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West Louisiana Chenier Plain History
William F. Tanner (1)
The "a-b-c..." model, which is based on littoral transport ofsand, provides a powerful method for coastal analysis. Where long parallel beach ridges are present, however, it may not be applicable: the two-dimensional map-view model requires that dq/dx (a numerical evaluation of changes in the littoral drift load) not be zero, except at three sharply-defined points, whereas long parallel beach ridges were built under conditions where dq/dx was essentially zero at all points.
These parallel beach ridges, then, do not represent an important littoral drift system. Study of numerous sets of such ridges shows that they were built by on-shore movement of sand which came from deeper water. The equilibrium which they achieve, with the passage of time, must be considered primarily in a vertical plane, taken at right angles to the beach, rather than in the map plane. They represent a steepening, with time, of an initially very gentle slope offshore from the beach, leading to the suggestion that the present steeper slope is closer to equilibrium than the original gentle slope.
The chenier (beach ridge) plain of Cameron Parish, Louisiana, is composed in good part of parallel ridges. Because of the parallelism, as well as the large content of shell debris, it is thought that these cheniers were built of material which must be attributed to an offshore source. Because of local departures from parallelism, and the presence of the Mississippi River heavy mineral suite, these cheniers must have had a significant contribution from the littoral drift system. It is concluded that much of the chenier plain, other than silt-clay mud swales, was built by onshore migration of sand which acquired its offshore location when sea level occupied some lower position.
A plot of position vs age of the Cameron Parish cheniers indicates that growth of the plain has been slowing down, especially in the western part of the area, and that it may be close to a maximum width. The shoreface slope also may be close to a maximum angle. Both of these inferences suggest that the depositional history of this part of the coast may be essentially over, being replaced--either now or in the near future--by long-term coastal erosion.
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