The Ojo Alamo Sandstone of the San Juan Basin of New Mexico is composed of interbedded conglomeratic sandstone, sandstone, siltstone, and mudstone. Conglomerates are found in the western part of the basin; siliceous pebbles diminish in size both southward and eastward across the basin, becoming rare to nonexistent in the eastern part. There is great variation in the internal stratigraphy of the Ojo Alamo: Individual sandstone or mudstone beds thicken, thin, and pinch out laterally. The thickness of the Ojo Alamo varies from 6 m (20 ft) to more than 122 m (400 ft). The formation varies in composition from one to as many as five sandstone beds with interbeds of siltstone or mudstone. Dinosaur bone has been found within the Ojo Alamo at several sites in the western part of the basin. Paleocene pollen has been found within the Ojo Alamo in the eastern part of the basin. To date, no Cretaceous pollen has been found at or above the stratigraphic level of dinosaur bone within the Ojo Alamo Sandstone. Near Barrel Spring, in the southwest part of the basin, both dinosaur bone and Paleocene pollen have been found. One bone, found at the top of the Ojo Alamo, was loose on the surface, and its significance is therefore equivocal. Dinosaur bone, however, has also been found in place in the upper part of the Ojo Alamo about 1.6 km (1 mi) west of Barrel Spring, at about the same stratigraphic level as Paleocene pollen from a site just east of Barrel Spring. Because there is no apparent unconformity between the highest in-place bone level and the Paleocene pollen level in this area, the Ojo Alamo dinosaurs, if not reworked, are Paleocene in age at this site and probably throughout the San Juan Basin.

Lithofacies analysis of the Tertiary Ojo Alamo Sandstone and related strata in the San Juan Basin indicates that Laramide (Late Cretaceous–early Tertiary) volcanism and uplift north of the present-day San Juan Basin controlled sedimentation patterns of Upper Cretaceous and lower Tertiary rocks. Eight major lithofacies reflect changes in sedimentation that occurred during this time. The Ojo Alamo Sandstone is characterized in most areas of the San Juan Basin by a pebbly, trough-crossbedded lithofacies. A related channel-form sandstone and shale facies makes up the Ojo Alamo at Mesa Portales. Both lithofacies include both sediment derived from north of the present-day San Juan Basin and sediment eroded and reworked from (1) a carbonaceous shale and channel-form sandstone facies, (2) a shale and volcaniclastic sandstone facies, and (3) a volcaniclastic conglomerate and sandstone facies. The pebbly, trough-crossbedded lithofacies, which was deposited by streams on alluvial plains, differs in grain size, pebble composition, and transport direction on the east and west sides of the present-day basin. At least two distinct source areas for the streams are suggested by these differences. One source is in the area of the present-day Needle Mountains and western San Juan Mountains. A second source is located in the area of the central to eastern San Juan Mountains of southwest Colorado. Sediments deposited by alluvial streams in the western San Juan basin include sand- and pebble-size material. Initially, Ojo Alamo streams carried up to 25 percent volcanic pebbles reworked from the Animas Formation or from Upper Cretaceous andesitic flows in the source area. Later streams, however, carried an increasing percentage of quartz pebbles over volcanic pebbles. Lithofacies of the Ojo Alamo in the eastern San Juan Basin include channel sandstone and conglomerates and a channel-form sandstone and shale facies. Compared to sediments of the western alluvial complex, the eastern sediments (mapped as Ojo Alamo Sandstone, upper part of the Animas Formation, and Nacimiento Formation) are finer grained, contain few pebbles, contain less than 1 percent volcanic pebbles, and show different transport directions. Mudstone interbeds are thicker and more abundant, especially at Mesa Portales where an accompanying down-dip change in the alluvial system contributes to formation of the channel-form sandstone and shale lithofacies.

