Geology topics

S. Southworth

Publications and source records attributed to S. Southworth.

3 recordsLinked to original sources

USGS2009

Comparison of alternative representations of hydraulic-conductivity anisotropy in folded fractured-sedimentary rock: Modeling groundwater flow in the Shenandoah Valley (USA)

A numerical representation that explicitly represents the generalized three-dimensional anisotropy of folded fractured-sedimentary rocks in a groundwater model best reproduces the salient features of the flow system in the Shenandoah Valley, USA. This conclusion results from a comparison of four alternative representations of anisotropy in which the hydraulic-conductivity tensor represents the bedrock structure as (model A) anisotropic with variable strikes and dips, (model B) horizontally anisotropic with a uniform strike, (model C) horizontally anisotropic with variable strikes, and (model D) isotropic. Simulations using the US Geological Survey groundwater flow and transport model SUTRA are based on a representation of hydraulic conductivity that conforms to bedding planes in a three-dimensional structural model of the valley that duplicates the pattern of folded sedimentary rocks. In the most general representation, (model A), the directions of maximum and medium hydraulic conductivity conform to the strike and dip of bedding, respectively, while the minimum hydraulic-conductivity direction is perpendicular to bedding. Model A produced a physically realistic flow system that reflects the underlying bedrock structure, with a flow field that is significantly different from those produced by the other three models.

Hydrogeology Journal↗

USGS2003

Temporally and spatially uniform rates of erosion in the southern Appalachian Great Smoky Mountains

We measured 10 Be in fluvial sediment samples ( n = 27) from eight Great Smoky Mountain drainages (1–330 km 2 ). Results suggest spatially homogeneous sediment generation (on the 10 4 –10 5 yr time scale and >100 km 2 spatial scale) at 73 ± 11 t km −2 yr −1 , equivalent to 27 ± 4 m/m.y. of bedrock erosion. This rate is consistent with rates derived from fission-track, long-term sediment budget, and sediment yield data, all of which indicate that the Great Smoky Mountains and the southern Appalachians eroded during the Mesozoic and Cenozoic at ∼30 m/m.y. In contrast, unroofing rates during the Paleozoic orogenic events that formed the Appalachian Mountains were higher (≥10 2 m/m.y.). Erosion rates decreased after termination of tectonically driven uplift, enabling the survival of this ancient mountain belt with its deep crustal root as an isostatically maintained feature in the contemporary landscape.

Tennessee, North Carolina↗

USGS2003

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

Analysis of 10 Be and 26 Al 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 10 Be and 26 Al in quartz separated from alluvial sediment. 10 Be and 26 Al 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. 26 Al/ 10 Be ratios do not suggest significant burial periods for our samples. However, alluvial samples have lower 26 Al/ 10 Be 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.

Tennessee, North Carolina↗