LATE QUATERNARY STRATIGRAPHIC EVOLUTION, ALABAMA AND WEST FLORIDA OUTER CONTINENTAL SHELF
Copyright © 2012, Society for Sedimentary Geology (SEPM)
43
LATE QUATERNARY STRATIGRAPHIC EVOLUTION OF THE
ALABAMA AND WEST FLORIDA OUTER CONTINENTAL SHELF
PHILIP J. BART
Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A.
AND
JOHN B. ANDERSON
Department of Geology and Geophysics, Rice University, 6100 South Main, Houston, Texas 77005, U.S.A.
ABSTRACT: Approximately 3000 km of single-channel seismic data from the Alabama and west Florida outer continental shelf and upper
slope were analyzed to characterize the late Quaternary regional stratigraphic framework of this ramp margin. Seismic analysis shows that
thick delta lobes are located at several near-surface stratigraphic levels on the outer continental shelf. On the basis of the depocenter
locations, we infer that sediments delivered to the deltaic wedges on the Alabama shelf were derived from the confluence of the ancestral
Mobile–Tombigbee rivers. The smaller deltaic wedges on the west Florida outer continental shelf probably received sediment from the
confluence of the Perdido, Escambia, Blackwater, and Yellow rivers. Deep, incised, cross-shelf fluvial valleys do not exist offshore of west
Florida. There, outer-shelf depocenters probably received sediment by broad and shallow braided rivers. This situation is in stark contrast
to the Texas, Louisiana, and Mississippi continental shelves, where large incised fluvial valleys occupied the shelf during eustatic
lowstands.
Subsurface mapping shows that the sinuous trend of the Alabama and west Florida shelf edge is a result of the primary delta morphology.
We interpret the shelf-margin deltas as lowstand systems, and on the basis of our seismic correlation to chronologic control at a drill site in
Main Pass lease area 303, we conclude that the youngest shelf-margin deltas were deposited during the last glacial maximum. The lack of slope
canyons indicates that bypass was minimal in this area during the last glacial maximum. In a basinward direction, the clinoform toes and/
or aggrading bottomsets of the lowstand deltaic units interfinger with thin but regionally extensive slope wedges. The seismic evidence of
extensive erosion of the shelf-margin deltas suggests that slope wedges in this area may correspond to deposition during the last transgression
and present highstand. The youngest seismic-stratigraphic unit is shelf perched and has regional extent and great thickness. We surmise that
this shelf-perched unit represents a drowned coastal-plain system that was well established early during the last sea-level transgression.
INTRODUCTION
The purpose of this paper is to present preliminary results
from a study of the late Quaternary sequence stratigraphy of the
Alabama and west Florida margin (Fig. 1). Our results primarily
concern the seismic-stratigraphic units deposited on the outer
continental shelf and upper slope by the Perdido, Escambia,
Blackwater, and Yellow rivers during the last glacial cycle. This
investigation is part of a larger study aimed at characterizing the
natural lateral variability of late Quaternary stratigraphic sequences on the northern Gulf of Mexico margin. In contrast to the
late Quaternary shelf–slope margins of Mexico, Texas, Louisiana,
and Mississippi, the northeastern Gulf of Mexico margins have
an overall ramp-type geometry. In addition, the near-surface
stratigraphy is undisturbed by faulting (Martin, 1978), and although some diapiric salt is located in the vicinity of De Soto
Canyon (Harbinson, 1967), the Alabama and west Florida outer
continental shelves are not underlain by thick salt basins (Martin,
1978; Stude, 1978; Jackson and Galloway, 1984; Ingram, 1991).
Taken together, these factors make the northeastern Gulf of
Mexico an end-member setting quite different from the salt/
growth-fault dominated Texas, Louisiana, and Mississippi margins. This description of the late Quaternary sequence stratigraphy of the Alabama and west Florida margin is intended to form
the basis for comparisons with the shelf–slope stratigraphic results from studies of the northwestern and north-central Gulf of
Mexico presented elsewhere in this volume.
