Sand
Shell
Gravel
0
0.5
km
4m
Fig.
9.
Stratigraphic
cross
section
of
the
fl
ood
delta
at
Johns
Pass.
modest
tidal
currents
in
the
small
channels.
These
channels
may
be
the
fl
ood
-dominated
channels
of
the
ebb
deltas
or
the
numerous
channels
of
the
shallow
subtidal
fl
ood
deltas
such
as
at
Hurricane
Pass.
North
South
MP
-
9
MP
-
8
MP
13
M1'
-
20
11
,
2
-
19
MP-18
Limestone
Peat
Quartz
Sand
Muddy
Quartz
Sand
Shell
-
o
'Gravel
0
-
2
3
m
0.5
1.0
km
Fig.
10.
Stratigraphic
cross
section
of
the
flood
delta
at
Mid-
night
Pass.
58
R.A.
Davis
Jr.
et
al.
/Marine
Geology
200
(2003)
49-60
A
Ebb
Tidal
Delta
m
0
Marginal
Flood
Channel
Channel
-
Margin
Linear
Bar
1
2
Back
-
Barrier
Facies
3
Fig.
11.
Stratigraphic
model
of
(A)
an
ebb
tidal
delta,
and
(B)
a
fl
ood
delta.
The
ebb
delta
section
shows
relatively
high
concen-
trations
of
shell
beds
and
mud
is
nearly
absent
as
compared
to
the
fl
ood
delta
section.
Main
Ebb
Channel
B
Flood
Tidal
Delta
Channel
Facies
Flood
Lobe
Facies
Back
-
Barrier
Facies
m
0
1
2
.1a
5.4.
Quartz
sand
(WSS)
The
quartz
sand
facies
is
typical
of
both
fl
ood
and
ebb
deltas
and
is
the
most
widespread
of
the
three,
clean
tidal
-delta
facies.
It
is
commonly
1-2
m
thick
and
continuous
on
individual
tidal
deltas
but
discontinuous
on
the
Johns
Pass
fl
ood
delta
where
channels
cut
below
its
base
(Fig.
9).
This
facies
does,
however,
display
some
differences
in
its
character
between
ebb
deltas
and
fl
ood
deltas.
The
ebb
deltas
have
no
mud
and
shell
material
is
rare.
By
contrast,
this
facies
may
have
mud
on
the
fl
ood
delta
and
shells
are
generally
more
common
than
on
the
ebb
deltas.
This
facies
represents
the
wave
-dominated
conditions
of
the
ebb
delta,
and
the
combined
wave
and
tidal
conditions
of
the
fl
ood
delta.
Physical
energy
levels
on
the
ebb
delta
are
higher
than
on
the
fl
ood
delta
as
reflected
in
the
presence
of
mud
and
the
fact
that
there
are
some
sea
grass
stands
and
mangrove
mangals
on
the
fl
ood
deltas
(Fig.
11).
6.
Stratigraphic
models
Although
there
are
many
similarities
in
the
dis-
tribution
and
character
of
the
four
lithofacies
de-
scribed
above,
there
are
enough
subtle
differences
to
justify
separate
stratigraphic
models
for
the
fl
ood
and
ebb
tidal
deltas.
The
differences
in
geo-
morphology
and
in
the
dominant
physical
pro-
cesses
(waves
or
tidal
currents)
produce
distinctly
R.A.
Davis
Jr.
et
al.
/Marine
Geology
200
(2003)
49-60
59
different
stratigraphy
in
the
ebb
and
the
fl
ood
deltas.
6.1.
Ebb
delta
All
four
lithofacies
are
typically
present
in
the
ebb
tidal
delta
model.
The
basal
unit
is
the
MSS
facies
that
is
the
pre
-tidal
delta
unit.
The
contact
at
the
base
of
the
tidal
delta
sequence
is
typically
sharp
and
typified
by
a
shelly
concentration.
Although
there
is
no
physical
evidence
for
scour
at
the
base
of
the
unit,
the
nature
of
the
condi-
tions
at
initiation
of
storm
-generated
tidal
deltas
and
the
presence
of
the
shell
concentration
sug-
gest
such
an
origin
for
the
contact.
Most
of
the
volume
of
the
ebb
delta
is
com-
posed
of
the
quartz
sand
facies
that
represents
the
wave
-dominated
portion
of
the
sediment
body.
Gravel
size
shell
debris
is
rare
and
mud
is
absent
as
a
result
of
these
energetic
conditions.
