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

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  1. 1. LATE QUATERNARY STRATIGRAPHIC EVOLUTION OF THE NORTHERN GULF OF MEXICO MARGIN Edited by: JOHN B. ANDERSON Earth Sciences Department, Rice University, Houston, Texas 77251-1892, U.S.A. AND RICHARD H. FILLON Earth Studies Associates, 3730 Rue Nichole, New Orleans, Louisiana 70131-5462, U.S.A. Copyright 2004 by SEPM (Society for Sedimentary Geology) Laura J. Crossey, Editor of Special Publications SEPM Special Publication Number 79 Tulsa, Oklahoma, U.S.A. April, 2004
  2. 2. ISBN 1-56576-088-3 © 2004 by SEPM (Society for Sedimentary Geology) 6128 E. 38th Street, Suite 308 Tulsa, OK 74135-5814, U.S.A. Printed in the United States of America SEPM and the authors are grateful to the following for 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 the purchase price, making the volume available to a wide audience Boyd Professorship, Coastal Studies Institute and the Department of Oceanography and Coastal Sciences Office of Naval Research Pioneer Natural Resources SEPM (Society for Sedimentary Geology) is an international not-for-profit Society based in Tulsa, Oklahoma.Throughitsnetworkof internationalmembers,theSocietyisdedicatedtothedissemination of scientific information on sedimentology, stratigraphy, paleontology, environmental sciences, marine geology, hydrogeology, and many additional related specialties. The Society supports members in their professional objectives by publication of two major scientific journals, the Journal of Sedimentary Research (JSR) and PALAIOS, in addition to producing technical conferences, short courses, and Special Publications. Through SEPM's Continuing Education, Publications, Meetings, and other programs, members can both gain and exchange information pertinent to their geologic specialties. For more information about SEPM, please visit
  3. 3. 1LATE QUATERNARY STRATIGRAPHIC EVOLUTION, NORTHERN GULF OF MEXICO MARGIN: A SYNTHESIS LATE QUATERNARY STRATIGRAPHIC EVOLUTION OF THE NORTHERN GULF OF MEXICO MARGIN: A SYNTHESIS JOHN B. ANDERSON Earth Sciences Department, Rice University, Houston, Texas 77251-1892, U.S.A. ANTONIO RODRIGUEZ Department of Geological Sciences, University of Alabama, Tuscaloosa, Alabama 35487-0338, U.S.A. KENNETH C. ABDULAH Pioneer Natural Resources, 5205 N. O’Connor Blvd., Irving, Texas 75039-3746, U.S.A. RICHARD H. FILLON Earth Studies Associates, New Orleans, Louisiana 70131, U.S.A. LAURA A. BANFIELD BP, 501 Westlake Park Blvd., Houston, Texas 77210, U.S.A. HEATHER A. MCKEOWN Department 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. WELLNER Earth 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 northern Gulf of Mexico, from the Apalachicola River to the Rio Grande. The studies characterize deposition and strata formation associated with different 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 and base-level change, variations in delta growth and evolution across the shelf, lowstand delta and fan evolution, the evolution of transgressive deposits on the shelf, the preservation of these deposits, and the resulting differences in stratigraphic architecture. In this paper we summarize the key observations made in those studies and compare the paleogeography and deposystem evolution of the various study areas. The integration of the chronologies developed with key regional seismic surfaces allows comparison of stratal geometries produced by contemporaneous depositional systems operating under identical eustatic conditions. This synoptic comparison permits differentiation between eustatic and other controls on sedimentation and testing of many of the assumptions made in sequence stratigraphy. A set of paleogeographic maps and sequence and systems-tract models depict the major depositional systems of the Gulf margin during different stages of the eustatic cycle. These summary diagrams highlight considerable variability in stratigraphic architecture along the margin. For example, the relative proportion of highstand, lowstand, and transgressive strata differs between study areas. Thus, deposition and stratal packaging are more complex than most sequence stratigraphic models predict. However, for any given fluvial, deltaic, and fan system, the general style of deposition appears to repeat itself from one glacioeustatic cycle to the next. Thus, the results of this study can be used to test 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 Margin SEPM Special Publication No. 79, Copyright © 2004 SEPM (Society for Sedimentary Geology), ISBN 1-56576-088-3, p. 1–23. INTRODUCTION During the past two decades there has been a proliferation of sequence stratigraphic models, including the popular “slug dia- grams”.Thesemodelsrelateobservationsfromseismicdata,well logs, 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 is that there is some orderly stratigraphic motif for any given basin setting whereby the general style of deposition repeats itself with each eustatic cycle as long as other controlling factors (tectonics, climate,sedimentsupply,shelfgradient,subsidence,etc.)remain relatively constant. This seems like a reasonable assumption; however, the principal role of eustasy in controlling stratigraphic architecture 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 stratigraphic models by using outcrop data (e.g., Van Wagoner, 1995). Geolo- gists working on outcrops are forced to make important assump- tionsaboutpaleogeography,base-levelchange,andclimaticinflu- enceonsedimentsupply.Thereisalsoalackofchronostratigraphic control needed for detailed outcrop correlation. These problems typically lead to debate about stratigraphic interpretations (Van Wagoner, 1995,1998;Yoshidaetal.,1996,1998).Anotherapproach to stratigraphic modeling is that of using laboratory experimental models 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 this approach concerns the scale of these models, although Koss et al. (1994) argue that the concept of sequence stratigraphy is scale independent. There has also been a proliferation of quantitative models 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.
  4. 4. 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 are limited by different factors. What is needed to develop more predictive stratigraphic models are experiments in which sedi- mentation and strata formation can be related to known rates of eustasy, subsidence, and sediment discharge. It is important that these experiments be carried out in a broad range of depositional settings if the results are to be widely applicable for stratigraphic analysis. 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 to the shelf, (2) the shelf gradient and subsidence rates, (3) the eustatic and climatic history of the basin, and (4) oceanographic influences on sedimentation. Previous studies have shown that depositional geometries and stratal architecture of Quaternary strata do reflect, at least at first 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 geographic coverage 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 same eustatic fluctuations. The next logical step is to conduct similar experiments in a much larger basinal setting where regional variations in climate, sediment discharge, subsidence, neotectonics, and margin physiography and their influence on sedimentationandstrataformationcanbeexaminedandchrono- logically matched. This volume contains papers that present results from studies of late Quaternary strata of the northern Gulf of Mexico basin. Our objective is to compare and contrast deposition and strata formation within different linked drainage systems and margin depocentersthroughonecompleteglacioeustaticcycle.Thenorth- ern Gulf of Mexico margin is divided into eight distinct deposi- tionalsettingsbasedondifferencesinfluvial-drainage-basinsize, geology and physiography, climate setting, and margin physiog- raphy (Fig. 1, Table 1). We focused on the last glacioeustatic cycle (120 ka) for a number of reasons. First, this is the time for which sea-level change is best documented. Second, the strata of this age occur at shallow enough subsurface depths to be imaged using high- resolution seismic methods and they can be sampled using conventional coring and shallow drilling techniques. Lastly, Nueces R. 50-60 50-60 30-40 30-40 20-30 40-50 R io G rande R . Colorado R. Brazos R. PascagoulaR. 50-60 0 500 km Average Annual Precipitation (in) Bathymetry (m) Gulf of Mexico 200m 300m 400m 1000m 500m 100m 60-80 Mississippi River Drainage Basin Sabine R. Apalachicola R. TombigbeeR. GuadalupeR. Trinity R. Pearl R. PerdidoR. MobileR. RG CT ET WL MD MEL ALWF APL FIG. 1.—Map of fluvial drainage basins for the northern Gulf of Mexico margin. Also shown are margin bathymetry and values of mean 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. The different 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. The Mississippi River drainage basin is shaded.
