CHARACTERIZATION OF PSEUDOKARST IN BADLANDS NATIONAL PARK, SOUTH DAKOTA

Pseudokarst reveals extensive erosion of bedrock by percolating waters that occurs by mechanisms other than actual dissolution, which is restricted to Karst. As with karstification however, a variety of sinks and caverns can result. In the White River Badlands, poorly indurated siltstones and shales of the Tertiary Brule and Sharps formations, which form the notable ‘castles’, along with Quaternary alluvium present in the surrounding sod tables, host a variety of pseudokarst features formed by granular erosion. In this study we mapped the spatial distribution of these erosional features, and characterized them in order to develop a classification scheme. Different morphologies occur in the sod tables vs. the ‘castles’. In the Tertiary ‘castles’, three distinct styles exist. Along the margins of buttes, joints erode into aligned, vertical, tabular caverns suggestive of pressure-release causation, which we call “lateral retreat” pseudokarst. In the steep rills and ravines so prominent in the dramatic landscape, connected networks of many sinks are initiated below resistant strata, in a style that we refer to as “rill tunnel”. In addition, mass wasting in the rills can result in the wedging of boulders between tight walls, producing a tunnel and sink network that we term “chock stone”. Within the sod tables, which exist as remnants of relatively gently graded alluvial-colluvial fans forming adjacent to the ‘castles’, yet a fourth style of distinct pseudokarst morphology arises. The sinks and caves forming in sod tables are reminiscent of karst systems formed in subhorizontal carbonate units throughout the eastern US and mid-continent. Stages in development are discernable, as youthful examples include random individual sinks with no visible connections. The mature stage initiates with the presence of pipe outlets in the table-bounding arroyo walls in the vicinity of sinks. Mature development progresses to multiple sinks along one or more lineations, and visibly identifiable pipes connecting sinks, including the connection of sinks to pipe outlets on adjacent arroyo walls. Old Age involves the breaching and coalescing of sinks. Eventually this process should produce a collapsed valley and dissection of the sod table, but such evidence is elusive because fluvial processes rapidly overprint it.