Effects of Sea-Level Rise on Deltaic Sedimentation in South-Central Louisiana
John T. Wells, 1987. "Effects of Sea-Level Rise on Deltaic Sedimentation in South-Central Louisiana", Sea-Level Fluctuation and Coastal Evolution, Dag Nummedal, Orrin H. Pilkey, James D. Howard
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The birth of two new deltas in Atchafalaya Bay of south-central Louisiana has provided scientists with a unique opportunity to observe and measure processes of delta growth in their incipient stages. These regressive deposits, localized along a transgressive shoreline that is characterized by low-lying marsh and eroding barrier islands, have developed in a setting in which compactional subsidence accounts for approximately 90% of the relative rise in sea level. Unlike previous Holocene deltas of the Mississippi River and its distributaries, however, the deltas in Atchafalaya Bay may soon be growing under conditions of eustatic sea-level rise that is so rapid it will exceed rates of subsidence (1–2 cm/yr).
Extrapolation of delta growth under three sea-level rise scenarios (subsidence only, 1 cm/yr, and 2 cm/yr) indicates that as rates of eustatic sea-level rise approach or exceed rates of subsidence in south Louisiana, the subaerial deltas in Atchafalaya Bay will continue to grow but at slower rates. Even at the extreme rate of sea-level rise of 2 cm/yr, sediments will accumulate subaerially for another 80-100 yrs. Perhaps contrary to expectations, the slower a delta grows, because of rising sea level, the more likely it is to be limited by inefficiency of channels and an inability to deliver sediments to its distal areas than it is to be limited by receiving basin area. Thus, a decrease in subaerial growth rate is reflected by a decrease in delta size.
In addition to producing smaller deltas, high rates of sea-level rise will affect sedimentation processes by leading to thicker sand bodies and deposition of sands farther upstream. High-energy environments of deposition, such as natural levees which grow primarily during spring floods, will keep pace with sea-level rise. Low-energy environments of deposition, such as back bar algal flats, will remain or become subaqueous as waters rise faster than sediments are introduced. Furthermore, high rates of sea-level rise will delay the extension of deltaic sediments to the continental shelf. This, in turn, will slow the growth of downdrift mudflats to the west. Accelerated growth of downdrift sediments will occur when Atchafalaya Bay becomes sediment-filled (2035–2085 a.d.), thus allowing a greater volume of sediments to enter the dynamic shelf region seaward of the bay.
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Sea-Level Fluctuation and Coastal Evolution - This Special Publication is the result of a symposium in honor of W. Armstrong Price held at the first SEPM Midyear Meeting at San Jose, California, on August 12, 1984. The factors controlling relative sea-level change along our shores are varied and, at best, imperfectly understood. Yet, the relative rate of change is what controls shoreline erosion, the arrangement of sedimentary facies of the coastal zone, and the character of deformities within the coastal stratigraphic record. Therefore, these papers address sea-level changes, shoreline responses, and the controls on the three-dimensional geometry of the consequent lithosomes; in short, the architecture of the coastal depositional systems.