Vadose Zone Pseudokarst and Holocene Pedogenic Implications for Geomorphic Evolution of the White River Badlands, South Dakota

Our studies have focused upon the evolution of Holocene pediments in the White River Badlands. The modern Badlands landscape displays ‘castles' of intricately eroded Tertiary strata surrounded with lower elevation sod tables, representing the vestigial remnants of dissected pediments. To better understand the landscape we have pursued (1) description of landforms with investigation of genetic processes and (2) dating of paleo-landsurfaces.

Pseudokarst perforates both the Tertiary bedrock and Holocene sedimentary cover. We have classified this vadose erosion by morphology and the processes of granular erosion that carve secondary porosity without extensive mineral dissolution. Notably, evidence suggests that erosion by soil water produces a significant impact upon landscape morphology, contributing particularly to the dissection of pediments. Moreover, at old-age, pseudokarst in the sod tables is overprinted with fluvial erosion, leading to perhaps gross underestimation of erosion by soil water.

To better understand the timing of pediment development and rates of aggradation and degradation, twenty one radiocarbon dates on the bulk humate fraction of paleosols in the sod tables have been determined. These dates reveal intervals of pedogensis active at ca. ~900, 1100-1300, 1700-1900, 2300-2400, and ~3,600 RCYBP. These intervals suggest surface stability and moisture regimes conducive to organic carbon accumulation. The youngest date is most intriguing because it presents a maximum age for the onset of dissection of pediments into sod tables. The presence of several paleosols suggests pulsations of aggradation punctuated with intervals of pedogenesis. Then, incision, dissection, and separation of sod tables from their source areas in the adjacent ‘castles' imply that the sedimentary dynamic of the region has been thrust into an interval of degradation in the last thousand years, and potentially within the last several centuries. The resulting erosion rates are quite impressive and the causality of the erosive regime remains intriguing and elusive.