Paleocene palynomorph assemblages from the Nacimiento Formation, San Juan Basin, New Mexico, and their biostratigraphic significance

The Nacimiento Formation bears vertebrate faunas that are types for the Puercan and Torrejonian North American Land Mammal 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 biochronologic framework for regional correlation. Two palynomorph assemblages recovered from the Paleocene Nacimiento Formation in Kimbeto Arroyo, San Juan Basin, New Mexico, can be correlated to a Paleocene palynostratigraphic 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 in (6.5 ft) above strata that yield a basal Torrejonian (To1) NALMA vertebrate fauna and within a succeeding zone of normal magnetic polarity correlated with polarity subchron C28n. It yielded an assemblage including Cicatricosisporites sp., Corollina sp. (common), Laevigatosporites sp., Momipites triorbicularis (common), Pityosporites spp., Tricolpites anguloluminosus, Ulmipollenites krempii, and Zlivisporis novomexicanum. SJB03-17 contains Momipites tenuipolus, a taxon that is widespread in the lower Paleocene in the Western Interior. Its occurrence in this sample in the lower Nacimiento Formation is consistent with its known stratigraphic range. SJB03-19 yields Momipites triorbicularis, a taxon that indicates a correlation with Paleocene palynostratigraphic Zone P3 (the Momipites actinus-Aquilapollenites spinulosus Interval Biozone), a zone identified throughout the lower Paleocene of the Western Interior. The identification of Zone P3 in the Nacimiento Formation closely associated with a To1 vertebrate assemblage firmly establishes a correlation between a Paleocene palynostratigraphic and vertebrate biostratigraphic zonation. This is consistent with most recent age correlations proposed for the Nacimiento Formation and provides an important biochronologic correlation for the Western Interior.