7

Evidence for Mid-Holocene Change in Depositional 

Style in Mobile Bay, Alabama

Open-File Report 2012–1081

U.S. Department of the Interior

U.S. Geological Survey

Evidence for Mid-Holocene Change in Depositional Style in 

Mobile Bay, Alabama

By David Twichell, Kyle Kelso, and Elizabeth Pendleton

Open-File Report 2012–1081

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

Twichell, David, Kelso, Kyle, and Pendleton, Elizabeth, 2012, Evidence for mid-Holocene shift in depositional style in 

Mobile Bay, Alabama: U.S. Geological Survey Open-File Report 2012–1081, 17 p., available at 

http://pubs.usgs.gov/of/2012/1081/.

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endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.
iii

Contents

Abstract ......................................................................................................................................................................... 1

Introduction.................................................................................................................................................................... 1

Data Collection and Processing..................................................................................................................................... 2

Mapping of the Holocene Stratigraphy........................................................................................................................... 3

Holocene Stratigraphy ................................................................................................................................................... 4

Summary and Conclusion.............................................................................................................................................. 6

Acknowledgments ......................................................................................................................................................... 7

References Cited........................................................................................................................................................... 7

Figures

1. Map showing the bathymetry of Mobile Bay, Alabama, the paths of lowstand river valleys as interpreted by 

three authors, the extent of Pleistocene deposits around the bay, and the location of three sets of Holoceneaged beach ridges on Morgan Peninsula ......................................................................................................... 9

2. Map showing the extent of the seismic data and cores collected in 2010 and the tracks along which analog 

seismic data were collected in 1990 ................................................................................................................10

3. Seismic profile and interpretation showing the Holocene stratigraphy of the bay above the last glacial 

maximum surface ............................................................................................................................................11

4. Map showing the depth to the erosional surface cut during the last glacial maximum.....................................12

5. Map showing the depth to the top of the mid-Holocene R1 unit and the variations in reflection amplitude from 

the R1 unit .......................................................................................................................................................13

6. Seismic profiles showing (A) variations in reflection amplitude of the R1 unit and how it onlaps the bay side of 

Morgan Peninsula, (B) an east-west profile showing the flat-lying nature of unit R1 and that it pinches out on 

the last glacial maximum surface along the edges of the bay, and (C) a crossing of the northwestern 

submerged extent of the Little Point Clear peninsula.......................................................................................14

7. Vibracores from Bon Secour Bay showing variations in sediment composition associated with the R1 unit ...15

8. Isopach map showing the thickness of Holocene deposits in Mobile Bay, Alabama .......................................16

9. Isopach maps showing (A) the thickness of early Holocene deposits that accumulated between the last 

glacial maximum and R1 surfaces and (B) the thickness of late Holocene deposits that accumulated above 

the R1 surface .................................................................................................................................................17
iv

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Area

square kilometer (km2) 247.1 acre

square kilometer (km2) 0.3861 square mile (mi2)

Volume

cubic kilometer (km3) 0.2399 cubic mile (mi3)

Abbreviations

B.P. before present

FSSB full spectrum subbottom (profiler)

LGM last glacial maximum

USGS U.S. Geological Survey
1

Evidence for Mid-Holocene Change in Depositional 

Style in Mobile Bay, Alabama

By David Twichell, Kyle Kelso, and Elizabeth Pendleton

Abstract

The Holocene stratigraphy of Mobile Bay, Alabama, was mapped using a combination of 

high-resolution seismic data and sediment cores to refine changes in the bay’s evolution during 

this time. The base of the Holocene-era stratigraphy is an erosional surface formed during the 

last glacial maximum. Overlying Holocene deposits are primarily estuarine mud that has a finely 

laminated weak acoustic signature. One exception is a thin unit, R1, with varying reflection 

amplitude that can be traced throughout the southern part of the bay. The continuity of the unit 

throughout the southern part of the bay suggests a baywide change in sedimentation that was 

perhaps driven by rapid retreat of the bay-head delta in response to a sudden rise in sea level or 

an abrupt change in accommodation space due to basin geometry. Along the southern edge of the 

bay, the R1 unit increases in thickness and reflector amplitude towards Morgan Peninsula. The 

peninsula itself underwent a period of erosion and narrowing between 4,300 to 3,000 years 

before present, and the variation in reflector amplitude and the geometry of this part of the R1 

unit appear to reflect a period of increased overwashing of the peninsula during this period. 

