Relict Progradational Beach Ridge Complex on Cat Island in Mississippi Sound

James B. Rucker, Jesse O. Snowden

Comparative field and aerial photographic studies of the Mississippi-Alabama Gulf Coast barrier islands reveal that Cat Island, the western-most barrier island in the series, is unique in having a large number of prominent, forested beach ridges. The Cat Island ridge complex is composed of three distinct sets of subparallel east-west-trending ridges. The ridge set on the south side of the island is younger and less well-developed than the older two sets.

The beach ridge sands are characterized by an ilmenite-kyanite-staurolite heavy mineral assemblage, which indicates an eastern source, ultimately the southern Appalachian igneous-metamorphic complex. This mineralogy distinguishes the barrier island complex from the nearby Mississippi River deltaic sediments, which are characterized by an amphibole-rich heavy mineral suite. There is considerable evidence that the barrier island system predates the eastward progradation of the St. Bernard lobe of the Mississippi delta complex, which began its eastward progradation about 3,000 years ago and continued until its abandonment approximately 1,500 years ago. The low, poorly developed ridges along the southern shore of Cat Island and the low ridges and broad swales of Middle Spit mark the trans tion to a more protected, energy-reduced environment that accompanied the eastward progradation of the St. Bernard delta into the area immediately south and southeast of Cat Island.

The development of the St. Bernard delta complex not only modified the previously existing longshore current and littoral sediment transport, it also protected Cat Island from the erosional effects of waves and swell of the open Gulf of Mexico, with the exception of a narrow "window" to the southeast between Ship lsland, the barrier island to the east of Cat Island, and the Chandeleur Islands, which are reworked distributary sands of the St. Bernard delta. This restriction in the direction of wave approach has resulted in the partial reworking of the eastern end of the progradational ridge complex into a large northeast-southwest trending spit, thus giving Cat Island its unique "T" shape.

It is likely that during the pre-St. Bernard delta period of robust sedimentation, the other islands in the barrier chain also exhibited progradational ridges similar to those now found only on Cat Island. Unlike Cat Island, which has been protected and preserved by the St. Bernard delta, the other barrier islands have subsequently suffered a general reduction of sediment supply due to the weakening and eastward displacement of the littoral currents, and have been modified and reworked during the past 1,500 years by processes of island and inlet migration and storm-wave washover.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.