February 2008, Volume 30, Number 1 NEW MEXICO GEOLOGY 3 Abstract The Nacimiento Formation bears vertebrate faunas that are types for the Puercan and Torrejonian North American Land Mam- mal Ages (NALMAs). A biostratigraphy based on therian mammal distribution in the Nacimiento Formation can be tied to magnetostratigraphy. This is correlated to a geomagnetic polarity time scale, forming an important geochronologic and biochronolog- ic framework for regional correlation. Two palynomorph assemblages recovered from the Paleocene Nacimiento Formation in Kim- beto Arroyo, San Juan Basin, New Mexico, can be correlated to a Paleocene palynostrati- graphic zonation of western North America, providing an independent and significant biochronologic correlation. The first sample (SJB03-17) is from a carbonaceous mudstone ~1 m (~3 ft) below strata of the Puercan (Pu2) NALMAs and in a zone of normal magnetic polarity correlated with polarity subchron C29n. It yielded an assemblage including Arecipites sp., Corollina sp., Laevigatosporites sp., Momipites tenuipolus, Nyssapollenites spp., Pityosporites sp., Tricolpites anguloluminosus, Tricolpites spp., and Ulmipollenites krempii. The second sample (SJB03-19) is from a car- bonaceous mudstone approximately 2 m (6.5 ft) above strata that yield a basal Torrejonian (To1) NALMA vertebrate fauna and within a succeeding zone of normal magnetic polar- ity correlated with polarity subchron C28n. It yielded an assemblage including Cicatrico- sisporites sp., Corollina sp. (common), Laevi- gatosporites sp., Momipites triorbicularis (com- mon), Pityosporites spp., Tricolpites angulolu- minosus, Ulmipollenites krempii, and Zlivisporis novomexicanum. SJB03-17 contains Momipites tenuipolus, a taxon that is widespread in the lower Paleo- cene in the Western Interior. Its occurrence in this sample in the lower Nacimiento For- mation is consistent with its known strati- graphic range. SJB03-19 yields Momipites tri- orbicularis, a taxon that indicates a correlation with Paleocene palynostratigraphic Zone P3 (the Momipites actinus–Aquilapollenites spinu- losus Interval Biozone), a zone identified throughout the lower Paleocene of the West- ern Interior. The identification of Zone P3 in the Nacimiento Formation closely associ- ated with a To1 vertebrate assemblage firmly establishes a correlation between a Paleocene palynostratigraphic and vertebrate biostrati- graphic zonation. This is consistent with most recent age correlations proposed for the Nacimiento Formation and provides an important biochronologic correlation for the Western Interior. Introduction The Paleocene was a time of dramatic restructuring of terrestrial ecosystems and rapid diversification of many groups of ani- mals and plants following the Cretaceous– Tertiary (K–T) mass extinction. The San Juan Basin (Fig. 1), a foreland basin formed during the Laramide orogeny, contains a long and relatively continuous deposition- al record spanning the K–T boundary and the early Paleocene (Fig. 2). The record of mammalian succession of the Nacimiento Formation is particularly important as it is the longest and most complete for the early Paleocene of North America. This record has been crucial for constructing a biochronologic framework for the early Paleocene based on the succession of mam- malian faunas, the North American Land Mammal Ages (NALMAs). The continuing goal is to refine NALMAs and tie them to other biological and geological time mark- ers (Woodburne 2006). A biochronology of the early Paleocene of western North America, tied to the geomagnetic polarity time scale (GPTS), and in some places, con- strained by radiometric ages obtained from volcanic ashes, is well established (Lofgren et al. 2004). The type faunas for the two NALMAs ushering in the age of mammals, the Puercan and Torrejonian, are from the Nacimiento Formation. In an effort to refine early Paleocene biostratigraphy and biochronology, Wil- liamson (1996) proposed a therian mam- mal biostratigraphy based on lithostrati- graphic correlation of fossil localities in the Nacimiento Formation. This was tied to an existing magnetostratigraphy (Lindsay et al. 1981) that provided tie points to the GPTS. The resulting biochronology pro- vided the chief basis for the latest revision Paleocene palynomorph assemblages from the Nacimiento Formation, San Juan Basin, New Mexico, and their biostratigraphic significance Thomas E. Williamson, thomas.williamson@state.nm.us, New Mexico Museum of Natural History and Science, 1801 Mountain Road, NW, Albuquerque, New Mexico 87104; Douglas J. Nichols, dnichols@dmns.org, Denver Museum of Nature & Science, 2001 Colorado Boulevard, Denver, Colorado 80205; and Anne Weil, anne.weil@okstate.edu, Department of Biological Anthropology and Anatomy, Duke University, Box 90383, Durham, North Carolina 27708 (current address: Department of Anatomy and Cell Biology, Oklahoma State University Center for Health Sciences, 1111 W. 17th Street, Tulsa, Oklahoma 74104) FIGURE 1—Geologic map of the San Juan Basin, northwestern New Mexico (after Williamson 1996).