Average estuarine sedimentation rates decreased after the formation of the R1 unit, and the 

decrease coincides with a decline in the rate of sea-level rise. A similar change in depositional 

style at approximately the same time in neighboring Apalachicola Bay suggests a change that 

affected the northeastern Gulf of Mexico region and not just Mobile Bay.

Introduction

Mobile Bay is the largest estuary in the northeastern Gulf of Mexico, and sedimentary 

deposits within the bay provide a record of its Holocene history. Presently, the bay is partially 

enclosed from the Gulf of Mexico by Dauphin Island to the west and the Morgan Peninsula to 

the east (fig. 1). The core of Dauphin Island comprises Pleistocene deposits; Morgan Peninsula 

was formed by spit elongation during the Pleistocene and Holocene (Otvos and Giardino, 2004, 

p. 68–69; Rodriguez and Meyer, 2006, p. 58–68). The westward elongation of Morgan Peninsula 

would suggest that, through the Holocene, Mobile Bay became more sheltered from the Gulf of 

Mexico with time, yet a study of beach ridges on the peninsula identified an erosional period 

when this barrier may have been breached, allowing coarser material to be transported into the 

bay (Rodriguez and Meyer, 2006, p. 64–66). Behind Morgan Peninsula, the Holocene deposits 

that fill Mobile Bay have been the focus of several studies. High-resolution seismic profiles have 

been used to map the unconformity cut by rivers during periods of lowered sea level (Mars and 

others, 1992, p. 538; Davies and Hummell, 1994; Kindinger and others, 1994, p. 86; Greene and 

others, 2007, p. 147; fig. 1). Holocene deposits that rest on this unconformity have been divided 
2

into two units, a basal sandy bay-head delta unit that fills the narrow deep paleovalleys and an 

overlying muddy estuarine unit that covers the entire bay floor (May, 1976, p. 6–14; Mars and 

others, 1992, p. 533–540; Greene and others, 2007, p. 143–153; Rodriguez and others, 2010).

Rodriguez and Meyer (2006, p. 264–266) identified a silty to sandy interval within the muddy 

estuarine fill from a core in Bon Secour Bay, which they suggest was storm or tidal delta 

deposits that formed when the Morgan Peninsula was breached between 4,300 to 3,000 years 

before present (B.P.), but they could not define the full extent of these deposits with only one 

core and with limited seismic data.

The purpose of this study is to further refine the Holocene stratigraphy of the bay with 

particular emphasis on the presence and composition of potential storm deposits in the southern 

part of the bay. A reconnaissance grid of high-resolution seismic profiles and previously 

acquired seismic data were used to map Holocene sediment thickness throughout the bay and, 

where data quality was sufficient in the southern part of the bay, to refine the Holocene estuarine 

stratigraphy and its relation to Morgan Peninsula. Short vibracores were used to characterize the 

upper 4 to 5 meters (m) of the estuarine deposits, and published age dates (May, 1974, p. 12; 

Rodriguez and Meyer, 2006, p. 260; Greene and others, 2007, p. 143) were used to refine the 

history of the bay.

Data Collection and Processing

High-resolution seismic-reflection profiles were collected with a Geo-Star full spectrum 

subbottom (FSSB) Chirp system deployed in an SB–0512i towfish (0.5–12 kiloHertz frequency) 

(EdgeTech, undated) along 260 kilometers of survey trackline in the southern part of Mobile Bay 

during July 17–21, 2010, aboard the U.S. Geological Survey research vessel G.K. Gilbert. These 

data were logged digitally in SEG Y format (Norris and Faichney, 2002). Vibracores were 

collected from nine stations located along seismic profiles in the southern part of the bay on July 

23, 2010, aboard the G.K. Gilbert (fig. 2). Differential Global Positioning System navigation was 

used during acquisition of seismic data and cores. The survey did not cover the entire bay, but 

analog seismic profiles collected by the U.S. Geological Survey (USGS) in 1990 employing an 

Ocean Research Equipment. Geopulse system were used to map the northern part of the bay and 

complimented the new dataset in the southern part of the bay.