  5. 5. 3LATE QUATERNARY STRATIGRAPHIC EVOLUTION, NORTHERN GULF OF MEXICO MARGIN: A SYNTHESIS chronostratigraphic methods for the late Quaternary provide the level of resolution necessary for direct correlation of depositional systems in different regions. It is important that stratigraphic studies of this type encom- pass the full eustatic cycle so that the entire range of eustatically controlled and climate-controlled influences on sediment dis- charge and deposition can be examined. Results presented in this volume show that the sediment discharge of modern rivers in the northern Gulf of Mexico are a fraction of what they were in the past, so the modern depositional setting is not a good analogue for most of the eustatic cycle. Why the Gulf of Mexico? The northern Gulf of Mexico margin encompasses a broad spectrum of depositional settings and is, in our opinion, un- equaled as a natural laboratory for research aimed at improving ourunderstandingofsedimentationandstrataformationonlow- gradientcontinentalmargins.Thereareanumberofreasonswhy the Gulf is particularly well suited for research of this kind. 1. The margin receives sediment from a number of fluvial sys- temswithverydifferentsedimentdischarges,intermsoftotal sediment flux and proportions of suspended and bedload material (Fig. 1). These differences in sediment discharge are relatedtodifferencesindrainage-basinsize,geology,climatic setting, and fluvial geomorphology (stream gradient, sinuos- ity, channel morphology, etc.). For example, the rivers of east Texas have very extensive meanderbelts and floodplains where vast quantities of sediment are sequestered (Wright and Marriott, 1993). In contrast, the rivers of Alabama and west Florida are incised into mostly Pleistocene strata and have rather limited flood plains where sediment storage can occur. 2. The northern Gulf of Mexico margin spans five major climatic belts that run roughly NNW–SSE across the state (Thornthwaite, 1948). From east to west, precipitation reflects humid, moist subhumid, dry subhumid, and semiarid condi- tions (Fig. 1). Wet and dry phases, linked to glacials and interglacials, 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 Coast region have been summarized in review papers (e.g., DuBar et al., 1991; Toomey, 1993). These studies are highly general- ized,buttheyprovideafirstapproximationforclimatechange, 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 and timingofthesechangesundoubtedlyvariedacrosstheregion. The potential impacts of these climate shifts on sediment flux to the Gulf should be profound (Langbein and Schumm, 1958). The Louisiana, Mississippi, Alabama, and west Florida climates are believed to have remained relatively humid throughout the last glacioeustatic cycle. We can estimate sediment flux using the volumes of depositional units and their ages. 3. Subsidence and sediment supply are relatively high, particu- larly offshore Louisiana and Texas, and are reasonably well constrained(e.g.,Paine,1993).Thus,preservationpotentialof all 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 to meandering small coastal plain and piedmont rivers broad, meandering narrow, meandering broad, meandering steep, incised steep, incised (Mobile) small coastal-plain river (Escambia) steep, incised bedload/suspended load 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/fluvial dominated fluvial wave fluvial wave fluvial fluvial storm storm lithology muddy muddy muddy muddy mixed sandy sandy sandy structure growth faulting growth faulting salt and growth faulting salt and growth faulting salt and growth faulting minor diapirs and large scale growth faulting undisturbed by salt and faulting undisturbed by salt 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 locations are 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 basin of 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).
  6. 6. J.B. ANDERSON, A. RODRIGUEZ, K.C. ABDULAH, R.H. FILLON, L.A. BANFIELD, H.A. MCKEOWN, AND J.S. WELLNER4 high, and these strata are thicker than on most other continen- tal margins. For example, detailed work on the New Jersey continental margin has shown that strata formed during the falling stage of sea level (120 ka to 22 ka) are poorly repre- sentedontheshelfbecauseofreworkingduringthetransgres- sion (Duncan et al., 2000). 4. Shelfwidthsvaryfrom30kmto180km;shelfandupper-slope gradients are therefore highly variable as well. The Texas and Louisiana shelves have distinct shelf breaks, with the excep- tion of the central Texas margin, which lacks a distinct shelf break. The western Florida margin is more of a steep ramp setting. 5. Over 25,000 kilometers of high-resolution seismic data is available (Fig. 2), and there are abundant oil-company plat- form borings and long cores needed to test seismic facies interpretationsandforchronostratigraphicanalysis.Theonly area where long cores were not available at the time of these studies was on the Alabama and west Florida shelves. 6. The northern Gulf of Mexico region has a long history of sedimentological and paleontological research and has been the location of significant pioneering discoveries (e.g., Fisk, 1944; LeBlanc and Hodgson, 1959; Curray, 1960; Kennett and Huddlestun, 1972; Frazier, 1974; Berryhill et al., 1986; Kohl, 1986).Theseearlierstudieshaveprovidedanimportantframe- 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 seismic stratigraphic methods were used to describe the external forms andinternalstratalgeometriesofdepositionalunits(e.g.,Mitchum et al., 1977; Vail et al., 1977a, 1977b; Vail et al., 1997c). The nature of the bounding surfaces and the reflection configurations of the units were then combined with the timing of their formation to examine the relative roles of eustasy and other controlling factors (e.g., sediment supply and tectonism) on deposition. An analysis of this kind is necessary in order to understand the factors that influence the evolution and distribution of lithofacies in space and time. An independent chronology was established for the northern Gulf of Mexico through integration of biostratigraphy, tephro- chronology, radiocarbon dating, and oxygen isotope stratigra- phy. The chronology is linked to regionally extensive seismic reflectorsbytheintersectionofthelocationsofthesedimentcores with the seismic dataset. These ties create a robust chronostrati- graphic framework from which to interpret the temporal and spatial distribution of depositional environments on the shelf over the past 120,000 years. Seismic Stratigraphic Method and Terminology Sequence stratigraphy has provided a valuable means of subdividingstratigraphicpackages,buttheproliferationofterms and new methodologies has resulted in much confusion. Part of the problem stems from the attempts of researchers to link systems tracts to specific well-defined segments on a sea-level curve(e.g.,Haqetal.,1987),butitisunclearwhetherthiswasever the intention of the Exxon workers who developed the terminol- ogy (Van Wagoner et al., 1988). FIG.2.—SeismicdataacquiredduringthepastnineyearsbytheRiceUniversitygroup(areas1,2,3,4,5,8,and9),theLSU–OilIndustry consortium (area 6), and the University of Alabama (area 7). The corresponding area numbers and papers in this volume are as follows. 1 = Banfield and Anderson; 2 = Eckles et al., 3 = Abdulah et al., 4, 5 = Wellner et al., 6 = Fillon et al., Roberts et al., and Kohl et al.; 7 = Bartek et al., 8 = Bart and Anderson, and McBride et al.; and 9 = McKeown et al. 98 96 94 92 90 88 86 30 28 26 FlorAla EastTexas Trinity/ Sabine Western Louisiana Central Texas South Texas Gulf of Mexico . Apalachicola 1 2 3 4 5 6 7 9 8 East Texas Brazos/ Colorado
  7. 7. 5LATE QUATERNARY STRATIGRAPHIC EVOLUTION, NORTHERN GULF OF MEXICO MARGIN: A SYNTHESIS In the end, what is important is the adoption of a consistent means of stratigraphic analysis. In this volume, because there is a well-defined sea-level curve spanning the late Quaternary, the authors 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 linking systems-tract terminology and all discussions of stratigraphic features directly to the sea-level curve, the relative roles other allocyclic processes play (in particular, climate change and asso- ciated sediment supply) in influencing stratal stacking patterns can 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 above the maximum flooding surface and below the sequence bound- ary (Posamentier and Vail, 1988), it usually spans the period of falling sea level. Likewise, when defining systems tracts purely by their internal geometries (descriptively as opposed to geneti- cally),itisclearthatthelowstandsystemstractmayincludemuch of the time interval when sea level is rising (Nystuen, 1998). Plint and Nummedal (2000) have suggested a new systems tract, the falling-stage systems tract (FSST), which lies above and basinward of the highstand systems tract and below the lowstand systems tract. The term falling-stage systems tract is more intuitive than highstand systems tract when referring to the time when sea level is actually falling. But, the term has not been widely accepted. Plint and Nummedal (2000) point out that the most diagnostic criteria of the FSST are the presence of erosionally based shoreface sand bodies in nearshore areas. The erosion results from wave scour during sea-level fall (regressive surface of marine erosion). This approach stems from outcrop studies where such associations may be evident. When working with seismic data and core logs, however, it is not always possible to trace the base of the falling-stage sys- tems tract. A lingering controversy in sequence stratigraphy concerns which surfaces should be used to subdivide sequences and how these surfaces are identified. The Exxon model (Vail et al., 1977a, 1977b; Vail et al., 1977c) favors use of the sequence boundary. 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 and used as a sea-level proxy curve for the past 140,000 years. Curve A is the SPECMAP oxygen isotope curve that shows the last four glacial 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 of Barbados corals (Bard et al., 1990) OIS 2 OIS 3 OIS 5 e c a OIS 6 OIS 4 b d 20 0 -20 -40 -60 -80 -100 -120 -140 0 20 40 60 80 100 120 140 sealevel(m) U/Th dates of Huon, New Guinea corals (Chappell et al., 1996) New Guinea lowstand 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 Years BP (x 1000) Glacioeustasy(m) 0 -120 18O(0 /00PDB) -2.0 -1.0 0 1.0 2.0 TST LST HST (Late) 5e MFS 3 MFS Stage 3 A B Benthic δδδδδ18O curve, Norwegian Sea (Labeyrie et al., 1987) Composite benthic/planktic δδδδδ18O curve, Pacific Ocean (Shackleton, 1987)