METHODOLOGY
Correlating late Quaternary depositional sequences on the
northern Gulf of Mexico margins requires (1) data grids dense
enough to enable mapping of individual units, (2) grids with
regional extent sufficient to identify along-strike shifts in
depocenter locations, and (3) seismic data with sufficient stratigraphic resolution to image bounding surfaces and relatively
thin units (Anderson et al., 1996). In this study, we evaluated a
broad sector of the Alabama and western Florida outer continental shelf and upper slope using 3000 km of high-resolution
seismic data that were collected from the R/V Lone Star during
1994, 1995, and 1996 field seasons (Fig. 1). The spacing between
dip- and strike-oriented seismic profiles averages 20 km. Experience has shown us that this line spacing is adequate to map the
smallest late Quaternary seismic units of interest in this study.
The seismic sources were a 15 in3 watergun and a boomer, and the
seismic data were recorded with a single-channel streamer. The
boomer source provided detailed imaging of the uppermost
section, but because the depth of signal penetration was too poor
to image even the basal reflector of the near-seafloor surface unit,
the use of the boomer source was abandoned early during the first
field campaign.
The analog seismic data were plotted on an EPC printer, and
the digital seismic data were recorded and processed using a
Delph system. The filter cutoffs were set at 30 and 800 Hz. The
dominant frequency of the seismic data was between 130 to 200
Hz, providing a theoretical stratigraphic resolution of 2.5 to 4 m,
on the basis of Rayleigh resolution limit criteria and an average
sediment velocity equal to 1.5 km/s. Seismic data processing
included scaling and standard band-pass filtering to improve the
signal-to-noise ratio.
Seismic interpretations were made from shipboard EPC and
processed profiles plotted at a vertical exaggeration of 35:1. Our
seismic-stratigraphic analysis indicates that regional (several
tens of kilometers) reflectors bound prograding-wedge strata. On
Late Quaternary Stratigraphic Evolution of the Northern Gulf of Mexico Margin
SEPM Special Publication No. 79, Copyright © 2004
SEPM (Society for Sedimentary Geology), ISBN 1-56576-088-3, p. 43–53.
44
PHILIP J. BART AND JOHN B. ANDERSON
Pensacola
Bay
Mobile
Bay
Choctawathcee
Bay
20
0
25
kilometers
Fi
g.
20
6
40
30°
60
80
g.
Fi
10
5
0
Fi
40
g.
Fi
g.
4
2
20
g.
3
0
29.5°
Fi
60
LEGEND
80
500
10
0
00
10
De Soto
Canyon
1500
88°
bathymetric
contours (m)
seismic grid
seismic profiles
referred to in text
87°
FIG. 1.—Study area location map showing drainage basins, surface geology, bathymetry, and seismic grid.
the basis of these bounding discontinuities, the near-surface
stratigraphy was subdivided into seven units numbered from the
top down; for example, the youngest unit described is Unit 1.
Two-way travel times reported from the seismic data are approximate. The travel times were converted to depth using a
velocity of 1500 m/s.
