Abstract

For your sleeping pleasure, my latest abstract:

The Brian Head Formation represents the first widespread pulse of volcanism in the Tertiary of southwestern Utah. In the Casto Canyon area, about 20 km north of Bryce Canyon National Park, silicified beds are found within the upper part of the formation, a ~200 m thick sequence of volcaniclastic sandstone, bentonitic mudstone and thin discontinuous limestone beds. The sequence is primarily of fluvial origin and the limestones were deposited in associated freshwater wetland environments. The silicified layers are typically associated with the limestone beds. In order to understand the origin of the Brian Head silicified beds, they were studied in the field and sampled for petrographic and geochemical analyses.

Three types of silicified beds were recognized: thin (mm-cm scale), thick (up to 1.3 m thick), and silicified root mats. Petrographic analyses revealed a complex association of carbonate and silica mineralization. The paragenetic sequence consists of precipitation of: 1) microcrystalline calcite (micrite), 2) spar calcite, often replacing micrite 3) chert, including widespread replacement of spar and micrite, and 4) chalcedony. X-ray diffraction analyses confirmed the calcite and quartz composition, but also revealed opal.

Stable isotopic ratios of carbon (δ¹³ C_PDB from 0 to -2) and oxygen (δ¹⁸ O_SMOW from 25 to 33) in the calcite indicate precipitation by low-temperature groundwater in shallow buried palustrine sediments, possibly in a semi-arid climate. Isotope ratios of oxygen (δ¹⁸ O_SMOW from 21.7 to 29.28) in the silica indicate precipitation by elevated temperature fluids. Because the petrographic data indicates that the silica mineralization post-dates the calcite, it follows that hydrothermal fluids were also of groundwater origin. Estimation of the temperature of the hydrothermal silica-bearing fluid yields a range of 65-120^oC. Hydrothermal fluids dissolved silica from the abundant glass fragments in the volcaniclastic unit. Subsequent cooling of fluids caused the dissolution of spar and micrite within limestone beds and the precipitation of silica, thus forming the silicified layers of the Brian Head Formation.