SIOSEIS (undated), Seismic Unix (Stockwell and Cohen, 2007) and SeisVision 

(GeoGraphix, 2008) were used to process the raw Chirp seismic data. For digital interpretation, 

SEG Y trace data were imported into SeisVision, where reflectors representing the sea floor and 

key subsurface stratigraphic contacts were identified and digitized. The interpretations were 

exported from SeisVision as XYZ point files and imported into ArcMap (Environmental Systems 

Research Institute, Inc., 2011), where depth to the interpreted horizons was plotted. For the 

analog profiles, depth to the sea floor and other subsurface stratigraphic horizons was measured 

by hand, entered into a spreadsheet, and then imported into ArcMap. A 1,500-meter-per-second 

speed of sound was used to convert the vertical scale of the seismic data from milliseconds of 

two-way travel time to depths in meters. Point data were contoured by hand because of the wide 

spacing between the survey lines. The contours were digitized and then converted to a grid using 

the topo-to-raster tool in ArcGIS . Grid surface calculations were completed to compute the 

thickness and volume of different stratigraphic units. Vibracores were obtained in the Bon 

Secour Bay section of Mobile Bay by using a Rossfelder model P–3 coring system deployed 

from the G.K. Gilbert (fig. 2). Upon recovery, the aluminum barrel was removed from the rig 

and cut to 1-m lengths. The completed cores were transported to the USGS in St. Petersburg, 
3

Florida, for curation and description. Each core section was split vertically, visually described 

using standard sediment logging methods, and photographed. Core logs were generated to 

include a description of sedimentary texture, sedimentary structures, percent sand, physical 

characteristics, and stratification type.

Mapping of the Holocene Stratigraphy

The seismic profiles were used to map (1) the sea floor, (2) the unconformity interpreted 

to be cut by rivers during the last glacial maximum (LGM), and (3) a mid-Holocene thin unit, 

R1, with varying reflection amplitude that could only be mapped on the Chirp profiles collected 

in the southern part of the bay (fig. 3). Additional units associated with Morgan Peninsula were 

identified along the very southern margin of the bay. The sea floor was mapped on the profiles 

throughout the bay, whereas the subsurface horizons were mapped in the areas free of gas.

The coastal relief dataset compiled by the National Oceanic and Atmospheric 

Administration (2006) was used to characterize the bathymetry of Mobile Bay (fig. 1). The bay 

is shallow with most of it being less than 4 m deep. It is divided into eastern and western basins 

that are separated by the dredged ship channel that extends from the mouth of the bay to the port 

of Mobile at the head of the bay. The eastern basin reaches 4-m depths in the northeastern part of 

the bay. Bon Secour Bay, which composes the southeastern corner of Mobile Bay, is 2 to 3 m 

deep. The western basin occupies the southwestern part of the bay. This basin increases in depth 

toward the south and reaches 4 m deep near the bay entrance. Immediately behind the bay 

entrance, depths exceed 14 m, and the dredged channel is mostly greater than 10 m deep.

The erosional surface that was cut during the last lowstand of sea-level, the LGM surface, 

is shown in figures 3 and 4. At the head of the bay, borings show two shallow channels within 

the floor of a broader valley (Greene and others, 2007, p. 147). Available seismic data suggest 

that these two channels follow paths along the eastern and western margins of the northern and 

central parts of the bay where they are separated by a north-south trending plateau in the middle 

of the bay (fig. 4). The valleys converge, or nearly converge, in a broad basin that underlies the 

southern part of the bay. Differences in the paths of lowstand valleys published by Kindinger and 

others (1994, p. 86), Davis and Hummel (1994), and Greene and others (2007, p. 147) are in part 

due to gas masking the deeper stratigraphy under a large part of the central bay (fig. 2) and to 

insufficient seismic coverage to ensure that valleys were correlated with the correct Pleistocene 

lowstand (fig. 1).

