Cretaceous mammals have been recovered from the lowermost Kirtland Shale (Quarry 1 of the Fossil Forest study area). Of the 13 therian mammal taxa described, 6 are new ( Alphadon parapraesagus, n. sp., Ectocentrocristus foxi n. gen. and sp., Pediomys fassetti n. sp., Aquiladelphis paraminor, n. sp., Gypsonictops clemensi n. sp., and Cimolestes lucasi n. sp.). Faunal comparison with other known Cretaceous sites show that Fossil Forest mammals are most closely related to faunas described by Sahni (1972) from the Judith River Formation and by Fox (1977, 1979a, 1979b, 1979c, and 1981) from the Oldman Formation (Judith River equivalent). The age of the lowermost part of the Kirtland Shale must be near the Campanian-Maastrichtian boundary based on mammalian evidence. The long normal magnetochrons (30 and 31 of Lindsay and others [1981]) must, therefore, be renumbered. Two alternatives for renumbering are discussed; the fossil mammal evidence favors renumbering the magnetochrons as anomaly 33. This interpretation requires that one or more unconformities be present between the lower part of the Kirtland Shale and overlying formations on the south flank of the San Juan Basin. This results in the loss of approximately 6.4 Ma of rock record that probably includes the Cretaceous-Tertiary boundary event.

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 ± 3.1 to 74.4 ± 2.6 Ma; biotite concentrates—73.2 ± 2.7 to 76.1 ± 2.8 Ma; (2) lowest ash (sample 62): sanidine concentrate—75.0 ± 2.7 Ma; and (3) upper middle ash (sample 93): sanidine concentrate—69.8 ± 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.

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.