SURFING VERSUS CREEPING? - QUANTIFYING RESIDENCE TIME OF LARGE HILLSLOPE BLOCKS ON SOIL MANTLED HILLSLOPES

The presence of less erodible rock fragments on a softer more erodible hillslope is a common feature across many sedimentary landscapes. These rock fragments are sourced from less erodible lithologic units that outcrop above the more erodible slope forming unit. These large rock fragments can range in size from cobbles to large boulders. Extensive rock fragment cover can act as an armor that slows down and protects the hillslope from ongoing erosion. These large rock fragments can also act as point locations for downslope sediment storage. Therefore, knowing how long these large rock fragments and blocks reside on hillslopes allow for a more holistic understanding of the rate of hillslope regolith transport over geomorphic time scales. However, the rate at which this armor is transported and removed for hillslopes has been yet to be constrained via geochronology.

Cosmogenic geochronology techniques have allowed for the determination of erosion rates, rates of soil production, and surface exposure age across many landscapes. We use ³⁶Cl cosmogenic exposure dating to constrain the ages of large limestone blocks along a downslope topo-sequence. A sampling procedure was implemented to minimize the effects of block rotation as well as surface spalling. A total of ten samples were collected from hillslope blocks. Our study location are the soil mantled hillslopes in the Konza Prairie portion of Flint Hills of Kansas which resemble many locations across the region. The Flint Hills like other parts of the Great Plains is composed of repeating sedimentary layers of alternating erodibility. In the Flint Hills the shales weather into soil mantled slopes and limestones weather into bedrock benches and large rock fragments.

Preliminary results from a subset of the slope blocks samples suggest that there is a detectible downslope difference in surface exposure age of the large blocks. However future analysis and interpretation will be needed to determine the residence time of the large blocks on these hillslopes. Further results from ten samples taken from cliff proximal blocks will allow us to constrain rates of large block production and block-by-block cliff retreat. Together these two cosmogenic datasets will allow us to understand the production and transport of blocks in soil mantled landscapes.