There has not been much drilling on the Alabama and west
Florida outer continental shelf, and thus chronologic constraints for our seismic observations are poor. Our seismic
interpretation of the base of Pleistocene is based on seismic
correlation to Unconformity A defined on the West Florida
Slope by Mitchum (1978). Late Quaternary chronological control was provided by seismic correlation to a drill site in the
Main Pass lease area 303, which is 80 km to the west of the
study area. At this outer-continental-shelf drill site, Sydow
and Roberts (1994) defined surface 10 and the overlying pro 10
Lagniappe deltaic sequence. They surmised that surface 10 is
equivalent to the oxygen-isotope stage (OIS) 3 maximum flooding surface. Our seismic correlation of surface 10 to the area of
our study is based on three strike-oriented profiles from the
outer shelf. The correlation shows that the pro 10 Lagniappe
delta wedge pinches out towards the east, and thus the underlying surface, surface 10, is amalgamated with the tops of two
older delta lobe complexes (pro 20 of the Lagniappe delta, and
the Western Delta of Sager et al., 1999). This situation greatly
complicates the seismic correlation of surface 10 farther to the
east. We project surface 10 from the tops of pro 20 (Sydow and
Roberts, 1994) and the Western Delta (Sager et al., 1999) to the
base of a third shelf-margin delta that defines the shelf edge at
the western edge of our study area. This shelf-margin lobe, our
unit 2, is the Eastern Delta of Sager et al. (1999), and we believe
it to be chronostratigraphically equivalent to the Lagniappe
delta pro 10 lobe. Inasmuch as our correlation to the Main Pass
303 drill site is speculative, we show surface 10 on only one of
the seismic profiles (Fig. 2). The chronostratigraphic relationships we infer are summarized in Table 1. We acknowledge
that further chronologic control is needed to confirm our
interpretations.
BACKGROUND
The Alabama and West Florida Ramp Margin
The rivers entering this sector of the northeastern Gulf of
Mexico are the Mobile–Tombigbee rivers, which drain an area of
100,000 km2, and four small coastal-plain rivers (Perdido,
Escambia, Blackwater, and Yellow rivers), which drain a combined area of 20,000 km2. To the west, the Pascagoula River enters
the Mississippi coast, and to the east, several small rivers enter
into Choctawhatchee Bay. Offshore Alabama and west Florida,
the shelf edge is roughly oriented southwest–northeast. The most
striking bathymetric features are the narrow width and low
gradient of the margin. The continental shelf narrows in a west–
east direction from 100 km to 35 km (Fig. 1). Offshore Pensacola,
the average gradient of the outer continental shelf is 6 m/km. The
shelf edge depth ranges from 80 to 120 m. There are a few canyon-
375
225
150
75
225
150
525
450
375
38
onlapping
wedges
4
r-botto
m multi
ple
w at e
water depth (m)
28
shelf edge
unit 7
18
paleo-shelf edges
unit 6
unit 2
0
0
wa
t er
26
shelf edge
kilometers
unit 1
1
unit 2
NW
unit 6
V.E. = 32:1
kilometers
10
surface 10
0.2
0.1
0.7
0.6
0.5
two-way travel time (s)
bo
tto
m
3
16
V.E. = 10:1
unit 7
5
0.2
0.1
0.6
0.5
FIG. 2.—Interpreted line drawing and dip-oriented seismic segment of Profile 5 showing prograding wedges 2, 6, and 7, onlapping wedges on the upper slope, and shelfperched Unit 1 on the outer shelf.
water depth (m)
94-1
ul t
ipl
e
m
two-way travel time (s)
75
SE
LATE QUATERNARY STRATIGRAPHIC EVOLUTION, ALABAMA AND WEST FLORIDA OUTER CONTINENTAL SHELF
45
46
PHILIP J. BART AND JOHN B. ANDERSON
TABLE 1.—Stratigraphic relationships inferred from seismic correlations.
1
Sydow and Roberts (1994)1
Units 1 and 2
Sager et al. (1999)
Units 3 and 4 (pro 10)
Units 5 to 7 (pro 20)
Eastern delta
Western delta
Bart and Anderson, this study
Unit 1
Unit 2
Unit 3 (?)
Units listed were defined at the Main Pass 303 drill site.
like indentations in the trend of the shelf edge, but no contiguous
pathways connect these features to De Soto Canyon, the head of
which is at 450 m water depth (Fig. 1). Offshore west Florida, the
uppermost slope (i.e., from 100 m to 200 m water depth) dips to
the southeast and has a gradient of 1° to 3°. Beyond the toe of the
upper slope, the seafloor dips to the southwest at a gentle angle
(i.e., << 1°). Taken together, these factors demonstrate that the
Alabama and west Florida margin has a ramp geometry, which is
morphologically quite different from the Mississippi, Louisiana,
and Texas shelf–slope margins to the west.