On seismic profiles, the estuarine deposits that fill the bay show faint laminations that are 

separated into two major intervals by the thin R1 unit. The estuarine deposits underlying the R1 

unit are characterized by laminations with low to moderate relative reflection amplitude, while 

the deposits above the R1 unit are characterized by laminations with very low relative reflection 

amplitude. The R1 unit could be identified only on Chirp seismic data, and thus the mapped 

extent of the unit is limited to the southern part of the bay (fig. 5). The R1 unit is limited to being 

a single reflector throughout the central and northern part of the study area and broadens to two 

to three reflectors along the southern edge of the bay (fig. 3) where it onlaps Pleistocene deposits 

behind Dauphin Island and early to mid-Holocene deposits that compose Morgan Peninsula (fig. 

6A). Available data suggest that the surface of the R1 unit mimics the early Holocene estuarine 

deposits that floor the bay but does not drape up onto older deposits exposed at shallower depths 

along the bay margins (fig. 6B). Depth to the R1 surface is 7 to 10 m in the central and northern 

part of the study area (fig. 5). Limited Chirp coverage and gas preclude mapping the northern 

limit of the R1 unit.
4

In addition to the distribution and depth of the R1 unit, relative reflection amplitude was 

mapped as well (fig. 5). Reflector amplitudes are highest on the profiles collected from Bon 

Secour Bay east of Little Point Clear and a smaller area off the western tip of Little Point Clear. 

Reflector amplitude decreases from these two areas northward into the bay. Moderate amplitude 

reflectors extend farthest northward in the western part of the bay. The northern limit of the low 

amplitude reflectors could not be mapped because of insufficient data coverage.

Cores that penetrate the R1 unit show that the unit coincides with a bioturbated, sandy 

shell bed that interrupts an otherwise muddy section (fig. 7A). The bed is thickest and sand 

content highest in the cores located in the high-amplitude region. The sand content of this bed 

decreases northward, and the one core in the moderate amplitude region that penetrated the R1 

unit recovered shell hash at the depth of the R1 unit (fig. 7B).

The thickness of the Holocene sediment in the bay was mapped by taking the difference 

between the present sea floor and the depth to the surface cut during the LGM (fig. 8). Holocene 

sediment is thickest at the head of the bay and in the paleovalleys. Sediment filling the 

paleovalleys commonly is masked by gas, but in places free of gas, this interval is characterized 

by foreset bedding (fig. 3). This bedding is interpreted to represent bay-head deltas that retreated 

up the valleys in response to the rising sea. Bay-head deltas could not be mapped as a separate 

unit because of the extent of gas in the southern part of the bay and the lower resolution of 

profiles in the northern part of the bay. Borings through the thick Holocene section at the head of 

the bay indicate that much of this sediment is associated with the present-day Mobile-Tensaw 

Delta (May, 1976, p. 7–9; Greene and others, 2007, p. 146–147).

Immediately northwest of Little Point Clear, Holocene sediment reaches 8 to 10 m in thickness 

and is characterized by foreset bedding dipping primarily to the south (fig. 6C). Little Point Clear 

extended farther to the northwest earlier in the Holocene, and the southward dip of the foreset 

beds is presumably due to the southward accretion of beach ridges during the formation of this 

spit.

Holocene deposits overlying the basal bay-head delta unit are characterized by lowamplitude continuous acoustic reflections. The deposit is thickest in the south-central part of the 

bay where the two paleovalleys merge (fig. 8). The deposit is less than 4 m thick over the central 

plateau that separates the two paleovalleys and pinches out along the bay margins. Earlier cores 

(May, 1976, p. 4–21) and cores from this study indicate that this deposit is a silty clay that 

represents open-bay sedimentation (fig. 7).

The R1 unit divides the Holocene estuarine deposits into two units (fig. 9). In the 

southern part of the bay, Holocene sediment deposited prior to the deposition of the R1 interval 

first filled the paleovalleys and then spread more broadly across the bay floor (fig. 9A). The 

volume of the deposit below the R1 interval is 3.9 cubic kilometers (km3

). The unit overlying the 

R1 interval is thickest over the confluence of the two paleovalleys as well as over the valleys 

themselves (fig. 9B). The volume of sediment deposited above the R1 interval is 1.3 km3.

Holocene Stratigraphy

Holocene sedimentation in Mobile Bay shows a transition from bay-head delta 

sedimentation to fine-grained estuarine sedimentation (Greene and others, 2007, p. 143–152). 