  8. 8. J.B. ANDERSON, A. RODRIGUEZ, K.C. ABDULAH, R.H. FILLON, L.A. BANFIELD, H.A. MCKEOWN, AND J.S. WELLNER6 FIG.4.—A)SeismiclinesG-300XandLineG-90Yareusedtoillustratethekeyseismicstratigraphicsurfacesandgeneralizedstratalstackingpatternsonthemargin.The stage5emaximumfloodingsurface(mfs)andthestage3maximumfloodingsurfaceareprominentdownlapsurfacesrecognizedatthebaseofhighstanddeltasthat progradedacrosstheshelf,inthiscasetheancestralBrazosDelta.Thestage2sequenceboundary(SB)isalowstanderosionalsurfacethatoverlieshighstanddeltas. Thetransgressivesurface,whichseparatesthelowstandandtransgressivesystemstracts,mergeswiththesequenceboundaryontheinnershelf,sothattransgressive depositsliedirectlyonhighstanddeposits.Thetwolinesrepresentaneast–westtonorth–southsectionsoastofollowthemaindepositionalaxisoftheBrazosdelta (afterRodriguezetal.,2000).B)SeismiclineR93-51isastrike-orientedprofilethatextendsalongtheoutershelf.ItcrossesthelatehighstandandlowstandColorado andBrazosdeltasandthelowstandTrinity–Sabine–Brazosdeltaandillustratesthedifferentagesoftheseshelf-margindeltas. 10kilometers 40 80 120 160 200 Two-wayTravelTime(ms)LineR93-51 multiple seafloor earlyStage3 ColoradoDelta lateStage3 ColoradoDelta Stage2 SB Stage3 BrazosDelta 0 30 60 90 120 Depth(m) LineG300X 240 Stage2-1 Trinity/ Sabine/ Brazos Delta 100 200 300 400 SouthEast kilometers 1230 Two-WayTravelTime(ms) Stage2lowstand deposits Stage3late highstanddelta Stage5early highstanddelta Stage2SB Stage5emfsStage3mfs multiple LineR93-51 LineG-300XLineG-90Y Transgressive surface Stage2-1 Colorado Delta A B NorthWest WestEast
  9. 9. 7LATE QUATERNARY STRATIGRAPHIC EVOLUTION, NORTHERN GULF OF MEXICO MARGIN: A SYNTHESIS falling as well as the lowstand (Posamentier et al., 1988; Posamentieretal.,1992).Galloway(1989)hassuggestedthatthe maximum flooding surface, a condensed section formed during the maximum flooding of the shelf, is the more prominent and less diachronous surface and therefore should be used for division of sequences. Sydow and Roberts (1994) used maxi- mum flooding surfaces to subdivide the stratigraphic package of the Lagniappe Delta because those surfaces were easier to recognize than sequence boundaries, both on the slope and on the shelf. On the Texas shelf, the sequence boundary is the more prominent surface. Without question, this investigation dem- onstrates that it is at the maximum lowstand when the most basinward subaerial exposure and sediment bypass of the shelf occurs. This is also the time when maximum incision of fluvial valleys takes place. Sediments that are eroded on the shelf to form the sequence boundary are deposited on the slope to create a conformable section, 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 shelf during the late Quaternary occurred throughout the falling limb of the sea-level curve (Fig. 4). These deltas reached the shelf margin at different times, depending on the sediment supply of their fluvial feeders. Some (e.g., the Brazos Delta) were active during the falling stage of sea level; others (e.g.,the Rio Grande) were most active during the lowstand; and others remained active during the transgression (Trinity/Sabine/ Brazos deltas). For the most part, these deltas downlap the maximumfloodingsurfaceandliebeneaththesequencebound- ary (Fig. 4). During the fall in sea level, shelf accommodation decreases and deposition on the shelf shifts seaward, with erosion of inner- shelf strata contributing to the nourishment of outer-shelf strata. Thisconcepthasbeenaroundformorethanacentury(Chamberlin, 1898), but sequence stratigraphers have redefined it as a forced regression (Posamentier et al., 1992) and yet another systems tract, the forced regressive systems tract, has emerged (Hunt and Tucker, 1992). The lower boundary of the forced regressive systems tract (FRST) is the basal surface of forced regressions (HuntandTucker,1995),whichiswherePosamentieretal.(1992) and Posamentier and Morris (2000) would place the sequence boundary. It is easily confused with the maximum flooding surface. Along-strikevariabilityinlateQuaternarystratalarchitecture across the northern Gulf of Mexico margin is the rule rather and the exception. This variability results from differences in the timing and extent of delta progradation across the shelf, which is controlled by the long-term sediment discharge of rivers (Fig. 4). Figure 4 illustrates the problem. At this location on the shelf, the Brazos delta prograded across the shelf prior to progradation of the Colorado delta. This example illustrates why it is difficult to trace 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 to recognizeandmapregionally,evenwiththebesthigh-resolution seismic data. Chronostratigraphy Inthestudyareas,thechronologicframeworkwasdeveloped by integrating foraminiferal abundance variations, foraminiferal extinctions, tephrochronology, oxygen isotope stratigraphy, and radiocarbon 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 used to 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 the occurrenceoftwospeciesofplanktonicforaminifera,Globorotalia menardii and Globorotalia inflata, are used to define Ericson zones (Ericson and Wollin, 1968). This zonation was later modified by KennettandHuddlestun (1972),whoexaminedthequantitative presence or absence of the Globorotalia menardii complex and several other foraminifera species in piston cores from the Gulf. This modified zonation was used only when sufficient numbers of planktonic foraminifera were present (e.g., Banfield et al., this volume) (Fig. 5). The extinction of Globorotalia menardii flexuosa, interpreted as occurring near the oxygen isotope boundary between stages 5a and 5b (approximately 85 ka) and the first- appearancedatumofthecalcareousnannofossilEmilianiahuxleyi in stage 8 (approximately 260 ka) provide other important biostratigraphic benchmarks for this study (Kennett and Huddlestun, 1972; Poag and Valentine, 1976; Kohl et al., this volume). On the east Texas and western Louisiana shelves, planktonic foraminifera 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 this sector of the northern Gulf. Thus, detailed analyses of benthic foraminifera were conducted on cores and the paleobathymetric curves generated from these analyses used to refine our oxygen isotope and seismic stratigraphic interpretations in areas west of themodernMississippidelta(Abdulahetal.,thisvolume;Wellner et al. this volume). In areas east of the Mississippi delta, particu- larlyattheshelfedgeandontheupperslope,planktonicforamin- ifera are more abundant and provide the basis for additional oxygen isotope and biostratigraphic interpretation (Fillon et al., this volume; Kohl et al., this volume). Radiocarbon Dating.— Conventionalandacceleratormassspectrometerradiocarbon datesprovidefurtherinformationforthelastglacioeustaticcycle. The maximum age that can be determined using AMS radiocar- bon methods depends on sample quality but is about 40 ka None of the authors exclude any samples for which radiocarbon dates were obtained. Out-of-order dates can provide important infor- mation on flooding and erosion on the shelf (see discussion by Fillon et al., this volume). Oxygen Isotope Stratigraphy.— In this project, the global oxygen isotope curve is used as a proxy sea-level curve. In addition, oxygen isotope curves were generated within some study areas as a chronostratigraphic framework and for correlation between study areas (Abdulah et al.,thisvolume;BanfieldandAnderson,thisvolume;Ecklesetal., this volume; Fillon et al., this volume). Previous oxygen isotope stratigraphic studies have tended to avoid the continental shelf. This was due to concerns about incomplete stratigraphic section due to erosion, variable deposi- tion rates, paucity of planktonic foraminifers, diagenetic alter- ation, and fresh-water contamination overwhelming the isotopic signal (see discussion in Fillon et al., this volume). Furthermore, foraminifera are patchy in their down-core distribution on the Texas and Louisiana outer shelf and planktonic foraminifera are restricted to highstand deposits. In the ideal case, seismic strati-
  10. 10. J.B. ANDERSON, A. RODRIGUEZ, K.C. ABDULAH, R.H. FILLON, L.A. BANFIELD, H.A. MCKEOWN, AND J.S. WELLNER8 graphic analyses could be used to locate optimal core locations and compile the most complete stratigraphic section using care- fully selected cores from the outer shelf. But, with the exception of the Lagniappe delta study (Fillon et al. this volume; Roberts et al. this volume; Kohl this volume), the authors did not have the option of planning core locations. Despitetheseproblems,theresultsofstudiescontainedinthis volume indicate that a useful oxygen isotope record can be extracted from continental-shelf cores, given a seismic strati- graphic framework and results from paleontological and radio- carbon analyses (Fig. 5). That framework provides information on where within the section there are unconformities and yields information about where deposition was occurring on the shelf during a particular time interval. The oxygen isotope curves generated for this project were interpreted by comparing them with reference oxygen isotope curves of Globogerinoides ruber from Gulf of Mexico and Carib- bean cores. The two reference curves are from DSDP site 619 in the 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 record assembled from several deep-sea piston cores (Emiliani, 1978). In addition, to provide stratigraphic benchmarks, these oxygen isotope records also record the influx of 16O-rich meltwater into the northern Gulf during the retreat of the Laurentide ice sheet. Meltwater that flowed down the Mississippi River between 14 ka and 12 ka (Leventer et al., 1982; Fillon and Williams, 1984; Williams and Kohl, 1986; Brown and Kennett, 1998) created a negative spike in the planktonic oxygen isotope records of the Gulf of Mexico (Kennett and Shackleton, 1975; Leventer et al., 1982). At about 12 ka the outflow of meltwater shifted to the Atlantic Ocean, through the Champlain–Hudson Valley (Teller, 1987). Older meltwater spikes associated with stage 5 through stage 3 glaciation also have been identified in the Gulf of Mexico by Williams (1984), Trainer and Williams (1990), and Joyce et al. (1993). Our chronostratigraphic framework is generally not precise enough 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 13 transition meltwater event (Fillon et al., this volume). All of the isotope curves from the Texas shelf show that possible meltwa- ter spikes occur within the stage 3 interval of the cores. In general, the magnitude of oxygen isotope variations due to meltwater influx decreases westward away from the Missis- sippi River and is low east of the river. The possibility that these meltwater events indicate ice-volume-related eustatic events is discounted. This is based on the observation that none of the stage 3 deltas studied show the kinds of backstepping and prograding character that would have occurred if these deltas had 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 and global ice volume (Shackleton and Opdyke, 1973) provides a proxy for changes in global sea level (Shackleton, 1987) (Fig. 3). The conversion of the isotope curve to sea level assumes that sea 0 10 20 30 40 50 60 70 80 90 100 110 0.0 -1.0 -2.0 -3.0 -4.0 Depth(m) clay fine sand clay 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 6 5e 5d 5c 5b 5a 3 1 Z i ii iii vi missing section missing section iv v iv: 45,140 1,300 yr. B.P. at 14.0 m from AMS age date v: Possible 60,700 (Williams and Kohl, 1986) core B-2 2sb-ts 3mfs 5c 5d 5emfs W E Line 5 Two-waytraveltime(ms) 250 200 150 0 20 40 60 80 100 Depth(m) A core B-2 X Y B δ18O 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 stratigraphic surfaces sampled by this core. B) Oxygen isotope curve for Core B-2, showing chronostratigraphic benchmarks for seismic stratigraphic correlation of late Pleistocene strata on the shelf and upper slope (after Rodriguez et al., 2000). Also shown are Ericson zones (X–Z) as described by Kohl et al. (this volume).