RESULTS
Regional Stratigraphy on the
Alabama and West Florida Outer Shelf
Interpreted Seismic Profiles.—
In this section, we show five seismic profiles (Figs. 2–6) to
illustrate the general aspects of the near-surface stratigraphic
framework. Line drawings of the seismic profiles are shown
along with segments of the seismic data. The locations of the
cross-lines are indicated at the top of the line-drawing interpretations.
Four dip-oriented profiles (Figs. 2, 4, 5, 6) show the overall
topset–foreset stratal geometries of the prograding wedges at
the outer continental shelf. Figure 2 is from the western end of
the study area, and the other dip-oriented profiles are towards
the eastern end of the study area (Fig. 1). Figure 3 is a regional
east–west strike line that crosses all of the four dip-oriented
profiles on the outer shelf. The strike-oriented profile shows the
point-source character of the individual prograding wedges
and their stratal relationships along the strike of the outer
continental shelf.
Prograding–Wedge Unit 2 Offshore Mobile, Alabama.—
Figure 2 shows prograding-wedge Unit 2 at the shelf margin
offshore Mobile Bay, Alabama. This unit downlaps a surface that
we infer is correlative to surface 10, i.e., the OIS 3 maximum
flooding surface from the Main Pass 303 drill site. At this location,
Unit 2 has a maximum thickness of 200 milliseconds (ms). The
offlap break elevation of Unit 2 is at 115 ms (86 m) below sea level.
The prograding-wedge foresets either toplap or terminate near
the seafloor on the outer shelf. The profile also shows the Unit 6
and Unit 7 prograding wedges, which are found across the study
area. At this location, the offlap breaks for Units 6 and 7 are at 165
ms and 221 ms, respectively. The upper bounding surfaces dip
slightly in an offshore direction, and the top of the Unit 2
prograding wedges defines the seafloor on the outer shelf and
upper slope. On the middle shelf, a shelf-perched unit, 30 to 40 ms
thick, Unit 1, downlaps and buries Unit 2. Faint discontinuous
reflectors within Unit 1 suggest low-angle progradation to the
south and southeast. At its basinward edge, Unit 1 has an irregu-
lar taper before it pinches out at a depth of 105 ms (79 m). The
landward edge of Unit 1 extends beyond the limits of the study
area. On the upper slope, wedges of strata onlap the foresets of the
prograding wedges.
Regional Strike-Line Correlations at the Outer Shelf.—
Figure 3 shows a regional west–east strike line and at the
western end (left-hand side of Fig. 3) shows the Unit 2 strata seen
on dip-oriented profile of Figure 2. In strike view, Unit 2 has a
component of progradation to the east. This wedge is correlative
to the eastern delta of Sager et al. (1999). Seismic correlation of the
Unit 2 clinoforms and aggrading bottomsets along strike to the
east shows that the unit is stratigraphically higher than the cluster
of lens-shaped units, Units 3, 4, and 5. The maximum thickness of
Units 3, 4, and 5 is 150 ms. At this location, the internal reflectors
of these units are poorly imaged, but the faint reflections and the
overall geometry of the units suggest that the units represent
individual progradational centers with bidirectional downlap to
the west and east. Regional correlations suggest that the small
depocenter labeled Unit 2 on the far eastern end (right-hand side)
of Figure 3 is stratigraphically younger than Units 3, 4, and 5 and
probably is correlative to the larger Unit 2 depocenter at the
western end (left-hand side) of Figure 3. The offset stacking of
these units (Units 2, 3, 4, and 5) define the shelf edge offshore
Pensacola, Florida.
