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