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UPPER QUATERNARY STRATA OF THE UPPER CONTINENTAL SLOPE, NORTHEAST GULF OF MEXICO: SEQUENCE STRATIGRAPHIC MODEL FOR A TERRIGENOUS SHELF EDGE ROBERT D. WINN, JR.1, HARRY H. ROBERTS 2, BARRY KOHL3, RICHARD H. FILLON4, JASON A. CRUX5, ARNOLD H. BOUMA6, AND HOWARD W. SPERO7 1 Geology Department, University of Papua New Guinea, N.C.D., Papua New Guinea Coastal Studies Institute, Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A. 3 Tulane University, Department of Geology, New Orleans, Louisiana 70118, U.S.A. 4 Texaco, P.O. Box 60252, New Orleans, Louisiana 70160, U.S.A. 5 INTEVEP, Departamento de Ciencias de la Tierra, Los Teques, Edo Miranda, P.O. Box 76343, Caracas 1070A, Venezuela 6 Coastal Studies Institute, Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A. 7 Department of Geology, University of California, Davis, California 95616, U.S.A. 2

ABSTRACT: The shallow subsurface of the upper slope, northeastern Gulf of Mexico, shows abrupt changes in depositional environments, sequence stratigraphy, and sedimentation rates reflecting Pleistocene to Holocene glacioeustasy. Stratigraphic change is recorded in lithologic, sedimentologic, biostratigraphic, and oxygen isotopic data from a nearly continuous 245.5 m core from Viosca Knoll Block 774, a gamma-ray log of the borehole, and high-resolution seismic-reflection data. Mud with minor sand at the base of the boring (245.5 to ; 136 m) was deposited during several sea-level cycles corresponding to isotope stages 13 to 8, and includes a thick, probable stage 10 unit. A thick, coarsening-upward deltaic mud and sand at ; 136–59.9 m likely was deposited during oxygen isotope stage 8 glaciation. The section upward from ; 59.9 m contains: a deltaic silt and sand probably deposited during stage 8; a transgressive sand shoal recording the stage 8–7 transition; highstand mud and sand of stage 7; a relatively thin stage 6 lowstand mud and sand; stage 5 highstand mud and sand; stage 4–2 mud and sand; and Holocene sandy silt. Almost all contacts between systems tracts are transitional in core. Inferred lowstand deposits are lithologically and sedimentologically variable, reflecting degree of delta progradation and subsequent erosion, and consist of sand and (or) mud. Thicker, coarser intervals at the site represent shelf-margin deltas likely deposited during falling sea level. Falling sea level and lowstand sediment is characterized by mostly middle to inner neritic benthic foraminifera, cool-temperature planktonic foraminifera, and high oxygen isotopic values of the planktonic foraminifera Globigerinoides ruber. Transgressive and highstand systems tracts are thin. Rising-sea-level sections and highstand condensed intervals consist of clay or silt with carbonate concretions and with numerous pyritized microfaunal tests and small burrows. Condensed sections also contain abundant microfauna, mostly warm-temperature planktonic foraminifera, outer neritic to upper bathyal benthic foraminifera, and transitional to interglacial d18O values of Globigerinoides ruber. Some sandy turbidites appear to be interbedded with highstand mud.

powerful interpretive tools in hydrocarbon exploration. The study was undertaken in large part because very little integrated geological and geophysical data exist from the shelf edge of the northern Gulf of Mexico or from elsewhere. Exceptions include Lehner (1969), Sidner et al. (1978), Woodbury et al. (1978), Coleman and Roberts (1988a, 1988b), Sydow et al. (1992), Sydow and Roberts (1994), Armentrout (1996), and Morton and Suter (1996); however, the data in each of these studies is limited in some significant way. The effort of the Gulf of Mexico Outer Shelf/Slope Consortium centered on the acquisition and intensive analysis of four cores and integration with seismic-reflection data from the shallow subsurface. This paper summarizes analyses and interprets data from the second core obtained from a 245.4 m borehole in Viosca Knoll Block 774 (VK 774). The VK 774 site is approximately 63 km east of the modern Mississippi River delta (Fig. 1), on the upper slope, in 182 m of water. The first boring in Main Block 303 (MP 303) is discussed in Sydow et al. (1992), Sydow and Roberts (1994), and Winn et al. (1995). The results presented herein and in the MP 303 publications are the most geologically intensive analysis of nearly continuous shallow core from the Gulf of Mexico shelf margin to date that integrates lithologic, sedimentologic, biostratigraphic, paleoecologic, isotopic, high-resolution seismic, and log data. Biostratigraphic and isotopic results indicate that the VK 774 borehole encountered sediment likely deposited during the time corresponding to oxygen isotope stage 13 to the present (Fig. 2). We find that the modern upper slope at the site is underlain by a complicated sequence stratigraphy and a complex interbedding of sand, silt, and clay. Sediment was deposited in falling sea level to lowstand delta front and prodelta, transgressive sand shoal, and highstand environments. Sand is largely confined to deltaic intervals and immediately overlying reworked shoal deposits, with the exception of minor sand resedimented during sea-level highstands. Much of the VK 774 core consists of nearly uniform bioturbated mud. Interpretation of those intervals depends largely on biostratigraphic and oxygen isotopic data. METHODS

INTRODUCTION

A research consortium (Gulf of Mexico Outer Shelf/Slope Consortium) consisting of Amoco, Arco, BP, Chevron, Elf Aquitaine, Exxon, Marathon, Mobil, Texaco, and Union Pacific Resources investigated late Quaternary sedimentation on the outer shelf and upper slope in the northeastern Gulf of Mexico (Winn et al. 1991; Winn et al. 1995). Major objectives were to understand when and where different facies were deposited in response to changing sea level, how depositional sequences and important surfaces can be recognized from typical subsurface data, and under what conditions sediment is transported into deep water. A related goal involved the construction of depositional models for the shelf edge using near-surface data to aid in deciphering the deeper section. Such analog models are extremely JOURNAL OF SEDIMENTARY RESEARCH, VOL. 68, NO. 4, JULY, 1998, P. 579–595 Copyright q 1998, SEPM (Society for Sedimentary Geology) 1073-130X/98/068-579/$03.00

The VK 774 borehole (Fig. 1) was drilled August 26–September 5, 1989, from Fugro-McClelland’s M/V R.L. Perkins under contract to the Consortium. Both push coring and hydraulic-piston coring methods were used to retrieve samples (see Winn et al. 1995 for coring procedures). Core recovery was approximately 85% between 0 and 245.4 m depth. Greater retrieval would have been obtained except that only 0.6-m-long samples were taken for every 0.9 m of depth drilled in the mud interval below 150 m subsurface. In addition, some sand was lost from the core barrel during travel to the surface when coring through the sandy interval at 50–75 m. An additional 14.2 m was drilled without sampling after reaching 245.4 m depth to obtain a natural gamma-ray log to 245.4 m. The gamma-ray record was obtained through the drill pipe, and the lower 15.2 m of the pipe was too thick to record a meaningful signal. A final 0.5 m sample was obtained at

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FIG. 1.—Location of borings in Viosca Knoll Block 774 and Main Pass Blocks 303 and 288. The figure shows an isochron map of a late Wisconsinan delta deposited at the shelf margin east of the modern Mississippi River delta (Lagniappe delta of Kindinger 1988, 1989a, 1989b; isochrons from Sydow 1992). Inset maps show location of area relative to the Mississippi delta (upper left) and to the VK 774 Block boundaries (upper right). Seismic-reflection line through the VK 774 and MP 288 borehole sites in upper right inset is shown in Figure 6.

the bottom of the borehole. The only significant drilling problem occurred when the downhole-retrieval cable broke at 105.5 m subsurface. Tools attached to the line were lost then, and the borehole had to be redrilled before coring was resumed. Individual core segments were extruded from the metal coring tubes on the M/V R.L. Perkins, wrapped in foil, and placed in plastic tubes. These samples were sent to Louisiana State University for description, photography, X-ray radiography, sand-fraction determination, and sampling for isotopic and biostratigraphic analyses. Samples of sediment also were collected during coring and placed in cans with a sterilizing solution for later organic geochemical analysis. Samples for foraminiferal analysis were taken an average of every 1.2 m; calcareous nannofossils were analyzed from samples spaced approximately every 2.4 m; organic geochemical samples were taken approximately every 3 m. Results of most of the analytical work are shown in Figures 3 and 4. The gamma-ray record (Figure 3) is a composite log constructed from six separate runs of the recording tool through the boring. The boring was then cemented and the site abandoned. Although the work reported is collaborative, Winn is primarily responsible for final data integration and sequence stratigraphic interpretation; Roberts and Bouma provided lithologic information; Kohl contributed paleoecologic and biostratigraphic interpretation of foraminifera; Crux is responsible for calcareous nannofossil identification and interpretation; and Fillon and Spero provided and interpreted isotopic data.

