SILICIFIED LAYERS WITHIN THE VOLCANICLASTIC UNIT OF THE OLIGOCENE BRIAN HEAD FORMATION, SOUTHERN UTAH

The Brian Head Formation in Casto Canyon, about 20 km north of Bryce Canyon, Utah, contains silicified beds up to 1.3 m thick. The upper Brain Head Formation consists of about 200 m of volcaniclastic sandstone and mudstone, with thin discontinuous limestones. The sequence is primarily of fluvial origin; the limestones were deposited in freshwater wetland environments. Silicified beds are found throughout the volcaniclastic unit, increasing upward in number and bed continuity.

We recognized 3 types of silicified layers: thin (mm-cm scale), thick (~1 m thick), and silicified organic mats. The thin layers are mostly sub-parallel to strata, but are irregular, often cross-cut bedding, and appear to be fracture fills. Thin layers are generally white, but thicker sections of these layers are brown. Thick layers are typically discontinuous laterally; of 4 thick layers found in the study area only one is laterally continuous for more than 10 m. Thick layers have complex internal structures. Where thickest, these beds consist of m-thick multi-colored lenses at the base of associated sandstone or limestone beds. In the laterally continuous layer, thick lenses are found at the top and bottom. The strata surrounding the lenses are fractured, often brecciated. Associated beds are mottled and contain silicified rhizoliths. Silicified mats are about a cm thick and consist of 1-2 mm dia., subhorizontal string-like strands, similar to the rhizoliths.

Silicified layers are composed of microcrystalline quartz; thick layers contain rare vugs with quartz crystals. Preliminary petrographic analysis indicates the layers consist primarily of chert, chalcedony, and opal. Secondary calcite is pervasive throughout thin layers, silicified mats and rhizoliths.

Extensive mottling and rhizoliths indicate pedogenesis of the associated beds. However, the fracture fills (thin layers) and apparent absence of chalcedony pebbles in the volcaniclastic unit suggest the silicified layers are diagenetic. Extensive silicification of plant roots support an early diagenetic origin; at this point a hydrothermal spring origin cannot be ruled out. Further petrographic and geochemical analyses will provide a more detailed characterization of the beds and help refine an interpretation for the formation of the silicified layers.