The Lagniappe Delta complex formed on the Mississippi/Alabama continental shelf during the Wisconsinan glacial stage, within oxygen isotope stages 2-4. Data collected from four coreholes (Main Pass 242c1, 288c1, 303c1; and Viosca Knoll 774c1), which penetrate the Lagniappe Delta complex and older section provide the basis for our research. In our study of fossil foraminiferal assemblages from these coreholes we interpret paleobathymetric zones (inner neritic to upper bathyal) and paleo-water depths as well as subenvironments within the delta complex. The similarity of Pleistocene-Holocene fossil assemblages to those of the Recent allows us to use the Mississippi River Delta as a modern analogue. Five of the six modern Mississippi River Delta subenvironments are recognizable in the subsurface: fluvial, interdistributary bay, fluvial-marine, deltaic-marine, and sound. A marsh subenvironment is not present in our data set. An occurrence of warm, carbonate-bank microfauna in a cold, glacial interval in Main Pass 288 is a paradox explained by the close proximity of this site to the paleo-shelf edge during isotope stage 2 and the influence of a proto-loop current. Based on a reconstruction of the paleoenvironments, paleogeographic maps were drawn at three time slices: 83 Ka (stage 5a/5b boundary); 24 Ka (stage 2/3 boundary); and 19 Ka (late stage 2). These maps show the basinward migration of the shoreline during sea-level fall and the progradation of accreting lobes of the Lagniappe delta during the late Wisconsinan glacial stage.

Abstract As the sea level low of the latest Pleistocene glacial maximum was approached, fluvial systems worldwide migrated across continental shelves and built their deltas at or near the shelf-edge. Two excellent data sets consisting of high resolution seismic profiles and cores have been acquired in recent years from two of these shelf-edge deltas in very different climatic and physical process settings, the temperate Lagniappe (Mobile River) delta of the northeastern Gulf of Mexico and tropical Mahakam River delta of the eastern coast of Borneo (Indonesia). Both of these late Pleistocene deltas were built during the latest Pleistocene falling-to-low relative sea-level conditions. The Lagniappe Delta is defined by a complex clinoform wedge built in two simultaneous progradational thrusts resulting in western and eastern deltaic depocenters at the shelf-edge (Fig. 1). Both depocenters contain individual clinoforms sets interpreted to be both the products of autocyclic switching of dominant distributaries and a stepped fall in sea-level. Clinoforms lap down on outer shelf shale overlying an oxygen isotope stage 5 condensed section. The tops of clinoforms sets are extensively eroded resulting in a complex northeast/southwest-oriented fluvial scour. The eastern lobes of the Lagniappe Delta show evidence of wave reworking while the western depocenter is fluvially dominated. Figure 1. A schematic representation of Holocene and late Pleistocene stratigraphy of the Lagniappe delta built by the Mobile River. For illustration purposes, the incised fluvial network is shown to transverse view. This delta was deposited during the latest Pleistocene falling-to-low sea level. It is located at the shelf edge directly east of the active Balize lobe of the Mississippi River delta. The Lagniappe clinoforms built wedges on a relatively stable shelf under conditions of a low tide range, low wave energy, and seasonal storms. In this setting, siliclastic sedimentation dominated and carbonates had little impact on the sedimentary record. Dating (14 C methods) of both the eastern and western Lagniappe depocenters indicated an age of approximately 19 Ka BP. However, a deeper clinoforms wedge that was stratigraphically younger than the western and eastern deltaic depocenters occurred at a depth of approximately 119-126 m water depth. This clinoforms wedge has not been dated, but it is part of the falling-to-low-stand progradation. The latest Pleistocene Mahakam River Delta was deposited on a tectonically active shelf and in an equatorial setting characterized by moderate tide, extremely low wave activity, low storm activity, a strong north-to-south flowing oceanic current, and upwelling. Tropical ocean conditions and upwelling of nutrient-rich water provided environmental conditions suitable for developing a mixed siliciclastic-carbonate depositional system. Below the Pleistocene-to-Holocene ravinement surface, two deltaic depocenters developed during the latest Pleistocene falling-to-low sea-level, a northern depocenter characterized by numerous distributaries that created off-set and overlapping individual clinoforms sets through autocyclic processes and a larger southern depocenter characterized by an actuate delta front and a central northwest-southeast oriented distributary network resulting in a broad fluvial scour (Fig. 2). These Pleistocene deltaic deposits prograded to the shelf edge along most of shelf-slope break. Clinoforms of these falling-to-lowstand deposits lap down onto highly irregular shelf topography created by the growth of the calcareous green alga Halimeda. These isolated bioherms and aggregates of bioherms are seated on a prominent ravinement surface. A maximum flooding surface runs through these features. The success of Halimeda in building impressive shelf bioherms is attributed to the upwelling of nutrient-rich tropical water onto the Mahakam shelf thus stimulating plant growth as sea level rose across the shelf. Bioherm growth is terminated by siliciclastic influx as the delta progrades during a prolonged highstand or falling-to-lowstand conditions. During the lowstand turn-around, over half of the lowstand systems tract stratigraphy is related to delta plain aggradation. Modern equivalents to these delta plain sediments are found to have a lipid-rich organic content of sufficient concentration to be considered a source rock precursor. Figure 2. A schematic representation of Holocene and late Pleistocene stratigraphy of the Mahakam delta shelf. Timing of important surfaces is extrapolated from the MISEDOR boring (Carbonel et al. , 1987). In order to illustrate both interfluve and feeder systems the alluvial valleys are shown schematically as axial-transverse views.