  11. 11. 9LATE QUATERNARY STRATIGRAPHIC EVOLUTION, NORTHERN GULF OF MEXICO MARGIN: A SYNTHESIS level was 6 m higher than today during the last interglacial and 120 m lower than the present level during the last glacial. A 10 m sea-level change for every 0.1 per mil change in δ18O, after Shackleton and Opdyke (1973), provides the basis for sea-level curves used in this volume (e.g., Fig. 3). The proxy sea-level curve in Figure 3 is constrained by U–Th dated sea-level stands for the interval between stage 5e and early stage 2 by in situcorals (Bard et al., 1996; Chappell et al., 1996) and is more tightly constrained for the time interval between late stage 2 (ca. 18 ka) to present by radiocarbon-dated shell and peat samples (e.g., Bloom, 1983). The seismic stratigraphic results of this study show reasonable agreement with the isotope curve, with the exception of the amplitude of isotope stage 3. Data from this study show a sea-level stand of -15 m to -20 m for the highest sea-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 offshore southeastAustralia(Royetal.,1997)andwithpublishedsea-level reconstructions (Matthews, 1990; Moore, 1982; Chappell et al., 1996; Kaufmann, 1997) used in the Lagniappe delta study (Fillon et al., this volume). Despite its inaccuracies, the proxy sea-level curve, like that shown in Figure 3, is the most precise sea-level record for all of geological time. It is generally accurate to within ± 30 m for the timeintervalbetween120kaand15ka,andtowithin ±5mforthe past 15,000 years. Oxygen isotope curves also provide the most accurate indication of the timing and frequency of glacioeustatic change. Subsidence Rates In this volume we constrain tectonic subsidence rates on the shelf using maps of the elevations of the stage 5e maximum flooding surface (Fig. 4). The assumption made in these calcula- tions is that the relative water depth at the shelf break was approximately the same at the onset of stage 5 interglacial condi- tions as it is today. Stage 5e deposits occur at about 6 m above sea level on the modern coastal plain and are exposed or nearly exposed a short distance from the coast. Thus, subsidence rates are negligible along the coast. In general, all along the Gulf margin, the thickness of strata resting above the 5e maximum flooding surface increases more or less linearly across the shelf. This implies that subsidence rates increase linearly in an offshore direction. Measured rates of subsidence are highest on the Texas and Louisiana shelves and decrease from west to east across the Mississippi and Alabama–west Florida shelves (Table 1). A dis- cussion of the processes contributing to subsidence on the outer shelf is contained in the paper by Fillon et al. (this volume). RESULTS Dataandinterpretationspresentedinthisvolumeprovidethe basis 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 time for our paleogeographic reconstructions because they represent Deltas Channels and valleys 30 26 ? Shoreface deposits Shelf muds Legend kilometers 0 200 RGD CD BD 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 2 4 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 = Colorado Delta, BD = Brazos Delta, WLD = Western Louisiana Delta, LD = Lagniappe Delta, EMD = Eastern Mobile Delta, WFLAD = West Florida–Alabama Delta, and AD = Apalachicola Delta. East Texas Trinity/ Sabine Western Louisiana East Texas Brazos/ Colorado Central Texas South Texas FlorAla Apalachicola
  12. 12. J.B. ANDERSON, A. RODRIGUEZ, K.C. ABDULAH, R.H. FILLON, L.A. BANFIELD, H.A. MCKEOWN, AND J.S. WELLNER10 keyepisodesofsea-levelchange(Fig.3).Theindividualpapersin this volume provide more detailed paleogeographic informa- tion. Correlation of strata between study areas is accomplished using three prominent stratigraphic surfaces (Fig. 4). The oldest of these surfaces is the stage 5e maximum flooding surface (condensed section), which precedes the last high sea-level stand at approximately 120 ka (Fig. 3). The stage 5e maximum flooding surface is a downlap surface in most of the region (Fig. 4). It is recognized in outer-shelf and upper-slope cores by a sharp in- crease in planktonic foraminifera, by the presence of Globorotalia flexuosa, and by a characteristic oxygen isotope signature (Fig. 5). The highstand systems tract (in the Exxon sense) occurs between this flooding surface and the stage 2 sequence boundary (Fig. 4). There is another prominent flooding surface associated with the stage 3 sea-level rise, but no prominent erosional surface is reported that records the stage 4 sea-level fall (Fig. 3). On many parts of the shelf, a stage 3 flooding surface separates early highstand (stage 5) strata, which are confined mostly to the inner shelf, from stage 3 to stage 2 strata, which occur mostly on the outer shelf and slope (Fig. 4). The third prominent stratigraphic surface is the stage 2 se- quence boundary. It is a prominent erosional surface, marked by deep fluvial valleys and truncation of delta topset beds (Fig. 4). This surface is manifested in cores and platform borings by an abrupt change in lithology, a dramatic change in sediment shear strength (indicating prolonged exposure), general reduction of fossils, 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 outer shelf and slope (Fig. 7). They are separated from transgressive deposits by the transgressive surface. The transgressive surface correlates up dip to the first marine incursion onto the shelf (Fig. 4A).Ontheinnershelf,depositsofthetransgressivesystemstract often 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 at its maximum highstand position, a few meters above present sea level (Fig. 3). An ancestral beach-ridge complex, referred to as the Ingleside paleoshoreline, marks the location of this maxi- mum highstand (Graf, 1966). In Texas and Louisiana, the Ingleside paleoshoreline is located several kilometers landward ofthemodernshoreline.Thisisthemostextensivepaleoshoreline deposit of the last eustatic cycle. Its preservation suggests that it was never subjected to either transgressive or regressive shoreface erosion. North of the Ingleside paleoshoreline in Texas, the landscape is dominated by fluvial meanderbelts that spantheentireglacialeustaticcycle.Southofthispaleoshoreline the coastal plain is virtually flat. Older fluvial channels are mostlyburiedbeneathHolocenecoastal-plaindeposits.Stacked maximum highstand fluvial channels can be seen in the many sandpits that occur in the old Brazos and Colorado valley systems. After the last interglacial the ice sheets in both hemispheres began to expand, and sea level fell episodically. By the end of Stage 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 to 16 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 Incised valley Delta or wedge Slope deposit Legend FlorAla East Texas Trinity/ Sabine Western Louisiana East Texas Brazos/ Colorado kilometers 0 RGV CV BV T/SV Apalachicola Lowstand Systems Tract (Stage 2) 200 MC WMV EMV South Texas Central Texas
  13. 13. 11LATE QUATERNARY STRATIGRAPHIC EVOLUTION, NORTHERN GULF OF MEXICO MARGIN: A SYNTHESIS 98 96 94 92 90 88 86 30 28 26 Incised valley 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 interval of time when the paleoshoreline moved across the inner shelf is referred to as the early highstand. The early highstand saw increased sediment supply to the basin, as a result of fluvial incision,whichremovedsedimentsstoredinthelowerreachesof the drainage basins. During the early highstand, deposition was confined to the inner shelf where subsidence rates are low, hence preservationwaslow.Forthisreason,earlyhighstandstratahave a patchy distribution on the inner shelf of Texas and are generally absent on the Alabama and west Florida inner shelves (Fig. 6). Early highstand deposits are more extensive on the western Louisiana 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 directed toward the western Louisiana shelf. There, a large, sandy highstand delta existed (Fig. 6; Coleman and Roberts, 1990; Wellner et al., this volume). Likewise, highstand deltas were associated with the ancestral Rio Grande, Colorado, and Brazos Rivers in Texas (Banfield and Anderson, this volume; Abdulah et al., this volume), and with rivers east of the modern Mississippi delta (Fig. 6). As sea level fell during stage 5 (Fig. 3), these deltas prograded basinward and their updip portions suffered consid- erable erosion by rivers and streams and by waves (regressive shoreface erosion). Again, the relatively low subsidence rates on the inner shelf contributed to the poor preservation of the early highstand deltas. Erosion of the upper portions of these deltas removed the upper sand-prone fluvial and delta-front deposits, leaving mostly muddy distal bar and prodelta deposits on the inner 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 Trinity and Sabine, apparently did not construct large deltas on the continental 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 sand thatformedonthesteepshelfassealevelfell(BartandAnderson, this volume; McKeown et al., this volume). On the central Texas shelf, where there are no large rivers, depositionduringthisperiodwasdominatedbycoastalandshelf processes (Eckles et al., this volume). These systems were fed by sediment eroded from adjacent deltaic headlands and delivered to the central Texas coast by longshore currents flowing from the east and south. Prograded clastic shoreline and shoreface depos- itsarepreservedontheinnershelf.Overthelongterm,accommo- dationspacecreatedbysubsidencewasnotbeingfilled,resulting in 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 rose rapidly, culminating in the stage 3 flooding event (Fig. 3). At the peak of the flooding the paleoshoreline was located on the inner shelf, perhaps as high as -15 m (Rodriguez et al., 2000). Following the flooding episode, falling sea level again shifted the paleoshoreline to the outer shelf, causing incision of fluvial channels during stage 2. Deltas constructed during this period of falling sea level generally 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 River are taken from Frazier (1967). East Texas Brazos/ Colorado East Texas Trinity/ Sabine Central Texas South Texas Western Louisiana FlorAla Apalachicola
