M. Ramsey, D. Howard, P.R. Christensen, Department of Geology, Arizona State University, Tempe, Arizona 85287-1404
N. Lancaster, Desert Research Institute, 7010 Dandini Blvd., Reno, Nevada 89512
The analysis of dune field formation, composition and movement is critical for the interpretation of past climatic conditions and future desertification processes. The use of remote sensing over the past several decades as a tool to study dunes has given the geologist a synoptic view of the entire dune field and its sources. Thermal infrared (8-12 microns) remote sensing is extremely useful for studies such as these because it allows for mineral identification and mapping of alluvial material with relative ease. Specifically, primary dune-forming minerals such as silicates all have diagnostic absorption features in this wavelength region rendering them detectable. In addition, mixing of minerals on the surface result in a linear relation between a mineral's percentage in a given area (pixel) and the depth of its spectral features. Spectra can therefore be 'unmixed' in order to ascertain percentages and trace transport paths. Applied to airborne Thermal Infrared Multispectral Scanner (TIMS) data, this type of analysis produces mineral maps of the dune's surface.
The primary focus of this work was to use the Kelso Dunes, located in the eastern Mojave Desert, California (34o 55' N, 115o 45' W), as a test locale for the unmixing model. To verify the results and provide ground truth, forty-eight bulk samples were collected along a 9.5 km N-S traverse. Thirteen of these were chosen for thin section analysis to determine modal abundances and particle size. Results show clear mineralogic variations throughout the dune field and indicate the dunes contain significantly less quartz (40-50%) than previously thought (70% to 90%). Finally, in order to isolate the dominant mineralogic components for inputs into the model, quartz, plagioclase, K-feldspar and magnetite were separated from the bulk samples using heavy liquid and magnetic separation.
Mineral percentage images show very high concentrations (~45%) of feldspar within the dunes as well as on the immediate alluvial surfaces. Similarly, the magnetite and other mafic minerals seem to be the direct result of weathering of local amphibole-rich metasediments. By contrast, based on a higher concentration to the northwest, a dominant amount of quartz may come from the Afton Canyon Fan source 40 km away. This result is further supported by a similar petrographic analysis of two samples outside the dune field - Afton Canyon Fan and Devil's Playground.
This study represents a rigorous attempt to validate the linear mixing assumption of thermal infrared spectra for a real geologic surface. Model-derived endmember percentages agree to within an average of 5.4% of the point count figures. Applied to the image data, the model reveals a much less mature dune mineralogy with several local sources for the sand than previously suspected.
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Submitted: Geological Society of America Meeting
Date: October, 1994