TANNER, W. F., Florida State University, Tallahassee, FL
ABSTRACT: Louisiana Cheniers: Settling from High Water
There are two types of cheniers in Louisiana: one made largely of shells and shell debris, and the other of mostly quartz sand. Here, I deal with the second type of chenier.
Grain-size kurtosis shows near-shore wave energy level and also the extent to which particles settling from water was important. Kurtosis close to 3 (Gaussian) indicates moderate to high wave-energy density, and little or no settling. Kurtosis about 4 indicates low energy. As K rises above 4, the settling component increases. These settlements apply to suite-mean values of kurtosis.
Louisiana cheniers have a very high mean kurtosis (about 10), and a standard deviation of kurtosis that is far higher than any other known beach ridges (9.5, with nothing else above 6). These numbers place the Louisiana cheniers in a special class, like settling deposits from the waning stages of storms, just seaward of the surf zone.
A plot of kurtosis against standard deviation places the Louisiana cheniers with other known products of settling, such as the horizontally-bedded settling-lag (not swash-built) ridges on Mesa del Gavilan, near the mouth of the Rio Grande River (Texas).
One ridge does not represent a single storm, but was built over decades. Each ridge indicates a very small rise-and-fall couplet in sea level history; changes in ridge set height mark larger changes in sea level. High ridge sets represent a century or more of high sea level; low ridges or swales between sets indicate low sea level. The vertical difference between them typically is less to much less than 3 m.
AAPG Search and Discovery Article #90989©1993 GCAGS and Gulf Coast SEPM 43rd Annual Meeting, Shreveport, Louisiana, October 20-22, 1993.