GEOLOGIC SETTING OF THE NORTHERN GULF SHELF EDGE

Sea-level change, caused by glaciation and deglaciation, has largely determined Plio-Pleistocene sedimentation in the northern Gulf of Mexico by controlling the depositional sites of major fluvial systems (Fisk 1944; Frazier 1974; Sidner et al. 1978; Winker 1982; Suter and Berryhill 1985; Berryhill et al. 1986; Suter et al. 1987; Coleman and Roberts 1988a; Roberts and Coleman 1988; Winn et al. 1995; Morton and Suter 1996). Falling sea level shifted deltaic depocenters toward the shelf edge and resulted in deposition on the outer shelf, upper slope, and, at times, in deeper water. Areas incised during falling and low sea levels, in turn, were filled during lowstands and subsequent transgressions. In contrast, highstand conditions apparently were characterized by fine-grained deposition across most of the shelf, slope, and deep basin. Geotechnical and other shallow cores show that sand, gravel, and mud were deposited during low sea levels while carbonate- and pyrite-rich clay was deposited during high sea levels over most of the northern Gulf shelf (Coleman and Roberts 1988a; Roberts and Coleman 1988; Winn et al. 1995). Much of the sand was deposited in deltas, which are evident on high-resolution seismic lines as basinward-dipping, relatively steep clinoforms (Lehner 1969; Sangree et al. 1978; Sidner et al. 1978; Suter and Berryhill 1985; Berryhill et al. 1986; Morton and Price 1987; Kindinger 1988; Sydow et al. 1992; Sydow and Roberts 1994). However, the shallow

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stratigraphy under the outer shelf and upper slope is not well dated, nor is the exact stratigraphic response to sea-level change predicted precisely from previous studies. Several vertically stacked deltaic intervals imaged seismically on high-resolution records near the shelf edge generally are believed to represent deposition associated with the last several glacial lowstands, but disagreement exists concerning depositional timing in the absence of analyzed core material (cf. Suter and Berryhill 1985; Berryhill et al. 1986; Thomas and Anderson 1991; Morton and Suter 1996). The MP 303 boring, from base to top, recovered deltaic sediment probably deposited during a late Illinoian glaciation corresponding to oxygen isotope stage 6, a transgressive sand, a highstand clay, an upper Wisconsinan deltaic interval, fill of an incised valley, and an upper Wisconsinan to Holocene sand shoal (Winn et al. 1995). Erosional boundaries, cut at maximum lowstands, separate deltaic sediment in the two intervals, respectively, from overlying shoal sand and from an incised-valley fill. The VK 774 borehole shows that an equally complicated stratigraphy recording glacioeustatic changes underlies the upper slope in the northern Gulf of Mexico. VIOSCA KNOLL BLOCK 774 CORE

We divide the VK 774 core into several intervals based on lithology, sedimentary structures, biostratigraphic zonation and datums, and isotope values of planktonic foraminifera (Figs. 3–5). Age and Paleoclimates

FIG. 2.—Correlation of late Quaternary biostratigraphic zones and datums, oxygen-isotope stages, and continental glacial stages (modified from Emiliani 1966, 1971; Boudreaux and Hay 1967; Ericson and Wollin 1968; Kennett and Huddleston 1972; Ro¨gl and Bolli 1973; Gartner and Emiliani 1976; Thierstein et al. 1977; Constans and Parker 1986; Kohl 1986; Richmond and Fullerton 1986). Data are compared to the oxygen-isotope curve of Mathews (1990) and the sea-level curves of Aharon (1983), Chappell and Shackleton (1986), and of Labeyrie et al. (1987) and Shackleton (1987) as modified by Bard et al. (1990). Ages of biostratigraphic datums are from Berggren et al. (1995). See Appendix 1 for explanation for the placement of the 50% uphole reduction in G. caribbeanica datum. We recognize that considerable uncertainty exists in correlation of glacial and interglacial stage names to biostratigraphic, isotopic, and age data (e.g., Fillon 1984, 1985). In addition, Joyce et al. (1990) suggested that the G. flexuosa LAD may occur at approximately 68 ka within oxygen-isotope stage 4, instead of during oxygen-isotope stage 5. WISC. 5 Wisconsinan; ‘‘EOWISC.’’ 5 ‘‘Eowisconsinan’’; SANG. 5 Sangamonian.

Age relationships in the VK 774 core are indicated, in part, by abundances and the presence of microfaunal datums of calcareous nannofossils (Figs. 2, 4; Appendices 1, 2). The last appearance datum (LAD) of Pseudoemiliania lacunosa is at ; 223.5 m subsurface. The datum indicates that sediment at the bottom of the boring is older than ; 460 ka. The 50% uphole reduction in Gephyrocapsa caribbeanica datum, in turn, is between 50.3 and 48.8 m, and the first appearance datum (FAD) of Emiliania huxleyi is at ; 51 m (Boudreaux and Hay 1967; Gartner and Emiliani 1976; Thierstein et al. 1977). The lowest level of dominant E. huxleyi is identified at 35.1 m depth. The G. caribbeanica dominance datum and the FAD of E. huxleyi possibly could be deeper because few nannofossils were found in immediately underlying sediment. Biostratigraphic zonation of the VK 774 core also is based on presence and abundance of planktonic foraminifera of the Globorotalia menardii/ tumida group and of Globorotalia inflata (Ericson Zones and foraminiferal datums in Figures 2 and 4; see Appendix 2; Ericson and Wollin 1968; Kennett and Huddleston 1972; Thunell 1984; Kohl 1986). Globorotalia inflata is a cool-water foraminifer; the G. menardii/tumida group represents tropical to subtropical surface-water temperatures. The Ericson U/V Zone boundary is close to the LAD of P. lacunosa at the stratigraphic horizon where Globorotalia menardii becomes abundant upward relative to G. inflata (Fig. 2). Globorotalia inflata is dominant below 236.8 m in the VK 774 boring, absent in samples between 235–232.3 m, and few to abundant in samples at 230.7–214.2 m subbottom. Globorotalia menardii is very rare to abundant in the interval at 235–230.7 m, suggesting that the boundary between Ericson Zones U and V is between 236.8 and 235 m. The Globorotalia flexuosa FAD was not identified in the core; however, the G. flexuosa LAD appears to be at approximately 35.8 m. The interval from ; 236.8 to 113.7 m contains varying, although generally low, numbers of foraminifera and varying ratios of G. inflata to the G. menardii/tumida group, preventing precise zonation; however, all or part of the interval likely corresponds to Ericson Zone V because of the abundance of G. menardii in the interval (Ericson and Wollin 1968). Foraminifera are even sparser above 113.7 m, but G. inflata increases and G. menardii decreases above ; 113.7 m. This indicates sedimentation during cooler conditions associated with a glacial stade of Ericson Zone V or

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FIG. 3.—Lithology, sequence-stratigraphic interpretation, gamma-ray log, and percentages of sand, silt, clay, diagenetic grains, and shells and shell fragments in the sandsize fraction, VK 774 core. Percentage of foraminifera determined from the sand-fraction analysis is shown in Figure 4. Sequence-stratigraphic interpretation (SEQ. STRAT.) in Figures 3, 4, and 5 is from Figure 7. The gamma-ray record appears to underestimate natural gamma-ray radiation of the sediment in the upper approximately 15 m because of greater hole diameter there. Included with quartz grains are miscellaneous components (chert grains, lithic rock fragments, and uncommon volcanic glass shards). Sulfide grains are mostly pyrite. GLAUC. 5 glauconite; CARB. 5 carbonate grains; BRYOZ. 5 bryozoan fragments; GASTRO. 5 gastropods.


