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Sedimentary Environments, Evolution, and Stratigraphic Framework of Laterally Prograding Transgressive Barrier Complex: Timbalier Island, Louisiana

Thomas S. Isacks, Thomas F. Moslow

Timbalier Island is a beach-ridge barrier flanking the abandoned late Lafourche deltaic lobe on the south-central Louisiana coast. Twenty-five vibracores (5-9 m) and 12 short cores were acquired in a variety of subaerial, intertidal, and subaqueous environments of this barrier complex. These cores, coupled with detailed shoreline change maps, indicate that the island's migration, evolution, and stratigraphy are complex and variable.

Since 1887, Timbalier Island has laterally migrated approximately 6 km to the northwest, while the adjacent inlet (Cat Island Pass) migrated 2.5 km. Due to this extensive lateral progradation at the western end of the island, the following sequence is found: (1) bay/lagoon, (2) lowerspit platform/shoreface, (3) upper spit platform/shoreface, (4) foreshore, (5) backbeach, and (6) dune. An upward decrease in burrowing and increase in physical sedimentary structures, grain size, percent sand, and sorting are observed. None of the cored sequences resemble the tidal inlet channel-spit platform models observed elsewhere but, instead, mimic regressive shoreface sequences.

During the island's evolution, the interior beach ridges subsided in response to compactional subsidence and became vegetated by a Spartina and Avicennia (mangrove) marsh. In this central-interior part of the island, the progradational sequence is capped by an aggradational (0.5-1 m thick) marsh deposit.

In response to relative sea level rise, there is an overall landward component to island migration. In past years, the eastern two-thirds of Timbalier has experienced the highest rates of erosion. This eastern end of Timbalier has a sedimentary section that includes the lower progradational and aggradational sequences and a capping (0.75-1 m thick) washover and foreshore facies.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.