Clay Mineral Variations in Marginal Deltaic Plain, Coastal Hancock County, Mississippi

Gregory N. Bonn, David M. Patrick

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 dinfraction of the < 2µ fraction for each of 22 samples revealed the relative peak-height percentages overall as 50-75% smectite, 20-40% kaolinite, and 5-20% illite.

Core data combined with detailed near-surface field mapping indicate the study area as marginal deltaic, 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 marsh surface.

Generally, smectite decreases upward by about 20% throughout the sedimentary sequence, whereas 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. A slight increase in kaolinite, concurrent with a decrease in smectite in the near-surface organic-rich marsh deposits, indicates the Pearl River as an increasing source during their accumulation. A continuation of these mineral trends, revealed in the surface sediments, implies that the Pearl River was the primary source for clay minerals deposited in the study area subsequent t St. Bernard delta development.

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

AAPG Search and Discovery Article #91042©1987 GCAGS and GC-SEPM Section Meeting, San Antonio, Texas, October 28-31, 1987.