Dip-Oriented Profiles Offshore Pensacola, Florida.—
Figure 4 shows a dip profile that crosses the shelf margin at the
location of a broad embayment in the shelf margin (see Fig. 1). At
this location, the Unit 4 prograding-wedge strata are near the
seafloor. Isolated patchy amplitudes are seen near the Unit 4
topset surface, but topset development is minimal. Unit 4 has a
maximum thickness of 80 ms and exhibits well-defined
progradation. The Unit 6 and Unit 7 prograding-wedge bounding surfaces are apparent, but the internal stratigraphy is poorly
imaged. A very thin section of Unit 2 and Unit 3, and a relatively
thick section of Unit 1, cap the Unit 4 prograding-wedge strata.
Unit 1 pinches out at 100 ms (75 m) below sea level. The top of Unit
1 also shows a well-developed sand-ridge field occurring in a
water depth of 45 ms (34 m). There is a 15-m-high mounded
feature above the Unit 4 shelf edge in a water depth of 118 ms (89
m). Similar mounds are found over the topsets of the other shelfmargin prograding wedges but were not observed on Unit 1. On
the upper slope, onlapping-wedge strata bury the aggrading
bottomsets of the prograding wedges.
The Figure 5 profile crosses the margin where prograding
wedges of Units 1, 3, and 5 are vertically stacked on the outer
continental shelf. Prograding-wedge Unit 5 defines the shelf
edge at this crossing. Regional seismic correlation illustrates
that prograding-wedge Unit 3 defines the shelf edge to the
west of this location. Unit 5 has a maximum thickness of 240
ms, and Unit 3 has a maximum thickness of 180 ms. Stratal
300
SW
0
ter
wa
unit 4
10
unit 2
15
m
otto
unit 7
unit 6
tom mul tiple
5
kilometers
water- bot
52
A
13
14
30
multip le
8
onlapping
wedges
shelf edge
B
unit 4
unit 6
unit 5
unit 3
11
unit 6
32
51
unit 5
0
unit 3
shelf edge
7
unit 7
17
unit 2
48
47
unit 2
V.E. = 25:1
22
NE
0.3
0.2
0.1
kilometers
ipl e
0.1
0.4
5
V.E. = 25:1
unit 2
V.E. = 44:1
ult
ttom m
water-bottom multiple
49
unit 2
21
bo
terwa
9
FIG. 3.—Interpreted line drawing and regional strike-oriented seismic segment of Profile 18, showing the lateral pinchout of prograding wedge 2 at the outer shelf in the
western and eastern parts of the study area, and older prograding wedges offshore Pensacola.
B
75
0
water depth (m)
53
b
two-way travel time (s)
A
LATE QUATERNARY STRATIGRAPHIC EVOLUTION, ALABAMA AND WEST FLORIDA OUTER CONTINENTAL SHELF
47
75
225
150
450
375
water depth (m)
wa t e r
38
b otto m
4
wa
dges
g we
onlappin
unconformity A
ple
multi
28
m
tto
bo
r
te
onlapping wedges
31
3
ltip
mu
le
unit 4
30
0
unit 4
possible
bioherm
0
18
possible bioherm
unit 6
94-1
unit 6
V.E. = 32 : 1
kilometers
10
SRF
units 2 and 3
kilometers
isolated patchy
amplitudes
unit 6
unit 1
NW
unit 1
unit 7
1
0.2
0.1
5
V.E. = 16:1
0.6
0.5
0.2
0.1
0.4
FIG. 4.—Interpreted line drawing and seismic segment of Profile 14 showing the depocenter of Unit 4 prograding wedge strata capped by a thin flank of the Units 3 and
5 prograding-wedge strata. The upper surface of Unit 1 is locally reworked into a broad sandy-ridge field (SRF). Carbonate banks (bioherms) occur at the shelf margin
above the Unit 4 prograding-wedge strata.
