--> ABSTRACT: Sedimentary Texture, Structure, and Framework of Lacustrine Deltaic Deposits in Lower Atchafalaya Basin, by Phyllis L. Breland, D. M. Patrick, and L. M. Smith; #91036 (2010)

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Sedimentary Texture, Structure, and Framework of Lacustrine Deltaic Deposits in Lower Atchafalaya Basin

Phyllis L. Breland, D. M. Patrick, L. M. Smith

The lacustrine deltaic deposits in Six Mile Lake have been defined sedimentologically in terms of sedimentary texture, structure, and framework. Data sources include color infrared photography, vibracore samples, and field studies. Six Mile Lake is interconnected with Grand Lake on its northern end and both lakes are interdistributary lakes located in south-central Louisiana. The lakes lie between the former Teche Mississippi River meander belts and the La Fourche-Modern Mississippi River meander belt. Lacustrine deltaic deposition has been accelerated since Grand and Six Mile Lakes were included in the Atchafalaya basin floodway in 1928.

The sedimentary sequence of the deltaic deposits in Six Mile Lake is built on approximately 120-160 ft of lower Holocene backswamp deposits. Sedimentological analyses describe a lacustrine subenvironment with a lacustrine and fluvio-lacustrine facies and a lacustrine deltaic subenvironment with a subaqueous and subaerial bar facies.

A proposed sedimentological development model was designed from sedimentological data and field studies. The model includes four phases of development of the lacustrine deltaic deposits of Grand and Six Mile Lakes. The first phase is the southward translation of subaqueous bars over quiet-water lacustrine deposits and fluvio-lacustrine deposits. In phase two, vertical and lateral accretion deposits of fine sand, silt, and clay increase the size of the subaqueous bar, and it becomes subaerial and stabilized in the lake basin. Natural levee swales develop on the subaerial bars along with dense growth of vegetation. In phase three, subaerial bars merge and coalesce by lateral accretion processes to form lacustrine deltaic lobes, which are lineated by abandoned distributary channel suture . The abandoned interdistributary sutures add to the complexity of the subaerial-bar sedimentology. The model is completed by phase four when the subaerial bars and lobes eventually coalesce through continued lateral and vertical accretion until all of the original lacustrine area fills with the deposits and overbank sedimentation from the main Atchafalaya River channel. The resultant landform will resemble a floodplain or well-drained swamp.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.