LATE QUATERNARY STRATIGRAPHIC EVOLUTION OF THE ALABAMA AND WEST FLORIDA OUTER CONTINENTAL SHELF

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1 LATE QUATERNARY STRATIGRAPHIC EVOLUTION, ALABAMA AND WEST FLORIDA OUTER CONTINENTAL SHELF Copyright 2012, Society for Sedimentary Geology (SEPM) LATE QUATERNARY STRATIGRAPHIC EVOLUTION OF THE ALABAMA AND WEST FLORIDA OUTER CONTINENTAL SHELF 43 PHILIP J. BART Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A. AND JOHN B. ANDERSON Department of Geology and Geophysics, Rice University, 6100 South Main, Houston, Texas 77005, U.S.A. ABSTRACT: Approximately 3000 km of single-channel seismic data from the Alabama and west Florida outer continental shelf and upper slope were analyzed to characterize the late Quaternary regional stratigraphic framework of this ramp margin. Seismic analysis shows that thick delta lobes are located at several near-surface stratigraphic levels on the outer continental shelf. On the basis of the depocenter locations, we infer that sediments delivered to the deltaic wedges on the Alabama shelf were derived from the confluence of the ancestral Mobile Tombigbee rivers. The smaller deltaic wedges on the west Florida outer continental shelf probably received sediment from the confluence of the Perdido, Escambia, Blackwater, and Yellow rivers. Deep, incised, cross-shelf fluvial valleys do not exist offshore of west Florida. There, outer-shelf depocenters probably received sediment by broad and shallow braided rivers. This situation is in stark contrast to the Texas, Louisiana, and Mississippi continental shelves, where large incised fluvial valleys occupied the shelf during eustatic lowstands. Subsurface mapping shows that the sinuous trend of the Alabama and west Florida shelf edge is a result of the primary delta morphology. We interpret the shelf-margin deltas as lowstand systems, and on the basis of our seismic correlation to chronologic control at a drill site in Main Pass lease area 303, we conclude that the youngest shelf-margin deltas were deposited during the last glacial maximum. The lack of slope canyons indicates that bypass was minimal in this area during the last glacial maximum. In a basinward direction, the clinoform toes and/ or aggrading bottomsets of the lowstand deltaic units interfinger with thin but regionally extensive slope wedges. The seismic evidence of extensive erosion of the shelf-margin deltas suggests that slope wedges in this area may correspond to deposition during the last transgression and present highstand. The youngest seismic-stratigraphic unit is shelf perched and has regional extent and great thickness. We surmise that this shelf-perched unit represents a drowned coastal-plain system that was well established early during the last sea-level transgression. INTRODUCTION The purpose of this paper is to present preliminary results from a study of the late Quaternary sequence stratigraphy of the Alabama and west Florida margin (Fig. 1). Our results primarily concern the seismic-stratigraphic units deposited on the outer continental shelf and upper slope by the Perdido, Escambia, Blackwater, and Yellow rivers during the last glacial cycle. This investigation is part of a larger study aimed at characterizing the natural lateral variability of late Quaternary stratigraphic sequences on the northern Gulf of Mexico margin. In contrast to the late Quaternary shelf slope margins of Mexico, Texas, Louisiana, and Mississippi, the northeastern Gulf of Mexico margins have an overall ramp-type geometry. In addition, the near-surface stratigraphy is undisturbed by faulting (Martin, 1978), and although some diapiric salt is located in the vicinity of De Soto Canyon (Harbinson, 1967), the Alabama and west Florida outer continental shelves are not underlain by thick salt basins (Martin, 1978; Stude, 1978; Jackson and Galloway, 1984; Ingram, 1991). Taken together, these factors make the northeastern Gulf of Mexico an end-member setting quite different from the salt/ growth-fault dominated Texas, Louisiana, and Mississippi margins. This description of the late Quaternary sequence stratigraphy of the Alabama and west Florida margin is intended to form the basis for comparisons with the shelf slope stratigraphic results from studies of the northwestern and north-central Gulf of Mexico presented elsewhere in this volume. METHODOLOGY Correlating late Quaternary depositional sequences on the northern Gulf of Mexico margins requires (1) data grids dense enough to enable mapping of individual units, (2) grids with regional extent sufficient to identify along-strike shifts in depocenter locations, and (3) seismic data with sufficient stratigraphic resolution to image bounding surfaces and relatively thin units (Anderson et al., 1996). In this study, we evaluated a broad sector of the Alabama and western Florida outer continental shelf and upper slope using 3000 km of high-resolution seismic data that were collected from the R/V Lone Star during 1994, 1995, and 1996 field seasons (Fig. 1). The spacing between dip- and strike-oriented seismic profiles averages 20 km. Experience has shown us that this line spacing is adequate to map the smallest late Quaternary seismic units of interest in