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FIG. 4.—Lithology, sequence-stratigraphic interpretation, nannofossil abundance and zonation, paleobathymetric estimates (from benthic foraminifera), foraminiferal zonation and datums, and percentage of foraminifera in the sand fraction relative to other components (see Fig. 3), VK 774 core. See Appendix 1 for notes concerning nannofossil taxa. The following water-depth divisions are used in this paper: inner neritic 5 0 to ; 20 m; middle neritic 5 ; 20–100 m; outer neritic 5 ; 100–200 m; upper bathyal 5 ; 200–500 m. (See Appendix 2 for relative abundances in samples of Globorotalia inflata and the G. menardii/tumida group.)

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FIG. 5.—Oxygen-isotope stratigraphy and interpretation of the VK 774 core (see Appendix 2 and Figure 7). Data are presented relative to the PBD standard, and were obtained by analyzing 8 to 12 speciments of Globigerinoides ruber per sample. Intervals are assigned to oxygen-isotope stages on the basis of the isotopic record and on biostratigraphic datums identified in the core (Figs. 2, 4).

possibly of Ericson W. An Ericson V zonation is favored because an age corresponding to Ericson Zone W is older than suggested by the 50% uphole reduction in G. caribbeanica datum and the FAD of E. huxleyi. Both datums are at a shallower depth in the boring; some uncertainty exists concerning dating of the deltaic interval at ; 122–59.9 m, however, because of the generally low numbers of microfossils there. Increased numbers of foraminifera are present above 50.3 m at the approximate depth of the 50% uphole reduction in G. caribbeanica and the FAD of E. huxleyi at ; 51 m and below the lowest level of dominant E. huxleyi at 35.8 m and the LAD of the foraminifer Globorotalia flexuosa at ; 35.8 m. The four biostratigraphic datums indicate that the 50.3–35.8 m

interval was deposited during the time represented by part of Ericson Zone V and by Ericson Zones W and X (Fig. 2), but the zone is not easily subdivided from foraminiferal abundances. The upper boundary of Ericson Zone X is marked by the disappearance of G. flexuosa at ; 35.8 m. Globorotalia inflata is dominant in the 35.8–9.4 m interval and together with the LAD of G. flexuosa at 35.8 m indicates correlation of the 35.8– 9.4 m interval to Ericson Zone Y. Globorotalia menardii/tumida dominates the assemblage above 8.4 m, indicating Ericson Zone Z deposition; G. inflata is present in the sample at 9.4 m and absent in the next shallower sample at 8.4 m. These occurrences together bracket the Ericson Zone Y/ Z boundary.

UPPER QUATERNARY STRATA AND SEQUENCE STRATIGRAPHY, NORTHEAST GULF OF MEXICO SLOPE Paleobathymetry Paleoenvironmental bathymetry of the section was estimated by a comparison of benthic foraminiferal assemblages in the core to depth distribution of the same species and assemblages in the northern Gulf of Mexico (Fig. 4, Appendix 2; Phleger 1951; Walton 1964; Poag 1981; Murray 1991). The paleobathymetric inferences are based mostly on a comparison of core assemblages to shallow-depth and deep-depth limits, species diversity, and presence and dominance of benthic foraminiferal species and genera in the modern (see Robinson and Kohl 1978 for a detailed discussion). The paleodepth inferences in large part are dependent on the deepestwater benthic foraminifera in the samples because of the possibility of downslope faunal displacement. The percentage of planktonic forms in the samples also is used in paleodepth estimates. The assemblages recovered from the VK774 core are assigned to paleoenvironment zones (e.g., inner neritic, 0–20 m; middle neritic, 20–100 m; outer neritic, 100–200 m; upper bathyal, 200–500 m), and changes within the depth zones are inferred on the basis of minor changes in assemblage character. The environmental zones are recorded as approximate numeric water depths in Appendix 2, which were used to construct Figure 4. The paleo-water depths for the VK 774 section indicate changes in sea level, relative highstand and lowstand conditions, and general paleoenvironments. The results indicate relatively deep outer-neritic to upper-bathyal conditions during deposition of the intervals at 245.4–209.9, 139.6–121.9, and 49.8–0 m in the boring. Those sections are separated by sediment deposited in shallower paleoenvironments. Oxygen Isotopes Oxygen isotopic measurements were made on the planktonic foraminifer Globigerinoides ruber (Fig. 5, Appendix 2). Pleistocene oxygen-isotope variation of G. ruber mostly is a function of glacial ice volume caused by global changes in the isotopic composition of sea water (Shackleton and Opdyke 1973; Kennett and Shackleton 1975; Shackleton 1977; Williams 1984; Mathews 1990). The d18O record is influenced less by the local salinity and temperature of ambient water, although meltwater, largely from the Mississippi River, did significantly affect the isotopic record of G. ruber in the Gulf of Mexico at times (Kennett and Shackleton 1975; Leventer et al. 1982; Joyce et al. 1993). In general, values of 10.2‰ d18O PDB for G. ruber represent full glacial values for the late Quaternary; 20.45 to 20.9‰ values are transitional between glacial and interglacial ice volumes; and values of 21.5‰ indicate full interglacial conditions, in the absence of glacial meltwater and high-latitude rainwater brought down by rivers, or diagenetic alteration. The glacial–transitional and transitional– interglacial boundaries are shown in Figure 5. In addition, sharp negative shifts of 20.5 to 22.0‰ in d18O values for planktonic foraminifera in the northern Gulf, mostly occurring during early stages of deglaciation, are interpreted as caused by addition of isotopically very light meltwater or isotopically light rainwater derived from high latitudes. The nearby Mississippi River would have had a significant impact on sea-water d18O because its northern headwaters would have drained melting glaciers (d18O of , 220.0‰) and collected high-latitude rain water (d18O of 27.0‰). Alternatively, very negative d18O values of foraminifera could be the result of diagenetic alteration, but such measurements should be associated with negative d13C values (, 22‰ d13C PDB) from the same samples if they were altered after burial. Appendix 2 shows that the negative d18O readings of foraminifera from the VK 774 boring are not associated with negative d13C values. Relatively light d18O values for G. ruber of 20.5 to 22.0‰ for intervals i, ii, v, vii, and xiii in the VK 774 core (Fig. 5) indicate probable interglacial conditions. Sharp negative shifts of 20.5 to 22.0‰ in some d18O values for planktonic foraminifera in these zones were likely caused by addition of 18O-depleted glacial meltwater and/or high-latitude precipitation discharge.