Abstract The Pleistocene Fuji–Einstein system in the northeastern Gulf of Mexico consists of a shelf-edge delta that is directly linked to and coeval with two submarine channel–levee systems, Fuji and Einstein. There is a continuous transition between the channel fills and the delta clinoforms, and the seismic reflections of the prodelta are continuous with the levee deposits. Five smaller delta lobes within the Fuji–Einstein delta formed through autocyclic lobe switching that was superimposed on a single falling-to-rising sea-level cycle. The corresponding stratigraphic complexity is difficult to interpret in single downdip seismic sections, especially where elongated mudbelts are attached to some of the delta lobes. The two slope channel systems, Fuji and Einstein, deeply incise the shelf-edge delta. However, late-stage delta progradation was coeval with slope-channel development, and, as a result, there is no easily mappable, single erosional surface separating channel deposits from deltaic sediments. During early delta-lobe development, a gully field forms on the upper slope, directly downdip from the delta lobe. As the delta progrades, one of the larger gullies in the middle of the field captures most of the denser flows and gradually evolves into a sinuous channel. The larger delta-related slope channels source 2–4 km-wide submarine aprons where they encounter areas with lower gradients. If the slope gully or channel remains active for a long enough time, its corresponding submarine apron smooths out the slope and becomes incised by the later bypassing flows. The well-preserved and mappable 3D shelf-edge architecture provides a rare opportunity to understand relationships between deltaic and slope depositional systems.

