Dinosaurs, pollen, and the Cretaceous-Tertiary boundary in the San Juan Basin

A BSTRACT .— In the San Juan Basin, New Mexico, the Ojo Alamo Formation includes two members, the upper Kimbeto Member and the lower Naashoibito Member (previously assigned to the underlying Kirtland Formation). The Naashoibito Member produces dinosaur fossils, as does the De-na-zin Member of the Kirtland Formation, which lies unconformably beneath it. A lignite bed in the upper part of the De-na-zin Member has been identified as the horizon of a major unconformity and the source of some Paleocene palynomorphs. The overlying dinosaur remains in the Naashoibito Member thus have been assigned a Paleocene age, but new pollen data refute this interpretation. The dinosaurs from the Naashoibito Member are not well-known, but late Maastrichtian (Lancian) dinosaur taxa ( Torosaurus latus and Tyrannosaurus rex ), as well as the early Maastrichtian Torosaurus utahensis , are not demonstrably present in this unit, despite previous claims. Vertebrate biostratigraphy suggests an early Maastrichtian age for the Naashoibito Member of the Ojo Alamo Formation, and palynological analyses of this unit does not support a Paleocene age; thus there are no Paleocene (non-avian) dinosaurs in the San Juan Basin.