Geomorphology of welded tuffs, Chiricahua National Monument, southeastern Arizona.
AuthorHall, Douglas Benjamin.
Committee ChairBull, William B.
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractThe Chiricahua National Monument in southeastern Arizona presents several interesting questions to the process geomorphologist. First, why are columnar landscapes characteristic of the tuffs? Air photo analysis reveals that column geometries in both cross section and plan view are controlled by two locally radiating, vertical joint sets and a third horizontal joint set. Differential compaction of the tuff may also have led to jointing. The resultant joint planes provide avenues for water migration and consequent focusing of chemical weathering. Second, how can we arrive at the rate of formation of such landforms? The total amount of material erosionally removed from the entire Turkey Creek Caldera ash flow facies present in the monument watersheds was calculated using a digital elevation map. An average denudation rate of 0.017 mm/yr was calculated for the watershed based on the eroded volume. Third, the column ages determined by the denudation rate and the prevalence of different erosional processes, each with its own efficiency, during different climatic periods, suggest a two stage erosional history with rapid erosion during glacial periods and slow erosion during interglacial periods. Fourth, how mechanically stable are the "balanced rocks" and "hoodoos"? Strength parameters for the tuffs were measured. Six columns with very different profiles were measured and then modelled by finite element meshes. Despite their fragile appearances, all columns were well within their static mechanical failure limits. Failure by slip along joint surfaces is the most likely block failure mode, but is greatly minimized by erosional necking of columns into hourglass shapes. Necking causes the principal stresses in the column to rotate out of vertical and focus on the neck. The redirected stresses act as a normal confining stress across dipping joints that would, in the absence of the neck, imperil the column. Fifth, what can the preservation of columns tell us about the seismicity of southeastern Arizona? The dynamic loads of an earthquake should induce resonances that would greatly imperil the larger, thinner columns. The upper height limit of the column population indicates the past occurrence of an earthquake that triggered this resonant vibration and consequent failure. It appears, therefore, that the last column-destroying quakes occurred 2.4 My ago. (Abstract shortened by UMI.)