PRELIMINARY INVESTIGATION OF FLOW IN PSEUDOKARST, BADLANDS NATIONAL PARK, SOUTH DAKOTA

An investigation of flow phenomena producing diverse pseudokarst morphologies is underway in the Badlands. Pseudokarst (or piping) is a process that results in small caverns and sinks, whereby silt is removed by granular erosion in arid regions. While essentially ubiquitous at this locale, description is limited to occurrences in the Oligocene Brule Formation, the dominant cliff-forming unit, and in sod tables composed of Holocene alluvium.

Subsurface conduits in these units occur in relation to strata with substantial permeability contrasts. In the Brule Formation, many tunnels truncate on their uphill end just beneath dense caliche layers. A field experiment demonstrated plausibility of hydraulic tunneling as a causal mechanism. A rivulet flowed down an overhang adhering to the face, building momentum on the hydraulic jump, entered the softer underlying material, and re-emerge within a couple meters downslope. Tunneling downward lies in discord with the conventional mechanism of void enlargement proceeding up a flow path by dissolution or clast dislodgement under seepage pressure. Evidence to confirm void extension either up or down a flow path has been elusive.

In sod tables, conduits can extend for ten or more meters, produce multiple sinks that eventually coalesce, and this process is likely to be an under-rated mechanism for landform dissection due to overprinting of fluvial processes at geomorphic old age. Void development occurs at the contact of overlying Holocene alluvium upon the lower, better indurated Oligocene siltstone. These voids appear to extend upslope, perhaps following joints related to dessication, foundering, or diminished lateral support, which is being investigated.