Abstract

The Brian Head Formation represents the first widespread 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, an ∼200-m-thick sequence of volcaniclastic sandstone, bentonitic mudstone, and thin discontinuous micrite limestone beds. The sequence is primarily of fluvial origin, and the limestones were deposited in associated freshwater wetland environments. Silicified layers are typically associated with the limestone beds.

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 paragenetic sequence that consists of: (1) microcrystalline calcite (micrite); (2) spar calcite, locally replacing micrite; (3) non-fibrous microcrystalline quartz, including widespread replacement of spar and micrite; and (4) chalcedony. Stable isotopic ratios of carbon (δ13 C from 0 to −2 per mille [‰]) and oxygen (δ18 O from 25 to 33‰) in the calcite indicate precipitation in meteoric water. Calcite precipitation likely occurred in a palustrine setting shortly after burial, possibly in a semiarid climate. Isotope ratios of oxygen (δ18 O from 12.7 to 29.3‰) in the microcrystalline quartz are compatible with precipitation by 80–150°C microcrystalline quartz-bearing fluids. Because the petrographic data indicate that the microcrystalline quartz mineralization post-dates the calcite, it follows that elevated-temperature fluids were also of groundwater origin. Subsurface elevated-temperature fluids, possibly associated with volcanism of the Marysvale volcanic complex, dissolved microcrystalline quartz from abundant glass shards in the volcaniclastic unit. Subsequent cooling of fluids caused dissolution of spar and micrite within limestone beds and the precipitation of microcrystalline quartz, thus forming the silicified layers of the Brian Head Formation.

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