The
SG
facies
represents
main
ebb
channel
con-
ditions
and
may
be
as
thick
as
the
entire
sequence
(Fig.
11).
It
may
even
exceed
the
combined
thick-
ness
of
the
other
facies
if
the
ebb
channel
is
deeply
incised.
The
overall
thickness
of
the
ebb
delta
se-
quence
is
typically
only
a
few
meters
but
it
can
range
up
to
at
least
6
meters.
The
shelly
quartz
sand
facies
is
typically
present
near
the
upper
part
of
the
sequence
representing
marginal
fl
ood
chan-
nels.
Thickness
is
typically
less
than
1
m
but
it
may
be
completely
absent
in
some
ebb
delta
se-
quences.
These
ebb
delta
sequences
contrast
to
those
de-
scribed
from
the
mesotidal
South
Carolina
coast
(Imperato
et
al.,
1988)
where
mud
is
present
and
shells
are
not
a common
constituent.
6.2.
Flood
delta
The
fl
ood
delta
sequence
contains
at
least
two
or
three
of
the
lithofacies
discussed
earlier;
it
is
rare
that
all
four
are
present.
The
thickness
of
the
fl
ood
delta
is
typically
only
a
few
meters.
There
may
be
multiple
lobes
and
multiple
periods
of
activity
shown
in
the
stratigraphy
of
this
sediment
body
complex.
The
SG
facies
is
typically
absent
because
physical
energy
in
the
channels
is
not
high
enough
to
concentrate
large
shells.
The
base
of
the
sequence
is
the
muddy,
shelly
quartz
sand
on
which
rests
the
fl
ood
delta
itself.
The
sharp
contact
is
similar
to
that
of
the
ebb
delta
and
probably
represents
scour
as
the
over-
lying
fl
ood
delta
sediments
moved
over
the
back
-
barrier
environment.
Most
of
the
fl
ood
delta
se-
quence
is
comprised
of
the
quartz
sand
facies
with
some
of
the
shelly
quartz
sand
that
represents
the
small
channel
areas
where
concentrations
of
shell
debris
are
common.
There
is
noticeable
mud
in
several
places
in
the
form
of
fl
asers
and
burrow
linings.
The
presence
of
these
on
the
fl
ood
delta
reflects
the
lower
energy
conditions.
This
is
true
of
both
the
waves
that
are
only
those
developed
in
the
fetch
-limited,
back
-barrier
area,
but
also
the
tidal
currents
that
are
more
sluggish
on
the
fl
ood
delta
than
in
the
main
ebb
channel
of
the
ebb
delta
(Hayes,
1975,
1980).
7.
Summary
These
simple
but
useful
stratigraphic
models
are
important
tools
for
the
overall
management
of
barrier
-inlet
systems
in
that
they
provide
us
with
information
on
the
expected
thickness
and
stratigraphy
of
the
sand
bodies.
Such
information
is
important
in
considering
burrow
areas
for
nourishment,
dredging
for
navigational
purposes
and
environmental
characteristics
related
to
vari-
ous
plant
and
animal
communities
and
landfall
of
oil
spills.
The
models
also
provide
a
useful
tool
to
enable
researchers
investigating
ancient
coastal
deposi-
tional
sequences
to
make
decisions
about
environ-
ments
of
deposition,
especially
when
comparing
fl
ood
tidal
deltas
with
washover
deposits.
The
reader
will
fi
nd
it
very
helpful
to
compare
the
fl
ood
delta
model
with
the
model
presented
in
the
paper
on
the
stratigraphy
of
washover
depos-
its
presented
in
a
companion
paper
(Sedgwick
and
Davis,
2003).
Acknowledgements
The
research
summarized
for
this paper
was
funded
in
part
by
the
Pinellas
County
Commis-
60
R.A.
Davis
Jr.
et
al.
/Marine
Geology
200
(2003)
49-60
sion,
the
Florida
Department
of
Environmental
Protection,
the
Florida
Sea
Grant
Program,
and
the
cooperative
program
between
the
U.S.
Geo-
logical
Survey
and
the
University
of
South
Flor-
ida.
Numerous
people
from
the
Coastal
Research
Laboratory
assisted
in
the
fi
eld
work
including
G.
Creaser,
M.
FitzGerald,
A.
Gibbs,
T.
Griggs,
R.
Haney,
D.
Inglin,
J.
Pekala,
P.
Sedgwick,
B.
Sil-
verman,
and
P.
Wang.
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