REVISIONS TO THE TORNILLO GROUP (UPPER CRETACEOUS - EOCENE), BIG BEND NATIONAL PARK, TEXAS

Maxwell and others (1967) subdivided Udden’s (1907) “Tornillo Clay” into the Javelina, Black Peaks, and Hannold Hill Formations, and mapped their distribution in Big Bend Park. The formation contacts were placed so as to coincide approximately with the Cretaceous, Paleocene, and Eocene Period boundaries. Because of difficulties in discriminating the three formations of the Tornillo Group, and inconsistency in the mapped positions of the contacts between them, some recent workers have advocated demoting the units to member rank. These strata are important in documenting the progression of the Laramide Orogeny in Texas, and in containing the southernmost well-studied succession of Late Cretaceous through Eocene continental faunas in North America. Hence, it is important to properly subdivide and map these strata regardless of whether they are considered members or formations in rank. Over the past ten years, graduate students at Texas Tech University have re-mapped all of the major exposures of the Tornillo Group, and have found that several simple changes in the positions of the formation boundaries allow for consistent recognition and mapping of the three formations. The Cretaceous-Paleocene and Paleocene-Eocene boundaries do not occur at readily mappable stratigraphic contacts. The original type sections for all of the three formations must be revised, because each includes strata equivalent to the other units, as mapped elsewhere in Big Bend Park. The suggested changes do not require any revision of the stratigraphic nomenclature. Only emendation of the formation contacts is required.