Abstract Shelf deposition following lowstand delta building at the shelf edge has been documented for the northeast Gulf of Mexico between the Mississippi River and Apalachicola River deltas. Fifty-two vibracores, foraminiferal data, and bathymetry data have been used to detail the quartz-rich terrigenous clastic sediments that dominate the entire coastal to shelf depositional system. Because of low subsidence rates that characterize most of the study area, reworking and hydrodynamic winnowing occurred during repeated cycles of sea-level rise and fall in response to Pleistocene glaciation and deglaciation, producing sandy coastal-plain and continental-shelf deposits. Moreover, during the postglacial rise and present highstand in sea level, the eastern two-thirds of the shelf has been sediment starved, enabling additional reworking of the shelf sands during the passage of strong cold fronts and hurricanes, thus concentrating there a nearly uniform thickness of clean, multicyclic quartz sand known as the Mississippi–Alabama–Florida (MAFLA) shelf sand sheet. In the northeast Gulf of Mexico, variations in shelf width and morphology (e.g., shoals and shelf-edge deltas) are a function of glacio-eustatic changes in sea level, relative river discharge, size of drainage-basin area, and river location and frequency. Four surficial sediment types characterize the shelf: the MAFLA sand sheet, the St. Bernard prodelta deposit, the Chandeleur sand deposit, and outer-shelf carbonates. The MAFLA sand sheet dominates, covering about 75% of the shelf surface, and consists of a fine- to medium-grained quartz sand. Although relatively thin (3.5 to 5.5 m thick), the areal extent of the sand sheet is continuous and extensive (at least 400 km along strike; 60 km along dip), producing a sand volume of ∼ 7.2 × 10 10 m 3 . Furthermore, five major shoal complexes are located in the study area: Cape St. George shoals, Cape San Blas shoals, South Perdido shoal trend, North Perdido shoal trend, and St. Bernard shoals. The St. Bernard shoals consist of the Chandeleur sand deposit, whereas the other four formed within and consist of the MAFLA sand sheet. A composite stratigraphic column was compiled for the late Quaternary geology of the northeastern Gulf of Mexico shelf, which is up to 35 m thick and has seven depositional environments (Units 1–7) and five erosional surfaces. This composite stratigraphic column synthesizes the modern transgressive and highstand systems tracts preserved on the shelf. Unit 1 is a Pleistocene strandline deposit capped by a well-developed soil horizon, and represents the top of the last highstand and/or falling-stage systems tract. This unit is truncated by an erosional unconformity (Type 1 sequence boundary) produced when the entire shelf was subaerially exposed during the last sea-level lowstand, at about 18 ka. This unconformity was further eroded and reworked during the ensuing transgression to form a flooding surface (bay ravinement), thus creating a combined erosional surface (SB–FS). Overlying the sequence boundary is Unit 2, a fine-grained estuarine unit with occasional rip-up clasts and shell layers. The estuarine unit was planed off to form a regional transgressive surface of erosion (shoreface ravinement). Unit 3, a shelf sand sheet (MAFLA), sits on top of the transgressive surface of erosion and is dominated by fine-to-medium quartz sand up to 5.5 m thick with a distinctive shell bed and quartz pebbles at its base. The maximum flooding surface (MFS) overlies the MAFLA sand sheet and represents the boundary between the transgressive systems tract below (Units 2 and 3) and the highstand systems tract above (Units 4–7). Unit 4 is a prodelta deposit dominated by laminated silty clay that ranges in thickness from 12 to 16 m. The prodelta deposit grades upward into delta-front sediments (Unit 5) that are characterized by interlaminated silty sand and silty clay averaging 8 to 10 m thick. The delta-front unit is cut by channel-base diastems caused by the erosional scour of distributaries. Distributary sands (Unit 6) are 4 to 7 m thick and tend to be oriented shore-normal. Together, Units 4, 5, and 6 are coarsening-upward, shallow-water deltas, and represent parasequences within the modern highstand systems tract. In the western part of the study area, the shallow-water deltas overlie the MAFLA sand sheet, the result of the eastward progradation of the St. Bernard delta complex of the Mississippi River. The deltaic avulsion process (autocyclic) caused the St. Bernard delta complex to become abandoned, thus creating a local transgressive surface of erosion (parasequence boundary). The composite section is capped by a shelf sand shoal (Unit 7), which is a retrogradational parasequence up to 3.5 m thick within the modern highstand systems tract. The MAFLA sand sheet serves as an actualistic modern-day analog for shallow marine sandstones deposited under regional transgression in the ancient sedimentary record. These sandstones—commonly known as “transgressive lags” or “sheet sands”—are poorly documented with respect to sedimentary characteristics of recognition, stratigraphic framework, and reservoir architecture. This study provides additional insight to the geologic characterization of shelf sand sheets. Transgressive shelf sandstones can be significant hydrocarbon reservoirs in certain sedimentary basins of North America and elsewhere. This study offers examples that suggest that the preservation and resource potential of transgressive shelf sandstones commonly are misinterpreted in reconstructions of ancient sedimentary successions.

Abstract During lower-sea-level parts of glacioeustatic cycles, limited accommodation on the northeastern Gulf of Mexico shelf creates a stratigraphic framework within which the thickest deltaic and open marine section is confined to a narrow zone at the shelf edge. Key to understanding ancestral Mobile River delta complexes including the Lagniappe and older deltas is establishment of a 14 C and oxygen isotope stratigraphic framework. The δ 18 O record in a corehole penetrating deltaic and prodeltaic section on the upper slope, in Viosca Knoll block 774, contains normal glacial and interglacial values related to isotope stages 1–13 as well as evidence of meltwater spikes at the stages 1–2 and stages 13–14 glacial terminations. A second 14 C calibrated deltaic–prodeltaic δ 18 O record in an outer-shelf corehole located in Main Pass Block 303 contains normal glacial and interglacial values and a stages 1–2 meltwater spike in the upper part, but in the lower part contains evidence of diagenetic precipitation of isotopically light CaCO 3 related to pore-water and methane seepage. This added carbonate creates an anomalously light δ 18 O and δ 13 C overprint in the data but does not appear to negate the utility of the δ 18 O signal in resolving isotope stages 5–8. With the detailed chronology made possible by fine-tuning the δ 18 O stratigraphy it is possible to track the changes in relative sea level that are important to the development of deltaic morphology and stacking patterns and to the proliferation of benthic foraminiferal assemblages. We have been able to demonstrate that during deposition of the Lagniappe and related deltaic systems, relative sea level is controlled by glacio-eustatic sea-level variations, compaction-induced seafloor subsidence, and seafloor movements related to isostatic loading. Investigation of the complex relationships between these factors and the stratigraphic record over several glacial terminations is supported by graphical stratal-history analysis of corehole data. Temporal backtracking of deltaic–prodeltaic strata in the interval 0–500 ka reveals a northeastern Gulf of Mexico continental margin that is progradational at the scale of glacioeustatic sea-level change and individual lowstand deltas, such as the Lagniappe Delta, and retrogradational at the scale of shelf-margin evolution, sequence stratigraphy, and exploration geophysics.

