SPECTRAL CHARACTERISTICS OF CRETACEOUS ROCKS OF THE ALBUQUERQUE BASIN, NEW MEXICO

The Cretaceous rocks found northwest of Albuquerque, New Mexico were studied under a cooperative effort involving several state and federal agencies in order to determine the hydrologic balance existing today within the Middle Rio Grande Basin (MRGB). In an effort to combine the needs for general geologic mapping at a scale of 1:100,000 and the tracing of reservoir rocks supplying much of the county and city groundwater, we undertook an investigation using satellite imagery and spectral signatures to discriminate potential reservoir rocks. It was hoped that the spectral characteristics of component clays in the aquifer units might enable us to discriminate outcrops of the unit not previously mapped at smaller scales. Although some unmapped outcrops were identified, their location and geometry indicated that they could not add potential groundwater resources to the MRGB aquifer. It was also a goal to use this project as a test case for production of a digital geologic map based on calibrated satellite images. The result is a single CD-ROM containing the Collier Draw 7.5 min geologic map, stored in an ARC format. The rocks found in the Collier Draw area are Cretaceous sedimentary rocks that exhibit few diagnostic features in the wavelengths covered by the ETM+ instrument (i.e. the visible to near-infrared). Spectral plots of the sandstones and siltstones in the area display the typically bland curves for this part of the spectrum, but they also have a generally higher reflectance in all bands than the shales and carbonaceous units. The clays contained in the shales and weathered surfaces of other deposits are by far the dominant influence on the spectral signatures of those units in the area. The laboratory spectra of samples taken from these units show enough detail to identify several different types of clays that can be confirmed from XRD clay analyses. More subtle differences between clays with similar crystal structures (i.e. mixed-layered clays) are more difficult to distinguish in the ETM+ data and images. However, enough subtle differences, either in clay type or abundance of iron-oxides or carbonates, were evident to influence the signatures of the units in the area to the point where we were confident in using the imagery as a base for vectorizing unit contacts. These contacts followed well the stratigraphic break down of mappable units in the area.