ABSTRACT

The five barrier islands along the Mississippi-Alabama coast are located 10 to 14 miles (16 to 23 kilometers) offshore and separate Mississippi Sound from the Gulf of Mexico. The barrier islands in the chain are, from east to west: Dauphin Island, Petit Bois Island, Horn Island, Ship Island, and Cat Island. The islands are low sand bodies situated on a relatively broad Holocene sand platform that extends 70 miles (113 kilometers) from Dauphin Island on the east to Cat Island on the west. The platform varies in thickness from 25 to 75 feet (7.6 to 23 meters) and rests on Holocene marine clays or on Pleistocene sediments. The barrier islands are nourished chiefly by littoral drift from sand sources to the east. The barrier island chain predates the St. Bernard lobe of the Mississippi delta complex, which began to prograde about 3,000 years ago, and continued until it was abandoned approximately 1,500 years ago.

In contrast to the other islands, Cat Island at the western down-drift end of the Mississippi-Alabama barrier island chain, is characterized by more than 12 prominent east- west oriented progradational linear ridges. The ridge system of Cat Island is interpreted as a relict of an earlier stage in the life cycle of the barrier platform when there was a more robust littoral drift system and an abundant sediment supply. During the Pre-St. Bernard Delta period of vigorous sedimentation, the islands in the barrier chain probably exhibited progradational ridges similar to those now found only on Cat Island. Presently, only vestigial traces of these progradational features remain on the islands to the east of Cat Island. Unlike Cat Island, which has been protected and preserved by the St. Bernard Delta, the other barrier islands have been modified and reworked during the past 1,500 years by processes of island and tidal inlet migration, accompanied by a general weakening of the littoral drift and a reduction of the available sediment supply.

The four tidal passes, or inlets, that separate the barrier islands are broad low areas in the Holocene barrier island sand platform, ranging from 3.5 to 5.8 miles (5.6 to 9.3 kilometers) in width. The water depth in the passes is generally shallow, less than 15 feet (4.6 meters), except in the tidal channels where natural depths of more than 30 feet (9 meters) are often reached. Due to the water depths reached by the tidal channels, the nearly the full thickness of the Holocene island platform east of Cat Island has likely been reworked during the migration of the islands and tidal passes. Additionally, the down-drift islands have become increasingly sediment-starved due to sediment losses in the island passes. Shoals and lobes of sand, commonly found on the north (Mississippi Sound) side of the barrier islands, are interpreted to be the remains of interchannel shoals that mark former tidal inlet positions.