late quaternary stratigraphic evolution of the northern gulf of mexico margin

LATE QUATERNARY STRATIGRAPHICEVOLUTION OF THE NORTHERN

GULF OF MEXICO MARGIN

Edited by:

JOHN B. ANDERSONEarth Sciences Department, Rice University, Houston, Texas 77251-1892, U.S.A.

AND

RICHARD H. FILLONEarth Studies Associates, 3730 Rue Nichole, New Orleans, Louisiana 70131-5462, U.S.A.

Copyright 2004 bySEPM (Society for Sedimentary Geology)

Laura J. Crossey, Editor of Special PublicationsSEPM Special Publication Number 79

Tulsa, Oklahoma, U.S.A. April, 2004

ISBN 1-56576-088-3

© 2004 bySEPM (Society for Sedimentary Geology)

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Printed in the United States of America

SEPM and the authors are grateful to the followingfor their generous contribution to the cost of publishing

Late Quaternary Stratigraphic Evolution of the Northern Gulf of Mexico Margin

Contributions were applied to the cost of production, which reduced thepurchase price, making the volume available to a wide audience

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1LATE QUATERNARY STRATIGRAPHIC EVOLUTION, NORTHERN GULF OF MEXICO MARGIN: A SYNTHESIS

LATE QUATERNARY STRATIGRAPHIC EVOLUTION OF THENORTHERN GULF OF MEXICO MARGIN: A SYNTHESIS

JOHN B. ANDERSONEarth Sciences Department, Rice University, Houston, Texas 77251-1892, U.S.A.

ANTONIO RODRIGUEZDepartment of Geological Sciences, University of Alabama, Tuscaloosa, Alabama 35487-0338, U.S.A.

KENNETH C. ABDULAHPioneer Natural Resources, 5205 N. O’Connor Blvd., Irving, Texas 75039-3746, U.S.A.

RICHARD H. FILLONEarth Studies Associates, New Orleans, Louisiana 70131, U.S.A.

LAURA A. BANFIELDBP, 501 Westlake Park Blvd., Houston, Texas 77210, U.S.A.

HEATHER A. MCKEOWNDepartment of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A.

Present address: ExxonMobil Exploration Company, 233 Benmar, Houston, Texas 77060, U.S.A.AND

JULIA S. WELLNEREarth Sciences Department, Rice University, Houston, Texas 77251-1892, U.S.A.

Abstract: This volume presents results from several high-resolution stratigraphic investigations of late Quaternary strata of the northernGulf of Mexico, from the Apalachicola River to the Rio Grande. The studies characterize deposition and strata formation associated withdifferent fluvial and deltaic systems during the most recent glacioeustatic cycle (approximately 120 ka to present).

The Gulf margin region encompasses a variety of depositional settings characterized by different drainage-basin size, physiography,fluvial morphology, and structural and diapiric activity. The papers presented in this volume focus on fluvial response to climate andbase-level change, variations in delta growth and evolution across the shelf, lowstand delta and fan evolution, the evolution oftransgressive deposits on the shelf, the preservation of these deposits, and the resulting differences in stratigraphic architecture. In thispaper we summarize the key observations made in those studies and compare the paleogeography and deposystem evolution of thevarious study areas.

The integration of the chronologies developed with key regional seismic surfaces allows comparison of stratal geometries produced bycontemporaneous depositional systems operating under identical eustatic conditions. This synoptic comparison permits differentiationbetween eustatic and other controls on sedimentation and testing of many of the assumptions made in sequence stratigraphy. A set ofpaleogeographic maps and sequence and systems-tract models depict the major depositional systems of the Gulf margin during differentstages of the eustatic cycle. These summary diagrams highlight considerable variability in stratigraphic architecture along the margin. Forexample, the relative proportion of highstand, lowstand, and transgressive strata differs between study areas. Thus, deposition and stratalpackaging are more complex than most sequence stratigraphic models predict. However, for any given fluvial, deltaic, and fan system, thegeneral style of deposition appears to repeat itself from one glacioeustatic cycle to the next. Thus, the results of this study can be used totest and calibrate quantitative stratigraphic models and to predict reservoir occurrence within a sequence stratigraphic framework.

Late Quaternary Stratigraphic Evolution of the Northern Gulf of Mexico MarginSEPM Special Publication No. 79, Copyright © 2004SEPM (Society for Sedimentary Geology), ISBN 1-56576-088-3, p. 1–23.

INTRODUCTION

During the past two decades there has been a proliferation ofsequence stratigraphic models, including the popular “slug dia-grams”. These models relate observations from seismic data, welllogs, and outcrop studies to the character and spatial and tempo-ral distribution of depositional systems in the subsurface (e.g.,Jervey, 1988; Posamentier et al., 1988; Christie-Blick and Driscoll,1995; Miall, 1997). The underlying assumption in such models isthat there is some orderly stratigraphic motif for any given basinsetting whereby the general style of deposition repeats itself witheach eustatic cycle as long as other controlling factors (tectonics,climate, sediment supply, shelf gradient, subsidence, etc.) remainrelatively constant. This seems like a reasonable assumption;however, the principal role of eustasy in controlling stratigraphicarchitecture is now being reevaluated (Galloway, 1989; Walker,1990; Schumm, 1993; Wescott, 1993; Shanley and McCabe, 1994;Christie-Blick and Driscoll, 1995; Miall, 1997; Ethridge et al.,1998).

Many researchers have attempted to improve stratigraphicmodels by using outcrop data (e.g., Van Wagoner, 1995). Geolo-gists working on outcrops are forced to make important assump-tions about paleogeography, base-level change, and climatic influ-ence on sediment supply. There is also a lack of chronostratigraphiccontrol needed for detailed outcrop correlation. These problemstypically lead to debate about stratigraphic interpretations (VanWagoner, 1995, 1998; Yoshida et al., 1996, 1998). Another approachto stratigraphic modeling is that of using laboratory experimentalmodels to examine stratigraphic response to changing base level,discharge, and subsidence (e.g., Wood et al., 1993; Koss et al., 1994;Heller et al., 2001; Paola et al., 2001). The main criticism of thisapproach concerns the scale of these models, although Koss et al.(1994) argue that the concept of sequence stratigraphy is scaleindependent. There has also been a proliferation of quantitativemodels for generating different stratigraphic architecture by vary-ing such parameters as sediment input, subsidence, and eustasy(e.g., Thorne and Swift, 1991; Steckler et al., 1993; Harbaugh et al.,1999). But, there is a paucity of data to test these models.

J.B. ANDERSON, A. RODRIGUEZ, K.C. ABDULAH, R.H. FILLON, L.A. BANFIELD, H.A. MCKEOWN, AND J.S. WELLNER2

Thus, the various approaches to stratigraphic modeling arelimited by different factors. What is needed to develop morepredictive stratigraphic models are experiments in which sedi-mentation and strata formation can be related to known rates ofeustasy, subsidence, and sediment discharge. It is important thatthese experiments be carried out in a broad range of depositionalsettings if the results are to be widely applicable for stratigraphicanalysis. Factors that should be defined for each setting include:(1) the size, climatic setting, fluvial geomorphology, topography,and geology of fluvial drainage basins that supply sediment tothe shelf, (2) the shelf gradient and subsidence rates, (3) theeustatic and climatic history of the basin, and (4) oceanographicinfluences on sedimentation.

Previous studies have shown that depositional geometriesand stratal architecture of Quaternary strata do reflect, at least atfirst approximation, sea-level changes (e.g., Suter and Berryhill,1985; Coleman and Roberts, 1988a, 1988b; Boyd et al., 1989;Farron and Moldonado, 1990; Tesson et al., 1990; Hernández-Molina, 1994; Sydow and Roberts, 1994; Anderson et al., 1996).Although these studies were often limited in their geographiccoverage and paucity of sediment cores needed for chrono-stratigraphic analysis and verification of seismic facies interpre-

tations, they showed a variety of stratal responses to the sameeustatic fluctuations. The next logical step is to conduct similarexperiments in a much larger basinal setting where regionalvariations in climate, sediment discharge, subsidence,neotectonics, and margin physiography and their influence onsedimentation and strata formation can be examined and chrono-logically matched.

This volume contains papers that present results from studiesof late Quaternary strata of the northern Gulf of Mexico basin.Our objective is to compare and contrast deposition and strataformation within different linked drainage systems and margindepocenters through one complete glacioeustatic cycle. The north-ern Gulf of Mexico margin is divided into eight distinct deposi-tional settings based on differences in fluvial-drainage-basin size,geology and physiography, climate setting, and margin physiog-raphy (Fig. 1, Table 1).

We focused on the last glacioeustatic cycle (120 ka) for anumber of reasons. First, this is the time for which sea-levelchange is best documented. Second, the strata of this age occur atshallow enough subsurface depths to be imaged using high-resolution seismic methods and they can be sampled usingconventional coring and shallow drilling techniques. Lastly,

Nueces R.

50-60

50-60

30-4

0

30-4

0

20-3040-50

Rio Grande R.

Colorado R.

Brazos R.

Pas

cag

ou

la R

.

50-60

0 500km

Average Annual Precipitation (in)

Bathymetry (m)

Gulf of Mexico 200

m30

0 m

400

m1000

m

500

m

100

m

60-80

Mississippi RiverDrainage Basin

Sabine R.

Apalach

icola

R.

Tom

bigb

ee R

.G

uadalupe R.

Trinity R.Pearl R.

Per

dido

R.

Mo

bile

R.

RG

CT

ET WL MD

MEL

ALWFAPL

FIG. 1.—Map of fluvial drainage basins for the northern Gulf of Mexico margin. Also shown are margin bathymetry and values ofmean annual precipitation for the region. The Gulf margin is divided into eight distinct settings based on differences in (1)drainage-basin size, geology, and climate, which control sediment flux; (2) margin physiography; and (3) subsidence rates. Thedifferent margin segments are labeled as follows. RG = Rio Grande, CT = central Texas, ET = east Texas, WL = western Louisiana,MD = Mississippi Delta, MEL = Mississippi–eastern Louisiana, ALWF = Alabama–west Florida, and APL = Apalachicola. TheMississippi River drainage basin is shaded.


chronostratigraphic methods for the late Quaternary provide thelevel of resolution necessary for direct correlation of depositionalsystems in different regions.