The new seismic and core data collected for this study indicate that, at least in the southern half 

of the bay, the fine-grained estuarine deposits can be broken into two intervals separated by the 

R1 unit (fig. 5). This unit was sampled in Bon Secour Bay and dated at 4,300 to 3,000 years B.P. 

(Rodriguez and Meyer, 2006, p. 260–266). Drawing on that chronology and the seismic facies 
5

distribution of this horizon, we make inferences as to the origin of this thin sedimentary unit and 

use it to evaluate changes in Holocene sedimentation style and sedimentation rates in the 

southern part of the bay.

Sea level has risen approximately 20 m since the initial flooding of the bay, but the rate 

of sea-level rise has not been constant. Two recent curves of sea-level rise based on data from the 

northern Gulf of Mexico have been presented by Balsillie and Donoghue (2004) and Milliken 

and others (2008), showing that the average rate of sea-level rise decreased from 4 to 5 

millimeters per year (mm/yr) before 8,000 to 7,000 years B.P. to between 1 and 1.4 mm/yr by 

7,000 to 4,000 years B.P., at and after 4,000 years B.P. the rate of sea-level rise slowed to less 

than 0.6 mm/yr (Balsillie and Donoghue, 2004, p. 10; Milliken and others, 2008). The R1 unit, 

based on the age published by Rodriguez and Meyer (2006, p. 260–266), formed during the 

transition from a period of moderate sea-level rise to a period of slow sea-level rise. We used the 

volume of sediment that accumulated before and after the formation of the R1 unit along with 

dates of the LGM surface and R1 unit to calculate average sedimentation rates.

Approximately 8.2 km3 of sediment has accumulated during the Holocene in Mobile Bay 

(fig. 8). Of this, 5.2 km3 accumulated in the southern part of the bay covered by our highresolution seismic data (fig. 9). Of the 5.2 km3 that accumulated within the study area, 3.9 km3

accumulated below the R1 unit, covers an area of 590 square kilometers (km2), and has an

average thickness of 6.7 m. The deposit overlying the R1 unit has a volume of 1.3 km3

, covers an 

area of 476 km2, and has an average thickness of 2.7 m. Using 9,600 years B.P. as the time the 

bay initially flooded (Greene and others, 2007, p. 151) and 4,300 to 3,000 years B.P. for the age 

of the R1 unit (Rodriguez and Meyer, 2006, p. 260–266), sediment accumulated at an average 

rate of 1.0 to 1.3 mm/yr before the formation of the R1 unit and 0.6 to 0.9 mm/yr since the time 

of the formation of the R1 unit. The higher accumulation rate during the early part of the history 

of the bay is attributed to the higher rate of sea-level rise during this time, which provided more 

accommodation space for sediment accumulation than was available later when the rate of sealevel rise slowed. Whether sediment input to the bay by the Mobile-Tensaw River system was 

greater during the early Holocene or whether sediment input was constant and more sediment 

bypassed the bay during the late Holocene due to a lack of accommodation space cannot be 

determined with the available data.

The R1 unit was first identified by Rodriguez and Meyer (2006, p. 264–265) in Bon 

Secour Bay and was inferred to represent either storm deposits sourced from the peninsula or a 

tidal delta that formed once the peninsula was breached. The broader suite of seismic profiles 

available from this study allowed mapping of the morphology, distribution, and seismic 

amplitude of this unit. The conclusions of the mapping from the study are more consistent with 

the unit being formed by storms. The deposit is only 1 to 3 m thick along its southern edge where 

it onlaps Morgan Peninsula and thins gradually to the north to a single reflection that can be 

traced throughout much of the southern half of the bay. Reflector amplitude decreases gradually 

to the north as well (fig. 3). The distribution of the high-amplitude reflectors suggests two source 

areas to either side of Little Point Clear with the area to the east being larger (fig. 5), which 

floors a shallow depression behind the narrow part of Morgan Peninsula. The thin nature of the 

deposit, its broad extent throughout the southern half of the bay, the largest high-amplitude area 

occupying a slight depression behind Morgan Peninsula, and the gradual decrease in the 

amplitude of the R1 unit towards the north are factors more consistent with a storm deposit than 

a tidal delta.
6

A core from the high-amplitude area east of Little Point Clear penetrated a 50-centimeterthick bioturbated sand bed with shell fragments and may represent an overwash deposit (fig. 7A). 