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Heavy planktonic values (d18O between 20.45 and 0.2‰) occur in intervals iii, vi, ix, x, and xii in the core and indicate glacial conditions and very little isotopically light fresh-water discharge. Transitional glacial–interglacial values for G. ruber occur in intervals iv and xi. Lastly, too few foraminifera were recovered in interval viii to analyze. Seismic-Reflection Data The VK 774 boring penetrated several separate seismic-stratigraphic units (Fig. 6). Preliminary sediment-velocity analysis from data collected using sonobuoys during seismic-reflection acquisition suggest the following average interval velocities: 1585 m/s for 208–138 m subbottom; 1706 m/ s for 138–50 m; and 1517 m/s for the interval from 50 m depth to the sea floor (A.R. Tinkle, personal communication 1994). The seismic-stratigraphic units at the VK 774 site from bottom to top are as follows: (1) An interval of weak, moderately continuous, relatively steep seaward-dipping reflections is present below about 0.425 s one-way travel time (OWTT; Interval 1 in Figure 6). Interval 1 reflections are obscured by seafloor multiples below approximately 0.485 s at the VK 774 site. Interval 1 reflections are truncated by an erosion surface at ; 0.425 s. The surface is at ; 160 m subbottom as calculated using the above inferred interval velocities. (2) A zone of low-angle seaward-dipping, continuous reflections (Interval 2) discordantly overlies Interval 1. Reflections in Interval 2 are approximately parallel to the modern sea floor. The upper contact shown for Interval 2 is approximate because the clinoform reflections of Interval 3 converge at a very low angle with Interval 2, and it is difficult to pick the contact precisely. The zone extends from ; 160 to ; 125 m subbottom in the VK 774 boring. (3) An interval with relatively steep clinoform reflections under the upper slope extends from ; 125 to ; 60 m (Interval 3). The clinoforms dip at a maximum of 3–58. (4) A zone of low-angle seaward-dipping, parallel reflections extends from about 60 m upward to the sea floor (Interval 4). Interval 4 includes the seaward toe of a clinoform interval that directly underlies the present shelf edge northwest of the VK 774 site (Fig. 6). An isochron map of this delta is shown in Figure 1. Reflections are parallel to the present sea floor, which dips at almost 18 at the VK 774 site. The apparent dip of the sea floor at the VK 774 site in Figure 6 is 0.858. Inferred depths to seismic reflections and interval thicknesses are approximate. In particular, variable velocities within intervals may cause discrepancy between calculated depths to boundaries from seismic data and horizons in the VK 774 core. Although we attempted to directly intersect the VK 774 boring site during acquisition of the seismic line shown in Figure 6, weather and sea conditions during data acquisition and uncertainties in navigation may have resulted in the line being up to approximately 50 m from the borehole. Figure 6 shows that intervals change thickness and contacts are shifted relative to the sea floor over small distances; however, seismic boundaries were found to match lithological and biostratigraphic breaks in the VK 774 boring rather closely. Sediment, Depositional Environments, and Unconformities Age, depositional, and sequence stratigraphic interpretations of the VK 774 borehole are derived from sedimentologic, lithologic, biostratigraphic, isotopic, and seismic-reflection data (Figs. 3–6). Letter designations correspond to the lettering of the facies and sequence stratigraphic summary diagram (Figure 7). A. Pre-Illinoian, Stage 13 Highstand (?), Outer Neritic–Upper Bathyal Silty Clay and Minor Sand, 245.2 to ; 232.5 m Core Depth.—The base of the continuously cored interval in the VK 774 borehole consists of mostly bioturbated, structureless, silty clay with numerous pyritized microfaunal tests and traces, some macrofaunal burrows,

FIG. 6.—Northwest–southeast 300 joule minisparker seismic-reflection line through the VK 774 and MP 288 boring sites (see Figure 2 for line and shot-point locations). Vertical axis is one-way travel time. Approximate depths to seismic horizons are calculated from velocity measurements. Also indicated are seismic-stratigraphic intervals discussed in text. Acquisition filters were 300–1000 Hz.

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FIG. 7.—Lithology, stage, and sequence-stratigraphic interpretation of the VK 774 core, northern Gulf of Mexico. Unconformities indicated correspond to sequence boundaries formed at maximum lowstands. Also shown are intervals with probable turbidite sand beds. Seismic units are from Figure 6. Dashed lines indicate approximate and transitional contacts. Sequence boundaries are indicated by wavy lines only where there is evidence of significant erosion at the contact. HST 5 highstand; TST 5 transgressive systems tract; LST 5 lowstand systems tract; SB 5 sequence boundary; MFS 5 maximum flooding surface; FSL 5 falling sea level.

and significant areas of diagenetic carbonate precipitation. A sharp-based 2.1-m-thick, fine-grained, completely bioturbated sand is present at 236.5 m depth (Fig. 8A) and a thinner bioturbated sand is at 235 m. The 245.2– 232.5 m interval has abundant microfossils and a few fine-grained macroshell fragments. Paleobathymetric estimates from benthic foraminifera suggest mostly outer-neritic and upper-bathyal water depths, and possible water deepening during deposition. Globorotalia inflata is dominant in the zone, and G. menardii becomes important only at 234.9–231 m depth at the top of the interval and at the base of overlying sediment. Low isotopic values of planktonic foraminifera suggest interglacial conditions

during deposition, possibly with meltwater or high-latitude precipitationderived runoff events, in contrast to the cool climate suggested by the foraminifera. The fine grain size of the sediment, outer-neritic to upper-bathyal paleobathymetric estimates, and interglacial isotopic data together are interpreted as relatively strong evidence of highstand conditions, although cool-water foraminifera are dominant. The thin sand beds are interpreted as turbidite beds. The 245.2232.5 m interval is below the LAD of P. lacunosa, which indicates a Pre-Illinoian age and possible deposition during the highstand associated with oxygen isotope stage 13 (Fig. 2).

FIG. 8.—X-ray radiographs of VK 774 core sediment. Dark areas in Figures 8 and 9 represent sediment and features relatively transparent to X-rays such as clay (relative to sand), concentrations of organic debris, gas-expansion cracks, desiccation features, and thin areas of the core slab; light domains are regions more opaque to X-rays, such as shells, sand grains, and areas of diagenetic pyrite and carbonate. Sand also appears more granular than clay and silt. X-ray radiographs show that remarkably little disturbance of sediment occurred from coring and handling. A) Bioturbated, fine-grained, probable turbidite sand with abundant shell debris. Arrows point to shells. Deposition appears to have occurred during a sea-level highstand. Sample from 236.7 m depth. B) Soft-sediment clasts (at arrows) in medium-grained sand. Sand is inferred to represent sediment-gravity-flow deposition during a lowstand. 208.2 m. C) Bioturbated, structureless mud with pyritized microfaunal burrows and tests (at a) and incipient diagenetic carbonate nodules (at b). Sample from 169.8 m. D) Structureless mud with conspicuous pyritized microfaunal burrows (as at a), other small burrows (at b), and pyrite nodules (at c). Deposition was during a probable highstand. 138.5 m. E ) Bioturbated, probable turbidite sand with small shell fragments (at arrows). Burrows are evident at the top of the core segment. Deposition was likely during a relative highstand. 136.1 m. F ) Nearly structureless silty clay with contorted, swirled bedding (at arrow). Sample from base of a shelf-edge delta. Compare with (C) and (D). Note apparent lack of microfaunal and larger burrows. 118 m.