It is important that stratigraphic studies of this type encom-pass the full eustatic cycle so that the entire range of eustaticallycontrolled and climate-controlled influences on sediment dis-charge and deposition can be examined. Results presented in thisvolume show that the sediment discharge of modern rivers in thenorthern Gulf of Mexico are a fraction of what they were in thepast, so the modern depositional setting is not a good analoguefor most of the eustatic cycle.

Why the Gulf of Mexico?

The northern Gulf of Mexico margin encompasses a broadspectrum of depositional settings and is, in our opinion, un-equaled as a natural laboratory for research aimed at improvingour understanding of sedimentation and strata formation on low-gradient continental margins. There are a number of reasons whythe Gulf is particularly well suited for research of this kind.

1. The margin receives sediment from a number of fluvial sys-tems with very different sediment discharges, in terms of totalsediment flux and proportions of suspended and bedloadmaterial (Fig. 1). These differences in sediment discharge arerelated to differences in drainage-basin size, geology, climaticsetting, and fluvial geomorphology (stream gradient, sinuos-ity, channel morphology, etc.). For example, the rivers of eastTexas have very extensive meanderbelts and floodplainswhere vast quantities of sediment are sequestered (Wrightand Marriott, 1993). In contrast, the rivers of Alabama and

west Florida are incised into mostly Pleistocene strata andhave rather limited flood plains where sediment storage canoccur.

2. The northern Gulf of Mexico margin spans five major climaticbelts that run roughly NNW–SSE across the state(Thornthwaite, 1948). From east to west, precipitation reflectshumid, moist subhumid, dry subhumid, and semiarid condi-tions (Fig. 1). Wet and dry phases, linked to glacials andinterglacials, respectively, have a marked effect on the cli-matic belts, fluvial discharge, and fluvial and deltaic deposi-tion. Late Quaternary climatic conditions for the Gulf Coastregion have been summarized in review papers (e.g., DuBaret al., 1991; Toomey, 1993). These studies are highly general-ized, but they provide a first approximation for climate change,which can be related to variations in sediment flux with time.The drainage basins of Texas rivers have experienced semi-arid to humid climate shifts, although the magnitude andtiming of these changes undoubtedly varied across the region.The potential impacts of these climate shifts on sediment fluxto the Gulf should be profound (Langbein and Schumm,1958). The Louisiana, Mississippi, Alabama, and west Floridaclimates are believed to have remained relatively humidthroughout the last glacioeustatic cycle. We can estimatesediment flux using the volumes of depositional units andtheir ages.

3. Subsidence and sediment supply are relatively high, particu-larly offshore Louisiana and Texas, and are reasonably wellconstrained (e.g., Paine, 1993). Thus, preservation potential ofall systems tracts (highstand, lowstand, and transgressive) is

Study Area

(River)

South Texas

(Rio Grande)

Central Texas

(Guadalupe)

East Texas

(Brazos,

Colorado)

East Texas

(Trinity,

Sabine)

Western

Louisiana

Lagniappe

(West Mobile,

Pascagoula)

Florida Alabama

(East Mobile,

Escambia)

West Florida

(Apalachicola)

Drainage Basin

size (km2) 400,000 24,000

118,000 (Brazos)

110,000 (Colorado)

44,000 (Trinity)

13,000 (Sabine)

unknown

95,000 (Mobile)

95,000 (Mobile)

19,000 (Escambia) 60,000

climate semiarid semiarid semiarid

to subhumid

moist subhumid moist humid moist humid moist humid humid

Fluvial Morphology braided tomeandering

small coastalplain andpiedmont rivers

broad, meandering narrow,meandering

broad, meandering steep, incised steep, incised(Mobile)

small coastal-plainriver (Escambia)

steep, incised

bedload/suspendedload

high (mixed) low (mixed) high (suspended-Brazos

bedload Colorado)

moderate(suspended)

high (mixed) high (bedload) low (bedload) moderate(bedload)

modern discharge(m3/s-1)

123 75 226 (Brazos)

81 (Colorado)

730 (Trinity)

510 (Sabine)

2200 2200 (Mobile)

250 (Escambia)

650

Coastal-plain gradient

width (km)

low low

150

low

150

low

150

low

170

low steep

< 20 km

steep

75

Shelf

morphology

shelf-slope ramp shelf-slope shelf-slope shelf-slope shelf-slope ramp ramp

gradient (m/km) 1.8–2.75 1.2 0.6 0.5 0.5 0.5–3.3 1.2 (west) to 2.7(east)-6

1.3– 3.5

width (km) 90 80 100 160 180 140 100 (west)-35 (east) 75

storm/wave/fluvialdominated

fluvial wave fluvial wave fluvial fluvial storm storm

lithology muddy muddy muddy muddy mixed sandy sandy sandy

structure growth faulting growth faulting salt and growthfaulting

salt and growthfaulting

salt and growth

faulting

minor diapirs

and large scalegrowth faulting

undisturbed by saltand faulting

undisturbed bysalt and faulting

subsidence (mm/yr) high (0.1–5.0) high (0.1–5.0) high (0.1–4.0) high (0.1–4.0) high (0.1–5.0) low low (0.5) low (0.5)

sediment flux

(metric tons/year)

20,000,000 16,000,000 (B)

1,900,000 (C)

750,000 (S) 170,000

TABLE 1.—Characteristics of the various study areas. The modern Mississippi River Delta is not included. Study area locationsare shown in Figure 1. The drainage basin of the western Louisiana area, which is part of the ancestral Mississippi River

drainage basin, has changed, and its modern configuration is therefore unknown. Likewise, the drainage basinof the Lagniappe delta is uncertain, but it probably was nourished by both the Mobile and Pascagoula rivers.

Data on modern river discharge and sediment flux are from Milliman and Syvitski (1992).


high, and these strata are thicker than on most other continen-tal margins. For example, detailed work on the New Jerseycontinental margin has shown that strata formed during thefalling stage of sea level (120 ka to 22 ka) are poorly repre-sented on the shelf because of reworking during the transgres-sion (Duncan et al., 2000).

4. Shelf widths vary from 30 km to 180 km; shelf and upper-slopegradients are therefore highly variable as well. The Texas andLouisiana shelves have distinct shelf breaks, with the excep-tion of the central Texas margin, which lacks a distinct shelfbreak. The western Florida margin is more of a steep rampsetting.

5. Over 25,000 kilometers of high-resolution seismic data isavailable (Fig. 2), and there are abundant oil-company plat-form borings and long cores needed to test seismic faciesinterpretations and for chronostratigraphic analysis. The onlyarea where long cores were not available at the time of thesestudies was on the Alabama and west Florida shelves.

6. The northern Gulf of Mexico region has a long history ofsedimentological and paleontological research and has beenthe location of significant pioneering discoveries (e.g., Fisk,1944; LeBlanc and Hodgson, 1959; Curray, 1960; Kennett andHuddlestun, 1972; Frazier, 1974; Berryhill et al., 1986; Kohl,1986). These earlier studies have provided an important frame-work for this investigation.

Methods

The data set used for this study consists of over 25,000 kilome-ters of high-resolution seismic data (Fig. 2), lithological descrip-tions of hundreds of oil-company platform borings, and paleon-

tological, sedimentological, and geochronological data from sev-eral hundred pneumatic hammer cores (up to 5 meters length)and ten long (average 100 m) cores.

In all of the papers contained in this volume, classical seismicstratigraphic methods were used to describe the external formsand internal stratal geometries of depositional units (e.g., Mitchumet al., 1977; Vail et al., 1977a, 1977b; Vail et al., 1997c). The natureof the bounding surfaces and the reflection configurations of theunits were then combined with the timing of their formation toexamine the relative roles of eustasy and other controlling factors(e.g., sediment supply and tectonism) on deposition. An analysisof this kind is necessary in order to understand the factors thatinfluence the evolution and distribution of lithofacies in spaceand time.

An independent chronology was established for the northernGulf of Mexico through integration of biostratigraphy, tephro-chronology, radiocarbon dating, and oxygen isotope stratigra-phy. The chronology is linked to regionally extensive seismicreflectors by the intersection of the locations of the sediment coreswith the seismic dataset. These ties create a robust chronostrati-graphic framework from which to interpret the temporal andspatial distribution of depositional environments on the shelfover the past 120,000 years.

Seismic Stratigraphic Method and Terminology

Sequence stratigraphy has provided a valuable means ofsubdividing stratigraphic packages, but the proliferation of termsand new methodologies has resulted in much confusion. Part ofthe problem stems from the attempts of researchers to linksystems tracts to specific well-defined segments on a sea-levelcurve (e.g., Haq et al., 1987), but it is unclear whether this was everthe intention of the Exxon workers who developed the terminol-ogy (Van Wagoner et al., 1988).

FIG. 2.—Seismic data acquired during the past nine years by the Rice University group (areas 1, 2, 3, 4, 5, 8, and 9), the LSU–Oil Industryconsortium (area 6), and the University of Alabama (area 7). The corresponding area numbers and papers in this volume are asfollows. 1 = Banfield and Anderson; 2 = Eckles et al., 3 = Abdulah et al., 4, 5 = Wellner et al., 6 = Fillon et al., Roberts et al., andKohl et al.; 7 = Bartek et al., 8 = Bart and Anderson, and McBride et al.; and 9 = McKeown et al.

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FlorAla

EastTexasTrinity/Sabine

WesternLouisiana

CentralTexas

SouthTexas

Gulf of Mexico

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Apalachicola

1

2

34 5 6

7

9

8East TexasBrazos/Colorado


In the end, what is important is the adoption of a consistentmeans of stratigraphic analysis. In this volume, because there isa well-defined sea-level curve spanning the late Quaternary, theauthors have related systems tracts to specific periods on the sea-level curve. Several authors then apply the Exxon terminology,using the original tripartite scheme for the partitioning of uncon-formity-bounded sequences (Vail et al., 1977a, 1977b) (e.g., Figs.3, 4), and others leave this interpretation to the reader. By linkingsystems-tract terminology and all discussions of stratigraphicfeatures directly to the sea-level curve, the relative roles otherallocyclic processes play (in particular, climate change and asso-ciated sediment supply) in influencing stratal stacking patternscan be examined, and that is our ultimate objective.

