Figure 42. Stratigraphic cross section showing levels of Paleocene mammal fossils identified in lowermost part of Nacimiento Formation, southern San Juan Basin, modified fromWilliamson 1996. Magnetic-polarity columns and fossil levels for DNZW, WFKW, and BTW localities scaled directly from illustrations in original source: Lindsay et al. (1981), thus differ slightly from figure 9 of Williamson (1996). Fossil levels in GC and EFKW columns from Williamson (1996). Stratigraphic levels of Torrejonian mammals in MDC column from Simpson (1959, figure 1). Base of C29n not determined at BTW locality but projected to just above base of Ojo Alamo Sandstone. Top of C29n not determined at MDC locality but interpolated based on estimated rates of deposition for lower part of Nacimiento Formation. Boundary between Puercan and Torrejonian mammals (black dashed line) is placed between lowest Torrejonian fossil at MDC locality and highest Puercan fossil at GC locality. Boundary is estimated to be about 64.4 Ma because it is near top of magnetochron C29n. Column localities are shown on Figure 3.