this study. The seismic sources were a 15 in 3 watergun and a boomer, and the seismic data were recorded with a single-channel streamer. The boomer source provided detailed imaging of the uppermost section, but because the depth of signal penetration was too poor to image even the basal reflector of the near-seafloor surface unit, the use of the boomer source was abandoned early during the first field campaign. The analog seismic data were plotted on an EPC printer, and the digital seismic data were recorded and processed using a Delph system. The filter cutoffs were set at 30 and 800 Hz. The dominant frequency of the seismic data was between 130 to 200 Hz, providing a theoretical stratigraphic resolution of 2.5 to 4 m, on the basis of Rayleigh resolution limit criteria and an average sediment velocity equal to 1.5 km/s. Seismic data processing included scaling and standard band-pass filtering to improve the signal-to-noise ratio. Seismic interpretations were made from shipboard EPC and processed profiles plotted at a vertical exaggeration of 35:1. Our seismic-stratigraphic analysis indicates that regional (several tens of ) reflectors bound prograding-wedge strata. On Late Quaternary Stratigraphic Evolution of the Northern Gulf of Mexico Margin SEPM Special Publication No. 79, Copyright 2004 SEPM (Society for Sedimentary Geology), ISBN , p

2 44 PHILIP J. BART AND JOHN B. ANDERSON Mobile Bay Pensacola Bay 20 Choctawathcee Bay Fig Fig Fig. 4 Fig Fig LEGEND 100 bathymetric contours (m) De Soto Canyon seismic grid seismic profiles referred to in text FIG. 1. Study area location map showing drainage basins, surface geology, bathymetry, and seismic grid. the basis of these bounding discontinuities, the near-surface stratigraphy was subdivided into seven units numbered from the top down; for example, the youngest unit described is Unit 1. Two-way travel times reported from the seismic data are approximate. The travel times were converted to depth using a velocity of 0 m/s. There has not been much drilling on the Alabama and west Florida outer continental shelf, and thus chronologic constraints for our seismic observations are poor. Our seismic interpretation of the base of Pleistocene is based on seismic correlation to Unconformity A defined on the West Florida Slope by Mitchum (1978). Late Quaternary chronological control was provided by seismic correlation to a drill site in the Main Pass lease area 303, which is 80 km to the west of the study area. At this outer-continental-shelf drill site, Sydow and Roberts (1994) defined surface 10 and the overlying pro 10 Lagniappe deltaic sequence. They surmised that surface 10 is equivalent to the oxygen-isotope stage (OIS) 3 maximum flooding surface. Our seismic correlation of surface 10 to the area of our study is based on three strike-oriented profiles from the outer shelf. The correlation shows that the pro 10 Lagniappe delta wedge pinches out towards the east, and thus the underlying surface, surface 10, is amalgamated with the tops of two older delta lobe complexes (pro 20 of the Lagniappe delta, and the Western Delta of Sager et al., 1999). This situation greatly complicates the seismic correlation of surface 10 farther to the east. We project surface 10 from the tops of pro 20 (Sydow and Roberts, 1994) and the Western Delta (Sager et al., 1999) to the base of a third shelf-margin delta that defines the shelf edge at the western edge of our study area. This shelf-margin lobe, our, is the Eastern Delta of Sager et al. (1999), and we believe it to be chronostratigraphically equivalent to the Lagniappe delta pro 10 lobe. Inasmuch as our correlation to the Main Pass 303 drill site is speculative, we show surface 10 on only one of the seismic profiles (Fig. 2). The chronostratigraphic relationships we infer are summarized in Table 1. We acknowledge that further chronologic control is needed to confirm our interpretations. BACKGROUND The Alabama and West Florida Ramp Margin The rivers entering this sector of the northeastern Gulf of Mexico are the Mobile Tombigbee rivers, which drain an area of 100,000 km 2, and four small coastal-plain rivers (Perdido, Escambia, Blackwater, and Yellow rivers), which drain a combined area of 20,000 km 2. To the west, the Pascagoula River enters the Mississippi coast, and to the east, several small rivers enter into Choctawhatchee Bay. Offshore Alabama and west Florida, the shelf edge is roughly oriented southwest northeast. The most striking bathymetric features are the narrow width and low gradient of the margin. The continental shelf narrows in a west east direction from 100 km to 35 km (Fig. 1). Offshore Pensacola, the average gradient of the outer continental shelf is 6 m/km. The shelf edge depth ranges from 80 to 120 m. There are a few canyon-

3 LATE QUATERNARY STRATIGRAPHIC EVOLUTION, ALABAMA AND WEST FLORIDA OUTER CONTINENTAL SHELF 45 SE NW surface 10 shelf edge unit onlapping wedges unit 7 paleo-shelf edges V.E. = 32: water-bottom multiple shelf edge unit 7 V.E. = 10: water bottom multiple FIG. 2. Interpreted line drawing and dip-oriented seismic segment of Profile 5 showing prograding wedges 2, 6, and 7, onlapping wedges on the upper slope, and shelfperched Unit 1 on the outer shelf.

