This study in Breton, Chandeleur, and Mississippi Sounds combines the use of bottom-penetrating acoustic-reflection records with lithologic descriptions of piston and gravity cores for construction of sections showing boundaries of depositional facies units. Continuous acoustic records delineate morphology of buried horizons (lithologic breaks, changes in density, water content, or degree of lithification). Lithologic descriptions give internal structure and variations of particle size, color, components such as shells and wood fragments, and consistency. With these criteria, deposits were divided into the major broad facies units of sound deposits, delta deposits, basal and barrier island sands and Pleistocene clays. Delta deposits are recognized as being of two distinct periods of the St. Bernard subdelta, which has subsided beneath Breton and Chandeleur Sounds.
Probable buried distributaries of these delta phases are recognized in the records. Sand bodies connecting islands in Mississippi Sound are suggested as remnants of an extensive barrier island which fronted the area previous to the subdelta growth and had its beginnings about 7,000 B. P.