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Zircons from the Black Forest Bed, Petrified Forest Member, Chinle Formation, in Petrified Forest National Park, yield ages that range from Late Triassic to Late Archean. Grains were analyzed by multigrain TIMS (thermal-ionization mass spectrometry), single-crystal TIMS, and SHRIMP (sensitive, high-resolution ion-microprobe). Multiple-grain analysis yielded a discordia trajectory with a lower intercept of 207 ؎ 2 Ma, which because of the nature of multiple-grain sampling of a detrital bed, is not considered conclusive. Analysis of 29 detrital-zircon grains by TIMS yielded U-Pb ages of 2706 ؎ 6 Ma to 206 ؎ 6 Ma. Eleven of these ages lie between 211 and 216 ؎ 6.8 Ma. Our statistical analysis of these grains indicates that the mean of the ages, 213 ؎ 1.7 Ma, reflects more analytical error than geologic variability in sources of the grains. Grains with ages of ca. 1400 Ma were derived from the widespread plutons of that age exposed throughout the southwestern Cordillera and central United States. Twelve grains analyzed by SHRIMP provide 206 Pb*/ 238 U ages from 214 ؎ 2 Ma to 200 ؎ 4 Ma. We use these data to infer that cores of inherited material were present in many zircons and that single-crystal analysis provides an accurate estimation of

Dinosaur-bearing shale and sandstone exposed along Ojo Alamo Arroyo in the western part of the San Juan Basin were first called the Ojo Alamo Beds of Cretaceous age in 1910 by Barnum Brown, who wrote that these beds were overlain unconformably by the Puerco Formation of Tertiary age. W. J. Sinclair and Walter Granger differentiated four lithologic units in the •rocks that had been descri'bed as the Ojo Alamo Beds and lower part of the Puerco Formation by Brown. Their units are, in ascending order: (1) "shales with dinosaurs, lower horizon" (Brown's Ojo Alamo Beds); (2) lower conglomerate; (3) "shales with dinosaurs, upper horizon"; ( 4) conglomeratic sandstone with fossil logs (lower part of Brown's Puerco Formation). Units 1-4 were called the Ojo Alamo Beds by Sinclair and Granger, who reported that unit 4 was overlain unconforma•bly by shale of the Puerco Formation, with the reported unconformity probably marking the boundary between Cretaceous and Tertiary •rocks.C. M. Bauer assigned the "shales with dinosaurs, lower horizon'' to the upper part of the Kirtland Shale of Cretaceous age and defined the Ojo Alamo Sandstone to include the lower conglomerate, the "shales with dinosaurs, upper horizon" and the conglomeratic sandstone with fossil logs. Bauer reported that the upper dinosaur-bearing shale is a local lens enclosed between the two conglomerates of his Ojo Alamo Sandstone, and he placed the Cretaceous-Tertiary boundary at Sinclair and Granger's reported unconformity between his Ojo Alamo and the Puerco Formation. The dinosaur faunas of the Kirtland Shale and of Bauer's Ojo Alamo Sandstone were considered to be of Montana (Late, but not latest Cretaceous) age by Barnum Brown and C. W. Gilmore. J. B. Reeside, Jr., correlated the uppermost part of Bauer's Kirtland Shale with the McDermott Formation of Cretaceous age and stated that Bauer's Ojo Alamo Sandstone was unconformable with both the underlying McDermott and the overlying Puerco. Reeside assigned the Ojo Alamo Sandstone to the Tertiary(?), although it contained dinosaurs of Late (but not latest) Cretaceous age. Later, Reeside reassigned the Ojo Alamo Sandstone to the Cretaceous.The Puerco and Torrejon Formations in the vicinity of Ojo Alamo •were distinguished from each other by Sinclair and Granger only on the basis of their early and middle Paleocene mammal faunas. G. G. 'Simpson pointed out that no one has found a lithologic basis for mapping the rocks containing the two faunas as two separate formations. He proposed bhat the rocks containing the Puerco and Torrejon faunas be called, collectively, the Nacimiento Formation, as used by C. H. Dane.'The present writers found that the upper conglomeratic sandstone of Bauer's Ojo Alamo Sandstone rests on a widespread deeply channeled erosion surface cut in his medial dinosaurbearing shale which is a persistent unit, rather than being a lens as reported by Bauer. The present writers found also that the D1