MORONI FORMATION, SALT CREEK CANYON, CENTRAL UTAH: A PREVIOUSLY UNRECOGNIZED VOLCANIC CENTER

The Moroni Formation is an Oligocene (~35 Ma) volcanic-sedimentary unit that is exposed in central Utah in the Cedar Hills and north to Salt Creek Canyon (SCC) and Thistle. The Moroni is related to other mid-Tertiary centers in this region and was previously thought to be derived from the East Tintic center that lies ~50 km to the northeast. Recent fieldwork has revealed that the Moroni Formation in SCC is, in fact, derived from a previously unknown local center. The Moroni Formation in SCC consists of a volcaniclastic sequence with associated air fall tuffs that were deposited on significant paleotopography, a series of lava flows, and a trachyte dike that reaches ~100 m in thickness. The volcaniclastic sequence is divided into three units based on clast composition and field relations. The lowermost unit is a series of mass flow deposits composed of angular blocks (up to 2.5 m) of porphyritic andesite with rare intercalated tuff beds, and is interpreted as a volcano flank deposit. The unconformably overlying unit is composed of a series of mass flow deposits that grade into tuffaceous sandstones with channel fill conglomerates and numerous ash fall tuff beds; it is interpreted as an alluvial fan. Clasts consist of trachyte (up to 3 m), andesite, and minor quartzite and limestone. This second unit also includes an 18 m thick series of lava flows and agglomerates. The third unit is composed of tuffaceous sandstones and channel fill conglomerates that are also alluvial fan deposits. The clasts are welded tuffs, andesites, and rare quartzite. The dike contains fiamme and exhibits weak flow texture that suggests an explosive mode of emplacement that likely fed ash flow eruptions at the surface. The dike cuts the lowest unit and closely resembles the trachyte clasts in the second unit. These observations allow the volcanic evolution of the area to be deduced. The first phase of volcanism built a stratovolcano that provided material for the andesite mass flow deposits. This edifice underwent significant erosion and was intruded by the trachyte dike associated with an ash flow eruption; the ash flow tuff was eroded and provided clasts for the second and third unit. The quartzite and limestone clasts in the second and third unit were probably derived from strata exposed on Mt Nebo, which presented significant topography in the Oligocene.