Seismic Stratigraphy of Barrier-Island Arc Retreat Paths in Mississippi River Delta: ABSTRACT

Abstract

The stratigraphic record preserved in the retreat path of Mississippi delta barrier-island arcs is controlled by erosional shoreface retreat processes, relative sea level rise, and sediment supply. Marine reworking combined with subsidence of the abandoned delta generate a characteristic sequence of facies from flanking barrier islands, barrier-island arcs to inner-shelf shoals depending on age. The preservation of any part of these sequences is a function of the rate of relative sea level rise and the depth at which individual barrier environments accumulate below the base of the advancing shoreface. More than 500 km (300 mi) of high resolution shallow seismic profiles correlated with vibracores from retreat paths fronting the Isles Dernieres and Chandeleur barrier-islan arcs, show contrasting stratigraphic sequences preserved on the inner continental shelf (Mississippi delta). The Isles Dernieres barrier-island arc developed as a consequence of the Caillou Headland abandonment in the early Lafourche delta approximately 800 years B.P. The base of the advancing shoreface lies 3 to 4 m (10 to 13 ft) below sea level and truncates the entire Isles Denieres barrier-lagoonal sequence and the upper part of the Caillou delta plain. On the lower shoreface, channels can be seen projecting seaward under the central part of the island arc; associated with it is a beach-ridge plain extending eastward. On the inner shelf, a sand sheet up to 60 cm (2 ft) thick marks the retreat path of the Isles Dernieres. The Chandeleur barrier-island arc was generated by abandonment of the St. Bernard delta complex 1,500 years ago. The base of shoreface erosion lies 6 to 8 m (20 to 26 ft) below sea level, and truncates the entire barrier-lagoonal sequence in the central part of the system. On the downdrift flanks only the upper portion of tidal inlet and recurved spit complexes are truncated. Scattered outcrops of shell reefs and lagoonal deposits occur on the lower shoreface. Beyond the shoreface, a 1 to 5 m (3 to 16 ft) thick sand sheet, with gently seaward dipping interval reflectors, caps tidal inlet scars up to 10 m (33 ft) thick, as well as the basal portions of migrating barrier-island sequences associated with earlier shoreline positions. Differences seen in the two stratigraphic sequences are a function of distributary size and depositional history of each barrier-island arc. The Isles Dernieres developed from a series of small sand-deficient distributaries in the Lafourche delta complex, whereas the Chandeleur Islands developed from large sand-rich distributaries of the St. Bernard delta complex. End_of_Article - Last_Page 1470------------