RAPID ENVIRONMENTAL CHANGE EVENTS IN ALABAMA ESTUARIES

Coastal plain and piedmont incised-valley systems are characterized by similar incised valley-fill architecture in eastern Mississippi Sound, Mobile Bay, and Weeks Bay, Alabama. The Bayou La Batre, Fowl, Fish, and Magnolia drainage basins are small,65-410 square kilometers and confined to the coastal plain, while the Mobile drainage basin is 95,000 square kilometers and extends into piedmont areas. An erosional surface was mapped regionally throughout the area using high-resolution seismic data and rotary drill cores. A high-amplitude reflector that truncates underlying seismic facies characterizes the surface and it is sampled as a sharp contact with clay rip-ups, plant roots, and oxidation. The surface is interpreted as the oxygen isotope stage 2 sequence boundary that formed during the last lowstand in sea level. During this time, fluvial systems incised 20-33 m-deep channels through the estuaries. Broad terraces found at –15 m and –10 m separate the different fluvial systems in the estuaries.

Incised valley-fill is composed of thin (< 2 m) fluvial and/or alluvial plain deposits at the base, which are overlain by up to 10 m of central basin mud. The flooding surface between fluvial and/or alluvial plain deposits and central basin sediments is sharp. The absence of bay-head delta deposits in Mississippi Sound, Mobile Bay, and Weeks Bay indicates that estuaries formed suddenly and bay-head deltas shifted landward at catastrophic rates. For example, the Mobile Bay-head Delta began aggrading at its present location around 9,700 cal. yr BP and central basin sediments began accumulating in the incised valley south of the delta shortly after, around 9,200 cal. yr BP. Both bay-head delta and central basin sediments accumulated directly above alluvial plain deposits. Rapid bay-head delta backstepping events are documented in fluvial systems of various sizes; however, these events did not occur coevally. This implies that the forcing mechanism was not an increase in the rate of sea-level rise, but rather an autocyclic response to flooding gently sloping antecedent topography (terraces). Interactions between relative sea-level rise and antecedent topography plays an important roll in incised valley-fill architecture and estuarine evolution.