Alluvial Pedogenesis in Pleistocene and Holocene Mississippi River Deposits: Effects of Relative Sea-Level Change

摘要

Comparison of Pleistocene and Holocene soils developed in Mississippi River meander-belt deposits in Louisiana illustrates the influence of relative sea-level change on pedogenesis. Holocene and Pleistocene morphologies, lithofacies, and soil characteristics, and probably formed under similar hydrologic regimes. The Pleistocene Avoyelles Prairie meander belt is now terraced, but it was originally graded to a lowere relative sea-level position than the Holocene flood plain. Holocene alluvial soils are generally weakly expressed cumulative profiles with abundant redoximorphic features, and formed on an aggrading flood plain with a shallow fluctuating water table. The soil morphology varies with respect to landscape position and overbank lithofacies.Pleistocene alluvial paleosols show evidence for two stages of pedogenesis. During the first stage of pedogenesis, water-table fluctuations and flood-plain aggradation produced profiles with color mottles, iron and carbonate nodules, and weak B and C horizons, similar to Holocene flood-plain soils. The second stage of pedogenesis occurred after the Avoyelles Prairie was terraced and the regional water table fell a minimum of 30 m. Improved soil drainage created oxidizing and acidic conditions, removed free carbonates, dehydrated iron oxides, and transformed clay minerals. This stage produced Pleistoncene profiles with brighter matrix and mottle hues, lower pH values, larger and harder iron nodules, and greater kaolinite contents than comparable Holocene flood-plain soils.The primary influence on regional water-table lowering in the southern Lower Mississippi Valley was relative sea-level fall. Mississippi River flood-plain lowering adjacent to the Avoyelles Prairie most likely occurred between the end of oxygen isotope stage 3 and the maximum period of valley entrenchment, during oxygen isotope stage 2. Water-table fall affected pedogenic processes a minimum of 400 km upvalley of the continental shelf margin. Pleistocene Mississippi River paleosols represent excellent analogs for interpreting ancient alluvial paleosols for interpreting ancient alluvial paleosols formed during episodes of base-level fall and valley incision. As fluvial entrench ment terraced the meandering Avoyelles Prairie, a pedogenic threshold was crossed that altered the pathway of chemical weathering. The complexity of a multiple-step pedogenic history also highlights the fact that interpretation of relative profile development as a function of time can be problematic.