NEW KARST LANDSLIDES IN THE WHITE RIVER VALLEY, CARROLL COUNTY, ARKANSAS

During evaluation of environmental impact of a proposed major power line in 2012, five recent previously undocumented landslides were investigated that are uniquely related to karst, and present geohazards to development. The observed landslides range from 0.25 to 1.6 acres, and have widths up to 150 feet, and lengths from 200 to 560 feet. They display characteristic geometry of head-scarp, hummocky surface, and piled up toe which are indicative of a thickness of unstable material up to 20 feet thick.

Field mapping demonstrates that the slope failures occur in a landslide hazard zone controlled by topographic, geological and hydrological conditions along the south side of the White River Valley from Inspiration Point to Wolf Ridge. The topographic relief here is over 600 feet. The occurrence of landslides is confined to north-facing slopes underlain by Ordovician bedrock, topographically below the Saint Joe Limestone cliff and Chattanooga Shale bench. The slopes measure 14 to 20 degrees and are underlain by unstable surficial deposits composed of karstic residual weathering soils (regolith and chert mantle) and colluvium, all rich in clay that is readily fluidized by the addition of water. All of the slides had major movement triggered by the torrential rainfalls of April and May, 2011, as recorded by citizen scientists, culminating with the post-Joplin tornadic storms. In addition, some human disturbance of terrain appears related to all of the studied landslides.

Occurrences of pockets of clay-rich surficial deposits of colluvium and regolith are necessary for formation of the landslides, and these materials tend to occur in areas of karst dissolution with topo contours concave downslope toward the White River. In addition to infiltration of rainfall, the karst hydrological conditions on Wolf Ridge amplify slope instability by charging the slides with extra water from ephemeral springs at the overlying Chattanooga level, and from sub-slide fluidizing seepages, both actually observed after storm events.

Slope stability and landslides are a concern for construction economics and safety, a geohazard for construction activity, and a concern for short and long term environmental impact in this lovely and sparsely settled area. Further investigations of karst geomorphology may shed light upon these karst landslides.