PRELIMINARY RESULTS OF GEOLOGIC MAPPING OF THE SOUTHERN PORTION OF THE THREE PEAKS 7.5’ QUADRANGLE, IRON COUNTY, UTAH

The southern portion of the Three Peaks 7.5’ Quadrangle is being remapped as part of the 2021 USGS EDMAP program in conjunction with the Utah Geological Survey office in Cedar City. The southern portion of the quadrangle was last mapped by Mackin (1944-1950) and published posthumously in 1976. The Three Peaks laccolith, part of the Iron Springs Mining District, hosts economically significant apatite-magnetite dikes. The eastern flank of the intrusion is blanketed by Tertiary-modern alluvial fan deposits, potential aquifers for an area where water resources are being depleted. These issues warrant more extensive mapping of this area to: 1) refine stratigraphic relationships to further constrain Sevier deformation. 2) define mineralogic variability of the intrusion and dikes, and 3) refine mapping of modern deposits.

A preliminary geologic map was prepared using principal components analysis of remote sensing data (e.g., Landsat band ratios). 3-5 m DEMs were used to assist in mapping modern alluvial deposits. This was used to inform extensive “boots-on-the-ground” mapping.

Two major unconformities are recognized: 1) A disconformity between the ~100 Ma Marshall Creek breccia and the Middle Jurassic Crystal Creek Member of the Carmel Formation. Jurassic strata dip concordantly with lower Cretaceous strata, though localized relief on the unconformity is present. This unconformity may represent a forebulge. 2) A gentle angular unconformity between upper Cretaceous Iron Springs and upper Cretaceous to Paleogene Claron Formation. This unconformity suggests regional Sevier deformation had largely ceased by Claron time.

Based on color index (CI) of hand samples, enclave type and abundance, and magnetic susceptibility (MS), three distinct laccolith units were mapped: Facies 1 has a CI around 7 to 10, contains mafic enclaves, and yields MS values around 7*10-3 to 12*10-3 SI. Facies 2 had a CI around 4-8, contained intermediate to felsic porphyritic enclaves and microgranite dikes, and had MS values around 4*10-3 to 8*10-3 SI. Facies 3 was a leucocratic with sparse enclaves and MS values around 0.2*10-3 to 4*10-3 SI.

Intrusive contacts are generally gradational. Our mapping refined local Cretaceous stratigraphy, defined new laccolith units, and provided more detailed mapping of recent deposits.