CORRELATION CHARTS AS IMPORTANT TOOLS FOR HARMONIZATION OF GEOLOGIC MAPS: AN EXAMPLE FROM THE ORDOVICIAN OF THE APPALACHIANS

As digital seamless and regional geologic maps become the norm in geologic mapping programs, correlation of geologic map units is more important than ever to understand the stratigraphic and historical context of geologic nomenclature. Geologists evaluate historical stratigraphic precedence, changes in facies across regions, and the mappability of the foundational unit, the formation. A complete geologic map includes a correlation of map units to show the age and position of each mapped unit; this becomes more important for small-scale maps that cover large areas displaying lateral and age-correlative units. As different areas are mapped at various times and different entities, different geologic names tend to proliferate. With over 140 years of geologic mapping this tendency of proliferation is exemplified by the Ordovician rocks of the. The overall stratigraphic framework along and across the trend of the Central and Northern Appalachians is fairly consistent over hundreds of miles as tectonics and sea-level changes occurred. However, changes in facies and the timing of various paleoenvironments show larger-scale changes in stratigraphy that are reflected in local or 1:24,000-scale geologic mapping. Development of historical and regional correlation charts between central Virginia and West Virginia northeastward to east-central New York and western Vermont has shown the diachronous nature of regionally mapped units and helped with recognition of local units based on changes in lithofacies for the Appalachian Valley and Ridge province.