Louisiana Cheniers: Settling from High Water

Abstract

ABSTRACT There are two kinds of cheniers in Louisiana: one made largely of shell debris, the other mostly quartz sand. This report deals with the second. In other study areas, where swash-built beach ridges are made of quartz sand, suite-mean values of grain-size kurtosis show near-shore wave-energy level, and the extent to which settling from water was important. Kurtosis close to 3-3.5 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. Louisiana cheniers have a very high kurtosis (10), and a standard deviation of kurtosis (9.5) far higher than any other sandy beach ridges now known (nothing else > 6). These numbers put the cheniers in a class with settling deposits (from waning stages of storms) just seaward of the surf zone. A plot of kurtosis against standard deviation (of the grain diameter) places the Louisiana examples with other known products of settling such as the horizontally-bedded settling-lag (not swash-built) beach ridges on Mesa del Gavilan, near the mouth of the Rio Grande (Texas). One narrow sandy swash-built beach ridge does not represent a single storm, but decades of slow deposition by the swash. Each ridge indicates a small slow rise-and-fall pair in sea-level, perhaps 5-40 cm. Beach ridges commonly occur in sets of 5 to 20 or more. Differences in the heights of ridge sets mark larger changes in sea level. High ridge sets represent a century or more of high sea level; low ridge sets indicate a century or more of low sea level. The vertical difference between set elevations is typically less, to much less, than three meters. The Louisiana cheniers -- except where splayed -- are like beach ridge sets, rather than like the individual swash-built beach ridges that are seen in other study areas.