New Mexico Geological Society Annual Spring Meeting — Abstracts

A robust palynologic data base sharply defines the Cretaceous-Tertiary (K-T) interface at numerous localities in the San Juan Basin of New Mexico and Colorado. This important Era, Period, and Formation boundary is located at the base of the Ojo Alamo Sandstone-top of the Kirtland or Fruitland Formation throughout the New Mexico part of the basin and at the base of the Animas-top of the McDermott, Kirtland, or Fruitland Formation in the Colorado part of the basin. Over the last four decades, the last occurrence of diagnostic Cretaceous index palynomorphs (K-taxa) has been used throughout the Western Interior of North America to mark the K-T boundary in continental strata. The precision of this criterion was strikingly validated in 1981 when a cmthick interval at the palynologic K-T boundary in the Raton Basin was found to contain the K-T asteroidimpact fall-out layer, thus joining a bio-chronologic boundary with a rock-stratigraphic unit. Since that discovery, the fall-out layer has been found at dozens of other localities throughout the Western Interior at the palynologic K-T boundary.

The principal Cretaceous index palynomorphs in the San Juan Basin are Tschudypollis sp. (previously named Proteacidites). Tschudypollis sp. are present in large numbers in samples from Cretaceous Fruitland and Kirtland Formation rock samples, but are never found in the overlying Paleocene Ojo Alamo Sandstone (except for rare, reworked specimens). Moreover, Paleocene index palynomorphs Momipites tenuipolus and Brevicolporites colpella have been identified from the Ojo Alamo Sandstone at several localities in the basin. Lists have been compiled of all known palynomorphs from published and unpublished sources from Cretaceous-Paleocene strata in the San Juan Basin (Fassett, 2009, in press). These lists show that 244 palynomorphs have been identified from these strata; of these, 50 taxa (20%) are present only in Paleocene strata, 143 taxa (59%) are present only in Cretaceous strata, and 51 taxa (21%) are common to Cretaceous and Paleocene strata. Thus, the K-T interface is palynologically defined in the southern San Juan Basin at the contact between the Cretaceous Kirtland or Fruitland Formation and the base of the Paleocene Ojo Alamo Sandstone; in the northern part of the basin, this interface is between the Cretaceous McDermott Formation and the base of the overlying Paleocene Animas Formation. In addition, palynologic data in the San Juan Basin identify a significant hiatus at the K-T interface with all of the uppermost Cretaceous Maastrichtian Stage and the uppermost part of the Campanian Stage missing; in addition palynologic data suggest that the lowermost part of the Paleocene is also absent throughout the basin.

Reference: Fassett, J.E. (2009), New geochronologic and stratigraphic evidence confirms the Paleocene age of the dinosaur-bearing Ojo Alamo Sandstone and Animas Formation in the San Juan Basin, New Mexico and Colorado: Palaeontologia Electronica (in press).