Upper Cretaceous and lower Tertiary rocks exposed in the Raton basin, New Mexico and Colorado, contain a thin, indium-rich, kaolinitic clay bed that marks the palynologically defined Cretaceous-Tertiary boundary. The clay bed is presumed to have been deposited as the result of a catastrophic event, possibly the impact of an asteroid, at the end of the Cretaceous; it is preserved in sediments laid down in the quiet waters of ponds and coal-forming swamps. Occurring in a conformable sequence of nonmarine rocks, this extraordinary marker bed enables observation of depositional conditions of an instant of geologic time throughout a wide area in the basin. Investigations made at various sites where the boundary clay is present indicate that component elements of a dynamically aggrading fluvial system existed at the close of the Cretaceous. This depositional system was characterized by meandering rivers, broad floodplains, and levees. The levees were breached locally by crevasse splays that periodically invaded the floodplains and associated poorly drained and well-drained swamps.

At the end of 1980, seven complete cores were recovered from a 30-m (100-ft) interval in the Raton Formation at York Canyon, New Mexico. The interval cored spans the palynologically defined Cretaceous-Tertiary boundary, which is marked by a distinctive noble metal–bearing claystone in the Raton basin. Azimuthal orientation of the cores can be recovered both from the average directions of the most stable components of the remanent magnetization, with a root mean square error of 28°, and from the average direction of secondary components of magnetization removed by thermal and alternating field demagnetization, with a root mean square error of 33°. The natural remanent magnetization of about 95 percent of the core is dominated by a secondary normal polarity component. Polarity of the characteristic magnetization of each core, interpreted from 12 to 14 samples per core run, is reversed. No evidence of normal polarity characteristic magnetization was found in the 30-m (100-ft) interval sampled. The characteristic magnetization probably is a depositional remanent magnetization acquired during chron 29r. The noble metal–bearing boundary claystone in the Raton basin is interpreted to be part of a synchronous global deposit laid down at the end of the Cretaceous period.

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.