One of the most confusing terms is “highstand systems tract”.Because the highstand systems tract, by definition, rests abovethe maximum flooding surface and below the sequence bound-ary (Posamentier and Vail, 1988), it usually spans the period offalling sea level. Likewise, when defining systems tracts purelyby their internal geometries (descriptively as opposed to geneti-cally), it is clear that the lowstand systems tract may include muchof the time interval when sea level is rising (Nystuen, 1998).

Plint and Nummedal (2000) have suggested a new systemstract, the falling-stage systems tract (FSST), which lies aboveand basinward of the highstand systems tract and below thelowstand systems tract. The term falling-stage systems tract ismore intuitive than highstand systems tract when referring tothe time when sea level is actually falling. But, the term has notbeen widely accepted. Plint and Nummedal (2000) point outthat the most diagnostic criteria of the FSST are the presence oferosionally based shoreface sand bodies in nearshore areas.The erosion results from wave scour during sea-level fall(regressive surface of marine erosion). This approach stemsfrom outcrop studies where such associations may be evident.When working with seismic data and core logs, however, it isnot always possible to trace the base of the falling-stage sys-tems tract.

A lingering controversy in sequence stratigraphy concernswhich surfaces should be used to subdivide sequences andhow these surfaces are identified. The Exxon model (Vail et al.,1977a, 1977b; Vail et al., 1977c) favors use of the sequenceboundary. This surface of subaerial exposure and fluvial ero-sion forms during much of the time interval when sea level is

FIG. 3.—Composite oxygen isotope records (Labeyrie et al., 1987; Shackleton, 1987) calibrated with U–Th dates on corals (Bard et al.,1990; Chappell et al., 1996) and the Stage 3 paleoshoreline position on the Texas shelf (Rodriguez et al., 2000) are integrated andused as a sea-level proxy curve for the past 140,000 years. Curve A is the SPECMAP oxygen isotope curve that shows the last fourglacial cycles (from Imbrie et al., 1984). Curve B shows the isotope curve converted to sea level with sea-level datums noted.

OIS 1

HST (early)

230 Th age (kyr)

U/Th dates ofBarbados corals(Bard et al., 1990)

OIS 2 OIS 3 OIS 5 ec

a

OIS6

OIS4

b d

20

0

-20

-40

-60

-80

-100

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-1400 20 40 60 80 100 120 140

sea

leve

l (m

)

U/Th dates ofHuon, NewGuinea corals(Chappell et al.,1996)

New Guinealowstand deposits(Chappell et al.,1996)

Rodriguez et al., 2000

2

3

4

5a

5b

5c

5d

5e

6

7

8

9

10

11

12

0 100 200 300 400

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sy (

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0

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5e MFS

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Stage 3

A

B

Benthic δδδδδ18O curve,Norwegian Sea(Labeyrie et al., 1987)

Compositebenthic/planktic δδδδδ18Ocurve, Pacific Ocean(Shackleton, 1987)


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hig

hst

and

del

ta

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ge

2 S

B

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ge

5e m

fsS

tag

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mfs

mu

ltip

le

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falling as well as the lowstand (Posamentier et al., 1988;Posamentier et al., 1992). Galloway (1989) has suggested that themaximum flooding surface, a condensed section formed duringthe maximum flooding of the shelf, is the more prominent andless diachronous surface and therefore should be used fordivision of sequences. Sydow and Roberts (1994) used maxi-mum flooding surfaces to subdivide the stratigraphic packageof the Lagniappe Delta because those surfaces were easier torecognize than sequence boundaries, both on the slope and onthe shelf. On the Texas shelf, the sequence boundary is the moreprominent surface. Without question, this investigation dem-onstrates that it is at the maximum lowstand when the mostbasinward subaerial exposure and sediment bypass of the shelfoccurs. This is also the time when maximum incision of fluvialvalleys takes place.

Sediments that are eroded on the shelf to form the sequenceboundary are deposited on the slope to create a conformablesection, which may be quite thick. Should the sequence bound-ary be placed above or below this section? For the most part,delta development across the northern Gulf of Mexico shelfduring the late Quaternary occurred throughout the fallinglimb of the sea-level curve (Fig. 4). These deltas reached theshelf margin at different times, depending on the sedimentsupply of their fluvial feeders. Some (e.g., the Brazos Delta)were active during the falling stage of sea level; others (e.g., theRio Grande) were most active during the lowstand; and othersremained active during the transgression (Trinity/Sabine/Brazos deltas). For the most part, these deltas downlap themaximum flooding surface and lie beneath the sequence bound-ary (Fig. 4).

During the fall in sea level, shelf accommodation decreasesand deposition on the shelf shifts seaward, with erosion of inner-shelf strata contributing to the nourishment of outer-shelf strata.This concept has been around for more than a century (Chamberlin,1898), but sequence stratigraphers have redefined it as a forcedregression (Posamentier et al., 1992) and yet another systemstract, the forced regressive systems tract, has emerged (Hunt andTucker, 1992). The lower boundary of the forced regressivesystems tract (FRST) is the basal surface of forced regressions(Hunt and Tucker, 1995), which is where Posamentier et al. (1992)and Posamentier and Morris (2000) would place the sequenceboundary. It is easily confused with the maximum floodingsurface.

Along-strike variability in late Quaternary stratal architectureacross the northern Gulf of Mexico margin is the rule rather andthe exception. This variability results from differences in thetiming and extent of delta progradation across the shelf, which iscontrolled by the long-term sediment discharge of rivers (Fig. 4).Figure 4 illustrates the problem. At this location on the shelf, theBrazos delta prograded across the shelf prior to progradation ofthe Colorado delta. This example illustrates why it is difficult totrace the base of the forced regression. Kolla et al. (2000),Hernández-Molina et al. (2000), and Trincardi and Correggiari(2000) also found that the forced regressive surface is difficult torecognize and map regionally, even with the best high-resolutionseismic data.

Chronostratigraphy

In the study areas, the chronologic framework was developedby integrating foraminiferal abundance variations, foraminiferalextinctions, tephrochronology, oxygen isotope stratigraphy, andradiocarbon dates. This integrated chronology builds on previ-ous work in the Gulf of Mexico by industry and academic inves-tigators.

Biostratigraphy.—

Planktonic foraminiferal assemblages have long been usedto identify cold-water and warm-water intervals and to deter-mine the age of the intervals (Ericson and Wollin, 1968; Thunell,1984; Kohl, 1986; Kohl et al., this volume). Fluctuations in theoccurrence of two species of planktonic foraminifera, Globorotaliamenardii and Globorotalia inflata, are used to define Ericson zones(Ericson and Wollin, 1968). This zonation was later modified byKennett and Huddlestun (1972), who examined the quantitativepresence or absence of the Globorotalia menardii complex andseveral other foraminifera species in piston cores from the Gulf.This modified zonation was used only when sufficient numbersof planktonic foraminifera were present (e.g., Banfield et al., thisvolume) (Fig. 5). The extinction of Globorotalia menardii flexuosa,interpreted as occurring near the oxygen isotope boundarybetween stages 5a and 5b (approximately 85 ka) and the first-appearance datum of the calcareous nannofossil Emiliania huxleyiin stage 8 (approximately 260 ka) provide other importantbiostratigraphic benchmarks for this study (Kennett andHuddlestun, 1972; Poag and Valentine, 1976; Kohl et al., thisvolume).

On the east Texas and western Louisiana shelves, planktonicforaminifera are generally restricted to those stratigraphic inter-vals that record maximum flooding of the shelf. Benthic foramin-ifera, however, are generally more abundant in cores from thissector of the northern Gulf. Thus, detailed analyses of benthicforaminifera were conducted on cores and the paleobathymetriccurves generated from these analyses used to refine our oxygenisotope and seismic stratigraphic interpretations in areas west ofthe modern Mississippi delta (Abdulah et al., this volume; Wellneret al. this volume). In areas east of the Mississippi delta, particu-larly at the shelf edge and on the upper slope, planktonic foramin-ifera are more abundant and provide the basis for additionaloxygen isotope and biostratigraphic interpretation (Fillon et al.,this volume; Kohl et al., this volume).

Radiocarbon Dating.—

Conventional and accelerator mass spectrometer radiocarbondates provide further information for the last glacioeustatic cycle.The maximum age that can be determined using AMS radiocar-bon methods depends on sample quality but is about 40 ka Noneof the authors exclude any samples for which radiocarbon dateswere obtained. Out-of-order dates can provide important infor-mation on flooding and erosion on the shelf (see discussion byFillon et al., this volume).

Oxygen Isotope Stratigraphy.—

In this project, the global oxygen isotope curve is used as aproxy sea-level curve. In addition, oxygen isotope curves weregenerated within some study areas as a chronostratigraphicframework and for correlation between study areas (Abdulah etal., this volume; Banfield and Anderson, this volume; Eckles et al.,this volume; Fillon et al., this volume).

Previous oxygen isotope stratigraphic studies have tended toavoid the continental shelf. This was due to concerns aboutincomplete stratigraphic section due to erosion, variable deposi-tion rates, paucity of planktonic foraminifers, diagenetic alter-ation, and fresh-water contamination overwhelming the isotopicsignal (see discussion in Fillon et al., this volume). Furthermore,foraminifera are patchy in their down-core distribution on theTexas and Louisiana outer shelf and planktonic foraminifera arerestricted to highstand deposits. In the ideal case, seismic strati-


graphic analyses could be used to locate optimal core locationsand compile the most complete stratigraphic section using care-fully selected cores from the outer shelf. But, with the exceptionof the Lagniappe delta study (Fillon et al. this volume; Roberts etal. this volume; Kohl this volume), the authors did not have theoption of planning core locations.

Despite these problems, the results of studies contained in thisvolume indicate that a useful oxygen isotope record can beextracted from continental-shelf cores, given a seismic strati-graphic framework and results from paleontological and radio-carbon analyses (Fig. 5). That framework provides informationon where within the section there are unconformities and yieldsinformation about where deposition was occurring on the shelfduring a particular time interval.

