FORMAL STRATIGRAPHIC NOMENCLATURE FOR THE EDWARDS GROUP IN SOUTH TEXAS

Lower Cretaceous strata of north Texas have been formally subdivided into formations and members for a considerable time. However, in south Texas more informal descriptive units have historically been used with success because of the stable paleogeographic setting—a shallow, mostly lagoonal region protected behind the Stuart City Reef trend. As more detailed studies of the stratigraphy, hydrogeology, karst, and structure of the Edwards Group and its equivalents are conducted and expand regionally beyond the local strata, the descriptive nomenclature is often misleading and no longer applicable. For example, numerous localities throughout the Edwards aquifer include grainstone lithologies, even though the traditional Grainstone member is restricted to the uppermost Kainer Formation in the San Marcos Arch area. Few of the traditional descriptive units have precisely defined reference sections, and many derive their name from subsurface studies—both of which compound miscommunication and correlation problems.

Formal stratigraphic nomenclature and sequence stratigraphy are needed to more precisely correlate the various subdivisions of the Edwards aquifer with equivalent shelf-edge deposits of the Devil's River Trend and into the epicontinental Maverick Basin, as well as to improve correlations beyond the region with Lower Cretaceous strata in trans-Pecos and north Texas. The proposed formal stratigraphic nomenclature is based on properly measured and described reference (type) sections, with geographically derived names. These units are designed to correspond with the older descriptive terminology to avoid disrupting established stratigratigraphic concepts, but are much more conducive to regional mapping and correlation, diagenetic and hydrologic studies, and other analyses where facies changes are currently obscured by the traditional descriptive nomenclature.