150
water depth (m)
two-way travel time (s)
75
SE
two-way travel time (s)
48
PHILIP J. BART AND JOHN B. ANDERSON
225
150
75
225
unconformity A
375
onlapping
wedges
28
unconformity A
mu
ltip
le
wa
t
e
r-b
ott
om
150
water depth (m)
3
unit 3
31
ter
wa
m
tto
bo
29, 18
e
43
ipl
lt
mu
unit 5
unit 5
0
0
V.E. = 32: 1
kilometers
10
NW
kilometers
45
26
0.1
unit 3
0.5
V.E. = 21:1
5
0.2
0.1
0.5
0.4
FIG. 5.—Interpreted line drawing and seismic segment of Profile 11 collected along the axis of the Units 3 and 5 prograding wedges. Unit 3 forms a terrace at this cross
section but forms the shelf edge to the west. On the slope, the foresets and topsets of unit 5 and 3 are truncated. Unit 1 forms a terrace on the middle shelf.
water depth (m)
two-way travel time (s)
4
base-of-slope
channel
unit 1
two-way travel time (s)
75
SE
LATE QUATERNARY STRATIGRAPHIC EVOLUTION, ALABAMA AND WEST FLORIDA OUTER CONTINENTAL SHELF
49
unit 2
18
ple
multi
bottom
water
43
3
unit 1
kilometers
0
unit 6
V.E. = 63: 1
10
unit 1
150
kilometers
5
NW
V.E. = 32: 1
unit 7
0.2
0.1
0
94-1
75
unconformity A
unit 2
26
0.2
0.1
0
FIG. 6.—Interpreted line drawing and seismic segment of Profile 47 showing the eastern depocenter of the Unit 2 prograding wedge. A shelf-perched unit, Unit 1, caps
the prograding wedge at the outer shelf.
water depth (m)
150
75
45
water depth (m)
SE
two-way travel time (s)
1
two-way
travel time (s)
50
PHILIP J. BART AND JOHN B. ANDERSON
LATE QUATERNARY STRATIGRAPHIC EVOLUTION, ALABAMA AND WEST FLORIDA OUTER CONTINENTAL SHELF
terminations of the Unit 5 and Unit 3 strata and the underlying
strata are seen at the seafloor. The channel at the base of the
Unit 5 prograding wedge is strike oriented and dies out laterally. It does not extend up the foreset slope to the shelf edge or
downslope to the De Soto Canyon.
The profile in Figure 6 shows the stacking of the Unit 2
prograding-wedge strata in the eastern part of the study area. At
this location, Unit 2 has a maximum thickness of 100 ms. The
Unit 2 offlap break is at an elevation of 100 ms (75 m) below sea
level. On the outer shelf, the shelf-perched unit, Unit 1, overlies
the Unit 2 prograding wedge, and pinches out at a water depth
of 80 ms (60 m).
SEISMIC-STRATIGRAPHIC EVOLUTION
AND FRAMEWORK
The regional seismic-stratigraphic analysis illustrates that
discrete prograding wedges occur on the Alabama and west
Florida outer continental shelf and define its shelf-edge morphology. Subsurface correlations show that the wedges are
located at distinctly different stratigraphic levels. Given the
positions of these prograding wedges at the shelf margin and
51
the seaward dip of the topset that forms the upper bounding
surfaces, we infer that these units were deposited at or shortly
after the culminations of the past several sea-level falls. The
distinct lobate shapes of the prograding wedges in strike view
(Fig. 3) suggest that these units are fluvial–deltaic in origin. On
the basis of its location in the western part of the study area, and
the position of an incised valley in an updip position on the
inner shelf (Bartek et al., this volume), we infer that the ancestral
Mobile and Tombigbee rivers (Fig. 7) deposited the relatively
thick Unit 2 shelf-margin delta. Judging by its eastern location,
the thinner Unit 2 depocenter most likely was deposited by the
confluence of the smaller fluvial systems now draining into
Pensacola Bay (Fig. 7). We propose that the fluvial feeder to the
pro 10 Lagniappe delta may have been the Pascagoula River
drainage system.
