VOLCANIC STRATIGRAPHY OF THE FISH LAKE PLATEAU, UTAH

The Fish Lake Plateau in south-central Utah is predominantly underlain by a sequence of Tertiary volcanic rocks. Along the northwestern escarpment of the Fish Lake Hightop volcanic rocks are >600 m thick; to the southeast these units thin to ~250 m thick near Rabbit Valley at the eastern edge of the Fish Lake Plateau. The lowermost unit is composed of multiple welded ash-flow tuffs that collectively form a package up to 300 m thick that unconformably overlies the Flagstaff Limestone. These phenocryst-rich andesites are characterized by plagioclase, clino- and ortho-pyroxene, Fe-oxides, and iddingsite phenocrysts in a glassy, vesicle-poor matrix. The phenocryst-rich andesite is composed of 56-58% SiO₂, 8-9% FeO, 5-6% CaO, and 6-7% Na₂O + K₂O. The overlying unit is a heterogeneous glass-rich phenocryst-poor trachyte up to 250 m thick. Plagioclase is the dominant phenocryst with minor clino- and ortho-pyroxene. The trachyte is composed of 65-67% SiO₂, 4-5% FeO, 1-6% CaO, and 10% Na₂O + K₂O. The Osiris Tuff, a distinctive densely-welded porphyritic dacite with abundant biotite and minor quartz, caps the lower units. All Fish Lake volcanic units experienced a protracted pre-eruptive history as evidenced by magma mixing textures and post-eruptive deuteric alteration. The Fish Lake volcanic units are similar the regionally extensive 26-23 Ma Marysvale volcanics. At the southern edge of the Fish Lake Plateau younger olivine-bearing basalts, similar to those on the Awapa Plateau, unconformably overlie andesites, trachytes, and dacites.