Clay Mineral Composition and Distribution in the Atchafalaya Basin and Terrebone Marsh Areas, South-Central Louisiana

ABSTRACT The analyses of nearly two hundred vibracore samples taken from Holocene sediments in south-central Louisiana revealed strong statistical relations between clay mineral composition and depositional environment and lithology. The dominant clay minerals from most to least abundant in both the Atchafalaya Basin and Terrebonne Marsh areas were: smectite (70%), illite (18%), kaolinite (10%), and chlorite (1%). The higher concentrations of smectite occurred in the inland swamp (87%) and backswamp (82%), and the lower concentrations were found in the crevasse channel (44%) and lacustrine delta (57%). Generally, the percent smectite was inversely proportional to the relative depositional energy of the environment. The percent illite ranged from a low of 8 in the inland swamp to a high of 37 in the crevasse channel. Kaolinite was most abundant in the lacustrine delta (14%), and least abundant in the inland swamp (5%). The highest concentration of smectite (78%) occurred in deposits classed as clay, and the lowest (46%) occurred in organic-rich clay. Clay and organic-rich clay, respectively, also exhibited the lowest and highest concentrations of both illite and kaolinite. The high smectite content is attributable to the low energy of the clay deposits, while the low concentrations of smectite in both organic-rich and peat material are due to instability of smectite in low pH environments.