We conclude that the two Unit 2 depocenters we describe in
our study represent deposition during the last glacial maximum
(i.e., OIS 2) on the basis of our inference that the depocenters are
above surface 10 (i.e., OIS 3 maximum flooding surface from
Main Pass 303 drill site), and because the final phase of Unit 2
shelf-edge elevations are below a water depth of 70 m. The highstand systems tracts presumably are located on the inner shelf.
Pensacola
Bay
Mobile
Bay
20
0
50
kilometers
40
20
30°
100
Fi
g.
60
6
80
5
g.
Fi
LEGEND
Fi
40
g.
3
g.
cluster of older
shelf-margin deltas
(Units 3, 4, and 5)
approx. downlap
limit of unit 1
20
0
Fi
g.
Fi
60
4
2
80
Unit-2 shelf-margin
deltas & offlap breaks
29.5°
inferred location of
braided river at OIS 2
500
10
0
00
10
De Soto
Canyon
1500
88°
bathymetric
contours (m)
seismic grid
seismic profiles
referred to in text
87°
FIG. 7.—Time–structure map of the seafloor from the seismic grid, showing the locations of the Unit 2 shelf-margin deltas and the
Unit 1 backstepped coastal plain.
52
PHILIP J. BART AND JOHN B. ANDERSON
The upper surface of the Unit 2 deltas may be equivalent to a
OIS 2 sequence boundary, but because no well defined cross-shelf
valleys incise these shelf-edge deltas, we believe that the lowstand
valleys were shallow braided rivers roughly in equilibrium with
the elevations of the interfluves on the lowstand coastal plain.
Conversely, the general lack of incised fluvial valleys at the Unit
2 topset may indicate either that the upper surfaces of the
prograding wedges have been severely eroded during the subsequent sea-level rise or that the topset substratum is uniformly
sandy such that incised fluvial valleys are not imaged well with
the seismic sources used to acquire the data. Because no canyons
were found on the upper slopes of the Unit 2 prograding wedges,
we assume that a period of valley incision and sediment bypass
to the slope and deeper basin did not interrupt the construction
of the prograding wedges.
On the basis of its shelf-perched location and regional extent,
we assume that Unit 1 represents the backstepped strata of the
coastal plain relative to the position of the Unit 2 shelf-margin
depocenters. The local reworking of the upper surface of Unit 1
into sand-ridge fields (Fig. 4) suggests that marine currents
significantly remolded the continental shelf during the transgression and highstand. Conversely, the sand-ridge fields may be due
to the influence of waves and longshore drift with low sediment
supply (Harry Roberts, personal communication). Because the
onlapping slope wedge stratigraphically above the Unit 2
prograding wedge is of regional extent, we infer that the sediments were supplied as a result of erosion by strike-oriented
marine currents acting after the sediment supply to the shelf
margin was shut off during the sea-level rise that terminated the
last lowstand.
Stage A
primary fluvial
~100 m
feeder
coastal plain
shelf edge
coast line
depocenter
highstand
shelf
500 m
Slope
DeSoto
Canyon
Stage B
updip incision of
emerging coastal plain
~25 m
local modification
to shelf-edge trend
deltaic outbuilding and
braided fluvial system
downlap limit on
the upper slope
newly emergent
coastal plain
380 m
coastline
shelf edge
SUMMARY
Figure 8 is a four-stage model that summarizes our preliminary view of the Alabama and west Florida depositional systems
that evolved during the last glacioeustatic cycle. We believe that
the previous highstand coastal plain occupied a position similar
to today’s highstand depositional systems (Fig. 8A). During the
subsequent fall, the coastal-plain depositional systems prograded across the continental shelf into shoaling waters (Fig. 8B). At
the culmination of the last sea-level fall, the depositional systems
(i.e., Unit 2) advanced to the shelf edge but valley incision probably
was confined to the updip areas. Because we found no evidence of
valley incision into the Unit 2 shelf-margin deltas, we infer that
shallow braided rivers (Fig. 8B) fed the shelf-margin deltaic units.
