Stratigraphy of Transgressive Barrier Island Arc: ABSTRACT

Abstract

Isles Dernieres, a Holocene transgressive barrier island arc on the Louisiana coast formed as a result of dominant marine processes reworking deposits of the abandoned Caillou distributaries of the early Lafourche delta lobe. The island system exhibits a variable transgressive and regressive stratigraphy throughout its framework, which is discriminate from comparable Atlantic coast transgressive barrier sequences. Continuous subsidence and storm breaching divide the island system into 3 major island segments: Western, Central, and Eastern Isles Dernieres, each of which is separated by locally interjacent tidal inlets. Vibracore data reveal that Western Isles Dernieres represents a series of accreting sand spits that resulted from the erosion and longshore transport of the abandoned distributary channel deposits from the central deltaic headland. This 3 to 4-m (10 to 13-ft) thick sequence of sand spits and associated marsh deposits overlies interdistributary silts and clays. Central Isles Dernieres is a fluvial-deltaic complex that exhibits marsh and tidal-flat deposits capping a relatively thick sequence of levee and interdistributary bay sediments. A relatively thin beach-ridge plain lies locally submerged beneath sand spit and marsh deposits of Eastern Isles Dernieres. This beach-ridge plain formed during a regressive phase of island evolution. Erosion of the beach-ridge plain above the effective wave base presently provides an active sand source for the downdrift accreting sand spit at the eastern end of Isles Dernieres. A high preservation potential of the western and eastern transgressive sequences of Isles Dernieres is probably due to rapid subsidence and the consequent in-place drowning of the island segments. In contrast, Atlantic coast barrier counterparts often exhibit incomplete transgressive sequences owing to continued shoreface erosion. End_of_Article - Last_Page 511------------