GEOCHRONOLOGY

The presence of abundant dinosaur bone in the Ojo Alamo Sandstone in the San Juan Basin has bedeviled researchers for more than 80 years because all other paleontological data, and the physical stratigraphic relations discussed above, indicated that the Ojo Alamo was Paleocene in age. Indeed, had it not been for the presence of abundant dinosaur remains in the Ojo Alamo Sandstone, its Paleocene age would probably never have been questioned. Because the last occurrence of dinosaur bone has always been considered by vertebrate paleontologists to mark the end of the Cretaceous Period, various explanations were suggested to explain away the presence of these dinosaur remains in what otherwise appeared to be Paleocene rocks. (For a complete discussion of those explanations, see Fassett et al. 2002.)

The relative age of sedimentary rock formations was originally based on the fossils found in those rocks (Winchester 2001). This criterion worked extremely well for marine rocks containing abundant fossils of small, steadily evolving, mostly invertebrate life forms, but was much less useful for continental strata containing far fewer diagnostic fossils, such as those of vertebrates. Where present, the last occurrence of dinosaur fossils was traditionally used to mark the top of the Cretaceous and Paleocene mammal fossils helped to locate the base of the Tertiary. These vertebrate fossil are normally not abundant in continental strata, thus in most areas they did not allow for a precise placement of the K-T interface. Exacerbating the problem, the endemic nature of vertebrate faunas in the northern and southern parts of the Western Interior of North America made correlations of these fossils difficult. Plant fossils, being much more abundant in most continental strata, have proven to be a much more valuable biochronologic tool. In particular, fossil pollen and spores have proven to be a precise tool for locating the K-T interface in the Western Interior of North America.

Relatively recently, geophysical tools have been developed to precisely date sedimentary rock strata, including radiometric dating and paleomagnetism. In the San Juan Basin, ⁴⁰Ar/³⁹Ar dating of sanidine crystals from altered volcanic ash beds has provided a series of eight precise ages for Late Cretaceous strata ranging from 75.76 Ma to 73.04 Ma (Fassett and Steiner 1997, Fassett 2000). Thus far, in spite of much searching, only one dateable ash bed has been found in Paleocene strata (in the Nacimiento Formation) in the southeast part of the San Juan Basin. Several paleomagnetic traverses, however, have been conducted across the K-T interface in the basin, and the magnetochron reversal boundaries from those studies provide excellent geochronologic tie points for the ages of K-T-interface strata.

This report focuses on the radiometric dating of ash beds, the determination of remanent paleomagnetism of rocks adjacent to the K-T interface, and palynologic dating of rock strata to precisely locate the K-T interface in the San Juan Basin. It will be shown that these independent geochronologic tools are mutually supportive in locating this interface at or below the base of the Ojo Alamo Sandstone. Vertebrate paleontology, on the other hand, has not proven to be a very precise tool for biochronology in the San Juan Basin. Rather, the radiometric, paleomagnetic, and palynologic data for rock samples from strata adjacent to the K-T-interface have established a precise geochronologic framework that can now be used to more precisely assign ages to the vertebrate faunal assemblages in these strata.