Abstract

Although the east coast of North America has been a passive margin for over 200 million years, throughout the Appalachian Mountains features usually found only in regions experiencing active uplift such as high topographic relief, deeply incised river valleys, landslides, and elevated erosion rates dominate the landscape. Two contrasting hypotheses attempt to explain this contradiction: the Davis hypothesis of rejuvenated Cenozoic uplift along the eastern coast of North America and the Hack hypothesis of dynamic equilibrium where topographic relief is a consequence of spatial changes in surface lithology. I evaluate these hypotheses by characterizing longitudinal stream profiles of tributaries feeding the west branch of the Susquehanna River. Advances in the relationship between active tectonics and signatures that these forces imprint on surface features allow workers to identify dis-equilibrium conditions through perturbations in stream channel profiles called knickpoints. Here, I identify knickpoints, map them spatially, and determine if they are related to spatial variations in lithology (dynamic equilibrium model) or if they are transient signatures left from a pulse of recent uplift underneath the Susquehanna watershed (rejuvenated uplift model).

155 streams were analyzed across the Alleghany Front yielding 95 knickpoints with no correlation to lithology. Streams cutting transversely across synclinal features exhibited trends inconsistent with the dynamic equilibrium model. In these profiles, bedrock lithology from the stream headwaters to the stream mouth is symmetrical about a synclinal axis; however, stream profile metrics, specifically normalized channel steepness and knickpoint location, do not reflect the symmetry seen in stream channel bedrock. This suggests that lithology does not play a primary role in controlling stream profile metrics and the spatial distribution of knickpoints, and indicates that knickpoints represent a transient signal propagating throughout the Susquehanna watershed which could have been generated from a pulse of rejuvenated uplift. However, knickpoints are seen at a variety of elevations above baselevel, the lowest knickpoints occurring in younger, more resistant bedrock and the highest knickpoints occurring in older, less resistant bedrock. It was found that although variation in lithology does not generate knickpoints across the Alleghany Front, these changes significantly control the rate at which transient knickpoints migrate vertically upstream. Finally, it is recognized that the interplay between spatial changes in lithology and changes in knickpoint elevation can be used to estimate a history of how streams transverse plunging synclines evolved. Theoretical models of stream evolution are presented and implications are discussed.

Citation

Neely, A (2013): Characterizing the Recent Cenozoic Erosional History of the Appalachian Mountains through Spatial Variation in Stream Profile Metrics across the Allegheny Front. Bachelor of Science, Pennsylvania State University..