INNOVATION IN NEW GEOLOGIC MAPPING OF THE LOWER COLORADO RIVER CORRIDOR, SOUTHWEST USA

We are developing a regional surficial geologic map of the lower Colorado River corridor between Lake Mead and Yuma, AZ. Mapping is focused on ancestral Colorado River deposits, called the ‘LOCO group’, which are divided into six key stratigraphic units that record the river’s evolution over the last 5 – 6(?) Ma. The campaign includes new large-scale geologic mapping (ca. 1:24k) in select areas and intermediate-scale mapping (ca. 1:75k) elsewhere in the corridor, both coupled with multi-scale compilation of existing mapping. All new mapping and compilation are handled in a multi-editor, versioned GIS. Detailed map areas contain exemplary geologic arrays of LOCO group strata and help inform regional-scale map portrayals by establishing a solid stratigraphic framework. Our multi-scale compilation uses an automated script to aggregate the best available mapping in a given area. We handle areas where the aggregated mapping overlaps on a case-by-case basis. Compiled mapping is prioritized based on scale, completeness, and veracity. This involves varying amounts of contact refinement, unit division, generalization, and remapping for realistic portrayal.

For additional geologic analysis and map-checking, we use an automated script to extract the elevation range of all LOCO group map polygons from a 10-m DEM according to their geospatial position and project them onto the river's longitudinal profile. Each LOCO group unit tracks past positions of the river at a specific points in time and can reveal potential impacts of coeval or post-depositional tectonic activity. Geologic inferences from unit profiles rely on proper classification and mapping, and this sets a quality-assurance and field-checking priority for the mapping. Areas where an outcrop’s position deviates from a dominant trend are field-checked to determine if the deviation indicates a geologic control on the river’s long-term evolution or if the mapping, location, or DEM-derived elevation is incorrect. Points crucial for anchoring the vertical position of the unit profiles are surveyed in the field with high-precision GPS. The multi-scale maps, compilation, and profile datasets evolve and improve in tandem to inform our interpretations of the geologic evolution of the area while also identifying locations of related geologic resources.