The oxygen isotope curves generated for this project wereinterpreted by comparing them with reference oxygen isotopecurves of Globogerinoides ruber from Gulf of Mexico and Carib-bean cores. The two reference curves are from DSDP site 619 inthe Pigmy Basin offshore Louisiana (Williams and Kohl, 1986)and the southwestern Gulf of Mexico (TR-126-23; Williams,1984). The Caribbean reference curve is a stacked isotope recordassembled from several deep-sea piston cores (Emiliani, 1978).In addition, to provide stratigraphic benchmarks, these oxygenisotope records also record the influx of 16O-rich meltwater intothe northern Gulf during the retreat of the Laurentide ice sheet.Meltwater that flowed down the Mississippi River between 14ka and 12 ka (Leventer et al., 1982; Fillon and Williams, 1984;Williams and Kohl, 1986; Brown and Kennett, 1998) created anegative spike in the planktonic oxygen isotope records of theGulf of Mexico (Kennett and Shackleton, 1975; Leventer et al.,

1982). At about 12 ka the outflow of meltwater shifted to theAtlantic Ocean, through the Champlain–Hudson Valley (Teller,1987). Older meltwater spikes associated with stage 5 throughstage 3 glaciation also have been identified in the Gulf of Mexicoby Williams (1984), Trainer and Williams (1990), and Joyce et al.(1993).

Our chronostratigraphic framework is generally not preciseenough to allow identification of individual meltwater pulses,with the possible exception of the 14 ka to 12 ka meltwater event(Abdulah et al., this volume; Banfield and Anderson, this vol-ume; Fillon et al., this volume) and a 260 ka stage 12-stage 13transition meltwater event (Fillon et al., this volume). All of theisotope curves from the Texas shelf show that possible meltwa-ter spikes occur within the stage 3 interval of the cores. Ingeneral, the magnitude of oxygen isotope variations due tomeltwater influx decreases westward away from the Missis-sippi River and is low east of the river. The possibility that thesemeltwater events indicate ice-volume-related eustatic events isdiscounted. This is based on the observation that none of thestage 3 deltas studied show the kinds of backstepping andprograding character that would have occurred if these deltashad been subjected to eustatic changes of the magnitude indi-cated (tens of meters) by the spikes.

Sea-Level Record

The relationship between the oxygen isotope record andglobal ice volume (Shackleton and Opdyke, 1973) provides aproxy for changes in global sea level (Shackleton, 1987) (Fig. 3).The conversion of the isotope curve to sea level assumes that sea

0

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iii: Possible 42,800 to 44,300 (Williams and Kohl, 1986)

ii: 45,210 980 yr. B.P. at 10.7 m from AMS age date

i: 4,490 70 yr. B.P. at 5.8 m from AMS age date

vi: 85,000 yr. B.P. extinction of G. menardi flexuosa

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v: Possible 60,700 (Williams and Kohl, 1986)

core B-2

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FIG. 5.—A) A segment of seismic Line 5 from the south Texas shelf showing the location of core B-2 and the key seismic stratigraphicsurfaces sampled by this core. B) Oxygen isotope curve for Core B-2, showing chronostratigraphic benchmarks for seismicstratigraphic correlation of late Pleistocene strata on the shelf and upper slope (after Rodriguez et al., 2000). Also shown areEricson zones (X–Z) as described by Kohl et al. (this volume).


level was 6 m higher than today during the last interglacial and120 m lower than the present level during the last glacial. A 10 msea-level change for every 0.1 per mil change in δ18O, afterShackleton and Opdyke (1973), provides the basis for sea-levelcurves used in this volume (e.g., Fig. 3).

The proxy sea-level curve in Figure 3 is constrained by U–Thdated sea-level stands for the interval between stage 5e and earlystage 2 by in situ corals (Bard et al., 1996; Chappell et al., 1996) andis more tightly constrained for the time interval between latestage 2 (ca. 18 ka) to present by radiocarbon-dated shell and peatsamples (e.g., Bloom, 1983). The seismic stratigraphic results ofthis study show reasonable agreement with the isotope curve,with the exception of the amplitude of isotope stage 3. Data fromthis study show a sea-level stand of -15 m to -20 m for the highestsea-level position during isotope Stage 3 (Rodriguez et al., 2000).This is consistent with earlier studies in the Gulf (Suter et al.,1987), offshore New Jersey (Wellner et al., 1993), and offshoresoutheast Australia (Roy et al., 1997) and with published sea-levelreconstructions (Matthews, 1990; Moore, 1982; Chappell et al.,1996; Kaufmann, 1997) used in the Lagniappe delta study (Fillonet al., this volume).

Despite its inaccuracies, the proxy sea-level curve, like thatshown in Figure 3, is the most precise sea-level record for all ofgeological time. It is generally accurate to within ± 30 m for thetime interval between 120 ka and 15 ka, and to within ± 5 m for thepast 15,000 years. Oxygen isotope curves also provide the mostaccurate indication of the timing and frequency of glacioeustaticchange.

Subsidence Rates

In this volume we constrain tectonic subsidence rates on theshelf using maps of the elevations of the stage 5e maximumflooding surface (Fig. 4). The assumption made in these calcula-tions is that the relative water depth at the shelf break wasapproximately the same at the onset of stage 5 interglacial condi-tions as it is today. Stage 5e deposits occur at about 6 m above sealevel on the modern coastal plain and are exposed or nearlyexposed a short distance from the coast. Thus, subsidence ratesare negligible along the coast. In general, all along the Gulfmargin, the thickness of strata resting above the 5e maximumflooding surface increases more or less linearly across the shelf.This implies that subsidence rates increase linearly in an offshoredirection. Measured rates of subsidence are highest on the Texasand Louisiana shelves and decrease from west to east across theMississippi and Alabama–west Florida shelves (Table 1). A dis-cussion of the processes contributing to subsidence on the outershelf is contained in the paper by Fillon et al. (this volume).

RESULTS

Data and interpretations presented in this volume provide thebasis for reconstructing summary paleogeographic maps (Figs.6–8) that display the major depositional systems (systems tracts)on the shelf and upper slope during the intervals 120 ka to 22 ka,22 ka to 16 ka, and 16 ka to 4 ka. We selected these periods of timefor our paleogeographic reconstructions because they represent

Deltas

Channels and valleys

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Shoreface deposits

Shelf muds

Legend

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0 200

RGD

CDBD WLD

WLD

LD

EMD

28

AD

94 88

Highstand Systems Tract (Stage 5e-3)

98 96 92 90 86

1

2 3

1

1

2

2

3

1

24

WFLAD

FIG. 6.—Paleogeographic map showing major depositional systems that existed on the shelf 120 ka to 22 ka (stages 5e–3 highstand;Fig. 3). The different lobes of individual deltas are numbered in chronological order. RGD = Rio Grande Delta, CD = ColoradoDelta, BD = Brazos Delta, WLD = Western Louisiana Delta, LD = Lagniappe Delta, EMD = Eastern Mobile Delta, WFLAD = WestFlorida–Alabama Delta, and AD = Apalachicola Delta.

East TexasTrinity/Sabine

WesternLouisiana

East TexasBrazos/Colorado

CentralTexas

SouthTexas

FlorAla Apalachicola


key episodes of sea-level change (Fig. 3). The individual papers inthis volume provide more detailed paleogeographic informa-tion.

Correlation of strata between study areas is accomplishedusing three prominent stratigraphic surfaces (Fig. 4). The oldestof these surfaces is the stage 5e maximum flooding surface(condensed section), which precedes the last high sea-level standat approximately 120 ka (Fig. 3). The stage 5e maximum floodingsurface is a downlap surface in most of the region (Fig. 4). It isrecognized in outer-shelf and upper-slope cores by a sharp in-crease in planktonic foraminifera, by the presence of Globorotaliaflexuosa, and by a characteristic oxygen isotope signature (Fig. 5).The highstand systems tract (in the Exxon sense) occurs betweenthis flooding surface and the stage 2 sequence boundary (Fig. 4).There is another prominent flooding surface associated with thestage 3 sea-level rise, but no prominent erosional surface isreported that records the stage 4 sea-level fall (Fig. 3). On manyparts of the shelf, a stage 3 flooding surface separates earlyhighstand (stage 5) strata, which are confined mostly to the innershelf, from stage 3 to stage 2 strata, which occur mostly on theouter shelf and slope (Fig. 4).

The third prominent stratigraphic surface is the stage 2 se-quence boundary. It is a prominent erosional surface, marked bydeep fluvial valleys and truncation of delta topset beds (Fig. 4).This surface is manifested in cores and platform borings by anabrupt change in lithology, a dramatic change in sediment shearstrength (indicating prolonged exposure), general reduction offossils, and a characteristic oxygen isotope signature (Fig. 5).Preservation of lowstand deposits on the shelf above the se-quence boundary is minimal, except in incised valleys. Deposits

of the lowstand systems tract are confined mostly to the outershelf and slope (Fig. 7). They are separated from transgressivedeposits by the transgressive surface. The transgressive surfacecorrelates up dip to the first marine incursion onto the shelf (Fig.4A). On the inner shelf, deposits of the transgressive systems tractoften rest on a surface that is the composite sequence boundary–transgressive surface.

120 ka to 70 ka (Early Highstand)

Approximately 120,000 years before present, sea level was atits maximum highstand position, a few meters above presentsea level (Fig. 3). An ancestral beach-ridge complex, referred toas the Ingleside paleoshoreline, marks the location of this maxi-mum highstand (Graf, 1966). In Texas and Louisiana, theIngleside paleoshoreline is located several kilometers landwardof the modern shoreline. This is the most extensive paleoshorelinedeposit of the last eustatic cycle. Its preservation suggests thatit was never subjected to either transgressive or regressiveshoreface erosion. North of the Ingleside paleoshoreline inTexas, the landscape is dominated by fluvial meanderbelts thatspan the entire glacial eustatic cycle. South of this paleoshorelinethe coastal plain is virtually flat. Older fluvial channels aremostly buried beneath Holocene coastal-plain deposits. Stackedmaximum highstand fluvial channels can be seen in the manysandpits that occur in the old Brazos and Colorado valleysystems.

After the last interglacial the ice sheets in both hemispheresbegan to expand, and sea level fell episodically. By the end ofStage 5 the paleoshoreline was located on the middle shelf, at

FIG. 7.—Paleogeographic map showing the major depositional systems that existed on the shelf and upper slope during the 22 ka to16 ka (stage 2) lowstand (Fig. 3). Lowstand incised valleys are labeled as follows. RGV = Rio Grande, CV = Colorado, BV = Brazos,T/SV = Trinity–Sabine, WMV = west Mobile, and EMV = east Mobile. MC = Mississippi Canyon.

