ABSTRACT

The preservation of coastal facies in the retreat paths of barrier island arcs is controlled by shoreface translation, age of the transgression and the thickness of the transgressive sediment package in relation to the advancing ravinement surface. Barrier island arcs in the Mississippi River delta plain experience an average relative sea-level rise of .55 cm/yr and shoreface retreat rates range between 5 m/yr and 15 m/yr. Young barrier island arc sediment packages are thin and have undergone limited shoreface retreat. Because of more time available, older barrier island arcs are thicker and have experienced significant movement of the shoreface landward. If the barrier island arc sediment package lies above the advancing ravinement surface, the entire sequence is truncated and a thin reworked sand sheet is preserved in the retreat path. In thick barrier island arcs, portions of the transgressive sequence can lie below the ravinement surface. In this situation, the superstructure of the barrier is truncated, leaving the basal portion of the transgressive sequence preserved.

From more than 400 km of high-resolution seismic profiles, a variety of transgressive stratigraphic sequences from sand sheets to truncated barrier islands to sand-filled tidal inlet scars have been identified in the retreat paths of Mississippi delta barrier island arcs. The Isles Dernieres represent a recently detached barrier island arc in the early stages of transgression. The Chandeleur Islands are older and represent an advanced stage of barrier island arc retreat. This paper describes the stratigraphic character, the transgressive facies preserved, and differences observed in the retreat paths of the Isles Dernieres and Chandeleur Islands.