North American Paleocene land mammal ages are the Mantuan, Puercan, Torrejonian, Tiffanian, and Clarkforkian. These ages (and associate stages) are subdivided into 16 zones or subzones, varying in duration from 0.1 to 2.9 m.y., defined by widespread species. Although gross evolutionary changes during the first four of these ages are about equal, their durations are very unequal. As defined by magnetostratigraphy and fossil occurrence, the Mantuan is about 0.2 m.y., the Puercan about 1.1 m.y., the Torrejonian about 3.1 m.y., the Tiffanian about 6.1 m.y., and the Paleocene part of the Clarkforkian about 1.3 m.y. in duration. Puercan encompasses normal magnetozone 29, Torrejonian zones 28 and 27, and the Tiffanian–Clarkforkian boundary falls in zone 25. The type Rio Chico Formation of Patagonia is of mid-Tiffanian to Clarkforkian age. Problems in the identification of magnetozones in the San Juan Basin have arisen because an unconformity is present between the Kirtland Shale and the Ojo Alamo Sandstone, and for some years an extra normal chron was falsely identified. When this hiatus is taken into account, marine and terrestrial fossil correlations agree with magnetozone correlations throughout Upper Cretaceous and Paleocene rocks. The Danian stage in marine rocks in North Dakota is equivalent to Mantuan through early Tiffanian; the Thanetian is exactly equivalent to mid-Tiffanian to early Clarkforkian. The rate of Paleocene sedimentation in the major basins of North America does not depart from linearity much more than the contemporary rate of seafloor spreading. Terrestrial rates of sedimentation vary from a peak of 568 bubnoffs (b) (meters per million years) for the Hoback Formation at the Rocky Mountain front to 99 b in the San Juan Basin and 15 b in the Black Peaks Formation in Texas, compared to 2.7 b at Gubbio, Italy. Sedimentation rates along a transect through the Bighorn, Powder River, and Williston basins follow the equation Y = 200X −0.25 , where Y is the sedimentation rate in bubnoffs and X is the radial distance in kilometers from the Absaroka thrust. Absolute taxonomic and morphologic rates of evolution of the most rapidly evolving mammals during the Bugcreekian–Mantuan interval across the Cretaceous–Paleocene boundary peak at 5 genera per m.y. and 3.85 darwins (a rate of measurement defined in the text), the fastest rates known in the fossil record, and decline exponentially to more normal rates of 1 species per m.y. and 0.5 darwins by the Tiffanian. Range charts of 299 species of ungulates, primates, and multituberculates permit ready identification of zones. Seven new species of multituberculates are described, and shape and metrical properties of latest Cretaceous–Paleocene neoplagiaulacid multituberculates are summarized for ease in identification.

During the past year we have been measuring trace element abundances and searching for anomalously high iridium (Ir) concentrations in continental sedimentary rocks that span the Cretaceous-Tertiary boundary in the Raton and San Juan Basins of northern New Mexico and southern Colorado. Using neutron activation and radiochemical separations, we have identified anomalous concentrations of Ir in samples from two sites in the Raton Basin: in a drill core at York Canyon, about 50 km west of Raton, New Mexico, and in a road cut near the city of Raton. In both cases the anomaly occurs essentially at the base of thin coal beds, across a thickness span of only a few cm and at the same level at which several species of Cretaceous pollen become extinct and the ratio of angiosperm pollen to fern spores drops sharply. The Ir surface density ranges from 8 to 40 × 10 −9 g cm −2 . In the York Canyon core the Ir concentration reaches a value of 5.6 × 10 −9 g/g of rock over a local background of about 10 −11 g/g; the Pt abundance distribution is similar to that for Ir, while Au reaches its maximum concentration about 10 cm below the Ir peak. Se, V, Cr, Mn, Co, and Zn are about two-fold more abundant at the anomaly zone than in adjacent zones, and mass spectrometric 244 Pu analysis showed the 244 Pu/Ir atom ratio ⩽ 1 × 10 7 . In the San Juan Basin we have located a small Ir spike (55 × 10 −12 g/g over a local background of 8 × 10 −12 g/g) that is accompanied by high concentrations of Co and Mn. It is thought to be due to geochemical enrichment processes.

Available evidence suggests that dinosaurs in the San Juan Basin survived the event responsible for the creation of an iridium-enriched zone that has been discovered at or near the Cretaceous/Tertiary boundary at several locations throughout the world. The iridium zone has been found at two separate localities in the Raton Basin coincident with the palynologic Cretaceous/Tertiary boundary. The iridium zone has been searched for in the San Juan Basin at several localities where the palynologic Cretaceous/Tertiary boundary has been bracketed, but it has not been found. It is unlikely that the iridium zone will be found in the San Juan Basin, because stratigraphic studies indicate that the rocks which could have contained the iridium zone were eroded prior to deposition of the Ojo Alamo Sandstone. The significant fact is that dinosaur bone has been found within the Ojo Alamo Sandstone in the San Juan Basin stratigraphically higher than the palynologic Cretaceous/Tertiary boundary; thus, this bone apparently postdates the event that created the iridium zone.

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.