98 96 94 92 90 88 86

30

28

26

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Delta or wedge

Slope deposit

Legend

FlorAlaEast TexasTrinity/Sabine

WesternLouisiana


kilometers

0

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CV

BV

T/SV

Apalachicola

Lowstand Systems Tract (Stage 2)

200

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WMV

EMV

SouthTexas

CentralTexas


98 96 94 92 90 88 86

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Delta or wedge

Legend

0 200

Kilometers

MAFLA

Texas mud

Sand banks

100m

Transgressive Systems Tract (Stage 1)

MFLA

TMB

Shelf muds

approximately 60 m below present sea level (Fig. 3). This intervalof time when the paleoshoreline moved across the inner shelf isreferred to as the early highstand. The early highstand sawincreased sediment supply to the basin, as a result of fluvialincision, which removed sediments stored in the lower reaches ofthe drainage basins. During the early highstand, deposition wasconfined to the inner shelf where subsidence rates are low, hencepreservation was low. For this reason, early highstand strata havea patchy distribution on the inner shelf of Texas and are generallyabsent on the Alabama and west Florida inner shelves (Fig. 6).Early highstand deposits are more extensive on the westernLouisiana shelf, where subsidence rates are high (Wellner et al.,this volume).

During the Stage 5 interglacial high-sea-level episode, a sig-nificant portion of the Mississippi River drainage was directedtoward the western Louisiana shelf. There, a large, sandyhighstand delta existed (Fig. 6; Coleman and Roberts, 1990;Wellner et al., this volume). Likewise, highstand deltas wereassociated with the ancestral Rio Grande, Colorado, and BrazosRivers in Texas (Banfield and Anderson, this volume; Abdulah etal., this volume), and with rivers east of the modern Mississippidelta (Fig. 6). As sea level fell during stage 5 (Fig. 3), these deltasprograded basinward and their updip portions suffered consid-erable erosion by rivers and streams and by waves (regressiveshoreface erosion). Again, the relatively low subsidence rates onthe inner shelf contributed to the poor preservation of the earlyhighstand deltas. Erosion of the upper portions of these deltasremoved the upper sand-prone fluvial and delta-front deposits,leaving mostly muddy distal bar and prodelta deposits on theinner shelf. The only exception to this was the western Louisiana

Delta, which includes widespread sandy facies on the inner shelf(Coleman and Roberts, 1990).

Rivers with relatively low sediment fluxes, such as the Trinityand Sabine, apparently did not construct large deltas on thecontinental shelf during the early highstand. On the Alabama–west Florida shelf, early highstand deposits are thin and patchy.They may be the remnants of a once extensive braided sheet sandthat formed on the steep shelf as sea level fell (Bart and Anderson,this volume; McKeown et al., this volume).

On the central Texas shelf, where there are no large rivers,deposition during this period was dominated by coastal and shelfprocesses (Eckles et al., this volume). These systems were fed bysediment eroded from adjacent deltaic headlands and deliveredto the central Texas coast by longshore currents flowing from theeast and south. Prograded clastic shoreline and shoreface depos-its are preserved on the inner shelf. Over the long term, accommo-dation space created by subsidence was not being filled, resultingin the narrow, steep shelf physiography of this region.

70 ka to 22 ka (Late Highstand “Falling Stage”)

Approximately 70 ka, during stage 4, sea level fell then roserapidly, culminating in the stage 3 flooding event (Fig. 3). At thepeak of the flooding the paleoshoreline was located on the innershelf, perhaps as high as -15 m (Rodriguez et al., 2000). Followingthe flooding episode, falling sea level again shifted thepaleoshoreline to the outer shelf, causing incision of fluvialchannels during stage 2.

Deltas constructed during this period of falling sea levelgenerally contain more sediment than early highstand deltas,

FIG. 8.—Paleogeographic map showing the major depositional systems that existed on the shelf during the 16 ka to 4 ka (stages 2–1)transgression (Fig. 3). TMB = Texas Mud Blanket; MAFLA = MAFLA sheet sand. The transgressive deltas of the Mississippi Riverare taken from Frazier (1967).


East TexasTrinity/Sabine

CentralTexas

SouthTexas

WesternLouisiana

FlorAla Apalachicola


deposited in the interval 120 ka to 70 ka, despite the fact that stage3 was no longer in duration than stage 5. During stage 3, deposi-tion was greatest on the outer shelf where subsidence rates, andtherefore preservation potential, are high (Fig. 4). Also, cannibal-ization of stage 5 deposits by fluvial processes and regressiveshoreface erosion resulted in a large offshore flux of sediment,and hence contributed to the volume of the stage 3 deposits (Fig.4A). Large stage 3–stage 2 deltas were associated with the RioGrande, Colorado, Brazos, western Louisiana, west Mobile–Pascagoula (Lagniappe delta), east Mobile, Perdido–Escambia,and Apalachicola rivers (Fig. 6).

The Rio Grande, Colorado, Brazos, and western Louisianadeltas of this age are characterized by clinoforms that dip at lowangles (less than 3°) and are composed mainly of prodelta muds(Fig. 9A). The topset beds of these late highstand deltas do,however, include extensive, sandy fluvial feeder channels anddistributary-mouth bars. Indeed, the sandy mouth bars of thesedeltas cover vast areas on the outer shelf (Fig. 6; Abdulah et al.,this volume; Banfield and Anderson, this volume; Wellner et al.,this volume). The Lagniappe delta also contains thick andextensive delta-front sand deposits (Sydow and Roberts, 1994;Roberts et al., this volume). West Florida–Alabama late high-stand deltas have relatively steep clinoforms (greater than 3°)and little or no bottomset (prodelta) beds (Fig. 9B). These fea-tures, in conjunction with seismic facies interpretation, indicatethat late highstand deltas on the ramp-like outer shelf are sand-prone (Bart and Anderson, this volume; McKeown et al., thisvolume). On this slowly subsiding part of the margin, accom-modation space was minimal, so these deltas experienced largelateral shifts.

The larger river deltas of the northern Gulf of Mexico shelfexperienced virtually continuous growth during the stage 3 sea-level fall (Figs. 4A, 6). These deltas were situated in a shelf-marginposition at the end of the late highstand (Fig. 4). However, thetimes at which individual deltas reached the shelf margin varyacross the region, presumably as a result of the different sedimentdischarge of the rivers. The diachronous progradation of deltasacross the outer shelf is illustrated in seismic profile R93-51, astrike-oriented profile collected across the outer shelf (Fig. 4B).This profile shows that the younger late stage 2 Colorado andTrinity–Sabine–Brazos shelf-margin deltas onlap the older stages3–2 Brazos delta. The stage 3–early stage 2 Brazos delta progra-ded across the outer shelf first, followed by the other deltas. Thus,not every shelf-margin delta is in the lowstand systems tract. Thisis an important point because it is implicit that not every shelf-margin delta is associated with a slope fan or basin-floor fan. Italso explains why shelf-margin deltas can be large, despite thebrief time span of the lowstand.

Line R93-51 (Fig. 4B) also illustrates how dip lines across theshelf margin may image the superimposed clinoforms of tempo-rally offset deltas. This results in different stratigraphic architec-tures at different locations along the outer shelf and upper slope.The diachronous response of deltas to sea-level rise along theGulf margin is similar to that observed in the Adriatic Basin byTrincardi et al. (1994).

On the central Texas shelf, sandy prograding clastic shorelineand shoreface deposits grade offshore into shelf muds; there areno outer-shelf sand bodies in that area. Prograding shorelinesassociated with the early fall in sea level were unable to keep pacewith the rapid fall that occurred during stage 4. This was largelydue to reduced sediment supply during this time (see discussionin Eckles et al., this volume). By the late highstand, the Rio Grandeand Colorado deltas, which had served as longshore sedimentsources for the central Texas shelf during the early highstand,were not being eroded as extensively as they were during the

early highstand. Thus, sand deposition occurred in differentenvironments and at different times across the Texas shelf.

22 ka to 16 ka (Lowstand)

During the lowstand, streams and rivers cut their deepestincised valleys, to produce the Stage 2 sequence boundary, andsignificant volumes of sediment bypassed the shelf (Fig. 7). Thestage 2 sequence boundary is a prominent surface throughout thenorthern Gulf of Mexico and constitutes a definitive surface forseparating highstand and lowstand systems tracts. On the basisof global sea-level curves (Fig. 3), the paleoshoreline in thenorthern Gulf was situated at or near the shelf break (approxi-mately -120 m water depth) during the last glacial maximum.Thus, the shelf was subarially exposed. The exact age and dura-tion of the stage 2 lowstand is still uncertain, but it spanned onlya few thousand years. Prominent features of the lowstand includeincised fluvial valleys, lowstand deltas, slope fans, and othersediment-gravity-flow deposits (Fig. 7).

Lowstand fluvial valleys of the northern Gulf of Mexico varyconsiderably in their morphology. In general, the low-gradienteast Texas and western Louisiana shelves are characterized byvalleys that become broader and shallower in a seaward direc-tion. On average, these valleys are 40 m deep at the presentshoreline, which is about the same depth of incision as during theprevious lowstand (Blum and Price, 1998). The similarity indepths of incision of different rivers, which vary in terms ofdischarge and gradient, indicates a similar response to base-levelfall regardless of these differences.

The Rio Grande and Colorado rivers remained relativelyfixed in their locations throughout the highstand, resulting inbroad channels that subsequently were incised during the maxi-mum lowstand. These incised channels deepen offshore, cuttinginto late highstand and lowstand deltas to produce sandy slopefans. In contrast, the Brazos and western Louisiana rivers di-verted from their late highstand channels into different locationsat the beginning of the lowstand, leaving their broad feederchannels and associated highstand deltas isolated on the shelf.There are no slope fans downdip of these features. The Trinityand Sabine rivers have occupied the same valleys throughout theeustatic cycle, and apparently during previous eustatic cycles.Broad, terraced cross sections and a deep, U-shaped incisioncharacterize these channels and sediment bypass during re-peated eustatic lowstands has nourished fans within slopeminibasins (Anderson and Rodriguez, 2000).

