Sediment accommodation control on estuarine evolution: An example from Weeks Bay, Alabama, USA

The evolution of depositional environments is strongly controlled by the rate at which sediment accommodation is created; however, this factor is seldom quantified in studies of estuarine evolution. From a detailed map of the exposure surface, or bay line, and a precise sea-level curve, we calculated sediment accommodation over the last 8.4 k.y. in Weeks Bay, Alabama, and compared sediment accommodation with the late Quaternary evolution of the bay derived from seismic and lithologic data. The stratigraphy of the Fish and Magnolia paleovalleys, the two fluvial systems that discharge into the bay, is composed of multiple sequences. Due to data limitations, only the upper sequence boundary was mapped regionally. This surface formed in response to the oxygen isotope stage 2 lowstand, and it is overlain by alluvial and estuarine deposits that are separated by an exposure surface. Estuarine sediments were first deposited at 7200 cal yr B.P., and no bayhead delta sediments are recognized above the bay line, indicating that the Fish and Magnolia bayhead deltas back-stepped across the bay. Bayhead delta back-stepping occurred when the rate of sea-level rise was decreasing. A map of the exposure surface shows it to be broad and relatively flat. When this antecedent topography was flooded, the rate at which sediment accommodation was created increased rapidly, forcing the bayhead delta to step back. The morphology of the land being inundated is the most significant control on changes in sediment accommodation and, therefore, the most important factor to quantify when predicting the future response of coastal systems to sea-level rise.