4 46 PHILIP J. BART AND JOHN B. ANDERSON TABLE 1. Stratigraphic relationships inferred from seismic correlations. Sydow and Roberts (1994) 1 Sager et al. (1999) Bart and Anderson, this study Units 1 and 2 Unit 1 Units 3 and 4 (pro 10) Eastern delta Unit 2 Units 5 to 7 (pro 20) Western delta Unit 3 (?) 1 Units listed were defined at the Main Pass 303 drill site. like indentations in the trend of the shelf edge, but no contiguous pathways connect these features to De Soto Canyon, the head of which is at 450 m water depth (Fig. 1). Offshore west Florida, the uppermost slope (i.e., from 100 m to 200 m water depth) dips to the southeast and has a gradient of 1 to 3. Beyond the toe of the upper slope, the seafloor dips to the southwest at a gentle angle (i.e., << 1 ). Taken together, these factors demonstrate that the Alabama and west Florida margin has a ramp geometry, which is morphologically quite different from the Mississippi, Louisiana, and Texas shelf slope margins to the west. RESULTS Regional Stratigraphy on the Alabama and West Florida Outer Shelf Interpreted Seismic Profiles. In this section, we show five seismic profiles (Figs. 2 6) to illustrate the general aspects of the near-surface stratigraphic framework. Line drawings of the seismic profiles are shown along with segments of the seismic data. The locations of the cross-lines are indicated at the top of the line-drawing interpretations. Four dip-oriented profiles (Figs. 2, 4, 5, 6) show the overall topset foreset stratal geometries of the prograding wedges at the outer continental shelf. Figure 2 is from the western end of the study area, and the other dip-oriented profiles are towards the eastern end of the study area (Fig. 1). Figure 3 is a regional east west strike line that crosses all of the four dip-oriented profiles on the outer shelf. The strike-oriented profile shows the point-source character of the individual prograding wedges and their stratal relationships along the strike of the outer continental shelf. Prograding Wedge Unit 2 Offshore Mobile, Alabama. Figure 2 shows prograding-wedge Unit 2 at the shelf margin offshore Mobile Bay, Alabama. This unit downlaps a surface that we infer is correlative to surface 10, i.e., the OIS 3 maximum flooding surface from the Main Pass 303 drill site. At this location, Unit 2 has a maximum thickness of 200 milliseconds (ms). The offlap break elevation of Unit 2 is at 115 ms (86 m) below sea level. The prograding-wedge foresets either toplap or terminate near the seafloor on the outer shelf. The profile also shows the Unit 6 and Unit 7 prograding wedges, which are found across the study area. At this location, the offlap breaks for Units 6 and 7 are at 165 ms and 221 ms, respectively. The upper bounding surfaces dip slightly in an offshore direction, and the top of the Unit 2 prograding wedges defines the seafloor on the outer shelf and upper slope. On the middle shelf, a shelf-perched unit, 30 to 40 ms thick, Unit 1, downlaps and buries Unit 2. Faint discontinuous reflectors within Unit 1 suggest low-angle progradation to the south and southeast. At its basinward edge, Unit 1 has an irregular taper before it pinches out at a depth of 105 ms (79 m). The landward edge of Unit 1 extends beyond the limits of the study area. On the upper slope, wedges of strata onlap the foresets of the prograding wedges. Regional Strike-Line Correlations at the Outer Shelf. Figure 3 shows a regional west east strike line and at the western end (left-hand side of Fig. 3) shows the Unit 2 strata seen on dip-oriented profile of Figure 2. In strike view, Unit 2 has a component of progradation to the east. This wedge is correlative to the eastern delta of Sager et al. (1999). Seismic