David M. Patrick

Lower Mississippi River tributaries: contributions to the collective science concerning the “Father of Waters”

Abstract The geological and geomorphic information preserved in the tributary valleys of the lower Mississippi River (LMR) contributes to our understanding of the lower valleys Quaternary geological history. Prominent Pleistocene terraces are preserved in the tributary valleys. Fisk first formulated his four terraces framework on the Red River. Caution needs to be followed in projecting the Red River terraces across the entire Lower Mississippi Valley (LMV). The tributary system cannot be assumed to operate in a synchronized fashion in response to changes in climate and base level. To compare the collective contribution of the tributaries of the LMR, the streams are described in terms of. (1) their characteristics, (2) geomorphic development, (3) process and response of the tributaries to and from the LMR, and (4) engineering investigations and implications. The characteristics of the tributaries are a direct function of their drainage basin size and geology. The tributary system drains portions of six physiographic provinces. Synoptically, the tributaries can be viewed as two groups: the eastern and western tributaries. All of the eastern tributaries are intra-regional, i.e., they drain only one physiographic province, the Coastal Plain, and therefore, have a restricted sediment source. Generally, the eastern tributaries are more numerous and shorter than the western tributaries. The longer western tributaries drain outside the Coastal Plain. The extra-regional nature of the western tributaries adds to the variability of discharge and sediment types. The sediment record of the tributaries reflects response to the trunk Mississippi. During glacial outwash flushes, many of the tributaries were alluvially drowned, producing alluvial cones expressed as flattened longitudinal profiles. More recently, a number of tributaries in the state of Mississippi have experienced episodes of accelerated channel erosion. The effects of navigation and flood control modification of the Mississippi River on the tributaries have not been fully studied. Therefore, fluvial geomorphic research in the tributaries is essential to understanding ways to mitigate the adverse effects of river engineering, thereby designing engineering works in balance with the alluvial architecture and processes of the stream system.

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

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.