  14. 14. J.B. ANDERSON, A. RODRIGUEZ, K.C. ABDULAH, R.H. FILLON, L.A. BANFIELD, H.A. MCKEOWN, AND J.S. WELLNER12 deposited in the interval 120 ka to 70 ka, despite the fact that stage 3 was no longer in duration than stage 5. During stage 3, deposi- tion was greatest on the outer shelf where subsidence rates, and therefore preservation potential, are high (Fig. 4). Also, cannibal- ization of stage 5 deposits by fluvial processes and regressive shoreface 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 Rio Grande, 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 Louisiana deltas of this age are characterized by clinoforms that dip at low angles (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 and distributary-mouth bars. Indeed, the sandy mouth bars of these deltas 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 and extensive 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, indicate that late highstand deltas on the ramp-like outer shelf are sand- prone (Bart and Anderson, this volume; McKeown et al., this volume). On this slowly subsiding part of the margin, accom- modation space was minimal, so these deltas experienced large lateral shifts. The larger river deltas of the northern Gulf of Mexico shelf experienced virtually continuous growth during the stage 3 sea- levelfall(Figs.4A,6).Thesedeltasweresituatedinashelf-margin position at the end of the late highstand (Fig. 4). However, the times at which individual deltas reached the shelf margin vary acrosstheregion,presumablyasaresultofthedifferentsediment discharge of the rivers. The diachronous progradation of deltas across the outer shelf is illustrated in seismic profile R93-51, a strike-oriented profile collected across the outer shelf (Fig. 4B). This profile shows that the younger late stage 2 Colorado and Trinity–Sabine–Brazosshelf-margindeltasonlaptheolderstages 3–2 Brazos delta. The stage 3–early stage 2 Brazos delta progra- dedacrosstheoutershelffirst,followedbytheotherdeltas.Thus, noteveryshelf-margindeltaisinthelowstandsystemstract.This is an important point because it is implicit that not every shelf- margin delta is associated with a slope fan or basin-floor fan. It also explains why shelf-margin deltas can be large, despite the brief time span of the lowstand. Line R93-51 (Fig. 4B) also illustrates how dip lines across the shelf 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 the Gulf margin is similar to that observed in the Adriatic Basin by Trincardi et al. (1994). On the central Texas shelf, sandy prograding clastic shoreline and shoreface deposits grade offshore into shelf muds; there are no outer-shelf sand bodies in that area. Prograding shorelines associated with the early fall in sea level were unable to keep pace with the rapid fall that occurred during stage 4. This was largely due to reduced sediment supply during this time (see discussion inEcklesetal.,thisvolume).Bythelatehighstand,theRioGrande and Colorado deltas, which had served as longshore sediment sources 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 different environments and at different times across the Texas shelf. 22 ka to 16 ka (Lowstand) During the lowstand, streams and rivers cut their deepest incised valleys, to produce the Stage 2 sequence boundary, and significant volumes of sediment bypassed the shelf (Fig. 7). The stage2sequenceboundaryisaprominentsurfacethroughoutthe northern Gulf of Mexico and constitutes a definitive surface for separating highstand and lowstand systems tracts. On the basis of global sea-level curves (Fig. 3), the paleoshoreline in the northern 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 only afewthousandyears.Prominentfeaturesofthelowstandinclude incised fluvial valleys, lowstand deltas, slope fans, and other sediment-gravity-flow deposits (Fig. 7). Lowstand fluvial valleys of the northern Gulf of Mexico vary considerably in their morphology. In general, the low-gradient east Texas and western Louisiana shelves are characterized by valleys that become broader and shallower in a seaward direc- tion. On average, these valleys are 40 m deep at the present shoreline, which is about the same depth of incision as during the previous lowstand (Blum and Price, 1998). The similarity in depths of incision of different rivers, which vary in terms of discharge and gradient, indicates a similar response to base-level fall regardless of these differences. The Rio Grande and Colorado rivers remained relatively fixed in their locations throughout the highstand, resulting in broad channels that subsequently were incised during the maxi- mum lowstand. These incised channels deepen offshore, cutting into late highstand and lowstand deltas to produce sandy slope fans. In contrast, the Brazos and western Louisiana rivers di- verted from their late highstand channels into different locations at the beginning of the lowstand, leaving their broad feeder channels and associated highstand deltas isolated on the shelf. There are no slope fans downdip of these features. The Trinity and Sabine rivers have occupied the same valleys throughout the eustatic cycle, and apparently during previous eustatic cycles. Broad, terraced cross sections and a deep, U-shaped incision characterize these channels and sediment bypass during re- peated eustatic lowstands has nourished fans within slope minibasins (Anderson and Rodriguez, 2000). On the ramp-like central Texas shelf, distinct fluvial channels are evident only on the inner shelf, a result of the fact that gradients on the inner shelf were lower than river gradients and outer-shelf gradients (Eckles et al., this volume). The steep ramp- like west Florida shelf is characterized by extensive but shallow braided channel complexes on the inner shelf and discrete late- highstand to lowstand channels on the outer shelf (McKeown et al., this volume). This difference between the central Texas and westFloridafluvialgeomorphologiesreflects,inpart,differences in substrate conditions on the shelf. West Florida rivers flowed across sandy substrates so that they were laden with bedload material. On the central Texas shelf, rivers cut into muddy sediments 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 overall eustatic 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 single shelf setting (e.g., Posamantier, 2001), can therefore be mislead-
  15. 15. 13LATE QUATERNARY STRATIGRAPHIC EVOLUTION, NORTHERN GULF OF MEXICO MARGIN: A SYNTHESIS FIG.9.—SeismiclinesG-410andAPL19areusedtocontrastthestratigraphicarchitecturesoftheA)eastTexasandB)westFloridamargins,respectively.TheEastTexas shelfischaracterizedbycontinuousdeltaprogradationduringthehighstandthroughtheearlytransgressionwithaprominentlowstanddeltaandslopefan.Theramp- likewestFloridamarginischaracterizedbyprominenthighstanddeltasandapaucityoflowstandstrata. 0 100 200 NS multiple MFS-7 SB-6 1kilometer v.e.~33x MFS-5 - TS APL19 Delta B Upperslopemaximum 1km V.E.=17x fault 5eMFS h b Two-WayTravelTime (ms) 100 200 300 ravinementsurfaceSB2 WaterBottomMultiple Sediment gravityflow 5eMFS a b G-410 Shelf-edgedelta g fedc TS A APL19 SB-2 Two-WayTravelTime(ms) Two-WayTravelTime (ms) A B V.E.~33x 0km1
  16. 16. J.B. ANDERSON, A. RODRIGUEZ, K.C. ABDULAH, R.H. FILLON, L.A. BANFIELD, H.A. MCKEOWN, AND J.S. WELLNER14 ing.Furthermore,subdivisionofvalley-filldepositsintodifferent systems tracts (e.g., Van Wagoner et al., 1990) offers serious challenges (Blum, 1993; Blum and Valastro, 1994; Ethridge et al., 1998). The shelf-margin deltas of the northern Gulf margin show a complex pattern of progradation and aggradation that varies from one delta to the next. This complexity reflects the part of the eustatic cycle over which these deltas were constructed. Some deltas were active during the late highstand through the early transgression, such as the Colorado, Rio Grande, and Lagniappe deltas. Other deltas, such as the Trinity/Sabine/Brazos delta, were nourished during the lowstand and a significant part of the transgression (Fig. 4B). 