correlation of the Unit 2 clinoforms and aggrading bottomsets along strike to the east shows that the unit is stratigraphically higher than the cluster of lens-shaped units, Units 3, 4, and 5. The maximum thickness of Units 3, 4, and 5 is ms. At this location, the internal reflectors of these units are poorly imaged, but the faint reflections and the overall geometry of the units suggest that the units represent individual progradational centers with bidirectional downlap to the west and east. Regional correlations suggest that the small depocenter labeled Unit 2 on the far eastern end (right-hand side) of Figure 3 is stratigraphically younger than Units 3, 4, and 5 and probably is correlative to the larger Unit 2 depocenter at the western end (left-hand side) of Figure 3. The offset stacking of these units (Units 2, 3, 4, and 5) define the shelf edge offshore Pensacola, Florida. Dip-Oriented Profiles Offshore Pensacola, Florida. Figure 4 shows a dip profile that crosses the shelf margin at the location of a broad embayment in the shelf margin (see Fig. 1). At this location, the Unit 4 prograding-wedge strata are near the seafloor. Isolated patchy amplitudes are seen near the Unit 4 topset surface, but topset development is minimal. Unit 4 has a maximum thickness of 80 ms and exhibits well-defined progradation. The Unit 6 and Unit 7 prograding-wedge bounding surfaces are apparent, but the internal stratigraphy is poorly imaged. A very thin section of Unit 2 and Unit 3, and a relatively thick section of Unit 1, cap the Unit 4 prograding-wedge strata. Unit 1 pinches out at 100 ms ( m) below sea level. The top of Unit 1 also shows a well-developed sand-ridge field occurring in a water depth of 45 ms (34 m). There is a 15-m-high mounded feature above the Unit 4 shelf edge in a water depth of 118 ms (89 m). Similar mounds are found over the topsets of the other shelfmargin prograding wedges but were not observed on Unit 1. On the upper slope, onlapping-wedge strata bury the aggrading bottomsets of the prograding wedges. The Figure 5 profile crosses the margin where prograding wedges of Units 1, 3, and 5 are vertically stacked on the outer continental shelf. Prograding-wedge Unit 5 defines the shelf edge at this crossing. Regional seismic correlation illustrates that prograding-wedge Unit 3 defines the shelf edge to the west of this location. Unit 5 has a maximum thickness of 240 ms, and Unit 3 has a maximum thickness of 180 ms. Stratal

5 LATE QUATERNARY STRATIGRAPHIC EVOLUTION, ALABAMA AND WEST FLORIDA OUTER CONTINENTAL SHELF 47 A unit 7 V.E. = 25:1 0.3 SW unit 7 unit 4 unit 3 shelf edge A shelf edge unit 5 onlapping wedges V.E. = 44:1 NE water- bottom multiple ttom multiple water-bo 0 10 B water-bottom multiple B unit 3 multiple unit 4 unit 5 bottom 0 5 V.E. = 25:1 water FIG. 3. Interpreted line drawing and regional strike-oriented seismic segment of Profile 18, showing the lateral pinchout of prograding wedge 2 at the outer shelf in the western and eastern parts of the study area, and older prograding wedges offshore Pensacola.

6 48 PHILIP J. BART AND JOHN B. ANDERSON SE unit 1 NW possible bioherm SRF unit 4 units 2 and onlapping wedges unconformity A unit V.E. = 32 : water-bottom multiple 3 possible bioherm unit 4 isolated patchy amplitudes unit V.E. = 16:1 onlapping wedges water bottom multiple 0.4 FIG. 4. Interpreted line drawing and seismic segment of Profile 14 showing the depocenter of Unit 4 prograding wedge strata capped by a thin flank of the Units 3 and 5 prograding-wedge strata. The upper surface of Unit 1 is locally reworked into a broad sandy-ridge field (SRF). Carbonate banks (bioherms) occur at the shelf margin above the Unit 4 prograding-wedge strata.

