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 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 During the stepped fall of eustatic sea level from oxygen isotope stage 5 to the latest glacial maximum (oxygen isotope stage 2, approximately 18 ka), the Mobile River (possibly joined by the Pascagoula River) entrenched a delivery network into the continental shelf and deposited a complex delta that prograded to the shelf edge just east of the modern lobe of the Mississippi River delta. Over 4500 km of high-resolution 2D seismic profiles and four coreholes (MP303c1, MP288c1, MP242c1, and VK774c1) through these deposits have provided the data base for interpreting the depositional architecture, stratigraphic units, stratal surfaces, stacking patterns, and sediment characteristics of a beautifully preserved late Pleistocene shelf-edge delta (the Lagniappe delta). The complex clinoform wedge that defines this delta built in two simultaneous progradational thrusts, defining western and eastern deltaic depocenters. Each depocenter contains constructional subcomponents composed of clinoform sets interpreted to be the products of both a stepped sea-level fall and autocyclic processes. The coreholes and associated seismic profiles define a coarsening-upward sedimentary sequence that thickens to over 100 m at the shelf edge and thins rapidly to less than 20 m on the upper slope. Clinoforms representing the stage 2 delta deposits downlap onto the toes of fifth-order late stage 3 clinoforms, which in turn downlap an outer-shelf shale underlain by a calcareous condensed section (oxygen isotope stage 5). Tops of the clinoforms are extensively eroded, resulting in a complex NE–SW oriented fluvial scour. Graveliferous sand fills this scour relief at the MP303c1 site. While clinoform development at the shelf edge is interpreted as a late-falling-stage depositional response, infilling of the complex fluvial scour has previously been assigned to the lowstand turnaround and early rise (oxygen isotope stages 1–2 transition). However, dating of the clinoforms in both the eastern and western depocenters by AMS 14 C methods indicates an age of approximately 19 ka, i.e., just prior to the glacial maximum. Age relationships between the Lagniappe clinoform wedge and a similar clinoform package that forms a protrusion of the shelf edge to the east has never been satisfactorily resolved. Correlation of reflectors of known age from VK774c1 along high-resolution seismic profiles acquired in 1997 to the first clinoform set to the east of the Lagniappe delta depocenters shows that these eastern clinoforms are much older than oxygen isotope stage 2 clinoforms of the Lagniappe system. The most probable correlation suggests that these clinoform sets were deposited during oxygen isotope stage 8 or stage 6. Even though eustatic sea-level fall to the latest Pleistocene glacial maximum was impressive and rapid, researchers working on the Lagniappe delta have not previously reported evidence of fluvial incision of the shelf break and no large slope channels or canyons have been associated with the delta as sediment feeder systems to deep-water depositional sites. Furthermore, previous observations argued for a maximum fall of eustatic sea level during oxygen isotope stage 2 that was considerably less than the generally accepted -120 m level. Dates from both the western (MP303c1) and the eastern (MP288c1) parts of the delta suggest formation times of approximately 19 ka and at elevations of about –85 m to –89 m below present sea level. High-resolution seismic data collected by the authors in 1997, however, demonstrate that clinoform sets developed at approximately the –120 m level along the eastern progradational track of the Lagniappe delta. The deeper clinoform wedge is stratigraphically younger than the 19 ka lobes and formed at an elevation of approximately –119 m to –126 m below present mean sea level. With no recent faulting evident, these observations suggest a sea-level lowering of over 30 m in a 1–3 kyr period if the age of the glacial maximum is accepted as 16–18 ka. We believe that this sea-level fall is much too rapid to be accounted for solely by an increase in global ice volume; nevertheless, we are convinced that our age and level data are reliable.

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 A 300 ft (92 m) continuous boring collected in the Main Pass lease area, Block 303, provides excellent data for a better understanding of shelf margin deltaic deposition and facies architecture in relation to glacio-eustatic cycles. The boring penetrated a complete late Pleistocene deltaic progradational wedge characterized by sand-rich sediments that are represented as well-developed clinoforms on high-resolution seismic records. A thin shell-rich transgressive sand overlies the delta and its superimposed gravel to sand-filled channel. Below the delta, a shale and calcareous shale section occurs that represents prodelta and distal shelf deposits. It comprises about half the boring and accounts for most of the time represented by the sedimentary column sampled. Sediments of the boring span six oxygen isotope stages or three sea level cycles. Lithologically, the boring reveals an upward succession of shale to carbonate-rich shale to silty-shale and thin sandy laminae to sands (fine to medium grain size) of the delta to a thin organic shale to a massive/cross laminated channel fill to a carbonate-rich silty shale. Biostratigraphic data indicate that this upward lithologic succession reflects water depths of inner shelf to outer neritic to marginal marine and back to outer neritic.