STRATIGRAPHY ACROSS THE ELLICOTT LINE—RECONCILING GEOLOGIC MAPS OF SOUTHERN ALABAMA AND THE FLORIDA PANHANDLE

Recent work of the USGS-funded East Gulf Coastal Plain Stratigraphic Reconciliation Initiative (Geological Survey of Alabama, Mississippi Office of Geology, Florida Geological Survey) has identified several issues in need of resolution in the region of the Alabama-Florida state line (31^st parallel or “Ellicott Line”).

Revisions in stratigraphic nomenclature and/or rank—Includes historic or regional use of differing terminology for the same lithostratigraphic units (Ocala Limestone in Florida vs. Crystal River Formation in Alabama), abandonment of some restricted units (Mobile Clay and Ecor Rouge Sand in Alabama), and naming of informal units (Miocene Coarse Clastics in Florida). Recognition of age-equivalent lithostratigraphic units and regional facies changes—Examples are the Lower Oligocene Glendon Limestone of Alabama and Bridgeboro Limestone of Florida, and the Upper Cretaceous Tuscaloosa Group of Alabama and the Atkinson Formation of Florida. Allostratigraphic units and geomorphology—Formal designation of some allostratigraphic units, including alluvium of major river valleys and the extensive karst residuum of the Dougherty Karst Plain, would be helpful in Quaternary and geoarchaeological work. The relation of some surface lithostratigraphic units to geomorphic surfaces (alluvial and coastal terraces) needs clarification (e.g., Citronelle Formation/Terrace).

Reconciliation of surficial geologic maps—One of the most challenging issues is the differentiation of middle-late Miocene and Pliocene units across southeastern Mississippi, southwestern Alabama, and northwestern Florida. This long-standing problem is a function of the lithologic similarity, reworking, deep weathering, and (generally) non-fossiliferous nature of most units, and differences in cross-state nomenclature and usage. At issue is the relation of the Mid-Upper Miocene Hattiesburg/Pascagoula formations, and Pliocene Graham Ferry/Citronelle formations of Mississippi; with undifferentiated Miocene and Pliocene Citronelle, Pensacola Clay, “Coarse Clastics” of Alabama and Florida; and the upper Alum Group of the Florida panhandle. Recent subsurface mapping and projection of formation contacts to the outcrop should help resolve discrepancies in surface maps.