7 LATE QUATERNARY STRATIGRAPHIC EVOLUTION, ALABAMA AND WEST FLORIDA OUTER CONTINENTAL SHELF 49 SE NW unit , onlapping wedges base-of-slope channel unconformity A unit 3 unit V.E. = 32: water-bottom multiple unit unit 5 unconformity A water bottom multiple V.E. = 21:1 0.5 FIG. 5. Interpreted line drawing and seismic segment of Profile 11 collected along the axis of the Units 3 and 5 prograding wedges. Unit 3 forms a terrace at this cross section but forms the shelf edge to the west. On the slope, the foresets and topsets of unit 5 and 3 are truncated. Unit 1 forms a terrace on the middle shelf.

8 50 PHILIP J. BART AND JOHN B. ANDERSON SE NW unit unconformity A V.E. = 32: unit 7 unit 1 two-way travel time (s) V.E. = 63: water bottom multiple FIG. 6. Interpreted line drawing and seismic segment of Profile 47 showing the eastern depocenter of the Unit 2 prograding wedge. A shelf-perched unit, Unit 1, caps the prograding wedge at the outer shelf.

9 LATE QUATERNARY STRATIGRAPHIC EVOLUTION, ALABAMA AND WEST FLORIDA OUTER CONTINENTAL SHELF 51 terminations of the Unit 5 and Unit 3 strata and the underlying strata are seen at the seafloor. The channel at the base of the Unit 5 prograding wedge is strike oriented and dies out laterally. It does not extend up the foreset slope to the shelf edge or downslope to the De Soto Canyon. The profile in Figure 6 shows the stacking of the Unit 2 prograding-wedge strata in the eastern part of the study area. At this location, Unit 2 has a maximum thickness of 100 ms. The Unit 2 offlap break is at an elevation of 100 ms ( m) below sea level. On the outer shelf, the shelf-perched unit, Unit 1, overlies the Unit 2 prograding wedge, and pinches out at a water depth of 80 ms (60 m). SEISMIC-STRATIGRAPHIC EVOLUTION AND FRAMEWORK The regional seismic-stratigraphic analysis illustrates that discrete prograding wedges occur on the Alabama and west Florida outer continental shelf and define its shelf-edge morphology. Subsurface correlations show that the wedges are located at distinctly different stratigraphic levels. Given the positions of these prograding wedges at the shelf margin and the seaward dip of the topset that forms the upper bounding surfaces, we infer that these units were deposited at or shortly after the culminations of the past several sea-level falls. The distinct lobate shapes of the prograding wedges in strike view (Fig. 3) suggest that these units are fluvial deltaic in origin. On the basis of its location in the western part of the study area, and the position of an incised valley in an updip position on the inner shelf (Bartek et al., this volume), we infer that the ancestral Mobile and Tombigbee rivers (Fig. 7) deposited the relatively thick Unit 2 shelf-margin delta. Judging by its eastern location, the thinner Unit 2 depocenter most likely was deposited by the confluence of the smaller fluvial systems now draining into Pensacola Bay (Fig. 7). We propose that the fluvial feeder to the pro 10 Lagniappe delta may have been the Pascagoula River drainage system. We conclude that the two Unit 2 depocenters we describe in our study represent deposition during the last glacial maximum (i.e., OIS 2) on the basis of our inference that the depocenters are above surface 10 (i.e., OIS 3 maximum flooding surface from Main Pass 303 drill site), and because the final phase of Unit 2 shelf-edge elevations are below a water depth of 70 m. The highstand systems tracts presumably are located on the inner shelf. Mobile Bay Pensacola Bay Fig Fig. 3 Fig. 2 Fig. 4 Fig. 5 cluster of older shelf-margin deltas (Units 3, 4, and 5) LEGEND approx. downlap limit of unit 1 Unit-2 shelf-margin deltas & offlap breaks 29.5 inferred location of braided river at OIS bathymetric contours (m) De Soto Canyon seismic grid seismic profiles referred to in text FIG. 7. Time structure map of the seafloor from the seismic grid, showing the locations of the Unit 2 shelf-margin deltas and the Unit 1 backstepped coastal plain.

