MINERALOGICAL ANALYSIS OF FOSSILIZATION OF PETRIFIED WOOD IN THE CHINLE FORMATION

The Triassic age Chinle Formation is well-known for its fossil plant and vertebrate remains, preserved relicts of a forested terrestrial ecosystem more than 200 million years old. The famous fossilized trees in the unit have been well-sampled and tied to chronostratigraphy and biostratigraphy, however aspects of the fossilization mode, mineralogy, and fossilization process of this treasured paleontological deposit have not been thoroughly documented.

Here we report findings from our study of fossil wood specimens from the Chinle Formation near Holbrook, Arizona, from localities north of Flagstaff, Arizona, and from Petrified Forest National Park itself. Four specimens, from three different stratigraphic members of the formation (the Shinarump Member, the Sonsela Sandstone, and the Petrified Forest Member) were prepared and analyzed by means of petrographic thin section of radial, transverse, and tangential cuts, using x-ray diffraction (XRD), and through use of the scanning electron microscope (SEM) and associated energy dispersive spectroscopy (EDS). Using these methods, we describe the observed wood anatomical features and the permineralization and replacement fossilization chemistry and mineralogy. We also explore the relative timing of the taphonomic stages of fossil wood preservation.

Specimens display internal tree ring and cellular anatomy preserved by dominantly quartz (silica) permineralization, minor traces of elemental gold in some perservation mineral phases, and evidence of multiple mineral replacement stages in process of fossilization. The best preserved wood appears to have been permineralized quickly, and prior to compressive structural deformation. We also present data regarding growth ring anatomy of the sample woods, considering how these features could reflect seasonality and stressors in the tree’s growth history. These analyses consider earlywood/latewood ring thickness, cell counts, and transitions, as well as the presence of false rings.

This characterization of the Chinle fossil wood adds to the growing understanding of this Triassic paleoecosystem and its diagenetic preservation in a river basin paleoenvironmental setting at low paleolatitudes.