The one core from the moderate-amplitude region that penetrated R1 recovered a thin shell bed 

from this interval (fig. 7B). This shell bed could either be a more distal part of the same storm 

overwash bed or a localized lag resulting from repeated resuspension and reworking of bay-floor 

sediments by storms. Isphording and Isphording (1991) noted that the floor of the bay deepened 

by approximately 0.5 m following the passage of Hurricane Frederic in 1979. While much of the 

resuspended material was exported from the bay, coarser shelly material in the estuarine deposits 

may have remained in the bay to form thin shell lag deposits.

Studies of cores recovered from lakes on Morgan Peninsula (Liu and Fearn, 1993), in 

northwestern Florida (Liu and Fearn, 2000), and in the Pearl River Delta, Mississippi (Li, 1994), 

found an increased abundance of coarser-grained laminations in muddy lake or delta deposits 

during the period between 4,000 and 3,000 years B.P. These coarser laminations are interpreted 

to be overwash deposits generated by storms. The timing for this period of increased storm 

activity coincides with and may be the cause of the period when the Morgan Peninsula 

underwent erosion, resulting in narrowing and possible breaching (Rodriguez and Meyer, 2006, 

p. 266–268). The R1 unit, which is of the same age, may show the response of the bay floor to 

this period of increased storm activity.

An alternative explanation is that the R1 unit is of composite origin. Throughout much of 

the bay, the R1 unit is a single reflector that may represent a change in sedimentation in the bay 

in response to a rapid retreat of the bay-head delta between 4,300 to 3,000 years B.P. Below the 

R1 unit, the fill is characterized by moderate amplitude reflections; above the unit, reflection 

amplitudes are low (figs. 2, 6A). A similar stratigraphy has been identified in Apalachicola Bay. 

There, sandy deltaic deposits and correlative deposits dating from before the formation of the 

delta show moderate amplitude reflections and are covered by fine-grained estuarine deposits 

that show low amplitude reflections (Twichell and others, 2010). The surface that separates the 

deltaic and correlative deposits from the estuarine deposits formed approximately 4,400 years 

B.P., and the stratigraphy suggests rapid shoreward migration of the deltas at this time (Osterman 

and others, 2009). The rapid retreat of the deltas in Apalachicola Bay may have been in response 

either to a slight but rapid rise in sea level (Balsillie and Donoghue, 2004, p. 1–23) or to the 

interaction of a gradually rising sea level with the geomorphology of the bay, resulting in a rapid 

increase in accommodation space (Rodriguez and others, 2008, p. 3983–3991). The 

stratigraphies of the two bays suggest a regional change that affected both bays in a similar way. 

The thicker part of the R1 unit along the southern edge of Mobile Bay appears to be storm 

deposits transported into the bay during the time (4,000–3,300 years B.P.) when both bays were 

also being subjected to rapid headward retreat of bay-head deltas.

Summary and Conclusion

The Holocene stratigraphy of Mobile Bay, Alabama, has been mapped using recently 

acquired high-resolution seismic data and cores complimented with historical datasets. The 

Holocene estuarine deposits are separated into two units by a reflector package (the thin 

stratigraphic unit, R1) that can be traced throughout the southern half of the bay on the recently 

acquired seismic profiles. This reflector package is shallowest along the southern edge of the bay 

where it onlaps the bay side of Morgan Peninsula and Dauphin Island, and it is deeper in the 

central part of the bay where it drapes the underlying bay-floor deposits but does not onlap older 

deposits along the edges of the bay. The reflector amplitude of the surface of this unit is highest 
7

along the southern edge of the bay and is the lowest to the north in the central part of the bay. 

The variation in reflector amplitude and the geometry of the unit suggest the R1 unit may be the 

result of increased storm activity in the bay, which resulted in increased transport of coarse 

material into the bay from the south. Another possible result of increased storm activity was the 

formation of shell lag beds from resuspension and winnowing of bay-floor sediment. Rodriguez 

and Meyer (2006, p. 260–266) dated the horizon at 4,300 to 3,000 years before present, during a 

time when Morgan Peninsula underwent severe erosion and narrowing. This erosional period in 

the history of the peninsula appears to be indicated in the estuarine deposits of the southern part 

of the bay as well. Average estuarine sedimentation rates decreased after the formation of R1;

this decrease coincides with a decline in the rate of sea-level rise and appears to be controlled by 

accommodation space in the bay.

