PALEOBOTANICAL RESOURCES OF THE GRAND STAIRCASE - ESCALANTE NATIONAL MONUMENT, UTAH

Although, the paleobotanical resources of the Grand Staircase-Escalante National Monument (GSENM) of southern Utah generally are not very well known, limited research has already shown that some of the strata exposed there contain an abundance of plant megafossils and palynomorphs including an undescribed Late Triassic petrified forest that is the second largest in the world.

Plant fossils are particularly abundant in the terrestrial Chinle Formation of Late Triassic age and have been known in the GSENM since the early days of the last century. They occur principally in the Shinarump, Monitor Butte and Petrified Forest Members of the formation and include leaf compressions, pithcasts, petrified wood, and palynomorphs. With some notable exceptions, the flora is typical of that found in the Chinle Formation elsewhere on the Colorado Plateau in other parts of the Southwest. Plant megafossils that have been identified in the GSENM include horsetails (Equisetites spp., Neocalamites sp.) ferns (Cladophlebis spp., Phlebopteris smithii, and Cynepteris lasiophora), cycadophytes (Zamites powellii, Z. tidwellii, Eoginkgoites davidsonii, Williamsonia sp.), conifers (Araucarioxylon arizonicum, Woodworthia arizonica, Brachyphyllum spp., Pagiophyllum spp., Pelourdea sp.), and several unassigned forms (Dinophyton spinosus, Schilderia adamanica). The undescribed petrified forest is shown on maps of the northern part of the monument as the Wolverine Petrified Wood Natural Area. Here most of the wood is weathering out of a thin bed of sandstone in the Petrified Forest Member that may correlate with the Black Forest Bed in Petrified Forest National Park, AZ. The wood occurs in prone logs that range up to 26 m or more in length and 1.5 m in diameter. Most of the logs represent A. arizonicum but a few are assigned to W. arizonica. The wood is generally black in color and well preserved. Orientation measurement of 100 in situ logs in the main horizon is somewhat random although about one quarter of them have an azimuth of between 0° and 60.° Preliminary study of the structure in the logs of the Wolverine Petrified Forest indicates that they do not contain annual rings which is typical of most trees now growing in humid tropical parts of the world, such as southeast Asia.