Geochronologic and geochemical study of volcanic ashes from the Kirtland Shale (Cretaceous), San Juan Basin, New Mexico

Several volcanic ash units that have recently been discovered in the Kirtland Shale (Cretaceous) in the northwestern part of the San Juan Basin, New Mexico, occur some 20 m (sample 62) to 39 m (sample 54) above the uppermost rocks of the Fruitland Formation. The ashes are of extremely great value in attempting to work out the stratigraphy of the rocks in this area, especially as there is some question concerning the interpretation of faunal assemblages and paleomagnetic signatures; contradicting "dates" of earliest Tertiary to latest Cretaceous have been suggested for the part of the Kirtland Shale sampled. A systematic study of the Potassium-Argon geochronology of some of the ashes has been undertaken with the following results: (1) highest ash (sample 54): sanidine concentrates--72.4 + or - 3.1 to 74.4 + or - 2.6 Ma; biotite concentrates--73.2 + or - 2.7 to 76.1 + or - 2.8 Ma; (2) lowest ash (sample 62): sanidine concentrate--75.0 + or - 2.7 Ma; and (3) upper middle ash (sample 93): sanidine concentrate--69.8 + or - 2.5 Ma. Petrographic and scanning-electron microscope studies show the biotite and sanidine to be primary phases, containing negligible amounts of detrital material. The rare earth element and other chemical studies of the ashes show them to be slightly different from each other, and this information may allow rare earth element distribution data to be used for purposes of ash correlation. We interpret the Potassium-Argon ages as suggesting a Late Cretaceous age for the part of the Kirtland Shale sampled, a view consistent with recent paleontological and paleomagnetic studies.