PALEOMAGNETISM OF ROCK STRATA ADJACENT TO THE CRETACEOUSTERTIARY INTERFACE IN THE SAN JUAN BASIN, NEW MEXICO AND COLORADO

Multiple paleomagnetic sections have been obtained through rock strata adjacent to the CretaceousTertiary (K-T) interface in the San Juan Basin in New Mexico and Colorado. Paleomagnetic studies of uppermost Cretaceous strata in the southern San Juan Basin have revealed the presence of an interval of reversed-polarity in the Kirtland and Fruitland Formations immediately beneath the K-T interface and an interval of normal polarity of undetermined length in underlying Cretaceous strata. (These polarity intervals were incorrectly assigned to magnetochrons C30r and C31n by some previous authors.) Subsequently, 40 Ar/ 39 Ar ages for sanidine crystals from eight altered volcanic ash beds in this same stratigraphic interval revealed that these two magnetochrons were unequivocally C32r and C33n. Because two ash-bed ages were above the magnetochron reversal and six were below, the age of the reversal was precisely determined to be 73.50 ± 0.19 Ma, thus establishing a new precise Upper Cretaceous tie-point for global geologic time scales. A magnetic-polarity section obtained in the northeast part of the San Juan Basin in Colorado (the Chimney Rock section) through the upper part of the Lewis Shale, Pictured Cliffs Sandstone, and lowermost Fruitland Formation also located magnetochrons C33n and C32r. A 40 Ar/ 39 Ar age for an ash bed in this section confirmed the identity of these magnetochrons. A cross section across the basin from southwest to northeast shows that the C32rC33n reversal exactly parallels the Huerfanito Bentonite Bed present 335 m below in the underlying Lewis Shale indicating that there was no differential tectonism resulting in erosion or non-deposition of strata between these two geochrons in the San Juan Basin. Magnetic-polarity studies of the Paleocene Ojo Alamo Sandstone reveal that there is a thin, reversedpolarity interval in the lowermost part of this formation overlain by a relatively thin normalpolarity interval overlain by a thin reversed-polarity interval (all within the Ojo Alamo Sandstone), overlain by a longer normal interval extending into the lower part of the overlying Nacimiento Formation. On the basis of palynology, vertebrate paleontology, and physical geology the magneticpolarity intervals in the Ojo Alamo Sandstone can be confidently labeled magnetochrons C29r and subchrons C29n.2n, C29n.1r, and C29n.1n. Thus, not only is all of the Ojo Alamo Sandstone Paleocene in age; these data also indicate that approximately 0.3 m.y. of lowermost Paleocene time is not represented by rock strata in the southern San Juan Basin. Because the age of the highest Cretaceous ash bed, about 5 m below the base of the Ojo Alamo and the K-T interface is 73.04 Ma, the duration of the K-T hiatus in the southern San Juan Basin is estimated to be 7.84 m.y. (73.04 – 65.2 = 7.84).