RATE OF CLIFF RETREAT AND BLOCK FALL USING PAIRED SCHMIDT HAMMER METHODS AND TCN DATING IN ROUGH RIVER BASIN, KENTUCKY

Geologic hazards, such as rockfall and landslides, are prominent in west-central Kentucky. While human activity causes most mass movements, high-density slope failures are typically the result of susceptible local geology, topography, and rainfall intensity. The weak shales underlying significantly fractured sandstone caprock, coupled with the wet environment on the slopes of the Rough River, have led to massive clusters of sandstone rockfall. However, the exact mechanisms of cliff retreat and sediment accumulation in the area are not clear. In addition, both the continuity and the timing of these events are uncertain.

To better understand block fall mechanisms, we used Schmidt Hammer exposure dating (SHED) technique coupled with terrestrial cosmogenic nuclide dating (TCN) on two sites in the Rough River basin. In-field observations suggest potential pathways of boulder detachment and continued downslope movement. At site one, slope debris present as larger (> 20 m) sandstone blocks detached from the cliff then move downslope with the surrounding colluvium. Dissolution along joints release blocks from in situ bedrock and transport of clay-bearing sediment into fractures between the cliff and the sandstone block exerts pressure during wetting cycles. Continued wetting and drying cycles of clay-rich colluvium float the sandstone blocks downslope, typically in upright positions. The mean SH R-value for the boulder surfaces is ~31. Eight TCN samples were retrieved from the side that was attached to the cliff, which would yield a minimum age of block detachment.

The second site (HTC) consists of boulder fields that represent multiple stages of cliff evolution. The boulder fields act as "conveyors" of smaller block clusters, that are moving towards the river. During transport, sandstone blocks continue to be chemically weathered until they eventually disintegrate within the Rough River. R-values are in the range of ~31, however, boulder fields at higher elevations tend to have R-values of ~35. We acquired 20 TCN samples across the boulder fields in three transects from the contemporary river to the cliff. By coupling TCN dating results with SH measurements we aim to determine if sandstone block detachment is catastrophic and quantify the rate of cliff retreat and block/cluster downslope movement.