On the ramp-like central Texas shelf, distinct fluvial channelsare evident only on the inner shelf, a result of the fact thatgradients on the inner shelf were lower than river gradients andouter-shelf gradients (Eckles et al., this volume). The steep ramp-like west Florida shelf is characterized by extensive but shallowbraided channel complexes on the inner shelf and discrete late-highstand to lowstand channels on the outer shelf (McKeown etal., this volume). This difference between the central Texas andwest Florida fluvial geomorphologies reflects, in part, differencesin substrate conditions on the shelf. West Florida rivers flowedacross sandy substrates so that they were laden with bedloadmaterial. On the central Texas shelf, rivers cut into muddysediments and delivered little sand to the outer shelf and slope;they were suspended load-dominated rivers.

Incised fluvial valleys vary widely in their morphology, de-pending on when these valleys were cut within the overalleustatic cycle, differences in relative fluvial and shelf profiles,and substrate conditions (muddy versus sandy shelves). At-tempts to characterize incised fluvial valleys based on a singleshelf setting (e.g., Posamantier, 2001), can therefore be mislead-


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ing. Furthermore, subdivision of valley-fill deposits into differentsystems tracts (e.g., Van Wagoner et al., 1990) offers seriouschallenges (Blum, 1993; Blum and Valastro, 1994; Ethridge et al.,1998).

The shelf-margin deltas of the northern Gulf margin show acomplex pattern of progradation and aggradation that variesfrom one delta to the next. This complexity reflects the part of theeustatic cycle over which these deltas were constructed. Somedeltas were active during the late highstand through the earlytransgression, such as the Colorado, Rio Grande, and Lagniappedeltas. Other deltas, such as the Trinity/Sabine/Brazos delta,were nourished during the lowstand and a significant part of thetransgression (Fig. 4B).

16 ka to 4 ka (Transgression)

The final episode of sea-sea level change occurred betweenapproximately 16 ka and 4 ka During this period, melting icesheets contributed just over 100 m of sea-level rise to the oceans.There are several different depositional systems associated withthe transgressive episode in the northern Gulf of Mexico (Fig. 8).These include shelf-margin deltas, fluvial- and wave-dominateddeltas, thick shelf muds, incised-valley fills, sand banks andridges, and transgressive sheet sands.

Transgressive, backstepping deltas are associated with thoserivers with the largest sediment fluxes (Mississippi, Rio Grande,Colorado, Brazos, and Apalachicola). The absolute age of thesedeltas appears to differ, on the basis of their location on the shelf,and their development appears to have been rapid and short-lived. These deltas prograded across the shelf at a time when sealevel was rising rapidly (Fig. 3). Episodes of delta growth suggestincreased sediment supply, believed to have been caused byclimatic changes, which occurred at different times in the differ-ent study areas (Abdulah et al., this volume; Banfield and Ander-son, this volume).

Just as the morphology of incised fluvial valleys varies acrossthe northern Gulf of Mexico shelf, so do the types of sedimentsthat fill these valleys. Transgressive deposits in the Trinity–Sabine areas are confined to the incised valleys or to areasimmediately adjacent to the incised valleys. In interfluve areas,the transgressive ravinement surface is amalgamated with thesequence boundary, as evidenced by a thin (< 1 m) marine mudunit lying directly on Pleistocene (Stage 3) deposits. The Trinity–Sabine incised-valley-fill facies architecture consists of discon-tinuous, backstepping fluvial and estuarine (upper and lowerbay) facies separated by aggradational valley-fill deposits (Tho-mas and Anderson, 1994). The estuarine facies include upper tolower bay deposits as well as tidal-inlet sands. Flooding surfacesoften are manifested by an absence of one or more of the valley-fill facies, and individual flooding events result in updip faciesshifts of many tens of kilometers (Thomas and Anderson, 1994).On the Mississippi–eastern Louisiana shelf, cored transgressivedeposits range in thickness from 1 m to 9 m and comprise back-stepping estuarine, sound, and neritic facies (Fillon et al., thisvolume; Roberts et al., this volume). In that area, only the latterpart of the transgression appears to be represented. Radiocarbondates range from 12.4 ka to 8.24 ka (Fillon et al., this volume).Underlying coarse fluvial sediments filling the late stage 2 inci-sion surface are undated but are suspected to be of last glacialmaximum age (Roberts et al., this volume). Similar backsteppingfluvial/estuarine/marine facies characterize fluvial-valley suc-cessions in western Louisiana (Nicol et al., 1994) and offshoreAlabama (Bartek et al., this volume).

The Brazos and Colorado rivers have larger sediment sup-plies than the Trinity, Sabine, and probably Mobile rivers. The

Brazos and Colorado rivers filled their incised valleys with fluvialdeposits and abandoned them to occupy more shallow valleys.The result of these fluvial avulsion events has been the formationof multiple transgressive fluvial channels on the shelf and thesequestering of a significant volume of fluvial sediments in thesechannels. Preservation of these transgressive channels varies anddepends on their depth of incision, which in turn is controlled bywhere sea level was when avulsion took place. Older channels areincised more deeply and hence have better preservation. Trans-gressive ravinement led to decapitation of fluvial channels andmouth-bar facies and reworking of these facies into widespreadshelf sand bodies.

On the east Texas shelf, large banks lie adjacent to and abovethe Trinity–Sabine incised valley. These banks have been inter-preted as submerged paleoshorelines composed of a back-barrierestuarine unit at the base, a fore-barrier, lower shoreface and ebbtidal delta unit above, and a storm-reworked unit at the top(Nelson and Bray, 1970; Rodriguez et al., 1999). A transgressivesection cored in Main Pass Block 288 at the Lagniappe shelf edgecontains this same succession (Roberts et al., this volume).

Sand ridges characterize the south Texas inner shelf. Theseisolated inner-shelf sand ridges formed in situ at present waterdepths (Rodriguez et al. 2001). The different origins of east Texasbanks and south Texas ridges are attributed to variations in thedepth of shoreface ravinement (deeper in south Texas), shelfgradient (steeper in south Texas), and accommodation space(lower in south Texas) (Rodriguez et al., 2001).

An extensive (24,000 km2) transgressive-sand-ridge fieldcovers most of the Mississippi–Alabama–Florida shelf in thenortheastern Gulf of Mexico (the MAFLA sand sheet; McBrideet al., 1999; McBride et al., this volume). The MAFLA sandsheet is bounded below by the transgressive ravinement sur-face and above by the modern sea floor (the maximum flood-ing surface) and is composed predominantly of reworkedpolycyclic late-highstand and lowstand sand (McBride et al.,1999).

On the central Texas shelf and south Texas outer shelf, theyoungest deposits of the transgressive systems tract consist ofwidespread marine muds of the “Texas Mud Blanket” (Shideler,1981; Eckles et al., this volume). The mud blanket is up to 45 mthick in central and south Texas and is composed of sedimentfrom the Rio Grande to the south and from as far east as theMississippi River (Shideler, 1981).

4 ka to Present (Late Holocene Highstand)

The present-day sedimentary environment represents a maxi-mum highstand, which is a unique part of the total glacioeustaticcycle (Fig. 3). Currently, the shelf is flooded and extensive coastalbarriers exist. Onshore, broad meanderbelts occur on the low-gradient coastal plain and provide storage for vast quantities ofsediment. These meanderbelts, however, are not nearly as exten-sive as the broad alluvial plains that formed during the previousmaximum highstand (Stage 5e).

During the present highstand, the Mississippi River, with itshuge sediment supply, has prograded far onto the outer conti-nental shelf. A significant amount of the fine-grained sedimentthat is delivered to the Gulf by the Mississippi River is trans-ported to the west in wind-driven surface currents influencedby the Coriolis effect. These fine-grained sediments are depos-ited on the central and south Texas shelves as the Texas MudBlanket. Elsewhere on the shelf, sedimentation is at a minimumand the most important process occurring today is the forma-tion of a condensed stratigraphic section (maximum floodingsurface).


DISCUSSION

Fluvial Response to Sea-Level (Base-Level) Change

Within the northern Gulf of Mexico Basin, there is a fairlystrong correlation between the long-term sediment supply ofrivers and the size of their drainage basins. Currently, baysoccupy the fluvial valleys of rivers with low sediment discharge,such as the Trinity, Sabine, and Mobile rivers (Galveston Bay,Sabine Lake, and Mobile Bay, respectively). Rivers with greatersediment discharge, such as the Brazos and Colorado rivers, havebegun to construct prominent deltaic headlands across the coastalplain (Fig. 8). These differences reflect the inability of smallerrivers to fill their valleys as fast as sea level rose during thepostglacial transgression.

The response of Gulf Coast rivers to eustatic (base-level) changeduring the past 120,000 years varied across the shelf. Highstandfluvial channels are widespread on the shelf, particularly on thelow-gradient Louisiana and east Texas shelves (Berryhill et al.,1986; Anderson et al., 1996). In general, channels that extend fartheronto the shelf are deeper and narrower than channels that extendonly short distances across the shelf. This indicates that incisionoccurs throughout the falling limb of sea level.

During the last highstand, several of the larger rivers of theregion formed large deltas as sea level first began to fall. Thesediment discharge of these rivers was significantly greater thantoday. Estimates of sediment flux of the Rio Grande and Brazosrivers during the last several highstands were in the range of 1.4to 2.9 km3/yr and 0.3 to 3.2 km3/yr, respectively. However, thisincrease in sediment flux did not occur everywhere. ApalachicolaRiver sediment flux rates appear to have varied little between theprevious highstand and current highstand, averaging 0.4 km3/yrand 0.5 km3/yr, respectively (McKeown et al., this volume).

Differences in modern and ancient sediment flux of the largerTexas rivers versus the Apalachicola River are attributed todifferences in the sediment storage capacity of these rivers. Texasrivers have much more extensive alluvial plains with significantsediment storage capacity (Blum and Price, 1998). In contrast,west Florida rivers, including the Apalachicola River, occupyrelatively high-gradient channels that are incised into Pleistocenestrata. The coastal plain is narrow and flood plains are small,hence their sediment storage capacity is relatively small.

During a fall in sea level, sediments stored in alluvial drainagebasins of Texas and Louisiana are eroded as rivers incise, theirtributaries branching out into the alluvial basins (Koss et al.,1994). The result is a large increase in sediment flux during thefalling limb of the sea-level curve. In contrast, the sediment fluxof west Florida rivers remained relatively unchanged (McKeownet al., this volume).

