Journal Title

Journal of Geology

Publication Date



Detailed paleomagnetic and rock-magnetic analyses have been performed on samples from multiple sections through the Cretaceous/Tertiary (K/T) boundary and throughout a 750 m thick sequence of late Cretaceous through middle Paleocene continental deposits in the San Juan Basin, New Mexico. Curie temperatures have been determined for magnetic separates from 52 levels and, with only two exceptions, they range from 180 to 300°C. Along with microprobe and X-ray analyses these data indicate that the detrital ferrimagnetic mineral is titanohematite with composition 0.45 < x < 0.60. This magnetic mineralogy indicates derivation of the continental San Juan Basin sediments from a volcanic (probably dacitic or andesitic) source. These mineralogical data, along with other geological data and the pattern of magnetic polarity zonation in multiple sections across the basin, argue strongly for deposition of the late Cretaceous and Paleocene continental deposits in the San Juan Basin as a clastic wedge derived from a source to the north or northwest. Demagnetization experiments, coupled with the mineralogy of the magnetic minerals, indicate that revision of our previous correlation of the San Juan Basin stratigraphic sequence with the magnetic polarity time scale is required. This revision indicates that the K/T boundary (recognized above the highest stratigraphic occurrence of dinosaur fossils) occurs within a reversed polarity zone correlative with magnetic polarity chron 29R. This correlation is consistent with the K/T boundary in the marine sedimentary sequence at Gubbio, Italy. Puercan (early Paleocene) fossil mammals occur within a normal polarity zone correlative with chron 29N and Torrejonian (middle Paleocene) fossil mammals occur within polarity zones correlative with chrons 27N, 27R, and 28N. With this revision, consistent and sequential placements within the magnetic polarity time scale have been accomplished for all North American land mammal ages in the Paleocene through early Eocene interval.


Paleomagnetism; Mineralogy

Publication Information

© 1985, University of Chicago Press. View original published article at 10.1086/628979">10.1086/628979.





Document Type

Journal Article