SPECTRORADIOMETER ANALYSIS OF CLAYS FROM THE PETRIFIED FOREST MEMBER OF TRIASSIC CHINLE FORMATION, SOUTHWESTERN UTAH

Bentonitic mudstones and siltstones of the Triassic-age Petrified Forest Member (PFM) of the Chinle Formation are well known for their colorful sedimentary layers and abundant petrified wood. The smectites in the PFM are the product of weathered volcanic ash which also provided excess silica to petrify buried trees. Fluvial studies indicate a source to the south of the Colorado Plateau despite problems identifying the volcanic source terrane. The distribution of smectites relative to other clays (e.g., kaolinite), however, has provenance and engineering implications (e.g., swelling clays).

We analyzed 24 samples from 10 locations in southwestern Utah between St. George, Cedar City and Zion National Park. The analyses were conducted using a portable ASD FieldSpec Pro Spectroradiometer which measures light over 0.4-2.5 microns in 2151contiguous channels. Characteristic spectra from samples were reduced to absolute reflectance by ratioing to a National Bureau of Standards halon plate. This nondestructive, rapid analytical technique takes advantage of absorption and scattering of photons resulting from OH-, H2O, and Al-OH vibrational processes within clay structures.

We identified montmorillonite, kaolinite/smectite clays (ranging from 12-95% kaolinite), and montmorillonite-illite mixtures by comparing spectral features from our results with lab reference spectra (U.S. Geological Survey Spectral Library). Kaolinite-rich clays in the PFM are most abundant at northern and western sites, consistent with the model of a volcanic detritus source south of the Colorado Plateau region, rather than a source to the southwest of the study area.