GEOMETRY, SIZE, AND FRACTIONATION – CONSTRAINING THE TRANSPORT PROCESSES OF TILE AND CUBIC SHAPED LIMESTONE BLOCKS IN THE FLINT HILLS, KS

One of the key advances in modern geomorphology has been the formulation of geomorphic transport laws. These mathematical expressions describe the movement of material across landscapes and are used and tested at landscape evolution timescales via modeling. However, while landscape evolution models using these transport laws can replicate key geomorphometric characteristics, a multitude of climatic and geologic boundary conditions remain hidden behind few transport law terms. In the case of the non-linear hillslope transport law, climate and lithological factors remain all lumped into a sediment transport coefficient (D). The geomorphic community is therefore currently teasing out how individual factors such as climate, lithology, and vegetation can affect how hillslope sediment moves downslope. Yet, there is so far little investigation on how the geometry and size of rock particles may influence the hillslope processes that eventually move them downslope. This may be a significant factor in landscapes formed from two or more lithologies of differing hardness. These lithologic differences can result in cliffs and benches that produce large rock blocks that may armor below hillslopes.

We combine field-based measurements and statistical analysis to understand how the shape and size of rock blocks leads to departures from idealized size-distance relationships of large blocks on hillslopes. The study focuses on hillslopes in the Konza Prairie Biological Station portion of the Flint Hills of Kansas. Fortuitously, our location allows for the examination of shape impacts due to a distinct two-meter-thick limestone layer that breaks up about equally into cubic and tile shaped clasts. Our measurements consist of over 800 slope block observations spread over 30 slope transects.

Result suggest that block geometry has influence over soil interactions and ultimately the downhill movement pathway. Surprisingly, we find that the expected decrease in clast size with increasing distance from the bedrock bench is violated in this setting. Furthermore, evidence suggests that cubic shaped clasts do not role downslope as initially postulated, rather they are carried downslope via processes related to soil creep. We propose based on field observations and measurements of surface weathering that for limestone blocks the process of fractionation must be considered to explain size-distance observations. Further investigation will need to be preformed to constrain the total role of fractionation on the downslope transport of large rock blocks.