Digital method for regional mapping of surficial basin deposits in arid regions, example from central Death Valley, Inyo County, California

Abstract

Derivative maps generated from DEM's and panchromatic remote sensing data (TM Landsat 7 or SPOT) can be used to characterize surficial basin deposits in arid regions dominated by basin and range topography. Results indicate the technique is useful for rapid digital mapping of surficial deposits where a first order, systematic subdivision of bedrock, alluvial fan units, and playas is unavailable at regional scales. Digital mapping can provide information about relative age and material properties of units that in part can be derived from the position of units within the basin. This automated mapping, implemented in a GIS system, involves an iterative process applied to a combination of digital elevation models (DEM) and satellite image data, such as SPOT or the high-resolution panchromatic Band 8 of Landsat 7 scenes. The method first discriminates the region into first-order terrains consisting of bedrock mountain highlands, basin piedmonts, and playa-basin interiors based on user-defined slope cutoffs applied to DEM data. The basin areas are subsequently classified into surficial map units such as active channels, ground-water discharge zones, and multiple age alluvial-fan piedmont units based on reflective properties of the associated surfaces in the satellite imagery. The surficial units are differentiated through systematic classification based on specific user-defined ranges of spectral values for each unit. The spectral ranges used in the classification are largely dependent on the composite effects of surface characteristics and material properties, including depositional morphology and texture, pavement development, degree of surface clast varnishing, and (or) properties of exposed soils of the alluvial fan units. We have used the slope-curvature properties derived from the DEM data to discriminate the bajada areas that exhibit non-unique spectral characteristics. Slope curvature is particularly effective at differentiating young undissected surfaces from older dissected piedmont units. Available geologic maps and field observations may be used both to iteratively calibrate the spectral classification scheme and to provide additional verification of the digital map output. Digital mapping combined with detailed field studies in selected areas provides useful regional maps of surficial units until time and funding is available for more field intensive studies. In addition, anomalous areas on the thematic maps indicate where more detailed field or air photo work is warranted. The technique successfully distinguishes between bedrock, alluvial fans (generally multiple fan units), active washes, playas, playa rimming marshes and seeps and other active and inactive discharge zones in arid basin and mountain regions. Limitations occur in the subdivision of some fan units where the dominant detrital clast lithologies are not susceptible to varnish development.