Holocene flood-plain soil formation in the southern lower Mississippi Valley: Implications for interpreting alluvial paleosols

Holocene Mississippi River flood-plain soils in central Louisiana provide important insights on alluvial pedogenesis in an aggradational setting, insights that are useful for interpreting alluvial paleosols. The flood-plain soils are younger than ca. 5400 yr B.P. and are grouped into two categories: (1) meander belt and (2) backswamp soils. Meander belt soils are Entisols, Inceptisols, and Alfisols developed on point-bar ridges and natural levee crests; they consist of 3–5-m-thick, brown, silty and sandy, moderately well drained profiles with multiple parent materials, common yellow-brown and gray mottles, and a few iron and calcite nodules. Backswamp soils are Vertisols formed in flood-plain depressions away from active and abandoned Mississippi River channels and consist of up to 10-m-thick, gray, clayey, poorly drained profiles that have abundant yellow-brown mottles, iron and calcite nodules, gypsum accumulations, and slickensides. The thickness of the soils and the presence of multiple parent materials show that as the flood-plain aggrades, concurrent or alternating episodes of sedimentation and pedogenesis produce cumulative profiles. Profile thickness is more closely related to sediment accumulation rates than to the depth of chemical weathering, and the cumulative nature of the soils explains the difficulty of recognizing discrete paleosol profiles in many ancient alluvial deposits. Comparisons of soils formed on the three youngest Holocene Mississippi River meander belts (Saucier and Snead, 1989) show that the soils record episodes of meander belt growth, abandonment, and relocation. Meander belt 1 soils form as the active meander belt develops and consist of either vertically stacked Entisols or a single Inceptisol. The soils contain few calcite nodules and form over intervals of a few hundred years. Meander belt 2 soils are Alfisols, lack free carbonates, and form over intervals of a few thousand years following meander-belt abandonment and soil leaching. Meander belt 3 soils are Alfisols or Vertisols that contain common calcite nodules, and the profiles are developed in 1 to 2 m of backswamp mud that overlies meander-belt silt and sand. The mud inhibits profile leaching and leads to poor drainage and calcite precipitation. This change in soil hydrology complicates interpretations of the effects of time on meander-belt soil development. Backswamp soils are strongly influenced by seasonal episodes of soil wetting and drying, as demonstrated by the gray colors and the abundance of mottles, nodules, and slickensides in the soils. The distribution of calcite nodules in the backswamp and meander-belt soils is more closely related to seasonal water-table position than to mean annual precipitation, which suggests that not all calcic paleosols in ancient alluvial deposits represent semiarid or arid climates. Slickensides attest to seasonal rainfall, but the composition of the backswamp soils primarily reflects source-area lithology and depositional influences, rather than the effects of chemical weathering. In general, these soils provide an incomplete record of the humid subtropical climate of Louisiana. Although short-term sediment-accumulation rates decrease away from Mississippi River channels, the backswamp soils are not better developed (i.e., more mature) than the meander-belt soils, because poor drainage and slow but continuous sedimentation inhibits weathering. Collectively, these observations suggest that flood-plain soils that developed in aggradational settings, such as that of the Mississippi River flood plain, are best suited for interpreting depositional and hydrologic histories of ancient alluvial deposits, rather than paleoclimates.