MEASURING ESTUARINE SEDIMENT FLUX AND ACCOMMODATION-SPACE CREATION TO CALCULATE EVOLUTIONARY THRESHOLDS OF DEPOSITIONAL ENVIRONMENTS: AN EXAMPLE FROM MOBILE BAY, ALABAMA

Estuarine evolution during the Holocene is dominantly controlled by the interplay between sediment flux and accommodation-space creation. Often, profound changes in depositional environments are observed in estuarine stratigraphic records. For example, flooding surfaces recognized in core by a sudden decrease in sedimentation rates and entire reorganization of the estuarine complex has been documented in Galveston Bay, Texas. Mississippi Sound, Mobile Bay, and Weeks Bay, Alabama show similar changes. The stratigraphy of estuaries bounded by variable antecedent topography should inherently be characterized by backstepping environments, regardless of the rate of sea-level rise or sediment supply. Rates of shoreline transgression and associated accommodation space will suddenly increase as flat topography is inundated. At approximately 8.8 ka, the area of Mobile Bay rapidly increased 51% when sea-level inundated flat topography at -14 meters. This produced a low-energy environment conducive for the onset of central bay sedimentation, and resulted in a sharp decrease in deltaic sediment accumulation rates from 6.7 mm/yr to 1.0 mm/yr. Mississippi Sound and Weeks Bay show similar changes, but given shallower depths of flat antecedent topography, these events occurred later. From detailed maps of antecedent topography, sediment accumulation-rate curves, and a robust sea-level curve, the true forcing mechanism(s) behind estuarine changes can be obtained. This is important to constrain so that thresholds governing backstepping of estuarine depositional environments can be quantified.