ABSTRACT

The Isles Dernieres is a transgressive barrier-island arc in the Gulf of Mexico about 93 mi (150 km) west of the modern Mississippi delta. Since the mid-1800s, the Isles Dernieres has undergone extensive fragmentation and area loss, and the entire beach face has retreated northward at an average rate greater than 33 ft/yr (10 m/yr).

Much of the Isles Dernieres consists of highly dissected, salt-marsh muds, lying at or slightly above sea level, that are fronted along the Gulf of Mexico by a veneer of beach sand. Cores taken across the island and in the nearshore show that the contact between the beach sand and underlying marsh deposits is nearly horizontal under the back of the beach, and dips seaward near the berm crest. The difference in elevation between the top of the marsh deposits landward of the beach and the sand-mud contact under the toe of the beach is approximately 7 ft (2 m). An earlier researcher concluded that the seaward dip of the sand-mud contact was created by compaction of the muddy deposits by the sand overburden as storm waves carried sand onto the backshore.

Because the location of the sand/mud contact is important in calculating the volume of sand lost from the beach during storms, we vibracored along five shore-normal transects to map the contact in a central part of the Isles Dernieres. The point where the contact begins to slope seaward, the dip point, is not always in the same place relative to the berm crest as would be the case if compaction were the dominant process within the study area. Rather, the dip point lies just under the beach face in the western part of the study area and under the backshore in the eastern part. The dip point was found to follow the northern bank of a lake that appears in earlier aerial photographs. Although compression of the marsh deposits may occur, the location of bayous and lakes may be more important in controlling the elevation of the contact. Thus, where the overwashed sand has filled in a depression in the marsh, the contact will be low; where there is no depression, the contact will be high. In the latter case muddy deposits can even crop out on the beach face.