Regressive and Transgressive Sand Bodies Associated with Lafourche Delta in South Louisiana: ABSTRACT

Abstract

The beach ridge plains and coastal barrier systems along the seaward margin of the wave-dominated Lafourche delta represent regressive and transgressive events associated with multiple delta lobe progradations and abandonments over the last 1,500 years. Two major delta lobes, the Early Lafourche and the Late Lafourche, can be recognized by the subaerial expression of the abandoned deltaic plain. Associated with each delta lobe is a regressive beach ridge plain deposited during distributary progradation and coastal barrier system deposited during distributary transgression. The Cheniere Caminada beach ridge plain is associated with the Late Lafourche delta lobe. It consists of more than 70 subparallel beach ridges in an arcuate fan-shaped configuration which flare seaward along the eastern levees of Bayous Lafourche and Moreau. Radiocarbon dates indicate beach ridge building began approximately 600 years ago when Bayou Lafourche built seaward of the older Bayou Blue shoreline and started intercepting westward longshore sediment transport, resulting in the formation of Cheniere Caminada. Near the fan apex, beach ridges are 7 to 8 m (23 to 26 ft) thick and thin westward to 2 to 3 m (6.5 to 10 ft) thick. A typical beach ridge stratigraphic sequence coarsens upward with shoreface silty sands overlain by a cap of washover and aeolian sands. Beach ridge growth ceased approximately 300 years ago when Bayou Lafourche was abandoned. Distributary abandonment initiated the transgression at Bayou Lafourche and the development of an erosional deltaic headland, the Caminada-Moreau coast, with a set of symmetrical, flanking barriers, the Timbalier Islands to the west and the Caminada Pass spit and Grand Isle to the east. Reworking of distributary and beach ridge sand bodies by shoreface retreat supplies the sand source required for coastal barrier generation. Shore-parallel transport distributes sand from the headland source into downdrift marginal spits, tidal deltas, and flanking barrier islands. Cores show that the flanking barriers (Timbalier Island and Grand Isle) increase in thickness from 2 to 3 m (6.5 to 10 ft) near the headland to 5 to 6 m (16 to 20 ft) at their downdrift ends. A typical flanking barrier strat graphic sequence shows a coarse-grained tidal inlet fill overlying a finer grained interdistributary bay fill and underlying a thin cap of washover and aeolian sands. In the Early Lafourche delta, only remnants of an older regressive beach ridge plain, Chenier Caillou, can be recognized owing to its advanced stage of transgression. A series of relict, partially submerged beach ridges associated with the Caillou headland can be seen spreading seaward on their western margin in the central Isles Dernieres indicating the dominant longshore transport direction was westward. Cores show a regressive sequence 7 to 8 m (23 to 26 ft) thick, similar to Cheniere Caminada, where shoreface silty sands underlie washover and aeolian deposits. The Isles Dernieres represent the transgressive barrier system of the Early Lafourche delta lobe. Abandonment of the Caillou distributaries occurred approximately 600 to 800 years ago. Due to long-term subsidence, the Caillou headland is now submerged below sea level and the Early Lafourche barrier system has evolved into a transgressive barrier island arc separated from the mainland by an intradeltaic lagoon. Sediment dispersal consists of seaward transport into an inner-shelf sand sheet, landward transport into washover deposits, and shore-parallel transport into tidal deltas and marginal recurved spits. Cores show the central Isles Dernieres consist of a thin washover sand sheet 1 to 2 m (3 to 6.5 ft) thick transgressing over delta plain and beach ridge deposits. The downdrift ends of the Isles Dernieres islands are thicker, up to 4 to 5 m (13 to 16 ft), and overlie fine-grained interdistributary bay fills. In the subsurface of Grand Isle and Cheniere Caminada, lies the transgressive barrier system of the Bayou Blue distributary abandoned 1,200 years ago. The sand source for barrier generation is two Bayou Blue distributaries which lie - 10 m (-33 ft) below sea level adjacent to Barataria Pass. The transgressive sand body strikes northwest and is overlain by a sequence of regressive interdistributary clays and silts, regressive beach ridge sands (Cheniere Caminada), and a transgressive flanking barrier sand (Grand Isle). Radiocarbon dates indicate Bayou Blue Barrier was actively transgressing landward 920 years ago. The identification of regressive beach ridge plains along the Lafourche delta suggests that these wave-dominated delta plain components may be stratigraphically more significant in the Mississippi delta than recognized before. Sequential delta lobe abandonments have led to the development of imbricating regressive End_Page 530------------------------------ and transgressive sand bodies in the Lafourche delta complex. End_of_Article - Last_Page 531------------