224-2 A Geomorphic Mechanism for Stalling Escarpment Retreat Using Cosmogenic Radionuclides and River Grain Size Measurements

Session: From the Cosmos and Back: Quantifying Processes and Rates of Landscape Change (Posters)

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Abstract:

The evolution of topographic escarpments provides critical insights into geomorphic processes and landscape dynamics. Passive margin Great Escarpments, for example, have motivated the development and progression of numerical landscape evolution models, with recent work suggesting that their long-term evolution is related to elevated biodiversity observed along many escarpments globally. Despite advancements and focus over the last three decades, there are still significant knowledge gaps and conflicting opinions around escarpment motion. A majority of cosmogenic radionuclide derived erosion rates across escarpments indicate comparably low values on both sides of the main drainage divide, indicating slow to no motion. In contrast, landscape evolution models predict erosion rates > 1 mm/year, assuming erosion is only a function of slope and drainage area (a proxy for discharge), where upland rates are near zero and the escarpment is rapidly retreating inland. In this work, we conduct a case study of a prominent escarpment in Puerto Rico where erosion rates are comparable across the main divide, despite dramatic differences in topographic steepness. We quantify grain size distributions using the Wolman pebble count in 46 drainage basins along the escarpment. Results show that D₈₄ grain size fractions are ~100 to 800% greater on the escarpment side of the divide than their upland counterparts with a clear dependency on topographic asymmetry. From this, we calculate components of erodibility using the normalized channel steepness index (k_sn) and erosion rate measurements from paired basins to determine controls on across divide erodibility. Our findings indicate that sediment caliber is critical in explaining erodibility and k_sn variations across the drainage divide. We infer that steeper slopes on the escarpment face are largely attributed to coarser sediment in channels, forming an armor layer that limits bedrock erosion. In contrast, the shallow upland draining rivers are devoid of this protective layer of coarse sediment, enabling more efficient bedrock incision. These differences in sediment caliber on escarpment- and upland-draining rivers can explain comparable erosion rates in the context of drastic topographic asymmetry and slow or stall retreat along the Puerto Rican escarpment, and possibly Great Escarpments globally.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7467

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