16 ka to 4 ka (Transgression) The final episode of sea-sea level change occurred between approximately 16 ka and 4 ka During this period, melting ice sheets contributed just over 100 m of sea-level rise to the oceans. There are several different depositional systems associated with the transgressive episode in the northern Gulf of Mexico (Fig. 8). These include shelf-margin deltas, fluvial- and wave-dominated deltas, thick shelf muds, incised-valley fills, sand banks and ridges, and transgressive sheet sands. Transgressive, backstepping deltas are associated with those rivers with the largest sediment fluxes (Mississippi, Rio Grande, Colorado, Brazos, and Apalachicola). The absolute age of these deltas 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 sea level was rising rapidly (Fig. 3). Episodes of delta growth suggest increased sediment supply, believed to have been caused by climatic 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 across the northern Gulf of Mexico shelf, so do the types of sediments that fill these valleys. Transgressive deposits in the Trinity– Sabine areas are confined to the incised valleys or to areas immediately adjacent to the incised valleys. In interfluve areas, the transgressive ravinement surface is amalgamated with the sequence boundary, as evidenced by a thin (< 1 m) marine mud unit 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 lower bay) facies separated by aggradational valley-fill deposits (Tho- mas and Anderson, 1994). The estuarine facies include upper to lower bay deposits as well as tidal-inlet sands. Flooding surfaces often are manifested by an absence of one or more of the valley- fill facies, and individual flooding events result in updip facies shifts of many tens of kilometers (Thomas and Anderson, 1994). On the Mississippi–eastern Louisiana shelf, cored transgressive deposits range in thickness from 1 m to 9 m and comprise back- stepping estuarine, sound, and neritic facies (Fillon et al., this volume; Roberts et al., this volume). In that area, only the latter part of the transgression appears to be represented. Radiocarbon dates 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 glacial maximum age (Roberts et al., this volume). Similar backstepping fluvial/estuarine/marine facies characterize fluvial-valley suc- cessions in western Louisiana (Nicol et al., 1994) and offshore Alabama (Bartek et al., this volume). The Brazos and Colorado rivers have larger sediment sup- plies than the Trinity, Sabine, and probably Mobile rivers. The BrazosandColoradoriversfilledtheirincisedvalleyswithfluvial deposits and abandoned them to occupy more shallow valleys. The result of these fluvial avulsion events has been the formation of multiple transgressive fluvial channels on the shelf and the sequestering of a significant volume of fluvial sediments in these channels.Preservationofthesetransgressivechannelsvariesand depends on their depth of incision, which in turn is controlled by wheresealevelwaswhenavulsiontookplace.Olderchannelsare incised more deeply and hence have better preservation. Trans- gressive ravinement led to decapitation of fluvial channels and mouth-bar facies and reworking of these facies into widespread shelf sand bodies. On the east Texas shelf, large banks lie adjacent to and above the Trinity–Sabine incised valley. These banks have been inter- pretedassubmergedpaleoshorelinescomposedofaback-barrier estuarine unit at the base, a fore-barrier, lower shoreface and ebb tidal delta unit above, and a storm-reworked unit at the top (Nelson and Bray, 1970; Rodriguez et al., 1999). A transgressive section cored in Main Pass Block 288 at the Lagniappe shelf edge contains this same succession (Roberts et al., this volume). Sand ridges characterize the south Texas inner shelf. These isolated inner-shelf sand ridges formed in situ at present water depths (Rodriguez et al. 2001). The different origins of east Texas banks and south Texas ridges are attributed to variations in the depth of shoreface ravinement (deeper in south Texas), shelf gradient (steeper in south Texas), and accommodation space (lower in south Texas) (Rodriguez et al., 2001). An extensive (24,000 km2) transgressive-sand-ridge field covers most of the Mississippi–Alabama–Florida shelf in the northeastern Gulf of Mexico (the MAFLA sand sheet; McBride et al., 1999; McBride et al., this volume). The MAFLA sand sheet 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 reworked polycyclic late-highstand and lowstand sand (McBride et al., 1999). On the central Texas shelf and south Texas outer shelf, the youngest deposits of the transgressive systems tract consist of widespread marine muds of the “Texas Mud Blanket” (Shideler, 1981; Eckles et al., this volume). The mud blanket is up to 45 m thick in central and south Texas and is composed of sediment from the Rio Grande to the south and from as far east as the Mississippi River (Shideler, 1981). 4 ka to Present (Late Holocene Highstand) Thepresent-daysedimentaryenvironmentrepresentsamaxi- mum highstand, which is a unique part of the total glacioeustatic cycle (Fig. 3). Currently, the shelf is flooded and extensive coastal barriers exist. Onshore, broad meanderbelts occur on the low- gradient coastal plain and provide storage for vast quantities of sediment. These meanderbelts, however, are not nearly as exten- sive as the broad alluvial plains that formed during the previous maximum highstand (Stage 5e). During the present highstand, the Mississippi River, with its huge sediment supply, has prograded far onto the outer conti- nental shelf. A significant amount of the fine-grained sediment that is delivered to the Gulf by the Mississippi River is trans- ported to the west in wind-driven surface currents influenced by the Coriolis effect. These fine-grained sediments are depos- ited on the central and south Texas shelves as the Texas Mud Blanket. Elsewhere on the shelf, sedimentation is at a minimum and the most important process occurring today is the forma- tion of a condensed stratigraphic section (maximum flooding surface).
  17. 17. 15LATE QUATERNARY STRATIGRAPHIC EVOLUTION, NORTHERN GULF OF MEXICO MARGIN: A SYNTHESIS DISCUSSION Fluvial Response to Sea-Level (Base-Level) Change Within the northern Gulf of Mexico Basin, there is a fairly strong correlation between the long-term sediment supply of rivers and the size of their drainage basins. Currently, bays occupy 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 greater sedimentdischarge,suchastheBrazosandColoradorivers,have beguntoconstructprominentdeltaicheadlandsacrossthecoastal plain (Fig. 8). These differences reflect the inability of smaller rivers to fill their valleys as fast as sea level rose during the postglacial transgression. TheresponseofGulfCoastriverstoeustatic(base-level)change during the past 120,000 years varied across the shelf. Highstand fluvial channels are widespread on the shelf, particularly on the low-gradient Louisiana and east Texas shelves (Berryhill et al., 1986;Andersonetal.,1996).Ingeneral,channelsthatextendfarther onto the shelf are deeper and narrower than channels that extend only short distances across the shelf. This indicates that incision occurs throughout the falling limb of sea level. During the last highstand, several of the larger rivers of the region formed large deltas as sea level first began to fall. The sediment discharge of these rivers was significantly greater than today. Estimates of sediment flux of the Rio Grande and Brazos rivers during the last several highstands were in the range of 1.4 to 2.9 km3/yr and 0.3 to 3.2 km3/yr, respectively. However, this increaseinsedimentfluxdidnotoccureverywhere.Apalachicola River sediment flux rates appear to have varied little between the previoushighstandandcurrenthighstand,averaging0.4km3/yr and 0.5 km3/yr, respectively (McKeown et al., this volume). Differences in modern and ancient sediment flux of the larger Texas rivers versus the Apalachicola River are attributed to differences in the sediment storage capacity of these rivers. Texas rivers have much more extensive alluvial plains with significant sediment storage capacity (Blum and Price, 1998). In contrast, west Florida rivers, including the Apalachicola River, occupy relativelyhigh-gradientchannelsthatareincisedintoPleistocene strata. The coastal plain is narrow and flood plains are small, hence their sediment storage capacity is relatively small. Duringafallinsealevel,sedimentsstoredinalluvialdrainage basins of Texas and Louisiana are eroded as rivers incise, their tributaries branching out into the alluvial basins (Koss et al., 1994). The result is a large increase in sediment flux during the falling limb of the sea-level curve. In contrast, the sediment flux of west Florida rivers remained relatively unchanged (McKeown et al., this volume). Incisedfluvialchannelsalsoexistinthetransgressivesystems tract of the Louisiana and Texas shelves. In Texas, these channels are associated with the ancestral Brazos and Colorado rivers. Avulsion of these rivers during the transgression resulted in channels being cut to shallower depths as sea level rose. The younger channels have been deeply eroded by shoreface erosion andextendoffshoreintomarinemuds.Oldertransgressivechan- nels extend offshore into transgressive deltas (Abdulah et al., this volume) and reflect times when sediment supply was greater. Variationsinsedimentsupplyduringtransgressionswerecaused by climatic changes. Climate Control on Sediment Supply to the Basin Climate variation is known to be a major controlling factor on fluvial processes (Schumm, 1965, 1993; Hall, 1990; Ethridge et al., 1998). Indeed, Blum (1993) and Ethridge et al. (1998) conclude that, because the influence of sea-level (base-level) change on fluvial morphology diminishes in a landward direction, the climatic influence on sediment delivery to the basin may be greater than that of eustasy. The fluvial response to changes in climate can be very com- plex. For example, the same river can show different responses along its course because of changes in gradient and changes in sedimentloadasitflowsthroughdifferentclimaticbelts(Schumm and Brakenridge, 1987). In general, the sediment flux of fluvial systems increases with increasing mean annual precipitation (Schumm,1965).Butthemannerinwhichthesedimentdischarge of rivers responds to climate change is more complex than this simplerelationshipsuggests.Schumm(1965)arguedthatachange from semiarid to arid conditions would result in a decrease in sedimentdischarge.Incontrast,achangefromhumidtosemiarid climates 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 basin there is a fairly strong correlation between the long-term sedi- ment flux of rivers (i.e., over a complete glacioeustaticcycle) and the size of their drainage basins. This is in itself an important finding because the sediment discharge of modern rivers shows little correlation to drainage-basin size (Hovius, 1998), although the correlation is stronger when the relief of the drainage basin is considered (Milliman and Syvitski, 1992; Morehead and Syvitski, 1999). The timing and volume of sediment influx to the northern Gulf of Mexico margin during the last eustatic cycle varied from one river to another. This diachronous influx of sediment is undoubtedly a