Transport and Weathering of Large Limestone Blocks on Hillslopes in Heterolithic Sedimentary Landscapes

Geomorphic transport laws that describe the movement of material across landscapes generally hide the role of lithological and climatic boundary conditions behind proportionality terms. The geomorphic community aims to characterize the role of these boundary conditions and include them in mechanistic transport laws. A more detailed mechanistic understanding is needed for hillslopes formed in heterolithic rock that weathers into weak regolith and hard blocks, such as in hogbacks. The properties of such blocks may determine both the process and rate of their transport downslope. We focused on soil mantled hillslopes in the Flint Hills of Kansas in the United States, where we studied the role of block shape and size under a limestone cliff that breaks up approximately into equal amounts of cubic and tile-shaped clasts. Based on previous studies on the transport of rock clasts, we hypothesized that cubic and tile-shaped blocks should be transported differently, leading to different distributions of size and orientation with increasing distance from the cliff. Block shape does have significant influence over how soil collects and is stored on the hillslope near blocks. Yet, few cubic blocks appear to be transported through tumbling in contrast with assumptions in recent modeling work. We found complex relationships between block size and distance from the cliff face and we propose that this is due to weathering via fragmentation. This process produces discrete smaller fragments from a larger parent block. We suggest that at least in Kansas both cubic and tile-shaped blocks are transported primarily by creep processes.