Incised fluvial channels also exist in the transgressive systemstract of the Louisiana and Texas shelves. In Texas, these channelsare associated with the ancestral Brazos and Colorado rivers.Avulsion of these rivers during the transgression resulted inchannels being cut to shallower depths as sea level rose. Theyounger channels have been deeply eroded by shoreface erosionand extend offshore into marine muds. Older transgressive chan-nels extend offshore into transgressive deltas (Abdulah et al., thisvolume) and reflect times when sediment supply was greater.Variations in sediment supply during transgressions were causedby climatic changes.

Climate Control on Sediment Supply to the Basin

Climate variation is known to be a major controlling factor onfluvial processes (Schumm, 1965, 1993; Hall, 1990; Ethridge et al.,

1998). Indeed, Blum (1993) and Ethridge et al. (1998) concludethat, because the influence of sea-level (base-level) change onfluvial morphology diminishes in a landward direction, theclimatic influence on sediment delivery to the basin may begreater than that of eustasy.

The fluvial response to changes in climate can be very com-plex. For example, the same river can show different responsesalong its course because of changes in gradient and changes insediment load as it flows through different climatic belts (Schummand Brakenridge, 1987). In general, the sediment flux of fluvialsystems increases with increasing mean annual precipitation(Schumm, 1965). But the manner in which the sediment dischargeof rivers responds to climate change is more complex than thissimple relationship suggests. Schumm (1965) argued that a changefrom semiarid to arid conditions would result in a decrease insediment discharge. In contrast, a change from humid to semiaridclimates would result in an increase in sediment discharge be-cause of a decrease in vegetation cover, particularly grasslands.

As previously noted, in the northern Gulf of Mexico basinthere is a fairly strong correlation between the long-term sedi-ment flux of rivers (i.e., over a complete glacioeustatic cycle) andthe size of their drainage basins. This is in itself an importantfinding because the sediment discharge of modern rivers showslittle correlation to drainage-basin size (Hovius, 1998), althoughthe correlation is stronger when the relief of the drainage basinis considered (Milliman and Syvitski, 1992; Morehead andSyvitski, 1999). The timing and volume of sediment influx to thenorthern Gulf of Mexico margin during the last eustatic cyclevaried from one river to another. This diachronous influx ofsediment is undoubtedly a result of the different climate set-tings of the rivers. Currently, climate varies from humid tosemiarid across the study area, with a nearly threefold differ-ence in mean annual precipitation across the region (Fig. 1). It islikely that the magnitude and timing of climate change differedacross the region during the last glacial–interglacial cycle (120ka to present).

Perlmutter et al. (1998) generated computer models that illus-trate how differences in the timing of sediment discharge duringa glacial-eustatic cycle result in differences in the volumes andcharacter of different systems tracts. Their work is based onmodern sediment yields, which are quite low compared to othertimes in the last 120,000 years. Also, they do not take into accounteffects of vegetation changes and rates of soil development anderosion and the delayed response of these changes on sedimentyield. Their work did show, however, that sediment supply frommodern rivers varies by more than two orders of magnitudeduring a single climate cycle and that variations in the timing ofsediment supply differ between climatic belts. Their work alsoshowed that the greatest changes in sediment yields occur in theregions where climate varies from arid to subhumid, whichincludes the Texas coastal plain.

In general, all of the larger Texas, Louisiana, and Mississippideltas experienced continuous growth throughout the highstand.Climate changes did occur along the Gulf Coast as the ice sheetsto the north expanded, but any impact of these climate changes onsediment supply to the Gulf was overshadowed by the effects ofsea-level fall. Sea-level (base-level) fall was the driving force indelivering sediment to the margin. Indeed, detailed work on theancestral Brazos delta has shown that growth of the delta duringthe last highstand was strongly regulated by fifth-order eustaticfluctuations (Anderson et al., 1996; Abdulah et al., this volume).No sediment budgets for the lowstand have been obtained,owing to bypass of the shelf during the lowstand. So, the impactof climate change on sediment supply to the basin at this time isunknown. However, sediment fluxes of the different rivers ap-


pear to have varied significantly during the transgression. Thisdifference is attributed to climatic influence on sediment supplyrather than eustasy.

Blum and Price (1998) point out that, within the Colorado Riverdrainage basin, the postglacial sea-level rise was accompanied bywarmer temperatures and increased tropical-storm frequency andmore flashy discharge regimes. The net effects of these climaticchanges were degradation of soils and general denudation ofupland landscapes within the drainage basin. Sediments removedfrom the upland areas of the drainage basin were transportedbasinward. Floods delivered some of this sediment into the exten-sive flood-plain settings of the coastal plain (Blum et al., 1994; Blumand Valastro, 1994). There, these sediments are sequestered untilthe next eustatic fall. But there was also a significant flux ofsediment to the Gulf, as recorded by the transgressive deltasassociated with the ancestral Brazos, Colorado, and Rio Granderivers (Abudlah et al., this volume; Banfield and Anderson, thisvolume). The backstepping nature of these deltas suggests epi-sodic sediment flux that is probably controlled by higher-fre-quency climatic events. The most recent event occurred around 11to 9 ka, when both the Colorado and Rio Grande deltas experiencedsignificant phases of growth (Abdulah et al., this volume; Banfieldand Anderson, this volume). These periods of increased sedimentflux to the basin undoubtedly resulted from a shift to drier climatesand associated reduction in the area of grasslands following theYounger Dryas climatic event (Banfield, this volume; Snow, 1998).On the Lagniappe shelf at this time the transgressive floodingevent continued unabated.

Stratigraphic Models

Differences in fluvial response to sea-level change and climatehave resulted in very different stratigraphic architectures acrossthe northern Gulf of Mexico margin. Similar along-strike variabil-ity in systems tracts are observed on other continental shelves (e.g.,Trincardi and Correggiari, 2000; McMurray and Gawthorpe, 2000).

It is possible to group the different study areas of the northernGulf of Mexico margin into seven type sections on the basis oftheir gross stratigraphic architecture. These type sections areillustrated using stratigraphic slug diagrams (Fig. 10). Table 1provides key information about each area, and Table 2 summa-rizes the important depositional features for each area by systemstract. These models are generalized, but they serve to illustratethe very different stratigraphic architectures that exist on themargin and their relationship to the margin setting. Our generalmodels can be used to predict the distribution of reservoir-scalesand bodies on the shelf and for stratigraphic correlation. Wehope that by providing well-documented case studies our resultswill inspire other researchers to develop quantitative deposi-tional and reservoir models for the shelf (e.g., van Heijst et al.,2001). For greater details on these different areas, refer to theindividual papers in this volume.

A West Florida–Alabama Margin.—

A steep, ramp-like profile and slow subsidence rates charac-terize the west Florida margin (Bart and Anderson, this volume;McKeown et al., this volume). High-gradient, bedload-domi-nated rivers and streams deliver sediment to the margin. Thecurrent climate is humid and is unlikely to have been significantlydrier during glacial times.

There has been minimal preservation of early highstanddeposits on the west Florida shelf. Inner-shelf sediments wereeroded and transported seaward, where they are incorporatedinto late-highstand deltas that prograded into relatively deep

water (up to 70 m water depth). A lowstand delta and fancomplex is lacking. During transgression, sandy delta tops wereeroded and the sands spread across the shelf as an extensive sheetsand (MAFLA Sheet Sand; McBride et al., this volume).

The Latium shelf of Italy is perhaps a good analogue to theWest Florida shelf. The largest sediment supplier to the shelf isthe Tiber River. It is narrow (30 km), has no abrupt shelf break,and has a narrow coastal plain with several small rivers draininghighlands. Thus, the coastal plain is unable to stockpile largeamounts of sediment. As a result, changes in base level andclimate appear to have had a rapid influence on sediment supplyto the shelf (Chiocci, 2000). Also, there are no canyons dissectingthe margin and no basin-floor fans. Lowstand deposits weresupplied to the slope through gullies as a line source rather thanthrough canyons. This has resulted in a uniform thickness ofsediments on the upper slope, possibly also reflecting strongerlongshore transport during the lowstand.

B. Lagniappe (Mississippi–Eastern Louisiana Margin.—

During the late highstand and lowstand, sediments from thePascagoula and parts of the Mobile drainage system were depos-ited offshore of Mississippi and western Louisiana to constructthe Lagniappe delta. The delta prograded across a relativelybroad, low-gradient shelf that experienced relatively low subsid-ence. During the maximum lowstand, deposition shifted to theeast and constructed a shelf-margin delta (Roberts et al., thisvolume). Progradation of this shelf-margin delta continued intothe initial transgression (Fillon et al., this volume; Kohl et al., thisvolume). These deltas contain extensive delta-front sand bodies,but no associated lowstand fan has been identified. Currently thedeltas are being buried beneath muds shed from the MississippiRiver.

C. Western Louisiana–Brazos Margins.—

These areas are characterized by broad, low-gradient marginsthat have experienced moderate to rapid rates of subsidence(Abdulah et al., this volume; Wellner et al., this volume). Large,suspended-load-dominated rivers deliver sediments to thesemargins.

Early-highstand fluvial channels occur on the inner shelf, butsandy delta topset beds (mouth bars) are mostly confined to theouter shelf. Extensive delta-front sands are associated with late-highstand deltas. These deltas were abandoned, because of flu-vial avulsion, prior to the lowstand. Thus, there are no lowstanddeltas or fans on these margins. Transgressive deposits are mostlymuds, with the exception of incised-fluvial-valley fills and iso-lated sand banks.

D. East Texas—Trinity–Sabine Margin.—

The east Texas shelf is a broad, low-gradient shelf that hasbeen nourished with sediment from the mixed bedload/sus-pended load-dominated Trinity and Sabine Rivers throughoutthe eustatic cycle and by the Brazos River during the lowstand(Abdulah et al., this volume).

Highstand deposits are thin and composed of mud. Shelfbypass through the Trinity–Sabine–Brazos valley has resulted inthe development of a large lowstand delta and slope fan complexwhose distribution on the margin has been strongly regulated bysalt diapirs (Wellner et al., this volume). Transgressive depositsare mostly confined to the incised fluvial valley and includeextensive fluvial sands and backstepping bayhead deltas andtidal deltas (Thomas and Anderson, 1994). The only exceptions


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FIG. 10.—Seven stratigraphic models (A–G) are used to illustrate the different stratal architectures of the northern Gulf of Mexicomargin. See text for discussion. Dots indicate sand-prone areas.


are isolated sand banks that occur adjacent to the incised valleys(Rodriguez et al., 1999).

