ABSTRACT

Clay minerals sampled from each sedimentary unit present in five piston cores collected from the marshland of coastal Hancock County, Mississippi, were analyzed to determine vertical differences in the clay mineral suite. X-ray diffraction of the less than 2 fraction for each of 22 samples revealed the relative peak-height percentages overall as 50 to 75% smectite, 20 to 40% kaolinite, and 5 to 20% illite.

Core data combined with detailed near-surface field mapping indicate the study area to be a marginal deltaic environment, consisting of a regional transgressive and localized regressive-transgressive sequence of estuarine, beach, nearshore, lagoon-bay, brackish marsh, natural levee, and active shoreline sedimentary environments. Clay minerals selected for vertical suite analysis represent nearshore through brackish marsh environments, ranging in depth from 4 m to the present-day marsh surface.

Generally, the percentage of smectite decreases upward by around 20% throughout the sedimentary sequence while kaolinite correspondingly increases. This relationship suggests that the lower muddy sand and silty clay units, which contain the most smectite and are indicative of the nearshore and lagoon-bay environments, reflect the St. Bernard Phase of the Mississippi River Delta as the dominant source during their deposition. As light increase in kaolinite, concurrent with a decrease in smectite in the near-surface organic-rich marsh deposits, indicate the Pearl River as an increasing source during their accumulation. A continuation of these mineral trends, revealed in the surface sediments, imply that the Pearl River was the primary source for clay minerals deposited in the study area subsequent to St. Bernard Delta development.

Recent field observations suggest that presently, most of the clay minerals are inherited from eroding Cenozoic Coastal Plain formations in Mississippi and Louisiana prior to downstream transport by the Pearl River into Lake Borgne and the surrounding marshlands.