ABSTRACT The Lagniappe Delta was formed during the Wisconsinan glacial within oxygen isotope stages 2–4. In a study of fossil foraminiferal assemblages from four coreholes penetrating the Lagniappe Delta and related older section, we interpret paleobathymetric zones (inner neritic to upper bathyal) and paleo–water depths within the delta complex. The similarity of fossil assemblages to those of the Recent allows us to use the Mississippi River Delta as a modern analogue. We recognize five of the six modern Mississippi River Delta subenvironments, including fluvial, interdistributary bay, fluvial–marine, deltaic–marine, and sound. A marsh subenvironment was not present. Calcareous-bank faunas (CBFs) are also abundant in several of the coreholes. CBF foraminiferal taxa, which require warm, clear, shallow water, occur not only in interstadials but also in a prograding clinoform interval in the Main Pass 288 corehole, correlated to the last glacial (isotope stage 2). This apparent paradox is explained by lowered sea level and bathing of the narrow shelf edge by a warm marine current, possibly a proto–Loop Current during parts of the late Pleistocene. The fact that living CBFs do not now occur in modern sediments of the northern Gulf of Mexico supports our thesis that minimum temperatures on the present continental shelf are lower than those of the late Pleistocene–early Holocene. Deep-water occurrences of living CBFs from the northern Gulf of Mexico, cited in published reports, are shown to be relict faunas associated with Pleistocene hardgrounds, calcareous banks, or carbonate pinnacles. The physical parameters controlling the occurrence of living CBFs in the Gulf of Mexico are consistent with those controlling their worldwide distribution. Foraminifera are used, herein, to recognize unconformities through abrupt vertical changes in the benthic paleoenvironment, sharp variations in temperature-sensitive planktonic species, changes in preservation of benthic foraminiferal tests (non-abraded vs. abraded), and abrupt termination of age-constrained planktonic foraminifera.

Abstract Stratigraphic analysis of sedimentary basins is critical for correlation in a basin, for reconstructing the geohistory of a basin, and for developing a successful petroleum exploration strategy for a basin. In studying only the shelfal areas of basins that are characterized by carbonate or mixed carbonate and siliciclastic deposition and in which stratal patterns are driven by low-frequency, tectonic-eustatic events, a stratigraphic analysis based on the cyclicity recorded in the strata (transgressive-regressive cycles) has utility for correlation, for geohistory interpretation, and in formulating petroleum exploration strategies. This is the case for the Cretaceous section in basins of the northeastern Gulf of Mexico. In utilizing the concept of transgressive-regressive (T-R) cycles, eight T-R cycles are recognized in Cretaceous (upper Valanginian to lower upper Maastrichtian) strata of the northern Gulf of Mexico. The cycles consist of a transgressive (aggrading and backstepping) phase and a regressive (infilling) phase. These T-R cycles are useful for intrabasin correlation of Cretaceous strata in the Mississippi interior salt basin and for interbasin correlation of Cretaceous strata in the Mississippi interior salt basin and the East Texas salt basin. Six regional Cretaceous unconformities and associated hiatuses (late Valanginian, middle Cenomanian, late Turonian to middle Coniacian, middle Campanian, late Campanian to early Maastrichtian, and late Maastrichtian) and nine regional transgressive events (early Aptian, early Albian, middle Albian, early Cenomanian, late Cenomanian to early Turonian, late Santonian, early middle Campanian, early late Campanian, and early Maastrichtian) have been identified as major events in the geohistory of these basins. Hydrocarbon production from Cretaceous reservoirs in the northeastern Gulf of Mexico can be categorized as to the phase of T-R cycles. Sandstone reservoirs associated with the early transgressive aggrading phase have accounted for 42% of the 7.437 TCF of natural gas produced from Cretaceous reservoirs in this region. Sandstone reservoirs associated with the late transgressive backstepping and regressive infilling phases have accounted for 63% of the 2 BBO produced from Cretaceous reservoirs in this region. These findings indicate that the primary natural gas exploration target in the northeastern Gulf of Mexico should be Cretaceous sandstone reservoirs of the transgressive aggrading phase of T-R cycles and that the principal oil exploration target in this region should be Cretaceous sandstone reservoirs of the transgressive backstepping and regressive infilling phases of T-R cycles. These exploration targets are associated with structural traps related to salt movement.

Abstract The deep water northeastern Gulf of Mexico reflects one of the last frontier exploration areas in the U.S. Gulf. Analysis of 5860 km of 2-D seismic data over the deepwater DeSoto Canyon and Lloyd Ridge protraction areas allows us to evaluate the stratigraphic and tectonic framework of