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UPPER QUATERNARY STRATA AND SEQUENCE STRATIGRAPHY, NORTHEAST GULF OF MEXICO SLOPE B. Pre-Illinoian Stage 12 Glacial (?), Upper Bathyal to Outer Neritic Silty Clay, ; 232.5–221 m.—The 232.5–221 m interval also consists of bioturbated silty clay with abundant foraminifera and nannofossils, very conspicuous pyritized microfaunal tests and burrows, other pyritized, small burrow forms, and diagenetic carbonate concretions, similar to the underlying clay. In addition, a thin sand bed is present at 226.2 m depth. The lower contact of the 232.5–221 m interval is transitional. The clay grades to an upper 1 m silt, and microfossils decrease in abundance as sand content increases upward. The interval is differentiated from immediately underlying sediment mostly on the basis of a different paleoecologic biostratigraphy and isotope geochemistry. Benthic foraminiferal assemblages indicate shallowing from upper bathyal to outer neritic water depths during deposition. Globorotalia menardii increases relative to cool-temperature G. inflata at the top of the underlying sediment and in the 232.5–221 m interval, possibly recording the boundary between Ericson Zones (U/V), but specimens of G. inflata are still numerous. Heavy isotopic values of G. ruber over the 232.5–221 m zone suggest deposition during onset of glaciation, which is consistent with the continued importance of G. inflata, although not G. menardii. The LAD of P. lacunosa is in the interval at ; 223.5 m depth. The nannofossil datum, glacial isotopic values, shallower depositional water depths, and continued presence of cool-water foraminifera are interpreted as indicating sedimentation during glacial oxygen isotope stage 12 (Fig. 2). C. Pre-Illinoian Stage 11, Transgressive (?) Silt, ; 221–218 m.— This thin interval is very similar sedimentologically to immediately underlying silt. Deposition likely also occurred in outer neritic water depths, but a negative isotopic value for planktonic foraminifera in the interval is interpreted as recording deposition during transitional climatic conditions and oxygen isotope stage 11. D. Pre-Illinoian Glacial Stage 10 (?), Prodelta to Lower Delta-Front Mud and Turbidite Sand, ; 218–160.3 m.—The 218–160.3 m interval consists of dominantly silty clay and silt (Fig. 8B, C). The lower contact is transitional. Silt is the dominant lithology at 218–208.1 m, 205.4–184.1 m, and 163.4–160.3 m; silty clay is present at 208.1–205.4 m and 184.1– 163.4 m depth. The clay and silt are bioturbated and structureless and contain areas of concretionary diagenetic carbonate, abundant pyritized microfossil burrows, some larger burrow traces, and moderate to small numbers of microfossils and macrofossil fragments. A few thin, silty sand beds up to 0.6 m thick are also present. The sand beds are bioturbated and mostly structureless, but the sand at 208.7–208.1 m contains sediment clasts up to 6 cm long (Fig. 8B). Benthic foraminifera recovered from the interval indicate dominantly outer-neritic to middle-neritic paleobathymetry and, on average, shallower water than for the underlying section. Shallowing water depths are indicated for the upper part of the interval. Benthic foraminifera in the sand at 208.7–208.1 m are a very shallow marine to brackish assemblage, in contrast to the outer-neritic assemblage recovered in immediately underlying and overlying clay. High isotopic values for planktonic foraminifera from the 218–160.3 m interval indicate transitional to glacial conditions. The 218–160.3 m interval corresponds to an interval of weak to moderately prominent, seaward-dipping reflections (Interval 1 in Figure 6), which are inclined more steeply than immediately overlying reflections. The top of the interval on seismic-reflection data is at ; 160 m, as calculated from the inferred seismic velocities. The base of a clay at 160.3 m is chosen as the upper contact, but the discontinuity is more evident on seismic-reflection records than in the core. The 218–160.3 m interval represents outer-neritic to middle-neritic deposition during a relative lowstand associated with glaciation, probably during oxygen isotope stage 10. Slight increase in grain size and shallowerwater foraminifera relative to underlying mud, oxygen isotopic values suggesting glacial conditions, and relatively steep clinoforms on seismic-reflection data indicate marine deposition in a delta that entered the area during a glacial lowstand. The fine grain size of the sediment and absence

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of upper delta-front sand indicates that deposition occurred in a prodelta to lower delta-front setting (e.g., Coleman 1976). Shallower parts of the delta may have been present, but they were eroded during subsequent lower sea level if deposited at the VK 774 site. Shallow-water foraminifera recovered in some samples in the interval likely were transported downslope after initial deposition (e.g., Coleman et al. 1983). The mostly thin sand beds in the interval, in turn, including the bed at 208.7–208.1 m, are probably turbidites. E. Pre-Illinoian Stage 10 to 9 (?), Transgressive–Highstand Shelfal Clay, ; 160.3–155.8 m.—A bioturbated, structureless clay at 160.3–155.8 m is finer grained, but similar in appearance to underlying mud; however, the interval contains planktonic foraminifera with low oxygen isotopic values suggesting deposition during transitional to interglacial conditions. The sediment apparently lies above an unconformity evident on seismic-reflection data. The interval is correlated to a zone of mostly strong, slightly seaward-dipping reflections (base of interval 2 in Figure 6) that drape underlying delta clinoforms. Benthic foraminifera indicate deposition of the 160.3–155.8 m interval in middle neritic paleodepths. Deposition is inferred to have occurred during submergence of the delta interval during the oxygen isotope stage 10 to 9 transition. F. Lower Illinoian, Early Glacial Stage 8 (?), Regressive Shelfal Mud, ; 155.8–139 m.—The core consists of bioturbated silt and clay at 155.8– 139 m, and is coarser than underlying clay. The interval is similar to underlying sediment, however, in that it is bioturbated and has numerous pyritized microfaunal tests and small burrows traces and some areas of secondary carbonate. The mud also has low numbers of foraminifera and nannofossils and a middle-neritic to inner-neritic benthic foraminifera assemblage. Planktonic foraminiferal d18O values are relatively high, indicating glacial conditions. The interval corresponds to part of the parallel, slightly seaward-dipping reflections of Interval 2 in Figure 6. Isotopic evidence of glaciation, relatively low numbers of microfossils, increased coarseness, and apparent shallowing of water during deposition are consistent with shelf sedimentation during slightly lowered relative sea level compared to the underlying clay. Stratigraphic position suggests deposition during an early stage of a glaciation possibly corresponding to oxygen-isotope stage 8. G. Lower Illinoian, Highstand Outer Neritic–Upper Bathyal Silt and Sand, ; 139–136 m.—The slightly fossiliferous shelfal mud at 155.8–139 m is overlain by a silt and sand at 139–136 m that contain numerous foraminifera, nannofossils, and echinoid and bryozoan fragments. The silt is bioturbated and structureless and contains abundant diagenetic pyrite (Fig. 8D); sand is fine-grained and bioturbated (Fig. 8E). Foraminifera from the interval indicate upper bathyal–outer neritic conditions, deeper than the underlying section. Furthermore, increased numbers of G. menardii in samples at 137.9–136.3 m, which indicate warmer ocean temperatures, and the low d18O values for planktonic foraminifera in the interval suggest transitional to interglacial conditions and a sea-level highstand. The 139–136 m interval is inferred to record deposition during a minor highstand within the time represented by oxygen-isotope stage 8. The sand at the top of the interval is interpreted as a highstand turbidite deposit (Fig. 8E). Bioturbation of the sand prevents determining how many separate depositional events are represented by it. H. Early Illinoian, Stage 8 (?), Lowstand Shelf-Edge Delta, ; 136– 59.9 m.—The 136–59.9 m interval coarsens upward from a silt and clay to silt with some sand beds to a sand. The lower silt and clay extend from ; 136–110.6 m subsurface (Fig. 8F). The lower contact is placed just above a fossiliferous sand bed that has an upper bathyal foraminiferal assemblage (Fig. 8E). The ; 136–110.6 m silt and clay is structureless or has very minor contorted remnant bedding. The base of the lower silt and clay contains moderately abundant foraminifera, nannofossils, and echinoid and bryozoan fragments, although less than the underlying highstand sediment. Microfossil and macrofossil content decrease upward. In addition, pyritized microfaunal burrows and macrofaunal traces are present, although

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FIG. 9.—X-ray radiographs of VK 774 sediment. A) Thin-bedded delta-front sandy silt. Carbonate concretions (at a) and concentrations of plant debris (at b) are indicated. A few, small, faint burrows are evident at the base of the sample. 109.7 m. B) Parallel-laminated, upper-delta-front sand. Dark area on X-ray radiograph is largely the result of the granular nature of the sand, although some plant debris is present. Minor inclination of lamination may be due to postdepositional disturbance. 64.9 m. C) Transgressive shoal sand. Sand is bioturbated and silty, and contains small fossil fragments (as at a). A large burrow form is indicated at b. Note similarity to bioturbated, poorly sorted, shell-rich turbidite sand in Figures 8A and 8E. 50.9 m. D) Bioturbated, shelly silt deposited seaward of Wisconsinan shelf-edge clinoforms. Also note microfaunal burrows. 30.5 m.

