Erosion of an ancient mountain range, the Great Smoky Mountains, North Carolina and Tennessee

Analysis of ¹⁰Be and ²⁶Al in bedrock (n=10), colluvium (n=5 including grain size splits), and alluvial sediments (n=59 including grain size splits), coupled with field observations and GIS analysis, suggest that erosion rates in the Great Smoky Mountains are controlled by subsurface bedrock erosion and diffusive slope processes. The results indicate rapid alluvial transport, minimal alluvial storage, and suggest that most of the cosmogenic nuclide inventory in sediments is accumulated while they are eroding from bedrock and traveling down hill slopes.

Spatially homogeneous erosion rates of 25 - 30 mm Ky⁻¹ are calculated throughout the Great Smoky Mountains using measured concentrations of cosmogenic ¹⁰Be and ²⁶Al in quartz separated from alluvial sediment. ¹⁰Be and ²⁶Al concentrations in sediments collected from headwater tributaries that have no upstream samples (n=18) are consistent with an average erosion rate of 28 ± 8 mm Ky⁻¹, similar to that of the outlet rivers (n=16, 24 ± 6 mm Ky⁻¹), which carry most of the sediment out of the mountain range.

Grain-size-specific analysis of 6 alluvial sediment samples shows higher nuclide concentrations in smaller grain sizes than in larger ones. The difference in concentrations arises from the large elevation distribution of the source of the smaller grains compared with the narrow and relatively low source elevation of the large grains. Large sandstone clasts disaggregate into sand-size grains rapidly during weathering and downslope transport; thus, only clasts from the lower parts of slopes reach the streams. ²⁶Al/¹⁰Be ratios do not suggest significant burial periods for our samples. However, alluvial samples have lower ²⁶Al/¹⁰Be ratios than bedrock and colluvial samples, a trend consistent with a longer integrated cosmic ray exposure history that includes periods of burial during down-slope transport.

The results confirm some of the basic ideas embedded in Davis’ geographic cycle model, such as the reduction of relief through slope processes, and of Hack’s dynamic equilibrium model such as the similarity of erosion rates across different lithologies. Comparing cosmogenic nuclide data with other measured and calculated erosion rates for the Appalachians, we conclude that rates of erosion, integrated over varying time periods from decades to a hundred million years are similar, the result of equilibrium between erosion and isostatic uplift in the southern Appalachian Mountains.