SIGNIFICANCE OF LATE TRIASSIC CHARCOAL, AND LATE TRIASSIC AND LATE JURASSIC WOOD PETRIFICATION PROCESSES AND MINERALOGY, SOUTH-CENTRAL UTAH

The Late Triassic Chinle Formation and Late Jurassic Morrison Formation crop out extensively near the town of Hanksville, Utah, and contain an abundance of well-preserved petrified wood. Enclosing rock types include siltstones, sandstones, conglomeratic sandstones, and conglomerates. Petrified wood samples were collected and analyzed using thin section microscopy and x-ray fluorescence spectrometry (XRF) in order to describe the petrification processes and mineralogy of the samples. Of particular interest is the presence of a nearly complete charcoal rind encasing one of the in-situ petrified logs. Triassic-aged charcoal is extremely rare, not only in Utah, but throughout the world. Due to the rarity of Triassic charcoal, scanning electron microscopy (SEM) was used in order to validate that the sample was indeed charcoal. SEM analysis revealed structures that are indicative of charcoal, even after being mineralized, without a significant amount of silica like the petrified log it encased. All petrified wood samples show a combination of replacement and impregnation, however, Triassic petrified wood shows mostly replacement of cell walls by silica and other accessory minerals probably due to high wood decay rates, whereas Jurassic petrified wood shows mostly cell impregnation by silica and other accessory minerals due to lack of wood decay.