Controls on Location of Sand-Mud Contact Beneath Barrier Island: Central Isles Dernieres, Louisiana

John R. Dingler, Thomas E. Reiss

The Isles Dernieres is a transgressive barrier island arc situated about 150 km west of the modern Mississippi delta. Since the mid-1800s, the islands of the Isles Dernieres arc have undergone extensive fragmentation and area loss, and the entire beach face has migrated landward at an average rate that exceeds 10 m/year.

Much of the Isles Dernieres consists of highly dissected, salt-marsh muds, lying at or slightly above sea level and covered by a veneer of sand along the gulf shoreline. Cores taken across the gulf side of the island from the marsh to the nearshore show that the sand-mud contact is nearly horizontal under the back of the beach, and sand bars lie on a gently sloping muddy bottom seaward of the beach face. Between those two nearly horizontal surfaces, the contact dips seaward. The difference in elevation between the muddy surface behind the beach and the toe of the beach is approximately 2 m.

Earlier work suggested that the contact begins dipping under the berm crest, presumably as a result of compaction of the muddy deposits by the sand overburden. Because the location of the sand-mud contact is important in calculating the volumetric decrease of an island during storms, we collected core and probe data along several shore-normal transects to map the contact in a central part of the Isles Dernieres. Analysis of that data shows that the point where the contact begins to slope seaward is not fixed relative to the berm crest; rather it varies from the beach face to the backshore. Although compression of the marsh deposits may be a factor, 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, and muddy deposits can even crop out on the beach face.

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