Acknowledgments

We appreciate the assistance provided by David Bennett, captain of the U.S. Geological 

Survey research vessel G.K. Gilbert; Dana Wiese, who maintained the electronics; and Chandra 

Dreher and Jordan Sanford, who assisted with collecting the cores. The report benefited from 

reviews by Lawrence Poppe and Laura Brothers.

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9

Figures

Figure 1. Map showing the bathymetry of Mobile Bay, Alabama (National Oceanic and Atmospheric Administration, 2006), 

the paths of lowstand river valleys , the extent of Pleistocene deposits around the bay, and the location of three sets of 

Holocene-aged beach ridges on Morgan Peninsula. The bathymetry of the bay is from the National Oceanic and 

Atmospheric Administration (2006), and the paths of lowstand river valleys are interpreted by Kindinger and others (1994), 

Davies and Hummel (1994), and Green and others (2007). Inset map shows the location of Mobile Bay in the northeastern 

Gulf of Mexico.
10

Figure 2. Map showing the extent of the seismic data and cores collected in 2010 and the tracks along which analog 

seismic data were collected in 1990 in Mobile Bay, Alabama. The bathymetry of the bay is from the National Oceanic and 

Atmospheric Administration (2006). Locations of profies in Figures 3 and 6 are shown by labeled heavy black lines.
11

Figure 3. A, Seismic profile and B, interpretation showing the Holocene stratigraphy of Mobile Bay, Alabama, above the 

surface of the last glacial maximum. Profile location is shown on figures 2 and 5.
12

Figure 4. Map showing the depth to the erosional surface in Mobile Bay, Alabama, cut during the last glacial maximum 

(LGM).
13

Figure 5. Map showing the depth to the top of the mid-Holocene thin R1 unit and the variations in reflection amplitude 

from the R1 unit in Mobile Bay, Alabama.
14

Figure 6. Seismic profiles from Mobile Bay, Alabama. A, Variations in reflection amplitude of the mid-Holocene thin R1 

unit and how the unit onlaps the bay side of Morgan Peninsula. B, An east-west profile showing the flat-lying nature of unit

R1 and that it pinches out on the surface cut during the last glacial maximum (LGM) along the edges of the bay. C, A 

crossing of the northwestern submerged extent of Little Point Clear Peninsula (yellow shading) showing the initial spit 

(labeled 1) and subsequent sets of southward dipping foreset beds that may represent three different stages of beach-ridge 

development (labeled 2, 3, and 4); the R1 unit onlaps the northern extent of the spit. Red dashed lines show R1, and blue 

dashed lines show the LGM surface. Profile locations are shown on figures 2 and 5.
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Figure 7. Two vibracores charts from Bon Secour Bay, Alabama, showing variations in sediment composition associated 

with the mid-Holocene thin R1 unit. Core 10MB05 was collected from an area where the reflection amplitude is high, and the 

0.5-meter-thick sand bed corresponds to the seismic reflector. Core 10MB07 was collected from an area of moderate 

reflection amplitude, and a thin shell bed corresponds to the seismic reflector. Core locations are shown on figure 5.
16

Figure 8. Isopach map showing the thickness of Holocene deposits in Mobile Bay, Alabama.
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Figure 9. Isopach maps of Mobile Bay, Alabama. A, The thickness of early Holocene deposits that accumulated between 

the last glacial maximum and the surface areas of the mid-Holocene thin R1 unit. B, The thickness of late Holocene 

deposits that accumulated above the surface of the R1 unit.
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For more information concerning this report, contact

Director

U.S. Geological Survey

Woods Hole Coastal and Marine Science Center

384 Woods Hole Road

Quissett Campus

Woods Hole, MA 02543-1598

WHSC_science_director@usgs.gov

508-548-8700 or 508-457-2200

or visit our Web site at

http://woodshole.er.usgs.gov
Twichell and others— Evidence for Mid-Holocene Change in Depositional Style in Mobile Bay, Alabama—Open-File Report 2012–1081