result of the different climate set- tings of the rivers. Currently, climate varies from humid to semiarid across the study area, with a nearly threefold differ- ence in mean annual precipitation across the region (Fig. 1). It is likely that the magnitude and timing of climate change differed across the region during the last glacial–interglacial cycle (120 ka to present). Perlmutter et al. (1998) generated computer models that illus- trate how differences in the timing of sediment discharge during a glacial-eustatic cycle result in differences in the volumes and character of different systems tracts. Their work is based on modern sediment yields, which are quite low compared to other times in the last 120,000 years. Also, they do not take into account effects of vegetation changes and rates of soil development and erosion and the delayed response of these changes on sediment yield. Their work did show, however, that sediment supply from modern rivers varies by more than two orders of magnitude during a single climate cycle and that variations in the timing of sediment supply differ between climatic belts. Their work also showed that the greatest changes in sediment yields occur in the regions where climate varies from arid to subhumid, which includes the Texas coastal plain. In general, all of the larger Texas, Louisiana, and Mississippi deltasexperiencedcontinuousgrowththroughoutthehighstand. Climate changes did occur along the Gulf Coast as the ice sheets tothenorthexpanded,butanyimpactoftheseclimatechangeson sediment supply to the Gulf was overshadowed by the effects of sea-level fall. Sea-level (base-level) fall was the driving force in delivering sediment to the margin. Indeed, detailed work on the ancestral Brazos delta has shown that growth of the delta during the last highstand was strongly regulated by fifth-order eustatic fluctuations (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 impact of climate change on sediment supply to the basin at this time is unknown. However, sediment fluxes of the different rivers ap-
  18. 18. J.B. ANDERSON, A. RODRIGUEZ, K.C. ABDULAH, R.H. FILLON, L.A. BANFIELD, H.A. MCKEOWN, AND J.S. WELLNER16 pear to have varied significantly during the transgression. This difference is attributed to climatic influence on sediment supply rather than eustasy. BlumandPrice(1998)pointoutthat,withintheColoradoRiver drainage basin, the postglacial sea-level rise was accompanied by warmertemperaturesandincreasedtropical-stormfrequencyand more flashy discharge regimes. The net effects of these climatic changes were degradation of soils and general denudation of upland landscapes within the drainage basin. Sediments removed from the upland areas of the drainage basin were transported basinward. Floods delivered some of this sediment into the exten- siveflood-plainsettingsofthecoastalplain(Blumetal.,1994;Blum and Valastro, 1994). There, these sediments are sequestered until the next eustatic fall. But there was also a significant flux of sediment to the Gulf, as recorded by the transgressive deltas associated with the ancestral Brazos, Colorado, and Rio Grande rivers (Abudlah et al., this volume; Banfield and Anderson, this volume). 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 11 to9ka,whenboththeColoradoandRioGrandedeltasexperienced significant phases of growth (Abdulah et al., this volume; Banfield and Anderson, this volume). These periods of increased sediment fluxtothebasinundoubtedlyresultedfromashifttodrierclimates and associated reduction in the area of grasslands following the Younger Dryas climatic event (Banfield, this volume; Snow, 1998). On the Lagniappe shelf at this time the transgressive flooding event continued unabated. Stratigraphic Models Differences in fluvial response to sea-level change and climate have resulted in very different stratigraphic architectures across the northern Gulf of Mexico margin. Similar along-strike variabil- ityinsystemstractsareobservedonothercontinentalshelves(e.g., TrincardiandCorreggiari,2000;McMurrayandGawthorpe,2000). It is possible to group the different study areas of the northern Gulf of Mexico margin into seven type sections on the basis of their gross stratigraphic architecture. These type sections are illustrated using stratigraphic slug diagrams (Fig. 10). Table 1 provides key information about each area, and Table 2 summa- rizestheimportantdepositionalfeaturesforeachareabysystems tract. These models are generalized, but they serve to illustrate the very different stratigraphic architectures that exist on the margin and their relationship to the margin setting. Our general models can be used to predict the distribution of reservoir-scale sand bodies on the shelf and for stratigraphic correlation. We hope that by providing well-documented case studies our results will 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 the individual 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. The currentclimateishumidandisunlikelytohavebeensignificantly drier during glacial times. There has been minimal preservation of early highstand deposits on the west Florida shelf. Inner-shelf sediments were eroded and transported seaward, where they are incorporated into late-highstand deltas that prograded into relatively deep water (up to 70 m water depth). A lowstand delta and fan complex is lacking. During transgression, sandy delta tops were erodedandthesandsspreadacrosstheshelfasanextensivesheet sand (MAFLA Sheet Sand; McBride et al., this volume). The Latium shelf of Italy is perhaps a good analogue to the West Florida shelf. The largest sediment supplier to the shelf is the Tiber River. It is narrow (30 km), has no abrupt shelf break, and has a narrow coastal plain with several small rivers draining highlands. Thus, the coastal plain is unable to stockpile large amounts of sediment. As a result, changes in base level and climate appear to have had a rapid influence on sediment supply to the shelf (Chiocci, 2000). Also, there are no canyons dissecting the margin and no basin-floor fans. Lowstand deposits were supplied to the slope through gullies as a line source rather than through canyons. This has resulted in a uniform thickness of sediments on the upper slope, possibly also reflecting stronger longshore transport during the lowstand. B. Lagniappe (Mississippi–Eastern Louisiana Margin.— During the late highstand and lowstand, sediments from the Pascagoula and parts of the Mobile drainage system were depos- ited offshore of Mississippi and western Louisiana to construct the Lagniappe delta. The delta prograded across a relatively broad, low-gradient shelf that experienced relatively low subsid- ence. During the maximum lowstand, deposition shifted to the east and constructed a shelf-margin delta (Roberts et al., this volume). Progradation of this shelf-margin delta continued into the initial transgression (Fillon et al., this volume; Kohl et al., this volume). These deltas contain extensive delta-front sand bodies, but no associated lowstand fan has been identified. Currently the deltas are being buried beneath muds shed from the Mississippi River. C. Western Louisiana–Brazos Margins.— Theseareasarecharacterizedbybroad,low-gradientmargins that 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 these margins. Early-highstand fluvial channels occur on the inner shelf, but sandy delta topset beds (mouth bars) are mostly confined to the outer 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 lowstand deltasorfansonthesemargins.Transgressivedepositsaremostly muds, 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 has been nourished with sediment from the mixed bedload/sus- pended load-dominated Trinity and Sabine Rivers throughout the eustatic cycle and by the Brazos River during the lowstand (Abdulah et al., this volume). Highstand deposits are thin and composed of mud. Shelf bypass through the Trinity–Sabine–Brazos valley has resulted in the development of a large lowstand delta and slope fan complex whose distribution on the margin has been strongly regulated by salt diapirs (Wellner et al., this volume). Transgressive deposits are mostly confined to the incised fluvial valley and include extensive fluvial sands and backstepping bayhead deltas and tidal deltas (Thomas and Anderson, 1994). The only exceptions
  19. 19. 17LATE QUATERNARY STRATIGRAPHIC EVOLUTION, NORTHERN GULF OF MEXICO MARGIN: A SYNTHESIS ¥ramp setting ¥bedload-dominated rivers and streams low subsidence onlapping wedge (hemipelagic mud) TST-sheet sand sand ridges carbonate mound TS/SB HST deltaMFS A West Florida ¥broad, low-gradient shelf ¥moderate subsidence ¥large, mixed load river MFS HST delta SB/TS LST delta TST delta incised valley TS LST-slope fan E East Texas-Colorado ¥narrow, steep shelf ¥moderate subsidence ¥large, mixed load river MFS SB LST delta fan HST delta TST delta incised valley TST marine mud G South Texas-Rio Grande SB F Central Texas ¥narrow, steep shelf ¥moderate subsidence ¥small, bedload-dominated rivers HST marine mud HST shoreline/ shoreface deposits marine muds MFS Transgressive mud blanket TST/LST mud B Lagniappe ¥broad shelf, low gradient ¥low subsidence ¥rivers with moderate bedload TST (muds) slumps late highstand- lowstand delta (sandy mouth bars) onlapping wedge MFS TS ¥broad, low-gradient shelf ¥moderate subsidence ¥small suspended load-dominated river HST marine mud LST turbidites within minibasins LST/early TST delta SB/TS TST-sand banks SB incised valley D East Texas-Trinity/Sabine MFS TS TST early HS TST MFS MFS late HS C Western Louisiana/Brazos ¥broad shelf, low-gradient ¥moderate subsidence ¥river with dominant suspended load FIG. 10.—Seven stratigraphic models (A–G) are used to illustrate the different stratal architectures of the northern Gulf of Mexico margin. See text for discussion. Dots indicate sand-prone areas.
