Stratigraphy and depositional history of the Tornillo Group (Upper Cretaceous-Eocene) of West Texas

Fluvial strata of the Tornillo Group preserve a succession of Late Cretaceous, Paleocene, and early Eocene continental faunas and floras and provide a record of the Laramide orogeny in the southern part of the North American Cordillera. Contacts between units in the Tornillo Group (Javelina, Black Peaks, and Hannold Hill formations) have proven difficult to identify, but minor adjustments to the stratigraphy allow each to be readily mapped and provide a means to assess intraformational thickness variation and syndepositional deformation within the Tornillo Basin. The Javelina Formation is thin in the southwestern part of the basin, and the Black Peaks Formation thins toward both southwestern and northeastern sides, suggesting that development of the monoclines that bound the basin began in latest Cretaceous through Paleocene time. An obscure structure extending southeastward from Grapevine anticline divides the basin into northeastern and southwestern segments. The Javelina Formation thins southwest of this structure and lacks lacustrine facies found to the northeast. The upper half of the Black Peaks Formation is absent southwest of this line, and northeast- facing monoclinal folds that affect the Hannold Hill Formation in the same vicinity are truncated at the base of the overlying Canoe Formation. Depositional limits of the Hannold Hill Formation probably did not extend to the southwest. The middle Eocene Canoe Formation is largely unaffected by contractive deformation that affects the Tornillo Group. Although incipient Laramide-age deformation broadly defined the Tornillo Basin during latest Cretaceous through Paleocene time, deformation here occurred mostly during the early Eocene.