New Mexico Geological Society Annual Spring Meeting — Abstracts

A rich mammal, dinosaur and plant fossil record is contained within the Cretaceous and Paleocene rocks of the San Juan Basin (SJB) in NM. Details of this record have been hampered by lack of adequate geochronology that has also lead to highly contested conclusions about the dinosaur record. Direct dating of sedimentary rocks using a key volcanic ash near the base of the Nacimiento Formation and utilizing detrital sanidine dating from the Naashoibito Member of the Kirtland Formation, the Nacimiento Formation and the Ojo Alamo Sandstone is providing much needed radiometric control on the chronostratigraphy. The ash near the base of the Nacimiento yields concordant U/Pb zircon and ⁴⁰Ar/³⁹Ar sanidine dates and confirms that it is early Paleocene and within magnetochron 29n. Detrital sanidine dating of the Naashoibito reveals that it is not older than latest Cretaceous and provides a key result to constrain the paleomagnetic data. Coupling the ash date, the detrital sanidine data and a composite paleomagnetic record demonstrates that the Naashoibito (i.e., the dinosaur fossil bearing sediments of the Ojo Alamo Sandstone) is not younger than latest Cretaceous and thus does not support Paleocene dinosaurs.

Detrital sanidine dating of the Ojo Alamo Sandstone (senus stricto) near Mesa de Cuba reveals a single date that is early Paleocene seemingly providing the first radiometric constraint that at least part of the Ojo Alamo is Paleocene. We caution that a single grain is not ideal, however the date is in agreement with other Paleocene indicators. Detrital sanidine dates within the Nacimiento at Mesa de Cuba always contain a population of Paleocene grains suggesting that this method can provide ages at or near the depositional age of the rocks. Additionally the data suggests that the base of the Nacimiento is not time transgressive.

The data and conclusions from the SJB are not possible without ultra high precision Ar/Ar geochronology obtained on the new ARGUS VI multi-collector mass spectrometers housed at the New Mexico Geochronology Research Laboratory. These instruments can now provide data that will greatly expand the ability of the argon dating method and can also provide the precision necessary to answer important questions regarding mass extinctions and evolution rates. We seek to fully intercalibrate methods of geochronology, paleomagnetism and astronomical tuning at unprecedented levels of precision, and eventually, accuracy.