Seismic stratigraphy and Late Quaternary evolution of Mobile Bay and Mississippi Sound, Alabama -- A record of large- and small-scale fluvial systems through multiple sea-level cycles

Examination of the Mississippi and Alabama shelf, mapping of offshore incised valleys and shelf-edge deltas, and determination of their feeder systems has been the subject of numerous investigations focusing on the Mobile River with considerable variation. To address this controversy approximately 750 km of high-resolution seismic data, 11 rotary drill cores, 16 vibracores, and 1 GeoProbe core were collected from Mobile Bay, the Mobile Bay-head Delta, Mississippi Sound, and along Cedar Point Peninsula to map the headward components of previously published offshore valleys and to compare the incised-valley fill to the idealized model of Zaitlin et al. (1994).;Seismic data show that the Late Quaternary stratigraphy is composed of four unconformity-bound stacked seismic units. This study focuses on the upper two Seismic Units. The older unconformable surface is an exposure surface sampled in cores and interpreted as the Oxygen Isotope Stage 6 Sequence Boundary. Mapping of the Stage 6 Sequence Boundary shows a complex network of sinuous channels that flowed across Mobile Bay and eastern Mississippi Sound separated by a well-developed terraced morphology. The youngest unconformity is an exposure surface sampled in cores and based on 14C data is interpreted as the Oxygen Isotope Stage 2 Sequence Boundary of the last lowstand in sea-level. Mapping of the Stage 2 Sequence Boundary indicates that all systems re-incised their older lowstand valleys in approximately the same locations and are again bound by a well-developed terrace morphology.;Lithologic data show that the valley-fill sequences differ from the idealized model. The Stage 6 to 5e valley fill is composed of alluvial sediment capped by bay-head delta facies whereas Stage 2 to 1 valley fill is solely composed of central basin sediments. The absence of Stage 2 to 1 bay-head delta facies implies backstepping of bay-head deltas from the Alabama shelf to the northern shorelines of Mobile Bay and Mississippi Sound. Based on sea-level curves and 14C data, these backstepping events must have occurred before 9.5 ka in Mobile Bay and 7.0 ka in Mississippi Sound. The two estuaries, while different in their orientations and timing of initial inundation, have similar evolutionary histories.