Paleomagnetic results from the San Juan Basin, New Mexico, indicate that dinosaur extinction in that area occurred after marine extinctions at the Cretaceous/Tertiary boundary in Italy and in other marine sequences. Evidence of diachronous Cretaceous extinction in the San Juan Basin is strongly dependent upon correlation of San Juan Basin magnetozone γ + with magnetic anomaly 29.

Study of magnetic minerals in San Juan Basin sediments by Butler led to the realization that the dominant carrier of detrital remanent magnetization in those sediments is titanomagnetite with Curie temperature of about 180°C. In addition, isothermal remanent magnetization acquisition was studied in order to detect minor content of hematite in the sedimentary rocks.

Results show no clear evidence for overprinting associated with magnetozone γ + or any other normal magnetozones. However, sediments of magnetozone γ + and adjacent sediments have a higher than normal hematite content. More detailed tests are planned, utilizing knowledge already gained of magnetic properties in those rocks, for better identification of possible magnetic normal overprinting. Those tests, when applied to magnetozone γ + will provide a more secure determination for synchronous or diachronous extinctions at the end of Cretaceous time.