VERTISOLS AND ALLUVIAL STRATIGRAPHIC MODELS: SOIL AND PALEOSOL INDICATORS OF DISCHARGE VARIABILITY (Invited Presentation)

Clay-rich paleosols (> 30% clay) dominated by smectites with high shrink-swell potential are termed “Vertisols” by the US Natural Resources Conservation Service (NRCS) in USDA Soil Taxonomy and are widespread today in regions of alluvial sediment deposition where precipitation is either seasonally distributed, or in which there is a seasonal soil-moisture deficit caused by soil-water removal due to higher evapotranspiration (generally summer). During periods of soil moisture surplus associated with flashy or prolonged rainfall events and higher alluvial discharge, water ponds on the soil surface (termed “epi-saturation” by soil scientists) due to slow drainage of the relatively impermeable clays, and if gilgai “= little water hole” micro-topography is developed on the soil surface, water ponding is localized as small, closely spaced (<5-7 m) wet depressions separated by subaerially exposed areas with slightly higher topography and better drainage. Soil features, termed “redoximorphic”, developed during water ponding are mainly associated with Fe reduction and include Fe- depleted root channels, soil cracks, and ped faces, whereas during periods of improved soil drainage and aeration, Fe²⁺ oxidizes to Fe³⁺ forming reddish-orange soft masses and linings of mainly poorly crystalline Fe oxides and oxyhydroxides, including lepidocrocite, limonite and goethite. Over time, and with many cycles of saturation and aeration, Fe concentrations may form hard masses (termed nodules or concretions). Redoximorphic features can form (or disappear) in as little as a few hours to a few days, hence the preservation of these features in paleosols formed in alluvial successions can provide a “snapshot” of the last moisture conditions on the floodplain prior to soil burial and preservation as a paleosol. Regional studies of Texas Vertisol “climosequences” indicate distinctive trends in drainage indicators that are a function of mean annual precipitation. Paleo-Vertisols have exceptional preservation potential in the rock record, probably because they form relatively rapidly in low parts of the landscape are composed of cohesive clays that resist erosion. Examples provided demonstrate the viability of using Vertisols and paleo-Vertisols to reconstruct paleoprecipitation, and possibly infer discharge variability.