both are less prominent than in the underlying unit. In contrast, carbonate concretions are more numerous in the 136–110.6 m zone. Paleobathymetric data suggest outer-neritic to inner-neritic water depths for the zone and shallowing water upward. Isotopic values in the interval are relatively high, suggesting glacial conditions. The sediment at 110.6–75.3 m is dominantly silt interbedded with thin and medium-bedded, fine- and some medium-grained sand (Fig. 9A). The interval is burrowed to (less commonly) bioturbated and has well-developed carbonate concretionary layers mostly parallel to bedding. Small numbers of foraminifera, macrofossil shells, and sparse glauconite are components of the sand. Abundant plant debris is present, commonly concentrated

in thin layers. Silt is mostly laminated to very thinly bedded. Sand is structureless, faintly to well laminated, or contorted. Contorted, disrupted, and inclined stratification dipping more than a few degrees is interpreted as the product of synsedimentary deformation and slumping. Benthic foraminifera represent mostly an inner-neritic assemblage. Fine-grained sand at the top of the 110.6–75.3 m interval grades upward to medium-grained sand in the 75.3–59.9 m zone. The sand is thin to medium bedded, mostly structureless or parallel laminated, similar to sand in the underlying sediment, and unburrowed to slightly burrowed (Fig. 9B). Some beds are contorted, and lamination is inclined in places up to 208. Ripple lamination is uncommon. Plant debris is a minor component of the

UPPER QUATERNARY STRATA AND SEQUENCE STRATIGRAPHY, NORTHEAST GULF OF MEXICO SLOPE sand. In addition, a few small shell fragments and a few foraminifera and nannofossils are present. Interstratified with the sand are burrowed, thin silt zones that contain diagenetic pyrite and secondary carbonate. A shallow marine to brackish benthic foraminifera fauna was recovered from the 75.3–59.9 m interval. Sediment at 137.4–59.9 m corresponds to the uppermost part of Interval 2 and the high-angle clinoforms of Interval 3 in Figure 6. The contact between the two zones is at ; 125 m. The boundary is approximate because the higher-angle clinoforms are tangential at very low angles to underlying reflections. Sedimentary structures (cf. Coleman 1976; Winn et al. 1995) and the relatively steep clinoforms on seismic data indicate deposition of sediment at ; 125–59.9 m on a delta front. The 136–75.3 m interval represents a prodelta to lower delta-front facies, and overlying sand was deposited on an upper delta front. Bioturbated, structureless clay and silt in the prodelta and lower delta-front interval likely were deposited mostly by hemipelagic settling from river flood water distally on a delta. Contorted bedding in the interval is due to postdepositional downslope slumping, and some sand and silt beds in the interval likely are turbidites. In addition, some structureless mud beds (Fig. 8F) that lack burrow traces also may have been deposited by sediment gravity flows. The parallel-laminated sand in the upper delta facies, in turn, was deposited from suspension from river flood water immediately seaward of a distributary mouth (Fig. 9B). Lamination at deposition would have been nearly horizontal because the sea floor at distributaries dips only a few degrees. Higher dips evident now in core in some places likely are due to postdepositional failure and slumping. Contorted and convoluted stratification also is due to sediment loading and slumping. Most structureless sand beds are the consequence of very rapid deposition from flood water, in-place soft-sediment liquefaction, or postdepositional downslope failure and resedimentation. The 136–59.9 m deltaic interval is inferred to have been deposited during an early Illinoian glaciation and oxygen-isotope stage 8 because of the heavy isotopic values at the base of the interval and its position below the FAD of E. huxleyi and the level of G. caribbeanica dominance (Fig. 2). I. Illinoian Transgressive Delta Sand, ; 59.9–52.6 m.—Above 59.9 m is a sand and silty sand to silt that has small numbers of foraminifera and nannofossils, and contains minor amounts of glauconite, oxidized grains, and some plant debris. Sand is mostly medium grained and structureless or parallel laminated, similar to sand just below 59.9 m. Many beds also are contorted. The lower contact of the interval at 59.9 m approximately corresponds on the seismic-reflection profile to the truncation surface between relatively steep delta-front clinoforms below and overlying strong, continuous parallel reflections (base of Interval 4 in Figure 6). The contact is transitional in the core, and separates two similar-appearing sands, although the sand immediately above 59.9 m is more silty. Foraminifera indicate a shallow-water paleoenvironment for the 59.9–52.6 m interval. No isotopic measurements were made on foraminifera from the zone. Sedimentary structures are interpreted as indicating delta-front deposition. Parallel-laminated beds and structureless sand likely had an origin identical to similar beds in the upper part of the delta immediately below. A rippled zone is present at 52.6–52.7 m. Rippling may have formed during impingement of river flood water on the sea floor or by shelf reworking of very shallow-water deposits. The 59.9–52.6 m zone represents either deltaic deposition in a bay after distributary-mouth progradation seaward through the area, or the interval formed during transgression and depositional backstepping. The former interpretation would indicate deposition during oxygen-isotope stage 8 and the latter during the early stages of the isotope stage 8 to 7 transition. A back-stepping origin is preferred because of the presence of an inner-neritic to middle-neritic foraminiferal fauna, instead of brackish water, in the interval. J. Illinoian Stage 8–7 Transition Transgressive Shoal Sand, 52.6–

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49.8 m.—The core shows a change from medium-grained sand with few shells and abundant primary sedimentary structures below 52.6 m to a medium-grained sand with more glauconite, shells (visible in the core, although no increase is noted in sand fraction in Figure 3), and burrowing above (Fig. 9C). The lower contact is placed at 52.6 m in a medium-grained sand where estimates of paleowater depth suggest ocean deepening. Sediment at 52.6–49.8 m is mostly structureless, apparently because of burrowing, with only minor remnant bedding. The increase from no nannofossil recovery to abundant nannofossils occurs between samples taken at 52.1 m and 51.6 m depth. The sand fraction shows a change upward from few to abundant foraminifera at 51.6–50.5 m. Paleobathymetric estimates suggest deeper water depths than the underlying unit during deposition. An isotopic measurement on foraminifera in the interval suggests transitional ice volumes. Increased concentration of shells, glauconite, and microfossils and increased burrowing in sand, deepening paleobathymetry, transitional isotopic values, and stratigraphic position above a delta indicate deposition of the interval in a sand shoal during a transgression. The upper contact is placed in the zone having the 50% reduction in Gephyrocapsa caribbeanica datum, which occurs in samples taken between 50.3 and 48.8 m. Deposition is interpreted to have occurred during the oxygen-isotope stage 8 to 7 transition. K. Middle Illinoian Stage 7 Highstand Mud and Sand, ; 49.8–41.6 m.—The transgressive sand is overlain by a dominantly mud interval at 49.8–41.6 m. The lower contact is transitional over less than 0.5 m. The mud consists of bioturbated, structureless silty clay with abundant foraminifera, nannofossils, pyritized microfaunal tests, and a few small shells. Microfaunal and macrofaunal burrows are present. A fine-grained sand is present at 48.1–46.5 m. The sand contains small numbers of shells, and is bioturbated and structureless, with the exception of some faint parallel bedding and lamination at the top of the unit. Paleobathymetric estimates indicate upper-bathyal water depths during deposition, and isotopic measurements indicate interglacial to transitional ice volumes. Deeper water is indicated than for the underlying sand shoal. Evidence of highstand conditions and nannofossil datums suggest deposition during oxygen-isotope stage 7. The sand at 48.1–46.5 m is interpreted as a turbidite unit. L. Upper Illinoian, Stage 6 (?) Lowstand Mud and Sand, ; 41.6– 37.1 m.—A thin sand was cored at 41.3–40.5 m subsurface; the rest of the interval is silty clay or silt. The 41.6–37.1 m interval is bioturbated with pyritized microfaunal tests and burrows and macrofaunal burrows and lacks primary sedimentary structures. Nannofossils and foraminifera are moderately abundant. Benthic foraminifera indicate deposition in outer-neritic depths, slightly shallower than the underlying highstand sediment, and oxygen-isotope measurements are positive, suggesting a glacial climate. The interval is below the FAD of G. flexuosa at 35.8 m. That datum and evidence of moderately shallow water depths and glacial ice volumes suggest deposition during oxygen-isotope stage 6. The interval is inferred to be the deeper-water depositional equivalent of the lower delta cored on the shelf in MP 303 (Fig. 1; Winn et al. 1995). M. Sangamonian(?)–‘‘Eowisconsinan’’ Highstand Mud and Sand, ; 37.1–31.2 m.—The 37.1–31.2 m interval consists mostly of silt and clay with muddy sand at 36.9–35.4 m and 34.8–34.4 m. The mud is bioturbated and structureless, and contains large numbers of foraminifera and nannofossils and small amounts of detrital plant debris and shell fragments. Macrofaunal burrow traces, pyritized microfaunal burrows, and some diagenetic carbonate zones are also present. Sand is fine grained, muddy, and bioturbated. Benthic foraminifera from the interval suggest a return to upper bathyal water depths, and isotopic measurements are mostly light, suggesting interglacial to transitional conditions during highstand deposition. The zone contains the LAD of G. flexuosa and the Ericson X/Y Zone boundary at 35.8 m and the stratigraphic interval where E. huxleyi becomes dominant at 35.7–35 m, constraining the age of the top of the interval to

