PALYNOLOGICAL EVIDENCE FOR A LATITUDINAL MOISTURE GRADIENT IN THE LATE JURASSIC MORRISON FORMATION

The Late Jurassic Morrison Formation extends over much of the Western Interior, from central New Mexico into Montana and equivalent units in Canada. In contrast to its rich fauna, plants are rare, but palynology has proven to be a valuable aid to understanding Morrison vegetation and paleoclimate. Hotton & Baghai-Riding (2010) hypothesized that the Morrison Formation in New Mexico and Arizona were drier overall compared regions to the north, based on abundance of Classopollis and relative low diversity and abundance of spores in the former. We present here analysis of an additional nine samples from the upper Salt Wash and lower Westwater Canyon Members of northwestern New Mexico. Samples are well-preserved and relatively diverse, containing typical Morrison taxa. Spores typically comprise from less than 1% to about 10% of a given sample, and only about 20 distinct spore taxa are recognized among the nine samples. The most common spore type is Cyathidites spp. (here probably mostly Dicksoniaceae). Pilasporites marcidus (Equisetaceae) and Crybelosporites vectensis are also relatively common. Rare spore forms include Retitriletes spp. and Leptolepidites spp. (probably Lycopodiaceae), Ischyosporites spp. (Schizeaceae), Todisporites minor (Osmundaceae) and Rotoverrusporites major. In contrast, Classopollis (Cheirolepidaceae) or sometimes Exesipollenites (?Taxodiaceae) dominate all samples. Bisaccates (representing Pinaceae, ?Podocarpaceae, and perhaps seed ferns) and araucariaceous pollen are subdominant. Equisetosporites (Gnetophyta) and Cycadopites (Ginkgophyta, Cycadales or Bennettitales) are present but rare. Classopollis, produced by the extinct conifer family Cheirolepidaceae, is generally treated as an indicator of aridity based on xerophytic features of the megafossils and facies associations. Araucariaceae and other conifers are considered to indicate more mesic conditions, based primarily on analogy with modern representatives. Morrison palynofloras in New Mexico contrast with assemblages from Utah, Colorado and Wyoming, where Classopollis is rare and spores display greater abundance and diversity.