Sedimentation in a Transgressed Interdistributary Bay on Mississippi-Lafourche Delta Lobe: ABSTRACT

Abstract

Caminada Bay is a shallow south Louisiana bay, located between the distributary levees and beach-ridge plain of the late Lafourche Mississippi delta (900-500 y.B.P.) on the west and the Bayou Des Families levees (3,000-2,000 y.B.P.) on the east. Small distributaries traversed the region of the present-day bay, probably during both early and late Lafourche time. The Lafourche delta actively prograded until about 500 y.B.P. when it was abandoned by the Mississippi River; the area of the present bay then entered its transgressive phase. Grand Isle, which separates Caminada Bay from the Gulf of Mexico, was subsequently built by sand derived from local distributary-mouth bars and erosion of the Lafourche beach-ridge plain to the west. As a consequence of the cutoff in fluvial ediment supply and continued subsidence, the lower delta plain of the Lafourche lobe is currently being transformed into the rapidly expanding Caminada Bay. Visual description, x-ray radiography, and resin peels of 23 vibracores (4 to 8 m, 13 to 26 ft, in depth) reveal the following vertical succession of sedimentary units beneath the bay: (1) basal paleobay deposits, (2) an intermediate unit representing Lafourche delta lobe progradation, and (3) upper transgressive bay deposits. The deepest unit penetrated (paleobay deposits) consists of mottled or bioturbated very fine sand and clay and shell beds. The intermediate unit (delta lobe) consists of intricately interlaminated very fine to fine sand, silt, and clay. It features lenticular, wavy, and flaser bedding. The upper transgressive unit consists of banded organic-rich clay with in-situ and detrital peat. Expansion of the bay by wave erosion of the many marsh islands removes most of the organic-rich upper unit. A thin unit of bioturbated muds is currently being emplaced on the bay floor. End_of_Article - Last_Page 475------------