Shelf-Margin Delta Morphology as a Paleo Sea Level Indicator: Implications on Late Quaternary Relative Sea-Level Changes in the Northeast Gulf of Mexico

Abstract

A new technique to determine relative sea level history at siliciclastic settings is proposed in this dissertation. The clinoform break (defined as the topset-foreset transition point) of the modern Mississippi River Delta shows a direct relationship with sea level elevation. The analysis of Holocene lobes 8 and 9 belonging to the St. Bernard delta complex of the Mississippi Delta suggests that delta clinoforms can be significantly modified by post-abandonment processes, and hence a detailed stratigraphic evaluation is necessary to determine whether the clinoform geometry is a result of wave erosion or any other post-abandonment processes. The strategy was tested on Lagniappe delta, where a detailed understanding of stratigraphy and some age control was available. The relative sea level curve at the Lagniappe mimics the pattern and magnitudes of eustatic curves for the last glacial cycle. However, there is an offset in timing of the Lagniappe relative sea level record and eustatic curves proposed by δ18O and numerical modeling studies. This offset is probably a reflection of the isostatic adjustments associated with the northeast Gulf of Mexico. To extract the eustatic record from relative sea level history and to obtain a more refined understanding of isostatic adjustments affecting sea level changes, a global grid of relative sea level records must be generated. The clinoform break technique opens up such a possibility to develop relative sea level records from tectonically-stable passive margins where shelf-margin deltas have been identified and described. Evaluation of undated clinoform breaks from shelf-margin deltas offshore Mobile and Pensacola bays suggests that in the absence of age control a clear understanding of relative sea level changes during the last glacial period may not be obtained.