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FIG. 10.—Model for Late Quaternary shelfmargin sedimentation in the northern Gulf of Mexico showing relationships between sequence stratigraphy, lithology, gamma-ray log response, microfossil abundances, paleobathymetry from foraminiferal analysis, oxygen-isotope data, and high-resolution seismic-reflection records. TST 5 transgressive systems tract; HST 5 highstand systems tract; FSL 5 falling sea level; LST 5 lowstand systems tract; MFS 5 maximum flooding surface.

oxygen-isotope stage 5 (Fig. 2). Deposition of the base of the interval may have begun in the Sangamonian. N. Wisconsinan Lowstand Mud and Minor Sand, ; 31.2–8.9 m.— The inferred Sangamonian–’’Eowisconsinan’’ highstand mud and sand are overlain by interbedded clay, silt, and sand at 31.2–8.9 m. The lower contact is transitional, and the interval coarsens upward. Clay and silt are mostly structureless with some bioturbated, remnant very thin to thin beds. Several zones are contorted, suggesting slumping (Fig. 9D). Pyritized microfaunal burrows are common, and low to moderate numbers of microfossils and some macrofossils are present in the mud. Sand is the dominant lithology above 11.7 m depth. The sand is fine grained, contains some shell debris, and is mostly bioturbated. Gastropods and bivalves in the sand are up to 3 cm long. Paleobathymetric estimates for the 31.2–8.9 m interval indicate mostly outer neritic water depths, which is slightly shallower than paleodepth estimates for immediately underlying sediment. In addition, isotopic measurements are mostly high, indicating a glacial regime, and dominance of G. inflata suggests a cool ocean. The upper contact of the interval is placed at the LAD of G. inflata, which denotes the Wisconsinan–Holocene boundary (Fig. 2). The 31.2–8.9 m depth corresponds to the slightly seawarddipping reflections of Seismic Interval 4 of Figure 6. Decreased microfaunal content relative to underlying highstand sediment, due to dilution by clastics, and glacial ice volumes and cool watertemperature indicators indicate deposition during lowered sea level relative to immediately underlying sediment. Biostratigraphic datums indicate that the interval was deposited during the general time corresponding to oxygen-isotope stages 4–2. Relatively light oxygen-isotope values for planktonic foraminifera at 17.4–15.2 m may record ice conditions of isotope stage 3 (Fig. 2). Mud and sand in the upper part of the 31.2–8.9 m interval are the deeper-water equivalent to the late Wisconsinan delta shown in the time-interval map of Figure 1 and the shelf-edge delta immediately underlying the sea floor on Figure 6. O. Holocene Transgressive-Highstand Sandy Silt, 8.9–0 m.—The top of the borehole samples a bioturbated, nearly structureless silt and clay, which is sandy in places. The lower contact is placed between a silty sand (below 8.9 m) and a silt (above). The 8.9–0 m interval is finer grained than the underlying unit, and contains slightly increased numbers of fine shell fragments, significantly greater numbers of foraminifera, and more diagenetic carbonate relative to the underlying zone. Minor horizontal lamination and some contorted stratification are present, but most of the interval is bioturbated. Burrow sizes are variable, and include inclined, straight, and curved forms (up to 2 cm wide) and small, pyritized microfaunal burrows.

Paleobathymetric estimates indicate upper-bathyal to outer-neritic water depths during deposition, the latter consistent with present bathymetry. In addition, warm-water G. menardii and G. tumida are abundant in the faunal assemblage, and isotopic data indicate interglacial conditions. The interval corresponds to the upper part of Seismic Interval 4 in Figure 6, which appears to onlap the upper slope. Biostratigraphic datums, interglacial biostratigraphic indicators, and the onlapping pattern indicate deposition in transgressive and highstand systems tracts corresponding to the Holocene and oxygen isotope stage 1. FACIES AND SEQUENCE STRATIGRAPHY

Lithologic, biostratigraphic, and seismic stratigraphic data summarized in Figures 3–6 are used to derive the sequence stratigraphic interpretation of Figure 7 and the sequence stratigraphic model of Figure 10. The borehole is interpreted as preserving a moderately complete depositional record from a highstand correlative with oxygen isotope stage 13 to the Holocene. Highstand condensed deposits in the VK 774 core are evident as thin, bioturbated, dominantly clay to silty clay intervals with conspicuous pyritized microfaunal tests and burrows, some larger burrow traces, and large numbers of foraminifera and nannofossils (Figs. 8D, 10). Contacts with underlying transgressive sediment and overlying falling sea level and lowstand systems tracts are transitional because of bioturbation and because of continuous deposition with sea-level change. Planktonic foraminifera in highstand intervals mostly indicate transitional or warm climatic conditions and ocean temperatures, with foraminifera of the Globorotalia menardii/tumida group typically dominant relative to G. inflata. Sharp negative shifts in oxygen isotopic data in the intervals represent meltwater events or the addition of river water from high-latitude precipitation. Benthic foraminifera generally indicate bathyal and outerneritic paleowater depths. A few bioturbated sand beds are also present (Fig. 8A). Highstand deposits corresponding to isotope stages 13, 7, 5, and 1 are identified in the VK 774 core (Figs. 2, 7). Deposits corresponding to oxygen-isotope stages 11 and 9 are not as evident, possibly because of homogenization with bounding sediment from burrowing organisms, or because samples are too widely spaced to detect very thin condensed intervals corresponding to the two stages. Highstand clay and silt was the result of hemipelagic settling distant from deltas in upper-slope to outer-shelf environments. Minor contorted bedding in highstand mud sections is due to slumping, and minor sand in the intervals likely was deposited from turbidity flows, although bioturbation of the latter has destroyed definitive sedimentologic evidence of trans-

