LABORATORY ANALYSES OF SILICIFIED LAYERS WITHIN THE VOLCANICLASTIC UNIT OF THE OLIGOCENE BRIAN HEAD FORMATION, SOUTHWESTERN UTAH

The Brian Head Formation in southwestern Utah contains prominent silicified beds. In the Casto Canyon area, about 20 km north of Bryce Canyon, the beds are found within ~200 m thick sequence of volcaniclastic sandstone and mudstone, with thin discontinuous limestones of the upper Brian Head Formation. The sequence is primarily of fluvial origin; the limestones were deposited in freshwater wetland environments. Three types of silicified beds were recognized during field study: thin (mm-cm scale), thick (up to 3 m thick), and silicified organic mats. The thin units are typically sub-parallel to sandstone and mudstone beds, but many are fractures that cross-cut bedding. The thick beds typically are laterally discontinuous (<10 m, although one bed extends more than 100 m) and are associated with rhizoliths and limestone beds, which often have bioturbated and brecciated tops. Silicified mats are about a cm thick and consist of 1-2 mm dia. rhizoliths.

The four processes of silicification in continental settings are pedogenic, groundwater, hydrothermal and evaporite replacement. In order to understand the origin of the Brian Head silicified beds, petrographic and geochemical analyses were conducted.

The paragenetic sequence of mineral formation of the silicified layers is: 1) precipitation of micrite, 2) precipitation of spar and recrystallization of micrite into spar, 3) replacement of spar and micrite by chert and chalcedony. X-ray powder diffraction analyses confirm the calcite and quartz composition. The sequence is clearly seen within rhizoliths. Micrite root molds outline former roots. Spar filled root casts. The removal of spar through replacement by chert can be identified by the corroded boundaries and scalloped edges of spar root casts.

Rhizoliths, invertebrate bioturbation and breccias indicate pedogenesis of the palustral limestone beds, but carbon (δ¹³ C_PDB from 0 to -2) and oxygen (δ¹⁸ O_SMOW from 25 to 33) stable isotope analyses of the micrite and spar support precipitation from shallow groundwater of meteoric origin. Since silica replaces spar it is also likely to have a groundwater origin. The temperature of the silica-bearing fluid will be constrained by O-isotope analyses of the silica.