CHALLENGES STEMMING FROM LARGE-AREA SEAMLESS MAP COMPILATION: EXAMPLES FROM THE CORTEZ AND SHIPROCK 1°X2° QUADRANGLES, FOUR CORNERS REGION, USA

Large-area, seamless, digital geologic maps such as those produced using the U.S. Geological Survey Seamless Integrated Geologic Mapping (SIGMa) extension to the Geologic Map Schema (GeMS) present significant challenges. These map products typically require input from source maps of various scales, extents, vintages, and stages of refinement. Compiling disparate source maps into a unified product necessitates addressing inconsistencies stemming from identification of rock units by the original map author(s), discrepancies across source map boundaries, revisions to unit nomenclature, and formal or informal rock unit definitions. Here, we explore some of the challenges that arise from seamless compilation of maps that include Paleozoic and Mesozoic strata of the Colorado Plateau in the Four Corners region, USA.

We highlight and discuss several stratigraphic issues, most of which result from reassignment, refinement, or deprecation of rock units through time, as well as geographic variability in nomenclature for correlative units. For example, deprecation of antiquated terminology and reassignment of existing units is exemplified by the lower parts of the late Paleozoic Cutler Group. The Rico, Halgaito, and Elephant Canyon Formations have all been variably reassigned to the lower Cutler beds. However, in this case, we propose retaining the Halgaito and Elephant Canyon Formations, as well as a transitional unit, to highlight facies change from siliciclastic to calcareous dominated lithologies, respectively. Examples of variable names for correlative units include the Late Jurassic Wanakah and Summerville Formations or Cretaceous Dakota Sandstone and Naturita Formation. Further, updated stratigraphic work and geochronology may result in reassignment of a contact location and unconformities, such as within the Jurassic Entrada Sandstone and Carmel Formation or at the base of the Morrison Formation. In these examples, compilation of a seamless, large-area map and accompanying internally consistent database necessitated compromise; we found that consultation and collaboration with State Geological Surveys and active researchers facilitated useful discussions and guidance.