  20. 20. J.B. ANDERSON, A. RODRIGUEZ, K.C. ABDULAH, R.H. FILLON, L.A. BANFIELD, H.A. MCKEOWN, AND J.S. WELLNER18 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 margin is supplied with sediments by the Colorado and Brazos rivers, which have relatively high sediment flux rates and extensive coastal-plain sediment storage capacities. The Colorado River had a high bedload contribution to the margin relative to other eastTexasrivers.Theareaexperiencedsignificantclimaticchanges during the last glacial–interglacial cycle; specifically, it was drier during interglacial times (Toomey, 1993). Subsidence rates are relatively high (0.1 to 4.0 mm/yr). Thin, sandy early highstand deltas on the inner shelf and thick, sandy late highstand deltas on the outer shelf characterize themargin(Abdulahetal.,thisvolume).Sedimentbypassduring the lowstand resulted in a sandy Colorado lowstand delta and slopefancomplex.Thetransgressivesystemstractincludeslarge, sandy deltas and sand-filled incised fluvial valleys. The high sedimentyieldoftheColoradoRiverduringtherecentglacial-to- interglacial transition is credited with the increased sediment supply that led to the formation of transgressive deltas on the shelf. F. Central Texas Margin.— The central Texas margin has a relatively steep profile and lacks a distinct shelf break. The margin experiences moderate rates of subsidence (Eckles et al., this volume). Small, mixed bedload/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 is believed to have been more humid during glacial times than it is today (Toomey, 1993). Sand-prone deposits are mostly confined to the inner shelf andconsistofearly-highstandprogradingshorelineandshoreface deposits.Theoutershelfismud-dominated,therearenolowstand deltas 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. The Rio Grande River has been the principal supplier of sediment to the margin during the late Quaternary and, for that matter, throughout most of the Tertiary. Of the various study areas, it has the most persistently dry climate. Extensive highstand deltas with large delta-front sand bodies occur on the shelf, including both wave- and fluvial-dominated deltas(BanfieldandAnderson,thisvolume).Averythicklowstand delta and fan complex occurs on the shelf margin and upper slope,whichincludesthicksandunits.Thetransgressivesystems tract includes incised fluvial valleys and transgressive deltas that have 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 in shelf 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- Tract Descriptions Highstand (HS) • early HS preservation • areally extensive muddy deltas lobes (extensive prodelta) (< 0.5° angle) and associated distributaries (aggrading sigmoidal clinoforms at seaward margin) (wave/fluvial dominated) • prograding shoreline (converging longshore currents) • muddy deltas (extensive prodelta) with minimal delta lobe switching during early fall • sandy deltas with associated distributary systems during late fall • minimal HS preservation outside incised valleys • minimal HS preservation on inner shelf • preserved sandy deltas with several lobes (< 5° angle) and associated distributaries (aggrading sigmoidal clino- forms at seaward margin) (fluvial) • preserved thick, lobate silty to fine sand deltas with several lobes and associated distributaries (aggrading sigmoidal clinoforms at seaward margin) • minimal HS preservation on inner shelf • preserved sandy deltas with several lobes (> 3° angle) and associated distributaries (fluvial and wave) HS Sediment Supply (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, narrow incised channels • no major cross- shelf incised fluvial valleys or slope canyons • no LS deltas • incised fluvial valleys • muddy and sandy shelf-margin delta (Colorado only) • muddy and sandy slope fans (Colorado only) • single incised fluvial valley (point source) • sandy shelf- margin delta • slope fans • shallow, broad fluvial system with terraces • no cross-shelf incised fluvial valleys or canyons • no slope fans • broad, shallow incised fluvial braid system (line source) • no cross-shelf incised fluvial valleys or canyons • no slope fans • broad, shallow incised fluvial braid system with terraces (line source) • no cross-shelf incised fluvial valleys or canyons • distributaries are incised to shelf break • no LS fans Transgressive (T) • deposition not confined to incised valleys • localized sandy delta lobes (fluvial/wave dominated) • LS wedge (backstepped deltas) • valley fill • 40-m-thick mud blanket • reef trend ~ 60 m • sandy and muddy incised-valley fill • deltas with lobe switching (fluvial/wave) • sandy and muddy incised valley fill • sandy banks preserved adjacent to incised valleys • muddy shelf- margin deltas • incised-valley fill • sand ridges • two delta lobes • carbonate mounds • hemipelagics • shoreline-parallel shelf currents • valley fill • sand ridges • carbonate mounds • thin, laterally extensive slope wedges (shoreline- parallel upper slope- currents) • valley fill • sand ridges • backstepped delta lobes • thin slope wedges (not extensive) • hemipelagics TABLE 2.—Summary of the depositional styles of systems tracts for the various study areas in the northern margin of the Gulf of Mexico.
  21. 21. 19LATE QUATERNARY STRATIGRAPHIC EVOLUTION, NORTHERN GULF OF MEXICO MARGIN: A SYNTHESIS 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,whichisillustratedusingsevenstratigraphicmodels.The models 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 and minimal sediment discharge by rivers, the exception being the Mississippi River. Sedimentation on the margin is mini- mal, and the modern sea floor is a condensed stratigraphic section (maximum flooding surface). The only exception is the wave-dominated central Texas shelf, where a thick Ho- locene mud blanket is accumulating. Thus, the modern setting provides only a glimpse of how strata are formed on the margin. 3. During the last falling limb of the sea-level curve, the larger rivers of the Gulf Coast Region had sediment fluxes up to an order of magnitude greater than their current sediment fluxes. These rivers nourished large highstand deltas. In general, sediment fluxes of rivers increased with falling sea level. This was partly the result of cannibalization of early highstand deltas by fluvial incision and regressive shoreface erosion. If climate regulated sediment yields during this time, the effects were overshadowed by sea-level change. 4. The central Texas shelf was wave-dominated during the last highstandandwascharacterizedbyprogradingclasticshore- line and shoreface deposits. Progradation of these deposits across the shelf occurred only during the early highstand, largely because erosion of the Colorado and Rio Grande deltas nourished the central Texas deposits. Elsewhere along the Gulf margin, coastal deposits are mostly confined to the maximum 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 a significant amount of sediment delivered by these rivers beingsequesteredontheshelf.Incontrast,riverswithsmaller sediment supplies are more inclined to occupy the same channels throughout the eustatic cycle. This results in greater sedimentbypassoftheshelf,whichequatestoslopeandbasin floor fan systems (Anderson and Rodriguez, 2000). 6. Abandonment of highstand shelf-margin deltas by some of the larger rivers (Brazos and western Louisiana fluvial sys- tems) prior to the Stage 2 lowstand resulted in an absence of lowstand delta and slope fans in these regions. 7. Over the length of a glacioeustatic cycle, the overall sediment flux of a river is determined by the size of its drainage basin. The climate effects on sediment flux of different rivers are more apparent during the transgression; however, the effect of climate on sediment flux varied across the margin. 8. Geomorphology (width and cross-sectional geometry) alone is not a suitable criterion for distinguishing highstand and lowstand fluvial channels. Lowstand fluvial geomorphology varies widely across the shelf. These differences are the result of differences in shelf gradient, drainage-basin size and cli- mate (which controls discharge), substrate conditions on the shelf, and the interval of the eustatic cycle during which the river occupied the channel. 9. Idealizedtripartiteincised-valley-fillsuccessions(e.g.,Wright and Marriott, 1993; Zaitlin et al., 1994) are the exception rather than the rule on the northern Gulf of Mexico shelf. Along the margin, valley fills range from those that are dominantly fluvial (e.g., the Brazos valley) to those that are dominantly marine (e.g., central Texas valleys). This is due to the different sediment supplies of these rivers and their capacity to keep pace with the rate of sea-level rise during transgression. 10. Ourdatashowthatfifth-ordersea-levelfluctuationshavehad a marked influence on sedimentation on the continental shelf. 11. The modern physiography of the margin is largely a product of sediment supply. A broad shelf and distinct shelf break characterize portions of the margin where sediment supply is high, 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 the shelf.Largeriverswithhighsuspendedloadsconstructmuddy deltasinwhichsandsaremostlyconfinedtomouthbarsinthe topset portions of the deltas and to the point bars of the meandering rivers that nourished these deltas. Smaller, bedload-dominatedrivers,suchasthoseofwestFlorida,have constructed sandy deltas. 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