The Fish Lake Plateau in central Utah is underlain by a thick succession of Tertiary volcanic rocks; the Osiris trachyte is a densely-welded, phenocryst-rich ash flow tuff that occurs at the top of this sequence. It crops out on the summits of the Fish Lake Hightop and Mt. Terrill in the north, and as large exposures in the southeastern part of the Fish Lake Plateau. The unit was mapped in the southeast and samples were collected for petrographic, geochemical, strain and Ar⁴⁰/Ar³⁹ analyses. The unit's thickness ranges from 30-120 m. Isopach maps of the Osiris show that it thins to the north and thickens in the southeast, suggesting that a heterogeneous landscape existed on the Fish Lake Plateau prior to eruption. A basal vitrophyre (up to 1.5 m thick) occurs at many locations. The Osiris contains at least one thin (<1.5 m) interlayered ash fall deposit, and the basal vitrophyre is also underlain in some locations by a thin ash fall layer. The Osiris trachyte covers a large area of the Marysvale Volcanic Field, a region of ~10,000 km² in central Utah that encompasses the Fish Lake Plateau. The region is characterized by felsic pyroclastic deposits, lava flows and domes, mostly erupted between 23 and 26 Ma. The Osiris is related to a catastrophic caldera-collapse eruption of Monroe Peak in the early Miocene, an event that blanketed more than 5,000 km², totaling ~250 km³ in volume.

The Osiris contains abundant plagioclase, distinctive large biotite phenocrysts, and minor sanidine and pyroxenes in a microlitic feldspar and glassy matrix. It is composed of 62-65% SiO₂, 3-4% FeO, 2-3% CaO, and 9-10% Na₂O + K₂O. The contrast between the well-developed phenocrysts and fine-grained matrix of the Osiris shows a complex pre-eruptive history, probably characterized by extensive magma mixing. Fine-grained rims on many plagioclase grains indicate that they were incorporated into the fine-grained matrix melt from another source, and zoning in plagioclase indicates extreme, rapid temperature changes. Measurements of aspect ratios of fine-grained fiamme showed XZ ratios of 4-18, and strain analyses indicate the unit experienced a flattening strain during cooling. Ongoing Ar⁴⁰/Ar³⁹ radiometric dating is being conducted using sanidine phenocrysts.