No significant channelized bypass of the continental shelf occurred during the last glacioeustatic lowstand. This leads to the
conclusion that the Alabama and west Florida deltaic systems
adjusted constructively to falling sea levels in the late Quaternary
eustatic cycle. On the basis of available age control, we surmise
that Unit 2 deltas on the outer shelf correspond to deposition at
the culmination of the last glacial maximum (i.e., OIS 2).
After the lowstand, sea-level rise caused the coastal-plain
depositional system to backstep onto the shelf during the early
transgression (Fig. 8C). As the sea-level rise initially flooded the
Stage C
~50 m
backstepped
coast line position
430 m
flooded
coastal plain
DeSoto Canyon
shedding to
shelf-detached onlapping
wedge
Stage D
bay
new
highstand
coastline
~100 m
flooded coastal
plain
drowned
reefs
→
FIG. 8.—Four-stage depositional model summarizing the general
features of the Alabama and west Florida stratigraphy associated with the last glacial cycle. Stage A corresponds to the last
highstand. Stage B is near the culmination of the eustatic
lowstand. Stage C represents an intermediate position of the
coastal plain during the last sea-level rise. Stage D represents
the current highstand.
onlapping
slope wedge
500 m
backstepped
coast line position
DeSoto Canyon
LATE QUATERNARY STRATIGRAPHIC EVOLUTION, ALABAMA AND WEST FLORIDA OUTER CONTINENTAL SHELF
53
Coleman, J.M., eds., Framework, Facies, and Oil-Trapping Charactershelf margin, waves and marine currents eroded and transported
istics of the Upper Continental Margin, American Association of
fine-grained sediments from the flooded coastal plain basinward
Petroleum Geologists, Studies in Geology no. 7, p. 193–223.
to form thin but regionally extensive onlapping wedges on the
upper slopes. The regional extent of the onlapping wedges along SAGER, W.W., SCHROEDER, W.W., LASWELL, J.S., DAVIS, K.S., REZAK, R., AND
the upper slope suggests to us that deposition of the slope units
GITTINGS, S.R., 1999, Mississippi–Alabama outer continental shelf
was unrelated to the position of shelf-margin-delta units.
topographic features formed during the late Pleistocene–Holocene
Mounded features overlying the shelf-edge prograding
transgression: Geo-Marine Letters, v. 12, p. 41–48.
wedges are interpreted as bioherms formed during the transgres- STUDE, G.R., 1978, Depositional environment of the Gulf of Mexico
sion. Bands of carbonate mounds have been identified along the
South Timbalier Block 54 salt dome, and salt dome growth models,
shelf edge/outer shelf at water depths of approximately 120 and
Gulf Coast Association of Geological Societies, Transactions, v. 28,
90 m offshore Mississippi and Alabama using side-scan sonar
p. 627–646.
(Ludwick and Walton, 1957; Laswell et al., 1990; Sager et al., SYDOW, J., AND ROBERTS, H.H., 1994, Stratigraphic framework of a late
1992). The lack of similar mounded features on the Lagniappe
Pleistocene shelf-edge delta, Northeast Gulf of Mexico: American
delta lobes (Harry Roberts, personal communication) may have
Association of Petroleum Geologists, Bulletin, v. 78, p. 1276–1312.
been due to the proximity of the area to turbid and cold meltwater
that issued from the Mississippi River during the last glacial
maximum and transgression, which would have inhibited coral
development. These mounds obviously postdate deposition of
the Unit 2 shelf-margin deltas, and the carbonate growths may
have been coeval with the development of the Unit 1 coastal plain.
Grab samples from the area show the presence of crustose coralline algae, serpulid worm tubes, bryzoans, foraminifera, and
isolated hermatypic corals, indicating that the carbonate mounds
have been drowned (Ludwick and Walton, 1957; Gittings et al.,
1992). Figure 8D represents the late transgression to early highstand, when the depositional systems attained their present
position.
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54
PHILIP J. BART AND JOHN B. ANDERSON