E. East Texas—Colorado Margin.—

West of the Trinity–Sabine incised fluvial valley, the marginis supplied with sediments by the Colorado and Brazos rivers,which have relatively high sediment flux rates and extensivecoastal-plain sediment storage capacities. The Colorado Riverhad a high bedload contribution to the margin relative to othereast Texas rivers. The area experienced significant climatic changesduring the last glacial–interglacial cycle; specifically, it was drierduring interglacial times (Toomey, 1993). Subsidence rates arerelatively high (0.1 to 4.0 mm/yr).

Thin, sandy early highstand deltas on the inner shelf andthick, sandy late highstand deltas on the outer shelf characterizethe margin (Abdulah et al., this volume). Sediment bypass duringthe lowstand resulted in a sandy Colorado lowstand delta andslope fan complex. The transgressive systems tract includes large,sandy deltas and sand-filled incised fluvial valleys. The highsediment yield of the Colorado River during the recent glacial-to-interglacial transition is credited with the increased sedimentsupply that led to the formation of transgressive deltas on theshelf.

F. Central Texas Margin.—

The central Texas margin has a relatively steep profile andlacks a distinct shelf break. The margin experiences moderaterates of subsidence (Eckles et al., this volume). Small, mixedbedload/suspended load rivers deliver sediment to the region,along with converging coastal currents that deliver sands from

adjacent east Texas and south Texas coasts. The climatic setting isbelieved to have been more humid during glacial times than it istoday (Toomey, 1993).

Sand-prone deposits are mostly confined to the inner shelfand consist of early-highstand prograding shoreline and shorefacedeposits. The outer shelf is mud-dominated, there are no lowstanddeltas or fans, and a transgressive mud unit blankets the shelf.

G. South Texas—Rio Grande Margin.—

The south Texas margin has a broad, low-gradient shelf. TheRio Grande River has been the principal supplier of sediment tothe margin during the late Quaternary and, for that matter,throughout most of the Tertiary. Of the various study areas, it hasthe most persistently dry climate.

Extensive highstand deltas with large delta-front sand bodiesoccur on the shelf, including both wave- and fluvial-dominateddeltas (Banfield and Anderson, this volume). A very thick lowstanddelta and fan complex occurs on the shelf margin and upperslope, which includes thick sand units. The transgressive systemstract includes incised fluvial valleys and transgressive deltas thathave been buried beneath a transgressive mud blanket.

CONCLUSIONS

1. Highly variable fluvial morphologies, drainage-basin size,and climate settings characterize the northern Gulf Coast.These differences result in rather different long-term sedi-ment discharges of rivers. There are also major differences inshelf physiography and subsidence across the margin. Paleo-geographic maps for the highstand (falling sea level), low-stand, and transgressive systems tracts, which are related to

South Texas(Rio Grande)

Central Texas(Guadalupe)

East Texas(Brazos, Colorado)

East Texas(Trinity, Sabine)

Lagniappe(Mobile)

FLORALA(Mobile, Escambia)

West Florida(Apalachicola)

Systems-TractDescriptions

Highstand (HS) • early HS preservation• areally extensivemuddy deltas lobes(extensive prodelta)(< 0.5° angle) andassociateddistributaries(aggrading sigmoidalclinoforms at seawardmargin) (wave/fluvialdominated)

• progradingshoreline(converginglongshorecurrents)

• muddy deltas(extensive prodelta)with minimal deltalobe switchingduring early fall

• sandy deltas withassociateddistributary systemsduring late fall

• minimal HSpreservationoutside incisedvalleys

• minimal HSpreservation oninner shelf

• preserved sandydeltas with severallobes (< 5° angle)and associateddistributaries(aggradingsigmoidal clino-forms at seawardmargin) (fluvial)

• preserved thick,lobate silty to finesand deltas withseveral lobes andassociateddistributaries(aggradingsigmoidalclinoforms atseaward margin)

• minimal HSpreservation oninner shelf

• preserved sandydeltas withseveral lobes(> 3° angle) andassociateddistributaries(fluvial andwave)

HS SedimentSupply

(km3/1000 yr)

1.4 to 2.9 0.1 to 1.2 0.3 to 3.2 1.0 0.4 (100 kyr)0.9–3.0

Lowstand (LS) • incised fluvial valley• shelf-edge deltas(fluvial)

• slope fan

• shallow, narrowincised channels

• no major cross-shelf incisedfluvial valleys orslope canyons

• no LS deltas

• incised fluvialvalleys

• muddy and sandyshelf-margin delta(Colorado only)

• muddy and sandyslope fans(Colorado only)

• single incisedfluvial valley(point source)

• sandy shelf-margin delta

• slope fans

• shallow, broadfluvial system withterraces

• no cross-shelfincised fluvialvalleys or canyons

• no slope fans

• broad, shallowincised fluvial braidsystem (line source)

• no cross-shelfincised fluvialvalleys or canyons

• no slope fans

• broad, shallowincised fluvialbraid system withterraces (linesource)

• no cross-shelfincised fluvialvalleys orcanyons

• distributaries areincised to shelfbreak

• no LS fansTransgressive (T) • deposition not

confined to incisedvalleys

• localized sandy deltalobes (fluvial/wavedominated)

• LS wedge(backstepped deltas)

• valley fill• 40-m-thick mudblanket

• reef trend~ 60 m

• sandy and muddyincised-valley fill

• deltas with lobeswitching(fluvial/wave)

• sandy and muddyincised valley fill

• sandy bankspreservedadjacent toincised valleys

• muddy shelf-margin deltas

• incised-valley fill• sand ridges• two delta lobes• carbonate mounds• hemipelagics• shoreline-parallelshelf currents

• valley fill• sand ridges• carbonate mounds• thin, laterallyextensive slopewedges (shoreline-parallel upper slope-currents)

• valley fill• sand ridges• backsteppeddelta lobes

• thin slopewedges (notextensive)

• hemipelagics

TABLE 2.—Summary of the depositional styles of systems tracts for the various study areasin the northern margin of the Gulf of Mexico.


well known sea-level histories, serve to illustrate how depo-sition varied across the continental margin. There is consider-able variability in stratigraphic architecture across the mar-gin, which is illustrated using seven stratigraphic models. Themodels can be used to predict the occurrence of reservoir-scale sand bodies in continental-margin settings.

2. The current maximum highstand is characterized by signifi-cant sediment storage in low-gradient alluvial plains andminimal sediment discharge by rivers, the exception beingthe Mississippi River. Sedimentation on the margin is mini-mal, and the modern sea floor is a condensed stratigraphicsection (maximum flooding surface). The only exception isthe wave-dominated central Texas shelf, where a thick Ho-locene mud blanket is accumulating. Thus, the modernsetting provides only a glimpse of how strata are formed onthe margin.

3. During the last falling limb of the sea-level curve, the largerrivers of the Gulf Coast Region had sediment fluxes up to anorder of magnitude greater than their current sedimentfluxes. These rivers nourished large highstand deltas. Ingeneral, sediment fluxes of rivers increased with falling sealevel. This was partly the result of cannibalization of earlyhighstand deltas by fluvial incision and regressive shorefaceerosion. If climate regulated sediment yields during thistime, the effects were overshadowed by sea-level change.

4. The central Texas shelf was wave-dominated during the lasthighstand and was characterized by prograding clastic shore-line and shoreface deposits. Progradation of these depositsacross the shelf occurred only during the early highstand,largely because erosion of the Colorado and Rio Grandedeltas nourished the central Texas deposits. Elsewhere alongthe Gulf margin, coastal deposits are mostly confined to themaximum updip limit of flooding surfaces.

5. Rivers with high sediment supplies are prone to avulsion,during both falling and rising sea level. This results in asignificant amount of sediment delivered by these riversbeing sequestered on the shelf. In contrast, rivers with smallersediment supplies are more inclined to occupy the samechannels throughout the eustatic cycle. This results in greatersediment bypass of the shelf, which equates to slope and basinfloor fan systems (Anderson and Rodriguez, 2000).

6. Abandonment of highstand shelf-margin deltas by some ofthe larger rivers (Brazos and western Louisiana fluvial sys-tems) prior to the Stage 2 lowstand resulted in an absence oflowstand delta and slope fans in these regions.

7. Over the length of a glacioeustatic cycle, the overall sedimentflux of a river is determined by the size of its drainage basin.The climate effects on sediment flux of different rivers aremore apparent during the transgression; however, the effectof climate on sediment flux varied across the margin.

8. Geomorphology (width and cross-sectional geometry) aloneis not a suitable criterion for distinguishing highstand andlowstand fluvial channels. Lowstand fluvial geomorphologyvaries widely across the shelf. These differences are the resultof differences in shelf gradient, drainage-basin size and cli-mate (which controls discharge), substrate conditions on theshelf, and the interval of the eustatic cycle during which theriver occupied the channel.

9. Idealized tripartite incised-valley-fill successions (e.g., Wrightand Marriott, 1993; Zaitlin et al., 1994) are the exception ratherthan the rule on the northern Gulf of Mexico shelf. Along themargin, valley fills range from those that are dominantlyfluvial (e.g., the Brazos valley) to those that are dominantlymarine (e.g., central Texas valleys). This is due to the differentsediment supplies of these rivers and their capacity to keeppace with the rate of sea-level rise during transgression.

10. Our data show that fifth-order sea-level fluctuations have hada marked influence on sedimentation on the continental shelf.

11. The modern physiography of the margin is largely a productof sediment supply. A broad shelf and distinct shelf breakcharacterize portions of the margin where sediment supply ishigh, with the exception of the modern Mississippi delta.Narrow, steep shelves with a less distinct shelf break charac-terize areas with relatively low sediment supply.

12. The occurrence of sand bodies within deltas varies across theshelf. Large rivers with high suspended loads construct muddydeltas in which sands are mostly confined to mouth bars in thetopset portions of the deltas and to the point bars of themeandering rivers that nourished these deltas. Smaller,bedload-dominated rivers, such as those of west Florida, haveconstructed sandy deltas.

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