UPPER QUATERNARY STRATA AND SEQUENCE STRATIGRAPHY, NORTHEAST GULF OF MEXICO SLOPE port mechanism. Highstand deposits correlate to zones of relatively continuous, low-angle seaward-dipping reflections on high-resolution seismic data (Figs. 6, 10). Seismic reflections are ‘‘strong’’ in the condensed sections, likely because of significant impedance contrasts between only slightly altered mud and diagenetically altered calcareous nodular beds. Lowstand deposits are variable lithologically, in thickness, and paleoecologically in the VK 774 borehole. Thickness and lithologic variability are related mostly to the extent of delta progradation. For example, the late Wisconsin shelf-edge delta cored in MP 288 (Fig. 6) corresponds to a much thinner sandy mud in VK 774 (interval N of Figure 7). Sediment deposited during low sea levels tends to be slightly to significantly coarser than highstand mud, and has only moderate to small numbers of foraminifera and nannofossils, small amounts of macrofossil debris, and minor amounts of diagenetic pyrite and secondary carbonate. The deposits typically are bioturbated, except in proximal delta-front environments where structureless sand and parallel laminated sand are present (Fig. 9B). The deltaic interval from ; 136 to 59.9 m depth (Interval H, Fig. 7) contains the coarsest sediment in the borehole. Prodelta and lower-delta-front bioturbated mud in that interval grades upward to partially contorted, parallel-laminated and structureless upper-delta-front sand. The structureless and horizontally laminated sand is inferred to have been deposited from suspension from flood waters on a river-dominated delta front. Oxygen isotopic measurements of the planktonic foraminifera Globigerinoides ruber mostly are high (. 20.50‰ d18O) in the intervals, indicating transitional to glacial ice volumes and low sea levels. In addition, the intervals contain rare to abundant cool-water planktonic foraminifera (Globorotalia inflata). Benthic foraminifera in the sections dominantly represent middle to inner neritic assemblages. The thicker deposits of lowered sea level correspond to moderate- to high-angle seaward-dipping clinoforms on seismic-reflection profiles (Fig. 6) and represent shelf-margin deltas. The clinoform reflections are weaker than reflections from the condensed sections and are commonly contorted. The shelf-edge deltas cored in VK 774 (intervals D and H, which correspond, respectively, to Intervals 1 and 3 of Figure 6) and in MP 303 (a late Wisconsin delta corresponding to oxygen-isotope stage 2; Winn et al. 1995) likely were deposited during falling sea level. The paleobathymetric estimates from the lower parts of the deltas show changes in paleowater depths of approximately 100 to 200 m over stratigraphic thickness of 20– 30 m, suggesting deposition during falling sea level. In addition, the delta clinoforms of Intervals 1 and 3 on seismic-reflection data (Fig. 6) are truncated by well-defined erosion surfaces. Truncation indicates that progradation was followed by an episode of erosion, probably from subaerial exposure, although evidence of exposure was not observed in the core. The erosional surfaces are best interpreted as sequence boundaries formed at maximum lowstands corresponding to maximum glaciation. The paleobathymetric data and truncation surfaces indicate that regression of the deltas was ‘‘forced’’ by sea-level fall (e.g., Posamentier et al. 1992). Transgressive deposits are lithologically variable, thin, and bioturbated, and mostly contain large numbers of microfossils and macrofossil debris. Grain size of the transgressive systems tract sediment is mostly related to immediately underlying lithology. Sand accumulated only above sandy upper delta-front facies, probably as lenticular shoals. Transgression over the upper delta-front facies likely was associated with considerable reworking along the sequence boundary by marine waves and currents. Most of the transgressive deposits in the VK 774 borehole consist of silt, have benthic foraminifera that indicate water deepening, and are characterized by low d18O values in Globigerinoides ruber. The low d18O values of foraminifera indicate warm water temperatures and climatic conditions, consistent with rising sea level. In addition, sharp negative ‘‘spikes’’ in d18O values from foraminifera in the transgressive intervals record meltwater events and/or high-latitude rainwater runoff, the former also consistent with glacier melting and sea-level rise.

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CONCLUSIONS

The VK 774 core data, a gamma-ray log from the borehole, and highresolution seismic-reflection data from the area allow an integrated, geologically intensive analysis of shallow sediment from the modern upper slope. The subsurface at VK 774 consists of a complex interbedding of mud and sand that displays rapid vertical changes in lithology, depositional environments and processes, paleoecology, and sequence stratigraphy. The strata reflect glacioeustatic sea-level changes from the middle Pleistocene to the Holocene. Highstand, falling sea level and lowstand, and transgressive sediment intervals are recognized and are correlated to specific d18O stages. A schematic diagram summarizing the relationship between sequence stratigraphic development and the stratigraphic, biostratigraphic, and seismic stratigraphic record is shown in Figure 10. We believe that the model shown in Figure 10 is applicable to much the Plio-Pleistocene shelfmargin section in the northern Gulf of Mexico. We specifically conclude the following from the study: Transgressive and highstand deposits at VK 774 are thin and bioturbated, consist mostly of mud, and have calcareous concretions and large numbers of microfauna (Fig. 10). Foraminiferal assemblages indicate mostly outer neritic–upper bathyal paleobathymetries, and low d18O values indicate warm climatic conditions. Some very negative d18O values represent meltwater and/or river-derived high-latitude rainwater spikes. Transgressive sediment in places overlies sequence boundaries that cut shelf-edge deltas. The transgressive–highstand condensed intervals correspond to slightly seaward-dipping, continuous, relatively strong reflections on high-resolution seismic data. The transgressive and highstand intervals drape and separate sections deposited during eustatically low sea levels. Intervals deposited during eustatically low sea levels are thick (i.e., expanded), consist of sand and mud or just mud, contain few concretions, and have low numbers of microfauna (Fig. 10). The expanded sections also contain relatively high d18O values of Globigerinoides ruber, indicating glacial climatic conditions, and the intervals contain mostly middle-neritic to inner-neritic foraminiferal assemblages. Most of the sediment in the expanded sections is bioturbated, but parallel-laminated sand is present in places. The parallel-stratified sand likely was deposited from river flood water in a delta-front environment. Contorted bedding and possibly some structureless beds in the expanded intervals formed from downslope slumping and resedimentation. Thicker, coarser-grained sections at VK 774 correspond to moderate to high-angle clinoforms on seismic reflection data. The clinoform zones represent shelf-edge deltas that are inferred to have been deposited during falling sea level. This origin generally confirms models of Fisk (1944), Frazier (1974), Suter and Berryhill (1985), Berryhill et al. (1986), and Winn et al. (1995), who recognized the importance of delta deposition during falling sea level. It appears that much of the late Pleistocene shelf margin including the uppermost slope is constructed from shelf-edge deltas deposited during falling sea level and lowstands. In addition, correlation of the shelf-edge deltas with times of falling sea level as inferred from the oxygen-isotope and sea-level curves of Figure 2 suggests that deposition of most of the shallow sediment at VK 774 occurred during very short periods of time. Close sample spacing and a complete suite of lithologic, sedimentologic, biostratigraphic, isotopic, and seismic data were required to decipher depositional and sequence stratigraphic complexities of shallow sediment at VK 774. In particular, mud intervals under the upper slope are difficult to interpret in detail without the range of data. The VK774 results substantiate the difficulty in interpreting sequence-stratigraphic relationships from limited data. ACKNOWLEDGMENTS

Many people were important in initiating the Gulf of Mexico Outer Shelf/Upper Slope Consortium and in obtaining and analyzing the VK 774 core. We particularly

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thank J.M. Coleman (Louisiana State University), J.E. Damuth (University of Texas, Arlington), V. Kolla (Elf Aquitaine), J.A. Lopez (Amoco), C.L. Bowland (Arco), T.W. Neurauter, R.M. Slatt (Colorado School of Mines), A.J. Pulham (BP), S.L. Thompson (Chevron), and R.K. Sylvester (Texaco) for their assistance. Foraminiferal analysis was provided by J.R. Bailey, W.R. Hale, V.E. Loisel, and B.E. Robertson (Chevron). S.E. Nissen (Amoco) and J.C. Sydow (Louisiana State University) assisted in collecting seismic data. Seismic data were collected by Geoquip, Inc., under contract to the Consortium. 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Framework, Facies, and Oil-trapping Characteristics of the Upper Continental Margin: American Association of Petroleum Geologists, Studies in Geology no. 7, p. 117–137. Received 8 October 1996; accepted 31 December 1997. APPENDIX

1

Notes concerning nannofossil taxa referred to in this paper: Emiliania huxleyi (Lohmann 1902) Hay & Mohler 1967. Gephyrocapsa caribbeanica Boudreaux & Hay 1967. A broad definition of this species has been used herein; others might identify some of the more open-centered forms as G. oceanica. The 50% reduction datum of G. caribbeanica was calibrated using the more inclusive definition by an examination of nannofossils from ODP Site 625B, northeast Gulf of Mexico (Joyce et al. 1990). The datum was found to be just below the oxygen-isotope stage 7/8 boundary (Fig. 2). Gephyrocapsa oceanica Kamptner 1943 is a large, open-centered Gephyrocapsa with a central bar aligned within 208 of the short axis of the coccolith. Pseudoemiliania lacunosa (Kamptner 1963) Gartner 1969. APPENDIX

2

Values of d O ‰ and d C ‰ of G. ruber, bathymetric estimates from benthic foraminiferal assemblages, and distribution of the planktonic foraminifera G. menardii, G. flexuosa, G. tumida, and G. inflata. The G. menardii/tumida group includes G. menardii, G. flexuosa, and G. tumida. The data described in Appendix 2 have been archived and are available from the World Data Center-A for Marine Geology and Geophysics, NOAA/NGDC. See concluding paragraph of text for information on accessing the data. 18

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