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 ¹⁴C and oxygen isotope stratigraphic framework. The δ¹⁸O 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 ¹⁴C calibrated deltaic–prodeltaic δ¹⁸O 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 CaCO₃ related to pore-water and methane seepage. This added carbonate creates an anomalously light δ¹⁸O and δ¹³C overprint in the data but does not appear to negate the utility of the δ¹⁸O signal in resolving isotope stages 5–8.

With the detailed chronology made possible by fine-tuning the δ¹⁸O 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.