10 52 PHILIP J. BART AND JOHN B. ANDERSON The upper surface of the Unit 2 deltas may be equivalent to a OIS 2 sequence boundary, but because no well defined cross-shelf valleys incise these shelf-edge deltas, we believe that the lowstand valleys were shallow braided rivers roughly in equilibrium with the elevations of the interfluves on the lowstand coastal plain. Conversely, the general lack of incised fluvial valleys at the Unit 2 topset may indicate either that the upper surfaces of the prograding wedges have been severely eroded during the subsequent sea-level rise or that the topset substratum is uniformly sandy such that incised fluvial valleys are not imaged well with the seismic sources used to acquire the data. Because no canyons were found on the upper slopes of the Unit 2 prograding wedges, we assume that a period of valley incision and sediment bypass to the slope and deeper basin did not interrupt the construction of the prograding wedges. On the basis of its shelf-perched location and regional extent, we assume that Unit 1 represents the backstepped strata of the coastal plain relative to the position of the Unit 2 shelf-margin depocenters. The local reworking of the upper surface of Unit 1 into sand-ridge fields (Fig. 4) suggests that marine currents significantly remolded the continental shelf during the transgression and highstand. Conversely, the sand-ridge fields may be due to the influence of waves and longshore drift with low sediment supply (Harry Roberts, personal communication). Because the onlapping slope wedge stratigraphically above the Unit 2 prograding wedge is of regional extent, we infer that the sediments were supplied as a result of erosion by strike-oriented marine currents acting after the sediment supply to the shelf margin was shut off during the sea-level rise that terminated the last lowstand. SUMMARY Figure 8 is a four-stage model that summarizes our preliminary view of the Alabama and west Florida depositional systems that evolved during the last glacioeustatic cycle. We believe that the previous highstand coastal plain occupied a position similar to today s highstand depositional systems (Fig. 8A). During the subsequent fall, the coastal-plain depositional systems prograded across the continental shelf into shoaling waters (Fig. 8B). At the culmination of the last sea-level fall, the depositional systems (i.e., Unit 2) advanced to the shelf edge but valley incision probably was confined to the updip areas. Because we found no evidence of valley incision into the Unit 2 shelf-margin deltas, we infer that shallow braided rivers (Fig. 8B) fed the shelf-margin deltaic units. No significant channelized bypass of the continental shelf occurred during the last glacioeustatic lowstand. This leads to the conclusion that the Alabama and west Florida deltaic systems adjusted constructively to falling sea levels in the late Quaternary eustatic cycle. On the basis of available age control, we surmise that Unit 2 deltas on the outer shelf correspond to deposition at the culmination of the last glacial maximum (i.e., OIS 2). After the lowstand, sea-level rise caused the coastal-plain depositional system to backstep onto the shelf during the early transgression (Fig. 8C). As the sea-level rise initially flooded the FIG. 8. Four-stage depositional model summarizing the general features of the Alabama and west Florida stratigraphy associated with the last glacial cycle. Stage A corresponds to the last highstand. Stage B is near the culmination of the eustatic lowstand. Stage C represents an intermediate position of the coastal plain during the last sea-level rise. Stage D represents the current highstand. Stage A coast line Stage B primary fluvial coastal plain feeder highstand shelf updip incision of emerging coastal plain coastline Stage C flooded coastal plain Stage D new highstand coastline newly emergent coastal plain backstepped coast line position depocenter shelf edge Slope deltaic outbuilding and braided fluvial system backstepped coast line position shedding to shelf-detached onlapping wedge bay flooded coastal plain 500 m DeSoto Canyon ~100 m shelf edge local modification to shelf-edge trend 380 m ~25 m downlap limit on the upper slope onlapping slope wedge 430 m DeSoto Canyon ~100 m drowned reefs 500 m DeSoto Canyon ~50 m

