AGE AND CORRELATION OF THE UPPER CRETACEOUS TOHATCHI FORMATION, WESTERN NEW MEXICO

The Upper Cretaceous Tohatchi Formation is at least 160 m of nonmarine siliciclastic strata exposed in western New Mexico along the SE and E flank of the Chuska Mountains. The Tohatchi Formation conformably overlies the Menefee Formation, is unconformably overlain by the Paleogene Deza Member of the Chuska Sandstone and consists of a lower, sandstone-dominated member and an upper, mudstone-dominated member. A significant change in sandstone lithology and bedform at the base of the Tohatchi Formation is a mappable contact and justifies continued recognition of the unit as a distinct formation. Dinosaur fossils found throughout the Tohatchi Formation indicate a Late Cretaceous age, and extensive palynomorph assemblages refine this age assignment to early Campanian. The presence in the Tohatchi Formation of such species as Accuratipollis lactiflumis, Brevimonosulcites corrugatus, Callialasporites dampieri, Microfoveolatosporis pseudoreticulatus, Periretisynolporites chinookensis, and Rugubivesiculites reductus suggest links to upper Santonian assemblages of the Milk River and lower Eagle formations of Alberta-Montana. However, other Tohatchi species such as Aquilapollenites attenuatus, A. trialatus, A. turbidus, Pulcheripollenites krempii and Tricolpites reticulatus are more closely related to assemblages from the Pakowki Formation and Judith River Group of Alberta and the Claggett and Judith River formations of Montana. The palynomorph assemblages in the Tohatchi Formation thus fall within the Aquilapollenites senonicus Interval Zone of early Campanian age. Therefore, the Tohatchi Formation is not, as has been thought for 50 years, a correlative of part of the upper Campanian Pictured Cliffs-Fruitland-Kirtland formations succession to the east. Instead, the Tohtachi Formation is the uppermost part of the Mesaverde Group in western New Mexico, younger than the underlying Allison Member of the Menfee Formation locally, and older than the late Campanian turnaround of the Cliff House-Pictured Cliffs shoreline to the east.