Cretaceous-Tertiary palynology, eastern side of the San Juan Basin, New Mexico

Abstract

Early studies of vertebrate and plant fossils in the San Juan Basin confirmed a late Cretaceous age for the Kirtland Shale and a Tertiary age for the Nacimiento Formation, but resulted in disagreement over the age of the intervening Ojo Alamo Sandstone. Dinosaur evidence indicated a late, but not latest, Cretaceous age, but fragmentary plant megafossils suggested a Tertiary age. Pollen and spore florules collected from within, above, and below the formation tend to confirm a Tertiary age and reflect the environmental changes that accompanied local uplift at the Cretaceous-Tertiary transition. The most significant ecologic change takes place at the base of the Ojo Alamo Sandstone with the appearance of many podocarpaceous and ulmacoues pollen.The most significant change in terms of common forms occurs between the basal Ojo Alamo florule and one collected from a shale unit in the middle of the formation. The basal florule has only four forms in common with the overlying or underlying florules and could be either Cretaceous of Paleocene. The middle florule, however, has nine forms in common with the overlying Nacimiento florules, suggesting Tertiary affinity. Middle Montanan dinosaur bones and fragments, which occur in a similar shale unit on the western side of the basin, may have been reworked or erroneously identified.Proteaceous grains and Tilia are the dominant dicotyledonous types in the Kirtland Shale florule. Conifers are absent. The florule from the base of the Ojo Alamo Sandstone contains more than 70% Podocarpus pollen, ulmaceous pollen, and several other probable upland types. The middle Ojo Alamo florule contains a mixture of probable upland and lowland forms, as does a florule from the base of the Nacimiento Formation. A florule in the lower part of the Nacimiento Formation is similar to the florule at the base. Ulmaceous pollen, Momipites, and Cupanieidites are the most persistent dicotyledonous types in these three florules. A florule from the uppermost part of the Lewis Shale is a northern equivalent of the Kirtland or lower Ojo Alamo florule but is very different because of a more coastal environment on the opposite shore of the Lewis Sea.The classification system used in this study employs a combination of extant, organ, and form genera arranged in a phylogenetic outline. Eight new genera, Bombacacipites, Brevicolporites, Confertisulcites, Intertriletes, Kurtzipites, Navisulcites, Rectosulcites, and Ulmoideipites, are established, and several others are validated. Of the 88 fossil descriptions, 39 are new species and four are descriptions of dinoflagellates and marine microfossils Incertae Sedis from the Lewis Shale.