11 LATE QUATERNARY STRATIGRAPHIC EVOLUTION, ALABAMA AND WEST FLORIDA OUTER CONTINENTAL SHELF 53 shelf margin, waves and marine currents eroded and transported fine-grained sediments from the flooded coastal plain basinward to form thin but regionally extensive onlapping wedges on the upper slopes. The regional extent of the onlapping wedges along the upper slope suggests to us that deposition of the slope units was unrelated to the position of shelf-margin-delta units. Mounded features overlying the shelf-edge prograding wedges are interpreted as bioherms formed during the transgression. Bands of carbonate mounds have been identified along the shelf edge/outer shelf at water depths of approximately 120 and 90 m offshore Mississippi and Alabama using side-scan sonar (Ludwick and Walton, 1957; Laswell et al., 1990; Sager et al., 1992). The lack of similar mounded features on the Lagniappe delta lobes (Harry Roberts, personal communication) may have been due to the proximity of the area to turbid and cold meltwater that issued from the Mississippi River during the last glacial maximum and transgression, which would have inhibited coral development. These mounds obviously postdate deposition of the Unit 2 shelf-margin deltas, and the carbonate growths may have been coeval with the development of the Unit 1 coastal plain. Grab samples from the area show the presence of crustose coralline algae, serpulid worm tubes, bryzoans, foraminifera, and isolated hermatypic corals, indicating that the carbonate mounds have been drowned (Ludwick and Walton, 1957; Gittings et al., 1992). Figure 8D represents the late transgression to early highstand, when the depositional systems attained their present position. Coleman, J.M., eds., Framework, Facies, and Oil-Trapping Characteristics of the Upper Continental Margin, American Association of Petroleum Geologists, Studies in Geology no. 7, p SAGER, W.W., SCHROEDER, W.W., LASWELL, J.S., DAVIS, K.S., REZAK, R., AND GITTINGS, S.R., 1999, Mississippi Alabama outer continental shelf topographic features formed during the late Pleistocene Holocene transgression: Geo-Marine Letters, v. 12, p STUDE, G.R., 1978, Depositional environment of the Gulf of Mexico South Timbalier Block 54 salt dome, and salt dome growth models, Gulf Coast Association of Geological Societies, Transactions, v. 28, p SYDOW, J., AND ROBERTS, H.H., 1994, Stratigraphic framework of a late Pleistocene shelf-edge delta, Northeast Gulf of Mexico: American Association of Petroleum Geologists, Bulletin, v. 78, p REFERENCES ANDERSON, J.B., ABDULAH, K., SARZALEJO, S., SIRINGAN, F., AND THOMAS, M.A., 1996, Late Quaternary sedimentation and high-resolution sequence stratigraphy of the east Texas shelf, in De Batist, M., and Jacobs, P., eds., Geology of Siliciclastic Shelf Seas: Geological Society of London, Special Publication 117, p GITTINGS, S.R., BRIGHT, T.J., SCHROEDER, W.W., SAGER, W.W., LASWELL, J.S., AND REZAK, R., 1992, Biotic assemblages and ecological controls on topographic features in the northeast Gulf of Mexico, Bulletin of Marine Science, v. 50, p HARBISON, R.N., 1967, De Soto canyon reveal salt trends: Seismic-reflection profiles in Gulf of Mexico discover domal structures and a buried eroded slope: The Oil and Gas Journal, February, p INGRAM, R.J., 1991, Salt tectonics: An introduction to central Gulf Coast geology, in Goldthwaite, D., ed., Introduction to Central Gulf Coast Geology: New Orleans Geological Society, p JACKSON, M.P.A., AND GALLOWAY, W.E., 1984, Structural and Depositional Styles of Gulf Coast Tertiary Continental Margins: Application to Hydrocarbon Exploration: American Association of Petroleum Geologists, Continuing Education Course Note Series, no. 25, 226 p. LASWELL, J.S., SAGER, W.W., SCHROEDER, W.W., REZAK, R., DAVIS, K.S., AND GARRISON, E.G., 1990, Mississippi Alabama marine ecosystem study: Atlas of high-resolution geophysical data: US Department of the Interior, OCS Study/MMS , Minerals Management Service, Gulf of Mexico OCS Regional Office, New Orleans, Louisiana, p LUDWICK, J.C., AND WALTON, W.R., 1957, Shelf-edge calcareous prominences in northeastern Gulf of Mexico: American Association of Petroleum Geologists, Bulletin, v. 41, p MARTIN, R.G., JR., 1978, Northern and eastern Gulf of Mexico continental margin: Stratigraphic and structural framework, in Bouma, A.H., Moore, G.T., and Coleman, J.M., eds., Framework, Facies, and Oil- Trapping Characteristics of the Upper Continental Margin: American Association of Petroleum Geologists, Studies in Geology, no. 7, p MITCHUM R.M., JR., 1978, Seismic stratigraphic investigation of West Florida slope, Gulf of Mexico, in Bouma, A.H., Moore, G.T., and

12 54 PHILIP J. BART AND JOHN B. ANDERSON